Adding Zone with Fixed Speed Furnace

Discussion in 'HVAC Heating & Cooling' started by Kabra, Jun 10, 2021.

  1. Kabra

    Kabra New Member

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    I bought a new house a couple years ago and one of the bedrooms has three exterior walls and in the winter that room is 6* - 10* colder than the area where the thermostat is. The builder is offering to add a zone for that area (including an adjacent bedroom) on a fixed speed single zone system and bypassing 75% of the flow (heat and A/C) back into the return. My impression is that this approach is typically frowned on especially for a relatively small zone like this as it can freeze up the coil and possibly cause other problems. I am in the Denver area so humidity at least is quite low here. Thanks.
     
  2. fitter30

    fitter30 Well-Known Member

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    Single speed systems that are zoned are less energy efficient, tend to be louder ( fan running on high speed even with a bypass) and harder on the equipment by running lower coil temperature in bypass.
     
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  4. Dana

    Dana In the trades

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    HVAC contractors or insulation contractors have their own windows on the topic, but rarely see the bigger picture. Not all comfort problems are due to the HVAC, nor are they all about insulation or window performance. They have to work as a system, and the systems have to be in balance to work well. Micro-zoning the hell out of the place can sometimes work, but more often it creates other problems and ends up costing more in the end.

    Before you do anything, try to figure out WHY it's running 6F-10F colder (I'm assuming that's in winter?) than the rest of the house. Take the time and run room-by-room load calculations for this room and a few rooms adjacent to it using a freebie online Manual-J-ish type tool such as LoadCalc or CoolCalc (or if you want to play junior duct designer, use the load calculator on the BetterBuiltNW tool- it's free, but requires you to create an account.) Be aggressive on air tightness & R-value assumptions- try to be accurate, but think optimistically when guesstimating (is it R19 or is it R25 under the floor? etc.) The biggest errors are created by underestimating the performance of the insulation or windows or air tightness. For this exercise assume the house and ducts are hermetically sealed, completely air tight, unless you know from experience that it feels very drafty in a particular room.

    Then ake a look at both the sizes and lengths of the duct run(s) to those rooms. Are the duct sizes (in cross sectional area) roughly proportional to the calculated loads? Are the lengths from the plenum roughly the same? If the colder room is undersized or at the end of a long or twisty run it might be fixable with a duct tweak.

    To the extent possible, inspect for loose/disconnected ducts or squashed flex duct. If it's mostly flex duct, is the duct stretched pretty tight, or is it all twisted & floppy? Flex that is not stretched tight can easily have twice the friction (or more) that it was designed for, yielding very low flow.

    Does the room have a dedicated return duct & return grille? How big is the return relative to the supply duct? A doored off room with no dedicated return doesn't get enough flow when the door is closed and it pressurizes the room relative to the outdoors. In the (rare) case that the return duct is 3x or more bigger than the supply duct the room could be significantly depressurized relative to the outdoors, forcing outdoor air infiltration into the room whenever the air handler is running. If the returns are inadequate they can usually be improved by variations on "jump ducts" , creating paths for the return air to allow higher overall flow. Door cuts at the threshold are never enough except in very high performance homes (not your house.)

    It's also worth inspecting for gaps in the insulation and air leaks in from the outdoors into the walls, especially if there are exterior walls or parts of exterior walls that feel colder than others in winter. This is easier to do with infra-red cameras and calibrated blower doors, but a $50 pistol-grip infra-red thermometer and a large window fan can still get you there. (FLIR has a decent $200 IR camera that uses a smart phone or tablet computer as a display, if you're the type of person who loves gadgets. It's currently on sale for $150.) While it's easier to look for hot/cold spots in the walls during the fall when the outdoor temps are 20F or more cooler than the indoors, it's still possible to find stuff during the summer. Depressurizing the room with a window fan (blowing out), will cause leaky areas to change temperature toward the outdoor air temp, showing up as hot/cold spots at the air leak points. Missing insulation shows up as large patches of dramatically different wall or ceiling area.

    You may be able to know from the experience of living there roughly what the oversize factor on the furnace is. When it's ~ +5F outside (the approximate 99% outside design temp for Denver & surroundings) does the furnace run at least 40 minutes out of every hour (~1.5x oversizing factor) or does it run 8-10 minutes then stop for 15-25 minutes between cycles (2-3x oversized)? If you don't know but have last winter's fuel bills handy, you can calculate the "block load" aka "whole house heat load" by looking up heating degree-day data from a nearby weather station and running some arithmetic, as outlined here. (The AC oversize factor can be more readily estimated by measuring the duty cycle on afternoons when it's crossing through the ~91F mark, roughtly the 1% outside design temp for Denver.)

    If the furnace is 3x or more oversized and the room in question is at the end of a longer run the equipment oversizing could be preventing the room's temp from tracking with the rest of the house due to a low-low duty cycle. Bigger is not better- ASHRAE recommends a 1.4x oversize factor for the 99% design load, which would mean when it's +5F outside the furnace would be running (1/1.4= ) 71% of the time- that 43 minutes out of every hour at that temp (when the sun is down. since solar gain can lower that duty cycle). While a furnace & AC swap for something more appropriately sized would be an expensive fix, it's sometimes a necessary step in getting the room to room temperatures to balance.

    Nate Adams is a contractor in Cleveland OH (a location with comparable design temps to Denver) who has made a business out of fixing comfort issues looking at the whole house issues, not just the HVAC, and has written a decent book on the topic. In your situation it's worth reviewing his the short videos and free downloadable chapters from the book here:

    Home Comfort 101

    HVAC 101

    HVAC 102

    After ruling out gross errors in the duct design attacking these problems it's always important to fix the building issues first, starting with air sealing. Unless the house is reasonably air tight controlling heat & moisture flows within the house. The biggest leaks in the house aren't always obvious, but unless you're going there I won't go into it in detail. Fixing the insulation is best done AFTER the air sealing, since getting to the air leaks sometimes requires removing & replacing pre-existing insulation.
     
  5. Kabra

    Kabra New Member

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    Thanks very much for the responses. I intentionally left out the first 95% of the story so as not to complicate things. The question I asked is really towards the end of a very long trail where the builder's warranty isn't worth the paper it's written on until you force them to honor it. It started with purchasing several temp sensors that I calibrated and collected data showing that the problem was real. They then acknowledged there was a problem so they put a 1" larger duct to the room and that did almost nothing. Then was the phase where they send their warranty thug out that comes up with every reason under the sun why they aren't liable under the warranty to do anything and I just need to run the fan 24/7 and keep the bedroom door open. How do you think my teenage daughter liked the open door idea? My only option per the warranty is mediation and then arbitration. So then I hired an energy consultant that did thermal analysis, blower door and leakage testing, measured the CFM of all the registers and returns, and reviewed the Manual J specs. The bedroom in question had almost twice the heating CFM that the Manual J called for and the room was still extremely cold. While the house was tight overall there was some significant leakage around the rim in the basement because the insulation contractor did a really poor job of applying the foam to the seams so they agreed to have the contractor come back and apply a layer closed cell foam over the rim joists. That fixed the leakage but not the problem with the cold bedroom. There was no indication in the thermal analysis that there was any problem with the insulation. Incidentally, other homeowners with this same model have the same problem to one degree or another with the main variable the orientation of the house. This bedroom happens to be on the NW corner of the house and three of the four walls of the bedroom are exterior walls.

    So this is probably starting to paint a picture of why this led to the need for a new zone. The other thing I haven't mentioned which has to do with the orientation is that the thermostat is in the great room on the south and of the house. So on cold winter days with the sun so low in the horizon the sun pours into the great room keeping that area where the thermostat is nice and toasty so the thermostat won't kick the heater on all afternoon while the bedroom on the north end gets colder and colder all day long. This is where I've seen up to an 11* difference between the great room and bedroom. Overnight it's typically around 6* difference. Incidentally the newer tracts where this same model is still being sold have a different HVAC design that has a separate zone for this bedroom area. I think it's pretty obvious what was missed in the design is the thermostat location (in homes with the great room towards the south), and also the rate of heat loss due to the three exterior walls. I'm no Manual J expert but it seems that the CFM was probably calculated correctly but what seems to be missing is that the furnace needs to be cycled more for that room compared to the rest of the house and as we now know from experience just throwing more CFM in the room. Doesn't solve the problem.

    So the builder is trying to get off as cheap as they can and while there is no debate that an additional zone is necessary, they want to implement it with the existing fixed speed furnace which is a Bryant 912SB48080S17A-B and is the model usb before they started using ECM motors. My main concern of course is that I don't want unintended consequences trying to fix a 192sf bedroom area and compromise what is other than that heating problem in the bedroom, an HVAC system that works really well.

    Considering fitter30's comments, it sounds like how well it works is dependent on the details of how they implement the zone and adjust things. I think I can live with some imperfection in that zone as long as it doesn't cause problems in the rest of the house.

    As I mentioned the only problem is that bedroom doesn't get enough heat. The A/C is fine in that room as is so I was thinking of ways to keep the existing single zone for A/C and just use the 2nd zone for heating. This of course would also eliminate the concerns about overcooling the coil since there would be no bypass to the return for A/C. So I was thinking the thermostat for the new zone (which will be an Ecobee 5) could be setup without A/C. Then when the thermostat for the main zone calls for A/C it the zone controller would always open both dampers so in effect the entire system is a single zone for A/C. I'm not positive this can work but it seems like it's worth asking.

    This led me to another idea, and admittedly this might be overthinking things but again I think worth asking about. I'm thinking why not treat the entire system as a single zone, EXCEPT when the bedroom needs extra heat. So anytime the main zone calls for heat it could treat the entire system as one zone. Then when the bedroom zone needs additional heat it could run the system only for that zone with the bypass to the return. This would minimize the time that the small bedroom zone needs to call for heat by itself. It seems this would entirely eliminate the need for a damper for the bedroom zone. The only dampers that would be needed would be a one for the main zone (when the bedroom zone calls for heat by itself), and one for the bypass to the return under the same condition of bedroom heat only. I realize the bedroom would always get heat when the main zone calls for it whether the bedroom needs it or not but my sense is from experience that won't make a huge difference in the bedroom.

    I'm not sure if these ideas simplify things or add to the complexity but again it seems worth asking about. I know I'm also making this thread more complicated so I don't expectations for a response but sure would appreciate it if anyone cares to do so.

    Thanks again for all the input.
     
  6. Dana

    Dana In the trades

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    At 80,000 BTU/hr (in), 75,000 BTU/hr (out) the Bryant 912SB48080S17A-B is way more than 1.4x oversized for most normal sized newer homes in the US. (It's about 2x oversized for the design load for my way sub-code 2x4 framed 2400' + 1600' of insulated basement antique with mostly 1920 vintage single panes + 1980s vintage clear glass storms.)

    What was the whole house "block load" in the Manual-J? Does it correlate reasonably well with a fuel-use based load calc?

    What did the Manual-J call out for the heat load of the cold room? Is it a "bonus room" over the garage, mayhaps?

    If the water heater is reasonably close to the cold bedroom one could install some baseboard or flat panel rads. A 1/2 to 3/4 ton ductless mini-split heat pump is another possible solution. Depending on the complexity of installing a hydronic loop off the water heater it might be cheaper to do the mini-split. Any cold climate mini-split would do, but choosing a model with a reasonably low but still efficient minimum-output @ 47F would be preferred for both comfort & efficiency.

    Searching the NEEP database for models that put out 8000-12000 BTU/hr comes up with dozens of hits, but of those that modulate down to under 2000 BTU/hr @ 47F the LG LAU/LAN090HYV1 or Mitsubishi MUZ/MSZ-FS06NA-U1 (or FS09) are some of the better bets for decent efficiency at very low minimum speed. If it were serving a larger load it would be worth calculating the derating for altitude, but they are ALL overkill for all but the largest and lossiest bedrooms. The efficiency at minimum-output is the most important factor- there many are similar models that are fine at mid to max capacity, but fall off an efficiency cliff below 2000 BTU/hr delivering 1/3 the efficiency of the more efficient models.

    At pre-Pandemic Pricing a pretty good cold climate 3/4 tonner would run between $3-4K USD in my neighborhood, but it's hard to say what it would cost this week. Panel rads with a potable compatible isolating heat exchanger and pumps to sip some heat from the water heater could easily run about the same.
     
  7. Stuff

    Stuff Well-Known Member

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    A few questions I didn't see answered: Does the bedroom have a working return register? Is the room OK with the door open or not? Was a Manual J calculation done for the entire house or just the bedroom?

    From the book referenced above https://www.deanheatingandcooling.c...5937699/home_comfort_101_181118_spreadsv2.pdf
    I’ve tested a number of homes built in the last 10-15 years that are quite air tight, but are still very uncomfortable. The culprit? A huge furnace and air conditioner. It kicks on for a few minutes, then kicks back off. Often the rooms furthest from the furnace freeze in the winter because the heat didn’t make it there before the furnace turned off. The walls remain cold because heat comes in blasts, like a bucket of water being dumped over your head vs. a nice shower.​
     
    Last edited: Jun 13, 2021
  8. Kabra

    Kabra New Member

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  9. Stuff

    Stuff Well-Known Member

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    Thanks for the details.

    Some homes have a dump zone (basement) for this. Directing the bypass air back to the furnace can cause it to overheat and shorten its life.
     
    Last edited: Jun 14, 2021
  10. Dana

    Dana In the trades

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    So, what was the block load "whole house" Manual-J, and what is the ratio of the 75,000 BTU/hr output of the furnace to the Manual-J total load?

    This mattters! If the furnace is sized correctly the duty cycle would be high enough to overcome any problems related to higher loss room loads, provided the design cfm is being met. If it's running only a 20-30% duty cycle during "normal cold" (not 99% design condition) due to 2x+ oversize factor the room will always be cold, even if the duct design is fine.

    I disagree that there is no debate.

    Run the fuel-use based whole-house load calculation, and compare it to the Manual-J. Compare both to the 75K furnace output. There are many houses with 75K-out (or larger) furnaces serving 25-30K design loads with miserable outlier rooms like yours, a problem (often) better solved by installing a 35K-out furnace, since that increases comfort for the entire house.

    Breaking it up into zones can solve a cold room problem, but if the oversize factor is still 2-3x or more the comfort levels overall are lower than what they could/should be.

    Even the crummiest & smallest cold climate mini-splits will more than cover the loads on it's own, and will "play nice" with the main HVAC system turning down or off relative to how much the main HVAC system is delivering. In most of CO it really takes a cold climate version, not so much for the capacity issues, but for the defrost ice management in the outdoor unit (which should have a pan heater to keep ice from building up in the bottom during cold weather stressing the coils or even interfering with the blower fan blades.)

    For cost-sensitive applicatins, most cold-climate 3/4 ton Mideas (or any of the re-labeled versions from Carrier, Pioneer, Senville, Mr.Cool, etc.) cost less than $1500 for the hardware, and in normal times would cost ~$3K fully installed. Due to higher minimum-modulated output (this one can't go below 5000 BTU/hr @ 47F) it would cycle more often than some of the others ( rather than modulate), but most models still have decent efficiency at that minimum modulation, making the cost per heating BTU comparable to condensing natural gas (or even beating it during the shoulder seasons). Midea is a decent (independent, Chinese) manufacturer OEMing equipment to many other companies with decent standards, and has become the world's largest manufacturer of heat pumps & air conditioners. They have been in bed with Toshiba (the designer/manufacturer of Midea's refrigeration compressors) for a couple of decades, and with Carrier for over a decade. Mitsubishi or Fujitsu or LG cold climate 3/4 tonners would cost less than $4K at pre-pandemic pricing (but now it's a crap shoot).

    Some features of the newest-latest greatest Mideas sold under the Carrier nameplate are even preferable to some of the higher-priced Japanese & Korean vendors, being easier to maintain, and with a "follow me" temperature sensor in the hand held remote making the remote the room temperature sensor rather than sensing room temp via the temp the incoming air at the head/cassette, controlling it more like the thermostats US users are accustomed to. This feature also leads to more stable room temperatures, and avoids short-cycling the unit when it's installed in a cramped space. (Those features usually cost extra from the Japanese vendors.)
     
  11. Kabra

    Kabra New Member

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    Hi Dana, I don't see that the block load is calculated in the manual J. Interestingly the manual J specifies the heating output of about 75% of the rated capacity. So the 80K BTU furnace has a heat output just under 59K BTU. So I'm not sure what to do about the ratios you're asking about. In trying to keep this thread on the simpler side I didn't mention that there are actually two hvac systems. The one I've been referring to is called the "Day System" in the manual J and the other smaller system is referred to as the "Night System". The latter is also a Bryant 912 but 40K BTU. Both use Carrier compressors. I am including some snippets of some manual J pages and also the measured CFM after the ducts were upsized by 1" in Bedrooms 2 and 3. The larger duct resolved what was a pretty minor problem by comparison in bedroom 2 but didn't seem to make much of a dent in Bedroom 3 with the three exterior walls. The manual J refers to both the heat and cooling as Carrier models but as I mentioned the furnaces are Bryant.

    I agree that a mini split could solve the problem but I would rather not go that route for a bunch of reasons. As I mentioned the only problem I have is needing the furnace to cycle more in that one bedroom and there are two reasons for this. First and foremost are the three exterior walls, and second is the fact the thermostat is on the south end of the house in the great room which takes in sun on cold winter days. Let me know if there is any other information from the manual J that would be helpful.

    Thank you! Floor Plan.jpg Duct Layout.jpg Day System.jpg Night System.jpg CFM.jpg Whole House.jpg
     
  12. Kabra

    Kabra New Member

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    I forgot to mention that the house is one story ranch with a 3400 sf main floor, a 2300 sf unfinished basement and an 1100 sf conditioned crawl space.
     
  13. Dana

    Dana In the trades

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    At the bottom of the last page they give a block load for the "DaySystem" of 41,307 BTU/hr...

    ... and the block load for the "NightSystem" of 17,430 BTU/hr...

    ... with a combined block load for " Entire House" of 58737 BTU/hr plus "Other equip loads" of 6104. I suspect the last bit is what they're allowing to account for duct losses & air handler driven infiltration (?).

    The 75% of rated capacity may be a derating due to altitude (or something else?). The classic (and not accurate for most new equipment) derating is 4% for every 1000' of elevation above 1000'. So if you're at 5k' thats 4k' above 1k', so the derating would be:

    Rated Capacity - (.04 x 4)= 86% of rated capacity.

    Earlier they ascribe a

    If your elevation is 7k', that's 6k' above 1000', for a derated capacity of RC - (6 x 0.04)= 76 % of rated capacity, etc.

    Night System:
    [​IMG]

    ^^^^The Night System load page shows a block load of 17,430BTU/hr + 2367 BTU/hr of "Other equipment loads" of 2367 BTU/hr totaling 19797 BTU/hr. ASHRAE's 1.4x oversize factor recommendation would be then 1.4 x 19797 BTU/hr = 27,716 BTU/hr.

    The total square footage of conditioned space for for Night System is 2297 square feet. The load/square foot ratio is then 19797 BTU/hr/2297'= 8.6 BTU/per square foot @ 1F outdoors, 70F indoors. That would be about right for either a walk-out basement(?) or a fairly high performance building envelope.

    The fully derated output of the 40K furnace selected is 29440 BTU/hr which would a 1.5x oversize factor- not terrible, if both the load number and the altitude derating numbers are correct (usually not, if performed by an HVAC contractor rather than an engineer.)

    The problem room is on the Day System:

    [​IMG]

    The total load here is 45,043 BTU/hr for 4495 square feet of space, or (45,043/4495=)10 BTU/hr per square foot @ 1F outdoors, 70F indoors. That would be about right for a very tight 2x6/R20 house with better than code windows, or perhaps a better than code house overall(?).

    A derated 58,800 BTU/hr for a 45,043 BTU/hr load is a ratio of 1.3x, about as good as it gets for single-sped equipment (again, assuming the derating and load numbers are actually correct.)

    So, Bedroom 3 is the problem room with a design load of 2853 BTU/hr for a 164' room, a ratio of (2853/164=) 17 BTU/hr per square foot, 70% higher than the Day System average. It's still a pipsqueak load, and there is NO WAY to make it a separate zone using a 59K output furnace, and it's too big to rationally use a mini-split solution.

    The more scalable solution:

    At an average water temp of 120F (125F out, 115F return) typical fin tube baseboard puts out about 200 BTU/hr per running foot so 14' of baseboard running off any tank type water heater would cover 100% of that room's load, and would barely affect domestic hot water performance. But since it's also served by the ducted system it only needs to supplement to cover the shortfall- even 10' would probably be overkill 8' (1600 BTU/hr) of supplemental heat should do it. If cheap fin tube baseboard is too crummy looking for you, a 5' wide x 12" tall thin flat-panel rad like a Runtal UF-4-60 under the largest window would deliver ~1550 BTU/hr which is more than half the design load per the Manual-J.

    There are other radiators at different price points (Runtal is quite a bit more expensive than some others), but divide the rated output @ 180F by 3 to get the approximate output at domestic hot water temps.

    If it's close to the water heater it's usually possible to run it without an isolating heat exchanger between the potable heating system side. If it's a longer than 40' run to the water heater stagnation risks to the potable water are high, and the system needs to be heat exchanger isolated. (Isolation requirements are usually regulated by state & local codes.)

    [​IMG]
     
  14. Kabra

    Kabra New Member

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    Dana, thank you for all that analysis! The hot water approach sounds like something to consider as a last resort but you gave me an idea that I think might solve the problem of the tiny zone. So first let me clarify a couple of things from your analysis and see where that leaves things. This won't make a huge difference but it gives a more accurate picture.

    First, I am a little over 6K in elevation so that brings the calculation for altitude to 82% which is getting pretty close. Second is that the floorpan in the Manual J has an error concerning bedroom 3. In the snippet below I drew a red line to the right of the 95 CFM return. That line is the actual delineation of the contiguous bedroom area. Bedroom 3 (164sf), walk in closet (28sf), vanity area (36sf) and there are no doors between these areas. I included a picture taken from the common commode area of bath 2. Bedrooms 2 and 3 are Jack & Jill Bedrooms each having their own vanity and they share a commode and bathtub. So the contiguous area of Bedroom 3 is more like 228sf.

    Finally, note in the CFM numbers I provided in a separate spreadsheet. Bedroom 3 area has a register in the bedroom with a design rating of 72 CFM and the WIC design spec is 23cfm. That's a design spec of 95 CFM for that entire space. The actual CFM as measured by my energy consultant and after the larger duct was installed is 122 CFM for the bedroom and 67 for the closet or a total of 187 CFM or nearly twice the design spec and the bedroom is still cold! Note that the vanity area was not part of the calculated load for bedroom 3 in the manual J calcs and mistakenly included in bathroom 2. Not a huge deal but there is 8' or so of exterior wall. Also note that the ceilings are 10' in all rooms.

    Bedroom 3 Layout.jpg

    Bedroom 3.jpeg

    I'm going to talk about the idea you gave me to resolve the small zone in the next post.
     
  15. Kabra

    Kabra New Member

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    I'm not surprised that zone is too small on it's own. I wanted to give you a more realistic picture of what the actual CFM is going to that contiguous bedroom 3 area because it's double what the Manual J calls for. I'm not sure how that CFM converts to BTU and I imagine it's still pretty minuscule relative to a 80K BTU furnace.

    The first proposal from the A/C contractor was to use the basement as a dump zone for the zone. I balked at that because I'm afraid I would end up with a basement that's 85* all the time since not only would it get a massive amount of CFM but it would get it frequently because it would be controlled by a thermostat in bedroom 3. That's when they changed the approach to a bypass to the return but they want to put the damper in a location that would include bedroom 3, bathroom 2, and a good size register in the entry near the front door. This is asking for trouble though because all of these areas are fine the way they are and they're going to overheat if they're controlled by the bedroom 3 thermostat. And by the both of their proposals include A/C in this new zone as well. Again, everything works great the way it is except the bedroom 3 space. I don't want to cause new problems in other areas in an effort to fix that small bedroom space.

    So the idea you gave me is to dump a minimal amount of heating CFM to the basement / crawlspace. I'm thinking a hybrid approach of a bypass to the return for most excess flow, and an additional amount to the basement / crawlspace to make up for the small zone. currently I keep all the registers closed in the basement and crawlspace and it keeps the temperature cool but not cold which is how I like it. But I think there's some room to add some additional heat down there but I would want to keep it to a minimum.

    Also, please recall my idea early on about only using this zone for heating and not A/C. In the end I think the best solution is to set this zone up to ONLY provide additional heat when it's needed. All other times the dampers would work so that the system operates as a single zone. In fact I'm thinking there would not even be a damper to close off the small zone when the main thermostat calls for heating or A/C. The small zone would not even have A/C setup in the thermostat so could never call for it. The small zone would ONLY call for additional heat beyond what's supplied by the main thermostat when it's needed. The idea is for this zone to run as little as necessary. The one downside I can foresee is that the main thermostat could call for heat when bedroom 3 is up to temp. But from experience I'm not concerned about that because it would just add 1* at the most. Then because the room loses heat so fast it wouldn't be hot for long.

    As I said before maybe I'm over thinking this but it sure seems like it would be a good approach all things considered. And of course if it's doable.
     
  16. Dana

    Dana In the trades

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    If the foundation walls are insulated, the basement walls & crawlspace foundation walls are both air tight and insulated, with a balanced supply & return using the basement as a dump zone won't create a dramatic increase in fuel use. Heating basement under conditioned space lowers the load of the space above by raising the temperature of the floors a few degrees. To estimate the actual temperature the basement would rise to would take more analysis, including the design load of the basement.

    With such a low load & high cfm for Bedroom 3 as currently ducted & measured the duty cycle to the basement would still be pretty low, but it will cycle more frequently than the main zone.

    Those fears are well founded. If the room to room temperature balance is fine as-is it's best not to steal a defeat from the jaws of success. Excising the Bedroom 3/Basement zone from the main zone will likely require the balances to be tweaked slightly on the main zone, but not as much as if you start adding big registers in other locations.

    As stated previously, if the basement & crawlspace walls are insulated to the IRC 2018 code minimum (= R15 continuous insulation from the slab or stemwall footing all the way to the subfloor above) the additional heat loss of raising the temp down there 5F or even more aren't huge. If it's NOT insulated it's an efficiency disaster, and should be insulated even if you're not fully heating it.

    The code on unfinished basement & crawlspace insulation allows for insulating to R30 between the floor joists above in lieu of insulating the exterior walls. While that can fix most comfort problems, it's less efficient than air sealing and insulating the walls. If the house has insulation between the floor joists, removing that insulation when insulating the foundation walls tends to work better, especially when there are furnaces ducts & water heaters down there.

    Is Bedroom 3 above the crawlspace? Air leakage at the foundation sills & band joists (or outright venting to the outdoors) into thermal bypass spaces between the floor insulation & subfloor can result in heat loads dramatically higher than anticipated.

    What mid-winter low temperatures are the crawlspace & basement hitting?

    Without analyzing the duct design in greater detail, in principle (if not in practice) that could work, dampering off the main zone when just the bedroom zone is calling for heat. But that is often hard to pull off without introducing other imbalances.

    To hit Energy Star levels of duct design & implementation the room to room pressure differences need to be <3 Pascals under all operating conditions, independent of which zones are calling for heat, or whether the doors are open or closed. At pressures higher than that air handler driven outdoor air infiltration becomes a larger fraction of the total load, since any rooms that are pressurized >3 Pascals blow conditioned air out of any air leaks in the building envelope, and those negatively pressured at a magnitude that large will be sucking in outdoor air. Whenever zone dampers are thrown into the mix it becomes much harder to achieve.
     
  17. Kabra

    Kabra New Member

    Joined:
    Dec 25, 2019
    Location:
    Colorado
    Thanks for another excellent response.

    Yes, the foundation walls of the basement are insulated and I can tell what an efficient space that is because the temperature stays extremely consistent and comfortable all year long. I keep all the registers (2 in basement and 1 in crawlspace) closed all the time so whatever conditioning that space gets is leakage and whatever all the mechanical equipment radiates. My concern in the is space is not overheating it because that is where my exercise equipment is and again, it's just fine the way it is. But as you point out, warmer floors would be of benefit on the main floor in the cold season.

    Bedroom 3 is over the basement. The area over the crawlspace is the other side of the house that's served by the other HVAC system with the 40K BTU furnace. I've never actually measured the temperatures in the basement and crawlspace because it's always in a comfortable but cool range. I'm guessing maybe low 60s in the winter. Even then I keep the registers closed because it's not uncomfortable down there. Like you say there is some room to dump some of the heat down there when the new small zone is calling for heat. But I would want to keep that to a reasonable minimum to relieve sending all of the excess flow back through the return. Using only a dump to the basement without a return bypass I would think would overheat the basement given the size of the furnace and being controlled by the bedroom 3 thermostat.

    Regarding the idea of having all dampers open anytime the main thermostat calls for heat or A/C is that would be my fallback in the event there are unintended consequences when running the small zone. If worst came to worst I could keep the bedroom 3 thermostat off and the system would behave exactly like it does now. The one assumption is that the added dampers when in open mode won't make a significant difference.

    That's interesting to know about the room to room pressure differential. That said, 3 pascals seems almost infinitely small. If I calculate correctly that the equivalent of about four ten thousandths of 1 psi!

    I went to the building department yesterday to be sure this type of work requires a permit and it is an absolute requirement. My suspicion is that the builder has no intention of getting a permit and if so it will be interesting to see what happens when I point this out. It dawned on me how the changes the builder is proposing undoes so much of the original engineering of the system that has the Town's red approval stamp on it. From what I was told it sounds like new manual J and maybe D and/or S will need to be submitted.
     
  18. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    That's right- 3 pascals is about 0.00045 psi, which is still a big enough pressure to move some air. Expressed in inches of water column (a unit of measure commonly used in air based HVAC systems) 3 pascals is about 0.012" of water column, The typical pressure drop across a 1" thick HVAC filter is about 0.1" w.c. (0.036 psi) for a cheap fiberglass filter that's barely filtering out objects as big as houseflies, to 0.3" (0.1 psi or more) for very restrictive higher MERV filters. (Using thicker 4-6" deep pleated filter media allows much better filtering without excessive pressure drops.) The target design total pressure drop across the air handler for the entire system is typically 0.5" w.c.(~0.2 psi) but in practice it's common to find them running 0.8" w.c. (~0.3psi) or higher, resulting in lower cfm & lower efficiency. )

    The < 0.012" w.c. Energy Star spec for a room's pressure drop in a well balanced system is neither all that stringent nor very hard to meet (on simpler systems, anyway). The cross sectional area and "friction rate" of air flow through an entire room is a couple orders of magnitude smaller than a duct, so the induced pressure differences room to room from the air moving through the rooms should be no more than couple of orders of magnitude lower than the duct system as a whole. In a perfectly balanced system the room returns are pulling at exactly the same rate that the supplies are pushing, for both rooms on either side of the closed door resulting in a room to room pressure difference of zero. Nothing in the real world is ever perfect of course, and the Energy Star spec isn't crazy. Two orders of magnitude would be 0.012" x 100= 1.20 " w.c. which is substantially higher than the design pressure drop on the entire system. With a target design of 0.5" w.c. on the system and <0.012" room to room, 0.5" (system)/0.012"(room to room) = ~42, less than two orders of magnitude difference (but more than one order) relative to the drop on the ENTIRE system.

    When there are two zone vanes feeding the same supply register there may not be a single correct size for the return, so the room to room pressure imbalance may be out of spec under some operating conditions. That's usually fixable after the fact, but the fixes aren't always elegant.

    It doesn't take a lot of jump-duct to equalize pressures between rooms to that level if there are reasonably sized returns in every room with a supply register, but if the supplies & returns aren't reasonably balanced it becomes more than just threshold gap and crackage around an interior door. HVAC hackers who presume that a 3/4" gap under the door is going to handle the heating/cooling design flows for the entire room without inducing 0.1" or more of pressure on that path aren't living in the same physical universe as the rest of us. If the supplies and returns are there but still deliver bigger pressure drops under some operating conditions, it can be cheaper & easier to "fix" it with a jump duct hack than a complete re-do of the existing duct system.

    A single 2x4 partition wall stud bay jump duct on it's own moves about about twice the air flow of a 3/4" door cut at the threshold:

    [​IMG]

    To fully meet current code with a stud bay jump duct there should be air tight metal duct in the stud bay, but in retrofits that would require removing & repairing some drywall.

    If there is enough space above the door, transom jump ducts can be pretty easy to implement:

    [​IMG] [​IMG]
     
  19. Kabra

    Kabra New Member

    Joined:
    Dec 25, 2019
    Location:
    Colorado
    I always had an aversion to using jump ducts on bedrooms because of concern about sound transfer. I didn't know there was baffling to counter some of that but I'd be surprised if it's very effective. Seems like sound dampening and effective airflow don't go hand-in-hand. Your point is well taken about why something like this may be necessary to meet code if Bedroom 2 were included in the zone because it doesn't have a register.

    I had a good meeting this morning with the project manager who is the big cheese for the development. We had a good meeting of the minds and he seemed to like the ideas I had. One thing I did was calculate btuh per sq ft because that is about double the average of the house and about 2.5 times the other bedroom they want to include in the zone. That seemed to strike a chord with him. I also offered to put up some money if we could solve this with a real two zone furnace. So now I'm just waiting to hear back from him after he talks to the hvac contractor.
     
  20. Kabra

    Kabra New Member

    Joined:
    Dec 25, 2019
    Location:
    Colorado
    I finally figured out how to include quoted text!!! There are a couple of things I wanted to follow up on in your last post and ask an additional question. You mention above the 4-6" deep pleated filters that don't cause excessive pressure drops. I'd like to look into those and was wondering if you had a brand or product name so I can find them?

    You also mention a targeted total pressure drop across the air handler for the entire system of .5" w.c. and .8" being acceptable. My Manual J specifies "static pressure" of .80 in H2O and wanted to confirm that is the same .80 that you're referring to?

    One other question if you don't mind that I've been wondering for quite some time has to do with how or when a separate zone comes into play in the Manual J specs. Of course I'm referring to Bedroom 3 (164sf) which according to the spec has a heating load of 2853 btuh and calls for 72 cfm. Bedroom 2 (175sf) adjacent to it has a heating load of 1459 btuh and calls for 37 cfm or about 1/2 of what bedroom 3 calls for. The actual CFM delivered is 96 in bedroom 2 and 122 in bedroom 3. For completeness you may recall that there's also a small walk in closet (28sf) with no door that's part of bedroom 3 with a heating load of 925 btuh and calls for 23 cfm with 67 actually delivered. Bedroom 2 manages to keep a reasonable temperature with the rest of the house with that CFM but bedroom 3 is still really cold and we now know that it obviously needs to be it's own zone. So my question is how was this missed in the design? I would think the wrightsoft application would at least indicate there's a potential problem or is it completely up to the engineer or tech to make that judgement call. I just wonder how that was missed because it seems so obvious at least in hindsight.
     
  21. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    I have a 28" x 16" x 6" deep AprilAire 2400 on my (ridiculously oversized) 5 ton system that's pushing ~1800cfm. It's possible to get media as tight as MERV 17 that fits, but I'm usually running MERV 11 to MERV 13. I haven't measured the static drop across the filter in recent memory, but IIRC it was low enough that it was the least of my concerns. Even with MERV 13 filter media it's moving enough air that the AC coil never ices up. The replacement filters are a few 10s of dollars, but I only replace them every 12-18 months.

    That's right. The 0.8" spec should be considered an upper bound from a system design. If it's anywhere near that high when the filters are clean it's going to go over 1" if the filters get ignored it can easily go over 1.0", with ever decreasing flow.

    Zoning has zero effect on Manual-J, which only calculates the load. Manual-S is used for selecting the equipment (and where the cfm requirements get spit out, based on the calculated heat & coolth requirements from Manual-J. Ducts are supposed to be designed using Manual-D to guarantee that the requisite cfms can be delivered to each room.

    There is a lot of judgement calls when selecting equipment and designing ducts, and the design tools are just that- tools for the designers to use. Tools have become better, but they're not idiot proof. (Even if they were, the more idiot proof you make things the more creative the idiots become.:rolleyes:) Some designers are clearly better than others, and most systems work well enough that most clients don't complain (clearly not the case here.)

    Many times the ducts can be hacked in using crude rules of thumb (not Manual S & D) , then tweaked into reasonable balance using balancing vanes when commissioning the system. The success rate of rule of thumb methods is far from perfect.
     
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