Optimizing operation of heat pump for efficiency

[mooch91]

Our 3 ton HP system has 20 x 25" air filters. There is a 1" prefilter and then a 4" Merv 13 box filter. It's an Am Std. Heritage 16 system with a variable-speed DC air handler. Static pressure has never been an issue.

Mooch91, what is the air handler brand and model number? Does it have a multispeed (mulit-tap) motor and if so, what setting is it at?

@begreen

It's part of the GE Connect system. Model number is UUY60ZGDAB1.

It is the high-ESP version of the 5-ton air handler for the GE Connect outdoor unit. There is an identical Gree Flexx and Mr. Cool system.

It operates at a single speed but has 8 speed taps, currently on setting 5.

On setting 5, I measure about 0.5"WC static with both zones open, 0.65"WC when only one zone is operating, with the basement return grille open.

When I close the return grille, my static measurement increases to 0.6" with both zones, and 0.75" with one zone open.

Attached are the speed charts for the different settings. The lowest model in the chart is mine.

 

[begreen]

The SP appears to be high. It seems like the ducting is undersized for the volume of air being moved. Have you tried a level 3 setting?

 

[mooch91]

The SP appears to be high. It seems like the ducting is undersized for the volume of air being moved. Have you tried a level 3 setting?

@begreen

I was considering level 5 my "sweet spot". Static pressure about 1/2 - 3/4 of the range, speed between 1500-1800 cfm.

I measured static at level 4 around 0.45"WC, which would be giving me speed of about 1500 cfm max.

If I drop to 3, I'm probably dropping the speed off well beyond what would be necessary for a 5-ton system.

 

[begreen]

Is the 27" x 12" sizing of the return next to the air handler the OD or ID?

@begreen

I was considering level 5 my "sweet spot". Static pressure about 1/2 - 3/4 of the range, speed between 1500-1800 cfm.

I measured static at level 4 around 0.45"WC, which would be giving me speed of about 1500 cfm max.

If I drop to 3, I'm probably dropping the speed off well beyond what would be necessary for a 5-ton system.

The issue appears to be that the needs are switching from 5 tons, for both zones to 3 tons for a single zone. This was not apparent with the variable speed air handler, but it's more of an issue now. If so, I am wondering if a relay system could be setup to switch to level 3 for single zone operation?

 

[mooch91]

Is the 27" x 12" sizing of the return next to the air handler the OD or ID?

The issue appears to be that the needs are switching from 5 tons, for both zones to 3 tons for a single zone. This was not apparent with the variable speed air handler, but it's more of an issue now. If so, I am wondering if a relay system could be setup to switch to level 3 for single zone operation?

Approximate OD. I believe it's all 1" ductboard (fiber board).

 

[begreen]

Approximate OD. I believe it's all 1" ductboard (fiber board).

If so, then the ID is 25" by 10" which I think is about the proper area for 1450 cfm.

 

[mooch91]

If so, then the ID is 25" by 10" which I think is about the proper area for 1450 cfm.

I have had a feeling based upon feedback from others that the drop is undersized.

I've been wondering if a reconstruct of the drop lines, plenum box, and supply to the trunk lines might make an improvement? Trunk lines are only about 25x10, but since they split to two directions, they are each only carrying part of the load.

 

[EbS-P]

Thanks @EbS-P .

What you described is what I had expected, after reading up on the strategy the Bosch IDS / BOVA units use. I would have thought control for this system were similar. But I honestly think the Gree/Mr. Cool/Gree setups don't control to a target coil temp. Or at least not one that isn't 130 degrees+ in heating. I think they strictly ramp up over time, and that ramp rate may be affected by parameters like line temps and ambient temp. I have never seen this unit modulate down; it always ramps up to what looks like max (7000W) and hold, no matter how long it then runs.

Ideally, in my opinion, it would modulate to keep a set coil temp. If it finds it has ramped too fast and overshot the coil temp, it would back off on compressor output. Similarly if air flow were low, coil temp would rise, and it would reduce compressor output. Based on my observations, this is not the way this system is designed to work.

Attached is a pretty typical power consumption curve for a setback recovery (from 65 to 67) on a 27 deg F morning. Each bar is the average kW used over 1 minute of time. Have never seen a situation where the power consumption modulated down before the setpoint was reached, regardless of how long it has run. It always plateaus at max.
View attachment 309003

Everything is optimized for comfort (Hit set point ASAP) not cost savings. I think it might be possible to use a highly programmable thermostat like an EcoBee to do something better. But i really don’t know.

Two degree setback really isn’t worth much in my opinion. With a system like that picking the lowest comfortable setting and keeping it there is probably the most efficient. Your biggest efficiency gain with hi static pressure is probably running the blower lower for long.

 

[mooch91]

The issue appears to be that the needs are switching from 5 tons, for both zones to 3 tons for a single zone. This was not apparent with the variable speed air handler, but it's more of an issue now. If so, I am wondering if a relay system could be setup to switch to level 3 for single zone operation?

I had my concerns when switching from a two-stage to a single-speed air handler. But they were more about noise and dehumidifcation. I never considered static pressure because I was naive to it at the time.

Funny part is that the contractor didn't even know that he was selling me a single-speed air handler vs. the two-stage I had previously. I should have stopped right there...

 

[woodgeek]

Hmmm, yeah. I"m not an HVAC installer, but I think preserving total cross-section in a branching duct network is pretty standard. I assumed all your ducts were smaller than my ducts on a 4 ton bc it was engineered to run at higher SP. My trunks are about 24x24 IIRC.

I would be worried about the single speed air handler. Does it have slow start and stop, or is bang-on and bang-off?

Also it might be hard to get good dehumidification at low loads using high CFM in cooling. I have the opposite problem, a single speed compressor with a variable speed handler. No bang-on and bang off (nice gradual ramps up and down) but I can't spool down the compressor at low cooling loads, so I get iffy dehumidification.

Next system will be inverter rive compressor and variable speed handler.

 

[mooch91]

Hmmm, yeah. I"m not an HVAC installer, but I think preserving total cross-section in a branching duct network is pretty standard. I assumed all your ducts were smaller than my ducts on a 4 ton bc it was engineered to run at higher SP. My trunks are about 24x24 IIRC.

I would be worried about the single speed air handler. Does it have slow start and stop, or is bang-on and bang-off?

Also it might be hard to get good dehumidification at low loads using high CFM in cooling. I have the opposite problem, a single speed compressor with a variable speed handler. No bang-on and bang off (nice gradual ramps up and down) but I can't spool down the compressor at low cooling loads, so I get iffy dehumidification.

Next system will be inverter rive compressor and variable speed handler.

@woodgeek

Thankfully it is soft start and stop, so no concerns there.

Hindsight is always 20/20, and what I've learned now I would have certainly considered at the time I replaced the unit. I was unable to go to a true communicating system because I did not want to re-do all of the ductwork in the house...The existing zoning system is rather complicated and would have been an extensive re-work to allow for a higher end system. Add that to the already high cost of a full communicating system, and it would have been way out of budget. Frankly, I would have been satisfied with an in-kind replacement two-stage compressor, two-stage air handler, but the equipment wasn't available due to the common "supply chain issues" and I was in a pinch without A/C during the hottest months of the summer.

The GE Connect was sold to me as a better product at a price point equivalent to the two-stage units I was looking at. I think the variable-speed inverter technology is in fact better from the perspective of lower temperature operation, but it's packaged in an overall less flexible package to compete at a price point. In fact, knowing that the same unit is the equivalent DIY Mr. Cool, I think I actually overpaid.

 

[gthomas785]

Inverter drive is pretty useless unless you have a way to sense the heat load. It sounds like the unit is using "duration of heat call" as a proxy for demand since it cannot sense the coil temp or any other heat load related variables. Kind of like the 2-stage furnaces that when hooked up to a single stage thermostat will just kick into 2nd stage after fixed run time. It works to ensure the demand is satisfied, but it's not elegant or efficient.

It sounds like you could make your air handler 2-stage with a few mods if you're into that sort of thing.

 

[EbS-P]

@begreen

I was considering level 5 my "sweet spot". Static pressure about 1/2 - 3/4 of the range, speed between 1500-1800 cfm.

I measured static at level 4 around 0.45"WC, which would be giving me speed of about 1500 cfm max.

If I drop to 3, I'm probably dropping the speed off well beyond what would be necessary for a 5-ton system.

You have a good chance of having humidity issues. Sizing a heatpump for heating needs should rely on the strips to to meet demand at the design temp. The location where heating load and cooling load at design temp is the same is pretty far south. Like further south than Atlanta.

Is your choice bad? Probably not. It’s just been sized as a heater and not and AC. Inverter compressor probably does have some advantages at lower temps but really these units are just average efficiency and low temp output.

@woodgeek

Thankfully it is soft start and stop, so no concerns there.

Hindsight is always 20/20, and what I've learned now I would have certainly considered at the time I replaced the unit. I was unable to go to a true communicating system because I did not want to re-do all of the ductwork in the house...The existing zoning system is rather complicated and would have been an extensive re-work to allow for a higher end system. Add that to the already high cost of a full communicating system, and it would have been way out of budget. Frankly, I would have been satisfied with an in-kind replacement two-stage compressor, two-stage air handler, but the equipment wasn't available due to the common "supply chain issues" and I was in a pinch without A/C during the hottest months of the summer.

The GE Connect was sold to me as a better product at a price point equivalent to the two-stage units I was looking at. I think the variable-speed inverter technology is in fact better from the perspective of lower temperature operation, but it's packaged in an overall less flexible package to compete at a price point. In fact, knowing that the same unit is the equivalent DIY Mr. Cool, I think I actually overpaid.

You probably (hopefully) have decent support that can be lacking with the Mr cool units that one DIYs. Come the end of heating season I’d be switching to 4 tons and dropping air flow to 350 or possibly even 300 cfm per ton. Worst case you manually set the second zone damper and just condition that space all the time. Same for the basement vents.


The reality is the HVAC system is the single most expensive piece of equipment in a house. New ductwork will cost me 8k$. Adding a whole house dehumidifier and correctly ducting my heatpump hot water heater adds another 5k$. I’m even not replacing my 13 year old package unit. A like for like swap is probably another 8-9K, an upgrade prob starting at 11 with inverter units more than that.

I won’t pay a cent more than my 50% down until my contractor shows me my manual J and D. Without those it’s just guesswork. And to be honest if you do this kind of work long enough you get really good at guesswork.

If a contractor want to replace a system they should be doing a manual J. it’s code! If we as consumers hold fast to this our experience will be better. Had some friends get a new unit told the contractor that one or two rooms were always a bit warmer so without a lid calc they just upsized by a ton. Well they went on vacation a long vacation this summer set the thermostat to 80 like they had always done in the past and cam home to mold on everything! Alright I’m off my soap box.

I looked at the green flex unit specs and was thought that those might be an option but I won’t get anything that doesn’t have a variable speed blower.

 

[mooch91]

You have a good chance of having humidity issues. Sizing a heatpump for heating needs should rely on the strips to to meet demand at the design temp. The location where heating load and cooling load at design temp is the same is pretty far south. Like further south than Atlanta.

Is your choice bad? Probably not. It’s just been sized as a heater and not and AC. Inverter compressor probably does have some advantages at lower temps but really these units are just average efficiency and low temp output.

You probably (hopefully) have decent support that can be lacking with the Mr cool units that one DIYs. Come the end of heating season I’d be switching to 4 tons and dropping air flow to 350 or possibly even 300 cfm per ton. Worst case you manually set the second zone damper and just condition that space all the time. Same for the basement vents.


The reality is the HVAC system is the single most expensive piece of equipment in a house. New ductwork will cost me 8k$. Adding a whole house dehumidifier and correctly ducting my heatpump hot water heater adds another 5k$. I’m even not replacing my 13 year old package unit. A like for like swap is probably another 8-9K, an upgrade prob starting at 11 with inverter units more than that.

I won’t pay a cent more than my 50% down until my contractor shows me my manual J and D. Without those it’s just guesswork. And to be honest if you do this kind of work long enough you get really good at guesswork.

If a contractor want to replace a system they should be doing a manual J. it’s code! If we as consumers hold fast to this our experience will be better. Had some friends get a new unit told the contractor that one or two rooms were always a bit warmer so without a lid calc they just upsized by a ton. Well they went on vacation a long vacation this summer set the thermostat to 80 like they had always done in the past and cam home to mold on everything! Alright I’m off my soap box.

I looked at the green flex unit specs and was thought that those might be an option but I won’t get anything that doesn’t have a variable speed blower.

100% agree, I've learned a hard lesson with this purchase.

Unfortunately, I am NOT finding that I have decent support on this, so I might have done just as well with a DIY. The static pressure issue has been a functional issue since I had the system installed. The reason the vent was cut in to the return ductwork is that the indoor fan has periodically kicked out with a generic fault code. Without taking any measurements, cutting in the return vent was the solution the contractor attempted. I was not happy about introducing 57 degree basement air in to the system (thankfully the basement is relatively clean and any combustion appliances are direct vent), but that was the best they could offer to address the problem.

Coincidentally, I woke to NO HEAT last night. Indoor fan had tripped out again. Outdoor compressor was doing some on/off, but there was no airflow indoors. Pulled the air handler cover and saw the fan fault flashing.

I'm trying to find others to look at the system, specifically to look at the ductwork, because that's where I think the problem lies. Unfortunately I keep paying for service calls where I already know more than the tech they send, or no one wants to touch it because they didn't install it.

 

[EbS-P]

100% agree, I've learned a hard lesson with this purchase.

Unfortunately, I am NOT finding that I have decent support on this, so I might have done just as well with a DIY. The static pressure issue has been a functional issue since I had the system installed. The reason the vent was cut in to the return ductwork is that the indoor fan has periodically kicked out with a generic fault code. Without taking any measurements, cutting in the return vent was the solution the contractor attempted. I was not happy about introducing 57 degree basement air in to the system (thankfully the basement is relatively clean and any combustion appliances are direct vent), but that was the best they could offer to address the problem.

Coincidentally, I woke to NO HEAT last night. Indoor fan had tripped out again. Outdoor compressor was doing some on/off, but there was no airflow indoors. Pulled the air handler cover and saw the fan fault flashing.

I'm trying to find others to look at the system, specifically to look at the ductwork, because that's where I think the problem lies. Unfortunately I keep paying for service calls where I already know more than the tech they send, or no one wants to touch it because they didn't install it.

Ughhh…. Try the 4 ton and set blower to 300 cfm per ton so 1200 cfm. I’d tape a filter up the basement return if you haven’t already. Drawing basement air wouldn’t be my main concern.

Look at the contractor map on hvac talk.com.

 

[woodgeek]

Double Ugh.

If the basement is dry and semi- or fully conditioned, a little return air from the basement could help draw conditioned air from upstairs. I would be aok with that in terms of basement comfort and cold floors above. But a big basement return to avoid tripping blower faults... that a kludgey solution that your installer came up with as a (cheap) bandaid for his shoddy design work (and to avoid your callbacks).

Faults and callbacks are warranted. Installer needs to give you a working system, and one that is not a kludge. I get that the installer thought is was a 'drop in' and the undersized drops were a pre-existing condition... is that right?

One thing I like about variable speed blowers is that the CFM is often independent of the SP... which allows for more consistent/efficient operation (in theory). A fixed speed (dumb) blower will undershoot target CFM at high SP.

If I were you, I would plan on rebuilding/upsizing the drops (return and supply). Doesn't sounds like a $$$ job to me. Hopefully that will fix it.

Remember that CFM*SP is POWER. So reducing SP at fixed CFM means more efficiency.

If I were being a nerd, I would suggest putting rounded bends, like a rounded 'y', where the drops meet the trunks on the ceiling during the rebuild. This could be as simple as adding a 45 degree of duct board inside the 4 currently right angle turns.

If you want a whole house filter, you could put a larger filter holder in the rebuilt drop, even a 3" deep one, pretty easily.

 

[mooch91]

Double Ugh.

If the basement is dry and semi- or fully conditioned, a little return air from the basement could help draw conditioned air from upstairs. I would be aok with that in terms of basement comfort and cold floors above. But a big basement return to avoid tripping blower faults... that a kludgey solution that your installer came up with as a (cheap) bandaid for his shoddy design work (and to avoid your callbacks).

Faults and callbacks are warranted. Installer needs to give you a working system, and one that is not a kludge. I get that the installer thought is was a 'drop in' and the undersized drops were a pre-existing condition... is that right?

One thing I like about variable speed blowers is that the CFM is often independent of the SP... which allows for more consistent/efficient operation (in theory). A fixed speed (dumb) blower will undershoot target CFM at high SP.

If I were you, I would plan on rebuilding/upsizing the drops (return and supply). Doesn't sounds like a $$$ job to me. Hopefully that will fix it.

Remember that CFM*SP is POWER. So reducing SP at fixed CFM means more efficiency.

If I were being a nerd, I would suggest putting rounded bends, like a rounded 'y', where the drops meet the trunks on the ceiling during the rebuild. This could be as simple as adding a 45 degree of duct board inside the currently right angle turn.

If you want a whole house filter, you could put a larger filter holder in the rebuilt drop, even a 3" deep one, pretty easily.

@woodgeek

Yes, installer thought it was a drop-in replacement. Installer hasn't challenged me on the call-backs or said "not my problem", but truth is, I have absolutely zero confidence that the installer even suspects a ductwork issue or has any knowledge of how to address it. There have been a lot of signs throughout the install that lead me here. So that's where I'm at - practically considering trying to find someone else to look at it as a ductwork job. That's why I said I've been calling around and everyone wants to send out a service tech to look at it, if they are even willing. Those who have come have been inexperienced and end up looking at other things or shrugging their shoulders. I must not be asking for the right thing...

 

[mooch91]

Ughhh…. Try the 4 ton and set blower to 300 cfm per ton so 1200 cfm. I’d tape a filter up the basement return if you haven’t already. Drawing basement air wouldn’t be my main concern.

Look at the contractor map on hvac talk.com.

Thanks. The contractor map only shows a couple within a 75 mile radius. Unfortunately I live in a rural area.

Fan is either fighting too much static, getting warm from the heat itself, or some combination of the two.

 

[EbS-P]

Thanks. The contractor map only shows a couple within a 75 mile radius. Unfortunately I live in a rural area.

Fan is either fighting too much static, getting warm from the heat itself, or some combination of the two.

Or it’s defective? Can you get the static pressure in spec? If it is and it’s tripping off so then it’s a bad fan and should be replaced under warranty.

 

[mooch91]

Or it’s defective? Can you get the static pressure in spec? If it is and it’s tripping off so then it’s a bad fan and should be replaced under warranty.

@EbS-P

Possibly. Let me explain how I'm measuring static in case there are any experts in here who see something wrong.

Ideally, I'd measure static as the sum of the pressure immediately before the coil (on the return side) and immediately after the fan (on the supply side). The manufacturer charts don't indicate if the measurements include a filter, so I've assumed they do not.

It's easy enough to measure the pressure after the fan (hole in the duct - blue tape spot in the picture I posted), but not immediately before the coil without drilling my HVAC cabinet (because the filter sits in that slot below the coil). So I have been taking the filter out, making the measurements (at a hole in the return box - another blue tape spot in the picture), and then estimating that a spun fiberglass filter adds about 0.1"WC to it.

If my measurement and assumption for filter pressure drop are good, the current operating speed has me 0.52" if both zones are calling, 0.58" if zone 1 is calling, and 0.65" if zone 2 is calling. This is with basement supply and return open.

If I close the return vent in the basement, the static pressure measures 0.6" with both zones open, 0.68" with zone 1 open, and 0.71" with zone 2 open.

With a rating of up to 1.0"WC, I would think it's in range in either case. The zone panel is designed to soft start, meaning it leaves zone dampers open when it's done to avoid having high pressure issues, so I don't think there are transient spikes I'm getting. I'm not running AC at this point in the year, so I'm certain the coil is not freezing up (even when it defrosts, it doesn't stay there long enough to ice the indoor coil).

The fan has not tripped so far when I leave the basement supply and return open. If I close the basement return, it has tripped 3-4 times. It may run for a month before it happens. The trip last night happened within the first day of having that return closed.

Could my measurements be off? Sure, I'm no pro, I'm guessing at the midpoint of a number that bounces around on the manometer, and the manometer I'm using is a cheap one off Amazon. Could I be underestimating the filter pressure drop? I guess that's possible too. I found 0.1" as a typical pressure drop of a spun fiberglass filter, but that could be evaluated at a lower flow rate. If it's 0.2" or 0.3" or more, then my actual static pressure might be out of range.

Incidentally, I measure power consumption of the indoor fan motor at about 500-525W. It is rated for 750W, I believe. I've not seen it spike that high at all.

The other possibility I've been considering is that the high temperature differentials I'm seeing (40+ degrees) might be overheating the motor. If the compressor is ramping up uncontrolled, it may be creating too much heat in the cabinet and causing the motor to trip. On the old system, I'd never see over 90 degrees leaving air temperature. With this system I regularly see over 90, sometimes as high as 108-109 degrees F. That's with no heat strips operating.

 

[EbS-P]

Well it’s at this point we probably need to estimate a face velocity for your filter. You said 20x20 correct and running 1600 cfm. So you have volume divided by area and get the face velocity. 1600cfm /2.8 sq ft 570 ft per minute which is high and by this chart could a fiberglass filter could totally have . 16” drop So now we’re in that range of pressures that might result in a 1” static pressure.

You are using this blower as designed. It should not be tripping. High static pressure is a blower killer but that takes a time.

It’s under current, and at max SP it should just run. A quick cardboard and 2x2 return plenum to accommodate a 20x30 filter might be worth a try.

 

[mooch91]

Well it’s at this point we probably need to estimate a face velocity for your filter. You said 20x20 correct and running 1600 cfm. So you have volume divided by area and get the face velocity. 1600cfm /2.8 sq ft 570 ft per minute which is high and by this chart could a fiberglass filter could totally have . 16” drop So now we’re in that range of pressures that might result in a 1” static pressure.

You are using this blower as designed. It should not be tripping. High static pressure is a blower killer but that takes a time.

It’s under current, and at max SP it should just run. A quick cardboard and 2x2 return plenum to accommodate a 20x30 filter might be worth a try.

@EbS-P

Thanks for some confirmation. I did just repeat some measurements by disconnecting the condensate line and using it for the pre-coil (return) side measurement, and I'm seeing 0.75-0.8 with the return vent open and 0.8-0.85 with the return vent closed. So this should now "include" my filter and is consistent with the pressure drop info you described.

This also probably confirms that I'm only getting about 1370 cfm flow at this speed, way lower than a 5-ton should have. Probably explains high temperature differential.

One interesting thing I observe with my measurements is that as I reduce the static pressure on the return side (open basement vent), it shifts to the supply side. That would lead me to believe it's more than just a return issue, and that as I make the return more free-flowing, the fan draws/pushes more, and then the supply becomes limiting for the flow.

I suspect "opening up" both sides might be required,

Curious what you are suggesting with the cardboard - disassemble some of the existing duct and temporarily replace with cardboard?

 

[EbS-P]

@EbS-P

Thanks for some confirmation. I did just repeat some measurements by disconnecting the condensate line and using it for the pre-coil (return) side measurement, and I'm seeing 0.75-0.8 with the return vent open and 0.8-0.85 with the return vent closed. So this should now "include" my filter and is consistent with the pressure drop info you described.

This also probably confirms that I'm only getting about 1370 cfm flow at this speed, way lower than a 5-ton should have.

One interesting thing I observe with my measurements is that as I reduce the static pressure on the return side (open basement vent), it shifts to the supply side. That would lead me to believe it's more than just a return issue, and that as I make the return more free-flowing, the fan draws/pushes more, and then the supply becomes limiting for the flow.

I suspect "opening up" both sides might be required,

Curious what you are suggesting with the cardboard - disassemble some of the existing duct and temporarily replace with cardboard?

If it was easy enough to accommodate a larger filter in some fashion in the system yes. I was thinking back to my childhood house where the furnace had a direct return in the basement. Your case is a bit more difficult. I would be taking apart as little as a could.

Your ducts are just undersized. There is not much free flowing about the return trunk from the ceiling to the filter. That in my complete amateur opinion looks unprofessional. No elbows just dead 90s.

Options as I see them.

0 rule out mechanical malfunction (not sure how without a second blower motor)
0.5 re fab the return to the unit with some elbows at ceiling and into unit.
1 disable the 2nd zone control open the damper to it and run the whole space as a single zone.
2 add more basement vents. (Want to have finished living space down there).
3 add more vents and returns upstairs (still limited by undersized main trunks).

 

[mooch91]

You are using this blower as designed. It should not be tripping. High static pressure is a blower killer but that takes a time.

Thought about this a bit... Could be that the static and temperature are compounding each other? The blower may be operating within the pressure specifications, but if it's experiencing the heat of a higher-than-nominal temperature 5-ton coil, because of the low airflow going through, it might be the higher temperature that's getting to it.

 

[EbS-P]

Thought about this a bit... Could be that the static and temperature are compounding each other? The blower may be operating within the pressure specifications, but if it's experiencing the heat of a higher-than-nominal temperature 5-ton coil, because of the low airflow going through, it might be the higher temperature that's getting to it.

Yes it could but I don’t think that’s it. The system needs to be able to run at 100% output at 60 degrees outside temp. Do the math at the projected flow rate and known delta T and calculate btu output.

I don’t know if I trust the heating performance chart. Why would the system efficiency increase as the return temp increase???

Really you need to be able to measure face velocity and static pressure.