Looking to Use OTR Heating Vents / Returns in conjunction with LP forced air

[neoneddy]

Seems like keeping the OTR heat is a good idea.  If I'm going to keep it, I need to build around it / with it.  So, why not try and use it?

I've looked at what is currently there, attached is an idea I have illustrating a flapper valve as I call it, it flaps up when the OTR system is on, gravity brings it back down when the forced air is needed. 

Has anyone done this?

[bevans6]

My initial thought was "will the LP forced air furnace have sufficient flow volume to work with the OTR ducts" and that is a real question for me.  The RV furnaces I've seen suggest quite small ductwork compared to the OEM, and the bus fan is huge.  Other than that, cold air return for the LP furnace is important.

Brian

[TomC]

The difference between OTR capacity and LP furnace is way different. OTR typically uses 2-2hp blowers to get the air through all the ducts, and has unlimited BTU heating from the engine.
LP heat (I have a 40,000btu) uses a fractional horsepower 12v dc blower. If you use the existing ducts, the LP will just get lost and you'll ultimately not get any heat out of the duct.
Use the LP furnace independently with its' own 4" ducts. I have 4 outlets from the back to the front evenly spaced and it heats the bus quite nicely, and quickly. Good Luck, TomC

[neoneddy]

Do you have any photos you could share?

Do you block off the OTR ducts  when not needed?

[bevans6]

I just leave the OTR system as it is, and turn it on if I'm cold while driving.  My OTR thermostat control doesn't work, so heat wise it's on full or full off, and I just turn the fan on with the dash switch.  I do use the OTR dash heater for defrost and driver heat.  I have a Suburban propane furnace for when we are parked.

[neoneddy]

Yeah that's my plan as well, but I was asking about the venting.

Do you use the same vents or add new?

Pictures of the final install? Also aren't you in Stillwater,mn?
Edit: nope, must have been a similar bus in the profile pic.

[TomC]

I took out the OTR heating system and floor mounted ducts (no A/C since it was a Portland, Or) bus. I reused the big heat exchanger for my interior heat. I mounted the heater under my cabinets in the hall mounted fore and aft. I have 4 square vents (no ducts) into the hallway. It is powered by 2-14" 12v radiator fans with a thermostat behind the driver's seat that I can reach when driving. I also have a manual ball valve to turn on or off the coolant going to the heater. The defroster in the front also runs off this coolant line, but is unaffected by the heater ball valve. The defroster also has a ball valve to control heat.
The big heater works by the fans going on and off. The defroster works both by controlling heat by the ball valve and by turning on the fan. 9 times out of 10, the defroster puts out enough heat to keep the bus warm with just the air going down the road-no fan needed. I've used the big heater only a handful of times. Even though it is way back near the rear, when it comes on, you count to 10 and you can feel the heat in the front of the bus-truly a huge heater. So now no floor ducts. Only the 4" ducts for the propane furnace-of which I elevated the kitchen cabinets 6" off the floor to facilitate the heater ducts to run under the cabinets. Everything works quite well with minimum wasted space. Good Luck, TomC

[buswarrior]

Busnuts have taken advantage of the stock duct work in a variety of ways.

The trick is having fans with the right compromise between efficiency, voltage, air movement for YOU.

The needs for keeping the chill off on a fall evening, or winter camping in the Yukon, the compromise will swing...

Amongst busnuts, we have seen the big DC HVAC blowers replaced with smaller ducted AC fans, inverter powered going down the road, shore or generator power when camping.

Propane furnaces fed into the stock ducting, propane furnaces with separate ducting, propane furnaces with the fans upgraded, and not.

And then there are those who dislike the propane furnaces and feed hydronic heat to the stock heat exchanger and all the fun that entails...

I like your gravity damper idea, with strong enough fans, that will be fun!

happy coaching!
buswarrior