I have a heat pump with backup gas furnace. There is a ¾” PVC pipe coming from the unit going into a non-submersible pump, and a barbed fitting with a roughly ¼” clear plastic tubing running from it.

The pump plugs into a regular outlet, and there are two small gauge wires, (for control, or sensor?) running from it into the larger unit.

I presume that condensation drips from the coils down through the PVC pipe and into the reservoir of the pump, and that there is a small float or something that turns on the pump when it gets full and pumps it overboard somewhere through the clear tube.

The problem is this:

  • Yesterday I replaced my water heater, and in the process I needed to disconnect this pump for clearance. After I finished with the water heater I didn’t bother reconnecting things because I need to remount this pump in a different location and re-configure the PVC pipe bends to reach it.
  • Overnight it got pretty chilly. I don’t think it was cold enough to trigger the gas furnace, but quite chilly, and it seemed that there was no “normal” heat being circulated.


  • Is some level of water in the pump reservoir necessary to allow the heat pump and furnace to operate?
  • Are the two small gauge wires needed to tell the system that there is a pump installed, and that if not connected the system will not function?

Disclaimer: We have a fancy touch screen thermostat that I haven’t figured out. My wife controls the temperature in the house, (because she cares more…) and last night she was out of town. It wouldn’t be like her to turn it down during the day before she left yesterday, but it is possible that it is just set lower than normal, and with her not being here it wasn’t turned back up in the morning. (I would have suspected warmer than normal during the night due to her not turning it down...)

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    Condensate pump should have nothing to do with the heat pump running. In fact, I was expecting to see "and I found a flood this morning". Double-check that there are no breakers or switches turned off and then figure out the thermostat. Commented Apr 18, 2023 at 18:09

1 Answer 1


We don't know how the installer hooked up the condensate pump, but one way to hook it up is to inhibit both heating and cooling if the condensate pump fails.

If you have a Little Giant condensate pump (or similar) with two thin, low voltage wires coming out the top, those are the leads to the "High Water Level Switch." See the manual, page 4 for details. This pump is shipped from the factory with the wires connected NO (normally open) but it can be easily switched to NC (normally closed) by the installer.

The switch changes state if the water level rises much higher than the level at which the pump is set to empty the tank, so if it ever gets to that high, either the pump is not working or the drain tubing is clogged.

The installer could have wired the tank high water level switch normally closed and connected the Common wire of the thermostat (or the transformer's wire that supplies Common to the thermostat) to run through that safety switch. If that's true and the switch opens due to a high water level, your system will neither heat nor cool.

If you have a multimeter and you know how to use it, connect the meter to the two wires that are coming from the HVAC system that used to go to the short wires on the pump and see if you have 24-28 volts AC.

Now empty the pump's tank and measure resistance across the two short wires with nothing else connected. If it reads 0 ohms (or less than a few ohms), the internal switch is NC, normally closed. If you re-connect the pump, the HVAC should now work and the theory explained above is confirmed.

  • Thanks! I will give it a look this evening. Commented Apr 18, 2023 at 21:36
  • Confirmed! 25VAC from the furnace side leads, and since I had the pump off and empty I popped the reservoir off and measured resistance on that end as I moved the float. Some measurable resistance simulating empty, also at the pump-on first click, then open when it got to the last high-level click. For tonight I just connected those two wires together to simulate an empty tank and the heat fired right up. It's cool and dry, don't expect much of any condensation. I will fix it right later. Thanks again, I wish I had more upvotes to give! Commented Apr 19, 2023 at 1:24
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    @MichaelHall Excellent, I love when that happens! When I first saw your question I recognized your handle as a deep font of knowledge and well-written answers from aviation.se where I lurk but don't participate much, being an aviation fan but not a flier. I appreciate all you do over there.
    – MTA
    Commented Apr 19, 2023 at 2:59
  • That means more than you know, thanks. Commented Apr 19, 2023 at 3:56

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