To start off, I absolutely hate idea behind thermostatic mixing valve, but that's not a question.

The water in my house runs hot for about 10 minutes and then turns icy cold, and I have to turn it off and wait for an hour for the hot water to come back for ten minutes again. I assumed problem was with my boiler. So I called a repairman and he told me that boiler is functioning properly and that issue is with my thermostatic mixing valve. He explained to me that they have to be replaced once every 3 years or so and told me that it will cost 400$ to do so. His explanation was that government in Canada uses chemicals that break thermostatic mixing valve.

I am extremely broke, and can't dish out 400$, however I have kids that need to take a bath, and having hot water for only 10 minutes makes it really hard. Does my thermostatic mixing valve need replacing or is there any way I can fix this issue with water turning cold after ten minutes?

  • Is this anew problem or has it been going on for some time?
    – mikes
    Commented Dec 28, 2017 at 13:45
  • 1
    It's plausible that chemicals or sediments in your water supply reduce the lifetime of the valve. Depending on the exact cause, you may be able to clean or repair an existing valve rather than replacing it, or delay failure by installing a water filter before your water heater. But 3y seems short and $400 seems high. Call some other local plumbers. Commented Dec 28, 2017 at 13:51
  • Have you tried opening the valve to see if that alleviates the problem? This may be a safety issue but could at least verify that your tank is OK. Also may be worth considering removing the thermostatic valve and turning down your tank temp, if you can get enough hot water from a 120F tank. Commented Dec 28, 2017 at 13:54
  • @mikes this only started happening recently (one week) when temperatures hit -27C didn't happen last year with same temperatures however.
    – Quillion
    Commented Dec 28, 2017 at 14:09
  • @ShimonRura which valve should I open exactly? And I thought about getting rid of mixing valve, but that is apparently illegal.
    – Quillion
    Commented Dec 28, 2017 at 14:11

1 Answer 1


If it's an adjustable valve, some times fully opening and closing on a regular basis can help keep the guts free. After that attach a thermometer to the tempered water pipe, try running some Luke warm water at a sink while adjusting the valve, if temperature changes do not correspond with changing settings the valve is faulty. I would have a replacement on hand prior to disassembly, sometimes these things don't go back together nicely.

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