Airstream Heat Loss Calculations?

[Secguru]

1 - What is the total volume of water that will be in the lines under the floor?
2 - What is the total weight of the system including the water in the lines?
3 - How does that added weight change your gross weight and available payload?
4 - How will you heat the water tanks if reflective foil is between the water lines and the sub-floor?

[Gene]

Malcolm, I have heard you have to calculate bends when figuring out flow rate and pump capacity. My guess is 90˚ bends would be more restrictive than curves and sharp curves more so than gradual ones. Putting the Pex 3" apart may be much too close because of the tight bends. Like Bob, I know nothing about this. I'd start by checking the internet. Maybe Lowe's or Home Depot have books on installing this kind of system with tables explaining it.

Gene

[Jammer]

Great thread. Malconium, I'll be excited to hear how your project turns out.

I installed radiant heat in my 100 year old house a few years ago. We are in our third heating season with the system. What we're doing is similar to what you describe in that we have PEX tubing retrofitted underfloor.

I tend to obsess over BTUs and amps and stuff of that nature as some of my other posts will attest.

The heat loss calculators are worthless for anything other than conventional post-1980 residential construction in an area protected from the wind. Don't rely on them for anything else or you'll make expensive mistakes.

The only way to find out how much heat it takes to heat an Airstream trailer in particular conditions is to actually do it and measure the heat input. The amount of heat required is roughly the same regardless of source with the exception of forced air which has a loss of typically 10% due to the additional air leakage. Airstream is currently putting 34,000 BTU/h (input) furnaces that are 85% efficient into their 31' trailers. I don't see anyone complaining that these are overkill or, conversely, that they are too small. What we hear usually is that they are good down to aroudn 15 degrees in the wind at which point they'll run all the time. 34,000 BTU/h * 90% forced air efficiency * 85% furnace efficiency = 26,000 BTU/h.

Another data point is that the people who heat only the bedroom using an electric heater during relatively mild weather seem to be fairly happy with the 1500 watt "big heat" portable heaters. 1500 * 3.143 = 4,700 BTU/h. They're heating maybe a third of the trailer, so 4700 * 3 = 14,000/h BTU, in mild conditions (usually above freezing). So I think for planning purposes you aren't thinking in terms of enough heat.

You can measure system runtime ratios to scale the results. So if you sit in a trailer on a cold day and have the 34,000 BTU furnace running 40 minutes out of the hour, say, well then you can multiply the figures by 40/60 or 2/3 and that's how much heat the trailer needs.

I run my radiant heat system at home at 144 degrees. It uses 1/2" pex with the aluminum heat plates stapled up around it to the floor. I have water-flow measuring devices on each zone and can measure the temperatures on the inlet and outlet of each zone with an infra-red thermometer. The floor is a 3/4" solid wood subfloor with 3/4" wooden plank flooring above it and the usual mixture of area rugs and furniture you would expect. I have insulation to about R-20 below the pex over an unheated basement. What I have found is that the largest zone, which heats 300 s.f. of floor space, will not transfer more than 10,000 BTU/h. At this point the floor is quite warm to the touch although not unpleasantly so. The floor area on your airstream is somewhat less than that so you have to realize that you're just not going to get more than say 9,000 BTU from radiant floor heat no matter how well you run it, unless you get the floor too hot for it to be comfortable to walk on, with the attendant problems this poses for floor coverings, electronics, warpage, drying, etc.

Regarding flow rates, the rule of thumb is that you want to design the loops so that, with 1/2" pex, you get 1 GPM in 300 feet of PEX with a 10 degree drop which then heats 10,000 BTU/h. If you have lower BTU requirements you can still run 1 GPM and you'll get less than a 10 degree drop which is fine. It is possible to run up to 2 GPM through the PEX in which case you will get a 5 degree drop at 10,000 BTU or you can set up a longer loop or one with some baseboards and get 20,000 BTU at a 10 degree drop.

The pumps used in radiant heating can be throttled with a flow reducer or an ordinary valve, so that you get the flow rate you want. It shouldn't hurt the pump. If you have some sort of goofy pump that doesn't like to be throttled well then just plumb in a bypass loop around the pump with a valve you can adjust so that half the flow rate goes in a circle around the pump.

Much over 2 GPM and you risk abrasive damage to the piping system in 1/2". It's like sandblasting with whatever sediment made it past your filter and whatever corrosion products have become suspended in the water over time.

In general you do want to keep the domestic water and the radiant heat water separate, mainly because the dissolved oxygen in domestic water will tend to cause boiler corrosion. I run a glycol antifreeze at home having lived through the pipes freezing in a hydronic system in the house where I grew up. Not pretty. In a modern system the PEX will survive but any metallic components e.g. fittings in the system are likely to burst especially with repeated freeze-thaw cycles.

In our house we have had to add baseboard units in order to get enough heat. It was done after the fact and ended up being a big hassle and quite costly. Best to design them in if you think you'll need them and from the math I think you'll need them.

Before there was PEX people used 1/2" soft copper. Expensive and prone to leakage where solder joints are used, and generally many solder joints are required because you can't lace copper through a bunch of turns the way you can pex. In practice you won't get more heat output from copper than PEX, because the heat transfer through the wall of the PEX isn't going to limit the amount of heat you get, the amount of heat transfer through the floor is what will set the limit.

Our house has heat that actually works and every room is comfortable in wind and weather.That is rare for buildings in Minnesota because almost all of them are heated with forced air and there are cold spots and hot spots that move around depending on wind direction and whether the sun is shining. Aside from some pipe creaking at times due to expansion our system is silent.

Radiant systems don't inherently require much longer to heat up than forced air with the exception of system where the radiant coils are deep in a concrete floor. However in practice most forced air systems installed in houses are oversized while radiant systems are not and this makes for the situation where it takes several hours longer to heat up a cold house. A 60,000 BTU furnace costs about the same as an 80,000 BTU furnace and what happens is that the heating contractors use the same size furnace in most of the installs that they do, nobody calls to complain if it's too big and they get their chain pulled if it's too small. In practice the oversized furnace does pose a number of problems but that's outside the scope of our discussion here. I don't believe the same logic applies to Airstreams and would guess that the 34,000 BTU/h units being installed are a pretty good fit.

[Jammer]

Originally Posted by CrawfordGene

Malcolm, I have heard you have to calculate bends when figuring out flow rate and pump capacity. My guess is 90˚ bends would be more restrictive than curves and sharp curves more so than gradual ones. Putting the Pex 3" apart may be much too close because of the tight bends. Like Bob, I know nothing about this. I'd start by checking the internet. Maybe Lowe's or Home Depot have books on installing this kind of system with tables explaining it.

Gene

Better to put it 9" apart and use the aluminum heat plates. It will work just as well and be cheaper and easier to install

[Jammer]

Originally Posted by Secguru

1 - What is the total volume of water that will be in the lines under the floor?
2 - What is the total weight of the system including the water in the lines?
3 - How does that added weight change your gross weight and available payload?
4 - How will you heat the water tanks if reflective foil is between the water lines and the sub-floor?

100 feet of half-inch PEX holds somewhat less than a gallon of water. The weight of the PEX itself is inconsequential. I can pick up a 500 foot coil in one hand. It would be difficult to imagine that even with the pumps valves fittings that more than 50 pounds would be added to the trailer. Do the math, if you have a line every 9" across an 8" trailer you would have 10 or 11 lines which means between 300-400 feet of pipe, that means 4 gallons of water at 8.333 pounds a gallon, we're talking 34 pounds plus a few pounds for the pipe itself. With glycol antifreeze the weight would be somewhat lower because it isn't as dense as water.

The heat source -- water heater, boiler, etc., would be in addition to that and is really the only thing that has to be watched from a weight and balance standpoint.

[Jammer]

Originally Posted by ROBERTSUNRUS

Too fast of flow rate in a car radiator can cause overheating because it doesn't have a chance to transfer the heat. This could be the same in reverse for your system.

This just isn't true and you can come pay me a visit and we can set up a thermodynamics lab in my garage and measure it if you don't believe me.

With a faster flow rate yes the inlet and outlet temperatures will be closer together which may convince the uninitiated that less heat transfer is taking place, but the mass flow rate will be higher in terms of pounds of water per hour will be higher. For water BTU/h = pounds per hour * temperature difference in degrees F. In practice the rate will increase asymptotically as the flow rate is increased so there is indeed a point of diminishing returns but the heat transfer never goes down with increasing flow rate.

[Jammer]

Originally Posted by ROBERTSUNRUS

Hi, I know nothing about radiant heat or floor heating systems, But I do know that if you use a non-contact thermometer on your car/truck you will see a higher temperature reading on the metal thermostat housing and a brass radiator tank that you will see on a rubber radiator hose or a plastic radiator tank. This tells me that for heat transfer, you would be better off using copper tubing.

While what you say is true for a hose or tube that is in free air at room temperature. However for radiant heat the air surrounding the tube rapidly heats to within a few degrees of the temperature of the water inside the tube because the floor above the tube transfers heat more slowly than the wall of the tube does. There are a bunch of equations that explain this based on a characteristic thermal coefficient of the material and the area and thickness of the transfer region. Another way to think of it is that the 3/4" thick (or 1/2" or whatever) wood floor is more of a barrier than the PEX, rubber, whatever, and so it doesn't make much difference. Yes, copper will work better but it's an insignificant one and copper is just a hassle for radiant heat.

[Jammer]

Originally Posted by malconium

I had the inner skins off but unfortunately that is in the past tense now. They are all back on now except for the middle strip down the center of the ceiling. I also have the rough framework of my inner walls, shower and cabinets in the way now. Putting the tubing in the walls seems like a pretty interesting idea though. I am not opposed to the idea of adding some tubing in places where it might be visible. I had also given some thought to adding some tubing behind an aluminum overlay of some sort somewhat like the various kinds of wall radiators that are available.

Perhaps using some other type of tubing on the wall or ceiling areas that looked better than PEX tubing might be better. Maybe aluminum or copper tubing would be a good idea. I suspect that both aluminum and copper would make better radiators than PEX tubing.

The challenge is to find some visually appealing and reliable way to attach tubing to the walls or ceiling in a way that conducted heat to the inner skin so as to make it a radiant plate in my heating system. I do still want to keep the overall cost down too by the way.

Consider whether some of these would work with your decor:

Buderus Panel Radiators

Aluminum tubing will work but it is prone to corrosion so you would then have to have an isolated water system to keep the dissolved oxygen levels low.

[Jammer]

Originally Posted by CrawfordGene

I was thinking "why not put tubing under the cabinets"? Then I realized a lot of food gets stored in them and would get slow cooked at 130˚ (Airstream as crock pot?). Otherwise, heat under cabinets and furniture seems like a good idea because they would store a lot of heat, but you may have to be careful what you keep in them. Would a bed get too hot? Maybe fewer loops under them and more under the floor.

I have tubing under the cabinets in my house and it's not a problem. In practice the cabinets don't actually get that hot, maybe 80 degrees instead of 70, and most people use the cabinets closer to the floor for pots and pans. In a house you do want to avoid heat under the fridge and many installers just insulate the floor there to avoid having to disrupt the pipe pattern.

[Jammer]

Originally Posted by 2airishuman

DRAINING?

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

In general radiant systems are more prone to freezing than DHW systems because the pipe is in a less protected spot.

It is not realistic to expect to drain a radiant heat system without a source of high-volume compressed air.

[Jammer]

Originally Posted by malconium

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.

It's going to depend on how far you go and how cold it is. If it's 15 degrees out and you drive for two hours, you're going to have problems. All you need is one frozen spot to ruin your trip because then you can't run the heat and by the time you find it you'll have other frozen spots.

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?

Put this one in the same category as socks that are warmer because they wick away moisture

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.

It will work fine just be realistic about the amount of BTUs you need and the transfer rate per square foot you're likely to achieve

[Jammer]

Originally Posted by malconium

I can see that an expansion tank and air purge valve would make sense for a closed heating loop. Are they really necessary for an open loop systems such as what I am proposing here?

Once the system is hot it will have, say, a quart more water in it than when it was cold. Where's that going to go? If you already have a pressure tank (by any name, buffer tank, etc.) on the outlet side of your water pump maybe you're fine.

Yes you can get away without an air purge valve if you're willing to tolerate some noise. With fresh water coming in all the time the water heater will constantly generate air because it will come out of solution when the water is heated.

[Jammer]

Originally Posted by HiHoAgRV

How about measuring the thermal loss?

If you are lucky enough to have a time period where the outside temp is fairly cold and constant you can use small electric heaters to measure the thermal input to keep the trailer at a consistant temp. Use a Kill-a-Watt meter to totalize the watt/hrs over the time period and record the temperature difference between the inside and outside. You then have KWH/Delta degree constant for your trailer. KWH's convert to BTU's and you have a handy way of calculating how many BTU's it takes for a given temp difference. It's not perfect but it will give you something to go with...

If you use 1200W over 6 hours = 200WH = .2 KWH =700 BTU
for a 40 degree differental (20 outside, 60 inside)
700/40 = 17.5 BTU per delta degree F

If you have a 15,000 btu water heater and have 30 degrees of temp gain needed (40 outside and you want 70 inside) you need 525 btu (17.5 X 30)and have 15,000 available so it would run 525/15,000 =3.5% of the time

I have no idea where all of that came from...

This will work. I have a power-company type KWH meter and socket I bought from ebay for this type of thing.

You can also weigh your propane tanks before and after and do the math.

[Jammer]

Originally Posted by malconium

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's not. It's complete bunkum unless you're never going to take the trailer outside Arizona, and then only if you stay out of the mountains.

See the other posts I've made above. Another thing to consider is that, in Minnesota, typically it takes twice as many BTUs to heat a house as it does to cool it. If you stick to warmer areas where it doesn't get below freezing you can get away with the heating and cooling BTUs about the same. How many airstreams have been upgraded from a 9,000 BTU air conditioner to something bigger?

[Gene]

Jammer, thanks for a lot of good info. Makes me glad I'm not trying to figure out how to do this, but I am learning a lot about this type of heating system. Sad to see Malcolm can't use his cabinets as a crock pot.

Gene

[malconium]

Originally Posted by Secguru

1 - What is the total volume of water that will be in the lines under the floor?
2 - What is the total weight of the system including the water in the lines?
3 - How does that added weight change your gross weight and available payload?
4 - How will you heat the water tanks if reflective foil is between the water lines and the sub-floor?

The calculation that Kittrik made above determined that there is only about 1-1/2 gallons of water in the tubes on the floor. Water weighs something like 7 or 8 pounds per gallon if I recall correctly so this is a fairly negligible amount of extra weight for that part of the system. PEX tubing is pretty light stuff too. If I use a tank type water heater the 10 gallons there would be the most significant part of the weight of the system. The original water heater was probably a 6 gallon one so I would only be adding about 30 extra pounds by going to a larger water heater. The pump for the system is pretty light too. There would be some extra weight associated with the fact that I need to layers of sub-flooring but the top layer is likely to be 1/4" plywood which should not be all that heavy. I would guess that I might be adding about 200 lbs or so extra for the total system. The new axles that I have sitting waiting for installation have a 10% increase in gross capacity over the originals. That gives me approximately 700 lbs of extra load capacity. My Polyboard floor added about 250 extra pounds overall. Otherwise I am trying to keep the rest of my remodel as light as possible where I have a choice. I think I will be OK.

Regarding question 4 I think you may have misunderstood what I was saying there. In my installation I will be creating a sandwich with two sub-floor layers. The bottom layer will be the regular sub-floor. That layer in my case is now made out of Polyboard instead of plywood. The foil will be on top of that. Then comes the PEX tubing on top of the foil along with some 3/4" x 3/4" wood strips in between to help support the next layer. There will be a layer of 1/4" plywood on top with a seamless vinyl floor furnishing the final finish layer. I hope that clarifies what I meant.

Malcolm

[2airishuman]

lots of good info jammer,

the o.p. is clearly enthusiastic about trying this but i don't see these 3 relevant premises explained well here...

1. why specifically use radiant IN a 'stream?
2. careful cost comparisons of available rv heating systems.
3. an objective list of all the + and - of each approach to heating a travel trailer.

of course ALL that was requested was heat loss calculations, not an open debate on the nature of this project.

so there is NO need to address these issues...

but its pretty clear the heat loss calcs are a crap shoot disguised with math.

both hiho and now u have suggested the better way is to calculate how many btus are needed to heat a 31 foot 70s trailer.

to do that NOW the o.p. needs someone with a similar unit to step up, since the target trailer is in construction mode...

i still think there will be significant heat loss via the frame/ribs/shell UNLESS this is accounted for in the design.

from an earlier post it appears the o.p. has used a felt layer between the ribs and INNER skin,

my understanding is that this will enhance heat conduction/loss to the outside, not hinder it.
_____________

yes 2 1500 w (~4-5k btu) electric space heaters will do the job nicely in MOST pacific northwest weather...

and 3x5000 btus heaters would be toasty inside a 31,

except in the mountains or EAST of the cascades or during one of the common deep cold snaps that happen or during WINDY conditions.

i can confirm that 2 vented cats of ~6-7k btu will keep this size unit VERY toasty, down the upper 20s...

because most of the plumbing is INSIDE the unit, pipes are protected...

but the tanks are NOT warmed with space heaters so to deal with freezing tanks they DO need heat.

and the water tanks are NOT rigidly attached in the subframe, so the heating needs to allow for shifting,

unless the tanks are better fixed.

the forced air ducting warms the holding tanks slightly, with very little weight or highly technical design.

cats and furnace both run on propane the most common fuel source for most of rv doodads...

i like the outside the box thinking/approach that one of our canadian member smokeless joe took with an ALL DIESEL system...

http://www.airforums.com/forums/f227...ase-18448.html

amazing work in that thread some brilliant design and beautiful appliances...

but i'm not sure the unit has every been road tested/used even after 3-4 years.

as a last curiosity/aside MANY of the newer classics DO HAVE radiant heating in a very small section of the floor.

most of the pumping is on the street side in the newer classics, except the water pump, which is curbside along with the furnace and water heater.

this means a heater duct crosses UNDER the floor to curbside and pex water pipes cross over/back to the pump.

so IF i stand in just the right spot (where the floor vibrates) i can feel the warmth...

ok, maybe not.

cheers
2air'

[VIKING]

Warmboard is a radiant heat flooring company that uses pex at 12" on center, where most use 6" on center. Warmboard puts the pex into an aluminum tray that's imbeded into a grooved sheet of 1" plywood. The aluminum helps transfer the heat without the need for so much tubing and pumps. The maximum run in 1/2" pex is about 260' but 200' is better for consistent flow rates. Less than 6" centers causes the pex to lose it's shape and restrict flow. It helps to have it in a groove to maintain its shape. Sitting there by itself it may kink at such a tight bend.
I think you are going to run up against the issue of having to keep the floor uncomfortably warm to maintain the temperature in the Airstream. I think that is why the high-end coaches use baseboard heaters instead. Then you won't be walking on the heat source and you can have it at a higher temperature than for floor heat. It should take much less of a system if you were to use baseboard heaters, and it might make sense to have a dedicated system not part of the water system, oil filled, so no maintenance.
Not poo-pooing your Idea here. I just think the system may be over-done by the time you work out all of the issues, and another approach may actually be less expensive and more effecient. Feel free to ignore this entirely.

Best to you,
Rich the Viking

[Jammer]

Originally Posted by 2airishuman

1. why specifically use radiant IN a 'stream?
2. careful cost comparisons of available rv heating systems.
3. an objective list of all the + and - of each approach to heating a travel trailer.

I think that many of the advantages of well-designed hydronic systems over what the hydronic guys like to call "scorched air" are relevant in a 'stream, particularly:

1. Silent
2. Lower amp draw, potentially
3. More even distribution of heat throughout the trailer and better control options
4. Ease of dual-fuel (propane/electric) operation as befits a particular camping/parking situation

I think the problem with radiant particularly is that the ratio of floor area to exterior shell area is poor and there's no way to get even close on the BTUs. Anything above 8,000 from the floor just isn't realistic and this in a trailer that is going to need somewhere in the neighborhood of 20,000 BTUs/h up, to get the job done right.

Using panel radiators would allow a switch to a 180 degree operating temperature. There are several choices for shapes and sizes of panel radiators that would add up to 20,000 BTU/h in a reasonable number of pieces.

http://www.buderus.net/Portals/1/Man...ch_0208BTC.pdf

Pump wise the SID10B12 on this page would work about right and draws just under 1 amp.

That just leaves the not-so-small matter of a source of hot water. The typical RV water heaters top out at 12,000 BTU input and multiplying by .75 for efficiency leaves us in the range where we would have to install three of them, which isn't satisfactory on a) principle, b) weight, c) space.

Maybe two would do the job. That wouldn't be so bad.

[Gene]

The Airstream European models use baseboard heat for what that's worth. They are also smaller than current US and Canadian models.

Gene