Pre-purge

SweatyInToronto

Currently each our two conventional hydronics cast-iron gas boilers have S25 motors that run continuously, so most of the time one is acting like a chimney. I would like the motors to run on demand for better efficiency - and for less noise generation.

Most of the time only one boiler fires and the other would get cold if hot water weren't running through it constantly. If the motor were off most of the time, I guess there would be some waste while it heats up from a cold state. Would pre-purge help reduce this or it's really thermodynamically neutral?

I gather that thermal shock isn't a concern if hot water hits a cold boiler as much as cold water hitting a hot boiler... so is a pre-purge needed in this situation do you think?

EBEBRATT-Ed

Don't understand the question. What is an S25 a circulator?? How is it piped

STEVEusaPA

Why is the 'off' boiler getting flow. Like @EBEBRATT-Ed, I’d like to have more info on the equipment and controls.

SweatyInToronto

Yes, S25 is an Armstrong circulator - 1/12 HP. There are two, one on each boiler - each runs constantly, even when the control goes into WWSD. It's a direct return system (one loop basically). The main circ 1/4 HP (S45) is on the return, pumping through the boilers and towards the steel expansion tank in the ceiling. I've written before about making improvements but there is not a lot that's beneficial short of redoing the entire piping which is out of $cope. However, one thing that seems like it could be helpful running the boiler pumps only when the boiler fires up.

Any risks to this? It seems like it's standard operating procedure. Just not sure if pre-purge is necessary to have...

Thanks!

EdTheHeaterMan

S-25 being a circulator pump motor can be connected to the boiler relay that operates the gas valve. depending, of course, upon the type of relay that operates the gas valve.

As far as calling it pre-purge, that term is usually meant for the burner fan motor that would operate to clear combustible gasses from the combustion chamber prior to ignition and main fuel valve operation.

So your interpretation of "Pre-purge" should actually be called constant circulation. Regarding Constant circulation, it is not necessary and you can control the circulation pump top operate with the burner. Pump on and burner on at the same time and pump off and burner off at the same time.

One exception is the burner off and the pump on whenever there is a call for the burner & pump to operate and the water temperature is above the high limit setting, the pump should stay on and the burner stays off until the water temperature drops below the high limit differential (or cut in) temperature.

There should be check valves in the system to prevent reverse water flow thru the boiler that is not operating.

SweatyInToronto

Thanks EdtheHeaterMan, I was wondering about the check valves as I've seen them in the diagram.

There's no fan blower here so no pre-purge then in that context.

No relays either yet, as the Tekmar 261 just send the signals to the boiler directly based on the supply temperature. But we'll get relays that intercept and start pump with the igniter I hope.

If there are any contractors in Toronto, ON area that want to have a go at this and several other improvements, please send me a note.

Steamhead

Armstrong S25=B&G 100.