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Robert
Robert, RV Mechanic
Category: RV
Satisfied Customers: 13372
Experience:  18 yrs. RV experience including Cummins North West, Country Coach, Monaco Coach - vendor certified.
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Dometic Duo Therm, furnace will not come on. AC will not

Customer Question

Dometic Duo Therm, furnace will not come on. AC will not either, but the fan will turn on.
JA: Have you checked the Freon level?
Customer: No. After refilling both lp tanks, turned furnace on, but it never lit, then it shut off and wont do anything now.
JA: What is the model/year of your RV?
Customer: 2005 Keystone Cougar
JA: Are you fixing your Cougar yourself? What have you tried so far?
Customer: Yes I am. Checked fuses and breaker so far.
JA: Anything else you want the mechanic to know before I connect you?
Customer: Temperature is dropping FAST, going to be 28 degrees in NW Florida tonight.
Submitted: 7 months ago.
Category: RV
Expert:  Robert replied 7 months ago.

Hi my name is Robert.....

Sorry for the delay seeing if you still need help on this??

You say the fan is coming ON is that the fan for the AC unit OR for the furnace?

Robert

Customer: replied 7 months ago.
AC unit
Expert:  Robert replied 7 months ago.

OK

So if the fan for the furnace is not coming on at all when you select that on the thermostat it is going to be an issue with the thermostat, circuit board on the furnace, blown fuse, tripped breaker OR bad blower motor, when you troubleshoot these it is a process of elimination. IF you can get me the make and model of the furnace we can see if voltage is getting to the circuit board. You will need a test light or meter. IF you do not have a test light or meter then I would remove the circuit board and have a RV store/shop test the circuit board, they can fail.

IF you need more help I will try to respond back, I am in the Pacific Northwest and just started to get an ice storm... Not sure how long the lights will stay on....

Let me know if you need more help...... Robert

Customer: replied 7 months ago.
It is a Suburban sf-25f or sf-30f and a Dometic Duo Therm thermostat
Expert:  Robert replied 7 months ago.

OK

Let me get you a service manual..... Stand by

Customer: replied 7 months ago.
The puzzling thing is this, 2 minutes after replacing both lp cylinders and the furnace failing to light, now Neither AC or furnace engages when called
Expert:  Robert replied 7 months ago.

OK

Click HERE for a service manual follow the troubleshooting tree. Some models have a time delay relay and others the relay is part of the circuit board. When you turn the thermostat ON you should have voltage at the time delay relay or board. IF you have no voltage I suspect a bad thermostat.

IF you have the delay relay one post is always constant hot, that should be a red, then you have ground which is normally yellow, the blue is the signal from the thermostat and when the relay sees that you get voltage going to the other red for the blower motor.

Running out of LP should have nothing to do with this, as soon as you select furnace and raise the temp the fan should kick on for the furnace.

IF it is not a board issue on the furnace then it could be the thermostat OR circuit board inside the AC unit...

Hope this helps some, IF you are satisfied with my efforts please feel free to select a rating on the page, it is the only way I get credit for helping...

Robert

Customer: replied 7 months ago.
If i choose the phone call option what is my Total cost?
Expert:  Robert replied 7 months ago.

I do not handle the payments or billing, there would be another 29 added to the cost, so your total cost would be around 58..... Without 29....

I am unable to do a call myself but another may jump in and take the call....

I can try to help you in this format still, you can go back and forth as much as needed......

Robert

Also I want to point out 58 may seem like allot but shops will charge over 100 to diagnose the issue and you have the convenience of not bringing the unit in.

In this format I would only get credit when you select a star rating on the page but my goal is for you to be satisfied.

Robert

Customer: replied 7 months ago.
I just thought a phone call would be easier for me, but what the heck let's get this thing fixed. I am no service tech, but very good working with my hands, I'm the head of maintenance at our church, but we contract all of our HVAC out. Thermostat wire goes directly to ducted Dometic AC. Out of AC board is red and white going to furnace, I removed the wire nuts and closed that circuit and the furnace fan comes on but does not light.
Expert:  Robert replied 7 months ago.

OK

IS it trying to spark at all???

Robert

Expert:  Robert replied 7 months ago.

As the fan comes up to speed it will blow on a sail switch, when the sail switch closes it will send a signal to the board, after the board sees that signal it should spark and the gas solenoid should click. IF it is not sparking with the fan going could be a bad sail switch, circuit board or electrode.

I think you have multiple problems here, one being the thermostat or AC board and something going on at the furnace if the fan is running and it is not sparking.

Check your battery voltage as well, if the voltage was low under 11 volts the fan would still run but not fast enough to close the sail switch.....

Customer: replied 7 months ago.
I am going to run the Church and get all of my tools. I'll check back with you in a little bit, thanks Robert.
Expert:  Robert replied 7 months ago.

No problem, if I do not respond back it is because we are having a really bad ice storm, if that is the case I will respond as soon as I can.. Just a heads up I am not ignoring you...

Robert

Expert:  Robert replied 7 months ago.

Was just checking in to see if my answer helped?? IF you have more questions or concerns please feel free to contact me....

Robert

Customer: replied 7 months ago.
Good morning, thanks for checking in! We had a hard freeze in Florida, I've got HVAC problems at my church, roof is leaking, all of that took precedence over my furnace here. I have two services to get us through this morning, then I'll be back on the furnace problem and get back to you later. Thanks Robert!
Expert:  Robert replied 7 months ago.

Sounds good thanks for the update, we are still not out of the woods, we got 5" of snow, then an 1" of sleet now it is raining and 30F ..... So far the lights have stayed on.

Robert

Expert:  Robert replied 7 months ago.

Hi.... Was just checking in to see if my answer helped?? Please let me know if you have more questions or concerns....

Robert