Sizing DHW expansion tank for tankless heater

Question

I have a combined boiler for hydronic heating and DHW. Earlier in the winter, I started insulating as much of the pipes I can easily access.

This has led to hot water coming out of the cold water lines in the kitchen, which happens to be very close to the boiler. I understand now that this is happening because as water is heated, it expands, and being unable to expand further into the hot water lines, pushes back (towards the street).

The solution to this is to put a check valve (pressure reduction valve), stopping the back flow of hot water into the cold lines; this would also require a pressure expansion tank and T&P relief valve.

I don't know how to size the pressure tank. The tables and calculators I can quickly find seem built around using standard DHW heater/tanks, not a tankless coil.

So what size do I need?

Answer 1

The volume of water that is being heated by the on-demand heater is very small, so the smallest commonly available expansion tank will work just fine. Just make sure that you are getting an expansion tank for potable water, not the one made for hydronic heating systems.

Answer 2

If there is a drain near by your heater, you could install a combination shutoff and thermal expansion relief valve on the cold water line that feeds your heater. This would be installed downstream of the check valve. Here is an example of such a valve: Apollo Thermal Expansion Relief Valve.