Propane furnace data & test results

[ChuckMC8]

 A couple weeks ago I ran a test to  test how my furnace performs, thinking that guys in the planning stages could use the info.
     Click on my profile and go to my bus photo page to see my bus as reference-
MC8, dbl pane RV windows. Insulated with celotex (1- 1/2") Bus is finished inside.
Furnace is Surburban, 35K single. Propane. It is ducted with one leg to the front and rear and two in the center of the bus.
  Weater conditons, 19 degrees F by my porch thermometer.. The temp had not risen above freezing for several days. This is about as cold as it gets near Atlanta. My bus is stored inside building now :-) and it was colder than a  well diggers but inside. When its cold, it seems the bus is colder inside the building. Like a referigerator, I guess. A buddy in the HVAC business calls it "cold soaked"
My furnace is controlled by a Home Depo Digital thermostat, so it's digital thermometer is what I have taken the readings for this test.
    OK- Here's the deal, I was going for a ride in da bus, so I went out, plugged up the engine heater and turned on the furnace. I monitored the temp change every 15 minutes. Insde the bus-
9:15 temp 21
9:30------30
9:45------37
10:00-----42
10:15-----46
10:30-----50
10:45-----53
11:00-----56
11:15-----58
11:30-----60

I had to leave at 11:00, but it was getting comfortable inside. All that fabric and such has to asborb heat, so I think that heating the air in a shell would have been much quicker. On the flip side, tho, it would hold heat much longer sine the same fabric and such would radiate heat back into  the coach.

On the road, I  use a 12 volt ac/heater combo unit under the dinette seat that uses engine water (heater core) and the dash heat. As soon as the engine temp came up, it made plenty of heat inside the bus for going down the road.

If I were really tring to heat the bus from that cold temp w/o runing this test, I would have started the genset, plugged in the engine heater and I have two 120 volt plug in heaters that i use for campground heat as well as I would run the Propane furnace. It would have brought the inside temp up much quicker and exercised the genset with a decent load.

  I am not praising this setup, but I have read many times of guys wanting wanting to get opinions on heat requirements for planning purposes. I hope this helps someone-
Chuck MC8
         

[Tenor]

Great test Chuck!  It sure helps me do figuring!  Anyone else with different model furnaces/number of furnaces willing to do the same test?

[oldmansax]

Chuck,

Thanks for the info!

You said you have a Home Depot digital stat controlling the furnace. I have wanted to replace the old stat on my furnace ( same setup as yours) with a digital but haven't because standard electronic use 24V power I assumed the Suburban was 12V.

How did you do that?

TOM

[belfert]

Over on rv.net they have lots of posts describing how to put a digital thermostat on an RV furnace.  Go over there and search on Hunter thermostat. 

The trick is to get a model that has batteries so it won't need to be line powered off 24 volts.

[Jerry32]

I would have thought that 35K BTU would have brought the temp up faster than that with the bus sitting and out of the wind.  Jerry

[WEC4104]

I also have the 35K BTU Suburban furnace, mounted near the center of the bus, with one forward duct and one rearward duct.  (Although I only have a 35 footer).   I haven't performed a timed test like Chuck, but overall I think my heating results seem pretty similar.

I think the one aspect it points out clearly is the fact that the heat rise is not linear. In the first 1/2 hour he was able to raise the inside temp 16 degrees. But he was only able to climb 4 degrees in the last half hour. Assuming the thermostat had not begun to cycle on and off yet, it is apparent that getting the last few degrees gets more and more difficult.   

Fortunately, both Chuck and I have additional ways to add heat to the bus.  But if we were relying solely on the Suburban, it looks like it might take an extra 2 hours to get to the 66-68 degree range. (Whining threshold for my kids)

[JohnEd]

My Hydroflame had the wrong jets in it.  They also become restrictive and need changed out for max heat.  When plugged the furnace performs normally but has more of a roar.  There is a view port and the flame should be almost entirely blue with a tough of orange at the tips.  The HEIGHT OF THIS FLAME IS IMPORTANT.  That indicates how much gas is getting into the mix.  My initial problem was that my gas pressure was way off and had been turned down because the "stove" had jets installed that were too big.  Po shot himself in the foot having that adjusted.  Mine is a 48 K BTU and it heats a 30 foot S&S.  From 50 degrees I am up to 76 in 5 minutes or so...at least it seems that way.  The air coming out of that puppy will burn your fingers.

You must have all the outlets open and unrestrictive.  If you have flex dryer duct with crimps and many turns you are not getting the airflow to reach the spec BTU output of the furnace.

Watch your flame while the furnace is on.  It should never go out till you reach temp.  If it is cycling you have restrictive ducts OR your "over temp" safety thermostat is bad.  There are two safety thermostats involved.  One is for the "fire box" and the second is for the air being discharged.  If the ducts are too restrictive the "air temp safety" will trip.  If you are getting too much gas or the thermo is bad the other will trip.  These furnaces are going for $600 (some models) so they are really worth fixing and maintaining.

HTH

John

[Nusa]

Fabric and wood are decent insulators. All the exposed ceramic tile, counters, glass, and metal (especially any metal connected to the frame or areas outside the living space) are the real thermal sinks. Also, I notice your thermostat is mounted only a foot from the ceiling, which partly explains why the readings rose so quickly the first half-hour. If you took readings at table level, I think the curve would be flatter and colder.

[NJT 5573]

Most of these units have knock outs for at least 4 and usually 6+ ducts. 2 ducts won't get the job done and burns /wastes a lot of propane. Just like porting a set of heads, its all about airflow.

[JohnEd]

What NJT said!  Two would never let you get the full BTU out of the unit.

Knock out the slugs and vent the furnace to the cabin somehow in the interim.  Run ducts to proper grills when you get a chance.  If you are running only 2 of the 4 outlets you have probably cut the effective rate of the furnace in half.  In that case it is doing a wonderful job.

HTH,

John

[ChuckMC8]

Thanks for the kind comments regarding my 'heat test'
Heres the early morning follow up-
    1. My bus shop doesnt have sides on it yet, so while undercover, it was still in the wind. Odd, tho and I've experienced this in other buses, is that the bus under cover is lots colder than sitting in the sun, even on a very cold day. Its like a referigerator.  Next real cold snap, I'll park it out side and run the test again and see what the results are.

   2. Brian is right about the thermosat, it has to be amodel that uses 2 AAA batts for its own power. Mine controlls the a/c as well. Having the a/c on the thermostat is the best, you can set it to a very high tem in the summer, say 90 degrees (IIRC) and it wont let the bus get any hotter than that inside. I know thats to hot for personal use, but when its 102 degrees here, it keeps plastic inside from mellllllting!
No bust this summer, mines in the shade.

3. My ductwork is ran in 4" galvanized HVAC pipe from HD. The frontmost comes out in the dinette at your feet (nice) 2 in the kitchen (about 4' apart) and the last one goes back to the bedroom.

4. This furace came from a new wrecked Airstream Trailer, it had 4 ducts plumbed in in the factory installation.

5. If you're contemplating installing one of these, its very important to locate the furnace in your bus so  that the vent to the outside is located betweeen your bay doors when they are open.
   The exhaust gases are so hot they will damage the bay door in the open position.

6. Even when its very cold like it was on the day I ran the test, the furnace is quite adequate for my bus. The "cruical" period is bring the temp up from a dead start, or "cold soaked" position. One it gets to the desired temp inside, the furnace cycles normally, just as the one in my home does.

I know that I live in a moderate climate compared to our compatritiots who live farther north, but the info may still be useful........

  HTH Chuck

[DavidInWilmNC]

This is making me think I should just go with Propane instead of diesel hot air or hydronic system.  I also live in a very temperate climate and don't plan to spend much time in truly cold areas.  If I do, the propane furnace plus a couple of electric heaters should do the trick.  Chuck, what size propane tanks do you use?  How many?  I'm thinking that a couple of 30's or 40's (if they'll fit) would be good.  I'd keep a 20 as a spare to do the trade-out thing at WalMart if I can't find a refill station.  Anybody want to trade a low hours (was NOS when I got it a year or so ago) diesel furnace for a good propane one?   

David

[John Z]

David, if you have the room you may consider the 50 pound bottles. Mine are made by Manchester and measure just under 28" in height. They fit in the 4104 bays like they were made for it. One thing i really like about them is they are over the 40 pound mark, so are not required to have the idiot-proof OPD's on them. One less thing to go wrong. Two of them side by side on an automatic switching regulator fit nicely in my front curbside bay.

[ChuckMC8]

I moved my starting batts to the engine compartment- 2 group 31's start it great (8V71TA)
My propane setup is in the old battery compartment. I can just switch out a LP bottle nearly anywhere if I need one.
   Another benefit from moving batteries to the rear is that it frees up the 4 ought battery cables that run to the old battery compartment to connect to the house battery bank for charging with the bus alternator while underway.
Here's the pic- more on my profile HTH Chuck

[JohnEd]

David,

If I can I will opt for diesel hydronic.  You need propane to cook so one less fuel isn't the issue.  Energy cost and capacity is.  You can build in a "non road taxed" red diesel tank holding 50 to 75 gallons and save 50 cents a gallon.  Design in a transfer capability to fill from the road diesel for a backup.  My driving factor for this is that I NEED to be able to pre heat the engine and hydronis seems to be the way.  Also it lets me use the engine heat to heat the coach while under way. 

I used approx 100 pounds of propane to heat a 30 foot S&S in 20 degree weather using the gas for heating, hot water and cooking.  Pretty good, actually.

John