Furnace blows cold air after 45 minutes

[Stewfish]

1996 Suburban furnace model SF-42F


I have read and read and can't find this scenario

If it was the whole thing turning off I'd say it was a safety switch somewhere. If the fan didn't blow I'd say it was the sail switch. If a thermostat was bad and acting like it was actually warm enough, then I think the whole unit would go into standby and not blow the fan.

Scenario:
It works fine for 45 minutes and then it starts blowing cold. Everything works but its just cold air coming out. It does this when its cold in the RV. It happened last year so we used electric heat. Now that we are off grid on our property I need to fix it asap before winter.

Fridge and everything else on propane works fine

I have 4 batteries that charge from solar

Thanks everyone!

[Old-Biscuit]

When it starts blowing cold air (after blowing hot/warm air) does the Thermostat Open (reaches temp set point) and fan just keeps running???

Yes....either bad TDR (timed delay relay) or Control Module (circuit board)
*Depends on Model YEAR

NO....thermostat has not opened/temp set point not reached, fan runs but gas vlave closes-------bad control module


***when fan just blows cold does it restart if you turn thermostat off back on??

[Stewfish]

Originally Posted by Old-Biscuit

When it starts blowing cold air (after blowing hot/warm air) does the Thermostat Open (reaches temp set point) and fan just keeps running???

Yes....either bad TDR (timed delay relay) or Control Module (circuit board)
*Depends on Model YEAR

NO....thermostat has not opened/temp set point not reached, fan runs but gas vlave closes-------bad control module


***when fan just blows cold does it restart if you turn thermostat off back on??

Its a 1996

It turn off when cold. I have not looked inside the thermostat to see if the old school spring opens

I have been able to reset the heat for a short period when switching the thermostat on and off

[Ljwt330]

Originally Posted by Stewfish

Suburban furnace model SF-42F


I have read and read and can't find this scenario

If it was the whole thing turning off I'd say it was a safety switch somewhere. If the fan didn't blow I'd say it was the sail switch. If a thermostat was bad and acting like it was actually warm enough, then I think the whole unit would go into standby and not blow the fan.

Scenario:
It works fine for 45 minutes and then it starts blowing cold. Everything works but its just cold air coming out. It does this when its cold in the RV. It happened last year so we used electric heat. Now that we are off grid on our property I need to fix it asap before winter.

Fridge and everything else on propane works fine

I have 4 batteries that charge from solar

Thanks everyone!

If this is a relatively new problem, first appearing last year, I'd suspect the circuit board.

The furnace burner is shutting down, and could be it reached temp or the overheat switch is bad. Regardless, it should shut down completely. That failure, IMO, points to a circuit board problem.

Question. Can you manually shut it off and does it then begin heating again when you turn it back ON?

[Stewfish]

Originally Posted by Ljwt330

If this is a relatively new problem, first appearing last year, I'd suspect the circuit board. The furnace burner is shutting down, and could be it reached temp or the overheat switch is bad. Regardless, it should shut down completely. That failure, IMO, points to a circuit board problem.

It was a new to me rv last year.

I'm thinking just spend the money and get parts and then I will have spares besides what ever was bad. What should I order?

Sail switch
Board
Over heat switch
Over temp relay (is this the same as over heat switch?)
Time delay relay
Limit switch

[Stewfish]

How many over temp switches does it have?

[Ljwt330]

Originally Posted by Stewfish

It was a new to me rv last year.

I'm thinking just spend the money and get parts and then I will have spares besides what ever was bad. What should I order?

Sail switch
Board
Over heat switch
Over temp relay (is this the same as over heat switch?)
Time delay relay
Limit switch

Old-Biscuit is the expert on these matters and I would either wait for him to respond, or you may contact him.

[Old-Biscuit]

Normally I do not like 'throwing parts' at a problem
But on a 1996 Vinatge furnace I would

If just replacing parts these are the ones I would do on a 1996 Vintage Furnace
*Sail switch (it is a micro switch with small contacts)
*Board (Get the Dinosaur Electronics Fan50 board!!!)
*Time delay relay (internal coil fatigues from heat/cool cycles)

These 3 are same item...different names
Over heat switch
Over temp relay
Limit switch
**replacing limit switch is good idea for 1996 vintage due to heat exposure for past 25+yrs


Parts Tune Up KITS
https://pdxrvwholesale.com/products/...2f-tune-up-kit


Clean up the air & combustion wheels and if you really want to get into it....pull heat exchanger and wash it out
Pull burner assembly and check condition of burner and electrode



That old analog thermostat
Replace it with a Digital Thermostat'
Hunter/Honeywell/Lux etc non-programable/uses batteries for display
Home Depot/Lowes/ACE/Walmart etc

Very easy especially if thermostat is just for furnace
See this Link
https://heartlandowners.org/threads/...dge-m18.27348/

IF thermostat controls Furnace AND A/C Unit
See this LINK
RV Digital Thermostat Installation / Coleman and Suburban




Course.....Parts are expensive when purchased individually
Whole NEW SF42F......$780
https://www.adventurerv.net/suburban...us-p-1088.html

[Stewfish]

Originally Posted by Old-Biscuit

Normally I do not like 'throwing parts' at a problem
But on a 1996 Vinatge furnace I would

If just replacing parts these are the ones I would do on a 1996 Vintage Furnace
*Sail switch
*Board
*Time delay relay

These 3 are same item...different names
Over heat switch
Over temp relay
Limit switch
**replacing limit switch is good idea for 1996 vintage


Parts Tune Up KITS
https://pdxrvwholesale.com/products/...2f-tune-up-kit


Clean up the air & combustion wheels and if you really want to get into it....pull heat exchanger and wash it out
Pull burner assembly and check condition of burner and electrode



That old analog thermostat
Replace it with a Digital Thermostat'
Hunter/Honeywell/Lux etc non-programable/uses batteries for display
Home Depot/Lowes/ACE/Walmart etc

Very easy especially if thermostat is just for furnace
See this Link
https://heartlandowners.org/threads/...dge-m18.27348/

IF thermostat controls Furnace AND A/C Unit
See this LINK
RV Digital Thermostat Installation / Coleman and Suburban

Sounds good

I agree might as well put in a little maintenance for under another $100 in parts beside the board.

[Stewfish]

Originally Posted by Old-Biscuit

Normally I do not like 'throwing parts' at a problem
But on a 1996 Vinatge furnace I would

If just replacing parts these are the ones I would do on a 1996 Vintage Furnace
*Sail switch
*Board
*Time delay relay

These 3 are same item...different names
Over heat switch
Over temp relay
Limit switch
**replacing limit switch is good idea for 1996 vintage


Parts Tune Up KITS
https://pdxrvwholesale.com/products/...2f-tune-up-kit


Clean up the air & combustion wheels and if you really want to get into it....pull heat exchanger and wash it out
Pull burner assembly and check condition of burner and electrode



That old analog thermostat
Replace it with a Digital Thermostat'
Hunter/Honeywell/Lux etc non-programable/uses batteries for display
Home Depot/Lowes/ACE/Walmart etc

Very easy especially if thermostat is just for furnace
See this Link
https://heartlandowners.org/threads/...dge-m18.27348/

IF thermostat controls Furnace AND A/C Unit
See this LINK
RV Digital Thermostat Installation / Coleman and Suburban

I saw the tune up kit, whats the reasoning for it? What does the gasket seal up?

[Stewfish]

Originally Posted by Old-Biscuit

Normally I do not like 'throwing parts' at a problem
But on a 1996 Vinatge furnace I would

If just replacing parts these are the ones I would do on a 1996 Vintage Furnace
*Sail switch (it is a micro switch with small contacts)
*Board (Get the Dinosaur Electronics Fan50 board!!!)
*Time delay relay (internal coil fatigues from heat/cool cycles)

These 3 are same item...different names
Over heat switch
Over temp relay
Limit switch
**replacing limit switch is good idea for 1996 vintage due to heat exposure for past 25+yrs


Parts Tune Up KITS
https://pdxrvwholesale.com/products/...2f-tune-up-kit


Clean up the air & combustion wheels and if you really want to get into it....pull heat exchanger and wash it out
Pull burner assembly and check condition of burner and electrode



That old analog thermostat
Replace it with a Digital Thermostat'
Hunter/Honeywell/Lux etc non-programable/uses batteries for display
Home Depot/Lowes/ACE/Walmart etc

Very easy especially if thermostat is just for furnace
See this Link
https://heartlandowners.org/threads/...dge-m18.27348/

IF thermostat controls Furnace AND A/C Unit
See this LINK
RV Digital Thermostat Installation / Coleman and Suburban




Course.....Parts are expensive when purchased individually
Whole NEW SF42F......$780
https://www.adventurerv.net/suburban...us-p-1088.html

Do old analog thermostats go bad? It seems like they are bomber proof and it is currently working? I'm debating what the pros are for upgrading to a digital thermostat. Besides a digital display I can't think of any benefit, and if the batteries go out then there is no heat

Now if there was bluetooth of wifi to run it from an app on my phone in bed then that would be worth it

I got on chat with Honeywell and they said the T9 wifi thermostat will work with the two wires in an RV if you get a 24v transformer to power it (for the screen and wifi module).

[Old-Biscuit]

Analog t-stat
HUGE temp switch before it turns on...turns off

Digital...2* switch overall
1* above set point to turn off
1* below set point to turn on

Non-programable uses AA battery for the display.....last for YEARS
12VDC from trailer battery is power source for the furnace...same as analog

I replaced ours in 2007 and have had no isses with....love the tight temp control also


Gasket kit
Anytime you pull the burner assembly you should replace the burner access gasket

[Stewfish]

Well I got the parts in and took the unit out. When I took off the propane office tube I noticed that the orifice was a little clogged so I blew it out from the orifice side and a bunch of little no-see-em type looking dead things came out so I bet the orifice was just clogged just enough to start but not keep cycling well. However since I had all the parts and it's a pretty old unit I figure it might as well do some preventative maintenance and then now I have some spares (if everything was working) in case something does happens in the future.

The burner has cycled on and off a dozen times and seems to be running well for an hr so far

Thanks for all the help guys!