Air-source heat pump question

To cut to the chase, I’m looking at the possibility of installing an air-source heat pump to heat my house. Normally around here (Boston area, closer to the ocean than not), this isn’t much done, I suppose because the outside air temperature is often too cold in winter for these things to operate with the efficiency you’d want.

But here’s the thing: I have a garage, detached from my house, and set into the side of a hill, where I notice that the temperature never much drops below the mid-40s, I’d guess. What I’m wondering is whether it’s feasible to put the outdoor unit of the system in the garage, where it can take advantage of that relatively high temperature.

Here are some specifics of the situation The garage is about eight yards by eight yards by eight feet high. It has two bays where we park our cars, thus two garage doors, neither of them especially airtight. We go in and out of the garage let’s say about six times a day, so on those six occasions, one door will be open for a few minutes.

The part of the house we need to heat is two floors of five rooms each, plus a bathroom on each floor. These rooms are generously sized, with ten-foot ceilings on the first floor and eight-foot ceilings on the second. It’s an old house, so much of it is uninsulated, and parts of it are drafty. We currently have a gas-fired forced hot-air system (no central AC).

So I suppose the core of the question is how much air is moved by the heat exchanger, and whether the air in the garage would be so quickly depleted that the temperature there would soon drop to outside levels.

Anyone have any relevant expertise here?

Not an expert, but do have a air to air heat pump.

I’m pretty certain that the 'warm" air in the garage would soon be at or below the outside air temperature.

A ground source heat pump would be a better option if you have the area required. Around here, Missouri, they commonly cut a trench and bury the pipe in the trench. An alternative is to drop the pipe in a drilled well. My neighbor has his pipe submerged in his pond. That’s the best of the three as far as I’m concerned.

Heat pumps chill the “source” air (they essentially steal heat from the outside air, which cools it further). This will create a problem for your system as you envision it, as your garage is going to get pretty cold once you turn on the heat pump. The garage air might even get colder than the outside air, at which point efficiency is going to be even worse than if you had just used the outside air as your “source” to begin with.

Unfortunately, those are not really options for me, as I live in a densely built semi-urban area with a small plot of land, and a tricky hillside situation. My semi-warm garage is about all I’ve got!

Couldn’t you vent the heat-exhausted air outside the garage? I know it’s a little nonstandard, but maybe you could rig something up that would go through the roof of the garage. I just don’t know the volumes of air we’re talking about here. If it’s a large volume, then air from the outside would rush in and cool the garage down, which would defeat the whole purpose. On the other hand, if the volume is relatively small, the drawn-in air would be warmed by the earth-held heat of the garage.

Typical heat pumps have a COP of 3-4,
which means that for every 3-4 units of energy (heat) out, it only requires 1 unit of energy (electricity) to move it. The remaining 2-3 were “stolen” from the outside. So, in order to heat up your house by 10 degrees, roughly the same volume of outside (or, in your case, garage) air has to be chilled by 10 degrees.
Then more outside air must be continually chilled to account for the amount of heat leaving your house (through insulation, poorly sealed windows, doors, attic, etc.)

In short, this will not work.

Your idea, while logical, is basically the same idea that someone had a while back about using the cool air in the basement to cool the upstairs. Essentially you’re thinking about the garage as a heat exchanger of sorts, with the warm[er] earth it’s setting in as the heat “source.” The garage is warmer [in part] because of it’s orientation and so I [you] want to “capture” the heat and send into the house.

To say that this is inefficient doesn’t begin to describe the practical reality. At any given point your house has a given “balance point”—that is the BTUs you need to “replace”----BTUs that were lost to the environment from the house.

Whatever heat that is in the garage----heat that is greater than the ambient condition outside----would be captured rapidly by the heat pump, and since a heat pump is essentially an air conditioner that moves heat, you would very quickly bring the garage to the same temperature as outside. The reality is, the BTU needs of the house-----needs the heat pump must respond to via the thermostat-----are much greater than whatever nominal heat exists in the garage.

Your heating system—like any other—is replacing BTUs lost to the environment. This “heat transfer” can be slowed down by improving the envelope—windows, doors, insulation etc.

If you’re considering adding A/C anyway, I would strongly consider a heat pump—even in Boston. Sure, it won’t perform as well as if it was situated in Atlanta, but it will still produce BTUs cheaper than Natural Gas—even in cold temperatures.

The COP YamatoTwinkie is a moving target. The colder is outside, the lower COP. 3-4 may be common north of 30F, but in a Boston coldsnap your COP may be 2—or even a little lower. Basically, the colder it is outside the less operationally efficient it becomes.

Operationally is an important word because it means that the heat pump may not be able to extract enough BTUs from the ambient to replace the BTUs lost to the [same] ambient. In other words, it can’t keep up. That’s why you have a “back up” source of heat.

Compared to natural gas, however, the heat pump is still **economically efficient [vis a vis natural gas] Sure the heat pump isn’t as efficient at 10F (where your COP might be 2) than it was at 35F (where your COP might be 3.5), but even at 10F it’s still producing BTUs cheaper than natural gas.

So, understand that in Boston a heat pump may be operationally inefficient in the coldest temperatures, but compared to natural gas******* it is** not economically inefficient. **
(*******Ultimately, the cost NG and Electricity will determine if this statement is true. It is true in most places in the US. However, there are occasional pricing anomalies that may make one form of energy delievery cheaper/more expensive than another. I can supply you with the method of figuring that out in your locale if you wish)

You can go down with a ground source heat pump, too. That’s slightly more expensive, but at least you don’t need a lot of land.

Getting the drilling equipment onto my property is what’s problematic. And I think you’d be drilling through rock for a good part of the way down. I’m sure it’s not impossible, just expensive.

Anyway, I’m more or less abandoning the air-source-heat-pump-in-garage idea, thanks to the good advice I’ve received here. Though I could always convert my garage into a meat locker…

I think abandoning the heat pump in the garage is a smart move, however, depending the cost of the heat pump (vis a vis the same installation of a standard A/C system) I wouldn’t necessarily abandon the heat pump idea altogether, Boston or not.