Conditioning Second-Floor Rooms With Air-Source Heat Pump

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We are looking to replace our existing gas-fired furnace with an air-source heat pump, and are running into some of the common problems with heating/cooling the second floor in old houses: it is parceled out into three small-ish bedrooms and a bath; the existing ductwork is very limited (small supplies, no returns).

Our house is a 1920s Foursquare in Massachusetts (zone 5A, heating design temp 0F). About 1600sqft conditioned space. Unconditioned basement with R-0 walls and slab. Walls are 2×4 with R-11 blown-in cellulose. Vented attic with around R-60. Some air-sealing in attic and basement. Double-pane windows. I am attaching a floor plan with approximate location of existing supply and return registers. The stairway in the hallway is open.

I calculated the implied heating load based on our observed gas consumption, as explained in this article (https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new). Depending on the month, I obtain a peak heating load of 33-35k BTU/hr. With the ASHRAE 1.4x sizing factor, this gets me in the 45k range (this matches the size of the current furnace). I also ran a CoolCalc Manual J calculation and obtained a heating load of 40k and a cooling load of 25k (assuming a ducted solution).

Two companies proposed a ductless mini-split head in each room of the house for a very hefty price. Based on the recommendation in this article (https://www.greenbuildingadvisor.com/article/getting-the-right-minisplit), I’m not planning on going down that route.

Company C proposed a central heat pump with an air handler replacing the existing furnace. They’re planning on sealing the existing ducts (we’ll certainly do that) and increasing the size of the return registers near the bottom of the staircase. They argue that this will be sufficient to pull down the air from the second floor during cooling months.

They are proposing this system (https://ashp.neep.org/#!/product/25536) by American Standard, together with a back-up heating element.

My questions:

1. Could the box-swapping approach proposed by company C work, i.e. using the existing ductwork with minor modifications? Based on our duct layout as shown in the floor plan, and given that there are no return ducts on the second floor and only small supplies?
2. Does the system proposed by company C make sense? It seems a bit under-sized to me. This article (https://www.greenbuildingadvisor.com/article/ducted-air-source-heat-pumps-from-american-manufacturers) argues that American ducted heat pumps are inferior to Japanese units, particularly in cold climates such as ours. Given their cheaper price, are they good enough?
3. Are there better alternatives that don’t involve placing a mini-split head in every room? I’ve read a little about ducted mini-splits that could be placed on the second floor ceiling, but the rooms are only about 7’8” high and none of the companies I talked to were interested in entertaining this idea.

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Replies

1. | | #1

Hi Noe,

Just to your question about whether approach C will work - we have a similar setup at our house in northern New England (no return registers in bedrooms upstairs, original ductwork) and a heat pump retrofit has worked pretty well for the most part. The installer did find a way to add a return in the upstairs hallway by adding a new duct through two levels of closets in the house.

In our case, the long run times of the heat pump result in an evenly heated home for the most part. It actually gets worse for us in the bedrooms when the gas furnace
switches on at around 15 degrees F (and improves again as it gets towards zero F). Cooling evenly is more of a challenge but we’re working on addressing that.

Obviously each home is very different, just citing an example that it *can* work.

- Evan

1. | | #3

Good to know that working with the existing ducts is possible in practice. Unfortunately we don't have two levels of closets that would allow for a new return duct. We have an unused brick chimney, but presumably removing that and using the space for a return duct is too costly/disruptive.
Noe

2. | | #2

Make sure that the system will operate adequately at record lows. Some heat pumps will turn off or refuse to start, which, without lots of backup heat, could be an unpleasant surprise.

For example, what happens to a system with a spec of "This unit is designed to operate at outdoor ambient temperatures from ... from —10° F to 66° F in heating." when it goes below -10F?

3. | | #4

If you do a fuel switch understand it will take some adjustment of you expiations in that the old furnace most likely is blowing 130° air to warm the house when that duct is pointed at your favorite chair it feels great but a heat pump could be 89° air and moving more air. Getting the right sized duct to the right spot makes a big difference in heat pump comfort. Heat pumps work best when you do not change the temp setting so that can be an adjustment to the same temp 24-7.

My conventional heat pump heats my house down to 6°F with the resistance back up locked out.

My must have feathers list for a modern heat pump are.
1 Variable speed compressor.
2 Variable speed ECM indoor blower motor
3 Two electronic expansion valves.
4 Communicating thermostat by the same manufacture.

Mine is by RHEEM by I think every brand has a model that will have all 4 feathers.

I came to the conclusion that return ductwork is far less important than most of us assume when after almost 2 years I found parts of my return system had not been installed so my unit was getting all its return air from the basement. Installing the missing bits and testing that the returns were working had no affect on the comfort or operating costs that I could detect. We tend to leave most interior doors other people may not .

I like the idea of removing the unused chimney huge amounts of air are likely leaking from you home around this unused chimney. Pay someone remove the exterior parts and patch the roof the rest is simple manual labor starting at the top and working down.

Walta

1. | | #6

Thank you for this helpful list Walter.

2. | | #12

We have a retrofitted ducted heat pump with no return on the second floor, and we basically don't have AC up there. During the recent heat wave in Seattle you could feel the temperature increase on every step going upstairs. (The installers were quite clear that would be the case, and I have no beef with them.) The heating isn't a problem at all. That said, my husband went on sleeping upstairs during the heat wave and said it wasn't bad (he has more heat tolerance than I do; I moved downstairs for the duration), and it didn't get to a dangerous temperature by any means.

I agree that taking out an unused chimney could have other benefits.

4. Expert Member
| | #5

I would not oversize the heat pump from your fuel load based calculation, instead add in extra resistance heat strip to cover any polar vortex days.

The unit the installer suggest is decent but does loose steam in colder weather which would mean running the resistance heat a fair bit.

If available in your area for a reasonable cost, most of the hyper heat units (Mitsubishi, Daikin, LG Red, Midea) with a multi position air handler (these look like a standard furnace) will deliver 100% of their rated capacity down to 5F, some even to 0F. Equipment cost wise, these are only a big more expensive than comparable high end domestic units but installed costs could be more as some consider them "premium".

If your fuel calc numbers are correct, something like this could carry your house without a heat strip:
https://ashp.neep.org/#!/product/34582

If you don't mind a bit of DIY, these are excellent value:

https://mrcool.com/mrcool-universal-series-dc-inverter-heat-pump-air-conditioner-split-system/

Heat pumps need larger ducts than a fuel burner. Provided the ducts were properly sized for a big fuel burner, the existing ducting can work.

A return on the 2nd floor is more important for cooling performance. To get good cooling, you want a largish return near the ceiling. If you don't need a lot of cooling, you can skip this as the single main floor return will work for heating.

1. | | #7

Thank you for these suggestions Akos.

The size of the ducts is partly what worries me. I'm not quite sure how this is expressed, but the supply ducts have a cross-sectional surface of around 30-35 sq in. There are plenty of elbows and some forks in the ducts. Will these duct sizes work for a heat pump?

Unfortunately there is really no way to get a return duct up to the second floor, short of placing it in the vented attic (which I hear should be avoided) or ripping out our chimney.

If cooling is more limited in the second floor, we can probably live with that. The CoolCalc cooling loads for the upstairs were 1000-1500 BTU/hr per room (24k for the whole house).

EDIT: I am trying to understand how the CFM numbers produced by CoolCalc (about 50 for each upstairs room) relate to the size of our ducts (which allow for 100-125 CFM if I'm not mistaken) and the extent to which the blower motor in the air handler will be taxed. Is there any risk that the blower will have to work so hard as to negate all the efficiency gains from switching to a heat pump solution?

1. Expert Member
| | #9

You can measure the static pressure in the existing system to check. Online market places have a cheap digital manometer you can buy for this. Check the pressure just before the return air filter to see the drop in your return ducting and after the AC coil for the supply ducting.

The data sheet for your existing furnace will have the flow rate of the air handler to give you roughly the air flow through the ducting.

So for example if your furnace is around 1000CFM and you measure 0.8" WG, the same ducting would be 0.7" at the 900CFM required for the SVZ-KP30NA.

You can also increase the size of the return air filter, which is simple if you are changing the air handler. Going for a 4" filter frame reduces the pressure drop by about half VS a 1" filter, well worth it as they last much longer.

Generally bedrooms need very little air flow, the challenge with older ductwork is getting that air there. Lot of time these are not sealed and can have a lot of bends, which means even though the ducting can do 100CFM, you might only get 30. Lot of times you seal up part of the ducting in the basement and can do a bit of balancing on the supply trunk to provide more air for upstairs. Takes a bit of fussing but worth it for comfort.

Another way to get around undersized ducts is to reduce the heat load for your house. Insulating your basement can drop your overall heating load by 15% to 20% which would mean a smaller (cheaper) heat pump, lower operating costs and more importantly a much more comfortable main floor as the basement will be warmer.

2. | | #10

I just want to be sure you understand that once you have the heat load for each room from the manual J calculation that data is used in a manual D calculation to size the ducts going to each room.

In general you need about 400 CFM for each ton of equipment. The lower the air speed in the ducts the quieter the system will be. 900 FPM is often used as the highest sped deemed to be acceptable. Bigger ducts are better ducts but they cost more and take up more space. CFM = FPM x Duct Cross Sectional Area.

Also demanding to see these calculations will not win you any friends among the bidding contractors. Mostly they just connect to whatever they find in place and call it good and mostly it works.

Walta

3. | | #18

Thank you both. I will measure the pressure in the existing system. I gather from your responses that the results are often good enough, and that there are marginal adjustments that can be made (sealing the ducts; insulating the basement; increase the filter size) without ripping out the ducts or abandoning them in favor of a full ductless solution.

5. | | #8

“negate all the efficiency gains from switching to a heat pump solution?”

Depending on your local pricing generally form a dollars and cents point of view city gas almost always has the costs less to operate. If you can eliminate the gas meter and it monthly service fee you may come out ahead.

Walta

6. | | #11

Noe, option 3 will work fine (I've done the same thing) - you may not have perfect temperature balance between the two floors, but you probably didn't have that before either. Get multiple quotes from ducted heat pump installers - the Mitsubishi Diamond rated ones will be a good place to start. I'm surprised your fuel usage heat loss came in that high with R-60, double paned windows and some wall insulation - can you share a spreadsheet with the gas usage data?

1. | | #14

Paul, thanks very much for your response. I would very much appreciate if you could take a look at my calculations. I have used the statistical software R to run them, so I'm attaching a PDF - let me know if it's missing important information.
I've also added a page where I'm repeating the calculations but removing the average therms consumed over the summer months to reflect water heater and washing machine use.
The implied heating load for the 2021 winter months are a bit higher because we have a newborn that we wanted to keep extra cozy.

1. | | #21

Looks good! Call it 11,500 Btu/HDD, gives you ~29,000 Btu/hr heat loss before any oversize adjustment. You can try running a regression if you want, but I think you're in the neighborhood as it is. One question: you have therms by month, does your utility bill consistently on the first day of the month? Or did you summarize this from daily usage?

1. | | #23

Thanks Paul. Sorry, I'm not sure I'm following: did you take the 29'000 Btu/hr number as a rough average across the peak heating months (based on the adjustment for water heater usage)? I'm unsure whether this would speak for a unit in the 32k (as Akos recommended above) or 38k range.
About your question: My utility bills report monthly usage, but I adjusted for the different reading days when I summed up the HDD data (I didn't include that part in the printout that I shared here).

1. | | #24

Both the regression (Therms vs. HDD65) and the total Therms/HDD give you that result more or less (2,165 Therms /18,859 HDD = .1148 therms/HDD). So .1148*100,000*.93*65/24 = 28,915. The regression gives you ~27,000 Btu. There's no need to go overboard on the sizing, but inverter heat pumps will be more efficient running at part load. For example, the Mitsubishi 36k Btu Hyper heat ducted unit has a rated COP of 3.3 (!!!) at 0 degrees F (about 66% speed). But at max speed, it drops to 1.57 at that temperature.
And excellent! Just making sure the HDDs and Therms were properly matched up. Does your utility adjust the therms in any way? Some will adjust the meter readings up based on the gas's actual energy content.

2. Expert Member
| | #25

I'm getting similar numbers as you got (30 to 35 depending on month). For a 1600sqft house, that means you have some serious air leaks, more than just a unused chimney. It might be worth while to spend some effort to get the house sealed up.

If the place is balloon framed and a 1 1/2 story construction there are typically some very large air leaks into wall cavities and through these into the attic. Find these and seal them up.

https://www.finehomebuilding.com/2012/09/06/two-ways-to-insulate-attic-kneewalls

You generally hit outdoor design condition early in the morning, even a slightly undersized heat pump will catch up once the sun comes out, most houses have enough thermal mass that I doubt you'll lose more than a couple of degrees without a backup heater. The smallest heat strip most units take is around 5kW (17000 BTU), with that you'll have no problems to carry the house by under sizing the heat pump a bit in anticipation of future energy efficiency improvements.

3. | | #26

Thanks Akos,

What's a reasonable goal for BTUH/sq ft?
I'm wondering where the excessive leakage is coming from. The utility company did some air sealing in the attic and basement. We have a regular 2 story house, with a straightforward vented attic (no knee walls). There's a whole house fan that we put a foam lid on in winter. Perhaps the excessive leakage is from the uninsulated basement? We removed the ceiling insulation down there because of moisture issues.

Thanks Paul,
I take your point about the efficiency of VRF heat pumps at partial loads. It was easier to download the history of therms from our gas company rather than the BTUs delivered. I will double check that the results hold up when using the BTUs instead. Thanks for pointing that out.

4. Expert Member
| | #27

I have a century old balloon frame house mostly insulated 2.5 story in similar climate. That place is 13 BTUh/sqft but it does have an insulated basement.

Drafty basements can definitely do it especially if the place is balloon framed with no bottom or top plates.

Was the air sealing done with spray foam, rigid foam or mastic? Stuffing batts into openings does very little to limit air flow.

If the foam lid is not tight, the whole house fan can be a big air leak. The unconditioned sun room and the mud room can also be big sources of air leaks.

The best is to hire someone that does blower door directed air sealing. This is best done in colder weather as it is easier to find the leaks. You can also DIY it as well with a box fan mounted and sealed in a window and FLIR camera for locating the leaks.

2. Deleted | | #17

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7. | | #13

This comment by Dana Dorsett may be helpful (re not using the 1.4 multiplier for heat pumps): https://www.greenbuildingadvisor.com/question/heat-runs-to-unfinished-basement-pacific-nw-area#comment-43155

1. | | #15

Thank you for pointing me to that discussion thread, very helpful! A lot to digest..

1. | | #16

Make sure you understand the difference between a gas consumption based estimate and a Manual J and why you need up-sizing for the former and not the latter.

2. | | #19

The original article about that wasn't clear--the ASHRAE standard recommends 1.4X as a maximum size, not a target. The range recommended is 1.0 to 1.4. You could say the target is 1.2X.

8. | | #20

The concept of option C is fine, but the specific system they are spec'ing is a really bad choice which indicates they are incompetent and not who you want selecting your equipment. If you look at the graph of capacity vs. temperature, you see it drops way off at low temperatures. That means it's not a "cold climate heat pump" with the special type of "vapor injection" compressor that enables retaining capacity at low temperature. An example of one that retains capacity to low temperatures is this one:

https://ashp.neep.org/#!/product/29479

That's a Carrier, but there are lots of options.

9. | | #22

I have a system similar to option C, but it is a Mistubishi hyper-heat unit.

It works incredibly well. Blowing nice and warm all winter long, on even the coldest days.

It does have a backup system that almost never runs. I know this, because it was not physically plugged in for the first year, and we had some nice -30C polar vortex days that year.

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