Window A/C unit thermostat: is there a way to translate to °F?

I have a window A/C unit that I’ve been using to cool down my bedroom on these muggy summer days. The thing has a thermostat, with 1 being the highest setting and 9 being the lowest (lower numbers = higher temperatures before the compressor engages).

My usual way of setting the thermostat is to turn the thing on, forget about it until I start to feel chilly, and back the thermostat down until I here it click and the compressor shuts off. Has anyone ever figured out what those numbers correspond to in degrees? If so, why the heck don’t they put degrees on the dial instead of numbers that are essentially meaningless?

It’s a “GoldStar” A/C unit, if that matters.

In this case degree numbers would be useless because they wouldn’t correspond to the temperature of the room as a whole but only to the temperature at the window unit. If you want meaningful degree measurements you’ll need to put a thermometer somewhere in the room and do some trial and error with the unit’s temperature dial.

I have had several window A/C units with degree markings, and they are next to meaningless. They measure and set the ‘return’ air and the outside air temperature influences it also. If I want it to be 74-75 degrees in the bedroom I have to set the dial to 69-72 degrees, depending on the outside temperature. The lower the outside temp, the lower the dial must be set.

There is a lot of recirculation that causes this. The cool air mostly travels straight, but some circles back and lowers the return air temp and it shuts off. If there are obstacles nearby like a wall, the effect is more pronounced. Fan speed affects it, the lower speed recirculate easier.

Your method is just fine.

At one time I did convert a large window unit to a remote thermostat and it worked fantastic.


All my window a/c units have temperature readouts. One even reports the actual temperature. As Dennis said though, they have to fudge the readings a bit because the intake air is going to be cooler than the rest of the room due to some wrap around cold air. Even so, from a manufacturer’s point of view, a dial without degrees is easy and cheap because if the numbers mean nothing then it doesn’t need to be calibrated and nobody can complain that it’s a bit off.

But, often times, there’s no degree markings in refrigerants either, it’s just numbers or letters. Come to think of it, many water heaters don’t have degrees listed on the dial either.
Something I’ve noticed on commercial HVAC t-stats is that over time, they tend to drift. In a professional setting, I don’t pay that much attention to what the temp says on the dial (unless it’s way off, then I assume it’s broken), I just make small adjustments until the temp in the box is correct.
My theory with consumer units is that the average home owner isn’t going to be okay with that. They’ll set their fridge for 35 and someday a few years later, they might measure it and find that it’s 39 (or set their water heater for 120 and some day find that it’s 110) and instead of just adjusting it, they’ll either whine about it breaking or call for a warranty repair. Fridge t-stats for home use, IME, aren’t nearly as user replaceable as commercial ones (which are, more or less, stand alone units, just another in the myriad of of controls in the HVAC system, so a bad t-stat A)easily replaceable B)not a big deal to adjust and C)not all that expensive.

TL;DR, my WAG is that there’s not degree markings so if it’s not calibrated, no one cares, or even notices.

My usual way of setting mine is to wait until the room is comfortable then back off the temp setting slowly until the compressor shuts off. Then raise it back up slightly but not enough to turn the compressor back on. This puts the setpoint in the middle of the thermostat differential range (deadband.) Over a few days doing this gets it where I don’t need to move it again.

Even the units that have a thermostatic control based on temperature won’t be very accurate because the thermostat is in the unit. I think it’s measuring the air at the intake and that usually doesn’t represent the typical temperature in the rest of the room.

That’s why I said before that they tend to fudge the readings a bit. Mine read a bit high and I assume it’s for that very reason.

All the window ACs I’ve played with have the bulb mounted right on the coils. You can see it when you take the filter off. That probably makes the most sense since the unit would probably never shut off if it was on the intake, especially if it was really warm in the room and even more so if you had the flap open to bring in outside air.

Furthermore, now that I think about it, the air coming off the coils is going to be far cooler than the air in the living space. That’s probably why it doesn’t have actual degrees on it.

In big walk in coolers, we mount the t-stat bulb kind of in an ‘average’ spot. Even in reach in coolers where it’s closer to the blowers, the box is so small it’s still all pretty much the same temp (but if it ices over, you end up with strange problems).

That doesn’t make sense. Yes the bulb is right on the coil (maybe 1/4" in front of it) but it’s also in the air intake stream. The outdoor air flap is usually in the cold air supply plenum anyway so that’s irrelevant to the conversation. So I don’t see where the conclusion “the unit would probably never shut off if it was on the intake” comes from. That’s exactly where you do want it under the circumstances.

Ideally you’d have a remote thermostat somewhere in the room that’s not being blown on by the unit, or in the sun or near other heat sources. For many window units I’ve found they tend to work better if I pull the sensing bulb out in front of the air intake or off to the side, if it will reach. Some units have such cheap thermostats/controls that they short cycle and do other weird things.

Since they all come with remotes these days someone should put the thermostat in the remote so it can be left somewhere representative of the air temperature in the room.

I like that, since you’d most likely have it near where you are, like on a nightstand or end table. Though aren’t they pretty much all IR remotes? Those need a pretty good line-of-sight as it is, which wouldn’t be so practical. I have an indoor/outdoor thermometer whose transmitter lasts up to a year on four AAA batteries, doesn’t need line-of-sight, and can transmit about 30 feet. It’s about the size of a first generation iPod, or 2-3x the size of most a/c remote controls though.

I’d think that the kind of remote required for that would be a lot more complicated than the kind of remote that typically comes with a window AC unit. The thermostat needs to be in frequent contact with the main unit to make adjustments. If nothing else it needs to be RF instead of line-of-sight so it doesn’t require someone holding and pointing it all the time.

Line of sight would be a problem. It could work but not everyone would understand it. Maybe use Bluetooth instead.

ETA: Contact doesn’t need to be that frequent. It would use up the batteries faster, but it’s a brief transmission only a few times an hour, and intelligence should make adjustments less frequent too.

Unless there’s two bulbs it’s either in the intake or the exhaust, not both. The one I linked to shows it mounted on the evap coils, that’s the exhaust stream. Personally, I’ve never seen it in the intake, maybe it is in on some models, but both?

The problem I’d see with that is that it would need to be explained, so well, in the directions that I think a lot of people wouldn’t understand it. If you leave it falls under the couch cushions, it’ll take much longer to react to temp changes. If you (or a pet) sit on it, it’ll think the room is nearly a 100 degrees. If you’re holding it, set the temp and then go to bed and set it down on your phone, again, it’s going to think the room is extremely hot and never kick the air off. Sure, a lot of people will get it, but they’re also going to get calls from people that don’t understand why it can’t sit on their desk under a lamp or in the kitchen near the stove etc.

I’m going to go ahead and assume they already make a model like this, but too many people don’t understand heating and cooling as is that it just makes me think letting them physically hold on to the temp sensor would be an issue. Especially when, ISTM, the way it works now, is fine.

I guess if it works, it works, but the t-stat should be calibrated to be in the spot. Even in big commercial reach in coolers and freezers it’s mounted in the evap coils. I’d find a picture, but the edit window is going to run out.

No that’s the intake stream. The air goes through the grill, then the filter, past the bulb, into the evaporator coil, then through the fan and out the top.

You’re right, I was thinking about it backwards. As I said, I really don’t spend much time with window AC units. But then it makes even more sense. Since air passes over the bulb before it’s conditioned, the t-stat is sensing, more or less, room temp air.

More or less. I’ve pointed thermometers at the intake and the air tends to be 3-4 degrees lower than ambient due to spillover from the discharge. That’s why the thermostats need some fudging.

I did once come across a pair of enormous old window units at a church, probably dating to the 60s or 70s, where the evaporator coil was mounted in the top and the air was blown through it from behind. It also used an axial fan below instead of a squirrel cage. I can’t remember how the condenser coil was arranged but I think it was something similar. So if either coil got clogged with dust and dirt, you were kind of in trouble.

When I converted mine to a remote thermostat I used a inexpensive commercial PID controller that switched line voltage. Plugged the A/C into the controller and ran the thermistor down the wall about 15 feet. It switched the main power to the unit on and off as needed and held room temperature within a degree or 2 (you selected the on and off points), and humidity was far lower than the unit could provide with it’s internal, crude temperature control system

Unfortunately, you can’t do this with modern digital units, as they do not retain the proper settings when the power is switched off. You need it to power up with the lowest temperature set on the internal selector. All the modern A/C units I have tried power on with the temp set at 72, not low enough to turn on immediately. They might run for 2 or 3 minutes (pumping humid air all the while) before the compressor starts.