Still can't figure it out, furnace

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rickther

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Joined
Aug 19, 2022
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9
Location
WILLOW CREEK
So my furnace went out ( fan comes on runs 30 seconds, but won't light).
Bought new furnace, still the same.
bought new regulator for propane, still not working.
bought new converter, still not working.
what am I missing.
2014 dutchman rainier
everything else works great.
I do the work myself since I live in it and it is semi perm.
has proper gas pressure. (Manometer)
obviously getting electric ( fan runs great, for 30 seconds)
Had new furnace checked by tech in Redding and it checked out fine. (bench test).
I am at the end of nerves.
And before someone says it, yes the switch is on.
 
Did the igniter work during the bench test, and now long did the furnace run on the bench? If the furnace was not operated/run while on the bench, you really didn't see a bench test.
If the furnace ran properly on the bench but doesn't light installed in the trailer. It has to be a wiring issue in the trailer wiring
 
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Does the igniter do the "click-click-click.." trying to light the gas (also check volts on gas valve)? If not, then maybe the fan is not pushing enough air through the vents, you can check if the sail switch closes.
Fan needs full 12 - 13VDC to run up to proper speed.
 
There are a couple of files in the forum library that describe furnace operation step by step from "The Thermostat calls for heat" all the way to "The blower shuts off"

In addition to the make and model..... Where in that sequence of events. Does it fail.
 
rickther said:
So my furnace went out ( fan comes on runs 30 seconds, but won't light).
Bought new furnace, still the same.
bought new regulator for propane, still not working.
bought new converter, still not working.
what am I missing.
2014 dutchman rainier
everything else works great.
I do the work myself since I live in it and it is semi perm.
has proper gas pressure. (Manometer)
obviously getting electric ( fan runs great, for 30 seconds)
Had new furnace checked by tech in Redding and it checked out fine. (bench test).
I am at the end of nerves.
And before someone says it, yes the switch is on.
Click to expand...
The fan is running the purge cycle and then when the furnace for any number of reasons detects conditions aren't optimal for ignition it shuts down. Ordinarily it will try again before needing to be reset. The simplest and most common reason for this is the sail switch isn't closing. When that happens the furnace board ( brain) thinks the blower fan isn't turning (the sole function of the sail switch). So perhaps start there and eliminate that as a possibility first. If you can, short the sail switch wires to fool the board, if the furnace lights , there's the problem. If not the next most likely would be the flame sensor/ignitor and after that the board itself **make sure all the connections to the board are tight** in any case.
 
Since the problem has followed to a new furnace, I thought of this fix from some years ago. I trouble shot a Suburban RV furnace that would start and light, but after 5 or 6 minutes would shut off well before reaching the set temperature. After eliminating the ignitor board as a problem, I tried shorting the two thermostat wires together and found the furnace now stayed lit as long as I kept the wires connected. A new thermostat fixed the problem.
 
There are several possibilities that could cause the same issue with different furnaces. Have you monitored the voltage supply as it was running yet? Low voltage will cause the blower to run slow and if too slow the sail switch does not close and so the ignition doesn't fire and the gas valve does not open. Another possible cause could be some of the ductwork being blocked, crushed, or somehow restricted and preventing enough air movement.

It is possible, but very unlikely that both furnaces would have the same problem so look for things that are in the RV. The 12V side needs to have about 11V or a little more when the blower is running. A really bad battery could drag that voltage down, even if the converter is working properly. The fact that the blower starts would seem to say that the thermostat is calling for heat as that is what signals the blower to run, so that is unlikely. That can be checked by connecting the two blue leads together at the furnace, since the thermostat basically does that to call for heat. When connected the blower should start immediately followed by the valve opening and ignition probe sparking a few seconds later. The furnace should then operate continuously until the blue leads are separated again.

Am I right that you have a Suburban furnace?
 
When the blower starts, can you feel air flow from the vents? A blockage between the furnace and the outlet vents will keep the sail switch from opening and letting the gas flow.
 
I think the key here is that the problem continues even with a new furnace. That clearly points to a problem in the RV itself. Candidates are:
  • Low voltage - the fan doesn't spin fast enough to engage the sail switch
  • Poor air flow thru the inlet or out thru the ducts, again inhibiting the sail switch from closing
  • Poor combustion air intake/exhaust, preventing ignition
  • Poor gas pressure (too low or too high).
A critical piece of diagnostic info is whether the furnace attempts ignition or not, i.e. the sparker goes snap-snap-snap. If it does, the sail switch is closing and the first two possibilities are eliminated.
 
Poor airflow through the inlet or out through the ducts has nothing to do with the sail switch other than if the main blower isn’t turning, the blower for the sail switch isn’t either ( they’re on the same shaft). That’s the reason for the sail switch, to tell the board the blower is or isn’t functioning.
 
One thing I have found in 2 out of Two furnaces I've worked on (One Sticks and one RV)

The point spacing on the Furnaces both of them.... This is not to scale but it was supposed to be
Like: > < And it was more
like: > <

Adjusted per the manual and function was much better. Both were new
 
Onyrlef said:
Poor airflow through the inlet or out through the ducts has nothing to do with the sail switch other than if the main blower isn’t turning, the blower for the sail switch isn’t either ( they’re on the same shaft). That’s the reason for the sail switch, to tell the board the blower is or isn’t functioning.
Click to expand...
Insufficient airflow though the vents will stop the sail switch from operating. The system needs a certain air flow to allow the switch to close.
 
gwinger said:
Insufficient airflow though the vents will stop the sail switch from operating. The system needs a certain air flow to allow the switch to close.
Click to expand...
That sail switch is inches from the fan and that fan has nothing to do with airflow (circulation) its only purpose is to move the sail on the switch. If it doesn't move the sail or the board doesn't know it's moving the sail ( bad switch or bad board) the system takes that to mean the blower fan ( on the opposite end of that shaft) which circulates combusted air is not functioning.
 
Onyrlef said:
That sail switch is inches from the fan and that fan has nothing to do with airflow (circulation) its only purpose is to move the sail on the switch. If it doesn't move the sail or the board doesn't know it's moving the sail ( bad switch or bad board) the system takes that to mean the blower fan ( on the opposite end of that shaft) which circulates combusted air is not functioning.
Click to expand...
The fan is what 'blows' the air through the ducts (circulation). If the fan is not moving enough air, the switch will not "make". The switch tells the board that there is adequate air flow to allow the furnace to light.
Both fans run a the same speed. If anything is blocking either the blower,circulating fan, or the the combustion exhaust fan it will cause a reduction in air flow.
 
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gwinger said:
The fan is what 'blows' the air through the ducts (circulation). If the fan is not moving enough air, the furnace will overheat. The switch tells the board that there is adequate air flow to allow the furnace to light.
Both fans run a the same speed.
Click to expand...
There’s no combustion exhaust fan.
 
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Onyrlef said:
That sail switch is inches from the fan and that fan has nothing to do with airflow (circulation) its only purpose is to move the sail on the switch. If it doesn't move the sail or the board doesn't know it's moving the sail ( bad switch or bad board) the system takes that to mean the blower fan ( on the opposite end of that shaft) which circulates combusted air is not functioning.
Click to expand...
The circulation air and combustion air paths are separate but the two fans are driven by a single motor on a common shaft. If the circulation air is meeting a lot of resistance, the fan speed slows and both circulation & combustion air paths are affected. In essence, the sail switch is measuring the speed of both fans.
 
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