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Old 01-06-2010, 08:54 PM   #1
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Airstream Heat Loss Calculations?

I am giving consideration to heating my 1973 31' Airstream with hot-water radiant heat with PEX tubing in the floor. One challenge right now is deciding what type of water heater is necessary to heat the water. I did a little Internet surfing looking for a heat loss calculator so I could get an idea of how many BTU per hour I might need to replace to keep things warm. Check out the following link:

Home Heat Loss Calculator

I played with various numbers and temperature ranges and found that the calculator thought that my overall heat loss might be in the range of from 4000 to 7500 BTU per hour. If this is realistic it is a small enough amount that I think I could actually install a new Suburban 10 gallon tank type propane water heater to meet the need. It has an input of about 12000 BTU per hour. At even an 80% efficiency this would be about 9600 BTU per hour capacity.

If my calculations are correct then I could perhaps avoid the temptation to do something more exotic with a tankless water heater of some sort which I had been thinking I might need. Even the smallest tankless units have about 35000 BTU input and some have in the range of 90% efficiency.

So here is my question for the forum members:

Has anyone done any heat loss calculations for an Airstream that would help me calibrate whether or not my calculations are in line with reality?

Please keep in mind that the lowest outside temperature that I used in my calculations was 20 degrees. One nice thing about an Airstream is that you can move it to a warmer place if you do not like the temperature where you are. My wife and I do not intend to camp in really cold places if we can avoid it. Also note that I have installed reflective foil insulation in all the walls and roof and will be adding it under my radiant PEX tubing. In my heat loss calculations I estimate that I had an equivalent of R8 with the reflective foil even though the actual effect might be better. After all I am heating with radiant energy and reflective foil is especially good at blocking heat loss by radiation.

I would appreciate some "heated" discussion on this topic if you can get past the bad pun.

Thanks,

Malcolm
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Old 01-06-2010, 09:00 PM   #2
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Although I expect you know more than they do, Airstream puts hot water radiant heat in their European models and may be able to give you some ideas or facts. It's baseboard. In floor may be too advanced for them at Airstream.

Gene
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Old 01-06-2010, 09:31 PM   #3
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Just from my observations using 2 different rigs in cold weather:
1. You will have a lot of loss from the windows, lined drapes helps some.
2. Roof Vents another source, could be insulated.
In the Airstream last winter I camped when it was 16 degrees one night, the furnace ran all night. Unit was winterized. It was ok.

With the Avion, I still have issues with the single pane jalousie windows.

I am not sure if I've seen a manufacture use radiant heat in a trailer. That would be an alternative to the propane furnace.

Good Luck
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Old 01-06-2010, 09:35 PM   #4
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Hi, by sealing my entrance door and all of the possible mouse entry points, I think my trailer should be more efficient now. [maybe about 5%]
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Old 01-06-2010, 09:40 PM   #5
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I know that in a house, it takes two to three days for a radiant floor heating system to warm up the house. How long do you think it would take in a trailer? Maybe half a day? Can you live with that? Or would you use the furnace to get it up to speed, but then warm up the floor, and once the floor is good and warm and the whole rig is warm, then cut off the furnace? Would be OK if you're set up in one spot, but if you're mobile, might not be so practical.

I know radiant heat is nice heat, but it takes awhile for it to warm up.

Oh yeah, I wrote a program to do these calcs based off the power company's manual J calc book. That was about eight years ago, but I could dig if you think it'd help.

I think you could just look at the furnace that Airstream put in it. If they gave you a 30,000 btu furnace, then you probably need to put out somewhere around 30,000 btu's. We could recalc it, but they've probably already done the work for you.

At any rate, good luck with this. It will be a cool project for sure! It'll sure feel good on your toes on a cold night!

see ya on the road,
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Old 01-06-2010, 09:40 PM   #6
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Airstreams don't attract mice
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Old 01-06-2010, 09:54 PM   #7
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OK to run water heater while driving?

Jim,

Lag time is of course a concern with a radiant heating system. Many house systems have a lot of thermal mass around the pipes that does indeed take a long time to heat up. In the late 70's we built our own house in California with hot water radiant heating in the concrete slab downstairs and with 1-1/2" of light weight concrete upstairs. It did indeed take a couple of days to stabilize.

My systems should have a lot less thermal mass to get warmed up. The tubing will be between the sub-floor and the finished floor in about a 3/4" air gap. There will be reflective foil on the sub-floor which should help direct the heat upward. The thermal mass consists of the air and the finish floor. Time will tell of course as to how long it will take to heat up.

One thing I have wondered was if it was OK to let my water heater and run while I was driving. If I were to do that then I could keep the floor at a more or less constant temperature all the time which would be a good idea.

So here is a question:

Is it OK to let a standard tank type propane water run while driving?

Malcolm
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Old 01-06-2010, 10:13 PM   #8
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Originally Posted by malconium View Post
Jim,

Lag time is of course a concern with a radiant heating system. Many house systems have a lot of thermal mass around the pipes that does indeed take a long time to heat up. In the late 70's we built our own house in California with hot water radiant heating in the concrete slab downstairs and with 1-1/2" of light weight concrete upstairs. It did indeed take a couple of days to stabilize.

My systems should have a lot less thermal mass to get warmed up. The tubing will be between the sub-floor and the finished floor in about a 3/4" air gap. There will be reflective foil on the sub-floor which should help direct the heat upward. The thermal mass consists of the air and the finish floor. Time will tell of course as to how long it will take to heat up.

One thing I have wondered was if it was OK to let my water heater and run while I was driving. If I were to do that then I could keep the floor at a more or less constant temperature all the time which would be a good idea.

So here is a question:

Is it OK to let a standard tank type propane water run while driving?

Malcolm
One way to have the water heater work, when the trailer is in motion, is install a gasket around the two sides and the top of the water heater door.

I did that many years ago, and even the pilot would not blow out, during travel. I went accross the USA without that happening.

Strange thing though, the pilot light would still blow out, from a wind, when the trailer was parked, not often, but once in a while.

I used the same gasket as used on the screen door.

Andy
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Old 01-06-2010, 10:19 PM   #9
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yes it's ok to run a standard rv water heater while driving...

subject to ALL the issues (real or imagined) that go with running ANY flamed appliance while traveling...

fuel stations, tunnels, ferries and so on...
__________

this is an interesting project that MANY have asked about and posted 'to be used'

i've not seen ANY follow up by anyone who has done it.

i understand pnw weather and the need for something LESS than a full forced air furnace MOST of the time...

the issues (questions) 4me are these...

1-how much HEAT will be conducted INTO the frame/shell and OUT again without warming the interior?

there seems to be GREAT potential for heat loss this way.

2-IF it's freezing out and the system hasn't been turned ON, how do you do so without water freezing IN the lines ?

3-this will need to be DRAINED after any cold weather camping IF still cold and the trailer is stored, right?

4-how much moisture/condensation will collect IN the air space/extra sub floor layer when the system is NOT on ?

5-the frame/subfloor and flooring FLEX/ROTATED and otherwise MOVE around, will the system tolerate this movement?

6-this is a recirculating system right? if so why not use heated oil in the pipes?

7-how much MASS is added to the trailer?

8-IF radiant is so important to you why not buy the parts used on the euro models (they can be purchased from euro vendors)??

a "2 unit" vented catalytic heater seems so much easier to install and use and has MUCH more flexibility in heating.

good luck and post photos.

cheers
2air'


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Old 01-06-2010, 10:46 PM   #10
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Related to towing & heating -- I did this once in 16 degree weather with my Safari unwinterized. Successfully I might add. I learned that windchill & probably air infiltration led to an awful lot of propane use! I was a little (lot?) stoopid and had the thermostat in the upper 60s. Burned almost 30# of propane in a 340 mile drive; the furnace ran most of my first night at the destination and then the tank was empty.

If I did this again, I'd probably have the thermostat set around 45-50 degrees and leave a thermometer down by the open lower cabinets -- check it after 50 miles and at some interval afterward. Modify as seems appropriate...

It would be interesting to know how many air exchanges an Airstream undergoes while towing. The shell is chilled at speed + air exchanges ... I don't know how a slow-heating device is going to gain anything by operating while underway. I appreciate my propane furnace warming up the inside in 15-20 minutes.

What was the hot water system Safari Tim installed in his Ambassador on TheVAP.com?

trivia: the 16 degree drive was a mid-October surprise
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Old 01-06-2010, 11:08 PM   #11
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Here's the website, very pricey....

PrecisionTemp.com: For Recreational Vehicles and Boats
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Old 01-07-2010, 02:23 AM   #12
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Quote:
Originally Posted by JimGolden View Post
I know that in a house, it takes two to three days for a radiant floor heating system to warm up the house.
I disagree, having radiant for 15+ years in wood not concrete floors it takes 2-3 hours if the heat was off. If the doors were open going in/out maybe 15-20 minutes if the house was warm.

2-IF it's freezing out and the system hasn't been turned ON, how do you do so without water freezing IN the lines ?
3-this will need to be DRAINED after any cold weather camping IF still cold and the trailer is stored, right?


U could use anti freeze as they do in solar panels

4-how much moisture/condensation will collect IN the air space/extra sub floor layer when the system is NOT on ?

Shouldn't be any?

5-the frame/subfloor and flooring FLEX/ROTATED and otherwise MOVE around, will the system tolerate this movement?

Easily I would thinkI think it can be done but the alternatives are easier.
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Old 01-07-2010, 09:36 AM   #13
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Our hot water solar system at home uses mineral oil. Systems that recover heat from the ground also use mineral oil. Does anyone know what is used in European Airstreams? Water? Mineral Oil?

Gene
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Old 01-07-2010, 10:28 AM   #14
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here's mud in your eye...

or muddier water...my old E series van has two heaters...it would not be a stretch to consider putting an automotive type heater in the AS with hydraulic-type quick connectors such that the TV could transfer some of its heat into the coach while going down the road...it would be a physical plant challenge but could be done-just head on down to your local wrecking yard or auto parts store for a heater core and a blower motor and you are all set.
ol' bill
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