I recently had a new Worcester-Bosch combi boiler installed together with new radiators and pipes for the entire property. The radiators heat up fine but the moment I open the hot water tap, the radiators go cold. It’s as if the hot water gets diverted from the radiators to the taps.

I had a Worcester Bosch engineer came out and he found a broken diverter valve which he replaced but that hasn’t fixed the issue. He mentioned all combi boilers work this way and hot water will take priority over radiators.

However I’ve not seen this happen anywhere else. My previous place had a combi and radiators never got cold, my friend has the same boiler and doesn’t have the same issue. Any thoughts would be appreciated

  • It seems odd that your radiators would go cold when using DHW, How much are you using? Filling a large bathtub? If so, it's probably set for DHW priority, meaning it will turn off the flow to the radiators when using hot water. Perhaps there is a setting to change to treat both heating and DHW equally. I have that setting available on my boiler and it works great....keeps the flow for heating going and heats water for DHW. Apr 9, 2022 at 12:00
  • When washing the dishes for a few minutes is enough to make the heaters go cold. Do you by any chance now how to change the setting on the WB 4000 to treat water and heating equally? Apr 9, 2022 at 13:18
  • Sorry, I don't You'll probably need to get the installation/setup instructions from the manufacture or installer. Apr 9, 2022 at 14:58

2 Answers 2


There are systems where the same boiler provides both hydronic heating and hot water. But in my experience, those supply hot water via an indirectly heated tank, so even when set up with the tank having priority over the house a change isn't usually noticed until you have been running the hot water for a significantly long time. A long shower might indeed steal heating from the rest of the house, but the house has enough thermal mass that it shouldn't drift far before your shower is over... At least, not if the place is decently air-sealed and insulated. (I have such a system, though its house-heating function is largely taken over by a minisplit heat pump these days.)


All combi boilers work that way. They can either heat the hot water, or the central heating, but not both at the same time. The diverter valve determines where the heat goes, and that's controlled by the boiler detecting that you've turned a hot tap on.

Most people don't leave the hot tap running for long enough to notice the difference in temperature of the radiators. I never even noticed when running a bath, which can take some time.

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