Convert RV Trailer Heat From Furnace To Boiler

[coxid]

Have you looked at the furnace owners manual and compared the input (propane) input BTUs to the output (heat) BTUs? It may surprise you as to how efficient the furnace really is. X2 on the amount of power the furnace fan motor uses.

 

[DougMorford]

Have you looked at the furnace owners manual and compared the input (propane) input BTUs to the output (heat) BTUs? It may surprise you as to how efficient the furnace really is. X2 on the amount of power the furnace fan motor uses.

It's difficult to tell how efficient the stock furnace is for LP use.  I never saw in the manual that it said what it was.  It might be decent though, you're right.  I'm pretty sure it has an aluminum exchanger, and that fan does flow a lot of air over it.  It's likely a great solution if you're planning to run a generator.

I did end up installing a second water heater that runs off both LP and 115v, and I have it circulating through a cast iron radiator and a TRV.  The water heater has a claimed 80% LP heating efficiency.
Something I hadn't considered is how much these radiators weigh.  I'm guessing that when filled with water, it's 300lbs.  I had to run u-bolts through the floor to the frame to secure it.  A better option might have been a more modern panel radiator.  I didn't go that route though because they are pretty expensive, can't put out as much heat for the size, and don't have much mass so would make the water heater cycle a lot more (a storage tank would be needed).  I went with the 6 gallon water heater, but wish i had gone with the 10 gallon for the added energy storage capacity.  It would reduce the cycling frequency.  The water heater/radiator combo works fantastic though.  It's been a warmer fall (20-50 degrees), but so far just sitting in my driveway with the temp set at 45 degrees, it's been running off just batteries for a couple weeks and has plenty of charge left on the 100 ah battery.  I also plumbed in a 3 watt dc brushless pump to help with flow, and I have that controlled by a cheap 12v digital thermostat (inkbird).  It's helps when getting the system up to temp, and it would probably also kick in when the outdoor temp is pretty low or when the indoor temp is set pretty high.  The system flows sufficiently without the pump though when the indoor/outdoor temperature differential is around 25 degrees.

Having done this, I can see why a system like this isn't available.  It would be quite expensive to design and manufacture a system like this for general use in rvs, and the demand for it is probably low.  The main drawback is that in order to have the btu output needed, a large surface area is needed, ie, a 4' panel radiator or a 300lb cast iron radiator.  There's a reason the rv furnaces have those big fans in them;  they have to move a lot of air over those tiny exchangers for them to work. 

 

[TonyL]

For a couple of years now, top end and now mid range caravans (travel trailers) in the UK have used a wet system made by Alde. It uses a 12 volt boiler very similar to a domestic unit, finned distribution tubes and radiators. The liquid has anti-freeze in it. One of the best heating systems around and it has been tested down to very low ambient. The only issue I can see is routing the pipes and radiators with slides. The boiler does have a circulating pump but it draws a lot less current than the typical furnace heater.
TonyL

 

[WILDEBILL308]

Doug, I am going to agree with Gary Brinck on this. You are trying to use a tool that wasn't designed for the job. That cast iron radiator will never provide aduanet heat at the temps you are talking about. I doubt you will be able to keep everything from freezing using all the output from the stock furnace.
If you absolutely need to camp out in those conditions I would get a cold weather rated RV. I would add at least 4 Li batteries (like Battle Borne) I would have a RV with a onboard generator with auto start and a Aqua-Hot system. http://www.aquahot.com/products/rv.aspx
Bill

 

[Lou Schneider]

Having done this, I can see why a system like this isn't available.  It would be quite expensive to design and manufacture a system like this for general use in rvs, and the demand for it is probably low.  The main drawback is that in order to have the btu output needed, a large surface area is needed, ie, a 4' panel radiator or a 300lb cast iron radiator.  There's a reason the rv furnaces have those big fans in them;  they have to move a lot of air over those tiny exchangers for them to work.

You could adapt an automobile heater and fan, they're in a compact package that moves quite a bit of heat .

 

[DougMorford]

For a couple of years now, top end and now mid range caravans (travel trailers) in the UK have used a wet system made by Alde. It uses a 12 volt boiler very similar to a domestic unit, finned distribution tubes and radiators. The liquid has anti-freeze in it. One of the best heating systems around and it has been tested down to very low ambient. The only issue I can see is routing the pipes and radiators with slides. The boiler does have a circulating pump but it draws a lot less current than the typical furnace heater.
TonyL

Thanks.  That's a pretty cool system, and is essentially what I was shooting for.  Though I'm guessing it would cost many thousands to buy, and then I've have to do a major retrofit job.  That pump is going to draw as much as an amp I'm guessing too. 

You could adapt an automobile heater and fan, they're in a compact package that moves quite a bit of heat .

Do you mean like a heater core?  That would work fairly well too.  Though i think it might exceed the current draws I'm trying to attain. 

Doug, I am going to agree with Gary Brinck on this. You are trying to use a tool that wasn't designed for the job.

That's not how I interpreted his response.  That argument also seems irrelevant to me if the tool works for the job; which it does.

That cast iron radiator will never provide aduanet heat at the temps you are talking about.

False.  In fact, cast iron radiators are well known for excelling at low water temps.  Aside from their weight and appearance, they are far superior to modern wall mounted radiators.  The radiator I've used is rated to put out 2,590 btu/hr at 120 degrees, and 6,730 btu/hr at 160 degrees.  I did a heat load test on my trailer and found its coefficient of heat loss to be 0.15, which gives me a max delta temp of 28 degrees at a 120 degree water temp.  That means I can keep my trailer at 50 degrees with 120 degree water at down to 22 degree outdoor temps.  At the max water heater temp of 160 degrees, I can get a max delta temp of 73 degrees.  That's way more than I'll ever need. 

I doubt you will be able to keep everything from freezing using all the output from the stock furnace.

Your lack of faith disturbs me. All the plumbing is internal.  I'm thinking that airflow vents in the right places, combined with some insulation where the pipes are particularly close the the walls, will be sufficient to keep everything liquid.

If you absolutely need to camp out in those conditions

I'm not sure how to respond to this.

I would get a cold weather rated RV. I would add at least 4 Li batteries (like Battle Borne) I would have a RV with a onboard generator with auto start and a Aqua-Hot system. http://www.aquahot.com/products/rv.aspx
Bill

This is a hunting trailer.