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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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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Trailer: 2015 Oliver 23, Ram Cummins
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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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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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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
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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 2Ę worth.
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