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Old 06-22-2004, 09:02 PM   #1
Rivet Master
1973 31' Sovereign
Portland , Oregon
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Question Hot water radiant heat?

I am having to do a pretty extensive rebuild of my 1973 31' Sovereign. This will include rebuilding almost the entire interior (gutted by the PO). It will also require replacing the water heater and heater. I have some knowledge of and direct experience with hot water radiant heat. I know that it is possible to design a system that uses the water heater as the source of hot water. I have also been giving some thought to buying a continuous flow hot water heater. Most typical hot water radiant heating systems are installed in the floor. While I think that could be done with an AS it occurred to me that maybe the ceiling would be a better candidate. The whole inner skin is, after all, made out of aluminum which should make a tremedous radiating surface if I can just heat it up a bit. Modern radiant heating systems typically use a special variety of plastic tubing that should be very forgiving relative to vibration as the trailer moves. I think all I would have to do is to find a way to lay loops of tubing down the center panel of the ceiling in such a way as to get really good heat transfer from the hot water, through the wall of the tubing to the aluminum inner skin. The aluminum should be able to spread the head out and radiate it into the interior.

As an optional alternative it should also be possible to install some sort of wall mounted radiators here and there that would be heated with hot water. There are some nice units, for example, that double as a heated towel rack that might work nicely.

I would appreciate any thoughts on the topic, especially from anyone that has done anything along these lines.



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Old 06-23-2004, 06:14 AM   #2
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An interesting thought. Our home has infloor radiant heat. Hailed as the best thing to happen to heating since sliced bread, there are some shortcomings. For instance, it takes awhile to obtain heat if you turn on the system (in a "slab on grade" system like ours it can take many hours). An A/S furnace provides immediate heat. In such a small area as a trailer, it is warm in just a few minutes. I can imagine setting up camp on a cold fall evening and having to wait for the boiler, pump, and circulation to move the heated water to the "loops" you are discussing... I know when I am cold when I wake in the morning, I want heat "now!". Just a thought. And, there is nothing better, IMHO, than sleeping with the windows open. You don't want your boiler and pump running all night when you are trying to cool things down..

One thing I really don't like about the existing A/S furnace is the noise! I am looking at one of those propane catalytic radiant heaters as an add-on to my A/S. Garth Cane writes an article discussing the various options here.

Good luck and let us know how it works!

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Old 06-23-2004, 07:54 AM   #3
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Leakage issues aside, I think the next biggest challenge would be moving the heat from the ceiling to the floor. Ceiling fans are out, obviously.
Kind of the same issue with the heated towel bars and radiators. Can you locate them where they will be useful, and how do you move the heat evenly around the trailer?
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Old 06-23-2004, 08:04 AM   #4
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I think the double aluminum skins, aluminum ribs joining them and small insulation cavity will work against you. On cool days I can see the outlines of the ribs inside (condensation) and outside (no frost). If you put you heat source inside this cavity I bet you won't have to worry about clearing snow off the roof.

You are also going to add a lot of weight with the pump, water, lines, valves.

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Old 06-23-2004, 08:09 AM   #5
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gimme some of what youre smokin.....first off if you wanted to one would have to have a separate system and run glycol....2nd....the r- value of the insulation is insufficient, so if you like heating the outside, go for it....
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Old 06-23-2004, 08:16 AM   #6
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There have been, I think, two reports on these forums by people who have installed radiant floor systems.

The kind of thing you describe will require electric hookups, so it will not be suitable for boondocking. As long as you have electric power, why not direct electric radiant heat? We have a small fan driven electric heater that a PO installed under the wardrobe - rather neatly I might add. It is quiet, and very effective down to near freezing temperatures. At that point I (and you) will want the furnace anyway to provide heat to the holding tanks.

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Old 06-23-2004, 09:20 AM   #7
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One approach is described at

I do not think trying to couple the heat to the inner skin would be very easy to do. It would be much easier, as you noted, to use a few passive radiators in a loop around the trailer.

For a heat source, the RV-500 tankless should do fairly well. I hear the mfg is considering supporting hydronic heat with it. Its advantages include a 50kBTU/hr input (so you get the same amount of heat or a bit more than a typical RV furnace) and a modulated burner.

PEX tubing, such as used in most RV plumbing these days, should work for the system as it is used extensively in home hydronic systems.

A system such as described on the page link above should use less battery power than an equivalent furnace, expecially if you used passive radiators.

If you look around, you can also find systems that run on diesel that are designed for motorhomes.
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Old 06-23-2004, 10:07 AM   #8
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A possibly better solution

I just recently erased a link to a website where a fellow had used the water heater, a circulating pump, a very high-efficiency fan, and a small radiator to produce a hot-air system for a fair-sized trailer. He claimed great things for it. It would seem to me that that is a much better solution. He claimed it was virtually noiseless and used less 12v than the usual furnace. With some forethought, the current air distriobution system could be used to get some heat to the bathroom, bedroom, and tanks.

BTW, did you consider keeping the tanks from freezing?
John W. Irwin
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2013 Silverado 2500HD Duramax/Allison LTZ
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Old 06-23-2004, 11:14 AM   #9
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why not electricity

instead of using water with pex and a water heater and pump why not check out some of the mesh floor systems they are electrical and lay under the carpet and or vinyl it can be stapled thru and everything. might just be the answer, no noise at all, no worry about freezing pex lines. if you got creative you could probably wrap the holding tanks with it or something. i read about this stuff at a web site, search google for "in floor heat" there are a bunch of manufacturer sites.
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Old 06-23-2004, 01:39 PM   #10
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I used the electric floor mesh in my new bathroom. Cost was reasonable and it works great. The ceramic floor helps to equalize the heat. Won't run on 12V though.

I might try a combination of the electric mesh and hot water. Don't boondock that often in cold weather.
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Old 06-23-2004, 02:02 PM   #11
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I was really intrigued by the idea of using a hot water heater and a fan-radiator as a heat source. ( Someone built one and claims to use much less propane and electricity, and claims to get less hot and cold peaks. He used a car radiator, a low amp fan, a low volume circulating pump, and an old water heater ) Then I realized I would have water in the trailer in the cold season. I would have to keep the water warm 24/7 or keep draining it. I hadn't thought of using antifreeze. Duh!
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Old 06-23-2004, 03:22 PM   #12
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Thanks for the link to Hydronic heating in an RV. Our 345 has two surburban furnaces which literally wake you up at night if they both come on at the same time. We have two RV coach batteries on the house system both of which are close to drained after on evening of boondocking on a sub 32 degree night.

I just thought this was part of camping. Wake up early (and cold) to start the RV to generate enought power to fire the generator to charge up the batteries to run the furnaces again.

That article has me intriqued. Especially becuase I already have a ducted aux heater core from the 345 (was under the couch) sitting in my workshop....hmmm - good fall experiment.
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Old 06-23-2004, 03:28 PM   #13
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great idea

Nothing better than radiant heat. I think wall mounted or free-standing radiators like were used in old apartments would be perfect. If you can engineer it so that it uses convection to flow it would be even better and eliminate or reduce the 12v requirement.

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Old 06-23-2004, 06:35 PM   #14
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Some more thoughts...

I have read all the responses with interest. I thought I sould respond to some of them and perhaps add some additional food for thought. I also want to clear up some misunderstandings about thermal dynamics and radiant heating. Hopefully I have the facts right myself so please jump in with comments if I get something wrong here.

1.) It is true that radiant floor systems that are installed in a concrete slab do have a very long cycle time. That in fact helps them to be more comfortable since the floor continues to radiat heat even when water is not pumping through the floor. This is basically what happens with the oil filled electric radiator style heaters too.

2.) The response time of a radiant heating system does not need to be long providing the radiating material that gets heated does not have a large thermal mass (like a concrete slab has). My thought is to place platic tubing in direct contact with a radiating surface (such as the inner skin of my AS) that will warm up almost immediately. As soon as the radiant surface is warm the effect of the heat will be felt by the peopl near it. The air between does not first have to be warmed up either by the way.

3.) Radiant heating systems are inherantly very quiet since the principle noise is the pump that moves the fluid (and the noise of the water heater). They also do not stir up the air (a good thing for people with alergy sensitivities).

4.) One very good feature of radiant heating (and a commonly missunderstood feature too I think) is that it does not need to rely on air movement to heat a space. The heat energy is transmitted directly to the objects and people in the room by radiation (in the same way that we get heat directly from the sun when we go outside on a sunny day). In fact it also does not have to have a closed space to provide heat to people. Think about all the outdoor heaters you have ever seen. They are all radiant heaters and they manage to warm you up even though you are outdoors with no walls around you at all. This is one very good reason that radiant heating systems are so great for houses with high ceilings. Unlike heating by convection you do not have to heat the air in the top of the room before the people feel the warmth.

5.) In fact the double aluminum skins and small amount of insulation works against any type of heating system. It is my plan to change the insulation in my unit to be a lot more efficient at stopping heat loss by radiation. I intend to add a double layer of reflective foil insulation in the wall cavities instead of the fiberglasss. I have most of the inner skins already removed toward that end. If I do heat up a part of the inner skin I should also find a way to thermally isolate it from the ribs so that the amount of heat that I lose through conduction to them is minimized. I think I could probably add thin foam tape between the ribs and the part of the skin I heat up to help with this.

6.) I don't anticpate adding much more weight (if any) than the weight of my old forced air heater and full 10 gallon water heater. I might be able to make my normal water pump do double duty. I am thinking of using a continuous flow hot water heater (the RV-500 in fact). I will not need to have 10 gallons of water sitting in it. The tubing is pretty light and I don't think will need to hold all that much water (maybe the 10 gallons that used to be in my water heater would be enough). A single 12volt on-off valve should be pretty light in weight I would think.

7.) I don't think it would be necessary to run anti-freeze in the heat pipes any more than it should be necessary for me to run anti-freeze in my water pipes. I just need to make sure that they are drained when I winterrize and that they are either heated or insulated when the trailer is in use. It also would be pretty easy to add a thermostat to the water line to turn on the pump briefly any time the water temp falls below some preset limit. As I recall the RV-500 has some sort of feature (maybe an option?) that does something similiar.

8.) If anyone has any pointers to infomation on anyone that has put radiant heat in the floor of a trailer I would be interested.

9.) I would be very suprised if the electricity required to pump my water around would be greater than the electricity to push hot air around. Am I right that the standard furnace has a fan in it? I don't know. Maybe what I need to find is a propane powered pump.

10.) Its a good point about needing to heat the holding tanks. I think I should at least run a bit of tubing down through that area. This may also mean that it would be a better idea to put my radiant heat in the floor so it can heat the whole belly area. I would just need to put my reflective insulation underneath anything down there that I want to warm up.

11.) The approach with the automotive type radiator is one good way to go. It is not, however, a radiant system. It is a hot water system but it transfers heat from the radiator into the room by convection using a fan to push air. This means that there is a pump to move water to the radiator and a fan to move air across it. I think this approach, while probably better than the conventional furnace, will still use more electricity than a strictly radiant approach (with no fan needed anywhere).

12.) Electical radiant heating is a great way to go providing you are willing to have a heating system that only works when you are hooked up to power. I don't think I want to restrict myself to that requirement. If I were I might very well want to make an all electric trailer and avoid propane altogether. I do like the idea of using some electric in a place like the bathroom though.

Lets keep up the discussion. I am very interested in using radiant heat in my unit and fully intend to document whatever approach I decide to take for the benefit of anyone else that is interested.



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