Circulator control

dvg

Replacing old gravity boiler with wall hung condensing boiler in small (1200sf.) house.---2 pipe, single zone, big pipes, cast iron radiators.----wall hung pipeing instructions says use primary/secondary----Asumming the one only room thermostat controls the boiler and primary circulator and the system remains single zone, what and how is the secondary circulator controlled?------Is P/S really needed?-----Is there some major downside if I don't use it?

Leon82

The boiler can usually control both or a taco switch ing relay can control the primary pump also.

But one zone only you probably can omit it if the math works for flow rate thru the heat exchanger

Leon82

It will be worth it to spend time tuning the outdoor reset

JUGHNE

If it calls for P/S piping, you need it.
I have done a couple of these with Lochinvar water tubes.
Their wiring allows for boiler pump and system pump.
If system pump current exceeds the rating of the internal boiler relay, then another external relay may be required.

Have you reduced the piping down in size at the boiler for the existing piping?

EBEBRATT-Ed

You need primary secondary. You can start and stop both pumps with the thermostat.

Make sure the amperage draw of both pumps does not exceed the capacity of the internal boiler relay.

As @JUGHNE mentioned you can use a relay if needed to allow this to happen

dvg

So to be clear, p/s pumps are supposed to start and stop concurrently and not independently!

EBEBRATT-Ed

Yes, no reason they cannot start and stop at the same time