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Buderus GB142 Indirect DHW scalding when heating house

BillyW Member Posts: 1
We had a Buderus GB 142 installed this summer with an indirect DHW tank. The DHW temp is set at 120 and this is what we were getting up until heating season started. Now the hot water is 150-160 at the tap.

It looks to me that when a heating zone calls for heat, water is passively circulated through the DHW tank raising its temp to whatever the radiator water temp is set to. When this happens, the DHW circulator pump is off.

I am not sure what is wrong, so any help will be appreciated.


  • Paul Pollets
    Paul Pollets Member Posts: 3,656
    Ghost flow +

    You may need to install a check valve on the return or supply to the coil of the indirect tank. Some use a zone valve, some use a pump to activate the tank coil. If the pump already has a built-in CV, install one on the return to stop the ghost flow through the tank. You should also have a 3 way tempering valve (or anti-scald valve) on the HW outlet of the tank. This is required under most state codes for obvious reasons. The tempering valve assures that only 120 degree water is outbound to the fixtures.
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