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Old 08-05-2016, 01:30 PM   #11
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Tanks need to be vented to eliminate negative or positive pressure when drawing from or filling.
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Old 08-05-2016, 02:48 PM   #12
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Think of the tank and vent like this.
The tank is full of air so when you start putting water or waste in a 50-gallon(for example) you will only be able to get about 30 gallons into the tank because you are compressing the trapped air and it will create a back pressure that will stop the tank from accepting the full capacity of water/waste. A vent on a tank serves two purposes one for filling as described and for draining cause if the tank can't breathe/intake air when draining you will start getting a chugging effect like you experience when trying to pour water out of a milk jug,old style gas can or bucket with a pour spout and you will be at the dump station 3x as long waiting on your tank to drain.
The expansion tank is always a good idea in my mind but think about it like this?
Another name for an expansion type tank is a bladder type storage tank.
For example installed in this manner
If pump pressure is 65psi with a expansion/storage tank inline the pump will pressurize the storage/expansion tank (an extra 5-10 gal.) with a PRV(pressure reducing valve) installed downstream of the tank set at 50 psi.
Now the pump has to only pressurize the tank and your system feeds off of the extra 5-10 gallons of higher pressured storage.
Therefore your pump will only run full time when you are taking a long shower/tub or running a water spigot
Add an expansion/bladder type storage tank and save your pump some life and you won't have to hear everytime you get a cup and f water or fill the sink.
Just my opinion and some food for thought.
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Old 08-05-2016, 05:05 PM   #13
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Just my opinion and some food for thought.
Thank you for your thorough explanation! It makes so much sense. It evens out the water pressure and flow and reduces pump wear.

Do you think a bladder/expansion tank addresses any concerns of the finicky on-demand water heaters?
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Old 08-05-2016, 07:56 PM   #14
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Just thinking out of the box. A 50 gal pressure tank (fresh water) pressurized by air or city water when connected.

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Old 08-06-2016, 10:47 AM   #15
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Depends on what the heater is picky about?
If it water flow then probably not and if it water pressure then probably not.
The pump is going to move the same amount of water whether it is doing it at 10 psi or 50 psi.
Never had an on demand heater?
To make the heater happy I would think that the piping size needs to be adhered to to give the heater proper flow.
A return line system defeats the purpose of an instantaneous heater purchase?
In my opinion
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Old 08-06-2016, 01:15 PM   #16
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The instant hot water heaters are only instant if they are located immediately at the spigot they serve. The further away the spigot, the more water is sent down the drain while waiting for hot water to arrive. That is no big deal if you are hooked to city water but if you are boondocking it is as many gallons of water are wasted waiting for the hot water to arrive.

Also, because instant hot water heaters require both water pressure and water flow to operate the heater cycles on and off with each cycle of the spigot. This results in a choppy mix of either too hot or too cool water at the spigot.

By utilizing an electric solenoid water valve which directs hot water flow back to the water tank you eliminate waste of water and achieve an even flow of pretty constant temperature water at the spigot regardless of its distance from the heater. If you wish maximum flow of hot water (3 gal/min in my case) switching off the solenoid valve directs all the water flow to the spigot.

In my experience water seems to be the limiting factor when it comes to boondocking so anything that can be done to conserve water use becomes important. I've yet to use up my propane supply before running out of water but I suppose it could be done if one tried hard enough. I wrapped the hot water pipes leading to the various spigots with foam insulation but left them bare on the return line to allow the water to cool before going back into the fresh water holding tank. Jack
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Old 08-06-2016, 01:38 PM   #17
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Several of the folks I know who live with tankless water heaters always capture whatever cool water comes out and use it for other things. One even pours it back into his tank.
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Old 08-06-2016, 01:49 PM   #18
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Several of the folks I know who live with tankless water heaters always capture whatever cool water comes out and use it for other things. One even pours it back into his tank.
As I've combed through forums, this seems to be the general solution to the problem. A few posters have briefly discussed recirculating water but never to the point of describing the system or mentioning specific equipment. I suppose for those only spending a few weeks a year in their RV, catching the cool water for other purposes is just fine. For full-timing, I would think a recirc system would be desirable.
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Old 08-06-2016, 01:52 PM   #19
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In my experience water seems to be the limiting factor when it comes to boondocking so anything that can be done to conserve water use becomes important. I've yet to use up my propane supply before running out of water but I suppose it could be done if one tried hard enough. I wrapped the hot water pipes leading to the various spigots with foam insulation but left them bare on the return line to allow the water to cool before going back into the fresh water holding tank. Jack
Thanks, Jack! Agreed. This is what I was thinking of. Now to find the components and work into a system design.
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Old 08-06-2016, 02:47 PM   #20
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BF, here it is, $7:

1 2" 12 Volt DC Electric Solenoid Valve N C Plastic Body Water etc DDT CS 12VDC | eBay
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