mobilehomerepair.com

I couldn't help it with the title.My coleman furnace decided to puke tonight after family left from our little party.
Ok heres the details.
Coleman model # 7975C856
I have noticed the that it had been kicking the fan on for a second or 2.Maybe do this one or two times before the burner would kick on.The fan would then come on normally after the burner had warmed up the heat exchanger.The last couple of days it has tripped the upper limit ,and i had to reset it.
Tonight the upper limit was not tripped,but the burner will not come on.I have a strong pilot. the T-stat is calling for heat.I pulled the cover off the furnace and was checking things.I seem to be only getting approximatly 16v. coming from the transformer on the secondary side.According to the print on the furnace i belive it calls for 24v.
From what i have read online usually a transformer either works or it don't.Is it possible mine is just putting out to little voltage to allow the burner to come on?
I need to know were to go next with my trouble shooting if this is not the problem.
Thanks
Aaron Smith

Alright I just got it going after some time messing around.I had to help santa eat his cookies.
I don't know why ,but when i was checking the voltages I had 16.8v at everything ,but still no fire.That's why i was leaning on the bad transformer theory.I started checking again ,and this time the upper limit was giving me a voltage drop like it was open.
I took it out ,and sure enough it was now reading open.I took some LPS contact cleaner and soaked it really good.Wahla! it checks out ok.Put it back in and she fired up just like a new one.
I think i will have to be buying some service parts in the future.

Oh yeah I'm going to bed now.The kids are going to be waking me up in a couple of hours to unwrap presesnts

Well back to the same problem.The upper limit,or flame roll out switch what ever you want to call it.is finally dead.I will try and source one locally if i can.I didn't want to do it ,but i jumpered around it to get heat back in the house.
Can anyone tell me if the 16 + volts i am reading with the meter is ok?Furnace is running alright just wondered about the voltage from the transformer?

The xformer supply is 24VAC with no load .
Lift the hot side off on the secondary winding and measure across the windings.
Also look for a loose white neutral wire .
If there is 24vac then place the hot side back on and remeasure the Voltage to see what the drop is .
According to several wiring diagrams the only load there is the GAS CONTROL since the limit switches are supposed to be closed and therefore near zero ohms resistance .
The more I read these forum questions the more I'm convinced this furnace design requires careful attention to correct air filters (not too restrictive) and unblocked roof jacks , icing etc.

And it pays to put screening up around the air and exhaust pipes as birds love to nest there.

Maybe Robert can explain negative pressure measurements and why they are critical.

Hi,


That's why we always recommend manufacturers recommended filters. Correct airflow and checking roofjacks is a must in all type hvac units, not just MH's, but they are more critical and less forgiving.


Making sure roofjack caps are protected from birds is a must in all units.


Either get cap with screening or get it and add on.


This being DIY mostly forums, won't get into things too deep and there are limitations as to what info is divulged and/or allowed.



Thanks,
Robert

philly wrote:The xformer supply is 24VAC with no load .
Lift the hot side off on the secondary winding and measure across the windings.
Also look for a loose white neutral wire .
If there is 24vac then place the hot side back on and remeasure the Voltage to see what the drop is .
According to several wiring diagrams the only load there is the GAS CONTROL since the limit switches are supposed to be closed and therefore near zero ohms resistance .
The more I read these forum questions the more I'm convinced this furnace design requires careful attention to correct air filters (not too restrictive) and unblocked roof jacks , icing etc.

And it pays to put screening up around the air and exhaust pipes as birds love to nest there.

Maybe Robert can explain negative pressure measurements and why they are critical.

I too agree on the air filters.I believe the style i am running is too restrictive for this unit.The roof jack is in good operating condition,and has performed well over the years.
I didn't think of checking the transformer with out any load on it.that would acount for the voltage drop.
thanks for the great info guys

Hi,

Load or not, you'll still need and have 24vac from transformer to gas valve.


Thanks,
Robert