Heat question? How is it supposed to work?

[dgmelp]

Another new to us question - we've owned this motorhome for just a short time and trying to figure out how things are supposed to work (or if they are in fact working correctly).

The floor furnace goes on and blows, but the heat part didn't get the message - it is blowing cold air. This is a three part question;

1. How do we get the furnace itself to turn on with the blower?

2. Is there a way to increase the volume of air blowing out of the outlets (there are two outlets we've found that air comes out of when the blower turn on - one is from an outlet that is at the end of a flexible duct under the jack knife sofa, and the second is in the wall in the back bedroom.) The one in the front under the sofa has hardly a trickle of air coming out. Is there a filter we're supposed to replace somewhere?

3. There is another outlet in the floor near the exit door, but no air comes from there unless I turn on the rear heat switch and then only when the engine is running. Is this a separate system and tied directly to the engine heat?

By the way, we do have all the owner's manuals but couldn't find a reference to any of these questions in the manual - I guess we're supposed to know?

David

[Silverhobby]

Do you have your propane turned on? Is there propane in the tank? The furnace operates on propane and 12 volt power for the fan blower.

[dgmelp]

Propane was turned on and the tank is full...

David

[BarnacleBill]

1. How do we get the furnace itself to turn on with the blower?

We had the same problem and it was caused by low voltage.

The furnace is designed to operate at voltages between 10.5 to 13.5 VDC.

When the voltage from the batteries gets too low, the furnace fan will start but the burner does not light, so the furnace just blows cold air. Not too helpful.

The reason for the furnace not lighting is something called a sail switch.



The sail switch is an on/off device. (normally in the off position) It gets it's name from the "sail" or paddle that is attached to the switch mechanism. As the blower comes up to speed, it blows air onto the sail with enough force to push the switch closed, thus allowing electrical current to flow.

So with low voltage, the fan never blows hard enough to close this switch and turn on the gas for heat. The race is canceled because there is not enough air for the sail.

Our voltage drop was caused by the buss strip in the house battery compartment. The buss strip has four circuit breakers, a 50 amp for 12 volt distribution panel, three 30 amp breakers, one for the steps, one for the dock lights and one unused.



I found the first breaker (50 amp) that supplies the power to the 12 volt distribution panel was loose. There was evidence of arching and the heat had melted the plastic assembly that holds the buss bar and the breakers.

The breaker assembly must be something Airstream had made special. None of the auto parts stores had ever seen anything like it, and neither had the local RV dealer. I fixed the part myself.

I cleaned it up, used some super glue to reattach the broken plastic and it worked just fine.

Here is a link to a furnace troubleshooting article. This site has other info you will probably find useful as a new owner.

Troubleshooting the RV DSI Furnace

2. Is there a way to increase the volume of air blowing out of the outlets (there are two outlets we've found that air comes out of when the blower turn on - one is from an outlet that is at the end of a flexible duct under the jack knife sofa, and the second is in the wall in the back bedroom.) The one in the front under the sofa has hardly a trickle of air coming out. Is there a filter we're supposed to replace somewhere?

There is no filter. I found some holes in the ducting here and there and patched it with Red Green's secrete weapon, Duct Tape. Fixing the holes helped, but don't expect the same flow you have closer to the furnace like in the bathroom.

3. There is another outlet in the floor near the exit door, but no air comes from there unless I turn on the rear heat switch and then only when the engine is running. Is this a separate system and tied directly to the engine heat?

Yes.

[dgmelp]

Bill..thank you! That was incredibly helpful...you may have solved the problem as we do believe we have one of our batteries not working correctly (according to the monitor).....Now I have something to look at.

David

[BarnacleBill]

That's great. Check your voltage with some 12 volt stuff turned on, lights fan, etc., and if the voltage is 10.5 or under, there's your problem. It could easily be a bad battery.

[edglenn]

Bill's right about the sail switch.
If you don't have sufficient air flow from one of the ducts, you may not have sufficient airflow RETURNING to the unit. Thus, the airflow won't be enough to fully operate the sail switch which prevents the heater from igniting. (The return air provides COOLING to the unit. It's ON FIRE when it's heating, remember.)
You might have a broken/obstructed furnace duct. Maybe disconnected somewhere, letting air outside, etc. A varmit might have made a nest in there, too!!
A quick test is to "short-circuit" the airflow by disconnecting the ducts. This lets the air flow out, and right back to the unit. If there's an obstruction, this will eliminate it. You have to fix the problem, but this is a quick test of airflow, ONLY.
Could be low voltage, too. Or a problem with the gas-valve (bad valve, low propane pressure, bad wiring).
Just because you have a full tank of propane doesn't mean the FURNACE is getting any/enough. Does it have a seperate petcock valve? The gas-valve uses gas pressure to help keep it open, too.
Or the igniter isn't heating up, or the thermocouple is bad, or....
Well, you know where to start!

[jdalrymple]

To offer input on another of your questions, there is completely separate heating system that uses engine coolant for the heat source. I have found them under the sofa in several units. They are simply a heater core in a box with a fan to force air across the the exchanger.

They have heater hoses routed to the engine and a mechanical valve operated by a “choke” type push-pull cable (if you are old enough to remember manual chokes....) to stop the flow of coolant when heat isn’t required. Several I have seen leak, and the owners have simply disconnected the heater hoses from the core, and looped them together to create a bypass arrangement.

Good Luck!