Hot water radiant heat?

[malconium]

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.

Thanks,

Malcolm

[67caravel]

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!

[markdoane]

Malcolm,
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?

[74Argosy24MH]

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.

John

[norbert]

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....

[j54mark]

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.

Mark

[bryanl]

One approach is described at http://www.solarhaven.org/HPArticle.htm

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.

[Pahaska]

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?

[FireFighter]

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.

[markdoane]

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.

[mandolindave]

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!

[swebster]

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.

[71_safari]

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.

john

[malconium]

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.

Thanks,

Malcolm

[bryanl]

One potential concern is that the radiant system might also be a part of the fresh water supply. This means that some care needs to be taken to make sure you have radiators and such that don't have caustic metals (lead from solder) or other contaminant problems.

In a trailer, floor or skin area is in rather short supply. Overhead is also rather low. So you don't have much surface area to work with for heat dissipation and an inch or two for radiating materials might be a problem. This is one reason why forced air conductive heat transfer might be preferrable as it is compact and can quickley heat the air. With any passive radiator you are going to have a much slower air temperature change which is not the way an RV lifestyle usually goes.

[markdoane]

Originally Posted by malconium

. . . .
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.
. . . .

Malcolm

Malcolm,
I think there are lots of misconceptions about what 'radiant' heat is. In the strict sense, the heat you feel from the sun, or from an overhead sidewalk heater depends on the temperature of the emitter. In the case of the sun, the temperature is several thousand degrees, and even the sidewalk heaters are several hundred degrees. A necessary condition is that the emitter puts out lots of near IR radiation, which you can't achieve with hot water at 140F.
Even the old fashioned cast iron 'radiators' provide most of their heat by free convection, not radiative heat transfer. They heat up the air near the walls, the hot air rises to the ceiling, displacing the colder air, which scoots along the floor to the radiator, you know the rest. . . .

[norbert]

malcolm,
i know this is your toy, and you want to play with it, but you need to do some sober thinking...when you started this thread you did not specify any parameters as to the manner of use....i notice from your profile that you reside in portland....do you plan to only use your AS in this geographical area?...when a heating engineer designs a system, they use outdoor design temperature to size a heating system for a particlular area...in your case portland design temp is 10 degrees and an avg relative humdidity of 82%...given these conditions no amount of reflective insulation will work... it seems to me that all you are accomplishing is an exercise in mental masterbation.......
norby

[malconium]

Norby,

I think the whole point of having a travel trailer is to be able to travel around in it. We will probably not use it just in Oregon and I would like to be free to travel pretty much anywhere we want to. In general my guess is that we will want to travel to places where the weather is going to be nice when we are there. Why go otherwise?

Having said all that could you be more specific as to why you think reflective foil will not work for me? Of course the times that I would need it to work are principally situations where there is a difference between the temperature inside versus outside. I have been refering to information at the following website that describes some of the physics involved in thermal transfer in buildings.

http://www.tvmi.com/rfoil/physics.htm

This site indicates that the major component of heat loss through walls and roof (and heat gain too) is by radiation.

What am I missing here?

Malcolm

[norbert]

i guess we need to better define the use of the unit...therein lies wether or not insulation will meet the need...and return on investment...will it be cost effective?...consider tht your envelope of aluminium has different heat transfer characteristics as opposed to wood or brick...also consider heat migration from windows and ceiling vents...they are not double pane with argon gas between as some windows are manufactured....and also the wind chill factor...also voltage considerations...12v or 120v?...are you going to boondock or have an unlimited source of power?...heat migrates to cold not vice versa....maybe i am overdoing it....but AS was never designed to be an all weather camper...unless of course you want to be a snowbird....
norby

[malconium]

I did some research since I started this thread and have found out a few things that lead me to think that radiant heat is feasible in a trailer. It turns out that my best source of information is a local company called "Shelter Products". Supposedly radiant heat in the floor is used in some high end RV's.

There are several approaches that would work but the easiest system to use in an AS would install on top of the sub-floor and works just fine for a floating floor on top of it. The easiest solution is the most expensive solution of the ones that I looked at but still not too terrible. Here are the details:

1.) The system is based on an insulation panel that is 3/4" thick and comes in pieces that are 2' x 3'. The panel has a sheet of aluminum on top of it that has grooves molded in (at 6" on center) to accept 1/2" OD PEX plastic tubing. The panels come in a kit that covers about 100 square feet per kit. The kit price is $560 from Shelter Products. There are some additional pieces for around the edges where tubing would need to double back on itself. The tubing ends up being 6" on center or about 2 feet of tubing per square foot of floor area.

2.) The 1/2" PEX tubing comes in a roll of 300' for about $117.

3.) The heat transfer applies directly to the bottom of the floating floor so thermal lag time before the area is at maximum temperature should be no more than a maximum of 1/2 hour. Of course the heat would be felt a lot sooner than that.

4.) I was told that as a guideline the floor panels would need a maximum of about 25 btus of heat output from the hot water per square foot of floor panel. That means that 100 square feet would only need about 2500 btus from the water heater. When in use the floor tubing would actually act as sort of an extension of the water heater tank size (if not using a continuous flow water heater).

5.) A continuous flow water heater was not thought to be necessary to make the system work since the amount of heat needed was relatively low. Also the water returning to the water heater is not as cold as tap water either so a tank type heater would pretty much treat the whole system as just a little larger tank.

6.) Pretty much all that is needed in addition to the panels and tubing would be a suitable thermostat, a solenoid valve and a pump. I suggested the idea that the trailer's water system pump might be able to do double duty and that was thought to be a reasonable possibility. A single loop of tubing was thought to be OK for the 100 square feet.

My trailer is a 1973 31' Sovereign center bath model. I estimated that the actual floor area that I have after I substract cupboards, bed, couch area etc. was pretty close to the 100 square feet that the panel kit and 300' of tubing would cover. My total square footage is maybe 200 square feet. The guy I talked to felt that 100 square feet of floor panel would do the job just fine for the size unit. This means my total costs would be about the following:

Insulated panels $560
Tubing $117
Misc fittings $20
------------------------
Total $697

The above assumes that I can pick a replaement water pump (I need a new one anyway) that would do double duty. I was leaning in favor of a continuous flow water heater but I am having second thoughts about the need for it now. They cost about $875 and may not actually be necessary. I think more conventional tank types would cost about $300. The difference would go a long way toward paying for the heating system. I would also not need to buy a new LP heater either so this would make the overall approach fairly reasonable on the whole.

The only significant issue that I can think of now is that I lose an extra 3/4" of ceiling height because of the thickness of the floor panels. I noticed yesterday that the metal lip on the bottom of my door is 3/4" tall. This means that the floating floor would be sitting above the lip rather than below it. I would have to add some sort of trim strip at that interface.

It could also be that I would not really need any other undrfloor insulation. This would mean that I woul have to put 3/4" insulation on the other 100 square feet of my floor that is not covered by the heating panels but that should not be too much of a problem I would think. I am still leaning in favor of putting foil insulation about 3/4" below the floor anyway so I have to make up my mind what I want to do about that.

One other interesting note about this type of installation is that virtually all of the tubing would be accessible by just removing the floating floor if anything needed to be fixed.

Malcolm