HVAC Heat Pump
 

Technology Connections just released this video explaining that furnaces are massively oversized, and several methods to see how oversized your furnace is.

https://www.youtube.com/watch?v=DTsQjiPlksA

My furnace is a single-stage, meaning it's either on, or off. This makes calculating how oversized it is easy. On the coldest day this year (Jan 13), we hit a low of 16F and a high of 24F (about the coldest it gets here). I reviewed my furnace fan power consumption for that day, which came in at 5.56 kWh. My fan runs at almost exactly 700 watts, and only runs when the furnace is on.

My weather station
https://ambientweather.net/dashboard...b5b0a74/graphs

If my furnace ran at 100% capacity (on for 24 hours in the day), the fan would consume 16.8 kWh (24hrs x 700 watts).

So, my duty cycle on the coldest day is 5.56 kWh / 16.8 kWh = 30%

The furnace is rated at 100,000 BTU/hr, so 30% of that is 30,000 BTU/hr. That means that a 2.5 ton (12,000 BTU = 1 ton) heat pump would have just maintained heat running at 100% capacity.

The real problem I'm trying to solve is to even the temperature differential between upstairs and downstairs. It's typically 6-10 degrees cooler downstairs. With my furnace blower only running 10% of the time on average during the coldest month, there's not much mixing occurring.

My thought is to replace my 5-ton, 10 SEER AC unit and 80% efficient gas furnace/blower with an inverter heat pump. Inverter heat pumps can run at any output from about 35% to 100%. They are designed to run continuously instead of turning on and off. My assumption is that running continuously will keep the air mixing between upstairs and down, somewhat evening out the temperature differential. My other assumption is that I can close registers to force more heat downstairs without risk of stressing the ductwork since this system would be running substantially less pressure.

Other minor benefits include substantially less noise from the blower and furnace.

Other thoughts are to install a door between upstairs and down, and to increase soffit ventilation since mine is such a joke, leading the attic to get very hot in the summer.

It's like deja vu all over again.

That's the first thing I remember you posting on the forum.

Well, I joined in 2010, and bought this house in 2019, so there was 9 years of yammering about other things.

First thing I remember posting was a physics question regarding hill climbing. All engine efficiency and aero factors aside, is it more efficient to go faster up a hill. It takes a minimum amount of fuel burn to merely maintain position on a hill, and doing so would be 0% efficient. So you've got to burn the minimum plus something to go up it. The faster the hill is overcome, the less minimum fuel burn is needed. The analogy I gave was a rocket hovering, vs barely gaining altitude, vs going as fast as it can.

I spent too much time estimating how many BTUs I use for heating, converting that to therms, calculating my heating gas bill based on that, figuring out how many kWh I'd use if I heated with a 10 HSPF inverting heat pump... my estimate is I would save between $0-$100 per year. Very tricky converting between all these units. The SEER and HSPF should be simple % efficiency ratings (like a gas furnace) so the math is easy.

Anyhow, I estimated my furnace is on 6% of the year based on my blower on time, which amounts to 657 total heating therms. I also looked at my monthly gas bills, and knowing I have zero heating in July, Aug, Sept, I was able to roughly determine I use 20 therms/mo in hot water, range, dryer. Subtracting that from the total for last year, it comes to 665 therms used for heating.

I pay about $865/yr to heat on 80% efficiency gas. I estimate $790 heating on an inverting heat pump.

By first I meant earliest.

You hill-climbing example, is like mine of getting from a stoplight at 0MPG to an efficient cruise.

For seven years, I lived in a geodesic dome with a ceiling fan and a hot-water radiant floor under ceramic tiles. I found that optimal.

I haven't watched the video, but in my situation I let the house cool off at night and also when I'm not there. It can take several hours to get the temp back up. So an oversized furnace is a good idea in my mind.

I'm not sure if I'd save any money if I had a heat pump since gas is about a third the price of electricity.

Here's 1 thread where I talked about this;

https://ecomodder.com/forum/showthre...ing-40669.html

and another;

https://ecomodder.com/forum/showthre...irs-37573.html

and another;

https://ecomodder.com/forum/showthre...ing-40669.html

You can tell I'm finally getting serious about this problem.

I'm not a pro, but I have spent some time on HVAC-TALK and I am facing some of the same issues you discuss.
Regarding closing off returns... you SHOULD familiarize yourself with TESP measurements (static pressure in ducts) This is easy, but you need to buy, borrow or make a manometer. You also need to verify the TR (temperature rise from return (intake) air and supply duct air temperature. Compare that to the label on the furnace.
Fan speeds... you may be able to lower the fan speed for heat... ( but, not if you don't understand the function/purpose of the above two easy to preform tests.)
Do you know what your fan speed is? For example, a 80,000 btu furnace should have a minimum of 20x25 inch duct.....
What is the size of your return duct... should be equivalent to 16" round for a 80k btu and 1200 cfm.

Are the ducts rigid or flex?

Furnaces usually have a separate speed for heat and a faster speed for A/C....

You may save money hiring a HVAC guy to look over your ducts... the purpose of such is to "prep" your current ducts to a new system you 'might' buy. Since your current furnace is oversized, it's possible ducts will be efficient when you decide to buy a 50k btu unit.

And, it's possible....you might spend lots of money/time and not solve your temperature stratification cheaply...( newer units will have more efficient blowers that operate more cheaply (guess around 30%?) if you want/need to run the fan more often.

Good advice. Seems simple enough.

Don't know my fan speed. My furnace is 125,000 BTU, with 100,000 of useful heating. I have 2 returns shown in this photo. The cylindrical vertical one is from the upstairs ceiling, and the other is on the floor of the basement with the filters sitting on top.

https://www.diychatroom.com/cdn-cgi/...07-jpg.781672/

Flex branches; each one going to an individual register.

If mine does, it's not reflected in power consumption. I would think less speed is needed for AC considering 5 tons of cooling is nothing compared to 100,000 BTUs of heating.

One person suggested installing more registers both upstairs and down, and that way some of the upstairs/downstairs ones can be closed depending on the season. I basically need all of my cooling to come from upstairs in the summer, and 90% of heating to come from downstairs in the winter. I've got 10 registers upstairs, and 7 down.

Here's the downstairs, with 2 bedrooms and a bathroom not pictured (behind). The white door on the left leads to the utility room where the furnace is.
https://www.diychatroom.com/cdn-cgi/...53-jpg.781670/

The rest of the single main supply plenum
https://www.diychatroom.com/cdn-cgi/...10-jpg.781671/

Could you post a picture of the furnace spec plate?
ECM motors (variable speed) can be installed... (google time)....part costs less than $300.

Assumption...not enough return air. Could you provide:
1. Measurements of both return ducts (ie: 16" or 20x25")
2. Measurements of the return air grilles
3. Roof insulation depth or R-value. ( mine was R19; 8 inches fiberglass, I went balls out and it is now R70,;24-30" )
4. Type of attic vent, est size of vent, soffit ( opinion as to whether attic ventilation can be improved?)
5. Opinion on noise level from air rushing into return grilles. ( too noisy? )

The furnace must be sized for unusualy cold weather events unless you have robust backups.

https://ecomodder.com/forum/attachme...1&d=1710370162

I'm very interested in this aspect. Would be sweet if it varies automatically based on air temperature of the supply side. Move just the amount of air required to keep that temperature well within spec.

Both ducts are pictured above. The round return (upstairs ceiling) is 12" diameter and the square one (downstairs on the floor) is 17x20". Both grills and filters are 17x20"

I needed to get in the attic for another reason, so I took a peek. The label says R38, but looking at the inches of depth it looks closer to R30. Maybe it compresses after 20 years?

21x Standard 50in/sq vents. Knuckleheads added 3 more after I pointed out intake ventilation was inadequate.

https://ecomodder.com/forum/attachme...3&d=1588870239

Soffit vents are a joke. 2" round things with screens that have UPC tags on them covering half the intake. There are 2 of them spaced every 3' around the house.

https://ecomodder.com/forum/attachme...1&d=1710370162

Audible, but not loud.

17x20 is good for about 1600cfm and 12" is good for about 500cfm..... that checks out

Grille size 17x20 = 340sqin x 2 = 680cfm... problem
So the grille for the 12" round duct (capacitu = 500cfm) vs grille (680cfm) good.
Grille size for the 17x20 square duct (capacity = 1600cfm) vs grille capacity (680 cfm)..... not so good and this return is noises than the other.one.
So you have. .500+680cfm= 1180cfm. (Choked) for a 5 ton system which likely set to 1500 to 1700cfm....
What just happened...so now you will have excessive pressure (big deal, I'm in the same boat) This will make the blower motor work hard...drawing more amps.
I predict that if you tested static pressure you will be exceeding the .50wc spec.
The vents on the roof top are sufficient...the soffit are not. The total vent area, say 20x.5sqft =10sqft. You need roughly 10sqft soffit area, spread about the perimeter of the roof for your attic to vent properly...rather, the free area of the ridge needs to equal the free area of the soffit at the perimeter of the attic floor.

R30/38 would be minimum and I'll bet they recommend double that in current codes for new houses.

One problem with the ECM motors is that they do adjust speeds automatically....based on static pressure.....oops.... weren't we talking about that earlier.. I'm predicting high static pressure due to restricted return air....This might be an issue/might not.

So, since we are deficient in return air, look at the return plenum, or that 17x20 duct....can you tap into that and run an additional return somewhere where it might be benificial....?

What amount of airflow is called for with a 125,000 BTU 80% efficient furnace? It seems everything is engineered around the AC needs... but the furnace has way more tons of heating output than the AC has cooling.

The downstairs return is more noisy... actually when I went to replace the filter the first time, I found that ducting was not sealed with the wall, so it was mostly pulling unfiltered air from within the room where the furnace is. Perhaps that was left unfinished to gain sufficient airflow? I sealed it up so that all air goes through the filter from the living room.

I'll test the pressure and see where I'm at. Strangely, when I remove the air filters, the air handler draws more power.

Perhaps in the meantime I can install a higher flow filter?

Soffit ventilation will be on my to-do list for this summer. Looks like I need about 25 of the 16x8" vents.

I just looked up the specs on my current soffit vents. 0.8 sq/in LOL. I'd need like 1,800 of them to be adequate. I think there's about 100 on the whole house.

In the summer, I have been opening the access door to the attic.

Maybe I'll add some rolls. Currently have blown in fiber. I'd want to engineer some walkways though as I'm in the attic often.

Maybe. There is access between the ceiling and the floor above. Seems difficult, and you probably don't put a return in an exterior wall. There's no crawlspace.

' stratification '
 

In the 1970s, some addressed this issue by creating a 'chase', or vertical duct, with a low- power tube-axial fan, which would draw air from the up-stair's ceiling when the HVAC cycled off, and pull it downstairs, where it would diffuse it into the living space, mixing and blending, as a ceiling fan can do when it's rotation is reversed. Kind of a 'homogenator.'

I do the same. My heating cycle is 69F @ 4:30 am to heat the house by the time we get up about 5:30. That holds until 7 am when it goes back to 67F for the day. At 4:30 PM we heat to 72 F until 5:00 PM. (5 pm is when my electricity switches to peak rate*). We hold 69F to 7 pm and let the temp drop to 67F for the night. (It takes hours for the temp to actually fall down to 67F) (That is the schedule Mon - Thursday when we are both at work)

Rates are: $0.08 from 9pm - 7am / $0.16 from 7am - 5 pm / $0.41 from 5 pm - 9pm

There is a HUGE difference in efficiency. A furnace is 80 - 95% efficient. Heat pumps are 250% - 300% efficient.

I switched the thermostat off last Thursday evening 3/14 and haven't run heat since then.

We'll need some heat the coming week, but I might just use the gas fireplace as needed.

I let the temp drop to 66 at night, but this past week I've been taking all the solar gain I can get, driving the temperature in the evening to 76, and it only drops to 71 by morning.

Seems like the core of your problem is that you are trying to heat and cool a giant 2 story house with a single zone HVAC system. I've yet to see a house were that actually works with equal temperatures on both floors.

I seems the easiest solution is to split your HVAC by floor. You could do this by adding another ducted unit or by adding ductless heat pumps to one of the floors. (I would suggest the floor with less individual rooms.)

I replaced our 80% ducted furnace with a ductless heat pump about 7 years ago. I ended pulling that furnace out but shortly after I did that I thought about it and regretted that choice. The furnace could have been kept as a back-up or to supplement heating when the temperatures drop and the heat pump efficiency also drops.

Our corn / pellet stove was right sized for 5-15F, it would keep the house warmish down to -20F but its efficiency fell off a cliff running 100% continuously and ash output went up massively.

We would sort of run both gas and pellets in very cold weather which has the benefit of warmer floors while not going bankrupt on pellets

I see a couple options.
One is to buy a couple mini splits to supplement your upstairs cooling.
Two is to remodel a bit to greatly improve your duct system. Try to pipe in returns into increase circulation...you probably need twice as much cfm to the upstairs returns into the rooms. One method would be to chase a return into a closet to the attic and install ceiling returns. Another method, add jumper returns to the ceilings of both bedrooms and the great room.
One saving grace could be that the heater could later be changed to a lessor output...which means...
A good HVAC guy could look at your house and do a basic load calculation of each room and make a proposal to increase circulation, and the amount of air flow to each room needs. They are fairly good at this and it comes at a cost.... he will tell you what you need to know. If he's an older tech, he may be able to "derate" your furnace...something some guys won't do.
I'm guessing ventilating the attic properly might relieve some usage.
For your blower motor, you can get information by googling "ecm blower motor swap retrofit"

I found the following interesting. It basically says that HVAC manufacturers have no incentive to include efficient air handlers because they aren't factored into the efficiency rating of the furnace or AC. It also claims the added cost of an ECM air handler has a payback period of 2-3 years.

Simply improving air handler efficiency would save way more electricity than getting everyone to buy efficient refrigerators.

That was the first thing I started looking into, and didn't find enough info to make me comfortable swapping motors.

Here's some info I'm finding based on your search recommendation;

EDIT: Searching Youtube for PSC to ECM replacement only shows the exact opposite. For some dumb reason, it seems people are only interested in going from an efficient motor, to an inefficient one.

Well, almost all new manufacturer's air handlers have ECM blowers, so many, it's likely mandated. They sound great, especially in setups that allow for long duty cycles. Some people have allergies and leave the blower fan on 99% of the time and thats where the ECM blowers shine with their 30% better efficiency. Where they don't shine is in systems with high static pressure, or lousy ducts, as then they will only supply as much air as its pressure sensors allow.
Alas, it's possible a HVAC contractor will assess your actual needs, possibly suggest a 60-80k BTU high efficiency heat pump unit that will work in a 3.5 ton unit...which means...a 3.5k blower would be about 1400 cfm....possibly at 1400 cfm, your ducts would be sufficient.
And, there is still time to do return and supply Temps for the furnace to calculate temperature rise. Should be in the range of 40°-70°. Say, if the rise was close to 40°, you could lower the fan speed which could possibly circulate air better...being that the blower is operating longer.

I took a closer look, and the return is even worse than calculated, and it is way more noisy as you predicted.

https://ecomodder.com/forum/attachme...1&d=1711224365

The knuckleheads have the plenum placed well below the grill, and didn't seal anything. The foam is my attempt to seal it up. So the excess capacity of the return duct is made even worse by the grill being misaligned with it. Maybe I can bend the plenum up to the grill? Don't know how these are normally joined/sealed to it.

Could I simply add a 2nd 17x20 grill next to the existing one and feed it into the return?

Here's the other side of the wall, showing the return plenum sitting on a 2x4. Don't know why the forum put the image upside down.
https://ecomodder.com/forum/attachme...1&d=1711223556

My furnace control board schematic if that's useful at all. I can't find specs on the motor anywhere. Strangely, the more restriction there is, the less power it draws. I turned the fan only on, and it draws 750 watts. When I remove the filter and open the grill, it draws 800 watts.
https://ecomodder.com/forum/attachme...1&d=1711223556

To answer your earlier question about how much air flow is needed for a 125,000btu furnace... 1600cfm. And the airflow required for a 5 ton a/c is 2000cfm.... (and..17x20" duct is about 1600cfm).
So yes, your biggest gain with your current equipment would be to fill that 17x20" with as much air as possible. I think it would be very easy to open up the return grille to be large enough...(even going a bit bigger, say 2 20x30" return grilles.)
It also looks like you could tap the right side of the return just behind the grille to penatrate the bedroom wall, adding a return to each bedroom is recommended, It looks like you have high ceilings (with fresh paint ).ducting some return air from up high would be good if you didn't mind tearing out drywall....a standard 2x4 wall cavity lined with a 3.5x14" duct is only good for 100cfm.
Is it also possible to run a other 12" round return into the other floor somewhere?
Now I'm curious about the number of and sizes of your supply vent registers....

It's Oct 16th, and I still haven't run heat yet, though we've got a few days of rain in the forecast and cooler temperatures. My goal is to make it to November without running heat, but I've only accomplished that a couple times before.

I ran an experiment this year with the AC, using two, 15 SEER, one-ton window units upstairs. The goal was to utilize the 10 SEER five-ton less and hopefully somewhat even the upstairs and downstairs temperature.

Between May and August (4 months) the whole house AC ran for about 20 hours consuming 105 kWh. This compares to about 1,000 kWh in previous years.

The two window AC units combined consumed 504 kWh. That plus the whole house AC usage comes to 609 kWh this cooling season, saving about 400 kWh compared to only utilizing the whole house unit.

These results were good enough that I'll probably continue using window units in the summer.