Furnace blows hot, then cold. What gives?

[mayco]

Was your rig modified, furnace moved from its original location? If it is stock, the way it came from the factory, it really should operate properly without additional air flow or ventilation.

But, you say it WILL reach the set temp on the thermostat if you leave the access panel off? Hmmmmmmmmmm.

Since it goes all the way through the cycle till the burner lights, thats a good thing. Sooooo, About the only thing left that would shut it down would be the high limit switch. If you werent getting enough air flow through the furnace then the sail switch would prevent the furnace from lighting. Hmmmm, these old suckers can be frustrating but I wouldnt give up on your furnace.

I may be totally wrong but I still think it is your high limit switch tripping before it should be. Wish I could be more definitive. Good luck, youll get it, just keep at it.

Mike

[prairieschooner]

I went through the Suburban Heaters in our '82 Newell Coach a few years back.
It really is a good idea to get a good look inside the unit(s) and do a thorough cleaning. From what I remember the Burner needs to be checked and possibly cleaned annually. I found that some wasps (mud daubers) had gotten in through the vent and nested on the fan blades.
I found that Dinosaur Electronics makes an excellent replacement for the circuit boards. Here is a link, sorry don't remember what circuit board I used but they have a great customer service;
http://www.dinosaurelectronics.com/Ignitor_boards.htm

[trekerboy]

Originally Posted by mayco

Was your rig modified, furnace moved from its original location? If it is stock, the way it came from the factory, it really should operate properly without additional air flow or ventilation.

Yup. Our AS started life as a '79 Excella 500, and in the past year we've completely gutted it (even the floors and belly skin) to rebuild the interior from the ground up.

The way the furnace was originally positioned was immediately to the left as you entered from the front door; it was positioned under the oven. Here's a shot from the original owner's manual:



The idea is you'd be able to remove this wooden panel to reveal the furnace's rear access panel. it doesn't seem like there was any more or less ventilation to this thing even in the original location, but I could be wrong.

[nrgtrakr]

Again, I'd refer to the installation manual for the heater and see what it says about BOTH air out and air in - it sounds like it either can't blow enough air to keep from over heating (either obstructions in the ducts or not enough ducts), or the supply air is so restricted that it cannot blow enough air to keep from over heating. More ducts and more supply air are always better than not enough.

[altnbndr]

I just looked up your furnace "NT-24 SP" to check the spec. sheet. It shows that you need a minimum of : 3-4" round ducts ( or equivalent sized rectangular duct ) on the supply side, and 55 square inches of return opening feeding your unit. Looking at your pictures of your units install , I would lean towards a restricted or insufficient return air opening. You can cut one hole or many smaller ones as long as the sum of 55 square inches is met or exceeded that ties in to the common space of your cabin. Your unit also requires at least one inch of clearance on both right and left sides . The right side of the unit in your pictures looks a little tight.

[trekerboy]

Originally Posted by altnbndr

I just looked up your furnace "NT-24 SP" to check the spec. sheet. It shows that you need [...] 55 square inches of return opening feeding your unit. Looking at your pictures of your units install , I would lean towards a restricted or insufficient return air opening.

You and I are thinking along the same lines; thank you so much for looking into this for me!

I'm not familiar with the concept of "return air"... can you explain? Is this air that's drawn into the furnace to cool the heat exchanger? What is the different between "return air" and air that's drawn in from the outside of the AS?

We're getting closer to a solution...

[dznf0g]

In addition to return airflow minimum, there is a minimum hot air outlflow spec. I only see one duct coming out of the furnace. I believe you need to have a minimum of four 4" ducts, as well as good return flow.

[trekerboy]

Originally Posted by trekerboy

I'm not familiar with the concept of "return air"... can you explain?

Ok, I think the source of my issue is that I've not properly vented the compartment where the furnace is installed for "return air". I found this text in the Furnace Service Manual:

Return Air - The cabinet that the furnace may be installed in will have louvers or openings for the return air back to the furnace. When the furnace is installed, it is imperative that the return air louvers on the furnace cabinet openingare not obstructed.

Usually, these furnaces are installed under a counter, sofa or bed in order to be out of the way. A grille or opening must be built into the cabinetry or into the base area of the sofa or bed. Return air from the living area of the trailer is drawn in through the grille and into the return air openings in the furnace cabinet.

Insufficient return air will cause the furnace to overheat and cycle on limit.

I don't have any sort of ventilation built into the area where the furnace is installed (under the rear bed). And in a previous post when I had the top lifted off the bed the furnace worked fine. Although, even in the case where I had the top lifted off the bed, I still had to remove the access panel on the furnace to allow it to fully function properly (otherwise it would overheat and shut off the gas valve). This seems to indicate that perhaps I have a combination of restricted outflow (through the vents) and not enough return air to cool the thing.

I think my next step is to build some ventilation into the cabinet where the furnace is installed to allow more "return air". Then, I need to go through the process of seeing if there's restricted outflow anywhere.

Thanks for your help everyone! Getting closer!

[nrgtrakr]

Very good ...
If the fan is blowing air out through the ducts, where does that air come from? The air inside the trailer circulates through the heat chamber via the vent fan. Air coming in is called "return air".
But if your furnace specifications call for more ductwork than how you installed it, once you've supplied adequate return air, it could still shut down early from overheating, or the hot air coming out of the duct could be too hot and cause problems (melting plastics, burning wood).
The combustion chamber is vented to the outside. One vent provides the air for combustion, the other vent is the exhaust.

[trekerboy]

Originally Posted by trekerboy

I think my next step is to build some ventilation into the cabinet where the furnace is installed to allow more "return air". Then, I need to go through the process of seeing if there's restricted outflow anywhere.

Ok, so I thought this was a problem of not enough return air + restricted outflow through the vents, but now I'm not so sure... Here's what I've done since I last posted.

I got 2 atwood quiet blowers (which, by the way, are nowhere near quiet) to help pull air away from the heat exchanger in the furnace to compensate for all the twists, turns and ridges ducting I have. Each of those is attached to an adjustable voltage regulator to I can control the speed of the fans (slower=quieter). Then the fans are powered through a SPDT relay that's active when the furnace thermostat tells the furnace to turn on.

I also added to large return air vents to allow the furnace itself to suck in air from the cabin instead of just the limited supply that's under the bed. Here's a diagram of the air flow and furnace under the rear bed:


(click/tap to enlarge)

So basically, the goal with all of this was to try to pull as much air away from the heat exchangers on the furnace as possible, while also supplying enough "cold" air from the cabin to the furnace so that the furnace wouldn't overheat after about 11 minutes and start blowing cold air...

Well, guess what? It didn't work

Even though air is being pulled from the exchanger and blown out the heating vents with the force of a small leaf blower, the furnace is still turning off after about 10 minutes of use!

Not sure where to go from here. I can't determine if this is a problem with the furnace or a problem with the thermodynamic properties of the space in which the furnace was installed. I don't want to go out and buy a new furnace only to have the same problem.

One thing to note... prior to the furnace shutting down the air that comes out of the heating vents is really, really hot. So hot, in fact you can't hold you hand up to it for more than 5-10 seconds with it becoming so uncomfortable you need to pull away.

Could it be possible that my temperature limit sensor is working properly but the exchanger itself is just getting to hot (despite excellent airflow away from it)? Is there a way to adjust the flame in the furnace to lower the heating output?

[mayco]

You can adjust the air/gas mixture but no way that I know of to adjust the amount of gas dumped into the burner assembly.

[idroba]

There also is a possibility that your blower motor is just slow and tired and not moving enough air. Normally one would think that the sail switch would shut the system down altogether without enough air moving, but you might be on the edge of your performance envelope.

Another possibility is that the sail switch is not working, that is it says there is air movement when in fact there is not enough. Usually they fail in the open condition, but one could also fail in the closed position, telling the control board that enough air was moving, when in fact it is not.

I once found a bent "sail" control rod which would not allow the furnace to turn on. I can envision one being bent so it was always on, and thus not preforming it's function.

A long shot, but one more thing to check.

[trekerboy]

I called a suburban repair facility today to talk to them about my situation.

After speaking with them, I'm now thinking the heat exchanger is just getting old and burning up. I told them that the furnace itself (the outer casing) gets really, really hot, too hot to touch, and they said it should get warm but not too hot to touch. Also, the air that's coming out of the vents will burn your hand if you hold it in front of the vent for too long.

They couldn't give me a definitive diagnosis over the phone, but I'm glad it looking more like a faulty product than a faulty installation... If I had to relocate the furnace somewhere else in the camper it'd be a major tear-up for me.

So, next step is to take it in for service and see what they say. The heating element is only $30 if it has to be replaced, the control board somewhere around $100, so with labor I'll be at about $200 which is cheaper than replacing it. However, if it looks like everything else is starting to call apart in the furnace too, it might just be time to replace it.