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# How much cooler will a room get with an air source heat pump hot water heater?

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Member Posts: 11
I just signed up for an air source heat pump to replace my Boilermate hot water heater. I understand the basics of how it works, but I'm concerned at how much cooler my basement will be when the heat pump is running in the winter. It's a 50 gallon heat pump. The basement volume is 5000 cubic feet. I live in Massachusetts. Can anyone give me some guidance? I tried to do some basic heat transfer calculations (I'm a retired mechanical engineer), and the calculated drop in air temperature is kind of scary.

• Member Posts: 21
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My moms basement is a lot cooler. So cool that the heat pump portion of the water heater will not work and Hass to be switched to electric. She has an old house and not much insulation. 4 feet of the basement walls are above grade and it’s an old stone foundation with leaky single pane windows. However in the summertime her basement is dryer than it’s ever been thanks to her heat pump water heater.
• Member Posts: 7,397
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They make kits to duct the cool air outside.

A simple answer to your question would be to do a heat loss calc on the room vs. the btus extracted by the heat pump.

Obviously, this is something that only an engineer would try to figure out.
Bob Boan
You can choose to do what you want, but you cannot choose the consequences.
• Member Posts: 3,696
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IIRC the SlantFin heat loss app can handle partially-buried CMU walls. Might be a good place to start. Plus, then you could do a heat loss on your whole house & see just how oversized your heat source is.
• Member Posts: 11
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I did the calculations for exchanging the BTUs in 5,000 cubic feet of air to the BTUs in 50 gallons of water, heated from 50F to 150F. I then multiplied by 80%, which is how much of the energy in the heat pump comes from the ambient air in the room. The other 20% comes from the electricity used to run the heat pump. The result is an air temperature drop of 379F. Obviously that can't be right, but whatever it is, it's a big number. Thanks for the tip on the kits to duct cool air outside.
• Member Posts: 22,467
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How much hot water do you use? How often with the HP actually run each day?
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
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Your numbers are scary! Fortunately, you have overlooked a couple of things. Perhaps most important, you were figuring just the heat capacity of the air (I think correctly, but I haven't checked). But -- that is a very minor factor in the heat capacity of the space, most of which is in the structure and anything else you may have in there. The other thing is infiltration -- even a pretty tight space will have a couple of air changes per hour.

The space will get cooler -- but I'd be kind of surprised if it were more than a few degrees.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 11
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hot_rod said:

How much hot water do you use? How often with the HP actually run each day?

There are just two of us in the house, and we use probably 30 gallons of hot water per day.

Jamie Hall, thanks for the info. You're right, I'm just considering the air, not the walls and all the stuff in the basement.
• Member Posts: 3,384
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Hello, @Jamie Hall is right. This has been looked at and found to be a non-issue. I'm not positive I'm remembering correctly, but here: https://aceee.org/conferences/hwf/past is a link to all presentations at the Hot Water Forum, put on by ACEEE. Many presentations have been done on heat pump water heating, so there is a good chance you'll find useful info.

Yours, Larry
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I congratulate you, who as an old school ME, whose common sense caused you to doubt your calculations.
• Member Posts: 11
edited September 2019
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JUGHNE said:

I congratulate you, who as an old school ME, whose common sense caused you to doubt your calculations.

If there's one thing I learned after 36 years, it's to check your gut before believing any calculation.
• Member Posts: 11
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Hello, @Jamie Hall is right. This has been looked at and found to be a non-issue. I'm not positive I'm remembering correctly, but here: https://aceee.org/conferences/hwf/past is a link to all presentations at the Hot Water Forum, put on by ACEEE. Many presentations have been done on heat pump water heating, so there is a good chance you'll find useful info.

Yours, Larry

Thanks, Larry. Starting with that link, I came across the following excellent article about installation of heat pump water heaters in cold climates: https://contractors.efficiencyvermont.com/Media/Default/docs/programs/efficiency-vermont-heat-pump-water-heater-installation-guide.pdf
• Member Posts: 1,194
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That is an small basement at 5000cubic feet. But ultimately the compressor is at bet a 1/2 ton... like a small window unit or dehumidifier. Actually I suspect it’s just a smaller rotary modified to run on 134a for high temp output.

Point is that I have 2 dehumidifiers running each in about 900 cubic foot rooms and they raise the temp just 4-5f.

1/4-1/3 of the heat will come from latent heat in a typically damp basket. Other wise I’d expect a 70f in summer and 60f space to be 5-10f cooler and transfer the heat to the walls and mostly the ceiling.

In summer it helps cool your house slightly. In winter it will consume a little bit of heat from the 1sr floor.
• Member Posts: 11
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ratio said:

IIRC the SlantFin heat loss app can handle partially-buried CMU walls. Might be a good place to start. Plus, then you could do a heat loss on your whole house & see just how oversized your heat source is.

IIRC = 'if I remember correctly'?
CMU = ???
• Member Posts: 3,384
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Hi @samiam1955 , CMU means concrete masonry unit.... basically concrete block wall.

Yours, Larry
• Member Posts: 7,605
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Your numbers are off because you would need to calculate the mass of the room as it absorbs the energy.
Since energy cannot be created or destroyed, I am think that all the energy being saved by the pump is taken from the space.
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 11
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Thanks, @Larry Weingarten. My foundation is solid concrete, well below grade. @mikeg2015, the basement may be smallish, but at 5,000 cu. ft., it's well above the 750-1000 cu. ft. minimum recommended for heat pumps in various things I've been reading.
• Member Posts: 11
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Zman said:

Your numbers are off because you would need to calculate the mass of the room as it absorbs the energy.
Since energy cannot be created or destroyed, I am think that all the energy being saved by the pump is taken from the space.

Thanks, @Zman. I understand the laws of thermodynamics, but the conduction of heat from the cooler air in the room to the walls, floor, ceiling, and junk in the basement doesn't happen instantaneously. If it's all absorbed between one use of hot water to the next, I'm all set, but if it is still recovering to the original ambient temperature each time I use hot water, the room could always be uncomfortably cool.

It's impossible to do a transient heat transfer calculation of my whole basement, but there must be some kind of ballpark calculation that could be done, or even better, an empirical calculation that someone has come up with using data for average ambient air temperature change as a function of water usage and room volume. THAT'S what I'm looking for.
• Member Posts: 7,605
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To keep it very simple. Any time the outdoor temp is lower than the indoor setpoint, you will need to somehow replace the Btu's being stolen by the water heater. The buildings heat system will need to make up the difference.
Stealing from Peter to pay Paul...
The good news is that you will save on AC.
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 11
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Zman said:

To keep it very simple. Any time the outdoor temp is lower than the indoor setpoint, you will need to somehow replace the Btu's being stolen by the water heater.

Thanks again. I understand that part of it. The question is - while the heating system is recovering from the blast of cold air produced by the HP water heater every time I use hot water, how cold will my basement get? I know no one can give me an exact answer, but if someone can say "I have a XXXX cubic foot basement, I use YY gallons per day, and the temperature drop is typically between X and Y degrees", it would be a big help for me to make this decision.
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That's a question that cant be answered with any certainty. There's just too many variables
Infiltration
Water Pipes
Ceiling structure & Insulation
Frost level
Wind

• Member Posts: 11
edited September 2019
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pecmsg said:

That's a question that cant be answered with any certainty. There's just too many variables
Infiltration
Water Pipes
Ceiling structure & Insulation
Frost level
Wind

Thanks, @pecmsg. All of those factors affect the winter temperature in my basement, which I already know because I've lived here for 32 years. It's comfortable in the winter. On the coldest day I can work in my shop in a short sleeved shirt. The unknown, and what I am trying to ESTIMATE, is the CHANGE I can expect with a new HP water heater. Will it be 3 degrees, 10 degrees, or 30 degrees cooler? If it's 3 degrees, I'll be fine. If it's 10 degrees, I can adapt somehow. If it's 30 degrees, this is a bad idea. I can't be the first person in a northern climate who's asked this question before spending a lot of money on a new HP water heater.
• Member Posts: 2,261
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You're missing a lot in those calcs.
First you have to estimate how much hot water you use per day.
Second, figure out how many Btu's that takes.
Next, take the mean temperature of basement. Use that measurement as the surface temp of your walls and floor.
Then figure out the heat capacity of the walls and floor dropping the ambient temp in 1 degree increments. Once you have reached a heat capacity that is equal to the heat being absorbed by the water heater, you will have the new mean ambient temperature of your basement.

May not be exact but it will be very close.
• Member Posts: 11
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Then figure out the heat capacity of the walls and floor ...

Thanks @Harvey Ramer . That makes a lot of sense. To do that calculation, I need to assume a thickness for the walls and floor. I know the thickness of the walls. The slab for the floor was poured in 1974, long before I owned the house. Any suggestion of a thickness to use?

• Member Posts: 22,467
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plus the incoming water and setpoint temperature, sometimes called the “lift” in heat pump jargon
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 4,949
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pecmsg said:

That's a question that cant be answered with any certainty. There's just too many variables
Infiltration
Water Pipes
Ceiling structure & Insulation
Frost level
Wind

Thanks, @pecmsg. All of those factors affect the winter temperature in my basement, which I already know because I've lived here for 32 years. It's comfortable in the winter. On the coldest day I can work in my shop in a short sleeved shirt. The unknown, and what I am trying to ESTIMATE, is the CHANGE I can expect with a new HP water heater. Will it be 3 degrees, 10 degrees, or 30 degrees cooler? If it's 3 degrees, I'll be fine. If it's 10 degrees, I can adapt somehow. If it's 30 degrees, this is a bad idea. I can't be the first person in a northern climate who's asked this question before spending a lot of money on a new HP water heater.
Depending...…………..they could affect summer operation as well!
• Member Posts: 1,194
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You need ot look at it as a small 1/2 ton AC unit running indoors, but only to make hot water. Then subtract the latent capacity (25-30% in a typical damp basement). It’s in the winter that you’ll notice it.

Also, you can ignore standby losses as that returns the heat to the basement. In summer and fall, the incoming water is warmer, so there less capacity required.

On longer draws like multiple showers in a row or while doing laundry, it will likely use the backup heat. The recovery rate is pretty pathetic when all you have is about 2000-2500W of heating from the compressor.