kokilo

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