Furnace runs for 8 minutes at a time.

[RobG]

This is driving me nuts. For the past few months, my furnace has adopted this annoying feature in that it only runs for eight minutes at a time. To make it run more, you have to turn the thermostat DOWN (where it will then make that "click" sound) and then back up, or turn it off and back on. If you leave it alone, it will come back on in one hour and run for another eight minutes.

The furnace is a Suburban NP-30 if I remember the model correctly. Over the last 2-1/2 years it's been totally rebuilt. It has a new thermostat, new board (Dinosaur board), new sail switch, new motor, and new gas valve. Oh and a new limit switch.

A friend who works on RVs first said it's probably the limit switch. So at his recommendation, I removed it and jumpered across it to eliminate that as a possibility. Nope, didn't fix anything.

Next, we replaced the circuit board again... it already had a Dinosaur board, and that board even passed his board tester, but we put a new one in it just because. Again, no change.

Yesterday, I replaced the wall thermostat. And, still no change.

I'm running out of ideas... so thought I'd post here. I'm leaving for a trip in less than a week and I'll be dry camping for two weeks... I sure would like to have a working furnace!!

Rob

[Chris_Bryant]

The limit switch should never be able to shut the blower down- it just shuts the flame down. On a hunch, simply because the motor has been replaced, I would look carefully at the blower rotation- makes sure it is turning the right way. I would also look at the combustion blower wheel to make certain it is installed correctly- I have seen them installed backwards, with the opening out, rather than in.
The limit switch is the only part *meant* to be temperature sensitive in there, though if it is getting way to hot, all bets are off.
I'm not sure what the surface temp of the heat exchanger should be, but it would be pretty hot.

You can bypass the thermostat right at the furnace- just tie the 2 blue wires together.

[enblethen]

Locate the thermostat wires either at the furnace or thermostat, connect them together, furnace should start up and produce heat. To shut down, separate the wires, the furnace should shut off gas and flame, then go into cool down and shut off.

[RobG]

Silly questions... how do I bypass the thermostat? And then how do I shut it down manually? 🙂

I may pull the thing out and take it to the shop tomorrow and let them run it in there. I don't really want to set fire to my trailer a few days before I leave on a trip.

[enblethen]

By pass the thermostat and see if it shuts down. Stay close to unit and be prepared to shut down manually. If it shuts down then you know problem is in furnace for sure.
590 degrees is hot! are you sure your thermal cut out is bypassed?

[RobG]

I started doing some of the recommended diagnostics this morning. Some interesting findings..

I turned it on and observed the flame... nice, strong flame with the electrode in it, glowing orange. Blowing nice, warm air. Which it continued to do... and do... and do... after twenty minutes I shut it off.

What I noticed while it was running was that those metal "canisters" (for lack of a better description) where flame is and the air inside is ignited, gets seriously freakin' HOT. I used my infra-red temp probe and measured the middle one (of the three) at 590 deg at the top of it.

I'm starting to wonder if this thing IS getting too hot. BUT, like I said before, there's no limit switch in there right now. It's bypassed. So with the metal panel over it, it runs for 8 minutes and shuts off. What might make it do that if there's no limit switch working? But with the plate off so it just blows hot air directly into the trailer, it keeps running.

Here's a pic:



The black things in the middle are the canisters I'm talking about.

[enblethen]

Did you set the anticipator when you installed the new thermostat? Most should be set about .80
Setting the anticipator to 1.0 will make the furnace try to keep temperature exactly as set. This is wrong. The furnace should only run about 6 times an hour.

[wasatchmtnatvr]

Dinosaur boards don't like rapid cycling thermostats or too narrow of spread. Something in front of heat vent forcing heated air to blow toward t-stat tricking it? Did you adjust t-stat heat anticipator to 1.0. If not try that. If t-stat contacts open and then close within 1.75 minutes it may shut off furnace with you using a dinosaur board.

[RLS7201]

I also have a fan50plus in my Suburban SF-35 furnace. It some times shuts down after a given time. Have not measured the time frame. I find that I can pull the furnace fuse for a few seconds and the board will reset and refire the furnace. Try pulling the fuse and see if the furnace will refire. If so you have eliminated the thermostat as being the problem. I suspect Chris is right on.
I will be extremely interested in the resolution to your problem.

Richard

[coolmom42]

emcee wrote:
Sounds like the thermostat tolerance is too wide. Might be something like +/- 2 degrees default. What thermostat did you install? The tolerance may be adjustable.

Ex. In heating set to 70 degrees. Furnace comes on when temp drops below 68 then shuts off at 72. Will not come on again until temp drops below 68 again. When you lower the temperature setting manually you are resetting the thermostat and that's why it starts again.

That's what I'm thinking too.

[dougrainer]

Chris has more experience on Dino boards. BUT, from this post, it seems Dino ALSO can have operational problems. They are NOT the cure all that everybody seems to think. I would install the Suburban OEM board and test. You could also have a defective Gas valve. The furnace heats up the casing and the Gas Valve reacts to this heat and one of the 2 solenoids shuts down. But, you replaced the gas valve. When the furnace malfunctions after 8 minutes, what is the sequence that happens?
1. The flame stops first?
2. The fan and Flame stop at the same time?
3. The flame stops and you hear the Ignitor clicking?
4. Last, you have replaced ALL the parts. The problem is (my personal pet peeve), you installed an aftermarket Dino board. That makes a diagnosis difficult. From Chris's post, he seems to have seen this 8 minute problem before and ONLY with Dino boards as I have NEVER seen this type problem with OEM control boards. So, as I stated, install the OEM Suburban board and see what happens. If you replace ALL the parts with OEM and use just 1 off brand part, get rid of the off brand part and retest. Doug

[wrgrs50s]

It sounds like there is an imbalance in the warm air around the thermostat. Is it possible that there is a heater vent blowing toward the area of the thermostat heating up that area faster than the rest of the trailer causing the thermostat to kick off early?

If so you might try setting a fan in that area to circulate the warmer air away, or partially closing off heater vents nearest the thermostat.

If you completely close off a vent it may cause the limit switch to overheat.

[Chris_Bryant]

RobG wrote:
The whole furnace shuts off after 8 minutes.

Fan control board ? Um... other than the Dinosaur board, no idea. I know there's a connection on it for the Blower.

It's an NT-30SP now that you mention it.

The box the board came in says FAN50PLUS. This is the same board that's been in it for two years and it was working fine until recently.

OK- that is all good. Does it actually light? The eight minutes is about the time a fan control board (the Fan50Plus) will run before shutting down if the flame is not sensed. If the burner is igniting and burning well, and you can see the electrode is in the flame, I would do a continuity test on the ignition wire and electrode. There can be a break in one or the other, and the spark will jump it, but it will not pass the current needed to sense the flame.
If the flame is not good, look at LP pressure, etc.

[emcee]

Sounds like the thermostat tolerance is too wide. Might be something like +/- 2 degrees default. What thermostat did you install? The tolerance may be adjustable.

Ex. In heating set to 70 degrees. Furnace comes on when temp drops below 68 then shuts off at 72. Will not come on again until temp drops below 68 again. When you lower the temperature setting manually you are resetting the thermostat and that's why it starts again.

[enblethen]

Is the furnace protected by an auto resetting circuit breaker?