I work in a bakery. It gets warm in the summer, but not warm enough for bread to rise without extra heat. We have a 'proofer room' where the bread sits while the dough rises. The room is heated by two baseboard heaters, and another space heater. It's approximately 10 feet by 10 feet. Also, we run a humidifier in the room.

  1. I know that resistive heat is not the most economical way to heat a room, especially as warm as we need it. The electric bill agrees with me.
  2. We'd like the proofer room warmer, but the rest of the bakery cooler, if possible.
  3. Running a space heater in the summer feels foolish.

It seems like a perfect solution to place a through-the-wall type air conditioner (not the window type) in the wall between the rest of the bakery and the proofer room, to push the heat (and humidity) from the bakery into the proofer room. If I understand correctly, this would heat the room more efficiently than a space heater, with the added benefit of cooling the rest of the bakery at the same time.

One problem that I've already considered is that the temperature controls on the AC would be on the "cool" side of the AC. To prevent the room from overheating, I could plug the AC into a thermostat outlet outside the proofer, with a temperature probe inside the proofer. The outlet would shut the AC off once the proofer got to the desired temperature. I'd have to get an AC with a mechanical on/off switch that would run when the outlet turned it on, without having to push buttons.

I can't find any examples of someone doing this. Is this a good idea, or am I missing something?

We would hire the work out to someone else to do, so don't worry about my qualifications as an electrician etc.

  • 2
    Interesting concept but it seems that the AC would be cycling on and off because of cooling and it wouldn't be on long enough to heat the proofer room.
    – JACK
    Mar 15, 2020 at 0:18
  • 2
    "it wouldn't be on long enough to heat" That depends on how hot it is outside and what the thermostat is set to inside. If you've ever put your hand near the heat dumping part of an air conditioner...it's quite warm. There is no sense in wasting that heat if it can be used....as supplemental heat...not the main heat. Mar 15, 2020 at 4:32
  • I suggest a two hose portable, smaller holes in the wall. I have one and once in awhile I vent the cool out and the heat in and it puts out a lot of heat. I would say that one of the main drawbacks to your thought is temperature control in your rising room, can it get to warm?
    – Jon
    Mar 15, 2020 at 15:24
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    I suggest that you should get a better/more efficient heating and humidity control system for your proofing room, and a better/more efficient cooling system for everywhere else. Trying to hack something like this is IMO going to cost you more in the end because A) it's going to introduce dust/contamination into your proofing room, B) it's not going to work well for either purpose and C) eventually you're going to do the "separate system" installations anyways. Given that your business depends on the proofing room functioning properly I suggest that's your biggest need, so do that first. Mar 15, 2020 at 19:22
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    An ordinary air conditioner collects a lot of gunk on the "hot side." I would be very worried about contamination. Also what are you going to do with the water condensed out on the cold side. Engineering a special system would cost much more than it was worth.
    – MaxW
    Mar 15, 2020 at 19:43

12 Answers 12


I'm not an expert to tell you whether this is actually a good idea, but an AC unit used for heating is called a heat pump. There are two differences between "a heat pump" and "an air conditioner":

  1. Heat pumps contain a reversing valve which reverses the operation so that what would usually be the cold side gets hot and the hot side cools instead.
  2. The thermostat will be suitable for heating (turns on when too cold) as well as cooling (turns on when too warm), and also control the reversing valve to switch modes as needed.

In your case, you only care about the second feature. Thus, buying a heat pump to be installed through your wall is one possible solution which gives you a thermostat that works as desired without needing external control.

However, any prebuilt unit will also be designed to put the compressor noise on the "outside", which would be your bakery and might be less comfortable than making the proofer room noisy.

You could likely get a more pleasant result (noise in the proofer room instead) by getting a system designed and installed for the exact application by a HVAC technician; it would probably be slightly more expensive, but it wouldn't be particularly harder to install and maintain than a more usual cooling installation, I expect.

  • 3
    This is an excellent answer. If the noise of the compressor in a heat pump is indeed undesirable, and if you are successful in finding an A/C (solving the noise problem) with manual switches, then it will probably also have a simple mechanical thermostat built in. If that's the case, you could easily place an off-the-shelf thermostat in the proofer room and run its wires to the original mechanical thermostat. Dial the original thermostat up to 100 so it never kicks on on its own, and you have your solution. The A/C will be controlled by the proofer room, and the compressor will be there too. Mar 15, 2020 at 7:48

Yes, this would work, but I caution you to consider that the hot side of the A/C could produce airborne water droplets full of bacteria.

The typical through-window or through-wall air conditioner condenses water on the cool side and channels this water to a condensate tray on the hot side. The condenser fan typically has fan blades joined by an outer ring called a slinger ring. As the condenser fan spins, the slinger ring picks up condensed water from the condensate tray and slings it onto the heat exchanger, which assists in cooling the hot coils.

Unfortunately, the slung water does not completely evaporate, so the hot air coming out the back of the A/C is full of microscopic water droplets that will also include whatever bacteria happen to be growing in the warm, dusty, dirty water tray. Yecch!

Large commercial A/C units are known to spread Legionnaire’s Disease this way.

You might avoid contamination of your dough in the proofing room by opening the air conditioner’s outer case and modifying the internal condensate plumbing. Route the condensate directly to a sanitary drain, preventing any accumulation of water on the hot side.

It’s also likely that baking the bread would kill any airborne bacteria that stick to the dough, but who wants to take that chance with your customers’ health?

  • 1
    Not just bacteria, but potentially mold as well. The baking process might kill it, but that may not be sufficient for someone with a severe mold allergy who eats your bread.
    – bta
    Mar 16, 2020 at 21:56

This would work fine, go for it! The efficiency will come from the differences in temperatures between the heating and cooling elements and the room temperatures, the greater the difference the greater the efficiently (or effectiveness). An HVAC (Heating Ventilation, Air Conditioning) person or store will be able to help you with the calculations, even if they don't do the installation.


It sounds like a good plan. but you're right that the controls will be regulating the heat in the wrong room.

If you can find a unit that takes an external (to the unit) dumb thermostat, a thermostat could be positioned in the proofer room and used to command heat from the AC.

Else you're going to need to have the air conditioner modified to meet your needs. I would expect that a competent HVAC engineer can set something up for you, possibly based around an off-the-shelf air conditioner.


A split A/C would avoid big openings in the wall (only a small hole for a pipe/tube is needed), is not expensive and would allow to freely position the elements. And can be extended by a 2nd or 3rd unit if needed resp. for redundancy.

  • Check with the installer or manufacturer first, though. We have a split system for two rooms in our house, and it cannot run in different modes like OP wants. I.e. they have to both cool or both heat, or you can run one unit and have the other off. Mar 16, 2020 at 21:54
  • @GalacticCowboy Since the proofer room is assumed to be a room without people, my answer was meant to place the noisy compressor unit(s) in that room, resulting in a high COP.
    – xeeka
    Mar 17, 2020 at 4:20
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    @GalacticCowboy Then what is your definition of a "split A/C", because I've always understood it to mean any system where there is an outside box and inside box that are only connected by refrigerant lines. Small ones are called mini-splits, and most typical whole home HVAC systems are also "split". A window unit is an example of something that is not split - its all in one box.
    – JPhi1618
    Mar 17, 2020 at 15:31
  • +1, I recently came across a Technology Connections video where he was talking about mini-split systems exactly like this: a bunch of heads are connected to a single outdoor unit, and the individual heads can operate in any mode at any time. In OP's case, the system would spend most time pumping heat from the oven room into the dough room, with the outdoor unit making up any difference required. They do exist, apparently!
    – maples
    Jul 8, 2022 at 10:02

Unconventional Alternative

I see you have three problems, which you reasonably note may be related:

  1. Too much heat on the oven side
  2. Too little heat on the proofing side
  3. Too little humidity on the proofing side, due to heating the air

While a heat pump will certainly solve your problem, I claim that it might actually be overkill. You see, a heat pump is necessary when you wish to pump heat from a cold source to a warm sink (which is the opposite of the direction it will flow naturally). On the other hand, you already have a hot oven room and a cold proofing room, and really, you just want the heat to go from hot to cold. So let it. Just let the air circulate between the two rooms, until they reach equilibrium. Of course, this may not be appropriate, so let's examine the assumptions.


I'm not a professional baker, but a quick search indicates that the proofing room should probably run about 30°C/90°F, 75% relative humidity (RH). Is that correct? You would probably like the oven room to run closer to 20°C/70°F, but the ovens are putting out a bunch of heat, especially when you open them up to put stuff in or take it out. Also, hot product coming out of the oven will warm up the room.

If the oven room gets close to the desired proofing temp, then you really do just want to blow that hot air over into the proofing room. But even if the oven room doesn't get that hot, simple convection might work. After all, the oven room is probably not a uniform temperature. The air right next to the ovens is much warmer than the air on the other side of the room. The important point is that you mainly want to draw the air immediately surrounding your ovens into the proofing room, and blow the colder air from the proofer back over your ovens. If the air next to the ovens is close to the desired proofer temp, or even above (which is best), then this could work.

Evaporative Cooling

However, if you just blow air around in a circle, the ovens will still be as hot as the proofer, which only solves one of your problems. So let's introduce another tool. Instead of just driving air between the rooms through ducts using a simple fan, let's use a fan to blow air from the oven to the proofer, but an evaporative cooler to blow it from the proofer to the oven. This will do a few things.

First, the "swamp cooler" will cool air coming from the proofing room, so your oven room is not too hot. Second, it will humidify the air to 80-90% RH. You don't really want your oven room to be humid, but you do want the proofer room to be above-average (which is why you employ the humidifier). So you don't want to just blow the cool, humid proofer air into your oven room. You want to blow it onto your ovens to cool the ovens and heat the air.

Ideally, the now-warmed, extra-humid air will get sucked in by the fan and blown into your proofer room.


It's possible, even likely, that this setup will not give you the exact temperature and humidity that you are targeting. But, it is likely to move the most heat for the lowest cost, while also giving you a humidity boost, all for the cost of some minimal ducting, a fan, and an evaporative cooler (much cheaper than any AC or heat pump both to acquire and to operate). You may still need to run a heater in the proofing room, but hopefully you will use much less energy to achieve the desired temp. The same goes for the humidifier. And ideally, the oven room will cool down to a more pleasant level.

It's hard to say how to size the necessary components, but hopefully HVAC folks can perform the necessary computations for you and give you a reasonable estimate without selling you their most expensive hardware.


This isn't quite an "answer" but too long for a comment.

Very interesting and energy efficient idea, very creative!

OK, here goes my solution and I would welcome others (esp. the "big 3") to contribute and hopefully improve this suggestion. This would definitely be a "one off" approach. The bottom line parts are:

1) A thermostat (tstat) in the proofer room (potentially capable of Aux Heat).

2) A contactor (mechanical relay controlled by the tstat) wired to an outlet for the A/C unit.

3) A transformer to supply low voltage power to the tstat and contactor.

Here's how it would work.

The tstat in the proofer room would be supplied power by a low voltage transformer.
When the proofer room tstat calls for heat, the contactor would close, providing power to the thru the wall A/C unit. Then the condenser coils in the proofing room would be providing heat.

At this point we have 2 choices.
I assume your proofer room has existing thermostatic control, if so, you would want to set that existing thermostat several degrees lower than the A/C tstat to allow the more efficient A/C unit to get to the desired temps.

If you do not have existing thermostatic controls on the heaters in the proofing room (IE: you turn them on and off manually), the you could use the AUX HEAT capabilities of the new tstat to control ANOTHER contactor that would turn on the baseboard heating if the thru the wall A/C unit couldn't raise the temperature enough. Another benefit of using AUX HEAT, is if the bakery got too cold, you could simply turn off the A/C unit and when the tstat realized it wasn't get up to set point, it would automatically turn on AUX HEAT.

If you have existing thermostats in the proofing room and are reliable and in good shape, I'd keep them with a set point a few degrees lower than the A/C unit. AUX HEAT tries to be very accurate because it's an in-home comfort issue. Not knowing enough about how important exact temp controls are in the proofing room prevents me from offering better advice.

A few other comments: The "thru the wall" A/C unit would presumably have several settings (high, medium, low, etc.) If during a cool season, would the bakery provide enough heat to heat the proofing room? IE: On "high" would the A/C unit cool the bakery to much? I think your idea is basically sound, but it will take a qualified HVAC guy or electrician to wire this up according to code.

If this approach sounds good to you, I could provide diagrams, but I think you probably already get it! Good luck with a very creative idea!


I think this is a clever idea, and doable with a pretty simple hack. If you locate an ordinary hardwired thermostat in the proofer room, and tie the heating terminals on the thermostat to the cooling terminals on the air conditioner, you should get what you're looking for.

That will limit your options to units with terminals for an external thermostat.

I think a portable unit might be ideal, that way you just have to run a dryer vent through the wall - but I don't know if you'll find a portable that works with an external thermostat.

I think you're more likely to find a through-wall air conditioner that has these hookups, and you can easily find a 208V or 240V unit which will be a bit more efficient.

Worst case, you could use one of the wall-pack type heater / air conditioners used in hotels and motels, most if not all of those have terminals for an external thermostat.


The idea is good and pretty much possible.

Constructing it from off-the-shelf parts can be only a little tricky.

First, I am yet to see an AC unit capable of humidifying a room (drying is trivial and will happen by itself, but is not what you need).

The temperature controls are not necessarily on the cool side. Where I live, AC units are used more for heating and less for cooling, so all AC units on the local market have both heating and cooling function. The user controls the internal body of the AC unit, the thermostat is inside and the external body does the opposite (it blows cold when the the AC unit is used to heat the room).

It is possible that where YOU live, AC units on the local market are mostly cooling-only type. Just look harder and you will pretty much find one with both functions (I am not aware of consumer-grade AC units that are heating-only).

You will have to mount the internal part in the proofing room and the external part in the ovens room. The "external" unit will condense a lot of water and you will have to drain it somewhere. And because of the flour in the air, I am pretty sure you will have to clean the heat exchanger of the "external" part once in a while.

Be sure to check that the AC unit have the needed range of temperatures for the heating function. Most of them are up to 30 deg Celsius that may or may not be enough (I am not profoundly literate in baking, sorry).

Out of the available info I think that an 12000 BTU/h (3.5 kW heating/cooling power) AC unit will be just enough, but see the power rating of your space heaters and adjust the numbers accordingly. Inverter-type AC units are a lot better for your use case. You will probably have to use the lowest fan speed setting.

You will have to leave the humidifier alone.

You will probably save some 60-70% of the electricity used for the space heaters and you will also get some cold breeze in the ovens room.

p.s. a friend of mine that has a "server room" full of computers in his basement does something like that, except that he cools the server room and heats the rest of the house.

p.s.2 "in-wall" AC units are also called "split type" (that will help you in searching the Internet). They work best when their tubing is less than 3m (10ft) long, but it is pretty much possible to mount them even 10m (30ft) apart or even more if those who mount them are cappable of topping them with freon.


Thinking of using an AC system similar to Rimworld pc game?

It is not going to work, efficiently.

You are using R22 or R123a gas as a heating fluid?

Thouse systems operating at an ambient temperatures around 45-55 C?

And you wont be getting a thermal shut-off if you reached your desired heating temprature.

So you wont be having any cooling nor heating on the other side efficiently.

Just burning out your electricity bill.

Better off using your oven escaping heat to heat your dough resting room.


You could try two small portable units in each direction, ideally each of them half of what you need, that way you will have two thermostats controlling the temperature in each room. So one unit would provide a fraction of the heating and cooling and is uncontrolled and less than what you need and the other unit the top up and is controlled by thermostats


Yes, it would work. AC is a very simple concept. Take heat from one spot and put it somewhere else. if the somewhere else is another room, voila. this is actually what we end up doing a lot in server rooms, albeit with a way more expensiver unit "designed for it" but it is literally going to put the exact btu's removed from the cooled side into the heated side plus some bonus heat due to the electronics, compressor, and fan motor.

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