Furnace has insufficient exhaust ducting

[edubb]

Thought I'd pass this along for general info.  Our trailer has an Attwood 20K BTU furnace, pretty typical I'm sure.  But Forest River designed the trailer with two 4" heat ducts, and one 2" duct.  The 2" is worthless, and the Attwood manual says it doesn't count toward the required minimum 24 square inches of exhaust duct.  So we have the minimum required (two 4").  When I first saw this I almost laughed.  Heat a 26' trailer with two 4" ducts!!

When we camped in low 20 degree temps, I could tell the furnace was working harder than it should have to (it would cycle the gas on and off while the blower stayed on, I'm sure the furnace "firebox" reached max temp cutoff before the trailer reached full temp--in other words the furnace couldn't move the hot air out fast enough).

So long story short, I added another 4" heat duct run.  Had to relocate the fresh water tank slightly, move an electric outlet and the LP detector, but I got it done, and it works great!  All three 4" outlets blow hot air like crazy, so I was right to assume it could move more air.  The test is this weekend, we will camp at 4000', should be below freezing at night. 

 

[Karl]

Eric,

Congratulations on a job well done! Out of curiosity, what area did the 2" duct supposedly heat?

 

[edubb]

Thanks Karl, it was a fun project.  The 2" duct runs 12' from the furnace to the bedroom!  Can you believe that?  I'm sure they could have fit a 3", which would have helped. 

 

[Jim Dick]

Eric,

Just curious, was that 2" duct routed to the bathroom area? Most RV manufacturers do install a smaller duct to that area since it is smaller and a large duct will overheat it if the door is kept shut.

 

[edubb]

Eric,

Just curious, was that 2" duct routed to the bathroom area? Most RV manufacturers do install a smaller duct to that area since it is smaller and a large duct will overheat it if the door is kept shut.

I wish they had done that, and run a 4" up to the bedroom.  Poor design.  I'd like to replace the 2", but it would require dismantling half the trailer.

 

[Gary RV_Wizard]

Inadequate output ducting seems to be endemic to many makes & models or RVs. None of our trailers had enough (one was less than the furnace manufacturer's minimum) and one of our motorhomes had enough sq inches but the hoses were so twisted that hardly any air flowed. Finally got a factory rep to look at it and he took out almost 10 feet of excess duct hose after straightening the runs.  Heat flowed much better after that.  Still had short cycle problems, though, because a primary duct was cleverly located directly opposite the thermostat.

 

[edubb]

a primary duct was cleverly located directly opposite the thermostat.

Hah!  Sounds like my house.  I have a towel nailed over the thermostat, lift up towel to view thermostat!  Redneck engineering (I'm allowed to say that, I'm from Georgia )

 

[Karl]

Many years ago, a customer called and said her room got cold whenever she watched t.v. for any length of time. You guessed it - the old tube-type t.v. was sitting right under the thermostat.