Atwood Furnace runs great gets to temp gas shuts off and fan won't stop running

Hey guys thanks for reading....  Just bought a new pop up camper 1999 Camplite and it is awesome.  I have been a camper for quite some time, but this is the first pop-up I've ever owned.  Kids love it!!!

Okay, furnace seems to have a problem and I need a little help.  Don't have a huge budget so I don't want to take it in to get serviced, but I am very mechanical and can replace or repair things if I have a little assistance.  Here's my problem...

I have two propane tanks 100% completly full.  Also have a good 12 volt battery hooked up to a 6 amp battery charger full time so I know I have about 12-14 volts.  Furnace only runs off of the battery.  I can have 110volts plugged in and the furnace does nothing without the battery.  I also switch the tanks from one side to the other and get the same results.

When it is cold outside the furnace will turn on okay.  Kinda has a little bit of a poof when it lights, but it does light.  Scares me just a little bit cause it pops when it lights (sometimes).  After that furnace runs great and heats up the camper.  No soot, black smoke, or weird smells.  Problem is when the temperature gets to where it needs to be the gas shuts off.  Which is normal, but the furnace fan needs to run a little while longer to cool the parts inside which is exactly what it is supposed to do.  Problem is it never shuts off and just keeps blowing and blowing.  30 minutes later it is still blowing very cold air.  So I shut off the theromstat and turn it back on again and it turn on heats up the camper great and the gas turns off and the fan doesn't stop running and does the same thing over and over again.

I am guessing that I have a bad sensor or switch.  I figure it is something pretty easy.

Furnace is an Atwood hydro flame.  It has an automatic pilot light.  The label is missing, but I have an owners manual for a 7900-II, 8000-II, 8500-II, 8500-IV, and 8900-III.  So I am guessing that it is one of those models.  It has a light brown cover that you need to push down on to remove the front.

    Thanks a ton,
        Chad

I've written a furnace trouble shooting document and it is in the Forum library.
RV Furnace Trouble Shooting

The fan time delay may be a separate component or (on most recent models) controlled by the circuit board. They are typically time based rather than temperature controlled.

"I have two propane tanks 100% completly full."

I thought that propane tanks were only suppose to be filled to 80% capacity.

seilerbird said:

"I have two propane tanks 100% completly full."

I thought that propane tanks were only suppose to be filled to 80% capacity.

Click to expand...

Our propane tank gauge reads 100% when the tank is 80% full. Your's may be doing the same. All newer tanks, and older ones when refilled, are required to have a valve that doesn't permit filling in excess of 80%.

Question: Does the furnace start immediately or does the blower run for 10-20 seconds before the flame ignites? Have one person stand outside by the furnace cover and another by the thermostat. With the system reset (blower not running, no flame, thermostat set to a low setting - 60-65 degrees - have the person inside turn the thermostat to a high setting - 80-85 - and holler out the window "ON". The blower should come on soon thereafter, and the flame should ignite about 5-10 seconds after that. Is that what happens? That's the way is should happen. Try it several times and report back with the results

Furnace fan runs about 20 seconds before the flame turns on.  Kinda has a poof when it actually lights, but runs fine when it is running and gets up to the thermostat temperature and the flame shuts off.  Then like I said earlier the fan keeps on running and running and doesn't ever shut off.  Blows cold air and eventually I shut off the thermostat and wait a couple of seconds and turn it back on and it does the same thing over and over again.

    Thanks in advance for your help...

            Chad

You might want to remove the controll board and take it to an RV repair place that can test it for you.

Sounds more like the heat-exchanger temperature switch than the circuit-board....? I know from the description of symptoms that it is the supply (or circulation fan) that is "running-on, but when does THAT fan first come on. Does it come on immediately on a call for heat, or does it come on when the heat-exchanger warms a bit? If it comes on immediately, then either the temperature switch for the heat exchanger, or the relay for the circulating fan could be defective. The temperature switches for such things are usually of the "clicks-on" variety. Very inexpensive. When they fail, they usually fail to make contact because it is usually the bi-metal plate that cracks. Occasionally the crack is not severe enough to cause failure to make contact, but is enough to cause  it to not be able to break contact. It is certainly possible for the board to cause the symptom. It could be ignorantly holding the fan on trying to "make" the set-point temperature, but usually such functions are controlled by the permissives (temperature switches). I would opt to check that before spending the money for a new board... and would probably replace the "clicks-on" even if the problem was found to be the board for certain.

Thanks for all the replies.

A couple of more details.  I know the propane is full and I have a good 12 volts.

When I turn on the thermostat.  The blower fan turns on about 5 seconds after I flip the switch.  The pilot light is not lit yet.  The fan continues to run for about 15 seconds and then I hear the click, click, click and poof the furnace starts heating up.

I thought that this might be normal because my furnace at home does this, but from some of the replies I have received it doesn't sound like it should work this way.

I am sure a lot of you can understand that I don't want to take this thing in because next thing you know it will cost me $500 bucks and I just can't afford that right now.  This web site has been very useful.

    Thanks again,
          Chad

The combustion fan should run before ignition, but the circulating fan should not until the heat-exchanger is warm enough. Are you getting air from the diffusers before ignition?

We have a furnace like the one shown in the FURNACE TROUBLESHOOTING guide.  Its motor drives the combustion and circulating fans off of the same motor. 

We had a similar problem, except that the fan would turn off, after a long, long cool down period.  I found that the anticipator needed to be adjusted.  The first photo shows our thermostat and the second shows the internals.  The pen tip is pointing to the anticipator, which is now set to the lowest position.  This position shuts the fan off after the shortest time period.

You may have the same situation, check it out.

Bob Flight....

You got it!!!  I pulled off my thermostat and the anticipator just as you described was on the high setting actually touching the small rivet that attaches the metal band to the plastic housing.  I adjusted it mid-way and the furnace shuts off and turns on no problems.

I would think that it would just shut off after a while on the high setting, but since it was touching the rivet I think the resistance was just too much and would never allow it to shut off.

Thanks guys for all the responses...  This website is awesome...



 

Glad to hear you got it fixed and that it was a simple fix.  Thanks for the feedback.

We could use another whole trouble shooting guide just on thermostat problems. I intentionally left most of them out of the furnace guide because there are so many types and models in use over the years that cataloging all their idiosyncrasies may well be impossible. Any volunteers?

Great call Bob!! I thought it sounded more like a switch problem and you hit it right on the head.

I'm not willing to take all of the credit.  In part my suggestion came from experience.  In part it came from Gary's Furnace Troubleshooting Guide.  The last page provides a link to Bryant's Furnace Manuals.  Scroll down to the manual for the HydroFlame Furnace and review it's manual.  On sheet 21, it talks about the thermostat, see below.  The Atwood link also has alot of info.  I was not aware all of this info was in the library and found it very informative.

Lewis- I'd be concerned that you still have it set too high.  The sheet 21 discussion cautions about having the setting above 0.48 amps.  Ours has printing at that point (I had to use a magnifier to read it), which is below the mid point.  You may wish to double check yours.

Gary- I have only had to fiddle with the old contact style thermostats, and rarely at that.  The newer digital ones are mostly not adjustable or fixable.  For a Thermostat Setting Guide, could we just single out sheet 21 in the library?

We cannot legally copy the page to our site without permission from the publisher, but we could perhaps link to it. However, if since it is a page within a PDF fle, we can only link to the complete document and tell folks to look at page 21.

It doesn't really tell much about t-stats either, but at least it helps a bit with the anticipator adjustment, which is a feature of the old analog t-stats.

Gary is correct about both the copyright issue and the ability to link to a page within a pdf document.

BTW we have links to manuals, including Attwood - just click the Manuals button above.

Bob,

On page 21 of the manual it says the factory setting is 1 and it should not be set below .48 amps.  Therefore I would be safe at setting it at say .75 amps.

I think that you wanted to say below .48 amps and not above as your quote says below.  Or am I missing something?

"Lewis- I'd be concerned that you still have it set too high.  The sheet 21 discussion cautions about having the setting above 0.48 amps.  Ours has printing at that point (I had to use a magnifier to read it), which is below the mid point.  You may wish to double check yours."

  Thank for the info.  I did have it set at .4amps which is probably a little too low if I am understanding everything correctly.

Lewis- you are correct.  Ours is set for .6 and the fan shuts off after the furnace has cooled off, so we are not blowing cold air.  Glad it worked out.