New Furnace Issue

Hello. I have a 2017 Mallard M33, dometic touch thermostat, and i would assume a dometic furnace (although it was not marked as such). Last night i wanted to test the furnace prior to our upcoming camping trip as we have never used the heat before. Connected to good power with 2 full bottles of propane, the furnace fired right up. After checking the output, I noticed no heat was coming out of one vent and the cabinet was quite warm. I removed the panel and found that apparently the factory either didn't connect the duct or it had come loose in our previous travels. I turned the furnace off via the t-stat, let it cool, then hooked up the duct. I attempted to test again and now the furnace won't start. When I turn the temp up so that the t-stat calls on heat mode, I hear a click from the area of the roof A/C. Likewise, if I turn the temp back down on heat mode, I hear another click from the area of the roof A/C. Nothing else was moved or touched between the two cycles of the furnace.

Troubleshooting steps:
- I double checked the gas, Still on and confirmed by starting stove.
- Thermostat has power and is displaying temp.
- Used the t-stat to turn on the A/C, that works fine
- Grabbed my multimeter -- power at the t-stat (as mentioned), same power at the furnace coming from the t-stat
- Checking power by the furnace, I have power up to and through a cut off switch. After the switch is a circuit board with a red LED. The LED is off (not sure if it should be on) and I don't appear to have power passing this point.

Seems the issue may be the board, but it was weird that it worked 10 minutes prior and failed after only it's first use. I was unable to find any wiring diagrams to aid in troubleshooting. Any help would be greatly appreciated.

Hi whitebiker,

Welcome to the Heartland Owners Forum.

The click overhead is a relay that sends 12V DC to the furnace when the thermostat calls for heat. That starts the blower motor when things are working correctly.

You didn't mention checking the fuses, so I'd suggest verifying the 12V DC blade-type fuse that supplies power to the furnace via that overhead relay. If the fuse looks ok, check with your meter. If that's ok, you might check for wires to the furnace that may have been disturbed while working on the duct. There are usually several wirenuts connecting the furnace wiring to the trailer wiring. If the wire from the relay to the furnace came loose, that would interrupt power to the blower and board.

Yes, sorry, I forgot to mention that part in my initial post. I pulled the furnace fuse and verified with the meter, it is good. I will double check the wiring again in a couple hours, although I will say I traced them to no avail last night. Thanks for your thoughts.

danemayer said:

Hi whitebiker,

Welcome to the Heartland Owners Forum.

The click overhead is a relay that sends 12V DC to the furnace when the thermostat calls for heat. That starts the blower motor when things are working correctly.

You didn't mention checking the fuses, so I'd suggest verifying the 12V DC blade-type fuse that supplies power to the furnace via that overhead relay. If the fuse looks ok, check with your meter. If that's ok, you might check for wires to the furnace that may have been disturbed while working on the duct. There are usually several wirenuts connecting the furnace wiring to the trailer wiring. If the wire from the relay to the furnace came loose, that would interrupt power to the blower and board.

Click to expand...

Btw, the furnaces are usually from Suburban.

There's a troubleshooting guide written by Gary Brinck that's in this folder.

Just to follow up - I pulled the board located near the furnace, which was accessed from the external panel. CW tested it and said it failed 3 out of 5 times. New board was installed and the system is functional.

Good that the problem has been fixed. Our Suburban furnace board quit after about 8 years. BTW, the photo is of a Dometic furnace. As Dan said, the HL lines are typically Suburbans, even though the OEM thermostat may be from Dometic. They normally have two external ports (intake and exhaust) on the outside to the access panel. Check the paperwork that came with your rig, it should have the manual for the furnace (or at least tell you what it is).

I just had to replace the control board in our 2012. Interesting note: Camping World wanted $67.50 to test the old one and $126 with Good Sam, for the board, but the closest one was in Raleigh (1.5 hrs). I called a local shop and they had the board for the same price and tested the old one for free. I bought!