Airstream Heat Loss Calculations?

[malconium]

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

[Gene]

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

[Silverwanabe]

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

[ROBERTSUNRUS]

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%]

[JimGolden]

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,

[Silverwanabe]

Airstreams don't attract mice

[malconium]

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

[Inland RV Center, In]

Originally Posted by malconium

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

[2airishuman]

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'


-

[CanoeStream]

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

[Silverwanabe]

Here's the website, very pricey....

PrecisionTemp.com: For Recreational Vehicles and Boats

[LI Pets]

Originally Posted by JimGolden

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.

[Gene]

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

[wmarsha]

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

[malconium]

I can't us mineral oil or antifreeze coolant because my intent with the radiant heating system is to use the same water as is used for the shower and sinks. I was intending to install what is called an open loop system which uses water from the same water heater that is used for other things. One of the posts a little earlier in this thread has a pointer to how this type of plumbing is done. Otherwise I would need to have two water heaters which would add to the overall expense of the system.

When I was thinking about running the water heater while driving I was not so much thinking about pumping water around through the floor. Rather I was thinking about keeping the water in the tank hot so that I had a head start on heating the floor when I stopped. My thought is that if the water is already hot that it should not take all that long to heat up the floor.

I am hoping that water heaters with electronic spark ignition would not have as much of a problem blowing out. Is that true? Do they reignite themselves if the flame blows out? I also wonder how long a tank of water will stay hot in a modern tank type water heater.

Sure I will have to drain the water out of my system if I park for a while while it is cold. Since the radiant heating tubes are part of my hot water plumping I should be able to drain them easily enough. I might even be able to use the hot water pump to help with that. I will have to give that some thought.

As far as complexity of installation goes I agree that installing radiant heating in the floor is a bit more work than installing catalytic heaters or some other form of propane heater. I do not think that it will be more expensive however. I need to have a water heater anyway so as long as the one that I pick has enough capacity then that is almost a giveaway cost wise. PEX tubing is not very expensive. I think it runs about $10 per fifty feet at Home Depot and I might need 200 or 300 feet. The hot water pump is probably the most expensive item at about $209. I need a new thermostat regardless of what I do for heat - at least for my AC unit. Sure I need a few other odds and ends but nothing else that is all that expensive. If I do indeed decide to go through with this approach I will have to start a new thread and go into details about the whole job.

Yes I agree that the body ribs are going to leak heat faster than other parts of the wall. I did do something to help minimize that when I installed by reflective foil insulation though. I bought felt at the fabric store and attached strips of it on the ribs before I put the inner skins back on. I do not know how much that will help but it will no doubt help some.

I am still looking for feedback on the heat loss calculations...

Thanks,

Malcolm

[ventport]

Unless you run 2 water heaters, one with antifreeze for the floor heat, this sounds like a bad idea to me. If you do not run the system when you are going down the road how will you keep the tubing in the floor from freezing? I wonder how fast the system will make up for running an exhaust fan in the kitchen or bathroom or if the entrance door is opened during a strong wind. Floor heat is very slow in making up for large amounts of fast heat loss. The volume of air in a AS is very small compared to a house. I have floor heat in the mud room and greenhouse of my house that is run off of the domestic hot water heater. Though it works very well I have had many warm, not hot showers if the floor heat is running at the same time that I am showering. Also how will you keep the waste and fresh water tanks from freezing? Not that it can't be done, but I think it's an unproven system with more questions then answers and way too much trouble for what it accomplishes over a conventional forced air system.

[2airishuman]

Originally Posted by ventport

Unless you run 2 water heaters, one with antifreeze for the floor heat, this sounds like a bad idea to me...

i sure agree.

1 water heater=problems.

sure the modern igniters will RElight (up to 3 tries) and they don't tend to blow out while moving...

also one can typically HEAT the water tank in the am, turn it off and have HOT water ALL DAY, even on the road.

but most of the NEW water heaters don't have ADJUSTABLE TEMP CONTROLS...

which means the water is REALLY HOT without a mixer valve.

using the same water to shower/wash is a clever idea, adds WAY to many issues and complexity.

but then again the entire idea is more complex than needed to heat and predictably a disaster to a huge restoration job.

cats can be 'zoned' and it's common with the furnace or heat pump or cats to CLOSE OFF the rear bedroom.

just heat the front until bed time or 30 minutes prior.

typically there are NOT water pipes or internal plumbing to deal with at the EITHER end of long trailers.

DRAINING?

that's gonna be an issue since leveling/tilting will be needed, OR a blow out valve.

are up planning a compost toilet, wind farm and solar grid too?

i don't think ANY ONE can supply reliable "heat loss" calculations on a vintage unit REBUILT

and without considering the LARGE LOSSES into the metal and out the sides.

one would need to know your EXACT insulation and thermal barrier plan.

cheers
2air'

[malconium]

Originally Posted by ventport

Unless you run 2 water heaters, one with antifreeze for the floor heat, this sounds like a bad idea to me. If you do not run the system when you are going down the road how will you keep the tubing in the floor from freezing? I wonder how fast the system will make up for running an exhaust fan in the kitchen or bathroom or if the entrance door is opened during a strong wind. Floor heat is very slow in making up for large amounts of fast heat loss. The volume of air in a AS is very small compared to a house. I have floor heat in the mud room and greenhouse of my house that is run off of the domestic hot water heater. Though it works very well I have had many warm, not hot showers if the floor heat is running at the same time that I am showering. Also how will you keep the waste and fresh water tanks from freezing? Not that it can't be done, but I think it's an unproven system with more questions then answers and way too much trouble for what it accomplishes over a conventional forced air system.

You make some good points. Let me try to address some of the things you mention here...

1.) The tubing will not be under the floor - rather it will be between the sub-floor and the finish floor. The sub-floor will have a layer of reflective foil insulation on top of it and the PEX tubing will be above that. My guess is that this will be enough insulation to keep the pipes from freezing for the amount of time that it takes to get from place to place. The pipes should not be at more risk of freezing than any other pipes in the trailer. Also if I am traveling where it is exceptionally cold all I would have to do is to run the heating pump for a few minutes at each rest stop to warm them up. I could also leave the heating system on with a lower thermostat setting that would turn on the pump from time to time to keep the interior at a reasonable but low temperature. For example I could set the thermostat to something like 55 degrees. This would help keep other things in the trailer from freezing too by the way.

2.) It is my understanding that a radiant system could very well be comfortable again after loosing warm air faster than a forced air system simply because of the radiant effect. How quickly does your mud room feel comfortable again after you close the door to the outside?

3.) I think it would be pretty easy to turn off the floor heat for the short amount of time that it takes to take a shower. I will want to make a point of having an easily accessible on/off switch for this anyway. Probably if I pick the right thermostat I can just turn the heat off there.

4.) My fresh water and waste tanks are all directly under parts of my floor. My intent is to leave out the reflective foil insulation over these areas and put it instead under the holding tanks. I am anticipating that there should be just enough heat leaking downward in these areas to keep them from freezing.

5.) This is not really an unproven system. It is my understanding that most high end motor homes use hot water heating systems of one kind or another. Perhaps most of them use the little radiators and fans but I do know that some of them have tubing in the floor. I was told that by a VP of Precision Temp. Of course radiant heating in the floor has been around for a very long time. My first exposure to it was in the late 70's when I built it into a house that I designed and built for my family. At that time we used copper tubing because PEX was not available.

Malcolm

[malconium]

Originally Posted by 2airishuman

i sure agree.

1 water heater=problems.

sure the modern igniters will RElight (up to 3 tries) and they don't tend to blow out while moving...

also one can typically HEAT the water tank in the am, turn it off and have HOT water ALL DAY, even on the road.

but most of the NEW water heaters don't have ADJUSTABLE TEMP CONTROLS...

which means the water is REALLY HOT without a mixer valve.

using the same water to shower/wash is a clever idea, adds WAY to many issues and complexity.

but then again the entire idea is more complex than needed to heat and predictably a disaster to a huge restoration job.

cats can be 'zoned' and it's common with the furnace or heat pump or cats to CLOSE OFF the rear bedroom.

just heat the front until bed time or 30 minutes prior.

typically there are NOT water pipes or internal plumbing to deal with at the EITHER end of long trailers.

DRAINING?

that's gonna be an issue since leveling/tilting will be needed, OR a blow out valve.

are up planning a compost toilet, wind farm and solar grid too?

i don't think ANY ONE can supply reliable "heat loss" calculations on a vintage unit REBUILT

and without considering the LARGE LOSSES into the metal and out the sides.

one would need to know your EXACT insulation and thermal barrier plan.

cheers
2air'

Let me see if I can address some of the valid concerns you raise here...

1.) I fully intend to have a mixing valve on the output of the water heater where it connects to the domestic hot water part of the plumbing. That way I can set the temperature of the water heater higher if I find that it helps with the heating part of the system.

2.) I am not sure I understand your comment about using the same water to shower/wash. Was that what you mean or did you mean shower and heat? I hope that I did not convey the idea that I intended to reuse gray water.

3.) Actually it would be very easy to create zones with my radiant heating system. All I would have to do is have two separate branches of the heating loops running from the same pump. If I just have a simple manifold with two valves on it I can adjust the flow so I have whatever balance I want between the two zones including turning one off. I might do that for the adjust-ability part. Because radiant heating is a little slower to respond to changes I think it might be better to run the whole system all of the time - just with the right balance between zones.

4.) The original plumbing in my 31' unit started at the street-side rear corner with the water inlet. The water heater was under the head of the bed in the rear bedroom. It is true there was no plumbing forward of the kitchen but the plumbing was definitely strung out over about 2/3rds of the length of the trailer. If I recall correctly my service manual says something about tilting the trailer as part of the draining process. In my new arrangement the inlet will still be in the rear corner but the water heater will likely be under the kitchen sink. The regular part of the plumbing should not be any harder to drain out than the original was. I do agree that effectively draining the whole system does have some challenges but I think I am up to that. I do have an air compressor if it turns out that this would help and it seems like adding a blowout valve somewhere in the system might be a good idea. I will also give some thought to placing my hot water pump to see if there is a way as to be able to drain the heating pipes with it.

5.) Regarding the heat loss calculations I am not asking someone to try to second guess what it might be for my unit. I am just asking if anyone besides me has attempted to calculate heat loss for their trailer and has data to share.

Thanks,

Malcolm

[kittrik]

Malcom: I do radiant floor design and heat loss calculations for a living.
Post the number of windows and sizes and i can run the software program. How much insulation are you putting under the floor? I expect you will need hotter water in the floor tubing system than you will want in your domestic system. Generaly we see 130-140 degree temps in this type of install. Check out UPONOR'S quick trak radiant system. You may also think about using a small plate type heat exchanger for the radiant heat. It would eliminate the need for bronze pumps and you would not be running high pressure in your radiant lines. Add glycol to the radiant and not worry about freezing. I have not replaced the belly pan on my camper because i want to do radiant also.(someday)