Make or Break

terry_6

What is the point of making pressuretrols in 2 models? Why have 1 that breaks high and differentials down and the other that makes low and differentials up? They seem like the same end result to me...

John@Reliable

Why two pressuretrols?

One is for boiler control,the other less common is for steam unit heaters.

terry_6

Which is which? In the catalog both variations are offered for each model of pressuretrol. This is for control of a steam boiler.

John@Reliable

terry,

A PA404A is for steam PA404B is for unit fan heaters.
And also might find a *L404A which has a mercury switch. *Burnham steam boilers come with this type.The most common is the PA404A for most systems,it's not the best control for the job, but due to cost it's the control used. Hope this helps. P.S. When I say it's not the best control to use,I say this because most home systems only need oz of steam and not pounds which is what these pressuretrols are using. John@Reliable

Dale

What the other posts

said is correct about the A and B versions of the 404. The way I keep it straight is that when a steam boiler is off it is at zero pressure the same as the basement or the house. We do want the boiler to start so we choose a normally closed control like the A. Normally closed to me is if I have it in my hand and out of the box the switch is closed. This way a call for heat can send power from the transformer to the thermostat to the 404A through the hopefully closed low water cut off and on to the gas valve. The B model is normally open since we don't want the fan on the steam unit heater to start until steam pressure - heat - reaches the unit heater. This may sound too simple and I don't want to offend but I've seen several B's installed on boilers, some for years with the switch jammed shut because the person doing the repair couldn't figure out what else to do to get out of the basement. Honeywell doesn't make it easy either, they look the same. Good question