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:

  1. A sufficiently high flow rate through the loop to heat the home, and ensure proper functioning of the air separator
  2. 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?


You may consider boiler maximum power (30kW = 26kcal/h ), because you must be able to handle the heat your boiler can produce at full power.

difference in temperature between flow and return [usually 10°C, 20°C on condensing boilers] 20F (11K) is ok for standard boiler. If you have a condensing one I suggest you to increase the delta to 20K to both run your boiler at bigger efficency and reduce electricity need at the pump]

and get a pump that can hold that flow, in our case [10K delta] 10kcal/kg or about 2.6mc/h [1.3mc/h for 20K delta]

[26kcal/[1kcal/kg K] * [1Kg/L]] of water, then measure maximum water column you need on your system and choose a pump that can handle that flow for water column you have.

You should get zone valves with 'built in by-pass', so the flow is the same in any configuration (3 zone open 860L/h all trough the rads, if two zones are closed you have 1700L/h flowing trough by-passes and 860L/h flowing trough the only zone that is open and so on).

Anyway, 2.6mc/h (similar to your 12gal/min) won't fit well over 3/4" (20mm internal diameter) pipe because of excessive water speed (around 2m/s) that results in noisy pipes, it also will have quite high head (about 20cm wc per metre of pipe)

Anyway, flow [mc/h] = max power [kcal/h] / deltaT [K] (I use calories because 1Kcal heats 1Kilo of water 1K making calculation easy)

  • Thanks for the great answer! To make sure I'm clear - I want to install 3-way zone valves with the 'open' path going to the heating zone and the 'closed' path going directly to the return side of the boiler? – Kevin Lyons Oct 24 '18 at 17:26
  • Exactly: I suggest you to use thermo-actuated 3-way valves instead of motorized ones because the first are more 'polite' when opening and closing so you can avoid water hammer. My collector (not sure of english translation) is this one: stipbagni.com/idraulica/distribuzione-ricaldamento/… and uses MVA21 as thermal actuator. PS: you could use differential valves so 'bypass' is open only if required and not always (useful with condensing boilers to increment condensation by reducing return temperature) – DDS Oct 24 '18 at 20:43

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