Furnace questions

First of all I would like express my appreciation for all that I have learned by following this forum. We purchased a new 2012 Cyclone 300C two years ago and I joined the forum then but this is my first post.

Now to the topic. We have experienced furnace problems right from the beginning with the furnace not lighting and going into lock out etc. In the beginning often times it was operator error and I worked through those issues by reading through the various posts. Compounding the problem was the fact that these issues would pop up intermittently and never when a repairman was available. We where occasional campers and did not use the furnace much. We spent a couple of months in Texas this winter and had more issues. We finally where able to get a repairman who was very good in his his diagnosis routine. He brought his own tank and regulator to eliminate the variables in our fuel system and that lead to a replacement of the regulator and we seem to be better....BUT!

We had to leave and where unable to give it a lot of time but so far, so good but the one thing that is bugging me is that when the furnace fires it cycles. You can hear it ignite and burn and it will do this a number of times during a heating cycle. It appears that the "high limit" switch is shutting down the gas valve while the fan continues to run, then it will re-fire and repeat this, sometimes several times until the thermostat is "satisfied" and the furnace will shut down. MY QUESTION IS: Is the furnace supposed to fire and continue to run until the thermostat is satisfied OR is it normal for this "cycling" of the burn. I am thinking perhaps I have weak or undervalued high limit switch. Thanks rickster

Hi riskster,

If the furnace ignites and runs for less than 30 seconds before re-igniting, the problem may be with the flame sensor. After ignition, if flame is not sensed in 7 seconds, the gas shuts off and a 15 second purge cycle beings, followed by an ignition retry. After 3 tries, there's a lockout and a thermostat reset is required to clear the lockout.

If the furnace runs for a few minutes and shuts down, then re-ignites and runs again trying to satisfy the thermostat setting, it may be the high-limit switch. Before tearing into the furnace, make sure there are no restrictions on the airflow coming out of registers. If you have some registers partially or completely blocked, or ducts are kinked reducing airflow, that could cause overheating at the burner, triggering the high-limit switch.

If you have the air return blocked, you could have an intermittent failure at the sail switch, interrupting furnace operation.

If there is debris or insects have nested in the outside air intake, that could cause an intermittent problem.

Sometimes there is a little oil in the propane supply. I've had it accumulate in the hose coming out of the auto-changeover regulator where it loops before running along the frame. That causes marginal propane flow. Service techs are supposed to check propane pressure to be 11 water column inches using a test tool at the test port on the furnace. But many probably don't bother.

Thanks for the tips I had gone through all of that. No ducts are covered and they are flowing air as they are supposed to. The tech did check the flow which 12" so I think we are good there. So, are you saying the furnace should NOT cycle on/off but should run constant until the thermostat is satisfied? I have been told both ways on this question. I am very familiar with home heating systems and when working properly they do not cycle, but are designed to run constantly until the thermostat is satisfied.

When I say cycling it is not a short or quick cycle as it will run for 5-10 minutes before shutting down and then re-firing. During that time the air from the registers is adequately warm. Is there an ohm value for the limit switch allowing for it to be tested? Thank you, rick

Is there by chance a floor register near or close to the thermostat? Maybe it could be blowing the warm air directly on the thermostat. If so,, maybe cover and try it.

Jim M

rickster said:

Thanks for the tips I had gone through all of that. No ducts are covered and they are flowing air as they are supposed to. The tech did check the flow which 12" so I think we are good there. So, are you saying the furnace should NOT cycle on/off but should run constant until the thermostat is satisfied? I have been told both ways on this question. I am very familiar with home heating systems and when working properly they do not cycle, but are designed to run constantly until the thermostat is satisfied.

When I say cycling it is not a short or quick cycle as it will run for 5-10 minutes before shutting down and then re-firing. During that time the air from the registers is adequately warm. Is there an ohm value for the limit switch allowing for it to be tested? Thank you, rick

Click to expand...

Once it starts blowing hot air, it should continue uninterrupted until it reaches the set point and the thermostat signals for it to stop. There is a 2 minute cool-down cycle where the gas shuts off and the blower continues to run, but that shouldn't happen until the thermostat set point is reached.

Could also be caused by loose wires on the switches or control boards. If the 12V electrical system is not delivering 12V, that could cause problems. And the board itself sometimes is the problem.

The furnace in my current, as well as my previous rv's, have operated in this same manner. You can hear the flame cycle on and off while the fan is running. I believe this to be normal operation, as it blows heated air the whole time, it just isn't burning throughout the entire cycle. I don't think the burner needs to be burning the entire time the fan is running. I could be completely wrong, but it seems to work fine, so I never thought to question it.

We had the same problem in our 2011 sundance. We had a heat register right under the thermostat and when the furnace came on it blew hot air on the thermostat shutting down the gas and then blow cold air to turn the gas back on. I covered the register and all is fine.


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We ran a test by setting the thermostat at 88 degrees (as high as it will go) and the furnace continued to cycle on & off without ever coming close to satisfying the 88 degree setting. The registers are not restricted and there is a good volume of air coming out all the registers. In milder temps when the heating cycle is shorter is doesn't always cycle on and off prior to "satisfying" the thermostat. It is more common when the outside temps are colder causing a longer heating cycle to satisfy the thermostat.

My next step is to ascertain if this cycling of the burner is normal or not. The furnace works and will heat to the thermostat setting then shut down with the blower continuing for a period of time. If this "cycling" in NOT the normal way of operation then I sense I have a problem with the high limit switch. rickster

Here's the sequence of normal operation. Notice step 9.

Furnaces with Direct Spark Ignition:
Each step in this operation must be completed in the listed order before the next function
will occur. To properly diagnose a malfunction and correct it, it must be determined at what
step the operation of the furnace failed.
1. When the temperature drops to a set temperature, the wall thermostat contacts close.
2. The fan relay coil is energized in the thermostat relay, completing the circuit to the
blower motor. (Some models equipped with a time delay relay have a 5-25 second time
delay after the thermostat contacts close).
3. The motor starts and, after reaching 75% of its normal rpm, the room air blower wheel
or blade activates the sail switch (microswitch), sending current to the temperature
limit switch (a normally closed switch), and on to the module board.
4. When energized, the module board has a built-in 15-20 second delay, allowing the
combustion air to purge the combustion chamber.
5. The module board supplies a high-voltage spark through high-tension wire to the
electrode assembly, at the same time sending voltage to the gas valve, opening the
valve.
6. Burner ignites.
7. The sensor probe (left probe) signals the presence of flame and the spark stops. (If
flame is not established within 7 seconds, the system closes the gas valve and goes into
lockout).
8. The fan switch (normally open) closes on temperature rise. This does not apply to
models with the time-delay relay. These models will keep the motor running for 3 to 5
minutes by the delay of the relay.
9. The gas valve closes when either the limit switch or thermostat contacts open.
10. The blower motor goes off when the fan switch or time delay relay opens.

Click to expand...

Do you have wasp guard on your furnace fresh air intake and exhaust vents? If not did your service person inspect for insect nexts in the furnace? This might be a factor.

Not sure if it is relevant but in my experience in the HVAC world, if the furnace cycles off and on but the blower continues to run. There maybe a problem with the heat exchanger in the furnace. Could be a crack allowing ambient air into the furnace. I would recommend checking with a CO detector to make sure no unburnt gasses are getting into the RV.


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With my recent woes with the Suburban furnace in my Trailrunner it turned out to be mother board. However when the repair was complete I had the same symptoms you are experiencing. I was told by both the technician and several other members at the rally that this cycling is "normal" in suburban furnaces. I accepted that as good information and am happy with the way the furnace is working. If you are experiencing uninterrupted heating cycles of 5 to 10 minutes between re-ignition there is a likelihood that all is well. ED. :angel:

Thanks for that info. At this point I will continue on with how it is working, accepting that it is "normal". Just for all to know the furnace intact/exhaust etc. is in pristine condition as all that was thoroughly checked from the get go. It is to bad that Suburban is not more approachable about questions etc. but there approach is to take the RV to a qualified/certified (That's Suburban Certified) tech. We live 120 miles from the Blackhills which is a meca for camping and rv's and there are only two of these certified techs in the area and neither one are connected to the well known dealers/shops. We where fortunate to find the tech we did in Texas. He was very good and methodical, unlike some who just throw parts at the problem. Our whole problem started with a bad changeover regulator and I have been told numerous times that this particular one that Heartland uses has been very problematic. That said the tech went through diagnosis steps by hooking up his own, known to be good, tank and regulator before just replacing ours. When that eliminated the problems of the furnace not working consistently he installed a new one and tested the pressure from both regulators. All that said if it is normal for the burner to come on and off during a heating cycle I will consider things good to go. Thanks for the help and I enjoy the forum. During this process I read virtually all most all the previous posts regarding furnace problems. I gleaned a lot of info from all those but most of all learned how many have had issues with their furnace and heating systems. For now I will close the book on this as we look forward to parking our camper at Custer, S.D. for the summer. rickster