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We are planning to buy an AC unit based on inverter technology for heating our living room. I am not an expert as far as HVAC or heating technologies are concerned, however based on my understanding the outside unit is used to take air from outdoors, compress it and then use the resulting heat to heat the indoors, obviously with some magic (not really, but I don't know the details) happening in between.

We have an underground (UG) floor directly beneath the living room. The UG has windows, and we use it as a garage as well. I was thinking whether it would be a good idea to install the outdoor unit of this split system in the UG/garage instead of outdoors.

Based on my limited expertise on the field, the benefits of doing this are:

  1. It will be better protected than outside
  2. The air in the UG floor/garage might be warmer than the the one outdoors, especially when it will be snowing, therefore more heat for the outdoor unit to transform inside, and less need for electricity to make up for the difference.

Open questions (downsides):

  1. The air circulation in the UG/garage is not as good as outdoors (obviously), how does this impact the heating and air quality that will blow inside?
  2. How will the air that will flow out of the outdoor unit impact the air in the UG/garage.

Is it safe and sane to install the outdoor unit of an inverter AC indoors? Are you aware of any such installation?

To avoid any ambiguities about what this technology is, here I provide links from the manufacturer's website:

https://www.daikin.com.au/articles/category-1/air-conditioner-heating-and-cooling-guide

https://www.daikinindia.com/inverter-technology-0

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  • Is this AC a heat pump AC capable of both heating and cooling, and you're primarily planning on using it for heating? (In US English, AC typically implies cooling). Because during the cooling season, the outdoor unit actually exhausts heat, and it won't work well in an already-hot room. – Nate S. Sep 30 '20 at 22:52
  • Why don't you get a made-for-purpose heat pump that can switch modes without juggling? – ThreePhaseEel Oct 1 '20 at 1:13
  • @NateS. I amended the original post with links that describe the used technology. – Kristof Pal Oct 1 '20 at 10:22
  • @ThreePhaseEel I think that's exactly what this technology does (see the links I added to the OP). – Kristof Pal Oct 1 '20 at 10:23
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    While installing the outdoor section of the heat pump indoors may prolong its life somewhat by protecting it from the elements, remember that it's designed to live outdoors and the appropriate protections and cautions for it to live outside for 20-30 years are built right into the unit. The condenser for our AC unit has lived in our backyard for 25 years. It's time for it to be replaced, and when we do, it will be with a much more efficient unit, so there's no real need/desire for it to last much longer. – FreeMan Oct 1 '20 at 16:09
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Think of it this way. You're transferring heat from the garage air, or from the outdoor air, to the upstairs living space above the garage.

The garage may be warmer than the outdoors when you start the system, but there's far less available heat, and you'll quickly transfer all available upstairs. Soon the garage will be colder than the outdoors; you're basically making your garage into a walk in cooler. Yes, once it's cooler than the outdoors, heat will seep into the garage through the walls and windows and floor, but too slowly.

The practically infinite air outside, although less dense with heat than the air in the garage, contains far more energy. That's ultimately the source of the heat.

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  • Additionally, as the garage is cooled, the ceiling above it will be cooled. That will make the floor of the room above feel cold, and, if there are any pipes in that floor, it will risk them freezing, bursting, and flooding the garage. – FreeMan Oct 1 '20 at 16:10
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Heat pumps "move" heat, they don't generate it like an resistive electric furnace or a gas or oil furnace, etc. So in the winter, the HP will be moving heat from the UG/garage to the house and it could get very cold down there. If it got colder than the outside air, it would be less efficient than if the outdoor unit was outside. Lots of dependencies based on the size of the space, the size of the house, quality of the insulation, etc.

BTW, Heat pumps don't compress air to move the heat. Through the magic of evaporation and condensation of refrigerant, and compressors/valves to make that happen, lots of heat can be moved. As a painful, real world example, if you've ever been burned by steam as it condenses (changes state from a gas to a liquid), it transfers a lot of energy (heat) because a substance existing in a liquid form takes a lot less energy than as a gas. When the refrigerant condenses it releases that energy, when it evaporates, it absorbs energy. This happens in fan coils in the air handler and outdoor unit.

Bottom line, I wouldn't try what you are suggesting. You'd need a lot of outdoor airflow into that space to make it work, risky approach.

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    I agree with George it would not be safe or unsafe but the unit would not work well with a limited amount of air volume. The inverters are well sealed and made to work outside so installing it in an enclosed space would limit its effectiveness. – Ed Beal Oct 1 '20 at 6:59
  • @EdBeal interesting... on the contrary I was assuming that having the outdoor unit in an enclosed space would increase its effectiveness because the air would be slightly warmer than outdoors, therefore more heat to "extract" from there. Yet, the main question with the enclosed space is the airflow, and how crucial that is to the operation of this type of technology – Kristof Pal Oct 1 '20 at 10:27
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    You would be actively cooling that space while trying to heat the house. So after a little while of heating the house, it might well become cooler than the outdoor air. The 'not-magic' explanation of heat pumps makes this a bit clearer - heat is moved, so the side providing the heat blows air cooler than it takes in, out. – Ecnerwal Oct 1 '20 at 13:11
  • Your contrary idea is wrong on the enclosed space if you are trying to heat a 1000 sf home a garage is less than 1/2 to 1/3 the space the heat pump will reduce the heat in the garage faster than the house can be heated and then will not be able to provide heat unless there are emergency heating elements very inefficient. To use ambient heat a geothermal heat exchanger is required these are an order on magnitude more efficient but also can double the cost of a install depending on the type and local conditions. Notice the big fan and how much area the coil covers in the unit. It’s all air flow. – Ed Beal Oct 1 '20 at 13:42

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