Convert RV Heat to Boiler

[Doug Morford]

I hope it's ok that I'm asking this here since I'll be talking about a propane boiler. I know of no where else to ask about this; the rv guys are clueless about it.

My RV, like most from the 90's, has a central air propane furnace. It has two problems: 1) the heat exchanger it uses to transfer heat from the combusting propane to the cabin air is extremely inefficient. I think that has mostly to do with it being difficult to transfer heat to air without a lot of surface area. 2) The furnace uses a 12 volt blower in its operation. This blower realistically gives me only one night of reliable heating before it has drained my battery. I will be installing a second battery and buying a generator for recharging, but it's not the answer for elk hunting trips that last weeks in freezing temps. I need a heating system that uses preferably no electricity, little propane, and operates on a thermostat. I need to be able to leave the trailer unattended for a few days at a time and feel confident that my water system won't freeze.

My idea:
Use this RV hot water heater as a boiler:
Dometic Water Heat - Amazon

I already have this same water heater for domestic hot water, and it works great. Most importantly it uses zero electricity aside from lighting the flame.

The output from the top of the heater would make a short run of a couple feet to the top of a cast iron radiator sitting right next to the heater, and then the outlet at the bottom of the radiator would feed back to the bottom of the heater. The inlet to the radiator would be controlled by a TRV. I don't think any kind of pump would be needed with such a short run. The system would be filled with antifreeze to avoid having to winterize it.

My main concern is with pressure, though I think it would work and be safe as is. Air in the top of the heater tank acts as a small pressure reservoir, and the heater comes with a relief valve. Does anyone think pressure/ safety will be an issue? Is an added pressure tank needed?

Please let me know if anyone sees something I'm missing here.

Doug

 

[maple1]

I am thinking adding a small pressure tank would be an easy thing to do for peace of mind. Any time you heat water in a closed system, there is potential for bad to happen.

Generally speaking, I think you'd want to use as much space as possible for your rad. The bigger the better. To help promote convection. Might also be able to use a water heater tank, with insulation removed. Depends on your space.

 

[Doug Morford]

Thanks. I'll incorporate a pressure tank.

It's a small space, 140 sqft, so space is an issue. But I agree, something as large as possible for the radiator would be best. Though I think the one in the pic on the left is as big as I could practically fit in the trailer. What are you thinking when you suggest a water heater tank? I would think that for how much that would weigh, the surface area would be marginal.

 

[Doug Morford]

I guess one thing I'm concerned about as well:

The water heater only heats to 125 degrees. I think that's plenty hot as far as fuel efficiency is concerned. But is that going to be sufficient to keep my trailer a reasonable 50 degrees when it's quite cold out?

 

[Highbeam]

The ducted heat in your rv also heats the underbelly area and keeps things from freezing. Things like waste tanks and plumbing even.

Most people in your shoes either use a vent free catalytic propane space heater with an oxygen depletion sensor or they just run a Honda 2000 inverter generator all day and night to keep the furnace running.

you’ve got to pay to play. The alternative is to drain the water system and use the rv as a really fancy tent. With refer, furnace, locking door, tv, lights, everything but running water. It’s still pretty greatcompared to a wall tent!

 

[Doug Morford]

What's a tv?

All the plumbing and fresh water tanks are inside the cabin, so heating that will keep that stuff good. I'll be cutting vents everywhere needed to allow heat to get into cabinets and stuff. The grey and black water tanks I'll have to play with, but my plan is to dump enough antifreeze in them prior to use that they won't quite freeze. That's what I've been doing with the limited water I've been putting in them, and it seems to work fine. I'd like to find a graph of some sort that would let me know how much antifreeze i'd need per gallon of water at different temps to keep it from freezing. I'm not going to be doing this into the winter, just the fall. It only needs to endure down to probably 20 degrees. The stock furnace wouldn't keep the waste tanks from freezing anyway. It doesn't have a basement.

I've done the wall tent thing, and now i've upgraded to the "fancy tent" with a catalytic heater. It's miserable having to ration nearly freezing water while washing dishes, and my clothes never dry with that heater. It's like a sauna in there, and the windows are like a river. I'm sick of it. I'm not sleeping in there with that heater on either, I don't care what anyone says. Not to mention it doesn't have a thermostat to control the heat. It's just on on off. It's nice in a pinch, but not a great long term solution. The generator could work, but I'm not leaving it running while I'm not there.

The way I see it, this can work. And it will be well worth it if it does work. It's just a matter of finding a suitable heating solution to replace Dometic's BS furnace.

 

[maple1]

Thanks. I'll incorporate a pressure tank.

It's a small space, 140 sqft, so space is an issue. But I agree, something as large as possible for the radiator would be best. Though I think the one in the pic on the left is as big as I could practically fit in the trailer. What are you thinking when you suggest a water heater tank? I would think that for how much that would weigh, the surface area would be marginal.

I was just thinking about maximizing the water capacity with the space available to help prevent short cycling and help promote convection. If the up pipe out of the boiler has as much straight up as possible before it hits the emitter, that would drive convection better. I know nothing about the heater you are talking about though.

What about one of those Mr. Heater Big (Little?) Buddy heater things? They have one that varies between 4k & 18k btu, and you can hook to a big tank. I think it is thermostatic?

 

[moresnow]

Anyplace to hang one of the flat LP vented wall mount heaters. Maybe pull a window section and box in for the vent. They start with a grill lighter and stay lit on a pilot. Thermostatic. We have one in a log cabin that we have used for several years. Ours is non vented model which concerned me greatly the first few years. Relatively efficient. Take zero floor space etc. Big box store item.

 

[Doug Morford]

I was just thinking about maximizing the water capacity with the space available to help prevent short cycling and help promote convection. If the up pipe out of the boiler has as much straight up as possible before it hits the emitter, that would drive convection better. I know nothing about the heater you are talking about though.

What about one of those Mr. Heater Big (Little?) Buddy heater things? They have one that varies between 4k & 18k btu, and you can hook to a big tank. I think it is thermostatic?

The larger water capacity would be nice for cycle time. Weight and space are limited though, so I think I'll be stuck with probably 9 gallons total in the system: 6 in the heater and 3 in the radiator. As far as convection goes, the very top of the water heater tank sits at about 1' above floor level. The hot water outlet will rise vertically straight up to the top inlet of the radiator, which is at about 2.5' from the floor. That would be about 3' of pex from the heater to the rad, and then it would be a straight short run along the floor from the bottom of the rad to the inlet of the heater. I would think it will turn over pretty easily

A heater buddy run off two 30lb tanks is what i am currently using. I wouldn't mind that it's not vented (though that would be really really nice), but it doesn't have a thermostat and I haven't seen any other models that have a thermostat either.

 

[Doug Morford]

Anyplace to hang one of the flat LP vented wall mount heaters. Maybe pull a window section and box in for the vent. They start with a grill lighter and stay lit on a pilot. Thermostatic. We have one in a log cabin that we have used for several years. Ours is non vented model which concerned me greatly the first few years. Relatively efficient. Take zero floor space etc. Big box store item.

Those are nice. I think that is probably the best option. Two reasons I haven't gone that route: 1) Any that I have found are too big for the space I have on my wall. My trailer is tiny, and I only have one wall 26" wide to install this system against/through. There are a few I might be able to shoe horn in, but none of those use a thermostat. 2) I don't think I could buy everything needed to get one with a thermostat installed for under $1k. If I had probably a 5' longer trailer, I would go that route though regardless of price. But with price, space, and the fact that i like the thought of using a cast rad, I think I'm glued to this plan unless I'm convinced it's a fool's errand.

 

[maple1]

The larger water capacity would be nice for cycle time. Weight and space are limited though, so I think I'll be stuck with probably 9 gallons total in the system: 6 in the heater and 3 in the radiator. As far as convection goes, the very top of the water heater tank sits at about 1' above floor level. The hot water outlet will rise vertically straight up to the top inlet of the radiator, which is at about 2.5' from the floor. That would be about 3' of pex from the heater to the rad, and then it would be a straight short run along the floor from the bottom of the rad to the inlet of the heater. I would think it will turn over pretty easily

A heater buddy run off two 30lb tanks is what i am currently using. I wouldn't mind that it's not vented (though that would be really really nice), but it doesn't have a thermostat and I haven't seen any other models that have a thermostat either.

Well crap, OK - one of these Buddy things might be in my future & I was thinking they had one model that had a stat on it. I guess not after just reading a manual. Not sure where I got that idea.

Which model do you have, and how long will it run on the tanks? At what setting?

 

[Doug Morford]

Well crap, OK - one of these Buddy things might be in my future & I was thinking they had one model that had a stat on it. I guess not after just reading a manual. Not sure where I got that idea.

Which model do you have, and how long will it run on the tanks? At what setting?

I have this one, 4k-9k btu. In 40 degree weather, running about 5 hrs per day and off the other 19hrs, keeping 140 sqft at tshirt temps during those hours, a 30 lb tank had plenty left over, probably half left, after two weeks of use. Once i had the trailer warmed up, the heater easily kept it warm on the lowest setting. I don't see propane supply ever being an issue for me even in zero degree weather; just the battery. If the buddies had a stat, they would be the bees knees. I bet you could retrofit one with a thermostatically controlled valve, but i imagine it would be a heck of a project.

 

[Doug Morford]

They are extremely efficient since they aren't vented. If you compared the btu needs for your space, against the btu's available in a tank of propane, minus small inefficiency of the heater, I think you'd find that one of these heaters will keep you warm for a very long time on very little gas. Propane tanks are pretty easy to pack around considering how much energy is in them.

 

[Doug Morford]

I started to get really tired of the wall tent stove for that reason. Firewood is a bulky fuel when you're traveling all over the place with it.

 

[Doug Morford]

And that usage included making a LOT of coffee and bacon.

 

[Doug Morford]

I did end up installing this system in my trailer. Pictures are of the testing phase and where i ended up installing it. I'll post installed pics in a bit. I'm using a dometic 6 gallon lp/115v water heater and a CI radiator with a danfoss TRV. I wish I had gone with the 10 gallon heater for storage capacity, but it would have been much more difficult to install in the small cabinet it's in. The radiator was heavier than I expected, and I haven't quite figured out how I'm going to secure it. I'm thinking I'll go through the floor to bolt it to the frame. It circulates naturally pretty well up to around a 30 degree indoor/outdoor delta temp, but i also installed a 3 watt pump and 12v digital thermostat for when more is needed and for getting things warm faster. I'm happy with the way it turned out. It definitely puts out enough heat for most situations with just 120 degree water, and it's looking like it will be able to last around 3 weeks on a fresh battery.

 

[Highbeam]

Thanks for following up! Enjoy the bodhi, they make that just up the road from me.

 

[OhioDave]

Before I read through this entire thread I was thinking a couple of cheaper solar panels to solve battery drain. Not sure if thats still in the budget. But I think for a few hundred you get a small system.

 

[Doug Morford]

I do think I'll eventually be installing a 100w panel with a 20A MPPT controller. I'm pretty sure it will keep up with the heater I've installed in addition to the lights and pumps and stuff. However, considering I need to use the heater in the fall when the sun is low and the clouds are many (not to mention most camping spots are buried in dense forest), I think it would have taken a very large system to keep up with the stock furnace. It would have had to keep up with 50Ah or more per day. I think a generator would have been my only option, and I would have had to run it a lot. I'll likely still eventually get one, but it's not something I'm willing to rely on as my primary source of electricity when I'll possibly be gone for a few days at a time and I need to keep lines from freezing.

Slightly on topic: I've been a little disgusted with the options for mounting panels on rv roofs. I plan to make my own rotating and tilting mount using this downrigger swivel base and various other components.