I know there are newer campers that have combined water heater / furnace units but they are pricey and not necessarily an easy drop in unit. It got me to thinking that it would be an interesting experiment to see if a standard RV water heater and add furnace capability.Pros:1. Elimination of one large and complex component2. Opening up space for additional storage3. Possibly less heating hot/cold "cycles" at night because heated water in the tank would act as a stored reservoir of heat for the furnace.4. Possibly quieter overall (my furnace is very noisy while my water heater is nearly silent). Cons:1. May take longer to heat interiorThe idea I was thinking about is cutting access into the stainless water tank, running some stainless tubing channel inside with inlet connection and one outlet connection. Weld it all back together. Then running a line to a radiator / fan combination that ties into the existing ducting. Cycle the fluid with a water pump and use some sort of relay to activate the water pump with the fan when the thermostat calls for heat.Aside from the welding, I don't see this as something too difficult. I know there are some very skilled people on this forum and so I am wondering if others have toyed around with the idea themselves. This post is about whether the idea is feasible and could be done readily enough by a moderately skilled person. I'm not actually considering actually doing it (too many irons in the fire as it is) but might keep it in mind for a future project.

I have considered installing a recirculating pump like what's used in residential homes to reduce the wait time and wasted water at the shower. I suppose one could run the recirculating water through some baseboard heaters and heat the cabin that way. Might even be able to install a radiator in the space where the original forced air furnace is along with a much quieter fan.

I think your proposed solution will not transfer enough heat. You would probably need a heat exchanger to transfer enough energy from the water tank to your heating circuit.Also, how much energy (BTUs) does the water heater deliver? Will that be enough heat for the camper?Someone with more engineering knowledge than myself should be able to do some calculations on the energy needed to be transferred and the necessary surface areas.I would also consider moving away from a hot air heater towards a water filled radiator system like Alde.

A 12gal rv water heater produces approximately 12000 btu,some of that is exhaust . It is doubtful it would provide enough for cabin heat plus hot water in anything more than mild weather.

Not totally a bad idea, That system works great in homes and very fuel efficient.I’m not sure the RV hot water heater is capable of a hot water cycle and then a cabin heat cycle too often without it ending its life cycle prematurely.To manufacture one yourself would be dangerous as a hot water tank is a pressure vessel and requires some engineering to ensure you don’t blow your camper off of the truck.Pressure safety devices would also be required.Very good idea though, maybe something to run by the hot water heater manufacturers.Good LuckWes

There are a few products on the market. Truma has the hot air heaters covered, Alde the hot water (well, whatever fluid they have in there) heaters, both in combination with hot water. Combined, I think they have more than 90% market share here in Europe. I believe both brands are available in North America. I know Cirrus Campers offer an Alde heater.IMO, it should be much, much easier, safer and more reliable to just install one of these.In the OPs idea, he saves the space for the furnace, but he needs to find a space for the hot water to air heat exchanger and fan. And all of these components need to be sized correctly in order to produce enough useable heat. Obviously it can be done, I just doubt there is any real benefit over a store bought solution.

Build your own Water Heater / Furnace Unit? | Good Sam Community

JD5150
Explorer
May 03, 2019
I have worked around air to water heat exchangers most of my life. Hot air from engine exhaust blowing on a stainless or copper coil to heat water at 600 psi around 2 gallons or more a minute.

The idea you have most likely will not work. Someone mentioned that a 12 gallon RV water heater is around 12,000 btu. So lets say a 6 gallon one is 6,000 btu. By the time you use air flow/fan to blow the heat you will be down to 3,000 btu or less. The fan will cool the air some so there will be some loss of heat there from fan picking up cooler air and blowing through the radiator coil.

I just started looking at the Truma Combi eco plus. I'm still studying it to see how much juice it will take to run in electric and gas mode. I want to run it off solar and a 200 amp hour lithium battery pack plus gas if needed. It will do both at the same time
adamis
Nomad II
May 03, 2019
JD5150 wrote:
I have worked around air to water heat exchangers most of my life. Hot air from engine exhaust blowing on a stainless or copper coil to heat water at 600 psi around 2 gallons or more a minute.

The idea you have most likely will not work. Someone mentioned that a 12 gallon RV water heater is around 12,000 btu. So lets say a 6 gallon one is 6,000 btu. By the time you use air flow/fan to blow the heat you will be down to 3,000 btu or less. The fan will cool the air some so there will be some loss of heat there from fan picking up cooler air and blowing through the radiator coil.

I just started looking at the Truma Combi eco plus. I'm still studying it to see how much juice it will take to run in electric and gas mode. I want to run it off solar and a 200 amp hour lithium battery pack plus gas if needed. It will do both at the same time

The lower BTU output actually works to my advantage in this scenario. An Atwood 8 Gallon water heater is 8,800 BTUs, an Atwood Furnace is 35,000 BTUs. I don't need 35,000 BTUs constantly, there is a cycle time involved or I will be cooked out of the camper. Having a constant 8,800 BTUs burning with a lower fan speed and lower overall heat level but over a longer running time can have the equivalent effect as a blast furnace running for shorter periods of time.
Chris_Bryant
Explorer II
May 03, 2019
Hydronic systems run at a much hotter temperature than water heaters for a reason, and there are no stainless tanks- aluminum, which is two alloys sandwiched together, the interior alloy being more sacrificial, or steel, which is porcelain lined. Either tank cutting and welding would be problematic.
toddb
Explorer
May 03, 2019
I built a hydronic system in my old toy hauler. I started out using the water heater and then switched to a stand alone atwood water heater filled with glycol. It worked amazingly well, camped often to freezing and it maintained 65 in the trailer with a 50% duty cycle. The output was 140*, was really quiet, and didn't dry out the air in the trailer.

I'm currently collecting parts to utilize the water heater in the TC. Using the water in the tank is a bad idea, the anode crude plugs everything up. My plan is to pull the water heater and wrap a coil around it, topsflo circulation pump and 120mm fan/pc radiator. We use a use a similar setup at work to cool compressor heads, not super efficient but it does transfer a good deal of heat.
jimh406
Explorer III
May 03, 2019
My house propane system runs at around 170. You need several finned radiators and a circulating pump. They are good heat but warm up rooms slow.
adamis
Nomad II
May 03, 2019
toddb wrote:
I built a hydronic system in my old toy hauler. I started out using the water heater and then switched to a stand alone atwood water heater filled with glycol. It worked amazingly well, camped often to freezing and it maintained 65 in the trailer with a 50% duty cycle. The output was 140*, was really quiet, and didn't dry out the air in the trailer.

I'm currently collecting parts to utilize the water heater in the TC. Using the water in the tank is a bad idea, the anode crude plugs everything up. My plan is to pull the water heater and wrap a coil around it, topsflo circulation pump and 120mm fan/pc radiator. We use a use a similar setup at work to cool compressor heads, not super efficient but it does transfer a good deal of heat.

Nice! If and when you get to the project, take a few pics if you could so we could see how you did it!
Larry-D
Explorer II
May 03, 2019
Been done by some on the RPod forum but they didn't delete the furnace. Used a radiator/heat exchanger plumbed into the water heater's hoses.
JD5150
Explorer
May 03, 2019
adamis wrote:
JD5150 wrote:
I have worked around air to water heat exchangers most of my life. Hot air from engine exhaust blowing on a stainless or copper coil to heat water at 600 psi around 2 gallons or more a minute.

The idea you have most likely will not work. Someone mentioned that a 12 gallon RV water heater is around 12,000 btu. So lets say a 6 gallon one is 6,000 btu. By the time you use air flow/fan to blow the heat you will be down to 3,000 btu or less. The fan will cool the air some so there will be some loss of heat there from fan picking up cooler air and blowing through the radiator coil.

I just started looking at the Truma Combi eco plus. I'm still studying it to see how much juice it will take to run in electric and gas mode. I want to run it off solar and a 200 amp hour lithium battery pack plus gas if needed. It will do both at the same time

The lower BTU output actually works to my advantage in this scenario. An Atwood 8 Gallon water heater is 8,800 BTUs, an Atwood Furnace is 35,000 BTUs. I don't need 35,000 BTUs constantly, there is a cycle time involved or I will be cooked out of the camper. Having a constant 8,800 BTUs burning with a lower fan speed and lower overall heat level but over a longer running time can have the equivalent effect as a blast furnace running for shorter periods of time.

Lower BTU then it should work. Are you going to use a 12 or 110 volt pump to move the water through the heat exchanger? If so you can find pumps that can handle 140 degrees without damage. There are very few that can handle 180 degrees without damage.
SidecarFlip
Explorer III
May 04, 2019
JD5150 wrote:
adamis wrote:
JD5150 wrote:
I have worked around air to water heat exchangers most of my life. Hot air from engine exhaust blowing on a stainless or copper coil to heat water at 600 psi around 2 gallons or more a minute.

The idea you have most likely will not work. Someone mentioned that a 12 gallon RV water heater is around 12,000 btu. So lets say a 6 gallon one is 6,000 btu. By the time you use air flow/fan to blow the heat you will be down to 3,000 btu or less. The fan will cool the air some so there will be some loss of heat there from fan picking up cooler air and blowing through the radiator coil.

I just started looking at the Truma Combi eco plus. I'm still studying it to see how much juice it will take to run in electric and gas mode. I want to run it off solar and a 200 amp hour lithium battery pack plus gas if needed. It will do both at the same time

The lower BTU output actually works to my advantage in this scenario. An Atwood 8 Gallon water heater is 8,800 BTUs, an Atwood Furnace is 35,000 BTUs. I don't need 35,000 BTUs constantly, there is a cycle time involved or I will be cooked out of the camper. Having a constant 8,800 BTUs burning with a lower fan speed and lower overall heat level but over a longer running time can have the equivalent effect as a blast furnace running for shorter periods of time.

Lower BTU then it should work. Are you going to use a 12 or 110 volt pump to move the water through the heat exchanger? If so you can find pumps that can handle 140 degrees without damage. There are very few that can handle 180 degrees without damage.

The TACO IFC circulator in my shop PEX system will handle up to 210 degree fluid (which can be water or Glycol), no problem but it's 110 volt. However the amp draw is very low (like 60 watts in the high position and 15 watts in the low position. You could run it from a PSW inverter and run on 12 volts, no issue.

I could see circulating hot water from the HWH no problem through a fluid to air HX with a fan behind it, installed in the cavity where the existing furnace is now. Would be a fairly cheap install and add a t'stat to cycle the pump and fan. No mods to the HWH at all, other than a return line to the HWH for cooled water and let the HWH maintain the tank temp. Has some interesting aspects so long as the HWH can keep up with the demand.
SidecarFlip
Explorer III
May 04, 2019
Having said that I don't believe it would work with a Suburban HWH because of the sacrifical anode gumming up the HX but an Atwood with no anode rod, certainly.

In retrospect, I've never been fond of the suburban design just because of the anode, a cheap way to cut cost over a porcelain line tank like the Atwood, I always have to flush the crud from the anode rod from mine every year as it drops to the bottom of the tank. Cheaper is always the by-line in RV's.