Improper furnace install in 2720sd. Throttled.

This is a TM design fault which is probably in all 2720SD models.
(At first I thought that this was a manufacturing error...cold air return partly blocked due poor install but after analysis and testing it seems that TM decided to engineer in a 12.5 sq inch cold air return instead of the minimum 55 sq inches specified by Suburban for the NT20s furnace. This causes the furnace to cycle on the limit switch and run much longer to achieve called for temperature.)


The furnace in our 2006 2720sd had very low output. I would say it was less than a 1200 watt electric heater which puts out around 4000 btu's. Since the suburban nt20s is rated at 19,000 btu's something had to be wrong. At 32 degrees the furnace would not even get the interior of the TM to 55 or 60 degrees. Also the longer it ran the less it put out. It was cycling on the internal limit switch (180 degrees).

When I removed the plywood lid covering the furnace compartment, the furnace immediately started pushing more air at a higher temperature. With the lid open the temperature in the trailer reached 72 degrees quickly. When I replaced the plywood lid the problem came back.

The problem was that the cold air return's plastic vent to the furnace was installed improperly and was sitting almost directly against the furnace wall leaving about 2 or 3 sq inches of the cold air return open through louvers in the furnace side. If the cold air return had been placed a few inches further towards the middle of the trailer there would have been much better air flow (but still not enough when I checked further).

I wasn't about to cut another hole in the plywood so I modified the cold air return plastic vent by removing most of the collar which it didn't need anyway. This left the vent with 12.6 sq inches of air return. The furnace breathes better and the hot air from the ducts was much hotter and at better volume. I could now heat the trailer to 72 degrees with an outside temp of 32 degrees although I still thought the furnace output too low (and it still cycles on the limit switch which slows heating). After more investigation I found that the furnace output must be 25 sq inches ( 2 4" ducts) and the cold air return must be 55 sq inches. This means that the 12.5 inches provided by TM for cold air return is only 1/4 of the manufacturer's minimum requirement. I will have to increase the size of the cold air return.

Another thing I noticed while checking out the furnace flame etc was that when I had the cover off the end of the furnace the heat coming out was much better than when the end cover was on and the heat was coming out via the 2 ducts and louvered vents. In the 2720sd they use the S version of this furnace with side vents for ducts. Unfortunately there is no room to use the side vents so they cut 4" cutouts in the end cover and route the heat through 2 foot pieces of accordion flex hose to the end of the dinette where it exits via louvered vents. I believe that the furnace performance is being impaired by this Kludge as well. Most of the other TM models use a direct output furnace with no ducts so this will not be a problem for them.

I am going to replace the flex hose with semi-flexible 4" hose which has a smooth inner wall of full diameter. This is supposed to have 50% better air flow than the plastic flex with it's accordion inside surface. In addition I will seal the joints with metalized tape (right now there is a lot of leakage).

So the furnace was throttled at both ends thanks to TM's engineering and installation. If other owners have similar problems the corrections are quite easy. A 19,000 btu furnace should be able to put out as much as 4 or 5 1200 watt electric heaters.

Peter

Our 2010 Elkmont had similar problems.

For return flow I put in a big household return vent.
For out flow they were only using one of the 4" dia round side ports. I cut a much larger front vent directly into the living area and used the 4" side port only for the bathroom.

I also replaced the accordion flex hose with Al dryer exhaust tubing to the bath.

The furnace was also not screwed down.

Elkmont owners can find more details in the Elkmont section of the board.

Hope they've improved their furnace installation with the reorganization, it really needs it!

Cold air side: I replaced the 4" round return air register with a 8 by 10 inch brass colored return air register from Home Depot. It fit nicely and clears the table etc in the dinette.

Hot air side: Rather than replace the output ducts I took the ends off the hot air registers and stretched the hoses out until the kinks and accordion pleats smoothed out. I left a bit of slack for movement and refastened the ends in place. I also used aluminum tape to seal off gaps left by TM in the connectors.

Result: No more cycling on the 180 degree limit switch. Interior TM temperature went right up from 40 degrees until the thermostat said 72. The hottest the output I got was 169 degrees F with a cold air temp of 66 degrees. This is a TR (Temperature rise) of 103 degrees which is within limits for the NT20s furnace.
There is a change in noise from the cold air return...it is a bit louder but is now more white noiseish.
This good enough for me. Now I can go winter dry camping.

Possible improvements: Suburban ideally wants a TR of 90 degrees and this could be accomplished by rearranging the hot air output. The way TM taps the hot air from the furnace is not very well done. Two 4 inch ducts side by side on the front of the furnace would yield more heat than putting the ducts one above the other. The hottest air is near the top of the furnace and that duct is running 10 to 15 degrees hotter than the one on the bottom. Moving the bottom duct up to the top beside the other on would be an improvement but not something I want to do right now.

The attached documents helped me to correct the defect.

Peter

[Lesherp]

I have a 2720 SL. It is closed up right now in the garage and I have never looked at the furnace. I am not sure exactly what all you did without looking and am wondering if any of these problems you described might pertain to my trailer..

The 2720sl uses the nt20se furnace which has no ducts so the output side should be ok. I don't know how these get their cold air return. Take a look at how big the openings are for air to get into the furnace area.

The other difference in the nt20se is that the limit switch has a 170 degree setting rather than 180 degrees for the nt20s used in the 2720sd.

If your furnace is attempting to heat the trailer and the output temperature goes up 170 degrees and then drops to 100 degrees or so before cycling back to 170 degrees then back to 100 etc etc. it is cycling on the limit switch. You will hear clicking sounds as this occurs. This has nothing to do with the thermostat which keeps telling the furnace to make heat until the set temperature is reached. This cycling burns battery power because the fan keeps running all the time and it takes longer for the furnace to heat the room. In the case of a severely choked system such as I had the furnace could not heat the TM from 32 to 72 degrees. It made it to about 50 degrees and just cycled away making no more progress.

Peter