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I'm on the road currently, 3k miles in from NH to Colorado. Nights are dropping below freezing and the furnace will turn on the blower based on thermostat settings, but the burner isn't lighting. The unit is a duotherm with an electrically lit pilot, and I have the gas valve open near the unit. The fridge and stove work fine off propane. The house battery is at 12V-- I even tried running the furnace when plugged into city power but it still didn't light.

I took one trip a month ago where the furnace worked fine.  

What else can I do to troubleshoot and isolate the problem?  What does your furnace sound like?  Do you hear the distinct popping of the igniter?  Do you think it could be a failed solenoid or igniter?  How can I confirm?  Thanks!

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Does your furnace make the loud click to ignite the pilot light? 

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That whole unit looks pretty beat-up. Try cleaning all the contact surfaces. If the blower is purging the fire box but not igniting, it's either going to be the sail switch or the controller board. Either one will involve digging into the guts of the furnace.

You may want to look at a "Mr. Buddy" heater at someplace like WalMart as a temporary fix.

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All I hear is the click of the fan turning on. How many clicks do you typically hear before it lights up?

Edited by tofuricer

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You have a sight glass that allows you to look into the combustion chamber. Most furnaces will shut down after a cycle period if they sense no combustion. Leave the furnace running and see if you get heat, or it shuts itself off. If the blower does not eventually shut off on its own it's probably the circuit board. Best to take it to a furnace repair shop if your not sure what to do.

 

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1 hour ago, tofuricer said:

All I hear is the click of the fan turning on. How many clicks do you typically hear before it lights up?

You may not hear a click, if the gas valve opens they do so with a pretty good clunk then the thing should light if it does not the fan will continue to run until you turn the thermostat off. If the battery is up or it's on city power there should be enough fan to close the sail switch it then tries to light if you hear a good clunk and nothing happens chances are pretty high it's the board. 

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Thanks Maineah and Fred. 

Happy to report I have heat again. I believe the issue is a voltage issue since it started working again when I plugged in. I will measure the voltage at the battery in the morning and continue to measure until the furnace stops igniting.  

The noise I was looking for but didn't hear until tonight was a loud 5x rapidly-repeated click of the igniter which happens about 20 seconds after the fan starts.

I may try to add a boost converter when I can take mail order deliveries again as a fix, but may also just try a new sail switch. Pretty cool way of verifying airflow to gate the controller logic of the board.

Related, I have a mr buddy and 4 propane canisters for sale if Lowes won't take them back :-)

 

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Check your battery condition and also all 12v connections to the furnace (including grounds) to make sure the furnace is getting the full 12v+ when not plugged in. Check it with the engine running (as a test) to see if the alternator boosts the voltage enough.

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6 hours ago, tofuricer said:

Thanks Maineah and Fred. 

Happy to report I have heat again. I believe the issue is a voltage issue since it started working again when I plugged in. I will measure the voltage at the battery in the morning and continue to measure until the furnace stops igniting.  

The noise I was looking for but didn't hear until tonight was a loud 5x rapidly-repeated click of the igniter which happens about 20 seconds after the fan starts.

I may try to add a boost converter when I can take mail order deliveries again as a fix, but may also just try a new sail switch. Pretty cool way of verifying airflow to gate the controller logic of the board.

Related, I have a mr buddy and 4 propane canisters for sale if Lowes won't take them back :-)

 

Intermittent firing, still sounds like board to me. There are two proofing switches, one the over temp switch (NC) and the sail switch (NO) they are series wired. Down, load a circuit diagram from the furnace site and check the switches right at the board with wire colors using an ohm meter cycle the furnace a few times until it fails the reading should be some thing close to zero ohms with the fan running.

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Cleaning the sail switch will let it move easier and it will operate correctly with a little less airflow from the fan. Often times this enough ensure boondock operation through the night.

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low 12v dc voltage will make the gas valve and lighting process not work so well.  plugging into shore power should have boosted your 12v dc. Your dc cabin lighting should have gotten brighter too.   You should get the same result not plugged in but with the engine running.  After the furnace lights, the igniters are not used again but you still need 12v dc to keep the gas valve inside the furnace open.

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DanAatTheCape, you are spot on.  Unfortunately, in my recent 3 week tip from New Hampshire to Utah (and back) I wasn't plugging into shore power.  I only did it once to diagnose this issue.  As it turns out, my battery isolator seems to have failed which caused the alternator to not charge my house battery fully.  I do have 240W of solar charging the house battery, but that only moves the house battery voltage up to about 12.1V, but I need 12.4V in order for the heater's igniter.

A quick stop at Walmart, and $5 later I had a simple "switch assembly", using some 10 gauge wire, covered crimped spade connectors, and ring terminals.

Open (when engine is off)

IMG_3631.thumb.JPG.56d200c78dcf0e69e96f4281af447592.JPG

Closed (when engine is on)

IMG_3633.thumb.jpg.41680a7c905147741816f71b8d052ab1.jpg

When I got back home I replaced the battery isolator, but decided to leave the "switch" in place in case of draining the engine battery or if the new isolator fails. 

(If anyone decides to replicate this fix, I highly recommend cleaning those old rusty terminals with baking soda and a brillo pad or wire brush.  I wired it up and then when it didn't work I was really questioning my understanding of how this isolator worked.  Turned out I fully understood how the current was supposed to flow, except that part.)

 

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