Hot water space heater and Tankless - Fiberglass RV
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Old 11-27-2018, 09:01 AM   #1
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Name: JD
Trailer: Scamp 16 Modified (BIGLY)
Florida
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Hot water space heater and Tankless

As a retired engineer and having time to think etc I an thinking about designing a hot water heater using the tankless water heater as the source.
I have a heat pump that I use for heating and cooling so this is not a pressing problem for me.
I have a Fastar tankless system for hot water and it is quite capable of producing the heat the problem is th hot water circulation and the expansion tank required.
I have a hot water capable pump (new) that I bought for my old 2001 Prius inverter cooling circuit (12 volt).
My thought is that the Fastar will take care of heating to a setpoint and holding that as water is circulated. I plan to plumb a line from the last point in the hot water system (the sink) to a small radiator with a 12 volt fan to transfer the heat to the living space. I think I can find a new aluminum heat exchanger that will not have lead in it so that the water is not contaminated as it circulates.
I have an accumulator in the water line that will absorb the expansion from heating the water, but I think I need to add a reservoir with a high temp pop off valve like any water has.
A check valve in the cold water line ahead of the point where the warm water return reenters the heating circuit ahead of the Fastar.
The Fastar should regulate the temp headed to the heat exchanger and add the heat that is transferred to the camper interior.
A regular thermostat would control the pump and fan.
Of course water lines could be added under the floor for heating as well with insulation under the coils. Heat tracing might be necessary as well.
Any thoughts?
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Old 11-27-2018, 11:03 AM   #2
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Name: David
Trailer: 2013 Scamp 13 S1 BB
IL
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When you have the time and money for a project, it does not really matter how practical or expensive it is. You do it for your own pleasure and satisfaction. I think the best water heater/furnace combo is something like this ALDE but I don't see why your idea won't work. I think some of the new rv's actually use in floor hydronic heating which I think would take care of 2 issues, the cold floor and the heating.
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Old 11-27-2018, 11:32 AM   #3
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Name: Jerry
Trailer: Casita
Colorado
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Hot water heat

I have toyed with this idea for some time but have a question. How hot does the hot water have to be to transfer a sufficient amount of heat to maintain interior temperature with outside 20 F Temp? Circulating 110 degree water through a heat exchanger would probably not transfer enough heat. Circulating water at 175 or 180 would probably do th ejob but at a danger to user of the hot waterat a sink. What is the answer?
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Old 11-27-2018, 11:42 AM   #4
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Name: Charles
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Ohio
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Next thing ya know someone is going to install a wood burner in one of these things.
Oh wait! they have
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Old 11-27-2018, 11:50 AM   #5
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Name: JD
Trailer: Scamp 16 Modified (BIGLY)
Florida
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Part of the driving force is my bride of 40 years does not want LP in the trailer (enclosure).
The tankless water heater is on the tongue.
Hydronic heat from the floor is intriguing, but perhaps slow to warm up a cold trailer when on the move.
Small diameter tubing is the question and on the top of the floor with added covering or under?
Under the 3/4" plywood would take a while to transfer, but would keep the floor warm!.
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Old 11-27-2018, 12:25 PM   #6
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Name: JD
Trailer: Scamp 16 Modified (BIGLY)
Florida
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Here is a ready made heater core and fan, but more expensive than I had in mind.

https://www.amazon.com/H-603012-Volt...um+heater+core



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Old 11-27-2018, 03:34 PM   #7
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Name: Tom
Trailer: Sprinter 'til I buy
Denver, CO
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Be sure to check out the Truma Combi

It is a combined furnace-water heater in use here, and widely used in Europe. Below are 2 links, 1 to a search, other to a company video.

https://www.youtube.com/results?sear...ry=truma+combi

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Old 11-27-2018, 05:23 PM   #8
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Smith Valley, Nevada
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JD,

That method has been done many times in settings other than trailers. A couple of things will have to be true to make it work. The pump will have to be strong enough that the tankless heater recognizes the flow to start and the tankless will have to be capable of receiving hot water on it's cold inlet side while still modulating properly. The return side water, after your heat exchanger, should be nearly as hot as the heater output temp. It will probably only be about 10 degrees below the temp of the hot out from the heater. I don't know if your water heater is preset to a given delivery temp, but you might want to turn it up, if you can. You can play with this value as needed. If it's too low, the heater may not be able to throttle low enough and, of course, the hotter he water the more heat will be delivered. You will need an expansion tank, as you mentioned, but it cannot be exposed to the hot water if it is the usual plastic style used in trailers. A T in the line somewhere and a long sink pigtail going to it is fine. It also cannot be separated from the system by a check valve. It will be most affective if it is somewhere near the suction side of the pump. The water heater may have be on the output side of the pump. You can control the whole thing with a simple thermostat and a fan relay that turns on the pump when its time to heat, or if the pump is 12 volts, you can hook it directly to the thermostat on it's R and W terminals to complete the circuit, provided that it has an internal relay and runs on batteries. This is a very common programmable type and available at Home Depot for about $20. You should put a T with a hose bib after the pump and right after that a valve. This will allow you to purge the system of air by closing the valve and opening the bib. The only reason to include a tank in the system would be to stabilize the temp a bit, or reduce cycling of the heater. Otherwise it should not be needed. Normally, the cold line goes into the heater and the hot goes out to the faucets. In this case, just T into both of those lines for your hydronic loop. Service valves on the hydronic lines are optional, but a good idea. On the heating loop return line, add a check valve right before the tee. This can double as a service valve, but it's main function is to prevent water from going backwards through the heating loop as a shortcut around the water heater when a faucet is open. It's not needed if you're heating pump is a diaphragm style, like a Shureflow, but it is needed if it's a centrifugal pump. If you find a heat exchanger with no fan, or not a 12volt fan, you can always mount four muffin fans to the back side and wire them in parallel and even hook them up to a thermal switch attached to the heat exchanger near the fans. Then the fans would only run when hot water was present and the thermostat was calling. This would prevent it from ever blowing cold air and would blow the residual heat into the room after the thermostat was satisfied. PEX would be a fine choice to plumb the system with. I also like the idea of building a radiant heating system in the floor, but it will need some kind of freeze protection and it will cost a bit of headroom by adding an inch or more to the floor. The advantage of this would be excellent comfort, quiet operation and no wall space given up for the fan unit. Have fun building your system!
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Old 11-27-2018, 06:10 PM   #9
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Name: JD
Trailer: Scamp 16 Modified (BIGLY)
Florida
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The water supply comes from the street entry or onboard pump to the cold side of the sink faucet then the shower cold and from there to the tankless water heater.
From the tankless water heater back to the shower and then the sink.

One bit of information for any one planning on plumbing any water heater - don't use 1/2" line!3/8" is plenty for the flow rate in a camper and it will get hot water to the fixture faster!

Now back to the plan.

The place to tap off for the hot water is at the sink, the last tap in the line so that the flow will keep hot water in the line for instant use. The pump from the Prius can go from this point and then to the radiator. It might be a good idea to put at least a nice fat water hammer stopper here with a valve to purge if necessary.
From the radiator back to near the heater to keep the hot water from going to the cold taps instead a check valve upstream would be necessary here I think.
The Fastar has an automatic, adjustable flow switch and an adjustable thermostat, the question is how goo is it?
Only trying it will tell...
For most of my needs I don't think a lot of heat would be necessary, b ut welcome when needed.
If carefully built it should be quieter than a standard unit and probably no less efficient.
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Old 11-27-2018, 07:17 PM   #10
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Trailer: Xplore X22
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JD,
Yes, your system should be quieter than a conventional fan forced heater, depending on the fan used at the heat exchanger. No need for a water hammer device in the loop that I can see. What would cause a water hammer? If you use a diaphragm pump, then a water hammer device immediately after the pump would possibly smooth the flow from the pulses, but I'm assuming you are going to use a centrifugal pump.

I agree that smaller water lines are fine. 3/8" PEX (1/2" OD) is common and fine. 1/2" PEX is really 5/8" OD.
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Old 11-27-2018, 07:58 PM   #11
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Name: JD
Trailer: Scamp 16 Modified (BIGLY)
Florida
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The water hammer stopper is really an expansion tank and possibly just a place for air trapped in the system to collect and perhaps not cause problems.
I also don't think that water hammers would be a problem.
I looked at the You Tube video and it has given me some ideas as the piping the heat might be useful.
I find that even with my heat pump when it is cold the front is a little chilly.
I have space under the closet by the door where the fan and coil could possibly be hidden.
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Old 11-27-2018, 11:04 PM   #12
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JD,
Unless the system is completely free of air, you might have a problem. Whenever you run the tank dry or drain and re-fil the system, you'll have to deal with it somehow. It might be best to put an automatic air vent, like a Taco 400-4, 1/2" Hy-Vent on a tee. This should not be right after the pump because of turbulence, but it could possibly be on the suction side of the pump, with a T where you are going to tie in the expansion tank. Or some other place that is low pressure and non turbulent. It might be best to expand up to a 1" copper T and put the vent on the top of that T, and then reduce back down to the 1/2" PEX. Where the vent screws on, add another T to tie in the pigtail to the expansion tank Then you can purge the system by closing the valve downstream of the pump, opening the hose bib between the pump and the valve to purge, and then let it circulate while the vent clears the rest of the air. The 1" T will reduce the velocity of the water and help get the air up to the vent. Having the vent on the pump suction side, near the expansion tank connection, means the bubbles will be the largest at the vent, but the pressure must always remain positive there or the vent will let air in. Connecting the expansion tank there allows the vent to let any air out that may be in the expansion tank or the pigtail. It might also work for you to purge the majority of the air at the sink or shower faucet and have your purging valves at the water heater. All you have to do is purge it enough to get the pump to circulate. Then let it run for a while and clear the rest of the air out the vent. When it quiets down, because the air is mostly gone, and begins to heat, your done.
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Old 11-28-2018, 08:09 AM   #13
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Name: JD
Trailer: Scamp 16 Modified (BIGLY)
Florida
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I think that when I get the deck cleared after the first of the year I will collect the parts ans dummy the system up outside of the trailer where there is more space to make mistakes faster, always a plus.
I have the pump and fan now I just need a small NEW aluminum heater core and some plumbing. I think I can use PEX like I did in the rest of the camper (and while I am about it change the 1/2" PEX to smaller pipe.
The water hammer stopper / expansion chamber I talked about might be a large diameter PEX pipe that goes to a small faucet that could drain the air and dribbles to the sink. It is possible that the air in the system will get trapped in the line and cause no problems while fulfilling the need for space. The chamber might just be self maintaining as it would drain when the rest of the system was drained.
Interesting questions.
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Old 11-28-2018, 11:31 AM   #14
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My sense is that this topic falls firmly into the category of just because you CAN do something is not an automatic reason it SHOULD be done. I can put an elevator in a single story building, but should I? This violates my mod philosophy to make no mod that if it fails you can't go (or keep) camping. Many of my mods don't make any sense financially but they're largely harmless otherwise.

Not to pile on but this project moves the trailer from one I wouldn't pay extra for (i.e. the rivetless trailer) to one I would simply not buy because of the mod.

Neither of the above should discourage the thought experiment but since it comes up every so often I felt moved to chip in my 2 worth.
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Old 11-28-2018, 02:00 PM   #15
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JD,

The expansion tank should be a diaphragm type. An open tank will allow the air to absorb into the water and will become water logged. True, it will have a supply of new air as time goes by, but it's hard to match the rate of absorption to the rate of new air. A number 5 domestic water expansion tank would work well, but it is a bit big. So, the domestic water Shureflow, or equivalent, used commonly in trailers, would be fine. They have diaphragms and are smaller. But it will have to be kept away from the hot water by the longish pigtail I mentioned. 24" or so would be fine. If everything is set up right, you could refill the trailer after a winter shutdown and it would purge the majority of the air out a faucet, more would go out through the Taco air vent initially, and then a startup would bring the last of the bubbles to the air vent while circulating.

BTW, many trailers don't seem to have accumulators, or expansion tanks at all. Mine didn't and neither did the two previous ones. This meant that when the water heater was turned on, the system pressure went up to 100 PSI and popped the relief valve. NOT a good thing to be doing repeatedly. The small Shureflow accumulator fixed the problem, but is just barely big enough and the pre-charge pressure must be pretty close to right for it to be most affective, or effective enough.

I think your heating system will work well, it certainly won't be any louder than an RV space heater, could easily be more efficient, may take up less room, reduces the number of propane burning appliances and gives you instant hot water at the faucets when heating. What's not to like? Plus, it's a fun project based on your needs.

It's your trailer, so go for it. I don't like building things in ways that are predicted by a third party, who has no interest in the project, to be the least likely to reduce the imaginary re-sale value. That is not a standard that promotes creativity, practicality or efficiency. I never build things for some imaginary possible buyer that I will never meet. And further, If I like it, someone else will too, if that time ever does comes. Never base your life and creations on someone's prediction of resale value unless you are just building something to re-sell. So many things are "standardized" to some workable economic level that everyone can tolerate and get by with. That doesn't mean those things can't be much better or better suited to you.
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Old 11-28-2018, 03:26 PM   #16
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Name: JD
Trailer: Scamp 16 Modified (BIGLY)
Florida
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The $0.02 has been credited to your account. (NOT intended to be disrespectful)
You have a valid observation, but there is a reason for the quest and that is adding utility and dual purpose to a device giving the ability to drop another appliance off the trailer (for some, maybe).
I happen to have a heat pump and the only thing that this would give me would be the ability to heat when no mains power is available.
Also the XYL requires that no LP gas be within the camper shell and this lets me keep the LP (the only item is the tankless water heater) outside and still have an LP fired heating system that would run on 12 volts.
You would probably not be a customer for my much modified Scamp if I ever wanted to sell, but if you do that $0.02 credit is on the books.
Modifications on FrankenScamp
1. Mini-Split Heat Pump
2. Tankless water heater
3. Custom front bath
4. Custom Wood cabinets and overhead storage
5. Custom hatches for the
5A. Electric reel potable water hose
5B. Electric reel for the Macerating pump (gray and black)
5C. Storage from outside under street side bunk
6. Twin beds
7. Night stand/ dining table between bunks
8. Sat TV
9. King local OTA TV
10. Three solar panels on the roof with MPPT controller
11. Batteries under the driver's side bunk with venting
12.Custom closet by the door
13. Electric Swing compressor refrigerator
14. Microwave/convection oven
15. other small mods of no consequence.
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Old 12-05-2018, 11:38 AM   #17
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I was planning on a closed loop system which would also switch to open loop to supply hot tap water. No antifreeze. The issues that killed it for me were complexity, space, and freeze up. Forced air is sooo much simpler. You just need a larger fan. Propex is supposed to be quiet. It is an interesting undertaking!
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Old 12-05-2018, 08:47 PM   #18
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Name: Richard
Trailer: Casita
Iowa
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Hot water heat

In my 20 plus years as a professional automotive tech, my experience with heaters was that in a properly operating system with a 195 degree thermostat, the air output would be about 140 degrees. If you apply those temperatures to your project there could be some problems. Scalding water at the tap and water heater capabilities come to mind. Just food for thought.
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Old 12-05-2018, 09:25 PM   #19
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Name: JD
Trailer: Scamp 16 Modified (BIGLY)
Florida
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I don't think the temperature needs to be that high and a relatively large radiator could be used with maybe 110 or less as we are really looking to get the trailer air to the 70 degree range
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Old 12-06-2018, 01:13 AM   #20
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Quote:
Originally Posted by Iowan View Post
In my 20 plus years as a professional automotive tech, my experience with heaters was that in a properly operating system with a 195 degree thermostat, the air output would be about 140 degrees. If you apply those temperatures to your project there could be some problems. Scalding water at the tap and water heater capabilities come to mind. Just food for thought.
There is no connection between car engines operating most efficiently at 195 degrees, and the heating requirements of a camper. And there is no connection between car cooling systems and scalding tap water. I've never felt the need to run a home heating system, or my sink faucet, based on the temperature of my car engine. Not sure how one compares to the other or why it should. BTU delivery is determined by a number of factors, only one of which is input temp. When starting from scratch, and not using the car as the heat source, one can adjust any of the parameters as desired.
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