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Old 11-23-2010, 12:14 PM   #1
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The definitive fact-based furnace size thread

I have my traylah inside an unheated shed so it's in NO wind, and it's cold in Minnesota. A perfect time to measure heating performance.

With 2250 watts (7679 btu/h) of heat from two electric heaters, I observed, after a couple of hours for temperatures to stabilize, a 30 degree delta-t, that is a 30 degree difference between the outside temperature (30 degrees) and the inside temperature (60 degrees). The temperature was fairly even throughout the trailer.

I believe that most RV propane furnaces have an efficiency of around 60%, and they're rated in BTU/h input, so this would be the same heat actually delivered by an RV furnace of 12,798 BTU/h capacity.

Taking this as a starting point, I offer this table, based on a 70 degree desired inside temperature:

Outside temp Watts Unvented BTU/h Furnace BTU/h
60 750 2600 4300
50 1500 5100 8500
40 2250 7700 12800
30 3000 10000 17000
20 3750 13000 21000
10 4500 15000 26000
0 5200 18000 30000
-10 6000 20000 34000

Wind, of course, will require more heat.

I'll try to find a heat pump output chart somewhere (the BTUs decline as the outdoor temperature drops) and incorporate that.

This is for my 30' classic. Smaller trailers will require somewhat less heat.
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Old 11-23-2010, 01:01 PM   #2
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I have my traylah inside an unheated shed so it's in NO wind, and it's cold in Minnesota. A perfect time to measure heating performance.

With 2250 watts (7679 btu/h) of heat from two electric heaters, I observed, after a couple of hours for temperatures to stabilize, a 30 degree delta-t, that is a 30 degree difference between the outside temperature (30 degrees) and the inside temperature (60 degrees). The temperature was fairly even throughout the trailer.

I believe that most RV propane furnaces have an efficiency of around 60%, and they're rated in BTU/h input, so this would be the same heat actually delivered by an RV furnace of 12,798 BTU/h capacity.

Taking this as a starting point, I offer this table, based on a 70 degree desired inside temperature:

Outside temp Watts Unvented BTU/h Furnace BTU/h
60 750 2600 4300
50 1500 5100 8500
40 2250 7700 12800
30 3000 10000 17000
20 3750 13000 21000
10 4500 15000 26000
0 5200 18000 30000
-10 6000 20000 34000

Wind, of course, will require more heat.

I'll try to find a heat pump output chart somewhere (the BTUs decline as the outdoor temperature drops) and incorporate that.

This is for my 30' classic. Smaller trailers will require somewhat less heat.
Part of the heating energy, from the forced air furnace, is directed to the holding tanks, to keep them from freezing.

That's impossible to do with any other kind of heating.

I would be very very careful with the sub freezing temperatures.

Andy
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Old 11-23-2010, 01:57 PM   #3
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Great info with measurements to back it up!
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Old 11-23-2010, 03:45 PM   #4
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Andy, good point on the slight additional heating load.

I believe on my trailer, based on duct sizes and locations, that about 15% of the furnace output is directed towards the tanks. However, some of it ultimately returns still warm to the trailer, so I would guess that the additional load is about 10%.

I'm running my tests in a fully winterized trailer. While I expect to use my trailer occasionally this winter, I will be running it dry.
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Old 11-24-2010, 04:45 AM   #5
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Reading through the Atwood literature, I note that Atwood claims a minimum efficiency of 77% for the 8500 series furnaces used in current production trailers. I'll update the table accordingly.
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Old 11-27-2010, 10:13 AM   #6
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Moving on to the time required to heat a cold trailer, here's a set of measurements I made the other day. It was 15 degrees outside and the trailer interior was at this temperature as well. A few measurements in isolated places were a few degrees lower as it had been somewhat colder the night before.

I was running 1500 watts of electric heat in the bedroom, and another 1500 watts in the galley, and the 34,000 BTU/h Atwood furnace. I made temperature readings with an IR thermometer every 10 minutes or so and also recorded the air temperature measured by a thermometer near the thermostat. I had the door closed between the bathroom and the galley to better understand the extent of the imbalance in heat between the front and rear of the trailer.

Time Gaucho Dinette Head Bed. Air. Floor
0:10 28 ... 14 .... 14 . 16 . 25 . 22
0:15 37 ... 22 .... 21 . 25 . 30 . 20
0:20 43 ... 26 .... 22 . 28 . 35 . 23
0:30 53 ... 38 .... 25 . 39 . 43 . 21
0:40 58 ... 44 .... 40 . 42 . 49 . 36
0:50 62 ... 48 .... 44 . 46 . 58 . 39
1:00 72 ... 55 .... 49 . 51 . 65 . 44
1:10 79 ... 59 .... 52 . 54*. 69 . 48
1:20 77%... 61 .... 57 . 57 . 70 . 50
1:30 72 ... 67 .... 63 . 64 . 71 . 56
2:00 70 ... 69 .... 66 . 68 . 74 . 59
2:20 69 ... 70 .... 67 . 69 . 75

* denotes the point where the electric heater in the bedroom started to cycle off and on.

% denotes the point where the furnace started to cycle off and on. Also, I opened the door between the galley and the bathroom at this point.

So we conclude that, for a 55 degree difference between initial (15) and target (70) temperatures, it's going to take around 90 minutes for the trailer to warm up even with some extra heat in addition to the stock furnace. Without the electric heat we could expect to add 1/3 to these times (3000W = around 10,000 BTU/h, a little more than 1/3 of the 26,000 net BTU/h we get from the 34,000 BTU/h furnace at 77% efficiency) minutes.

As a rule of thumb for the frequently asked question of "how long will it take for the trailer to heat up," I would suggest 2 minutes per degree of temperature difference, with the stock furnace.

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Old 12-04-2010, 01:37 PM   #7
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Cieling to Floor Heat Variation

Interesting thread. I have two Suburban 30,000 btu furnaces in my 34' Classic. I used these to replace the 24,000 btu furnaces that were original equipment back in 86. I find my head is hot, and my feet are cold. The furnaces kick on, and blast heat until it is 80 at head level, but only 65 at the floor. Then the temps drop about 10 degrees before the furnaces kick on again. I purchased the higher btu furnaces as they were readily available. How can I get better heat distribution from floor to cieling? I live in Minnesota also, so heat is important to me.

I also take note that air temps are at "normal" in two hours, but the mattresses take much longer to get to room temp. Same with the floors and cabinetry.


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Old 12-04-2010, 02:34 PM   #8
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The furnaces that came out a year or two ago from Atwood (Excalibur XT series) that have multi speed fans should have solved this. A multi-speed fan can continue circulating the air when the combustion stops, thus mixing the cold and warm air resulting in a more uniform air temperature. The existing models all shut off the fan shortly after the combustion stops.

But an airforum.com member bought and tested one and found it was terribly noisy. He was in contact with engineers at Suburban who were unable to come up with an explanation or fix for him. He even went to the extent of installing a second furnace of the same type, with the same results.

Hopefully, this defect will be overcome at some point, and a good multi-speed furnace will be available.

The tests are in this thread, and start at post 16.
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Old 12-05-2010, 09:22 AM   #9
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Heat Variation Floor to Cieling

My Suburban furnace fan draws air from the outside. It shuts off just before the air starts to get cool after a heat cycle. I can't figure out how it could run as a circulation fan as it would just blow cold outside air into the trailer. I suppose the trouble with our Airstreams is the fact that there is no "cold air return" to the furnace. I need someway of circulating the air around in my Airstream so the hot air isn't at eye level, and the cold air doesn't settle at the floor level. Maybe these new Atwood furnaces are a more modern design.
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Old 12-05-2010, 10:51 AM   #10
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Good question. It never occurred to me to look into this part of the heating system.

I believe that in my home furnace (FAG) the outside air is only drawn in when combustion is happening. And I only think this because in every home we've had, I always leave the fan on in winter (and AC cooling season) to avoid the stratification of the warm air. It seems to do a good job. AND, the air that is being circulated in winter is only room temp and NOT cold.

However, the AS' heater could be working as you say, dbj, and drawing in outside (cold) air when the combustion is off.

I'll look into this next spring.
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Old 12-05-2010, 11:03 AM   #11
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My Suburban furnace fan draws air from the outside. It shuts off just before the air starts to get cool after a heat cycle. I can't figure out how it could run as a circulation fan as it would just blow cold outside air into the trailer. I suppose the trouble with our Airstreams is the fact that there is no "cold air return" to the furnace. I need someway of circulating the air around in my Airstream so the hot air isn't at eye level, and the cold air doesn't settle at the floor level. Maybe these new Atwood furnaces are a more modern design.
Suburban furnaces draw "outside air" to feed the combustion chamber only.

The inside air is circulated by the furnace fan, over the heated exterior of the combustion chamber.

At no time, is outside air ever brought into the coach for heating purposes.

If your furnace is as you say, I am surprised that you are still alive, since the carbon monoxide from the furnace burner, would be fed into the coach.

All USA safety standards prohibit what you described, regardless of the type of gas heating system.

Look at the air intake on the side of your Airstream.

The bottom opening is to allow air into the combustion chamber and the top opening is to allow the contaminated air to exhaust.

Andy
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Old 12-05-2010, 11:52 AM   #12
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The return air grille for my suburban furnace is located at floor level under the furnace. It is about a two inch slot the width of the sink cabinet. Outside air is drawn in only for combustion air. Inside air is recirculated.
Al
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Old 12-05-2010, 11:55 AM   #13
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Also, I have a Maxxfan in my living room area. It can be operated in both directions with the vent lid closed. This will circulate the hot ceiling area air and move it towards the floor if you get it going in the right direction. It will do this at a low fan speed which uses very little power.
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Old 12-05-2010, 12:10 PM   #14
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I also use the Maxair on ceiling fan mode. Pushes the warmer air down. ALSO use a ceiling fan in my house to circulate warm air from the wood stove. Sal.
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