Adding Zone with Fixed Speed Furnace

[Kabra]

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.

I really like this idea but can this type of filter work in my existing furnace or does it require some type of air purifier? I see there's some sort of a frame kit but I'm not clear on what exactly that's for.

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.

I've got a handheld manometer that I use to check pressure differential across the filter and I'm wondering if you have a favorite inexpensive mechanical one that I could mount on each of the units so I can see pressure when I walk by? This would also be handy to use initially after whatever changes are made to the system to monitor the system pressure drop under various conditions to be sure something stupid wasn't done. Anyway, one that is mounted seems like a good idea. I see Dwyer makes both a metal type of gage and the plastic type with the red fluid. Both seem pretty reasonably priced.

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.) 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.

Ok so I think what you're saying is that whether a separate zone is necessary as with my bedroom 3 is a judgment call? It seems to me the Manual J is a rate of flow but says nothing about how long or often that flow needs to run in a given space. I gather the assumption in a given manual J is that the design conditions of the various room are close enough to be be controlled by a single thermostat. I can't recall if I mentioned the ASHRAE FAQ I came across that sort of discusses this and makes it pretty clear at least when a separate zone is necessary. Here is the specific answer and a link to to the FAQ page if you want to look at the rest of the document.

"Areas or rooms having dissimilar load characteristics or different conditions should be controlled individually."

https://www.ashrae.org/File Library/Technical Resources/Technical FAQs/TC-02.01-FAQ-54.pdf

Thanks again for this information as it's all very helpful. I am told that I should be hearing something back from the builder today so we'll see what happens. This has been going on so long I know better than to get my hopes up.

 

[Dana]

I really like this idea but can this type of filter work in my existing furnace or does it require some type of air purifier? I see there's some sort of a frame kit but I'm not clear on what exactly that's for.

It's just a filter. The frame kit is the metal housing & cover that get installed on the return plenum (usually close to the air handler). The pleated media fits in a polyethylene box that slides out of the frame. The filter media is shipped with the pleats compressed like a closed accordion, and large plastic combs that snap into the frame keep them evenly spaced once it's installed. This video or perhaps this one might make it clearer.

It may be a bit of a PITA to replace the media compared to filters that come fully expanded with a cardboard frames that don't need to be carefully expanded & installed in a plastic box that slides out, but it's a once a year (or sometimes two) proposition.

I've got a handheld manometer that I use to check pressure differential across the filter and I'm wondering if you have a favorite inexpensive mechanical one that I could mount on each of the units so I can see pressure when I walk by? This would also be handy to use initially after whatever changes are made to the system to monitor the system pressure drop under various conditions to be sure something stupid wasn't done. Anyway, one that is mounted seems like a good idea. I see Dwyer makes both a metal type of gage and the plastic type with the red fluid. Both seem pretty reasonably priced.

I don't have any favorite duct-mounted manometers for tracking the drop across the filter, and haven't bothered to install any on my system, but they're a good idea. (More often found in commercial building systems to save time/money on the maintenance.)

Ok so I think what you're saying is that whether a separate zone is necessary as with my bedroom 3 is a judgment call? It seems to me the Manual J is a rate of flow but says nothing about how long or often that flow needs to run in a given space.

Manual-J doesn't specify the "...rate of flow..." of the air, but rather the rate of BTU needed to maintain temperature at the design condition. The air cfm needed is derived from the equipment selection (Manual-S) in combination with the Manual-J's load numbers.

I gather the assumption in a given manual J is that the design conditions of the various room are close enough to be be controlled by a single thermostat.

Nope. Manual-J is simpler than that- it only calculates the BTU requirements of the rooms, period, full stop. Manual-J makes no assumptions on if or how it will be zoned, or how that heat/coolth is to be delivered.

The zoning et al are left as an exercise for the HVAC designer. If the designer and the installers and the building contractors do a great job, controlling it all as a single zone can usually be achieved. But when the heat gain/loss characteristics of some rooms are very different from the average it makes it a bit more challenging- even more so when single-stage single-speed equipment is selected.

The fact that HVAC tools like Wrightsoft allow the designer to input a equipment selection and reports cfm requirements for individual rooms may give the impression that it's all part of ACCA Manual-J, but it is not. Manual-J only calculates the BTU/hr needed at the 1% & 99% design temps.

I can't recall if I mentioned the ASHRAE FAQ I came across that sort of discusses this and makes it pretty clear at least when a separate zone is necessary. Here is the specific answer and a link to to the FAQ page if you want to look at the rest of the document.

"Areas or rooms having dissimilar load characteristics or different conditions should be controlled individually."

https://www.ashrae.org/File Library/Technical Resources/Technical FAQs/TC-02.01-FAQ-54.pdf

Thanks again for this information as it's all very helpful. I am told that I should be hearing something back from the builder today so we'll see what happens. This has been going on so long I know better than to get my hopes up.

Again this is part of the judgement call. Just how different does it have to be to require separate zoning?

It depends.

Basements have very different loss characteristics than above-grade floors, and will often still have a modest heat load while other floors have cooling loads, and in some cases will still have neutral to negative cooling loads even at the 1% design condition. Basements usually need to be zoned separately to keep floor-to-floor temperatures within a reasonable range.

Similarly, top floors on multi-story houses will have different enough heat gain characteristics to require separate zoning to stay within range during the cooling season.

But there are counter examples of both, where the whole house from the basement to the conditioned attic can still be operated as a single zone without experiencing large temperature differences.

Right sized modulating systems with reasonable (3: 1 or better) turn down ratios can mitigate these issues by quite a bit by maintaining a nearly 100% duty cycle whenever there is a significant whole-house load. Whether that would work in your situation isn't something that's going to be easy to figure out with a "design by web-forum" approach though.

 

[Sponsor]

 

[Kabra]

Those videos did the trick to understand how the aprilaire filter setup works and the replacement process. I see what you mean about the replacement being a bit of an ordeal but what a great product it is. I'm going to consider putting it on my systems.

Manual-J doesn't specify the "...rate of flow..." of the air, but rather the rate of BTU needed to maintain temperature at the design condition. The air cfm needed is derived from the equipment selection (Manual-S) in combination with the Manual-J's load numbers.

I see what you mean and I think what gave me the impression the manual J specified CFM is because in the Manual JDS document for my house so much information is mixed together there's no clear delineation between J,D,S unless one has that knowledge as you described. I get it now and have a better understanding as I read through the document.

Right sized modulating systems with reasonable (3: 1 or better) turn down ratios can mitigate these issues by quite a bit by maintaining a nearly 100% duty cycle whenever there is a significant whole-house load. Whether that would work in your situation isn't something that's going to be easy to figure out with a "design by web-forum" approach though.

I understand this and it's really not an option nor is it necessary since the only problem I have is that one bedroom is too cold.

What I still fail to understand is how that bedroom runs 5* - 7* cold at night during the cold season when it's getting twice the CFM that it was initially designed to (95 as designed and 189 actual with the larger duct). Using the manual J data, on a per square foot basis that room calls for about 20 BTUh / sf vs. around 10 for the rest of the house. I don't know of a good way to figure that out and the best thing I have is detailed temperature data (one minute samples) with sensors in the great room where the thermostat is located, bedroom 3, and outside. I never showed you any actual data so I thought I'd create a chart for you. It's from 12/30/2020 when the overnight temp got down to 13* and is pretty typical for a fairly cold night. It was also a sunny day so I had sun pouring into the great room so the furnace never kicked on in the afternoon while bedroom 3 kept getting colder and peaked at 11* colder than the great room. You'll see at the bottom of the chart there is a line that shows the temperature difference between the great room and bedroom 3. If you wouldn't mind, I'd love to hear your insight on what you see.

 

[Kabra]

Regarding the meeting that I had week before last with the site manager, the only thing new that they came up with is offering the option of using a HotPod electric in-duct supplemental heater. Initially I thought this might be a good solution but after thinking about it, it's really just a space heater in the duct and just as inefficient. The 8" version (for the 7" duct) uses 1440W and produces 210 CFM with 23* temperature rise for 5000 BTUh. My sense is that thing will be running A LOT and will cost a small fortune, especially considering the sunny days where that room will rely solely on that in-line heater for heat in the afternoon.

A bit of an interesting tangent here. As I was writing the paragraph above it dawned on me to look at that chart and source data and based on the temperature rise as an indication, it appears that the furnace cycles about twice per hour and runs for about five minutes. Using the delta T equation along with the known CFM of 189 and temperature rise of 63* from the Manual S information, that works out to be just shy of 13,000 BTUh or a little over 1000 BTU supplied in the five minutes it runs each time. Since it runs twice per hour that would be about 2K BTUh where the manual J calls for 3778 for that space. The design further and in error excludes a 35sf vanity area from that room with one exterior wall. That adds 15% more sf but has only one exterior wall. So I'll add another 300 BTUh load for that calling the total load of 4078 BTUh. So if this logic is anywhere close to a reasonable estimate the current system supplies about half of what that room needs.

I am adding this edit because I determined something is missing from the estimate above and I don't believe it to be accurate. I found the same deficiency of supplied BTU to the entire house. I think the problem lies in how long the furnace runs each time it cycles. The temperature rise takes about 5 minutes to peak once it begins to rise. I don't know for sure but my sense is the temperature curve doesn't correlate well with respect to the duration of the furnace delivering heat. I've never timed it but from my recollection it does stay on longer than five minutes during a cycle. It was a good exercise though because I learned about the Delta T equation and further discovered the altitude at 6K feet makes a big difference in the Delta T factor. I came up with .24 x .06 x 60 = .864 rather than 1.08 which is a 20% reduction. Anyway the whole point of the exercise was to estimate actual BTUs delivered under those cold conditions.

 

[Dana]

A bit of an interesting tangent here. As I was writing the paragraph above it dawned on me to look at that chart and source data and based on the temperature rise as an indication, it appears that the furnace cycles about twice per hour and runs for about five minutes. Using the delta T equation along with the known CFM of 189 and temperature rise of 63* from the Manual S information, that works out to be just shy of 13,000 BTUh or a little over 1000 BTU supplied in the five minutes it runs each time. Since it runs twice per hour that would be about 2K BTUh where the manual J calls for 3778 for that space. The design further and in error excludes a 35sf vanity area from that room with one exterior wall. That adds 15% more sf but has only one exterior wall. So I'll add another 300 BTUh load for that calling the total load of 4078 BTUh. So if this logic is anywhere close to a reasonable estimate the current system supplies about half of what that room needs.

You are on the right path.

Without my having fully reviewed the data, if it's only running 10 minutes per hour at design condition the system as a whole would be (60min/10 min= ) 5x oversized.

If it's happening at a much more modest outdoor temperature, and/or periods when there is significant solar gains in the room where the thermostat is located that could be normal, EXCEPT that 5 minute cycles aren't nearly long enough a cycle to fully heat rooms at the far ends of the rooms. At the beginning of the cycle the air in the ducts and the duct walls would be only slightly above room temp, and it takes time to fully displace the air in those ducts, and heat up the duct walls to something close to the furnace output temp. Rooms with higher loads are likely to have higher volume ducts too. Do the math on the cfm, length, duct velocity and full volume of the duct path to the cold room- it's likely that for the first half of a 5 minute cycle the register output is tepid, nothing close to steady-state.

This is why modulating systems with very long duty cycles (under all load conditions) tend to even out the room to room temperature differences, and where single stage systems (even when right sized) cheat the rooms at the ends of the duct runs, especially rooms at the end with load per square foot ratios larger than the system average.

I'm a strong believer in measuring things to figure it all out. Tracking the exit air temp at the register(s) during several cycles might be enlightening. At the end of a 5 minute run the exit air temp will always be somewhat close to the design spec, but how long it takes to get there matters.

An inline duct booster FAN might be a (somewhat noisy) way to purge the tepid air quickly in the early part of a cycle reducing the amount of time it takes to get to steady-state. (A duct booster fan on a valved duct bypass from the supply to the return located somewhere close to the room, controlled so that it turns off when the incoming air at the fan reaches 110F has less of a noise factor than a duct booster running constantly over the cycle, and lessens the amount of tepid air going into the room.) Whether that approach can work here requires more analysis.

I'm still partial to the idea of a hydronic heating micro-zone running off the water heater as a sure-fire (but not ultra-cheap) fix. Even with the duct registers open and still running it's short cycles the room is unlikely to see a significant temperature overshoot when both are running, especially if the thermostat for the hydronic loop is located near the supply register so that it turns off as soon as warm air arrives from the air handler.

 

[Fitter30]

Don't know where u got your blower speed info but it either has a 4 or 5 speed blower motor. Venter moter is single speed. Take a pic of the wiring diagram it's on the inside of blower door.

 

[Kabra]

Don't know where u got your blower speed info but it either has a 4 or 5 speed blower motor. Venter moter is single speed. Take a pic of the wiring diagram it's on the inside of blower door.

Yes, you are correct. The furnace has 5 speed taps. Until recently I didn't really understand how these single stage furnaces worked and I somehow thought that also meant a fixed speed motor.

 

[Kabra]

Apparently we (the builder and I) finally have somewhat of a meeting of the minds as far as a plan to move forward. They will create a new zone with just the bedroom and the walk-in closet using a bypass to the return. I'm waiting for the details of exactly how they plan to implement this and I'm extremely skeptical about this given the 80K BTU furnace. Honestly, I will be surprised if they actually go through with this.

If they do go through with is and it's implemented the way I think and hope it will be, even if there are problems I at least get all of the ducting modifications for this new zone and as far as I can tell it should be easy enough to disable it by keeping all the dampers open with the exception of the damper to the return.

If by some miracle this small zone approach does work, I still plan to modify the operation of the dampers so the system continues to operate as a single zone system whenever the main thermostat in the great room calls for heat or A/C. The new zone will only be active as a separate zone when it calls for additional heat and the main zone does not. The point here is to get all the BTUs it can when the main thermostat calls for heat and then supplement as necessary operating as an isolated zone. This will minimize the time the system needs to operate in the bypass to return mode and it will never need to operate in that bypass mode for A/C since for cooling the single zone works just fine the way it is. I know this sounds a little crazy but this whole approach in setting up such a tiny zone is crazy in the first place. It's the best I can do without going the mediation and arbitration route.

I'm still partial to the idea of a hydronic heating micro-zone running off the water heater as a sure-fire (but not ultra-cheap) fix. Even with the duct registers open and still running it's short cycles the room is unlikely to see a significant temperature overshoot when both are running, especially if the thermostat for the hydronic loop is located near the supply register so that it turns off as soon as warm air arrives from the air handler.

As a backup plan if the zoning approach doesn't work, I did some looking at these types of systems and it looks like I could opt for a baseboard or wall version and for various reasons I prefer the wall version. The first time you mentioned this I think I recall you suggesting this could work with the existing water heater. I have a 50 gallon water heater that is somewhere around 40K BTU. While I don't know know with any degree of certainty what the BTU load actually is, I think I need to plan on 10K BTU since during sunny winter afternoons this hydronic heater would be the sole source of heat since the sun pouring into the great room tends to keep the heat from kicking on since that is where the main thermostat is. So I'm wondering if it's sensible to add that kind of load to the water heater or if I need a separate hot water source. Of course the other advantage of a separate source is that it can run at a higher temperature and I would think I can get away with a smaller heater unit in the bedroom.

 

[Stuff]

A lot of work but could they move this to the 40k furnace instead?

 

[Fitter30]

I really like this idea but can this type of filter work in my existing furnace or does it require some type of air purifier? I see there's some sort of a frame kit but I'm not clear on what exactly that's for.



I've got a handheld manometer that I use to check pressure differential across the filter and I'm wondering if you have a favorite inexpensive mechanical one that I could mount on each of the units so I can see pressure when I walk by? This would also be handy to use initially after whatever changes are made to the system to monitor the system pressure drop under various conditions to be sure something stupid wasn't done. Anyway, one that is mounted seems like a good idea. I see Dwyer makes both a metal type of gage and the plastic type with the red fluid. Both seem pretty reasonably priced.



Ok so I think what you're saying is that whether a separate zone is necessary as with my bedroom 3 is a judgment call? It seems to me the Manual J is a rate of flow but says nothing about how long or often that flow needs to run in a given space. I gather the assumption in a given manual J is that the design conditions of the various room are close enough to be be controlled by a single thermostat. I can't recall if I mentioned the ASHRAE FAQ I came across that sort of discusses this and makes it pretty clear at least when a separate zone is necessary. Here is the specific answer and a link to to the FAQ page if you want to look at the rest of the document.

"Areas or rooms having dissimilar load characteristics or different conditions should be controlled individually."

https://www.ashrae.org/File Library/Technical Resources/Technical FAQs/TC-02.01-FAQ-54.pdf

Thanks again for this information as it's all very helpful. I am told that I should be hearing something back from the builder today so we'll see what happens. This has been going on so long I know better than to get my hopes up.

Dywer either the plastic or metal filter gauge are both good and will last for years.

 

[Kabra]

A lot of work but could they move this to the 40k furnace instead?

That's actually a great idea to consider. My sense is the load of that bedroom is too much to add to the 40K system but it's definitely something worthy of thinking about. Thanks.

 

[Kabra]

Dywer either the plastic or metal filter gauge are both good and will last for years.

Thanks for the input. I decided to go with their 0" - 1" magnehelic gauge.

 

[Fitter30]

Thanks for the input. I decided to go with their 0" - 1" magnehelic gauge.

0 to 3" 1 /4" will be taken up just with a clean filter

 

[Kabra]

0 to 3" 1 /4" will be taken up just with a clean filter

My logic for getting the 0" - 1" was that anything beyond 1" is way out of range. In that case I can use a manometer to explore further. I hope I'm not missing something here.

 

[Fitter30]

My logic for getting the 0" - 1" was that anything beyond 1" is way out of range. In that case I can use a manometer to explore further. I hope I'm not missing something here.

No a clean filter will have a pressure drop. Dirty will have 1" + clean drop. The only filter rack and filters that I've ever change were in a air handler that was made for a radiation room . The rack and filter had a sticky substance that had 0% leakage. 30 filters 8 hours to change with myself and a inspector.