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I hope this is the right place - I'm looking at buying my first home in a climate where a heat pump would make a significant difference on our energy requirements for heating and cooling. I've done some research and I'm really excited about the idea of having a heat pump rather than rely on a furnace.

In looking at homes, however, some have identified as their heat or A/C source the home is equipped with an Air Exchange, rather than a heat pump. I've looked around and can find documentation suggesting their differences but ultimately they seem the same. Is anyone here familiar with heat pump and air exchange systems to identify the differences, or confirm they are the same? Thanks in advance!

Ryan
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2 Answers2

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Yes, the terms can refer to different things. There are two types of heat pumps (and A/C units in general). There are "air source" and "ground source." Ground source is often known as Geothermal and is much more efficient and sustainable.

Air Exchange could refer to air source A/C (or heat pump) but that is not very likely. Air Exchange is much more likely to refer to any system which pulls in outdoor air and exhausts indoor air to keep the air inside fresher. You hope that if a house has an "air exchange system" in place, it utilizes an HRV (or Heat Recovery Ventilator) because they are much more efficient. So much like a humidifier, an "Air Exchange" system typically has nothing to do with heating or cooling the house, but rather is an optional add-on to keep the air fresher.

A note on sustainability

Due to the comments below, I realized that I need to add a note here first. I have to admit that I don't really know much about grids and alternate fuel sources outside of Continental USA. When I talk about burning gas, I strictly am talking about propane and natural gas. I now understand that these resources are not available everywhere and other fuels such as oil are dirtier to burn. I also understand that some grids are much "Greener" than other grids. The following paragraph is just my experience and should just give some things to think about when analyzing what will be best for your location.

Some people have installed heat pumps and found adverse effects on their energy consumption and sustainability. It all depends on how warm it is, the cost of electricity (vs gas), and where the electricity is coming from. If you install enough solar to compensate for the extra electricity usage, then a heat pump is hands down the best thing to have (especially a ground source). Notice that air source heat pumps don't do well in cold climates (where you need it the most). If you are using electricity from the grid then it is helpful to know that most grids around here are much dirtier than using a high efficient furnace (look for 95% efficient or 90% at the lowest). So, (in my region) if you are considering the environment, you should weigh these factors to determine if it's better to use a high efficient furnace or a heat pump.

Maxfield Solar
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  • Note that gas isn't sustainable in the long term as it's a fossil fuel (unless it's biogas and that's very rare), so if you're considering sustainability you need to decide whether long-term sustainability is more important to you than short term, or affordability or safety etc. Where gas wins is compared to other fossil fuels - it's often more efficient and less toxic than coal or oil, for example. – Móż Mar 04 '16 at 08:50
  • @Mσᶎ That is true but in most areas, the utility grid is currently less sustainable than either natural gas or propane. The grid gets a huge percentage of it's power from fossil fuels but it does so quite inefficiently. It is much less efficient to first turn fuel into electricity and then convert that electricity into heat, than to go straight from fuel to heat in a high efficient furnace. – Maxfield Solar Mar 04 '16 at 15:15
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    Depends where your grid is, and whether you can choose sustainable options. We buy 100% wind power, for example, and in Aotearoa you get 60% hydro by default (on average). – Móż Mar 05 '16 at 06:17
  • Re your final note: It's rarely going to be better to use an electric heat pump than burning something on site. However, remember that not everywhere in the world has mains gas! (where I live there is no gas and oil by tanker - the other non-electric option - ends up costing about the same as electric heating) – Flyto Mar 08 '16 at 16:25
  • @Mσᶎ Thank-you for pointing that out. I have updated my answer because I hadn't thought of those of you who might be lucky enough to be on a sustainable grid. – Maxfield Solar Mar 10 '16 at 17:25
  • @SimonW Thanks for your comment. Around here, you can get a propane delivery no matter how far you live from a main. I didn't think about how many people don't have this convenience. I have updated my answer accordingly. – Maxfield Solar Mar 10 '16 at 17:28
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    also note that a heat pump can have efficiency over 100% since the heat is free from outside and not part of the calculation. Typically the energy spent to compress and expand the fluid is much less than the heat gained or lost by the house, resulting in >100% efficiency. – Kate Gregory Mar 10 '16 at 19:57
  • @KateGregory That is true if the air outside is relatively warm. It is all about the temperature differential. A heat pump will have one efficiency if the air inside is 10° warmer inside than outside. At a 20° differential, the efficiency is much lower. Here in the USA, most people want to keep it above 70°F so if the temperature is 30°F that is a 40°F differential. When you differentiel increases, not only does your need (demand) rise, but the efficiency drops substantially. This problem mostly disappears with ground source systems because the ground temperature is more constant. – Maxfield Solar Mar 10 '16 at 20:16
  • @KateGregory Then you need to calculate the efficiency at which that electricity was produced (unless it is supplied solely from renewables). At the end of the day, there are a lot of people that do benefit from heat pumps and it is their best option but it's just good to look at these factors first because they're not always better. – Maxfield Solar Mar 10 '16 at 20:19
  • @MaxfieldSolar i know about the cold thing. We had a heat pump, it was strictly spring and fall (I live in Ontario) and in the end we got rid of it. We use an electric furnace and the heat pump was only a supplement anyway. Just wanted to point out that a 95% efficient furnace is not the upper limit. – Kate Gregory Mar 10 '16 at 20:37
  • On that note, the difference can go both ways - here in Australia our loads are mostly air conditioning, and while it's very rare to see 30K between inside and outside, 20K is common (20K is about 32F, IIRC), we get 40 degree days in many parts of the country (and the north gets seasons with those temperatures). Same problem with efficiency, but there's not a lot of choice. Ground sourcing is rare but becoming more common, even though with a lower water table it is harder to do. Note that the question is not "in the continental USA", it's much, much more general. – Móż Mar 11 '16 at 01:34
  • @Mσᶎ Thank-you. Yes, I realize that the realm of my experience (a small portion of the USA) is only a small percentage. The clause containing "in the continental USA" is a weak attempt to humble myself and say "Hey, I don't really know about where you are but here are a few factors to think about and see if they apply to your location." If you tell me something specific about your area, that helps me see the issue from a different angle and while it may or may not apply here, I like broadening my knowledge database. I like an opportunity to think outside my little box and see if it applies. – Maxfield Solar Mar 11 '16 at 03:26
  • When you mention it depends on "the cost of electricity (vs gas)", I'm skeptical of what you're even talking about, as the cost has not much relation to the sustainability. The glaring thing you get wrong in this answer is that despite the great 95% efficiency of a NG furnace, that's just not very good. Heat pumps (even air source) are over 300% efficient, which means they compensate for even pretty bad electrical generation. We'd be better off burning the gas in a power plant and using the electricity for heat pumps even with all the losses. – Vectorjohn Mar 14 '19 at 17:02
  • Note that air source can refer to air-to-air or to air-to-water heat pumps, which are very different in practice. – gerrit Jul 17 '23 at 14:43
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A heat pump heats or cool your house by exploiting a cycle. For example to heat, it expands a gas to make that gas cold, then sends the gas through pipes outside to be warmed by the air or ground (the air may be much colder than your inside air, but the expansion trick has cooled the gas to colder than outside.) When it comes back in, warmer than it left, the heat pump compresses it, making it warmer than the air in your house, and then there is a warming effect on your house from this hotter fluid. It won't work in an arbitrarily cold environment. You can also reverse the process to cool your house in the summer even though the air outside is hotter than in.

An air exchanger is installed for an entirely different reason. Houses that use less energy to heat or cool get to that condition through many changes - good insulation, passive heating from sunshine in the winter, avoiding passive heating in the summer, thermal mass to help even out night and day temperature swings, and -- relevant to this question -- minimizing air leaks. This last one is pretty much the only one with a downside: it can result in the air feeling stale or not smelling great. So when a house is not turning over the air through various leaks and drafts, an air exchanger sends out house air and draws in outside air, in a way that deliberately causes them to exchange energy - the warm house air heats the cool incoming air on its way in - so that you get fresh air without losing as much heat as you would if your fresh air found its way in through leaks and gaps.

They are completely independent of each other - a house could have either, both, or neither.

Kate Gregory
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  • You've described air-to-air heat pumps, but air-to-water heat pumps store the heat in a water tank. This means you can use solar PV during the relatively warm daytime hours to run the heat pump, then use the hot water with radiators to heat your home during the night. – gerrit Jul 17 '23 at 14:45