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# Heat Pumps

Member Posts: 3,045
I have limited knowledge of heat pumps and more and more customers are asking for them. SpacePak just had a webinar on their products and I have a few questions for the HeatingHelp community to help me better understand them.

They explained that a heat pump is just like an electric boiler which explains their moderate amp draw (21 amps, 230 volts). Can a photovoltaic system on a 1,300 square foot house reasonably provide enough power for the heating, domestic use and charging an electric car?
Often wrong, never in doubt.

## Comments

• Member Posts: 17,828
And electric boiler? No. That term is -- in my understanding, @Alan (California Radiant) Forbes , used for boilers which use direct resistance heating. And thus a lot of amps!

That said. The commonly accepted usable figure for photovoltaics is 20% efficiency (used to be lower, and even now degrades over time) which means that for every square meter of photovoltaics you can get 200 watts of power. Note that that area is measured in a plane perpendicular to the direction to the sun -- not the plane of the array.

Therefore, at 230 volts, you will get slightly less than one amp for each square meter of array -- 0.86 amps, more or less. To be conservative, though, that should be downrated for aging to about 0.65 amps. For your 21 amps, then, you would want 32 square meters of array, measured in that perpendicular plane, or about 350 square feet.

Note that that is in that perpendicular plane. If the sun is 45 degrees off that perpendicular, you would need half again as much. -- say 500 square feet, more or less.

So yes, when the sun is shining you can get your 21 amps. from an array on your 1,300 square foot house.

When the sun is shining. Which, even in sunny California, it doesn't do all the time (here in New England, we use a guesstimate of 3 hours of usable sunshine per day; California -- depending on where you are -- would be more).

So... you have two choices: stay grid connected, and use the grid power when the sun isn't shining, or store enough electricity to keep the thing running when the sun isn't shining -- which translates to a bigger array plus a battery bank to store the power in.

I don't have the available solar for California -- sorry! But, as an example of the problem with the second option in New England, even if you are a hopeless optimist and you assume that you get those three hours of sun every day (you don't), you would need a 4,000 square foot array -- plus enough battery capacity to store a bit more than 100 kilowatt-hours of electricity. Even if we assume that you never have cloud cover -- you're in the Mojave somewhere -- you'd need a 1,000 square foot array, and at least 60 KW-hours of battery capacity.

That help?
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 3,393
A heat pump is an air conditioner that can run in either direction, to heat or to cool the interior space. That's how I would explain it in one sentence.

One piece of advice I can offer is don't trust the ratings on the heat pump, they are often very conservatively high in my limited experience. You want to see the actual draw before you do much math on your solar system.
1 pipe Peerless 63-03L in Cedar Grove, NJ, coal > oil > NG
• Member Posts: 2,379
Hi @Alan (California Radiant) Forbes , I asked about your question amongst a group of folks I deal with who are into "decarbonization". Here are their responses:

Hi Larry,
A central heat pump typically uses the same amount of power as a central AC. It is just working in reverse during heating. A 1300 sq ft house in the Bay area typically requires a 2.5 ton central heat pump. Alternatively people can use the mini-split heat pumps which are typically more efficient and draw less power but are usually more costly.

The power draw varies by how sophisticated the heat pump is. The more sophisticated ones with variable speed compressors will draw less power. But the typical 15-16 SEER basic single or dual speed heat pumps will draw about 10A, 240V for the 2.5 ton unit. Yearly heating consumption is about 2500-3000 kWh. So add this on top of the basic usage and an EV and you are looking at around 10000 or 11000 kWh roughly. This is assuming that other functions like water heating, cooking and drying are still using gas. Otherwise the electricity requirement will be even higher.

1 kW or solar produces about 1500 kWh yearly here. So to produce 11000 kWh a 7 kW system will be needed, That system will require about 500 sq ft of roof area. That may be available on a1300 sq ft home but depends on house orientation and shading.

The next response:

I have a 1600 sq ft home in Oakland – now all electric, heatpump for space heating and water, as well as an EV that we drive about 8K/year. We installed a 1.5 ton Carrier heat pump w/electric resistance back up (which I have turned off) and it keeps house at 68 on all but 2-3 of our coldest days when it struggles to get above 66 degrees until sun comes up.

A 7.2KW solar system provides almost exactly the right amount on annual basis - my monthly electric bill is \$9/month to power my home and car.

Hope that helps!

Yours, Larry

• Member Posts: 17,828
Nice commentary, @Larry Weingarten -- if only to illustrate the dramatic difference between the Bay Area and what an be done there and other parts of the country...
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 15,428
Probably be best to get a load calc, heat and cool first, then select potential HP, then do them numbers for amount of PV.

Roof space or ground mount availability may be the determining factor. And \$\$.

There are HP powered net zero homes in most every climate in the US, here is a Main example.

https://www.insourcerenewables.com/sites/default/files/pv_and_mshp_case_study.pdf

Last I visited the Bay area I saw a fleet of Tesla PowerWall mini vans at the Hampton I stayed at. Folks trying to get as much RE as possible before incentives dry up.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 17,828
Good artilce, @hot_rod -- but it is important to remember one key difference, which is often lost in the enthusiasm for net zero. It is not only possible, but not that difficult, to achieve in new construction. In fact, in many climates it is possible to achieve true zero with only a little more effort (the exceptions, so far, are the very humid climates of the south east)(but how about air conditioning, you say? The Persians had it 3,000 years ago. Fascinating technology). It is not, with rare exceptions, possible to achieve -- unless one has a lot of land -- as a retrofit on an existing structure.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 389
The killer for heat pumps is defrost power consumption. I don’t believe its factored into the ratings. If it is, its not enough. Below about 15-20F, the NET output and COP with defrost factored in and there little gain over strip heat considering the extra wear and tear.
• Member Posts: 3,393
> @motoguy128 said:
> The killer for heat pumps is defrost power consumption. I don’t believe its factored into the ratings. If it is, its not enough. Below about 15-20F, the NET output and COP with defrost factored in and there little gain over strip heat considering the extra wear and tear.

That’s why I love ground source heat pumps. No such problem
1 pipe Peerless 63-03L in Cedar Grove, NJ, coal > oil > NG
• Member Posts: 3,045
@Jamie Hall , @Larry Weingarten, @ethicalpaul, @hot_rod, @motoguy128

Thanks for all your valuable responses! And thanks, Larry for the great information you emailed me about California's incentive program.

What about maintenance costs? Do they brake down often? US vs. Chinese?
Often wrong, never in doubt.
• Member Posts: 389
Perhaps a bit anecdotal, but from what I’ve seen, there are a lot of 20+ year old AC units out there, and I don’t see a lot of 20+ Heat pumps.

You add 2 more expensive points of failure, the reversing valve and defrost board. The compressor will run I’d estimate about 3x longer than a AC system annually and 4-5x more run time in high lift Conditions and/or high head pressure conditions.

That being said, reliability is probably still better than a furnace and electric furnace heat kits last forever with just the occasional cheap sequencer replacement.

More rodent related failures on heat pumps since the control boards stay warm in winter and the Sump heaters probably attract them to the units.

It;s funny, a gas combustion appliance should be “more scary” to work on, but I’m always a lot more nervous working on electric heat kits. Maybe it’s because gas is contained in a pipe. But electricity is not contained in an exposed circuit in a heat kit.
• Member Posts: 17,828
Reliability is at least partly related to cost (there's a bling factor to be added for some brands...). It's also related to weight -- and, sadly, efficiency. One might think that the more efficient units might be more reliable, but unless they are engineered and built to very high standards, that's not so, since to achieve the efficiency they need to work much closer to the limits of the machinery. Cost enters in in choice of materials, choice of bearings, compressor design and materials, and the electronics -- a lot of money can be saved on units with less expensive materials (particularly bearings and electronics), but at the expense of reliability.

US vs. Chinese? Much as I might like to think so, I'm not sure that there really is that much difference. One does have to keep in mind, there, that the country of manufacture and the country in which the company says it is located are often not the same.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 4,359
To add to @ethicalpaul , another plus for geo is the condenser is not in the elements.
When my A/C goes I'll put in a high seer 2 stage heat pump. But I'll configure it to switch to my hot water BB at 39° OAT.
• Member Posts: 15,428

> @motoguy128 said:

> The killer for heat pumps is defrost power consumption. I don’t believe its factored into the ratings. If it is, its not enough. Below about 15-20F, the NET output and COP with defrost factored in and there little gain over strip heat considering the extra wear and tear.

That’s why I love ground source heat pumps. No such problem

GEO units do have a bit of efficiency over the A2WHP, but the gap is closing. The biggest issue between ground source and air is the \$\$ of the loop field. At some point that cost may offset the operating cost over 20 years.
Plus in a city location a loop field or wells may not be possible. Unless you directional more under several neighbors

Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream