Need some HVAC help

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Sam_Fear

Sam_Fear

Brother Rat
Joined
Oct 11, 2008
Messages
12,478
Location
Dixon, IA
Hopefully an HVAC guy can tell me I'm going in the right direction. (or not)

I got my used forced air furnace installed in my pole barn. Got it all plumbed and powered, but it's cycling. The blower doesn't come on until the limiter trips. When the limiter trips it shuts the gas off, the blower kicks on and then the gas comes back up. The fan times out and the cycle repeats in about 1-1/2 minutes.

The blower is controlled by a solid state timed Heatcraft controller and a Honeywell blower/limiter switch is used for the limiter switch. I haven't checked continuity across all the switches yet, but my suspicion is the Heatcraft timer is failing to switch on. Adjusting the ON delay does nothing.

Haven't figured out why triggering the limit switch engages the fan timer.

What I'm thinking of doing is connecting the blower to the Honeywell limiter switch instead of the Heatcraft timer.
 
I'm an "old" RV tech (and relying on memory) but the basics are alike.

Does your furnace have a sail switch?

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A sail switch proves air flow. If the switch is dirty, sticky or faulty, it can lie and allow the burner to run until it trips the limit switch.

Thermal fan switch?

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I suspect the normal sequence is broken and the limit provides another until it cools enough to start at the beginning.



.
 
No sail switch, it has a couple pressure difference switch. Already checked the thermal switches - all good.

It could be a pressure switch, but from what I could gather, those kill the gas. I may go back and recheck those switches again.

I'm starting to think it is the solid state switch - two wires had crimp connectors replaced and the screws don't match like someone had messed with it all before. So I went ahead with my plan of using the other fan switch. Seems to work well. Right now I just have a switch on the thermostat wires.
 
Thanks for the vote of confidence BH! :rolleyes:

I'm not going back to the shop tonight (7˚ and dropping) so no pics, but here's what the controllers are. The Honeywell was only being used as a limiter, and was set at 250 which seemed high. I backed it down to 200. The furnace is about 15 yrs old, but the heat exchanger was checked before it was removed a couple years ago.
 

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Normally Id say change the honeywell fan switch because it turns on and off the fan but if they are only using it as a limit then the other switch is being used a fan sequencer and timing out before the hi limit is reached.So either change the sequencer or rewire it to be run through the normal fan switch which may be bad and thats why the seq was added.
https://www.youtube.com/watch?v=jg0nK4QLpZo
 
I don't know much about this stuff, but I do know you don't cut the red wire,...... or is it the blue one?
 
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SS - Both parts are stock and wired as original according to the wiring schematic on the door, but yeah, that's how they were using them. And that's what I did - put the fan through the Honeywell. I just need to double check to make sure I haven't somehow bypassed a safety circuit.

Thanks fellas!

BH - I had Brown and Yellow wires.... so I cut them both!
 

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