Coleman heater keeps shutting off

[chivato06]

Hi I have a circa 1980 Mobile home...the furnace has had a AC update installed about 15 years ago, however the question i have is about the furnace ...its been working fine up till this year ..I light the pilot all seems well ,then it runs nice with a blue large flame then I hear a click like a relay and it shuts down...after about 30 seconds or a minute....light it again same thing ...used to just lose the main flame now the pilot also shuts down ....help!..... is there any specs to test the thermocouple, I have a Fluke VOM.I am an Aircraft Mechanic so I have some skills ....with mechanical ......thanks for your help Craig ,,, latest update I have learned that a 2 wire round switch is the part that cuts off the system it has a orange and black wire mounted on the left side of the housing..looks like it has a temp disc in it .. help

[Robert]

Hi Craig,

What is the complete furnace model number ? Is the limit that is tripping up next to or attached to the blower housing ?


Or is it about midway of the furnace ?




Thanks,
Robert

[chivato06]

Hi Robert..its a model 7656b series 856 , the limit switch? is inside the panel on the left side I see 2 one on the bottom and one above it both have 2 wires and have 2 screws and are round..I can make the unit work and blow hot, if I connect the 2 wires on the upper limit switch , however it doesnt shut down when you turn the themostadt to a low temp like 60...the flame goes to an idle ,but blower keeps going until you actually turn the on off switch off Craig

[Robert]

Hi Craig,


The top switch is a fan Switch. The bottom is a lower limit switch.



Fan Switch # 7975 - 3281 can be seen and ordered here by paging down :

http://www.aberdeenhomerepair.com/store ... -7975-3281



IF you need or wish to get the lower limit, after adding fan switch to cart, type in p/n 7680 - 3591 into the search window to see and order limit.


Sounds like maybe the fan switch is opening and turning the fan off which in turn causes limit to turn gas off.


I would also inspect flue and roof jack for leakage or damage or clogs. Inspect burner flame for any dancing of the flame.

Inspect burner orifice/spud behind the gas valve for any blockage from spider-web-bugs.

Do all the checks and if still not working correctly once all is ok, then order the parts and see if that resolves it.



Thanks,
Robert

[chivato06]

Hi Robert .....thanks for helping ...what exactly is the roof jack?....do I neeed to pull the gas valve off to inspect orifice / spud?...someone said the burner is under gased...caused by valve control..sound likely and why?..main flame is kinda wiggly but blue with some yellow tips..I noticed when I hit the gas valve (slight tap) while flame is going ,it gets a lot of yellow in it for a second or two..how does the fan switch operate?...heat?...or pressure ?........thanks for your skills.......Craig

[Robert]

Hi Craig,

Roof Jack = Furnace chimney/roof cap on roof.

Could be gas valve, low gas pressure or the orifice blocked. Most common is the orifice.


Some you remove the gas valve and others you don't. The orifice/spud is between back of GV and the burner, so you can see if you need to remove the gas valve and/or burner assembly to get to it.


There should be no to very little yellow at the tip.


Thanks,
Robert

[chivato06]

thanks Robert...going over to the house and inspect today , we dont get snow here so it would have to be leafs or something else on the roof vent....still trying to figure the nomenclature of that switch.....Craig have a good day......get back to you asap

[chivato06]

Hi Robert......after pulling off the gas valve ...I cleaned the spud , looks like a jet in a carb....the orifice was smaller than the actual brass opening.... cleaned some deposits off and put it back together...seems to stay on now and warmed the house up ...thank you for the help ...you saved us ...Craig .....really appreaciate your help.....

[Robert]

Hi Craig,

You're very welcome, happy to help. Sorry I missed the fan switch question. It closes when the heat chamber reaches a predetermined temp to allow air to warm up before being distributed through home.


Then, when burner shuts down, it keeps blowing to cool unit down to the off temp.


Thanks,
Robert