We have just had a new boiler and new bathroom fitted.

Shower and bath work fine - takes a minute or so for water to heat up, its a combi boiler and the boiler is on another floor to bathroom. However the sink tap in bathroom will not get hot unless either a shower or bath has been run.

My guess was that the sink tap does not create enough pressure to really get the boiler going, and the pipes need warming up.

To clarify the water from sink gets luke warm, at best, with no shower/bath. While if sink tap is run shortly after shower/bath the water is perfectly hot (~60c as expected)

Any tips ?

  • Do you have separate hot and cold taps on the sink or just a single mixer tap?
    – ChrisF
    Oct 6, 2010 at 12:09
  • mixer tap mixer tap Oct 6, 2010 at 12:11
  • Try turning the mixer tap all the way to the hot :)
    – ChrisF
    Oct 6, 2010 at 12:26
  • Do you have an aerator on the tap? They reduce the flow rate, so you could try removing it.
    – Niall C.
    Oct 6, 2010 at 12:45
  • 3
    Just to clarify for U.S. readers: is this an on-demand water heater that stays off until flow through the hot water pipe is detected, at which point it fires up and heats the water as it flows through? Oct 6, 2010 at 13:42

2 Answers 2


I think you are right. There is a minimum flow rate required before the boiler will actually kick on. I have exactly the same thing with my tankless hot water heater.

Aside from getting a separate "point of use" hot water heater for just the sink, The only way I'm aware of to get around it is to turn the water all the way over to hot and on full blast. That usually gets enough flow going to start up the water heater. Once hot water starts coming out, you can adjust the temp a bit (at which point the water heater will probably shut off, and all you have is the hot water left in the pipes).

  • Is the boiler's 'minimum flow rate' adjustable? I'd think that what you are describing is a big problem for an on-demand water heater in this day and age. If it's not, I'd have words with the installers - it's no use if it won't kick on when you need it! Oct 6, 2010 at 21:23
  • @Michael - It might be, depending on your model. I know on my tankless water heater it isn't adjustable. I think it probably has something to do with safety concerns - If too little water is flowing through the pipe when the burners are on, there's a chance it could actually boil the water. So the burners would have to be able to adjust depending on the flow rate, which of course would make the whole thing more complicated (and expensive). Oct 6, 2010 at 21:26
  • 1
    You may well be right - at the very least, it's not going to be adjustable too far down. On the other hand, if it won't turn on when it's needed, then the manufacturer needs a good kick in the pants - the device has only one purpose, and if it can't fulfill that purpose, it's been sold fraudulently. Oct 7, 2010 at 0:37

Eric was correct with the minimum flow rate, I would recommend removing you aerator, and removing some of the water savers that the companies have put in which are meant to slow down flow, this could be just enough to dive your problem!

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