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[WoodMOJoe]

I am well into my 2nd season of heating with my homemade OWB, and very happy with it.

When I built it I put a 190 degree snap disc limit switch in the fan circuit.  I either put it in the wrong spot (where it is "seeing" a heat hotter than my aquastat) or it is kicking out at a lower temp (defective), and kicking the fan out at too low a temp, so I took it out of the circuit within days of firing up the unit last year and have been running without a dedicated high limit. 

I am using the common Ranco aquastat for my fan control now and was wondering...can I use another identical ranco aquastat for a high limit switch on the fan, just in case the first aquasat malfunctioned?

And if so, how would I wire it in?

Or is there another way to have a high limit switch on the fan that would be better?

[slimjim]

yes you can butI would use caution on where the probe is placed, your snap disc may be telling you that flow rates have decreased resulting in a hot spot on the boiler.

[WoodMOJoe]

yes you can butI would use caution on where the probe is placed, your snap disc may be telling you that flow rates have decreased resulting in a hot spot on the boiler.

Thanks Slim but not sure what you mean by "flow rates may have decreased"?

The snap disc was only in the circuit for a day or 3 so I don't think the flow rates changed in that amount of time. 

I put the snap disc on top of the water tank, about 6" back from the plate the door is mounted on. 

If I go with a 2nd aquastat for high limit I can put the probe in a bung on the back of the tank, pretty close to where my original aquastat is plumbed in.

[slimjim]

 More than likely the snap disc is placed in a spot that has little flow and being on the top of the boiler it is considered at the crown point which on a conventional unit will be the hottest point, perhaps simply moving it to the back near the aqua stat would help?

[WoodMOJoe]

More than likely the snap disc is placed in a spot that has little flow and being on the top of the boiler it is considered at the crown point which on a conventional unit will be the hottest point, perhaps simply moving it to the back near the aqua stat would help?

That makes sense, I bet you are right as usual!  I would have to take apart the front of my furnace to get to the existing snap disc...doable but a PITA.

May try just installing another snap disc on the back near my existing aquastat bung tap, and abandoning the existing snap disc.  I can install it through my access door on the back and fish wire to it without taking anything apart I think.

Good ideas Slim, thanks for making me think.   

[slimjim]

Happy to help!