Furnace ground issue

j griff · 12-16-2011, 12:31 PM

This may or may not be the right subject. I may have a more basic ground issue - I am not an electrican and have a poor understanding of electrical systems.
My problem surfaced when I was dry camping (had a 15 amp elec connection to a home) over Thanksgiving and my gas furnace would not ignite. When the thermostat was set to gas heat, the furnace blower would start then not ignite. When we got home I took the MH in for service and they found the furnace board was bad. They replaced it and it bench checks just fine but in the coach acts as before. They decided the thermostat was bad and have replaced it with the same result. All other settings from the thermostat work OK.
During the Thanksgiving trip I noticed that my house batteries were very low and filled them with distilled water. Several days later they were quite low again and again I refilled them.
While in the shop troubleshooting the furnace the tech said the batteries were really hot. We replaced them. He says when the furnace is trying to start the batteries are droping to 9V. Winnebago says the furnace must be "losing ground".
I know I probably have not explained this very well but wouold appreciate any advice anyone might have for me and the repair tech. Thanks.

Dave78Chief · 12-16-2011, 12:48 PM

Battries hot / loosing water = replacement? This scenerio is typically due to high charging voltage which is boiling the batteries dry. That however could be due to a bad cell in a battery which should have been located with a battery tester. If charging voltage is in fact to high you are simply going to cook the new batteries also. Verify charging voltage is correct first. It is possible a high charging voltage caused the original furnace board to go bad. Some of what you describe sounds more like they are just throwing part at it hoping to fix the problem. A furnace that will work fine on the bench but causes the battery voltage to drop to 9VDC in the RV is not the fault of the furnace unless there is a real bad short created due to the installation. I am aware of someone who's fridge pressed down on the furnace when installed creating a short inside. Anyway, only a bad short can drop a charged battery voltage that much. That requires a heavy current drain like a engine starter (400 amps). Because of the battery heat / water loss, I would check the charging system real well first.

Dave

hometool · 12-16-2011, 01:03 PM

hi i cannot say i am an expert on this but i will toss out my thoughts. from what you stated it sounds to me like you may have a battery problem the furnace runs on battery power and at 9v may not be enuph for it to fully function. you said you were told the batterys were hot and using quite alot of water i suspect you have a cell or battery with a short ckt on one cell. a bad cell would cause the chargeing system to over charge the bank which would cause hot batterys and high water usage lee

bluepill · 12-16-2011, 01:17 PM

Quote:

Originally Posted by Dave78Chief

Anyway, only a bad short can drop a charged battery voltage that much. That requires a heavy current drain like a engine starter (400 amps). Because of the battery heat / water loss, I would check the charging system real well first.

Dave

A serious short circuit would cause circuit protection (Circuit breaker or fuse) to shut down the power to the furnace.

As already stated, this sounds like a classic case of overcharging. Get a sub $20 digital multimeter if you don't have one, and check the charging voltage with the converter on shore power and the alternator with the engine running. Anything above 14.8 volts DC is a problem.

TXiceman · 12-16-2011, 03:32 PM

I have ideas for a few areas to check, but if you cannot do the diagnose the problem, you need to find a better repair shop. You need one with real technicians and not pars changers.

It sounds as you have an over charge problem through your converter as part of the problem. But find a better tech.

ken

bobmac · 12-16-2011, 05:10 PM

I suspect the real problem is your battery. You didn't mention the age of the batteries. If you are using a "converter" as the charger (as most older rigs had these out of the factory), it charges at a constant voltage often well over 14 volts. This will shorten the useful life of a battery and as it gets older, will boil the water out of it often shorting a cell destroying the battery. To eliminate this, you are best to get a newer 3 stage charger that is much friendlier to a battery and should give it a longer life.

To eliminate the current battery as the source of your problem, I would disconnect the current batteries and substitute a know good battery (an automotive battery will do) to see if the furnace will start and run (leave the converter off while you are dong this). Alternatively, you could just disconnect the positive lead and use jumper cables to the disconnected positive lead and ground on the old battery but be very careful not to short the positive. The capacity of a single battery will be lower than the house batteries you have but if the furnace runs, you will have found the problem.

If this doesn't solve the problem, then it might be a defective"sail" switch in the furnace. Simplistically, this is a switch that "sails" (lifts) and closes a safety circuit when the blower is running. If the blower doesn't run, the switch doesn't lift and activates a circuit on the control board to shut down the flame so you won't burn the place up with no blower running. Good luck,

DancinCampers · 12-17-2011, 08:45 AM

I would check furnace operation with engine running. If it works I would agree house battery issue.

j griff · 12-17-2011, 09:58 AM

Update - first of all thank you to those who responded.
After conversations with Winnebago and others, the technician found a chaffed wire "in a channel", but only with the aid of a microscopic camera threaded through the tunnel. He said he has fixed the problem but did not have time to put it all back together before the end of the work day yesterday. This short apparently was the cause of the battery problem. The tech did test the furnace in the coach and it fired up as it should. Will get the full story after they get it all back together on Monday. Again - thanks for the helpful responses.

j griff · 12-19-2011, 07:55 PM

Final update. Turns out there are four connectors between the Thermostat and the furnace - NOT the two shown in the wiring diagrams. Long story short - not a lot of help from Winnebago Tech Support and lots of following the wires to find the other two connectors (one beneath the shower and one beneath the generator). Go figure .. Cleaned the "lost" connectors and all is well.