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I volunteer 3-4 days a week at private game preserve. Over the course of two years owner has provided a concrete pad W/E/S/Cable and internet. 


I find I have to be careful about what I say, mentioning that it takes so long to heat water for showers and dishes I show up to find installed a outside instant  propane hot water heater for showers. 


Wild idea I had, connecting the outside water heater to drain tube to the coach water heater. Drain tube from hot water heater in coach DOES NOT COME from the pressure relief valve. 


So before I try this, wondering if anyone else has considered this. If connected to coach heater, should not back feed to holding tank and only hot water faucets. 


Any thoughts, Trucken

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My first question would be if the exterior water heater has on-demand electronic ignition or not. Most portable "camp style" units will not. Residential and some RV units will.

If it does, it seems to me that it would be preferrable to pipe in an additional outlet along the bypass path (with an additional shut-off and back flow valve) rather than piping it through the existing water heater (in series). When connected to the exterior on-demand water heater you simply close off the supply lines and open the by-pass OR you have the option to run the interior and exterior WH's in parallel.

That would eliminate any potential risk to the existing back flow valve and pump as well as enabling you to have near instantanious hot water at your faucets without having to run through the 6(?) gallons of cold water currently in your water heater first. Not to mention that you'll be wasting the hot water "reserve" (that you paid to heat) left in the WH once you shut off the faucet.

Unless you plan to run them both continuously.

Trying to run them in series though, as you are proposing, would seem to present more questions and potential problems than just by-passing the interior WH altogether or running them in parallel. Ie., How will the interior WH thermostat respond? Since the exterior WH's thermostat is likely mechanical.. and only measured "locally", how consistent will output temperatures be?

The thought being... if the hot water coming in from the exterior WH is pumping in near the existing WH's thermostat probe will your WH fire enough so that the remaining gallons of cool/cold water aren't sapping all of the heat out of the water before it can leave the tank?

It wouldn't suprise me if a setup like that would run through "surge" temp cycles of too cool to too hot... which takes me back to KISS (keep it simple s*****). Not saying it "would", mind you.

If it does "not" have on-demand electronic ignition then you'll also have to take into account having to go out and light it up every time before any hot water demand.

The other question I would ask myself is... how long will you be in that situation and is it worth the effort plumbing in an additional inlet?

Getting back to the root of the problem though... taking so long for your WH to heat. Are you running it... or are you aware that you can run it... on both LP and electric at the same time? It will DRAMATICALLY reduce the heating time. Electric only is about the slowest way to heat water no matter who you are. ;)

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You have an interesting idea! I would think that connecting to the drain plug fitting could allow the hot water to backflow into the cold water system of the RV because most RV systems have the check valve for hot water on the outlet side. At the very least I would add a check valve to the cold water inlet just to be sure that didn't happen. To me, it would be better to plumb into the cold side of things with a diverter valve to supply the inlet side with hot water from that outside water heater. You do still need a check valve and make sure that wall water lines can withstand the temperatures of the hot water supplied. You should also put a pressure regulator into the supply to be sure you do not exceed the maximum test pressure of your water system. 

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I don't think you would get backflow into the cold water system.  But with a single pipe you won't get convection flow to warm your inside tank, so you would have to close the cold water inlet to the tank and run a hot water faucet long enough to drain the cold water from the water heater tank and replace it with hot water from the external heater.

It would be better to substitute the external heater for the internal one.  Maybe you could use the external low point winterizing drain on the hot water line as the inlet from the external heater.   Disconnect the existing hot water heater by removing the hot water outlet line and plugging both the tank outlet and the line you removed so cold water from the tank won't mix with hot water from the external heater.

Edited by Lou Schneider
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Well after thinking about in for a few days, decided it's not worth the effort to tap into the output of water heater. Coach heater is the one that seems to suck up the most propane, so think instead making a tap on h2o heater will make one on propane tank an get 100 lb tank. Owner has propane delivered once a month, they won't fill coach tank but will fill 100 lb cylinder. Can live waiting 20 min for  water to heat. 100 lb cylinder should last the winter, I leave for the summer. 

Thanks for the suggestion. 


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3 hours ago, Trucken said:

Owner has propane delivered once a month, they won't fill coach tank but will fill 100 lb cylinder.

We have been several places that supplied propane to us as volunteers. There are kit readily available to install a fitting to connect you to either a high-pressure supply before your pressure regulator or on the downstream side of the regulator if it is low pressure. If the tank comes with a regulator, which it most likely does, then you can just disconnect the propane line from your regulator and connect the supply from the tank to it. The supply should be set to 11" of water column to supply the RV appliances. You will find that propane also costs less when you get a larger amount. Look for "extend-a-stay" on Amazon.

Edited by Kirk Wood
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7 hours ago, Ronbo said:

100 lb is only 30gal. That is about the size if the permanently mounted tank in motor homes.

I suspect that he is thinking of the 100-gallon tanks that are commonly rented to customers by propane suppliers. With propane at 4.2#/gallon, a 100# tank would be 24 gallons.



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