Random Furnace Issue

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bermuda95

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Oct 30, 2010
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Hello folks.  I've read through some of the other threads on furnace troubleshooting but have not located anything that specifically describes this random/sporadic problem.

The furnace works sometimes, but not others.    The blower kicks on each time.  But sometimes there is heat and sometimes there isn't.
After a period of time working it stops providing hot air.  The blower keeps up though.  I shut it down at the t-stat and the some time later
it begins to work again.

This just started so I don't have exact times on how long it runs before failing to provide heat and how long it needs to be off before it ignites
again.  In in the middle of trying to diagnose that. 

There is plenty of propane and the two house batteries are fully charged.  This is a Hydroflame 8531 unit...

The last go-around went like this.  Last night about 6pm was working fine.  Tested through the ignition phase successfully.  Kept system running.  About 7pm noticed that there was no hot air again.  Shut it off at t-stat gave it about 90 minutes, tried again at the t-stat, did not ignite.  Just blower and cool air.  Left it off for night.  Used electric space heater.  Tried this morning at 7am or so and it started right up...Been running for almost an hour ok.  I expect it to go out again though.  Wondering if goes out after shutting off/trying to start up again or if it just stops pushing hot air while the blower is still running.....

Any advice is appreciated.

Thanks Chucky
 

bermuda95

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Oct 30, 2010
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update:  So it appears that it runs ok for about an hour.  It shuts off as part of it's normal cycle per the t-stat setting.  But then it just doesn't
ignite.  Just the blower and cool air.  Not sure how long it takes of rest before it'll ignite again.  But I'm timing that now....

Thanks.
 

DearMissMermaid

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My friend had a similar problem in the fifth wheel he bought.

His would come on, eventually blow out hot air, then cold air, then hot air and so on. The fan never shut down but the heat would be sporadic.

He finally got the dealer back out who claimed that is the way it was made... to run the fan nonstop whether the heat and flame was kicking in or not. So his furnace actually did work, just that the fan didn't cycle on and off.
 

Great Horned Owl

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It sounds like it might be caused by the sail switch not being activated. This would be from insufficient air flow due to low voltage at the blower motor, or the motor bearings going bad, or an obstruction in the ducts.

Joel
 

Alfa38User

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DearMissMermaid said:
snipped  .......

He finally got the dealer back out who claimed that is the way it was made... to run the fan nonstop whether the heat and flame was kicking in or not. So his furnace actually did work, just that the fan didn't cycle on and off.

Without knowing the furnace brand involved, that would difficult to to check but... From both Atwood (Hydroflame) and Suburban documentation I have on hand, that could happen if the thermostat was continuously demanding heat. If the thermostat drops its request after reaching the set temperature, then the furnace should stop after a timed cool down sequence. This odd condition might be caused by the furnace cycling on its internal overheat detector rather than on the thermostat perhaps, not sure of that though.

To the OP: I wonder if the flame detector/igniter could be involved, as in somewhat 'intermittent'.

When you get a failure to light, did you hear the igniter clicking (trying to light) and then a failure or no clicking, just the blower??.... Can you smell propane out at the exhaust after a failure to ignite?? You will have to FAST to detect these unless you just standing around waiting as lighting is attempted about 15 seconds or so after the fan starts... LOL

If the Hydroflame furnaces fail to ignite after 3 attempts (clicks) the furnace ignition is locked out for 1 hour according to my docs. (How accurate the actual 1 hour timing is is a question though!!) This timing sequence will be interrupted if the thermostat is lowered removing the request for heat and then reset so heat is again requested. Suburban documentation seems to agree with this info.
 

bermuda95

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Oct 30, 2010
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So it's been over 2 hours since it stopped blowing hot air.  I've had the t-stat shutoff for the 2 hours.  Just turned the t-stat back on and
after about 10 secs the blower started.  I put my ear right next to the furnace unit and waited for about 2-3 minutes but did not hear any "clicks".  So it seems there is no attempt to ignite the unit.  Again plenty of propane and battery charge.  I also had a burner going on the
stove, flipped the t-stat on, watched the burner flame and did not see it diminished in any way.  I read somewhere this is how you can determine if it's a regulator issue.   
 

John From Detroit

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There are several suspects in a case like this but SUSPECT #! is the spark gap on a Direct Spark Ignition furnace.. On mine it was 2x the specified setting.

On the oil fired furnace in my parents house 4x

Both acted just like yours.  (Well a bit difrferen on the sticks and bricks but that is due to a different operating sequence).

It might also be a failing solenoid, or a partially clogged oriface so do a full Routain maintenance cleaning and polishing the burner, oriface and check the spark gap, Clean the flame snesor rod (Which may be oneo of the spark rods) and make sure the connections, epically the one on the flame sensor (May also be ignition) wire are good.
 

Gary RV_Wizard

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Please read the Furnace Troubleshooting document in the RVForum LIBRARY, so that you understand the way the furnace is supposed to operate. It is, I think, an easy read.

There are a couple things that could cause these symptoms and the Sail Switch is one of them. It's purpose is to validate adequate air flow and it will not allow the furnace to ignite if it is not moving enough air, both for combustion and to circulate heated air through the RV.  Another is the High Limit switch, which prevents ignition if the heat exchanger box is too hot. A defective High Limit could have these symptoms. Some models have a separate fan control board that could also be defective, but that's rare (if only because the board is so simple).
 

bermuda95

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Oct 30, 2010
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Thanks folks.  We used a space heater to get us through our last camping trip of the season.  I had a mobile rv service come to the campground when the problem first occurred and of course the problem
was not happening then so not much could be done unfortunately other than clean the circuit board, plugs and contacts.  I didn't know it was a random issue at that point.  From what I have read here and
learned from talking with folks it sounds like it's either the sail switch or the high limit switch.  I checked all the ducts and return air intake to make sure there is no blockage.  I'll bring it to the dealer at some
point over the coming weeks to get diagnosed and repaired.  I'll update the thread when I find out what the problem actually is. 

Thanks for all your thoughts..

Chucky
 

Water Dog

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Sumner, TX
If it is the sail switch issue, it could be as simple as the squirrel cage being not tight on the motor shaft. First thing I would check if you wind up having to pull it out anyway.
 

John From Detroit

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Regarding the high limit switch... If the Thermostat (For that is what it is) has failed, yes, that could be it, you can bypass to test

However if it's sensing a real high limit... You have heat.
 
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