Combi boiler DHW setup correctly?

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New Member
Jul 17, 2018
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Canada B.C
I have had a Bosch combination Boiler for a few years now. It fires hydronic baseboards, works well.
I recently had an indirect Domestic hot water tank installed for this Bosch system to supply heat to as designed.
My issue is when the baseboards are heating some of the boilers hot water is entering the DHW tank loop when the HWT isn’t calling for heat.
I’m concerned as the household baseboard heating in winter can run for a month straight. In turn, this may over heat the HWT and cause an error on the Bosch unit. As “X” set point temp for HW most likely will be exceeded. Then no heating of any kind.
I understand this is most likely a path of least resistance thing. Any advice on how I can get the flow to stop going to the HWT when not needed?

The plumber that done the job doesn’t want anything to do with it.
Is their such thing as a recirculating pump that doesn’t allow any flow when not powered on / running?
I see motorized three position valves are sometimes used. I don’t think this will work in my application.

The HWT loop has its own Grundfos recirculating pump. Household also has Grundfos pump and individual zone valves.

Thanks for taking the time to read this.

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Well-Known Member
Dec 17, 2014
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New York
That water heater should have been piped as if it was another zone. With a zone valve using the existing pump. Your "plumber" should be fixing this issue not running away.