Old Suburban Furnace Question

[Jeffrey Stoecker]

Hello,

I have a NT-22A furnace in my '69 Overlander. Lately, its fan will come on briefly between heating cycles, even though the thermostat hasn't clicked. In fact, I've had the fan start when the thermostat is in the OFF position. The furnace doesn't try to light when it starts in these little spurts. The fan just blows for a second or two and then stops. Then later, the thermostat clicks and furnace will start as it should, completing its cycle.

I'm thinking that the fan switch may be going bad. If my understanding of this setup is right, that's the only way power would get to the fan with the thermostat off. Any ideas?

The unit has been apart and cleaned at the beginning of the winter. It's remarkably clean inside, and has worked beautifully, up to now, despite its age.

BTW, I checked and it has the safe metal crossover tube.

Any responses would be very much appreciated.

Jeff S.

[TomW]

Hi Jeff, and welcome to the forum.

Someone around here posted a nice, hand drawn Suburban furnace schematic, but I searched, and can not find it. The image below is from an old ’72 Suburban furnace, and may help your troubleshooting.

If by “fan switch”, you mean the thermostatically controlled fan switch, then I agree that is a possibility. On my unit, this switch latches power to the fan motor after the burner has run for some short period of time. It’s purpose is to keep the fan running after the burner shuts off in order to cool things down. It does, BTW, make an audible “click” when it engages.

I believe the fan motor is controlled by interrupting the negative side of the DC. If this is the case, you might have a wire with bad insulation touching the furnace chassis somewhere. Are you sure the thermostat is in good shape?

Good luck,
Tom

[nickcrowhurst]

Jeff, as Tom said, you need to be sure your thermostat is OK. I have done battle with my N22A furnace. On my trailer, the thermostat, when closed, completes a circuit to ground which energises the coil in the relay which switches on the power to the furnace fan via the safety temperature limit switch. I would first try to isolate the thermostat as the cause by removing the thermostat from the wall, and leaving the two (blue?) wires disconnected and hanging free, not touching each other(They carry 12 volts DC). I would leave it like that for a few hours and see if the fan flicks on occasionally. If it does, you have a a fault in the furnace, and I would first look for a short, as Tom said, and then replace the relay (just a few dollars from RadioShack). If it doesn't, then I would reconnect the blue wires, and check that the furnace operates correctly for a few hours as the wires are connected and disconnected manually. If it does, I would replace the thermostat, preferably with a digital one. Let us know how you get on, and good luck in the hunt. Nick.

[Jeffrey Stoecker]

Thanks for the replies and the schematic. I'll test the thermostat this weekend.

Jeff S.

[nickcrowhurst]

Jeff, I should have mentioned that the first place I would look for a short, after eliminating the thermostat as the problem, would be a short between the two (blue?) wires leading to the thermostat. These can be eliminated or confirmed as the source of the problem by pulling the (blue?) switching wire off its spade terminal on the relay, and then seeing if the fan comes on occasionally. If it still does, then these wires are not the problem. The two larger wires on opposite sides of the face of the relay should be 12v power and switched power. The two smaller wires should be the switched power from the thermostat to the relay coil, and opposite that will be the wire taking this coil switch circuit to ground. Both these latter can be disconnected to isolate the problem. Nick.

[scs]

electrical diagrams anyone.

[nickcrowhurst]

Originally Posted by scs

electrical diagrams anyone.

You might like to try http://www.airforums.com/forum...ace-18069.html?

and, in particular, http://mbligh.org/pics/furnace/schematic.png

Nick.

[balrgn]

Do you have this manual?

http://bryantrv.com/docs2/docs/NT22.pdf

[Creampuff]

Originally Posted by balrgn

Do you have this manual?

http://bryantrv.com/docs2/docs/NT22.pdf

Hey Balrgn;
Thanks for posting that. It's long (21 pgs.) but cool to have on file.

[Moonlight]

On a related subject the Pilot lighting cap on my same vintage Suburban Furnace (1975 Tradewind) has burned through any tips on replacement parts?

[Creampuff]

Originally Posted by Moonlight

On a related subject the Pilot lighting cap on my same vintage Suburban Furnace (1975 Tradewind) has burned through any tips on replacement parts?

There were two caps usually furnished with a Suburban- one a solid , and one with a mica 'window ' that was use for burner adjustment viewing. Most get lost over time and are hard to find. Which one are you referring to?
You may be able to solder a repair piece onto it.

[Moonlight]

The cap that was on my Furnace was solid it is toataly rusted out and flame was burning through. We were lucky it happened while we were in the trailer and awake or the outcome would have been "dire".
Does anyone know of a replacement for this part. I tried various metal caps from paint cans and such but they are not quite right. Furnace seems to be just fine so it would surely be a shame to have to replace for want of a Cap. Oh yeah the cap is toast there will be no way to solder a repair.

[Creampuff]

I'll see what I can dig up. Try searching Suburban on the web and call their parts dep't. It's probably obsolete but worth a try.
I'll PM you if I can find you one.