My kitchen sink hot water works inconsistently and mainly doles out cold water. However, if I turn the hot water in the bathroom sink the hot water starts working in the kitchen sink so I’ve just been turning on the faucet upstairs when I need hot water for dishes but would ideally get this issue fixed instead of having both taps running when I need hot water.

I have a tankless hot water heater which is over 10 years old. Perhaps it needs to be replaced but I’m wondering if there might be something else I can try before going that route.

Many thanks in advance!

2 Answers 2


Most likely, it's the switch that's detecting the water flow in order to turn on the heater. You may be able to replace only that switch, not the whole heater.


Eventually you'll have to replace the pressure-sensor diaphragm, but in the meantime if the heater has a simple temperature dial which is tied to the water flow-rate, then turning the dial hotter/slower should trigger the diaphragm's sensor more easily.
Also, turning the tap on fully will help it trigger.

Diaphragms can be bought separately or as part of a kit, eg

enter image description here

This one is only £10 as the full kit. Replacement is fairly simple - & even legal to DIY in the UK, where you're not allowed to touch the gas side of any appliance at all. You only need to switch off the water side to change these, the gas side is untouched & can stay on. Just switch off at the front panel.
Note that sometimes there's nothing wrong with the diaphragm, only that the needle is sticking a bit. A light touch of WD 40 will give it another 10 years if you can free it up.

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