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starcraft69

northern California(State Of Jefferson)

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Posted: 05/22/19 08:37am Link  |  Quote  |  Print  |  Notify Moderator

Old-Biscuit wrote:

Burner cycling on then off/back on/off is called 'limiting' which is when the airflow has been restricted and heat exchanger high temp limit switch is opening (too hot) then fan continues to run so temp drops...high temp limit switch closes and burner lights back off

You have something obstructing/blocking either the furnace discharge air flow OR the return air to furnace
Could be closed/blocked register, could be crushed ductwork, could be how/where stuff is stored


Fan runs cause high temp limit switch opened.burner keeps firing back off cause thermostat temp set point has not been met....high temp limit switch controlling burner shutting down/relighting



Listen to this man! This is the correct answer.


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jbrack

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Posted: 05/22/19 09:12am Link  |  Quote  |  Print  |  Notify Moderator

starcraft69 wrote:

Old-Biscuit wrote:

Burner cycling on then off/back on/off is called 'limiting' which is when the airflow has been restricted and heat exchanger high temp limit switch is opening (too hot) then fan continues to run so temp drops...high temp limit switch closes and burner lights back off

You have something obstructing/blocking either the furnace discharge air flow OR the return air to furnace
Could be closed/blocked register, could be crushed ductwork, could be how/where stuff is stored


Fan runs cause high temp limit switch opened.burner keeps firing back off cause thermostat temp set point has not been met....high temp limit switch controlling burner shutting down/relighting



Listen to this man! This is the correct answer.




This makes the most sense to me...and has been suggested to me before but none of the mechanics who have looked at the furnace have investigated this. None of my inside vent are blocked or restricted. I can see most of the ductwork from inside the belly but cant see the duct that feeds the one floor vent in the extreme rear of the coach. My next question is...if I continue to operate the furnace in this on/off fluctuating mode, will it cause any damage?

BFL13

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Posted: 05/22/19 09:46am Link  |  Quote  |  Print  |  Notify Moderator

People add muffin fans to their fridge vents to solve that problem if needed.

So to help with this, how about putting a fan behind the big furnace return air vent to suck more cooling air in to where that limit switch is mounted back there?


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Old-Biscuit

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Posted: 05/22/19 11:40am Link  |  Quote  |  Print  |  Notify Moderator

jbrack wrote:

starcraft69 wrote:

Old-Biscuit wrote:

Burner cycling on then off/back on/off is called 'limiting' which is when the airflow has been restricted and heat exchanger high temp limit switch is opening (too hot) then fan continues to run so temp drops...high temp limit switch closes and burner lights back off

You have something obstructing/blocking either the furnace discharge air flow OR the return air to furnace
Could be closed/blocked register, could be crushed ductwork, could be how/where stuff is stored


Fan runs cause high temp limit switch opened.burner keeps firing back off cause thermostat temp set point has not been met....high temp limit switch controlling burner shutting down/relighting



Listen to this man! This is the correct answer.




This makes the most sense to me...and has been suggested to me before but none of the mechanics who have looked at the furnace have investigated this. None of my inside vent are blocked or restricted. I can see most of the ductwork from inside the belly but cant see the duct that feeds the one floor vent in the extreme rear of the coach. My next question is...if I continue to operate the furnace in this on/off fluctuating mode, will it cause any damage?


YES!
Continued operation where high limit switch is 'controlling' the burner cycle can result in damage to the HEAT EXCHANGER.....cracks/warping---gasket leaks. That then results in CO Leakage into the INSIDE of RV,
CO is the 'silent killer'.

Every RV Furnace has a 'minimum ducting' requirements AND Return air requirements.
4" flex duct = 12 sq/ft (2" duct is NOT 1/2 of 4"---only 2 sq/inch)
16K/20K btu furnace---minimum 24 sq/inch (2 ducts)
25K/31K btu furnace---minimum 36 sq/inch (3 ducts)
35K/40K btu furnace---minimum 48 sq/inch (4 ducts)

Return Air......respectively
33 sq/inches, 65 sq/inches & 80 sq/inches



Furnace overfiring is another means of high limit switch cycling.
LP System pressure set too high will cause overfiring.
LP System should be set to 11"WC with at least 50% of propane appliances in service.



And then there is Thermostat Location
Does a register blow towards thermostat?
Is warm air coming thru hole where wires to thermostat exit the wall?


Is it time for your medication or mine?


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Old-Biscuit

Verde Valley

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Posted: 05/22/19 11:46am Link  |  Quote  |  Print  |  Notify Moderator

BFL13 wrote:

People add muffin fans to their fridge vents to solve that problem if needed.

So to help with this, how about putting a fan behind the big furnace return air vent to suck more cooling air in to where that limit switch is mounted back there?


High limit switch is a safety device that SHUTS down burner because heat exchanger is overheating.
Fan cooling area where high limit switch is located does NOT resolve the overheating issue.
Lack of sufficient air flow THUR heat exchanger is the issue...NOT that the high limit switch is actually doing it's job as intended.

Find/fix REASON high limit switch temp set point is being met.

jbrack

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Posted: 05/24/19 07:49am Link  |  Quote  |  Print  |  Notify Moderator

Old-Biscuit wrote:

BFL13 wrote:

People add muffin fans to their fridge vents to solve that problem if needed.

So to help with this, how about putting a fan behind the big furnace return air vent to suck more cooling air in to where that limit switch is mounted back there?


High limit switch is a safety device that SHUTS down burner because heat exchanger is overheating.
Fan cooling area where high limit switch is located does NOT resolve the overheating issue.
Lack of sufficient air flow THUR heat exchanger is the issue...NOT that the high limit switch is actually doing it's job as intended.

Find/fix REASON high limit switch temp set point is being met.


All of my duct work appears normal...the exhaust does not have an obvious obstruction when I look in with a flashlight. Would adding an additional floor vent help? I have a total of 4 vents now.

BFL13

Victoria, BC

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Posted: 05/24/19 08:09am Link  |  Quote  |  Print  |  Notify Moderator

Old-Biscuit said in my Atwood: (see link in my post after the OP)

"Mid...2 White Wires going into motor cover....Sail Switch, when it closes the DC from Sail Switch goes to High Temp Limit Switch and then to circuit bd via connector"

In the OP you said after they changed the main board it worked for a year until they changed the sail switch.

So the suspect would be the wiring they did for the sail switch?

dougrainer

Carrolton, Texas

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Posted: 05/24/19 01:42pm Link  |  Quote  |  Print  |  Notify Moderator

Limit switches have a habit of failing. Since the ductwork appears open, the fix is to replace the Limit switch. Doug

dougrainer

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Posted: 05/25/19 07:19am Link  |  Quote  |  Print  |  Notify Moderator

BFL13 wrote:

Old-Biscuit said in my Atwood: (see link in my post after the OP)

"Mid...2 White Wires going into motor cover....Sail Switch, when it closes the DC from Sail Switch goes to High Temp Limit Switch and then to circuit bd via connector"

In the OP you said after they changed the main board it worked for a year until they changed the sail switch.

So the suspect would be the wiring they did for the sail switch?


Possible, but I would change the Hi Limit switch. THEN that takes the Hi limit switch out of the possible problem. It is almost impossible for a sail switch to close then open in operation(killing the signal to stay running the burner) and then close again. Flame On and Off after running for a few minutes is almost 100 percent the Hi limit switch. Usually the Hi limit on this model, it sticks OPEN and does not allow the furnace to fire. Doug

BFL13

Victoria, BC

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Posted: 05/25/19 07:53am Link  |  Quote  |  Print  |  Notify Moderator

The limit switch must be working for it to be doing that intermittent burning? It can't be stuck open. Too sensitive maybe?

Changing the main board fixed the OP's intermittent problem for almost a year so it wasn't the ducts etc. Then it would not light at all after the fan came on (limit switch stuck open now?). New sail switch fixed that but back to intermittent action. (if they didn't also change the limit switch then it must still not be stuck open)

Has the replacement board from a year ago gone bad? What was that all about with the duff set of boards?--was that furnaces or fridges, can't remember.

The limit switch might be the most likely culprit, but it is the hardest thing to get at, so IMO it is the last thing to try for a DIY. In my case it was the main board at fault, which is the easiest thing to swap out, but last on the list of things to try according to the pros. Maybe getting at the limit switch is easy for a pro! [emoticon]

Is there no warranty on this furnace in a 2018 RV ?

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