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Old 01-11-2010, 12:05 PM   #57
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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'
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Old 01-11-2010, 12:14 PM   #58
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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
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Old 01-11-2010, 12:29 PM   #59
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Quote:
Originally Posted by 2airishuman View Post
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.
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Old 01-11-2010, 12:31 PM   #60
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The Airstream European models use baseboard heat for what that's worth. They are also smaller than current US and Canadian models.

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Old 01-11-2010, 12:42 PM   #61
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'panels' or baseboard heat does solve many of the issues...

but still doesn't address the tanks.

IF it's cold enough (even just occasionally) that the tanks might freeze, this needs to be considered.

so would one place a panel/baseboard under the tanks too?

a lot of this depends on how/when the user will be rving....

boondocking vs full hook ups and LENGTH of trip matter.

at least he's not thinking about trying to heat with the oven door open or a cook top flame...

cheers
2air'
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Old 01-11-2010, 01:13 PM   #62
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Easy, just put a baseboard element (only) in there wherever the ductwork is now.

http://www.aimradiantheating.com/sto...tOnly_New2.png

If it won't fit, wrap pex under over through etc. Rigid copper also works for situations where the flexibility of the pex poses a problem. Pex can be spiral wrapped around larger pipes or run in between hot/cold water pairs.
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Old 01-11-2010, 01:34 PM   #63
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2air you raise the vital topic of ROADWORTHINESS... anything in a 'stream must perform under what amount to constant earthquake conditions. It is for this among other reasons that cat heaters give me the willies. I feel good about PEX from this standpoint. It is flexible and has been widely used for fresh water in RVs. With the best available fittings and termination methods that involve the use of an expander and some type of compression ring, I think it would be fine. With the O-ring fittings like the Sharkbite and similar competing products I would not be as comfortable in a high vibration environment.

The panel radiators have no moving parts. The concern here would be the mounting brackets, which in Buderus as well as other brands rely at least somewhat on gravity, a fickle and unreliable fastener in a 'stream.

That leaves the pump. Now, even the best wet-rotor 120 volt circulating pumps will fail eventually though they are much more reliable than those spring-coupled, fiber-sealed horrors they used in the 1950s and 1960s. It is unclear to me whether the solar 12 volt pump manufacturers understand how to make a pump that can be relied upon to perform over a large span of years let alone in a high-vibration environment. They seem to be brushless designs which means there are electronics involved.

Finally there are the water heaters. If we were to assume the use of a standard RV water heater we would at least be in the same league wrt reliability as an RV furnace which admittedly is not saying a great deal. Nonetheless at least such a setup would fail safe. No CO in the room due to a hole in the heat exchanger of a furnace or fan failure in one of those vented cats, and the propane plumbing all stays outside the living area.
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Old 01-11-2010, 02:51 PM   #64
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the issue with WRAPPING any of the tanks is movement OF the tank during travel...

OR when full and access TO the tank for service/repairs is needed.

the current set up is basically a flex air tube loosely positioned by the fresh tank, that's it.

while lp gas lines can leak and fitting fail (there are s couple of important example of this here in other threads)...

lp/gas had been road tested and is already widely used for 4-5 systems in almost ALL travel trailers...

i favor the vented catalytic heaters but many use UNvented units that have NO FANS so the amp load is almost nil.

there are 30+ years of history and service data/user experience for cat heaters in rvs.

there are folks who have done VERY silly cat installations (using flex tubing or a free standing cat heater) that should be avoided...

rv grade cats are VERY LIGHT and have almost NO moving parts and can be surface mounted in many interior locations, safely.

but IF really concerned about lp gadgets and gas lines...

look no further than the fridge, stove, oven and water heater (and furnace) in the unit u just purchased...

i think baseboard radiant OR modular radiant, fluid filled heating elements have a lot of potential looking forward.

but would i want the basic structure of a stream FLOOR riddled with fluid filled tubes...

spread out over a surface that will absolutely flex/bend/shift/move and need repair eventually?

no.

cheers
2air'
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Old 01-11-2010, 03:22 PM   #65
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Jammer,

Thanks for all of the great feedback. You have given me a lot of food for further thought about how to proceed.

You suggest that installing aluminum plates on the PEX instead of putting the tubing closer together works better. Also it seams like other types of radiators or baseboard units are capable of transferring heat to the space better than PEX in the floor. That all being the case it is clear that not all types of radiant heat transfer mechanisms work the same. For one thing it is apparent that there would be a practical upper limit to how hot it makes sense to heat your floor while a standalone radiator could be permitted to get hotter. The hotter radiator might need some sort of shielding to keep people and things from coming in contact with it of course.

If I do want to continue with a heating system that distributes heat in my Airstream by direct radiation rather than convection then I suppose using some type of radiator like the ones that you gave the link to might be a reasonable option. My guess though is that their cost might be somewhat prohibitive. I will have to check that out.

I am also wondering if there is something that I could build external to the floor using PEX tubing that would do a better job of heat transfer than I could achieve by heating the floor. For example I have wondered if I could attach tubing to the inner skin in such a way as to make the inner skin be a heat transfer plate. It is aluminum after all. I would need some way to get maximum heat transfer from tubing to the inner skin for this to work though.

So I have some questions as follows:

1.) What is the practical limit as to how many BTU per hour can be transferred per foot of 1/2" PEX tubing?

2.) What is the ideal way to transfer heat from PEX tubing to the surface of a floor?

3.) What is the practical limit as to how many BTU per hour can be transferred through a floor per square foot of floor?

4.) What is the practical maximum temperature for a heated floor?

5.) What is the practical maximum temperature that PEX tubing could deliver to a non-floor radiant surface?

6.) What sort of attachment method could I use to fasten PEX tubing to the inner skin of my Airstream in a way that transfers heat well?

7.) Would it be better to use some type of metal tubing (rather than using PEX) for transferring heat to a non-floor radiator? All of the stand alone radiators that I have seen so far seem to be made of metal rather than any type of plastic.
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Old 01-11-2010, 03:30 PM   #66
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I think that I will want to give serious consideration to buying a watt meter and running some heat loss tests on my Airstream. It does seem like a good way to bypass a whole lot of speculation as to how much heat I really need. I do have a 1500 watt radiator type heater I could use too.

By the way earlier today I found out that the hot water pump that I ordered had not been shipped yet. I was able to cancel that order and place a new one for a pump that has an adjustable rate of flow. This will make it a lot easier to tweak my system. Check it out below:

East Coast Solar :: Laing D5 VARIO Bronze PV-Direct Circulating Pump $169.90

It is a little cheaper than the other one too.

Malcolm
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Old 01-11-2010, 03:43 PM   #67
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Yikes...

This thread has been moving too fast for me to keep up. While I was typing my responses to some things a bunch of new posts came in. I do not mind this though. I am perfectly willing to discuss any aspect of heating my Airstream. I do of course prefer that criticism be constructive and be accompanied by suggestions for alternative approaches when possible.

Lets keep the ideas coming...

Thanks,

Malcolm
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Old 01-11-2010, 05:11 PM   #68
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You suggest that installing aluminum plates on the PEX instead of putting the tubing closer together works better. Also it seams like other types of radiators or baseboard units are capable of transferring heat to the space better than PEX in the floor. That all being the case it is clear that not all types of radiant heat transfer mechanisms work the same. For one thing it is apparent that there would be a practical upper limit to how hot it makes sense to heat your floor while a standalone radiator could be permitted to get hotter. The hotter radiator might need some sort of shielding to keep people and things from coming in contact with it of course.
In most cases you can go up at least to 180 degrees without any shielding, which is usually considered the practical maximum design temperature for PEX.

Quote:
If I do want to continue with a heating system that distributes heat in my Airstream by direct radiation rather than convection then I suppose using some type of radiator like the ones that you gave the link to might be a reasonable option. My guess though is that their cost might be somewhat prohibitive. I will have to check that out.
I think you'll find that the ones I linked to are around $300 each in typical sizes. There are both cheaper and more expensive alternatives if you look around.

Quote:
1.) What is the practical limit as to how many BTU per hour can be transferred per foot of 1/2" PEX tubing?
Fourier's law would tell you the theoretical limit.

Conduction (heat) - Wikipedia, the free encyclopedia

The practical limit? I don't know, around 100 BTU/h/ft at 180 degrees, if you do everything right. Maybe a little more.

Quote:
2.) What is the ideal way to transfer heat from PEX tubing to the surface of a floor?
Aluminum plates like these:

PEX Fittings. Radiant Heat PEX Pipe, PEX Adapters, PEX Tubing Underfloor Systems. HousePEX PEX-A PEX-C. Heat Transfer Plates

There are various products that are heavier, or finned, or both, that are supposed to work a little better. Maybe they do, but there are still a lot of the plain aluminum plates being stapled up.

Quote:

3.) What is the practical limit as to how many BTU per hour can be transferred through a floor per square foot of floor?

4.) What is the practical maximum temperature for a heated floor?
They're interrelated, of course. I don't run the surface of the floor above 80 degrees or so, keeping in mind that below furniture and area rugs it's higher than that. I get around 30 BTU/sf.

Quote:
5.) What is the practical maximum temperature that PEX tubing could deliver to a non-floor radiant surface?
Around 180 degrees.

Quote:
6.) What sort of attachment method could I use to fasten PEX tubing to the inner skin of my Airstream in a way that transfers heat well?
One idea would be to use the regular aluminum heat transfer plates and rivet them on with the pex in between in a kind of sandwich. Anything that keeps the PEX in contact with the aluminum would work. Just supporting the PEX at the ribs and having it between the fiberglass and the wall would probably work fine.

Quote:
7.) Would it be better to use some type of metal tubing (rather than using PEX) for transferring heat to a non-floor radiator? All of the stand alone radiators that I have seen so far seem to be made of metal rather than any type of plastic.
[/quote]

I'd use pex rather than any metal tubing unless the rigidity of the tubing would be an advantage. For example, you can go five feet between supports for rigid copper if you have to, which is sometimes an advantage. There is also PEX-AL-PEX which I used once, which is more rigid than regular PEX and tends to stay where you put it. The lack of PEX radiators has more to do with the difficulty of manufacturing PEX in those shapes and the lack of rigidity due to low compressive strength than PEX's thermal properties.

PEX is also a nuisance in larger sizes and fitting-intensive environments and it's nearly impossible to get the tools in to terminate it in tight quarters. So, a switch to steel or copper might be called for in those situations.
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Old 01-11-2010, 05:17 PM   #69
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Malcolm, Check out www.radiantcompany.com they deal in home heating but much of the information is helpful. Jammer sounds like he installed a similar system as I did using the aluminum distribution plates. The system works great and I agree with much of what he says.

To answer a question raised as to why radiant heat, it is comfortable, quiet and efficient. Unfortunately it is not the cheapest system to install.

The two biggest issues in a trailer are a heat source and floor space vs heat loss to make the system work.

The standard water heaters are 6 or 10 gal. propane fired. Use something else and you are talking more money.

To address the limited floor space, you can add baseboard or radiator units, again this adds cost and hot things to sit near.

Heating tankage and domestic water needs from the same heat source is not really that difficult, tubing is not that hard to run. Again you just have to be creative.

The system I built in my home can use either wood or electricity for the energy source. It heats the house and domestic water and I can add a hot tub, pool, garage or out building simply by running tubing. It is also easy to add more heat sources if I so desire. Water supplied radiant is very flexible.

In an Airstream trailer the standard forced air propane is the cheapest system and it works. That is why they put them in at the factory. If you want water radiant heat it can be done, just like all the other modifications, renovations, and improvements people on this forum have made to their trailers.

This is a great thread with an awful lot of very good information.
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Old 01-11-2010, 09:36 PM   #70
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Curious fact about aluminum...

In doing some further research about various aspects of radiant heating I ran across the following statement with regard to how aluminum behaves relative to radiating heat:

"An under appreciated benefit of the aluminum conduction fins is its low emissivity (nearly zero). This means that when aluminum is warmed, it does not emit radiant energy to the extent that other materials do."

If true this very definitely suggests that it would not be a particularly good thing to attach PEX tubing to the inner skin of my Airstream with the expectation that the aluminum would make a good radiator of heat. It would appear that the use of aluminum fins under the floor is effective because aluminum is a very good conductor of heat. This helps spread the heat out more evenly to the bottom side of the flooring.

So if this is true what type of material should be attached to aluminum fins that would make a good radiator of heat? Does it follow in general that a good conductor is a poor radiator? Does it also follow that a pure conductor is a better radiator?

Thanks,

Malcolm
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