Furnace problem

Old-Biscuit wrote:

RustyJC wrote:

magic43 wrote:
If your voltage is OK then the odds are that you have a "sail switch" problem.

Going back to the OP's post #1, the furnace works fine when the RV is connected to shore power or the generator. It only fails to light on battery power. Since the fan motor never sees 120VAC, and if the 12VDC voltage supply to the furnace doesn't change irrespective of the power source (i.e., shore power, generator or battery), then the fan motor speed shouldn't change, either. I'm struggling to see how the sail switch plays a part in this problem IF the voltage to the fan motor and, thus, the fan motor speed doesn't change.

Rusty

Because shore/generator power supplies converter which is supplying the DC System.
No AC power than battery alone is supplying DC System

That is why OP needs to check actual DC Voltage with and w/o AC power.
Could be 13.7V when on AC and only 12V on battery
Fan DC motor could run slightly faster on that 13.7V then on battery 12V....make up sail switch/not enough to make up sail switch.

The circuit board is getting DC voltage when on AC but NOT getting voltage when on battery. (Assumption due to no spark----otherwise it would spark)
Fan speed & sail switch are usual suspects when DC doesn't get to circuit board.

Lower voltage/dirty-loose connections/contacts------resistance.


OP........
You can access furnace via outside cover.
INFO

Ummm....I suggested low 12VDC voltage as a potential problem back on post #3 of this thread. I understand full well the interrelationship between 120VAC shore/generator power, the converter and battery power alone. The point of my post above is that IF the 12VDC power supplied to the furnace doesn't drop on battery power alone (which is unlikely), then the sail switch probably isn't the cause of the problem - in other words, the furnace wouldn't run on shore or generator power if the sail switch itself was defective. Please see the post (which you quoted) to which my response was directed.

In all likelihood, 12VDC voltage IS dropping on battery power alone which could slow the fan motor sufficiently to prevent the sail switch from going permissive.

Rusty

RustyJC wrote:

magic43 wrote:
If your voltage is OK then the odds are that you have a "sail switch" problem.

Going back to the OP's post #1, the furnace works fine when the RV is connected to shore power or the generator. It only fails to light on battery power. Since the fan motor never sees 120VAC, and if the 12VDC voltage supply to the furnace doesn't change irrespective of the power source (i.e., shore power, generator or battery), then the fan motor speed shouldn't change, either. I'm struggling to see how the sail switch plays a part in this problem IF the voltage to the fan motor and, thus, the fan motor speed doesn't change.

Rusty

Because shore/generator power supplies converter which is supplying the DC System.
No AC power than battery alone is supplying DC System

That is why OP needs to check actual DC Voltage with and w/o AC power.
Could be 13.7V when on AC and only 12V on battery
Fan DC motor could run slightly faster on that 13.7V then on battery 12V....make up sail switch/not enough to make up sail switch.

The circuit board is getting DC voltage when on AC but NOT getting voltage when on battery. (Assumption due to no spark----otherwise it would spark)
Fan speed & sail switch are usual suspects when DC doesn't get to circuit board.

Lower voltage/dirty-loose connections/contacts------resistance.


OP........
You can access furnace via outside cover.
INFO

Rusty, you hit the nail on the head.

magic43 wrote:
If your voltage is OK then the odds are that you have a "sail switch" problem.

Going back to the OP's post #1, the furnace works fine when the RV is connected to shore power or the generator. It only fails to light on battery power. Since the fan motor never sees 120VAC, and if the 12VDC voltage supply to the furnace doesn't change irrespective of the power source (i.e., shore power, generator or battery), then the fan motor speed shouldn't change, either. I'm struggling to see how the sail switch plays a part in this problem IF the voltage to the fan motor and, thus, the fan motor speed doesn't change.

Rusty

If your voltage is OK then the odds are that you have a "sail switch" problem. As previously stated, this switch must operate to allow gas to flow to the burner. After 21 years, the furnace needs servicing anyway. Dust accumulates on the fan blades etc, and the holes in the burner tend to "rust shut". The fan motor needs oiling.

I would see no reason to replace the furnace.

The furnace is a Hydro Flame Excaliber 8935DCLP-II. The fan runs fine, no slower/faster than usual. I'm on my way home and am in Pueblo now; no heat needed. I'll rig up some jumpers when I get home on Sunday and start checking for weak connections. Thanks for the advice. Also, just how difficult is it to pull the old unit and install a new one?

What furnace do you have.....brand/model??

Furnace has fused DC power to it.
When t-stat contacts close calling for heat DC power goes t a timed delay relay.
This allows fan motor to start (DC Powered).
Fan comes up to speed and purges combustion chamber prior to gas valve opening.
As fan speed increases a 'sail switch' closes and DC power goes to circuit board. (Sail switch has an arm that sticks out into air stream...closes contact and dc goes to high limit switch which is always closed unless high temp in exchanger)
Circuit board now has DC power to send to gas valve and spark ignitor.
Main flame lights off.........flame is proven...circuit board keeps DC voltage on gas valve until t-stat contacts open.
DC power drops off of time delay relay....but DC power thru other side of relay keeps fan running to cool exchanger down. Coil inside relay cools off....shuts down power to fan.


Furnace works when AC Power is available.
Fan turns on when just using batteries...............BUT no heat.
Slow fan motor speed, sail switch not closing.
Dirty/loose contacts...high resistance

Does the fan sound like it is running as fast on battery power as it is on converter power??...there is a sail switch that will keep furnace from firing up if not enough air is being moved.

As others have suggested.. the furnace runs from the 12 volt side of life. If you need to have 13.7 coming into your batteries, like you would get from a charger, and it won't run from 12.7 that you might get from a super good battery, then start the troubleshooting.

My method would be, take your jumper cables, and bring them from the camper battery to near the furnace. Disconnect the negative to the furnace, and connect to the negative jumper. Did that make it run.. then it's the negative feed that is weak. If not.. connect up the negative as normal, and try using the jumper to connect to the positive jumper. If that made it run, the weakness is in the positive feed.

If neither of those help, try connecting the furnace to both jumpers. If that doesn't work, look for a bad furnace part. Something that requires a good voltage source to spark ignition.

Thanks; appreciate the replies. Again, I have new batteries, installed last month. I can watch hours of TV, lights, etc; and I have a digital volt meter, constantly visible. Available DC power is not an issue. I'll check the wiring and contacts. When the furnace is off, or when it's lit, I'm not sure how to check voltage. To my understanding, dc is only present to 'shoot' the spark. Can/ will someone elaborate?