Does a Heat Pump or Air Conditioner Condenser Need to Go Outdoors?

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hvac air conditioner condensing unit in attic 1 small

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?!

hvac air conditioner condensing unit in attic 2 small

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

hvac condensing units small indoor room small

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.

 

Related Articles

How the Heck Does a Heat Pump Get Heat from Cold?!

The Magic of Cold, Part 1 - How Your Air Conditioner Works

An Easy Way to Save Money — Let Your Air Conditioner Breathe!

 

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.

Comments

Franklin Menendez

Hi Allison, what about the rise of the heat pump water heater? 
Franklin

Curt Kinder

Allison - wow - just when one thinks one has seen all possible SNAFUs... 
 
Franklin - a typical residential HPWH has just a 1/2 to 3/4 ton compressor. It'll drop the temperature of a 10x10 room by 5 degrees or so.  
 
OTOH a typical whole house central air unit is 2-5 tons, so moves a whole lot more air and heat. 
 

Allison Bailes

Franklin M.: Curt's already provided the most important part of the answer to your question. Yes, the process is the same, but the HPWH has much less capacity. Still, placement of the HPWH is an important consideration. You definitely wouldn't want it in a small closet. 
 
Curt K.: Yeah, it's amazing the kind of stuff that's out there. I'd like to believe that was a DIY install in the attic, but there's plenty of evidence that pros do stupid things, too.

Eric

"no matter how much air that unit dumps outside . . ." - I hope it is dumping heat, not air. :)

Don Gatley

Heat Pump outdoor air placement reminds me of two buildings near our nations capitol. A project architect with one of the nation's largest developers asked me to climb up on a desk and look out and down on a large group of heat pump condensing units installed at the bottom of a 25' by 35' three story well created by several buildings. It was winter and each unit had about 4" of ice build up. That architect probably also understood a good bit of bldg. sci.! 
The second installation was at a historic site in Annapolis. In this case the 12 foot deep well was in the courtyard of a historic building. The grating over the well was designed for walking so the cold air discharge had to not only work against gravity but also against a 20% open grating.

Allison Bailes

Eric: Oops! Thanks for catching that mistake. I've corrected it. 
 
Don G.: Glad you discovered this article! I was going to tell you about it when we got together this morning, but then we didn't get together.  
 
Those are two more great examples of how to do it wrong! Nobody wants the condenser in view, but when people start getting creative about the placement, bad things usually happen.

Allison Bailes

Andrew M.: Wow! I hope those basement systems weren't used in summer. 
 

Robin Boyd

I was called in on a situation where 10 condensing units for a midrise office building were centrally located in a completely open garage under the building. 
 
The problem was that in order to protect these units, they were installed in an ell section where a stairwell and elevator shaft were located. Because of the location, there was a tremendous amount of recirculation of the heat expelled from these units. while the temperature outside of the building was around 80 degrees, the ambient temperature at the location where the units were was over 110 degrees.

Allison Bailes

Robin B.: I have a feeling that there are quite a few condenser placement problems like this. It seems like it wouldn't take that much thought to figure out that it wouldn't work, though.

Robin Boyd

Allison, you really would think it wouldn't take much thought...but this particular instance was like a bad sitcom. 
 
I was brought in to determine if the equipment was at fault. After my initial determination, I was brought back to a meeting with the mechanical engineer for the project, the builder, the building owners and the HVAC contractor. I was perplexed as to why. 
 
In the meeting there were proposals to do interior inspections of all of the offices and to discuss why the systems were not cooling the offices even though I had determined that they were operating as they were designed. 
 
I finally interrupted the "progression" of what we should be doing to evaluate the situation and requested that we continue our discussion in the garage where the units were located. 
 
Once in the garage, I had to keep corraling everyone back to where the units were because they wanted desperately to escape the heat. 
 
Even after that, some still did not understand that those units were not operating on an 80 degree day, but were operating at the temperatures in which they were actually exposed to, which was over 110 degrees.  
 
It simply amazed me that so many well learned persons in the building industry could not understand the basic concept of the situation. 
 
What it really boiled down to was attempts for each of those in the meeting to be able to blame another for why the problem existed so that someone specific could be made responsible for the cost of remedying the issue. Since that part was outside of my reason for being there, I just left, shaking my head.

Curt Kinder

In the land of the blind, a one-eyed man can be king. 
 
An HPWH can be in a small room, or even a big closet, but provision must be made to adequately ventilate it. Some HPWH models are ductable. 
 
There are some who scoff at the minor loss of performance occasioned by an HPWH in a confined, poorly ventilated space, but they should bear in mind the problems that arise when any surface or wall cavity's temperature drops below the dewpoint of surrounding air.

David Butler

The best place to locate an air conditioner condenser is where it won't see much sun, especially in late afternoon (outdoors, of course!) 
 
Here's an example of the impact 10 degrees has (from expanded performance table for a mid-SEER AC):  
 
entering air: 105F 
total output: 21.8k Btu/hr 
compressor: 1.77 kW 
compressor EER: 12.3 
 
entering air: 95F 
total output: 23.2k Btu/hr 
compressor: 1.55 kW 
compressor EER: 15.0 
 
When specifying heat pumps on new homes, I look for opportunities to locate the outdoor unit on the eastern or southeastern side. This gives maximum sun exposure in the AM when daytime heating loads peak and cooling loads are lowest. Then in the late afternoon, the unit will be fully shaded when heating loads are lowest and cooling loads peak.

Amy

I understand why the condenser is located outside.  
 
But, my brain got to thinking about the science of cold air being heavier then hot air. So this got me thinking.  
 
We have Central Air Stations (singular) in one location, usually in the basement or lower level, but wouldn't it be wiser if we separated those units out so cold (summer) was blown from the top down, and (heat) bottom up? 
 
This seems like a logical concept to work with the science, but I am not finding this anywhere. 
 
I am not a professional in this area, only on a quest to figure out how to cool my cape cod. 
 
Amy

David Butler

Amy wrote: 
> wouldn't it be wiser if we separated those units out so cold (summer) was blown from the top down, and (heat) bottom up?  
 
Having separate air handlers and ducts (heating vs. cooling) would be expensive. A properly designed duct system will have sufficient terminal velocity to distribute the air irrespective of diffuser location. Also, the more efficient the envelope, the less important diffuser design becomes.  
 
In any case, there are much more important considerations that should determine the duct system layout, especially in retrofit projects.

Amy

David, 
 
The theoretical question could be based on refocusing a design system to maximize the science of cold (down) and hot (up). Consider how technology might play a part in combining units in separate locations to act as one system using the same duct system. Might that be more efficient?  
 
Kind Regards, 
Amy

David Butler

@Amy, it's fun dream up ways to make things better, but trust me, you're wasting you're personal energy on this. If I had a magic "technology" wand, the last thing I would want is to further complicate forced air distribution systems. The fact that so few mechanical contractors can design/build a basic duct system that works should be a clue.

john lurcock

I took the top 2 pics....i didnt know at the time how popular the pics would get 

Ed

Suppose I was to put an ASHP in a small building, along with a wood stove to keep the air temp from falling. Would that not allow the heat pump to continue to work efficiently all winter long?

Robert

Would putting a wood stove in a separate building with the outdoor unit of a heat pump work? Yes, at least to a certain extent, but only for heating. Is it a good idea? I really doubt it. I'd be amazed if you would not come out better simply heating water with the wood stove and using that to heat the house, or running duct work and heating the air directly. 
 
A heat pump uses say one part of energy to move three parts of energy inside. This varies and drops off as the temperature difference increases. The key part is that you are using substantial energy to move that energy, whereas with a hot water type solution you are using much less energy to move the same amount of energy. 
 
Note that you may simply be able to buy a more efficient heat pump that works in extreme cold, although in many cases you want some second form of heat for the coldest days as a supplement. Right now, I believe the most efficient heat pumps compare to the most efficient natural gas solutions cost wise, but you'd have to check details locally to know the specifics. The advantage gas has is you end up with a device that is likely to last longer, as the compressor part of it will only be active a few months of the year.

David Butler

Robert is correct. Putting the heat pump in a building and then using a woodstove to heat the air would be just plain silly. That same heat could be used to heat the house directly without having to pay to operate a compressor. Plus you'd have to manage the temperatures to keep the head pressure from getting too high. 
 
It's a common misconception that heat pumps somehow create energy. It's true that a heat pump produces several times more heat than electric resistance heat. But it does NOT multiply the heat in the air. The heat extracted from the air is roughly the same as the heat produced by the indoor coil. 
 
Besides, cured firewood (if you have to pay for it) costs more per BTU than the heat pump. Those who have their own source and don't mind cutting and daily handling to heat their homes needn't look for better alternatives.

Liz

Would wrapping my outdoor heat pump loosely with electrical heat tape be helpful when outdoor temps drop below 25 degrees or more? Even those dreaded heat strips can't get our living room above 60 when it's frigid outside. I'd only plug the heat tape in on those icy, super cold nights.

Allison Bailes

Liz: Wrapping the outdoor unit with heat tapes would not be a good idea. If you're going to pay for electricity and turn it into heat, you'd be better off getting all of that heat into your house rather than losing a lot of it to the outdoors.  
 
If your heat pump and supplemental heat source can't keep your house warm 25° F, you need to find out what the problem is. Either the heat pump isn't sized properly, the heat strips are inadequate, or the heat pump isn't functioning properly. Find a good HVAC contractor to help determine how well it's working.  
 
If the house is too leaky and poorly insulated, you can help solve the problem by taking care of those issues.

pm suds

can across this great blog during the usual web query... looking for info and or advice on what to do about replacing a propane furnace in a very cold attic that has freezing issues 
with the condensation. the attic is 3rd story, not insulated, etc. a local hvac contractor talked about a heat pump being installed in the attic to utilize the existing duct system... any thoughts? thank you!

David Butler

@suds, not sure what you mean by a propane furnace having 'freezing issues with the condensation', but in most locations, a heat pump will save you a bundle in heating costs, compared to propane (depends on local rates).  
 
The heat pump goes outside, replacing your AC condenser (if you have central air), whereas the part that goes in the attic is referred to as the air handler. 
 
You may be able to use your existing furnace as the air handler for a new heat pump. That way, the furnace can provide supplemental heat when it's too cold for the heat pump to keep up with your home's heat loss. This is referred to as a dual-fuel system. A dual fuel heat pump that relies on propane for supplemental heat costs a bit more to operate than a heat pump with straight electric supplemental heat. But if your furnace is in good condition, the difference may not be enough to justify replacement. 
 
In any case, you need to solve the 'freezing condensation' problem before replacing any equipment. I can think of one possibility -- if your indoor humidity is too high, moisture will migrate into the duct system between cycles and condense on any cold surfaces. If you set your thermostat back at night, the furnace may be off long enough for exposed ducts surfaces to cool below the dew point. If this is what you're referring to, you need to ventilate your home to reduce wintertime humidity, and improve the insulation on your ducts (especially bare spots with no insulation). Heat pumps are less likely to experience this problem since they typically run continuously as it gets colder outside.

pm suds

david, thank you for your reply to my post. very much appreciated... the dilemma with this "high efficency" Duquesne propane furnace condensation revolves around 
the h20 that the unit itself kicks out. from the box (no humidifier)and from the exhaust stack. the condensation from the unit is plumbed into pvc, and the condensation from the exhaust stack, which runs diagonally approx 4 feet, then goes vertical for about 3 feet and through the roof, is collected via a T connection in the pvc, and dribbles into plastic tubing. that exhaust stack is cold 
as the attic is not professionally conditioned. both are then plumbed into a small pump which squirts the h20 into the sewer vent stack as there is no other place to go with it. my flip-out concern is with that little pump freezing, 
potentially catching on fire???. there is a pan under the furnace which is where the pump is located, so no h20 can escape into the ceiling beneath as the pan has a pvc drain pipe exiting the attic through the soffet. I installed a vent in the main trunk line which blows the air from the furnace onto the pump and plumbing, which keeps the immediate area around the pump and underside of the furnace  
at about 45-50 degrees. In an attempt to “condition” the 
attic I have attempted to block the air entering the attic by closing off the soffit vents, and the ridgeline vent with insulation. I keep a humidity monitor up there and it basically reflects the same amount of humidity that is outside. The humidity monitors in the house located on the 2nd fl read approx 25% max (no humidifers in the house). 
 
The existing unit is on it’s last legs, and I have no idea what to replace it with… not really keen on the idea of a heat pump with a backup propane furnace (natural gas is not available) unless that system is absolutely condensation free because I can’t afford the $12K spray foam conditioning solution. The house is located on the Chesapeake Bay so winter is ruthless and the home is completely exposed to the gale force winds (the reason behind attempting to close up the attic!!) Any advice is gratefully appreciated. Thank you for your time and consideration. 

David Butler

@@pm suds, you should give serious consideration to replacing your aging furnace with a heat pump. For a rough comparison, multiply your winter electric rate (exclusive of fixed charges) by 10. That's roughly the average operating cost per therm for a mid-efficiency heat pump (15-16 SEER with ECM blower), exclusive of supplemental heat (see below).  
 
For example, if marginal rate is 12 cents, each therm produced costs roughly $1.20. A therm is 100,000 Btu's.  
 
Now take the price of propane and divide by 0.916 (converts gallons to therms), and divide again by furnace efficiency expressed as as decimal. The result is your furnace's operating cost per therm. For an apples-apples comparison, you need to add about 5 to 10 cents to account for blower energy. So if propane costs $2.50/gal and your furnace is rated at 92%, then each therm produced by your furnace costs just over $3. 
 
Unless your home is super insulated and super tight, the heat pump will require supplemental heat. Even though electric supplemental usually costs a bit more per therm than propane (multiply winter electric rate by 29.3), you'll actually spend less with electric supplemental. Here's why: Electric supplemental only picks up a fraction of the load when the heat pump requires supplemental heat, whereas with a dual-fuel system, the heat pump is switched off when the thermostat determines it can no longer provide 100% of the heat, so the furnace ends up picking up 100% of the load at that point.  
 
Most folks don't' realize that even at 0F, a heat pump can still produce about 40% of it's design capacity at a cost that beats the socks off of propane. So propane furnaces don't make much sense, except in markets with extremely high winter electric rates and cheap propane. 
 
I can't condone your ad-hoc unvented attic. Building codes require unvented attics to have air impermeable insulation in contact with the roof sheathing. The risk is condensation or mold on the underside of the sheathing. It's most likely to occur after periods of mild, wet weather followed by an overnight cold snap. Or if high humidity bathroom air finds its way into the attic through ceiling leakage paths. Keep an eye on that sheathing and hope for the best!

pm suds

david, many thanks for the assist with the dilemma. and especially for the recommendation of electric supplemental.