Testing the pan float switch

Does anyone routinely test the overflow pan float switch by lifting it up and confirming the power gets cut?

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I never did until my evaporator drain clogged and almost flooded my office below it in the attic. When my HVAC guy came to fix things, he said that the wet switches (one in the aux. drain and one in the pan) were never connected to the control board. So now I test them all the time.
To make it easier, I test them when I have the gas furnace running. Lift the float and the gas burners shut off instantly if the switch is good.


thanks, that gets at my second question. Ill give that a try. ive tried to test them in the past with the AC running and it didnt seem to kill the operation instantly. after writing up a couple switches for “not operating correctly”, i figured there may be a delay I wasnt aware of and didn’t want to be wasting peoples’ time.

Go ahead and give it a shot. You’ll spend the next 30 minutes on the phone with customer service of the air handler manufacture trying to get the damn thing to start up again.

Every air handler has a different start up procedure once that switch kills the power.


sounds like a valid concern. do you disclaim it? it feels like something that must be addressed one way or another, or else I’m eventually going to have a very mad client on my hands when one fails and floods a home.

It is addressed in the NACHI SoP. Operate using normal controls. Your on your own after that.


Words of wisdom up there :point_up:t2::point_up:t2::point_up:t2::point_up:t2:


I do not disclaim.

I know this generation likes to say I feel this and I feel that but I’m kind of past that.


This is what I do.

I verify the AC was operational using normal operating controls (thermostat) at the time of inspection. Then I evaluate the unit. I report on issues. One of which may be the absence of a float switch.


No way I’m lifting it up.
The system will shut down (I guess).
Then usually a delay that could be up to 10 minutes… or it does not restart.
Then the inspector touched it last & the seller is not happy the AC is out.


Yes this is what happens. Panic mode for a careless inspector. When it’s 95° outside and there’s no A/C you’re a target.


If the gas burners shut down, it is installed improperly!

The only time your a/c makes water (in the amount to flood the ceiling below) is in the summer. The switch should be attached to the a/c thermostat wire, not the main power to the control circuit.
If the switch fails, something falls on it, a rat builds a house under it, you put Christmas decorations against it, you have no heat in the winter. All HVAC systems should Fail Safe. An a/c problem should never effect the operation of the heat.

As for a Heat Pump, if the compressor is shut down by the switch, the auxiliary heat should still have power to operate.


Thank you David!

It is not necessarily installed wrong. Don’t forget that condensing furnaces also create condensate(water). Once one of the rubber furnace condensate lines crack or come loose, water is going in the auxiliary pan below and a float switch wired to the furnace power would save the bacon, if the float switch still works by that time anyway. I mean the furnace doesn’t make as much water as an A/C in the summer, but people have to work, go on vacation, go visit family, and so on. It is not improbable for the homeowner to come home one day and have water damage.

If the homeowner puts tons of storage around the furnace when the small space is only there for servicing the unit, guess what is going to happen? They are going to bump into and dislodge/break condensate lines, block combustion air from getting to the furnace, or screw something else up. If they accidently damage/dislodge the float switch and the heat gets shut down? Isn’t that a good thing? Now they have to call an HVAC guy to fix it and he will tell them of all the problems they’ve caused and other potential problems. They are more likely to remember to not to do some of the things that caused the problem. IMO most people around here don’t get their HVAC systems looked at/serviced annually or EVER until there is actually a problem.

Here is a new construction from a month ago. It wasn’t cold enough back then to use the furnace, but it’s starting to get colder here in the evenings. I’m sure the homeowner would have eventually known about this issue down the road even if they didn’t do an inspection.


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And remember, if it is installed correctly, it will shut off the outdoor unit (if it is a split system) and not the indoor unit.

There should not be any time delay as the switch cuts power to the a/c. There is a fan delay sometimes however on some units, but the a/c is off. What your seeing is probably the fan not shutting off right away.