How to use thermostatic shower mixer with lower temp DHW from heat pump?

I’ve had a Nibe ASHP (F2040 12kW) installed, but have had difficulty getting a decent hot water temperature from my Bristan thermostatic shower mixer. To have a decent temperature in the shower I have to resort to running the cold water tap in the bathroom at the same time as the shower - not ideal! Hot water straight from the hot tap (kitchen, bath etc) is hot enough - so the problem is with the thermostatic cartridge in the mixer - I think it’s related to the temperature differential between the hot water supply and the design mixer output temperature. I don’t want to have to replace the mixer (it’s built into a fully tiled wet room and would cost thousands in re-tiling etc). Is there a way to replace the cartridge/adjust the cartridge/disable the safety cutout? I wondered if other heat pump owners had experienced similar issues?

Not likely to be temperature differential, Katherine, if both temps are about the same now as they were before you installed the HP. (And it’s nothing to do with the ASHP itself - your hot water doesn’t care where the heat has come from, and you choose how hot your tank should be.) More likely it’s differential pressure. Did your HP installer change anything on the DHW system? Sounds like your cold water pressure is now too high for the mixer. I think! (I’m no plumber).
PS if your mixer cartridge has developed a fault, this can usually be replaced easily (depends on which model of mixer you have).

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Thanks for the reply Sarah. We’ve had the problem since the ASHP was installed. We’ve had a pressure relief valve fitted, which is set at 3 bar. The mixer is a Bristan, and it has a removable cartridge. I wonder if it’s coincidence and that the problem might be with the cartridge as you suggest.

The clue is that the shower hot water is OK when you run a cold tap (and dropping the pressure in the cold supply to the mixer). Can you have a look at the mixer cartridge? It may just be a bit of debris from all your recent plumbing work. But a new cartridge can cost £100 or more… :lying_face:


Thank you! I will have a look…