New to living with a swamp cooler. I've been told by several sources (people and internet) that the swamp cooler and the furnace can't be on/active at the same time, and indeed my landlord switches one off and the other on at the same time.

However, the swamp cooler is not connected to the duct work that the furnace uses. The cooled air has a single entry point in the ceiling of the house. The two systems use separate thermostats as well. I do not understand why the swamp cooler and the furnace could not be active at the same time. (Not that I would run them at the same time.) Is it true that in two isolated systems as I describe, one must be all the way off and deactivated in order for the other to run? Or have I been told wrong?

  • If the heater is on, the swamp cooler can suck in the carbon monoxide from the vent Commented Feb 19, 2023 at 23:23

2 Answers 2


As long as the heat is turned off the air handler fan can be run while the swamp cooler is running.

Swamp coolers work on evaporation the water flowing over the plates causes the cooler and more humid air then the blower pushes this into the home.

Folks may think the fan on the furnace air handler can’t be run because you don’t heat and cool at the same time. When I lived in Ohio back in the early 80’s one place had a swamp cooler and the way it was set up the furnace could not be turned on. I added a fan switch to turn on and circulate the furnace air handler and our unit was much cooler. We had an office party at our place and the next week I was fixing all my coworkers units and eventually all the units in our block. I got a few attaboys from senior enlisted as it made the units much cooler and back then AC was not available in enlisted housing. So I think it may be the way it was set up the thermostat only had heat or cool. I came from the west coast and had been involved in building several housing tracks and had only seen 1 swamp cooler in my life so I did not know it was unusual and added the simple toggle switch to turn on the fan but the thermostat still controlled the gas valve and would turn the fan on if the switch was in the off position.

So possibly because it has always been that way and many did not know it could be done.

  • TIL. Sounds like what most everywhere would just call a humidifier, but the fantastic cartoon name makes it sound like a really super huge capacity humidifier. I guess these must be used in poorly air-sealed houses in the desert? Seems like it would be expensive to keep feeding it water in such a case...
    – J...
    Commented Sep 16, 2020 at 22:05
  • 3
    The water is pumped from a tray / tank below over the plates and recycled. The difference is that it makes the house cooler but yes increases the humidity. I have seen very few on the west coast but back East they were all over. Window air conditioning was what most had then if anything. Except higher end houses had central air.
    – Ed Beal
    Commented Sep 16, 2020 at 23:15
  • 2
    @J... They're called evaporative coolers. Informally, "swamp cooler" or just plain "cooler". Cheaper living arrangements are cooled by a swamp cooler; 2-3 bed homes typically AC or both AC and evaporative, and larger homes and stupid mansions exclusively with AC. For homes with both AC and evaporative cooling, the homeowner typically uses evap cooling during dry months, and AC in monsoon season.
    – Rich
    Commented Sep 17, 2020 at 0:52
  • 1
    @rich I agree they work by evaporation as I said, but on both sides of the US the people that have them call them swamp coolers, possibly because of the high humidity or possibly because if you don’t keep the system clean it stinks worse than a swamp. I learned some things in Ohio with a couple of years having one. Then classes at a hvac seminar where I learned about the dangers of not kept clean or sanitized.
    – Ed Beal
    Commented Sep 17, 2020 at 1:05

Short answer: Can you? Yes*. Should you? Probably not.

It's not so much a question of whether it's physically possible, but more of a rule of thumb to prevent the two systems from accidentally interfering with each other.

*If, as you say, the units are separate from each other, and have separate thermostats, then they should be completely capable of being run simultaneously. In my house, I can certainly switch on both units at the same time. Theoretically, if you had the cooler set to kick on above a certain temperature, and the furnace set to kick on below a LOWER temperature, then only one would kick on at any given time.

However, I think it's more of a safeguard to not have them switched on simultaneously. Traditionally, the heating thermostat is in a fairly central location, which also tends to be where the main swamp cooler vent is best located. This means that the temperature at the thermostat is going to be influenced by the direct stream of cold air, which will get warmer as it distributes through the house. So it's far more likely to hit it's "switch-on" temperature and turn the heat on, when you in fact wouldn't want that.

And of course if you accidentally set the heat too high (above the swamp cooler thermostat), it would heat the house to the point that the cooler would kick on and you'd end up in a very expensive feedback loop as your house is simultaneously trying to both lower and raise its temperature.

The other thing is, in cold weather, the swamp cooler should be winterized and covered, since it's a direct vent to external air and would lose a large amount of warm air generated by the heating system. If you're heating with the cooler uncovered, you'll be losing a lot of that heat out the roof.

Edit: Since most swamp coolers blast a the air into a single vent, the cooling isn't very evenly distributed throughout the house. Ed Beal's solution of having the air handler fan would be a good way to circulate the air to get that cool air a little more even. I personally ducted my swamp cooler into the individual rooms to get better distribution.

  • What is the difference in direct ducting and using the air handler? Using the existing air handler is much cheaper and can be done in a rental or my case base housing.
    – Ed Beal
    Commented Sep 16, 2020 at 22:01
  • My understanding of your use of the air handler is that after the cooled air is blown in through through the big vent, you use the air handler to circulate air throughout the house, before it escapes through the windows. In my house, I have no central air handler, as my heating is hot water baseboard. So I have run ducting directly from the swamp cooler plenum to down-vents in each room. I'm sure it would be easier to just turn on a fan to get the cool air into each room, but direct ducting the cooler works like a charm too.
    – MeGrasses
    Commented Sep 21, 2020 at 16:54

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