Adding Zone with Fixed Speed Furnace

Discussion in 'HVAC Heating & Cooling' started by Kabra, Jun 10, 2021 at 4:31 PM.

  1. Kabra

    Kabra New Member

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    Dec 25, 2019
    Location:
    Colorado
    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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    Retired service tech
    Location:
    Peace valley missouri
    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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    Dec 25, 2019
    Location:
    Colorado
    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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    Location:
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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

    Joined:
    Mar 7, 2013
    Location:
    Pennsylvania
    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 at 3:13 PM
  8. Kabra

    Kabra New Member

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    Dec 25, 2019
    Location:
    Colorado
     
  9. Stuff

    Stuff Well-Known Member

    Joined:
    Mar 7, 2013
    Location:
    Pennsylvania
    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 at 1:14 PM
  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.)
     
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