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Old 01-31-2015, 02:52 PM   #1
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water heater to heat fresh tanks?

I was just mulling over the different options for avoiding freeze damage to fresh water tanks and am a bit stumped. My trailer project will be getting a new frame and tanks so nothing is stock and rather than go with the traditional rv furnace with the ducting below the floor or electric tank heat pads I was wondering what else I could try. Yes I like to make things harder than they need to be...

I thought I'd had a brain wave when thinking how can I efficiently heat the water in the fresh tanks when it occurred to me that I would already have an on demand water heater. I was wondering if I could add a temperature sensor to the fresh tank tied to a solenoid valve coming off the water heater hot side back into the tank? Valve opens at 4 degrees C closes at say 8? Just guessing on the temps.

It seems too simple really so the bit I'm stumped with is would it be the amount of propane it would suck up or would it be the electric draw for the water pump that is going to kill my fantastic idea?
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Old 01-31-2015, 03:04 PM   #2
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Interesting idea. Heating 20 or 30 gallons of water will take some energy (which is calculable - see this).

Of course, this solution does nothing for the waste tanks; if you're worried about the fresh water tank freezing while you're camping, then you need to think about the waste tanks, too.
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Old 01-31-2015, 03:08 PM   #3
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It would work, the real question in my mind is is it really needed. I understand you live in the cold north but are you really going to be camping in cold weather? If so the idea could be developed. However you also have the gray and black water tanks to consider. How would you heat them?

Those are just my preliminary thoughts on the subject.
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Old 01-31-2015, 03:24 PM   #4
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Ah, as always I probably should have added more detail. At the moment I'm planning above floor black and grey with only dump valves below the deck. I really want the fresh down low for stability towing. The valves could be treated to some heat tape it's just the big heat pads that need a gazillion amps.

I'm not planning on camping in the cold but I'd like to be able to deal with a mildly cold night as they always seem to happen. In my Airstream fantasy life I get to travel to baking hot places to escape the frigid North but in doing so there's going to be a transition period of extreme cold misery.
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Old 01-31-2015, 03:37 PM   #5
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It would take forever (well, a day or two) to freeze 50 gal tank of water even in pretty cold outside temps. You still may not need it at all. But if you did go that route, a simple line from your water heater with a valve in it, as you mentioned, to allow you to pump water from your tank, up through the water heater and then back into the tank certainly would work. I am not even sure you need to automate it with a thermostat if it is only for very occasional use. Just put a switch on the valve, or put a manual valve in .

BTW, insulate around the bottom and sides of the new fresh water tank when you build it in. That is how the original ones were done and further slows down heat loss and minimizes the need for any additional heat.

From memory now, it takes one btu to cool one pound of water down one degree F. It takes 32 btu to be removed from that same pound of water to change it into ice. That is why it is extremely unlikely you will ever freeze your water tank. It simply takes a lot of cooling to change water to ice.
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Old 01-31-2015, 04:12 PM   #6
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Interesting, that makes plan A sound pretty doable. I'm looking at 4 x 13 US Gallon tanks which would I guess make it a bit more freezable but I think I'll go to the next step and run it by the on demand water heater vendors (PrecisionTemp) and see if they give it their seal of approval.

I might even attempt a bit of calculating with the link above and see how it pans out.
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Old 01-31-2015, 05:14 PM   #7
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"Mildly cold"? Define please.
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Old 01-31-2015, 05:34 PM   #8
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Just a thought, but why not get some tubing used in radiant heating, which I believe is just PEX, and run them around the tanks you want to heat. The hot water in the lines will come from your water heater. No water is wasted, and it would be a simpler system running off a loop added in to your hot water line. This way you could do the waste tanks as well as the fresh water tank. Again, just a thought.
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Old 01-31-2015, 07:22 PM   #9
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Thanks all. By mildly cold I'd say about five or ten below freezing or something like that. We do get some weird temp swings here. Sneakinup, I'm not sure how the PEX idea benefits much as I'd need more tubing, more water and there's more likelihood of expending more heat energy heating extra stuff, but you might have a better idea about this than I do especially as there is kit like the aquahot units that have dedicated hydronic heating set ups but are very expensive. I like plan A as I'd just need a 1/2" PEX line from the hot circuit to the tank furthest from the cold pick up point, a valve (solenoid $12) and maybe a temperature switch ($90) for automation.
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Old 02-01-2015, 11:51 AM   #10
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water bed heater? you would need to be "hooked up" to use it.
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Old 02-01-2015, 12:29 PM   #11
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Quote:
Originally Posted by truckasaurus View Post
Thanks all. By mildly cold I'd say about five or ten below freezing or something like that. We do get some weird temp swings here. Sneakinup, I'm not sure how the PEX idea benefits much as I'd need more tubing, more water and there's more likelihood of expending more heat energy heating extra stuff, but you might have a better idea about this than I do especially as there is kit like the aquahot units that have dedicated hydronic heating set ups but are very expensive. I like plan A as I'd just need a 1/2" PEX line from the hot circuit to the tank furthest from the cold pick up point, a valve (solenoid $12) and maybe a temperature switch ($90) for automation.

Not worth the trouble. We regularly camp in that sort of weather - even dry camping in it - and haven't had a problem with freezing. I doubt our furnace helps very much, either (other than keeping the valves from freezing).
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Old 02-01-2015, 12:42 PM   #12
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Zep and I were talking about this same concept a few years ago. Don't know if he ever followed up on it. Give him a PM.
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