I have a couple of 150L electric water heater cylinders in the cellar, fed by mains water pressure. They're designed to bleed a little from the bottom, as water inside heats up. If they were above ground, it would be easy to pipe this water to outside. As it is, their bottoms being below ground level, that's impossible.

I have to catch the water in a tray, and periodically empty it, every few weeks.

Looking for a fix-it-forget-it solution.

  • 1
    If the draining is related to expansion caused by heating, you could install an expansion tank which would absorb the expansion and maybe your relief valves (or whatever they are, I am not familiar with water heaters that are designed to leak water) would not need to open. Nov 13, 2020 at 15:00
  • One is available, but at around £100 I'd like a cheaper alternative. It also takes a fair bit of space. Also not sure whether that's pressurised too. If not, it'll just be a receptacle that eventually needs emptying.
    – Tim
    Nov 13, 2020 at 15:32
  • "They're designed to bleed a little from the bottom, as water inside heats up." Really? I would think any expansion would push back through the inlet pipe. Nov 13, 2020 at 15:42
  • It is a can with a sealed bladder partition, water on one side, and pressurized air on the other. Expansion caused by heating just causes the bladder to flex inward toward the air side, reducing or eliminating pressure increases. It is installed on the inlet piping, usually with a check valve upstream (and pressure regulator upstream of that, depending on expected mains pressure). Nov 13, 2020 at 15:45
  • @SteveWellens - the inlet pipe is at mains pressure, and I didn't think heated water was allowed to re-mix with cold anyway.
    – Tim
    Nov 13, 2020 at 15:45

3 Answers 3


This sounds like an ideal situation for a condensate pump. Those are miniature sump pumps with a built-in reservoir. You drain your seepage into this, and the pump pumps it up and away when the reservoir reaches a certain level. Just make sure you get one with the capacity to pump as high as you need to to reach the outside, a drain, or whereever you want to dispose of the water. These also usually have a 12V NO and NC switch that you could use to wire in an alarm if you wanted to.

photo of condensate pump


Does your cellar have a sump pump? If so, you should be able to divert the water into the pit and let the pump do its thing.

If not, then you will need to setup something like that with a place for the water to drain to, a pump with a level-sensing switch, and an outlet pipe to carry the water to a good drain.

BTW, are you sure that bleeding from the bottom is normal? I've never seen such a thing. Generally hot water heaters have a valve near the top to vent any overpressure.

  • I agree on both counts. The drainage is odd, but a common sump should do fine assuming the components are rated for the necessary temperatures.
    – isherwood
    Nov 13, 2020 at 14:08
  • No. If I had a sump, I'd certainly be using it! The bleed is normal on these heaters, and in fact, there's a kit that fits underneath to enable drainage automatically - it's a plastic cup that attatches to drain tube. But gravity is aginst me at that point..
    – Tim
    Nov 13, 2020 at 14:37
  • 1
    The type of sump suggested here is like a furnace condensate pump. Look into those.
    – isherwood
    Nov 13, 2020 at 16:30

Expansion tanks are $30.


They have a bladder that expands with increase in pressure and accommodates the additional size of the water. The bladder retracts when the pressure declines.

  • Access denied on your www address.
    – Tim
    Nov 14, 2020 at 11:24
  • weird still works for me. you can google "eastman 2 gal. thermal expansion tank" Nov 14, 2020 at 17:50
  • I need a minimum of 2m head for any pump, though.
    – Tim
    Nov 14, 2020 at 17:53

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