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Old 04-03-2016, 06:43 PM   #1
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Front furnace limit cycling

Just got back from a trip this morning during which we camped in 40 deg nights. Started the front furnace in my 390RB and noticed it lights and runs for about 90 sec and then apparently hits a limit, shuts down the burner but the fan continues to run. The burner stays off for about 3 minutes, relights and repeats the cycle. As a result it never reaches the requested temperature and just continues to run in this manner. This furnace has some rather small ducts under the loveseat so I'm wondering if the ducting is restricting flow or if I have a bad limit switch?

Anyone else have this happening?
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Old 04-03-2016, 07:57 PM   #2
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I had a similar situation with my other MH I first checked to make sure the inlet is clear. I changed the limit with a new one and that should be your first act if the inlet is clear since it is also the less expensive. You can also check ductwork for restrictions but unless you have critters making a nest in them I would doubt the ducting is causing the cycling. When I called the manufacturer they told me the fan motor will slow down with time so if the new limit does not correct it them check the fan blower and clean it if it's dirty if it's not replace the motor.


Wayne
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Old 04-04-2016, 08:24 AM   #3
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Edge, I haven't had the issue (yet), but please keep us posted about what you find. You and I have the same coach and I am interested in how it works out. Good luck and I hope it is an easy fix. -Glenn
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Old 04-06-2016, 08:07 PM   #4
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Tried the furnace with the two 4 in ducts disconnected from outlet vents. Cycle time improved from 90 sec on to150 sec on. So added another outlet vent, reconnected ducts and cycle time stayed at 150 sec on. Replaced limit switch, no change in time on. Blower seems to working fine but is probably the culprit. Also suspect propane regulator.


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Old 04-06-2016, 08:28 PM   #5
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As a test when mine acted up I bypassed the limit temporarily to test it everything worked fine and put out heat. When I removed the bypass went back to its old ways.


Wayne
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Old 04-06-2016, 10:47 PM   #6
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Well I do think somebody at Forest River screwed up on the duct arrangement on the front furnace. They took two 4 in ducts and placed a flow splitter on each to split the flow into two 2 in ducts (for a total of four). That's a drop from 25.4 sq in of flow to 12.5 sq in. Talk about a flow restriction. I removed those splitters and added a plenum that supplys the four two in ducts under the loveseat and another standard 10" x 2" floor register that discharges beside the loveseat. That gets the flow area up to about 32 sq in which meets the Suburban recommended minimum of 25 sq in.

Even with those changes and a new limit switch my run time is only about 150 sec before it cycles.

I have also noticed that the flex ducts that deliver the heat on the front furnace have about 18 in of excess that can be removed thereby eliminating several kinks and bends. I'll shorten those in the near future.

Measured the brass elbow that enters the heat exchanger chamber (its the propane supply) with an infrared thermometer since there is significant airflow leakage at the elbow. The front furnace runs about 20 deg F hotter than the rear (same model furnace). So the blower is suspect. I may try measuring the current draw on the front and rear to see if that indicates a weak blower before I start removing the furnace and searching for the best price on a new blower.
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Old 04-07-2016, 06:58 AM   #7
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Quote:
Originally Posted by edgewant View Post
Well I do think somebody at Forest River screwed up on the duct arrangement on the front furnace. They took two 4 in ducts and placed a flow splitter on each to split the flow into two 2 in ducts (for a total of four). That's a drop from 25.4 sq in of flow to 12.5 sq in. Talk about a flow restriction. I removed those splitters and added a plenum that supplys the four two in ducts under the loveseat and another standard 10" x 2" floor register that discharges beside the loveseat. That gets the flow area up to about 32 sq in which meets the Suburban recommended minimum of 25 sq in.

Even with those changes and a new limit switch my run time is only about 150 sec before it cycles.

I have also noticed that the flex ducts that deliver the heat on the front furnace have about 18 in of excess that can be removed thereby eliminating several kinks and bends. I'll shorten those in the near future.

Measured the brass elbow that enters the heat exchanger chamber (its the propane supply) with an infrared thermometer since there is significant airflow leakage at the elbow. The front furnace runs about 20 deg F hotter than the rear (same model furnace). So the blower is suspect. I may try measuring the current draw on the front and rear to see if that indicates a weak blower before I start removing the furnace and searching for the best price on a new blower.

Sounds like you have a good action plan. I had similar concerns about my previous non FR unit as they had run more duct length then needed but when unit was new it ran great and did not cycle like it did when it got older so did that issue really matter? Did yours ever work okay or has it always cycled?


Wayne
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Old 04-07-2016, 07:38 AM   #8
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Well my duct work on my front furnace was similar. They ran two 4" from the furnace then sized each into two 2". So now you have four 2". Then it really gets crazy. They attach all four 2" to a board directly under the L-sofa.

All four dump out together and can go nowhere!! Just under the sofa. TRAPPED there. Originally I would raise the sofa bottom up a couple inches and let some heat out.

Then I did away with the L-sofa and the 2" heat runs. Went to two 4" heat runs and installed them under a six inch cabinet on each end of the new couch I put in.

The new set up heats the front of the coach great now.

Russell
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Old 06-22-2016, 10:34 AM   #9
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Solved my problem. Changed motor and no improvement.

Added a third supply duct that discharges in the corner of the return. Doesn't really comply with Suburban recommendations but it works great. Furnace now operates until set point is reached. The added supply discharges plenty of air and doesn't interfere with return air.
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