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Atlee · 11-02-2021, 04:45 AM

During the weekend of Oct 23, the furnace stopped working on my 2021 Coachmen Freedom Express 20SE. The 20SE is a relatively small noslide 6000# GVWR, 24.5' long trailer. We really liked the floorplan, which is new in the 20SE model..

I bought it from a dealer in Florida in February as it was the only one in existence at the time. It didn't start showing up on dealer lots until this past summer.

The problem was the furnace bower was come on for 30 seconds and then stop. Propane would never ignite. Fortunately for us, General RV Center here in Ashland, VA ,who is a Coachmen and Dometic (Atwood) authorized sales/service dealer, found time to look at the furnace.

Turns out the "Blower motor has an internal ground". ... [Rep] from Atwood said the ohms on motor is low. ..." So, a replacement motor has been ordered and a date a month from now has been scheduled to replace the old motor. I'm crossing my fingers that, due to supply chain problems, etc, that it actually gets here in that time.

Also, interestingly, they found out if both drawers, which are directly above the furnace, are removed, the furnace will actually function. I'm guessing here, but the motor isn't strong enough to generate enough air through the rather small air return in the grille surrounding the single heat duct, but can with the huge hole where the drawers normally are.

Picture of the furnace grille.

OldWEB · 11-02-2021, 05:43 AM

You did not mention the model number.
But if the low ohms on the motor causes it to run slower (lose power) then they would be right in that it does not provide enough airflow to satisfy the sail switch monitoring the same.
If by opening the drawers eases the workload on the blower (intake air for the furnace blower is taken from that whole cavity the drawers are located in), I would suspect the intake airflow cfm requirements are not being met, that there is a 'starvation' of airflow for that model.

With a meter you can check the motor and the operation of the sail switch to confirm their diagnoses and as I have not seen that model of furnace grill before, it must be a low btu capacity furnace.

Persistent · 11-03-2021, 07:01 AM

Always include make and model number of the appliance for better advice.

I recommend you put a vent in the cabinet wall at the bottom next to the furnace. Even with a new fan motor the furnace will work more efficiently.

Or, you could put the vent in the other side of the cabinet and open holes between internal cabinet walls so air circulates better outside the cabinet.

I did this in my 20 foot TT. The return are now flows past my water pump and plumbing. The flowing return air keeps inside of cabinet above freezing.

glwinger · 11-03-2021, 03:56 PM

It looks like the grill on the front of the cabinet is both the outlet and inlet for the furnace. Poor design to begin with.

Is this the only heat vent in the trailer? Seems the furnace is trying to pull in the same air it is exhausting causing low return air pressure. This will cause the sail switch not to function properly and stop the furnace from igniting.
By removing the drawers more air is supplied to the furnace.

A short to ground in the motor should blow the fuse.

TXiceman · 11-03-2021, 06:51 PM

It looks like a poor design. You need to go back to the manufacture for a fix since it is still in warranty.

Ken

Persistent · 11-04-2021, 07:44 AM

The design for air flow is normal for small trailers. It normally works just fine. In this case the motor is defective. It does not work as well.

On the other hand it can be more efficient by venting the space behind the grill.

Old-Biscuit · 11-04-2021, 11:48 AM

Using a hole saw I would cut an opening in cabinet sidewall (to the right of furnace grill) and install a round register. Allow MORE unobstructed Return Air to furnace

New motor will like it......
*You can have too large discharge BUT RETURN SIZE ---More the better

Tomahawk · 11-04-2021, 08:21 PM

Back pressure will kill the fan motor. You should check the Heater Manufacturer's installation requirements. There is probably a requirement somewhere for minimum intake opening size. I'd guess the Trailer manufacturer did not meet that requirement. If you can't find a minimum opening size in the documentation, contact them and ask for it. The hole saw size thing OB suggested is a good one.

Dave Pelletier · 11-05-2021, 09:39 AM

I own an HVAC company but am unfamiliar with that exact furnace; are we SURE that all the return air isn't all drawn through the face of the unit? .....if it is, cutting holes in the cabinet won't do squat.

Dave

Atlee · 11-09-2021, 05:56 AM

The model number is Dometic DFSD20121. It is an 18,000 BTU model. It has 2 ducts, one down to where the holding tanks are, plus one direct discharge outlet into the living area.

Quote:

Originally Posted by OldWEB

You did not mention the model number.
But if the low ohms on the motor causes it to run slower (lose power) then they would be right in that it does not provide enough airflow to satisfy the sail switch monitoring the same.
If by opening the drawers eases the workload on the blower (intake air for the furnace blower is taken from that whole cavity the drawers are located in), I would suspect the intake airflow cfm requirements are not being met, that there is a 'starvation' of airflow for that model.

With a meter you can check the motor and the operation of the sail switch to confirm their diagnoses and as I have not seen that model of furnace grill before, it must be a low btu capacity furnace.

Atlee · 11-09-2021, 06:02 AM

As far as I know, that grille serves as both hot air delivery to the trailer and the air return. These types of heaters have been installed in smaller RV's for near forever.

Quote:

Originally Posted by glwinger

It looks like the grill on the front of the cabinet is both the outlet and inlet for the furnace. Poor design to begin with.

Is this the only heat vent in the trailer? Seems the furnace is trying to pull in the same air it is exhausting causing low return air pressure. This will cause the sail switch not to function properly and stop the furnace from igniting.
By removing the drawers more air is supplied to the furnace.

A short to ground in the motor should blow the fuse.

Tomahawk · 11-09-2021, 09:42 PM

OP said when he opens cabinets/drawers it increases the flow and the heater runs. Seems like it is drawing in air from somewhere other than the front grill. Maybe not by design, but that is what it is doing.

Persistent · 11-10-2021, 07:06 AM

Quote:

Originally Posted by Tomahawk

OP said when he opens cabinets/drawers it increases the flow and the heater runs. Seems like it is drawing in air from somewhere other than the front grill. Maybe not by design, but that is what it is doing.

Yes, that is what they do. That is how furnace designers make it work. RV designers decide whether to provide additional venting in the cabinets.

Furnace draws return air through grill on the back and sides. So the space around the furnace is the return air path.

If the space has no other air flow paths, the return air comes from slots around the main cabin discharge grill.

Opening a drawer or door into the space around the furnace allows freer return air flow. Load on the motor decreases.

Dave Pelletier · 11-10-2021, 12:12 PM

Quote:

Originally Posted by Persistent

Yes, that is what they do. That is how furnace designers make it work. RV designers decide whether to provide additional venting in the cabinets.

Furnace draws return air through grill on the back and sides. So the space around the furnace is the return air path.

If the space has no other air flow paths, the return air comes from slots around the main cabin discharge grill.

Opening a drawer or door into the space around the furnace allows freer return air flow. Load on the motor decreases.

It may well be that way but it's not how I've seen similar furnaces work before and it seems more than a bit foolish for the furnace manufacturer's to allow the coach builder discretion on whether or not to allow for return air. .....if this IS how it is then I'd imagine the installation instructions should have a minimum size return air opening spec'd....it's not as if supply static is much of a variable with 90+% of the air being discharged directly from the face.

....maybe I'm giving them more credit than they deserve.

Dave