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Old 06-23-2004, 07:55 PM   #15
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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.
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Old 06-23-2004, 08:19 PM   #16
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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. . . .
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Old 06-24-2004, 08:11 AM   #17
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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.......
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Old 06-24-2004, 01:09 PM   #18
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What am I missing here?

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
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Old 06-24-2004, 05:27 PM   #19
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am i being too inflexible in my view?

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....
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Old 07-06-2004, 03:46 PM   #20
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Radiant heat is feasible...

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
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Old 07-06-2004, 05:31 PM   #21
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Is a typical rv water pump rated for hot water use?

Can it pump through 300 ft. of 1/2" pex?

How are you going to switch the pump between the potable water and the heating system? Check the price on 12v solenoid operated 3 way valves, that will blow your budget.

How will you winterize 300 ft. of pex?

What are you going to do for heat for the holding tanks?

John
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Old 07-06-2004, 06:17 PM   #22
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"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."

to get the same amount of heat in the trailer as a typical furnace that means you would need about 1000 sq feet of this sort of heating.

to get the same amount of heat as a typical 110v space heater, which will barely help prevent pipes freezing in 10F - 20F weather, you would need 500 sq feet.

What I infer from the 2500 BTU (per hour assumed) per 100 sq feet is that floor radiant heat is a difficult way to move heat from water to a useful form. That makes me think you might be able to get a somewhat warm floor but keeping the water lines from freezing under the sink is going to be a problem. I am very curious to hear about how it works out in practice.
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Old 07-06-2004, 07:06 PM   #23
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i appreciate that you did some investigation.....and that now you probably realize that the idea of radiant heat will only be a means of making the inside of your coach a more even temperature..... in short this is only a auxillary system and if you want to do it, go for it, but my gut reaction is that you are trying to piss up a rope....once again as someone else on the forum has stated..."there is a fine line between a hobby and insanity"......
norby
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Old 07-06-2004, 07:13 PM   #24
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All good questions...

John,

I don't know if a standard RV pump has the necessary capacity - I have to find out more about that. I don't actually have to switch between potable and heating water. The system I am envisioning is a so-called "open system". This means that potable hot water is pumped out of the water heater, through the heating pipes and back to the water heater. The PEX tubing is fully qualified as potable - in fact it is exactly what someone would use to plumb their potable system if they chose to use plastic rather than copper. So a 3 way valve is not needed. All I need is to open a valve to the heat tubing when I want heat and get the pump to turn on - or maybe I just need a second pump for the heating system that would in esscence work as the valve too.

Winterizing is no harder for 300' of PEX than it is for anything else. I just have to make sure I can either drain or blow out the water. As for heating the holding tanks I could probably run a loop of the tubing by them. There might also be enough residual heat passing down through the floor to do the job.

Malcolm
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Old 07-06-2004, 07:25 PM   #25
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2500 BTU was what was needed...

Bryan,

I think you may have misinterpreted what I said about the 2500 BTU's. I asked the guy I was talking to to give me an idea as to how much energy I was likely going to need to supply from the hot water heater to heat the area serviced by the panels - I wanted to uderstand the loading on the water heater. His reply was that the radiant heating system is an extremely efficient means of heating people and that only about 25 BTU's per square foot of floor panel would be consumed from the water heater. He was not at all saying that this was all that the panels could transfer to the room - he was just giving me a guestimate as to what would actually be consumed by the panels in their cycling on and off over time in the process of heating my AS.

In actual practice hot water radiant heating systems are considered to be one of the most efficient ways to heat space. Also note that the PEX tubing is in close contact with large aluminum sheets that entirely cover the heating panels over most of the path of the tubes. The heat transfer mechanism is way more capable of transfering BTU's to the space than would ever be needed in practical usage.

Malcolm
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Old 07-06-2004, 07:30 PM   #26
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With all due respect...

Norby,

With all due respect I don't think you have a clue as to what you are talking about when it comes to radiant heating. My investigations do not lead me to the conclusions that you are suggesting at all. Perhaps you were swayed by Bryan's missunderstanding of the 2500 BTU's?

As far as there being a fine line between a hobby and insanity goes I can only quote the following:

"Only he that attempts the ridiculous can acheive the impossible".

Malcolm
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Old 07-06-2004, 08:35 PM   #27
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a noble quote...actually you unknownly will be achieving what is known as a 2 stage system if you keep your forced air system....cost factors usually dictate only a one stage system...given that cost seems to be no object.... i applaud you and wish you well in your endeavor...
norby
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Old 07-06-2004, 09:14 PM   #28
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Originally Posted by malconium
As far as there being a fine line between a hobby and insanity goes I can only quote the following:

"Only he that attempts the ridiculous can acheive the impossible".
Malcolm
I have to say I know NOTHING about radiant heating, but it sure looks to me like a solution in search of a problem. Then again, I will not criticize you if you promise not to make fun of some of the stuff I have done to my coach.

The question about valving and the water pump is, I think, related to the necessity of the pump to deliver hot water, cold water, and to circulate the heated water. I am finding it hard to envision how this can be done with a single pump.

Are you serious about the 300 feet of tubing on the floor, or is that just how much tubing comes in a roll? I don't think we have space for anything like 300 feet of Pex on the floor of any Airstream. Whatever it is, I think you underestimate the difficulty of truly purging that much tubing. You may want to consider some intermediate drain valves.

Finally, the roar of my water heater is not significantly quieter than my furnace. It likely depends upon location - mine is right under our bed while the funace is up near the front. I turn the water heater off at night as it wakes me when it fires up.

Mark
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