Pipng for Two Stage Boilers

Melton

I would apprecite advice on piping the primary loop on a two stage boiler system. The boilers are identical at 143,000 BTUs each. I plan to use the Tekmar controls that allow boiler rotation.

What is the best/most efficient way to divert flow through each boiler as it fires? Is it advisable to adjust the flow rate in the primary depending on how many boilers are firing?

Thanks.

Boilerpro_3

What I do

This piping layout is for typical cast iron boilers. Pipe both boilers reverse return as a Loop off the main system with closely spaced tees. Then each boiler gets its own pump and outlet check valve to prevent back flow through the off boiler when the other boiler is running. I also size my boilers for baseload design which usually ends up with boilers of two different outputs. The baseload boiler is always fired first and its pump is controled directly by the tekmar control to help eliminate short cycling under light loads by pumping the stored heat out of the boiler before firing. Often the stored heat in the boiler is enough to satisfy the heating needs.

Then, thermal shock issues need to be addressed... Both cold shock of a hot boiler and hot shock of a cold boiler. (Cold second stage boiler pump kicking on with a hot system). The least expensive solution to both these issues is the installation of thermostatic mixing valves (Esbe Thermix) on the return to each boiler. Boiler water is not allowed to go to system until boiler is warmed up (hot shock protection) and cold return water is mixed with hot supply at the boiler return to keep the return abaove 140F. This also will allow you to run full reset water temps to the system.

Here's an example...sans reverse return piping.

Boilerpro