I am installing a Takagi TK-1 for DHW and some space heating in my house. For the space heating, I have three baseboard heaters with 1/2″ copper tubing and fins. Two are 48″ long and one is 72″. They are plumbed in individual loops from a common manifold. (3/4″) Should I use a 1/12th HP pump or a 1/25th HP? I guess I need an expansion tank, too.
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Are you planning to use a heat exchanger? How are you going to handle the different water temps you want for DHW and heating?
I will have the tankleess set at 180 ° F and then use a mixing valve to mix some cold for DHW.
No heat exchanger.
I will have an expansion tank, though.
It sounds like you know something about hydronics, so I assume you know that most jurisdictions do not allow one open system for heating and DHW combined. Back to your original question.... I think you could use something in the Taco 00 range... 003, 005, 007.