I have the model 21786 condensate pump described below. The specs say it can push the water up 16' using a 3/8" PVC pipe.

Does this mean it will do it in any possible direction (my case 8' to the ceiling and then the remaining in the ceiling horizontally?

I am considering buying a 20' one as that will be enough to get the water across the hallway in a bathroom where I have the ceiling open and I can control the slope of a drain that I could build there.

Preliminary testing with the 16' model is not conclusive and I am having difficulties with building a properly sloped drain in the ceiling. Along the joists there is insulation and I can't reach that area to make room for the PEX pipe I am using so I can slope it properly. I guess the only solution is to use such a 3/8" PVC pipe and a more powerful pump that will at least push the water to the area where I can control the slope of a drain that I could build.

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The "lift" specification of a small pump is the height to which it will raise water above itself. It is another way of specifying the pressure that the pump produces at its output port. It is not a specification of the distance over which the pump will deliver water.

As your ceiling is eight feet high and the Mars-21786 can raise water up to 16 feet, it will satisfy your needs.

You do not need to build sloping drains or use larger tubing starting from 16 feet from the pump. You can run the 3/8" pvc tubing from the pump all the way to anywhere on your property where you can conveniently get rid of the water. A run of 150 feet is no problem, provided that no point in the tubing run is more than sixteen feet higher than the pump.

The effect of a long tubing run will be to slow down the rate of flow. But condensate pumps operate at a very low duty cycle, that is, the pump sits idle for minutes or even hours at a time, then runs for a minute or two to quickly empty its reservoir.

You need to be concerned more with the flow rate than with the output pressure. How much condensate does your equipment produce per hour?

I notice that the flow rate is not given for your model pump, but it certainly does not look like junk, so it will probably perform at least half as well as the more expensive Mars-21780, so 36 gph. Cut that in half for a really long drain tube, that's 18 gph.

The 21786 has a two quart reservoir. If it does what most do and starts when it's half full, it needs to pump about a quart before switching off again. At 18 gph this will take about 1 minute 12 seconds.

I suggest you do this: Run the 3/8" pvc tubing from the pump to whatever drain you find convenient, switch the pump on, pour water into the reservoir until the pump starts, and observe the flow at the drain. If anything more than a tiny dribble comes out, you're good to go.

  • so what happens if the tubing is just meandering on a flat surface above 8" (ceiling)? This is pretty noisy so I am not sure if having this running very often is a good thing. This used to run through the ceiling using s 3/8 up to the ceiling and there a smaller diameter pvc tube was inserted in That segment went across the ceiling, hallway and discharged in the bathroom. You could hear the pump struggling in the middle of the night. So having the pump work at its capacity is essential IMHO
    – MiniMe
    Sep 15 '20 at 3:04
  • The flow rate is not an issue it is not much but there was that weird tubing I mentioned
    – MiniMe
    Sep 15 '20 at 3:17
  • Yesterday I did one mistake, I took out the backflow valve you see in the specs page so that was making the pump work continously. I blamed that on the slope. I will try again tomorrow I put that vlave back although it is a little bit cracked on one side (it is rubber)
    – MiniMe
    Sep 15 '20 at 3:19
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    Don't use weird tubing. Use one piece of tubing all the way from pump to final drain. 3/8" is plenty large enough. - Put the check valve back. Order a replacement check valve now instead of waiting for the cracked one to fail. Sep 15 '20 at 3:27
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    Extremely long runs 'might' affect another thing, back-flow. Check valve or not it could seep back and refill it. If long horizontal run, I would make a high spot near the beginning, so it would flow forward. If running along under joists, I would put a hump close to the sub-floor at the start of the horizontal portion to help 'siphon' forward. A check valve might not work perfect every time. Restricting with too small tubing might increase pump noise as it increases the back-pressure... I would use black fabric hanger strapping, or something soft to isolate it from wood framing if too loud. Sep 17 '20 at 3:00

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