I'm thinking through an install of a new combination boiler in my house, and am confused with respect to picking the correct size circulator pump. I've found a few different references (e.g. https://www.taco-hvac.com/uploads/FileLibrary/SelectingCirculators.pdf) which lay out that you need:
- A sufficiently high flow rate through the loop to heat the home, and ensure proper functioning of the air separator
- A sufficiently low flow rate to avoid noise or undue strain on the pipes
As a rough example, let's say I have 3 heating zones with 3/4" copper pipe in my house. To make it as simple as possible, let's just say each zone is identical (same head loss, etc.). I calculate I need 12 gpm to heat the whole house with a 20F drop between supply and return water. When all 3 zone valves are open, the flow is divided among the zones and there is a flow of 4 gpm through each, which is fine for 3/4" copper pipe. I would expect if only one zone valve is open, that I would end up with 12 gpm through the single zone, which is way too high for the same pipe. What am I missing?