LP Appliances- Furnaces

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March 26, 2008

chris The RV LP Furnace is an amazing thing, producing a large amount of heat from a rather small appliance. To put it in to perspective- a 30,000 btu/hr furnace produces as much heat as 5 standard plug in electric heaters (even allowing for the heat lost out the vent). It does this safely, in a unit that takes up only a couple of cubic feet.

RV Furnaces have come a long way in the past 40 years, from the basic metal box with a burner, a pilot and a thermostat valve, to forced air pilot type models, to the modern electronically controlled models- even a 2 speed, 2 stage model, offering a high and low fan and variable flame.

While the basics of the flame safety systems are the same as in the water heaters, the furnace contains more safety devices, which can affect operation. Let’s look at them now…


Highlighted Schematic
The flame sensing used in the furnace is identical to the flame sensing used in the water heater- indeed most circuit boards are interchangeable (though not all- new models use a “Fan Control” circuit board, which turns off the blower motor if the furnace fails to ignite).

Looking at a typical schematic diagram for a furnace, we will find a couple more devices in the power line to the circuit board. The power that energizes the circuit board (ignition module) has to travel through the Thermostat, a hi temperature Limit Switch ( which shuts the power if the furnace over heats), and a Sail Switch (also called an ‘air prover’ switch), which will not close until the blower is running at at least 75% of its full speed, ensuring that there is enough for the flame to burn, and enough air moving across the heat chamber to keep it from getting too hot. Then the power goes on to the circuit board connector.

Being mechanical connections and switches, each of these devices has the potential to cut the voltage a small bit- and indeed, there is usually some voltage drop through all of these. The main circuit board needs above 11 volts to operate- ideally above 11.5, so especially running off battery power losing just a little bit of voltage across each of these items can mean the difference between cold and hot air.

Another factor is that people will sometimes replace a single part, eliminating its voltage drop, the furnace will run, but they really will not have fixed the problem- just delayed it for a bit- another component might have a larger voltage drop, or there could be a bad connection. The easiest way to test these items is with a volt meter- testing across the item- in other words, set your voltmeter to a low voltage range and put one lead on either side of the sail switch, thermostat or limit switch. Any voltage reading you get will be a voltage drop.

Often these voltage drops are caused by the quick connector hooking the wire to the device, and not the device itself, so many times simply removing the wire and reconnecting it will fix the problem.

Board ConnectorThe last “weak link” in the electrical chain is the connector to the circuit board. New designs place the circuit board in the return air flow for the furnace, which makes the board last much longer than older designs where the board was next to the combustion chamber (and got very hot), but as you can see in the photo, being in the return air flow makes for a dusty environment. I would estimate that a good 40% of the Atwood/Hydroflame furnaces I see are repaired by simply removing and cleaning this connector.

A few thoughts to wrap up- be aware that a thermostat can pass 9 or 10 volts to the furnace, which will be enough to activate the fan relay, but not enough to activate the ignition control board, so just because the blower fan runs, you cannot count out the thermostat as the problem for “blowing cold air”-

A multimeter is a useful and necessary tool in virtually every RV appliance troubleshooting procedure, and most RV furnace problems (though certainly not all) are in the control/safety systems, and can safely and easily be repaired by many people.

I have not gone in to the LP side of RV furnaces- working on the LP system takes more knowledge, both of safety procedures and of common practices, so it’s outside the scope of this post, but most times working on the LP system is not needed to diagnose and repair an RV furnace.

Leave a Reply

198 comments

  1. Sean

    My travel trailer has awallmounted thermostate that controls my furnace. Why is it so hard to maintain a normal set temp with this system. The ac is maintains this. Remember a Travel Trailer is new to me, I just moved up to it from tent.

  2. Timothy

    Sean, I am unsure if a TT system using DC voltage is similar enough to standard AC 24Volt Heat & Air circuits in a residence or any building to use very similar troubleshooting scenarios. If so, the Thermostat has a heat anticipator adjustment inside the stat. If its not set properly it will cause erratic temp. operation in heat only, while not affecting the cool mode at all. They have to be set by current draw in the heat circuit. We always set them at .4 and forget it. That seems to work as a rule. But with DC the same rules may not apply. Id have to look inside the stat cover or on the back of it to see if an anticipator is present.
    This anticipator gives the stat a positive cut in cut out in heat only.

    The above schematic for a furnace is nearly identical to a simplified burner in a home except its DC. In the last 10 yrs the HVAC industry has undergone huge changes with High Efficiency Equipment and the ever evolving EPA LAWS. Almost all indoor blowers now are what we call Ramp up motors. They have near 20 wires all fed from PC boards. Its a varible RPM motor. 2 stage gas valves for Hi & Low fire, 2 stage forced air combustion motor, then the varible RPM Blower. All Heat Pumps now use this Blower. Then when you get into commercial units it gets even more fun. Trane now uses all DC control circuitry. Sometimes I wish I could simplify my life with the likes of above schematics instead of what I see out here now. Chris, risking showing my ignorance of RVs what is a Framistan. When I saw this I thought of a widgit%$$^&*()^%#^#@[email protected]#

  3. Brian

    I appreciate that you blog “like a service manual” lol, makes for good reading if you ask me. Any chance you could expand the experience by explaining how to pull the combustion chamber out for cleaning/replacing? Does it pull out through the front like everything else or is it the one part where the whole shebang has to come out in order to get at it?

    We had a 10 year old motor finally croak – with a new motor the airflow is back up but the heat output hasn’t recovered. The voltage, gas pressure, orifice, burner etc are all in good shape – manual suggests heavy sooting or blockage in the chamber – if its blockage then it is way to deep to see – there is visible sooting in the exhaust tube which could extend further back – but again, can’t see that far in.

    Sure could use a “service manual explanation” on how to go about cleaning this up before the ski season starts!

  4. Philip West

    Brian,
    I’m with you! I have spent the better part of 4 days looking for a “procedure” on how to take out the motor, combustion chamber, etc. Have you found one yet? If I don’t get one, then I guess I will take it upon myself to “tear” into it and make a photo/step procedure and upload it so others can have one. You can email me if you have the procedure or website you may have found it on, to: [email protected]

    Thanks,
    Philip

  5. Kenn

    I have recently aquired a 1980 Coachman 3215 DrQuinstar 5th wheel rv, but the heater is missing, the model that it originally had was a Coleman model #4334749 what would be a good replacement that would fit that I would’nt have to do any extensive refitting work?

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