How do outdoor reset controllers regulate boiler output temperature ?

Maxl

I can think of two ways that an outdoor reset controller could regulate the output temperature of a boiler: (1) if the boiler has variable heat, (2) by changing the speed of the pump. I am an absolute newbe in the subject, and I imagine fuel fired burners are on/off devices, so the only way I can think of regulating the boiler's output temperature would be to change the speed of the pump (the faster the water flow, the cooler the supply temp).

Does that make sense ?

Gordy

well depends on the boiler. Most ci boilers in residential are fixed output. Until you get into the modulating condensing boilers which modulate output.

So with fixed output it's about turning the burner on, and off to maintain the desired supply temp. If 140 supply is desired then the controller shuts down the boiler at 140, and then relights when a determined differential is reached 20 degrees say so relights at 120. That's the short of it.

With modulating equipment the burner can modulate down as supply temp is reached, and maintain that temp at a lower output until setpoint is reached.