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Old 11-28-2015, 04:16 PM   #1
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Furnace seems to be running too much

I have a new to me 2000 36FDS and the furnace seems to kick on for 20 minutes then shut off. It only takes about 10 minutes and it's back running again. We set our temp low at 62 overnight but it still seems to be running a lot, even when I set it at a higher temp, as well. It does not seem to be pushing a lot of air out of the floor vents when it is running. Is there some place I can look that might make a difference or is this just standard. Our trailer pushed out more air so thats what I'm using as my basis for "lack of air".
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Old 11-28-2015, 06:06 PM   #2
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May not be quite the same for my 03 but here are a few suggestions. Depending on where the furnace is located, make sure the return air duct and fan cage are not obstructed. Check for obstructions or kinks in the output ducts. Thermostat on many Alpines are located forward of amidships so heating in colder weather is a challenge in the front--limited output vents. Try blocking or reducing hot air flow to the bedroom--not really needed. Last, WRV used dryer hose for duct work thru the basement--lots of heat loss. Suggest replacing it with insulated 4" residential duct [20-25ft box] from Home Depot or Lowes ......
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Old 11-28-2015, 07:09 PM   #3
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Thats a great idea. I'll start messing with it. I was looking at the ducting and that makes sense. I thought it looked a little on the cheap side! The pleasures of owning a used RV! I'm going to pull the furnace out when we get home and I can check that end out. Upgrading the piping seems like a no brainer as well. It's funny the bathroom is like walking into a sauna and both ends are cold. We closed the bathroom vents to help push more to either end. We'll see what happens tonight! Thanks for the direction.
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Old 11-28-2015, 07:24 PM   #4
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I am sitting here timing the on off cycles. 3 minutes off and 7 minutes running. It seems like an extremely short time to be off before starting up again.
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Old 11-28-2015, 07:35 PM   #5
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Check the anticipator or replace the thermostat.


Thermostat Heat Anticipator, Introduction
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Old 11-28-2015, 07:59 PM   #6
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Make sure none of the discharge vents are pointed at the thermostat, if they are the T'stat will think it's warm enough and shut off too early.
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Old 11-28-2015, 08:16 PM   #7
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Make sure the temp compensation setting on the furnace gas valve (is stamped on the valve=1.??? to 3.??? possibly) matches what is set on the thermostat. This may not be applicable any longer, but it was on my camper years ago. We are all assuming you have a propane furnace. Look up compensation values, and settings on the internet to see how they work. If it's really cold where you are, then the insulation in the walls is not done very well, and there are all kinds of air leaks especially is single pane windows, so your run times might be normal. Oh, your furnace is most likely working fine, so taking it out unless you are familiar with doing so might not be the best idea around. Based on original post, it sounds to me like it is doing its job.
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Old 11-29-2015, 07:45 AM   #8
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PS--not sure on the 2000 but the temp sensor for the front thermostat [controls the furnace] is behind the small white disc above the drivers seat. The sensor is pretty close to the window and pretty far from the heat vents--accordingly, it will take quite of bit of time for the "new" heat to reach the sensor and not very long for the area around the window to cool. So cycling in cold weather is likely. Might also consider some sort of temporary thermo blankets for the drivers side and front windshield area to reduce heat loss. My 03 is working pretty well after doing the aforementioned upgrades [not the thermo blankets]. Have considered adding another heat vent up front but floor location is an issue and it would take some work. Frankly--7-8 days at Quartzsite is about the only time we really need it so havent pursued it.....
PSS--just to be clear--you are talking about dry camping here [no electric] and you are not/cant use the heat pumps, or its too cold for heat pumps???????
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Old 11-29-2015, 01:29 PM   #9
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It does not seem to be pushing a lot of air out of the floor vents when it is running.

I fixed a similar “lack of air flow” problem with the furnace on our 2001 Alpine 36FDDS in which the burner cycled on & off every 5 minutes or so. The fan blower didn’t stop, but the burner flame cut off for a minute or two, and then relit again. An RV service tech replaced the thermal cut-off switch, but the cycling continued, so he said the unit was overheating. I read the Suburban install & owner’s manual and found that WRV ignored the specifications for both fresh air intake and heat duct outflow in our coach. I solved the cycling problem with fairly simple DIY mods.

Suburban specifications: The minimum open duct area for Suburban Model SF42F is 48 sq. in. for side ducts with a minimum of 4 used, or 72 sq. in. for the bottom duct. You must not count ducts 2” in diameter or less and any ducts ending in dead air space like the holding tank area. Each 4” duct is a maximum of 12.57 sq. in. (calculated by Pi x diameter 3.14 x 4). So 4 ducts = 50.29 sq. in. which is slightly over the minimum for side-ducting.

However, our Alpine only had one 2” side duct (not to be included in the count per Suburban), and the bottom duct goes into a distribution box that only has three 4” duct tubes attached. This makes a maximum duct outflow area of 38 sq. in. This area is further reduced by WRV’s use of flexible aluminum dryer vent tubes for vents, held up to the ceiling of the storage bay by cable ties that partially crimp the tube to a smaller diameter. The effective duct outflow area is probably less than 30 sq. in., far short of the requirement. In addition, the wooden grate covering the furnace compartment under the refrigerator has slots with a total area that is below the minimum required by Suburban for fresh air intake, but I don’t remember my calculation of this. I fixed the air flow problem with these mods:

1. The existing 2” side duct from the furnace split into a Y junction with one branch exiting at the dining area and the other going down into the basement storage. I removed the Y junction and made the 2” duct exit only into the basement, and then added another 4” duct to the side of the furnace and made that exit directly into the dining area.

2. In the basement storage area I removed the long flexible vent tube from the furnace distribution box running forward to the hole in the floor under the curbside window. I replaced this with 4” duct components from Home Depot – 90 degree elbow couplings at the distribution box and the floor hole; a semi-rigid aluminum tube from the distribution box with a slight curve around the HWH components and through the frame hole into the forward storage bay; and a 60” rigid vent tube through the forward storage bay to the floor coupling. We had previously removed the curbside couch, so the duct exits at a floor register under the desk we have there now.

Those two mods fixed the burner cycling problem and increased the warm air flow significantly, so I stopped there. The 2 other existing 4” flexible duct tubes run from the distribution box back into the waste tank compartment and through to the bedroom & bathroom, and would be more complicated to replace.

Fresh air intake grate: If I find with more furnace use during this winter that the burner cycling problem reoccurs, I will remove a few of the slats in the grate with a router to increase the air intake area without affecting the appearance too much.

Michael
2001 Alpine 36FDDS
Bellevue, WA
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Old 12-01-2015, 12:13 PM   #10
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I just got back into a place where we have internet. I will start working on rerouting the heater ducting. I should be able to get "Big Al" washed and into the shop tonight. I was dry camping as well so the heat pumps were not an option. When it is said it makes sense that the heater is not flowing enough and over heating. I'm going to start there and then work my way around. My neighbor's son is an AC guy and he is going to come over and make sure I'm all good on that end as well. I'm glad I wasn't on a month long trip somewhere, just a long weekend.
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Old 12-02-2015, 07:15 PM   #11
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Quote:
Originally Posted by mbweitz View Post
It does not seem to be pushing a lot of air out of the floor vents when it is running.

I fixed a similar “lack of air flow” problem with the furnace on our 2001 Alpine 36FDDS in which the burner cycled on & off every 5 minutes or so. The fan blower didn’t stop, but the burner flame cut off for a minute or two, and then relit again. An RV service tech replaced the thermal cut-off switch, but the cycling continued, so he said the unit was overheating. I read the Suburban install & owner’s manual and found that WRV ignored the specifications for both fresh air intake and heat duct outflow in our coach. I solved the cycling problem with fairly simple DIY mods.

Suburban specifications: The minimum open duct area for Suburban Model SF42F is 48 sq. in. for side ducts with a minimum of 4 used, or 72 sq. in. for the bottom duct. You must not count ducts 2” in diameter or less and any ducts ending in dead air space like the holding tank area. Each 4” duct is a maximum of 12.57 sq. in. (calculated by Pi x diameter 3.14 x 4). So 4 ducts = 50.29 sq. in. which is slightly over the minimum for side-ducting.

However, our Alpine only had one 2” side duct (not to be included in the count per Suburban), and the bottom duct goes into a distribution box that only has three 4” duct tubes attached. This makes a maximum duct outflow area of 38 sq. in. This area is further reduced by WRV’s use of flexible aluminum dryer vent tubes for vents, held up to the ceiling of the storage bay by cable ties that partially crimp the tube to a smaller diameter. The effective duct outflow area is probably less than 30 sq. in., far short of the requirement. In addition, the wooden grate covering the furnace compartment under the refrigerator has slots with a total area that is below the minimum required by Suburban for fresh air intake, but I don’t remember my calculation of this. I fixed the air flow problem with these mods:

1. The existing 2” side duct from the furnace split into a Y junction with one branch exiting at the dining area and the other going down into the basement storage. I removed the Y junction and made the 2” duct exit only into the basement, and then added another 4” duct to the side of the furnace and made that exit directly into the dining area.

2. In the basement storage area I removed the long flexible vent tube from the furnace distribution box running forward to the hole in the floor under the curbside window. I replaced this with 4” duct components from Home Depot – 90 degree elbow couplings at the distribution box and the floor hole; a semi-rigid aluminum tube from the distribution box with a slight curve around the HWH components and through the frame hole into the forward storage bay; and a 60” rigid vent tube through the forward storage bay to the floor coupling. We had previously removed the curbside couch, so the duct exits at a floor register under the desk we have there now.

Those two mods fixed the burner cycling problem and increased the warm air flow significantly, so I stopped there. The 2 other existing 4” flexible duct tubes run from the distribution box back into the waste tank compartment and through to the bedroom & bathroom, and would be more complicated to replace.

Fresh air intake grate: If I find with more furnace use during this winter that the burner cycling problem reoccurs, I will remove a few of the slats in the grate with a router to increase the air intake area without affecting the appearance too much.

Michael
2001 Alpine 36FDDS
Bellevue, WA
Thanks for your post.

I just did part of your mod and will see how things improve over our New Years trip.....normally gets nice and cold here in the high desert. I added 90 deg fittings at the collection box and the most forward vent and the rigid flexible hose (about an 8 foot length).

I also cleaned out the heater area under the refrigerator. Don't know if I ever did, but was surprised that it was not terrible.

I did a quick computation of the area of the inlet and it is 0.5x9x33=148.5 sq inches.
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Old 12-04-2015, 06:27 PM   #12
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The grate opening appears to be pretty good. I have an anemometer and measured the air speed with and without the grate in place. I measure about 5.1 mph with the grate in place and maybe another 0.1 to 0.2 mph with it not there. I just put the anemometer in the center of the front floor outlet. I wish I had thought of this before doing the mod.
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Old 12-07-2015, 11:14 AM   #13
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The Suburban Furnace Installation Manual states that the return air requirement for Model SF42F is 142 sq in, but that this can be reduced to 88 sq in if 5 ducts are used. So your grate measurement of 148.5 sq. in. is good.

The return air grate on our furnace is apparently smaller than yours. The whole wooden grate below the Dometic refrigerator measures 27.5" x 12.5". It has 33 slots, but each is only 6.5" long (+ small rounded ends) and 7/16" wide. So I calculated the total area as 98.8 sq in. This would be OK if there were 5 ducts installed, but the original WRV installation only had three 4" ducts, that I have recently increased to four ducts. So mine is still below the requirement, and I'll need to monitor it for thermal cut-off cycling.

Michael
2001 Alpine 36FDDS
Bellevue, WA
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