Does a Heat Pump or Air Conditioner Condenser Need to Go Outdoors?
Occasionally I get asked if it's OK to put the condensing unit for an air conditioner or heat pump in a garage or other room that's a buffer space. The thinking is that since the temperature may not be as hot in summer or as cold in winter, the system will operate more efficiently. I just saw yesterday that this same question came up in a column in Home Power magazine, so I thought this would be a good time to cover this issue (once and for all?) here.
The answer is no. In fact, the answer is an emphatic NO. Here's why:
The way an 'air source' heat pump or air conditioner works is that it exchanges heat with the air surrounding the condenser. In summer, it dumps heat into that air. In winter, it absorbs heat from that air. When the condenser sits outdoors, it's connected to a mass of air that's practically infinite. In other words, no matter how much heat that unit dumps outside, it's not going to change the outdoor temperature.
If you put the condenser in a garage, attic (as shown above), or other space, it's now connected to a finite mass of air. As it dumps heat into that air in summer, the temperature in the room will rise. As it pulls heat from it in winter, the temperature will drop. The smaller that room, the more temperature change you'll get.
What do you think happens to the efficiency and capacity of an air conditioner when it has to dump its heat into hotter air? It drops. What happens when the air gets too hot? The condenser may not be able to do its job - condensing the refrigerant so that it all becomes a liquid again. The refrigerant goes to the evaporator coil hotter and wetter and at higher pressure. That's a recipe for failure.
Condensing unit in an attic?!
I have no idea why anyone would put an air conditioner in an attic, as shown above, but the second photo of that unit shows another problem. That system not only is working with a smaller, hotter volume of air, but it's sucking blown insulation up against the coil, reducing the air flow. I guess they wanted to make sure that system failed as quickly as possible.
Even in a cold climate where you don't use the system for cooling, you can't do this. Not only is there not enough air, but if the temperature is higher, it's at least partially due to heat loss from the house. A better building enclosure is a much more practical way to keep the heat in your home in winter.
11 condensers in one small room
The photo above is from a Facebook page called HVAC hacks and other screw ups. They show lots of good photos of HVAC gone wrong. If you think I post some ugly stuff here, take a look at their page.
They posted the photo above yesterday. The caption said that in addition to the 6 condensers you see here, another 5 were in the room, too. That's 11 condensers in a room so small that the widest angle photo he could get shows only about a 10 foot section.
Wow! Not only is there a small volume of air, but you have 11 condensers fighting over the little that's there. Yeah, they've got louvers connecting it to more air (a parking deck? outdoors?), but that's not sufficient.
'Bright ideas' usually don't work
The upshot of all this is that if the idea ever occurs to you to put a condenser in a place other than the outdoors, don't do it. Put it outside and make sure it has plenty of space around it for good air flow.
Photo credits: I got the top two photos of the condenser in an attic from my friend Don Gatley, author of the great book Understanding Psychrometrics. The last one, as mentioned in the text, is from HVAC hacks and other screw ups.