Dometic thermostat/heat issues

[Forestrywrk76]

Purchased a used 2013 Cougar bumper pull to be used for work. Working in the mountains in AZ and I'm having a weird issue that's getting frustrating to troubleshoot.
Started noticing that the furnace would run perfect all night. Get up 5am, go start generator to make coffee, eat breakfast and pack a lunch and by 6 when I would leave for work it would be getting chilly as furnace had not ran. Oh well, time to leave.
Started to pay more attention. Get back to the trailer in the evening, start generator, bump up the thermostat, heat kicks on, make dinner, shower, sit down and watch a movie and realize I haven't heard the furnace and it seems to be cold again. Thermostat still set on 70, indoor temp 65 or lower! Mess around checking fuses, bottles, etc. Finally get it to come back on, go shut off generator and go to bed. Runs fine all night. Would come on everytime I bumped the temperature up a few degrees.
Moved trailer to a park with 50amp service last week no that we are getting close to snow and the problem persists. Had batteries tested, fine. Replaced t-stat and control board since the control board would click close to a dozen times before the furnace would kick on and SUCCESS!! Replaced that yesterday and no more clicking, furnace ran fine all night long. That was until this morning.
Got up and furnace was running, turned on the lights, made coffee, ate breakfast, temperature drops again and no heat.
New capacitive t-stat set at 70, reads 71, thermometer next to it 65!!!

So I had a idea that this was/is being caused by the difference of running strictly off of the batteries as opposed by being hooked up to the generator or shore power but that doesn't explain why it ran fine all night. On the generator I'd always shut it off before bed to save on fuel but now I'm on dedicated power and it seems to work just fine all night which made me have another thought.

Could this be caused by something with the overhead lights? Works great all night when the lights are off, acts up when you're awake and have the lights turned on.

Sorry for the novel but I'm stumped and no one has ever heard of something like this that I've asked.

Thank you

[jsb5717]

Welcome to the forum! Sorry for the issues you're having. Chasing gremlins can be difficult and frustrating. You might pull the sail switch and make sure it's completely clear of lint...just a troubleshooting point of eliminating items. It's not likely the sail switch since it does run some of the time, but still. It could possibly be intermittent and it's an easy item to check off.

Just for the sake of troubleshooting you might try getting up in the morning and not turn on the lights to see if the furnace runs normally.

You might also check the output of the converter just in case it's dropping voltage, which might have an impact on the TStat when the lights are turned on. Again, it's a stretch but is worth checking.

I might be tempted to suspect the thermostat itself. Dometic TStats are know to be glitchy. Can you post a pic of the one you've got? Many of us have swapped out our TStats for Microair Easy Touch to solve glitches.

Let us know how it's going

[Forestrywrk76]

I'm sorta leaning towards a voltage issue with the converter/charger. I've added a digital battery monitor and have never seen any other voltage pop up besides 13.6. The books says it should charge above 14 volts in bulk and drop to 13.1 I believe as a trickle charger.
When I had the batteries out to test them I turned the shore power back on and the monitor showed 13.6 volts and .42 amps and it was also 13.6 out at the ends of the battery cables.
I think my next step is that I will shut off the breaker to the converter/charger so everything is running off of 12volt only and see if that has any affect.
The t-stat I replaced was the older Dometic 3 button single zone and the local dealer only had the newer capacitive touch Dometic. Mine trailer uses the 3 wire setup so from what I've read it makes it harder to upgrade.
As far as the sail switch. When the camper SHOULD be calling for heat it isn't doing anything. No relay clicks, no blower running without ignition, nothing to show that its even being told to try and turn on. That's why I replaced both the t-stat and control board.
Just a weird gremlin for sure and to make it better the dealer doesn't have anymore 65amp converters in stock to replace the one I have.

[JRTJH]

I'm guessing your 2013 RV has the OEM WFCO converter? If so, it's probably a 30 amp, 8900 series power center with a 55 amp converter/charger.

Here's the explanation of the "converter modes" from the WFCO Owner's Manual:

"The three modes/stages of operation include:
Absorption mode/Normal operation
Nominal battery charge and supplies power to appliances

Bulk mode/Charge mode
Fast battery charge and supplies power to appliances

Float mode/Trickle charge
Trickle battery charge during storage

Absorption Mode: During this mode, the converter output is in the
13.6 Vdc range. This is the normal operation mode. This mode
provides the 12 Vdc and current required by the 12 Vdc RV appliances,
as well as slow charging the battery.

Bulk Mode: In this mode, the output voltage of the converter will
switch to 14.4 Vdc range for a maximum of four hours. If the converter
cycles between “Absorption and Bulk mode”, there could be a shorted
battery cell or other issues.

Float Mode: In this mode, the converter is charging the battery with a
trickle voltage of 13.2 Vdc. When the converter senses a demand (by
turning on lights), the converter automatically returns to the
“Absorption mode”."

Now, looking at "operation of the converter when the trailer is in use"...

You'll seldom see the "maximum of 4 hours" in bulk mode. 14.4VDC because the converter/charger will keep the battery charge state above the "requirement to enter bulk mode". A fully charged battery prevents that mode from triggering.

Almost all of the converter output will be in the "absorption mode" 13.6VDC.

You'll never see the float mode if you're occupying the trailer because there is a "demand on the system that triggers the converter/charger to remain in absorption mode. That demand will come from a light switch, the furnace triggering an ignition, the water heater ignition, the refrigerator control circuit or any other "use of battery power" while the converter/charger is in operation.....

ANY use of battery power will prevent the converter from entering the float mode.

There is a long explanation of the "Theory of Operation" located here: https://wfcoelectronics.com/wp-conte...eration-v2.pdf

A couple of notable points from that link:

In Bulk Mode, it may not be possible to observe the 14.4 VDC output because of the voltage-current relationship. (You typically will not be able to use a VOM to measure the higher voltage)

If the converter observes no significant variations in current draw for approximately 44 continuous hours, it will drop the output of the converter from 13.6V to 13.2V. (It's virtually impossible to avoid a "significant variation in current for 44 continuous hours" if you're inside the trailer on a daily basis)

[flybouy]

Just guessing but I'm guessing those lights aren't LED. Using the battery overjight then cranking up the generator (at 5 am? Bet the neighbors loved you) would kick the battery charger into bulk mode. Plugged into shore power turning on a bunchnof incandescent lights ontop of the draw of the furnace may do the same. The only way to troubleshoot isbyo actually trounleshoot. Start mojitoring the 12 vdc voltages during usage. Note commonalities when the failure occures.

Replacing the converter without diagnosing it first could be a waste of money. Find the cayse first, then decide if replacement is necessary. Just a note, the furnace and the converter doesn’t know the difference between day and night.

[Forestrywrk76]

Makes sense on all of that since I'm never away long enough to drain the batteries and things like you mentioned are putting a small amount of drain on the batteries.

[JRTJH]

It could be something as simple as lint on the sail switch preventing it from closing, maybe even a wire connector on the sail switch that fell off so it can't send the signal that it's closing. Possibly a melted wire on the limit switch that is grounded to the hot frame, always sending a "high limit signal" preventing the ignition sequence.... There's a lot of "might be's" so spending money to throw parts at it until it starts working could get to be very expensive, very quickly. Especially if, after doing all that, you find that "push on connector on the sail switch" dangling behind the furnace.....

[Forestrywrk76]

I don't think it's on that end of the system. When it calls for heat and does everything like it should the furnace fires up with out a problem. It acts like something is preventing that signal from being sent. But the curve ball is that it works all night long without issue. It's just when you're up and moving around with the lights on doing your thing.
I upgraded from a single battery to two 122AH batteries and right now as I sit here I have 4 lights on. My battery monitor is showing the charger is putting out 13.6volts at 1.51 amps.
If I shut the breaker off to the charger the monitor changes to 12.0volts and 9.25 amps. Shutting the lights off brings it back up to 12.8 volts and .21 amps.

[JRTJH]

I'd guess that your battery monitor is reading voltage across a shunt wired to the battery cable ??? If you're reading 1.51 amps with 4 lights on and the converter providing power, then turn off the converter and the reading goes up to 9.25 amps, I'd say you have incandescent lights (about 2 amps each) which is 8 amps with 1.5 amps running the rest of the trailer for a total of around 9.5 amps. So, if the amperage drops when the converter kicks in, the battery may not be getting a "charge voltage" to keep it going, at least that's what it looks like from your battery monitor readings. That could be from a loose connection at the DC fuse panel, a bad wire connection in the trailer belly or corroded contacts on one of the DC minibreakers on the A frame bulkhead or even a bad connection on a battery terminal.

I believe the 2013 Cougars had the "heat/cool interface unit" in the rooftop air conditioner. With the thermostat set to heat and the fan set to HI or LO, does the rooftop air unit fan turn on when the furnace starts up ??? If the unit is "wired correctly and the thermostat is operating correctly" the rooftop air conditioner fan should start when the furnace starts to run. It's part of the "Dometic functionality to improve heat circulation"... If it doesn't start, I'd look at the thermostat first, then the interface control box in the rooftop warm air return cavity.

[Forestrywrk76]

Yes it has the three wire thermostat that sends a signal to a control box at the AC unit. I have it set on AUTO at the moment so the AC fan doesn't come on but that portion does work.
I just put the new thermostat and control box in about 24 hours ago.
Also, I checked the sail switch. It is clean.
Mini breakers do have some corrosion. I'll take them apart and clean them up and see if the voltage maintains a closer value between the converter on or off with no load.

[Forestrywrk76]

Cleaned the corrosion off the connections. Checked the sail switch, it was clean. And checked all the connection screws in the distribution panel, they were tight.
Did notice since cleaning the corrosion that the amperage value on the shunt/battery monitor was slowing going down even with a couple lights on.
Guess I'll sleep on it and see how it reacts.

[Forestrywrk76]

So changing the t-stat and control board took care of the clicking and cleaning the connections was needed. I paid closer attention this morning and noticed that it seemed to once again not be working like it should.
Well during the night I'm all comfortable in bed and don't notice it but this morning while I made my coffee I paid closer attention to the actual temps.
It seems that the "cutoff" temperature is accurate and matchs with the indoor/outdoor thermometer I have next to the t-stat but its letting the temp drop 5-6° before kicking the furnace back on.

[JRTJH]

I don't think you ever posted which thermostat you installed when you replaced the Dometic thermostat. So, depending on what brand/model thermostat you connected to the system, you may have some "setup" or "initialization" process to perform before the HVAC in your trailer "recognizes the new thermostat". It may currently be running in a "limp mode" or a "default mode" with limited features.

If you installed a Dometic single zone digital thermostat, the initialization process is in the thermostat owner's manual. The temperature differential setting must be established before the furnace will operate properly.

That said, the "default setting is 2F. However, depending on where you installed the thermostat, it may be influenced by air from a nearby heat duct or it may be "insulated from cabin temperature" by being in a hallway or on a wall that is artificially hot or cold.

If you installed a Dometic single zone digital thermostat, take a look at the owner's manual for "initialization procedure". If you installed some other type thermostat, post a photo or the brand/model and someone may know how to better explain the specific setup process...

It sounds like you're in a "default mode" or a "limp mode" right now.

[Forestrywrk76]

Replaced the original 3 button Dometic model 3313192 with the newer style model 3316410 that came with the kit (3316230.700) that included the entire control kit.
Removed the old controller, plugged in the new one. Installed the thermostat and followed the instructions.
Placed a piece of foam behind it to close off the hole in the wall the wires come through in case it was picking up some heat from the wall.
Also changed the setting from D2 to D1 tonight to see if maybe it will reduce the 5-6° temperature drop before turning the furnace back on.

[LHaven]

One thing no one has addressed is your symptom where the thermostat claims the indoor temp is within specs, but wildly different than your independent thermometer.

Make sure nothing is generating local heat near the thermostat: the coffeemaker too close, the radio running on the other side of the partition, or even the morning sun coming in the window and hitting a surface under the thermostat.

[ingalmi]

Just a little curious, where is your thermostat in relation to your coffee pot? My coffee pot (heat) influenced my Dometic CT t-stat. Replaced with the MicroAir digital for lots of reasons but no more coffee pot influence. Who knows

[Fourbtgait]

Was following this as the Dometic are often finicky.
The Micro-Air sounded good but only one I saw is $260.

[JRTJH]

Originally Posted by Fourbtgait

Was following this as the Dometic are often finicky.
The Micro-Air sounded good but only one I saw is $260.

Yep, the price of the Micro-Air is definitely an incentive to keep working on replacement Dometic thermostats as long as the warranty is paying for the replacements... Once the Dometic warranty expires, then the "hard choices" begin....

[Fourbtgait]

Mines out of warranty. Don’t use it a lot and used to working with it.
Now if Micro-Air cut out Bluetooth etc might be good. Smaller trailer simpler is better

[precab]

Originally Posted by Fourbtgait

Was following this as the Dometic are often finicky.
The Micro-Air sounded good but only one I saw is $260.

My repair guy suggested going back to the analog thermostat after the capacitive touch ones keep timing out even replacing them they're horrible. We leave our hairless cat in the RV and can't afford a hot RV to get out of control with the CT thermostat

Analog swap out worked perfectly and it's the exact same three wires in the same three plugs

https://www.ebay.com/itm/NEW-Dometic-3313537-015-Cool-Heat-Fan-Single-Zone-LCD-Thermostat-/255381701446?mkcid=16&mkevt=1&_trksid=p2349624.m46 890.l49286&mkrid=711-127632-2357-0