Water diagram question - Fiberglass RV


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Old 05-21-2014, 06:30 PM   #1
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Name: Julian
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Water diagram question

My Scamp has a 20 gal fresh water tank. I am planning to add a 6 gal Atwood water heater. Will this increase the total usable volume of water to 26 gal or it will remain 20 gal due to the fact that the water pump will not be able to push the last 6 gallons that remain in the heater after the main tank is empty? I need to know if I need a larger waste water tank.

Looking at RV water diagrams it seems to me that as soon as the water runs out or the 20 gal tank, the pump will stop putting out pressure and there will be no water running out the hot tap. At the same time the water heater will remain full with 6 gal of hot water, which cannot be used. How do I get the last 6 gallons out of the water heater?
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Old 05-21-2014, 06:57 PM   #2
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Yer guess is right. Short of draining the water heater tank at the drain point, that 6 gallons is unavailable weight unless there is something in the supply tank to push it through.

Again, you can drain it.
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Old 05-21-2014, 08:12 PM   #3
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Just curious

This begs a question: Why not have "flow through" on-demand heaters that would not hold 50 pounds of water which could be considered dead weight? Does anybody make such a thing? Anybody here tried one?
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Old 05-21-2014, 08:46 PM   #4
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This could get messy, again.
There are at least 3 different 12 VDC, propane, tank-less heaters available. Some think them only suitable for a campground with full hook-ups though.
Girard Tankless Water Heater GSWH-1
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Old 05-21-2014, 08:55 PM   #5
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Quote:
Originally Posted by JulianK View Post
I need to know if I need a larger waste water tank.

Looking at RV water diagrams it seems to me that as soon as the water runs out or the 20 gal tank, the pump will stop putting out pressure and there will be no water running out the hot tap. At the same time the water heater will remain full with 6 gal of hot water, which cannot be used. How do I get the last 6 gallons out of the water heater?
Confirm once you are out of water in the Fresh water tank there is no pressure to move the water out of the Hot Water tank... carrying portable water totes you can fill up while out traveling around so you can refill the tank is the one way to solve the problem.

Julian what year is your Scamp? I have a stock 6 or 7 gallon hot water tank in mine (under the sink) and the grey water tank is 26 gallons but my fresh water tank is only 12 gallons which is Scamps stock tank. Did someone add a bigger tank to your scamp?
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Old 05-21-2014, 09:49 PM   #6
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Originally Posted by Paul O. View Post
This begs a question: Why not have "flow through" on-demand heaters that would not hold 50 pounds of water which could be considered dead weight? Does anybody make such a thing? Anybody here tried one?
Weight like that really isn't an issue. One would be hard pressed to detect any difference in economy with or without 50 lbs. of water on board.
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Old 05-21-2014, 09:53 PM   #7
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Bob, Are you saying that keeping the weight low on a fibreglass trailer is not important? What if the tow vehicle is close to or over its limit? Why do you think that many people travel with an empty water tank?
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Old 05-22-2014, 06:32 AM   #8
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Re: the OP's original question, I would confirm Bob’s description. Since the water pump is “upstream” of the water heater, when the fresh tank is empty, the 6 gallons in the water heater remains after the water stops coming out of the faucets. Draining the water heater is usually done through the anode rod fitting, or air pressure through the city water connection.
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Old 05-22-2014, 07:38 AM   #9
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Quote:
Originally Posted by David Tilston View Post
This could get messy, again.
There are at least 3 different 12 VDC, propane, tank-less heaters available. Some think them only suitable for a campground with full hook-ups though.
Girard Tankless Water Heater GSWH-1
Thank you, Dave, for the thread reference - very informative. I saw no vitriol there, although vitriol, as in H2SO4 (battery acid) is a valid topic on this forum!
Paul
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Old 05-22-2014, 07:46 AM   #10
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Originally Posted by David Tilston View Post
Bob, Are you saying that keeping the weight low on a fibreglass trailer is not important? What if the tow vehicle is close to or over its limit? Why do you think that many people travel with an empty water tank?
Nope, not at all. I was only addressing the effect of an additional 50 lbs of water on fuel economy.

And I understand the wisdom of not carrying a full tank of water, which can weigh upwards of 200 lbs, but many of those "Empty Tanks" will still
carry about 5 gallons to allow use of the loo and water needs at rest stops along the way.
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Old 05-22-2014, 09:00 AM   #11
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Thanks for the input. That sucks. It seems that it is not very easy to purge the water that remains in the water heater. Not only it is unusable, but I will have to carry it around. Do most people have water left in the heater the whole camping season?

To answer some of the questions addressed to me:

My Scamp is an empty shell right now - I am doing a complete make over. In my previous trailer, which was built by me, I had a "on demand" propane water heater, which has no tank. The problem I had with it was that it worked very inconsistently, because the pressure of an RV water pump is inconsistent. When I turned the hot water tap it would come on and of, on and off - several times per minute, so water was coming out in series with cold. Not a pleasant way to have a shower. This made it really unusable. This is why I decided to go with a water heater with a tank this time. However I did not consider its just discovered shortcomings. I don't like the fact that I will have to travel with extra 6 gallons. I may not feel it, but it adds up. And I think I can feel it uphill, because I tow with a 4 cylinder truck.
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Old 05-22-2014, 09:50 AM   #12
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An easy way to drain the water heater is to install a drain point in the line near the bottom fitting on the tank or in the water line where it is downhill from the tank. But you will have to remember to open a hot water tap to release the vacuum lock in order for the tank to drain. Many RV's are already built this way for draining the lines. It might leave a few quarts in the bottom of the tank, but that's about all.
And there are 4 gallon water heaters out there, that's what I recently installed in my Hunter.

16' Scamp/4 cylinder truck? Hmmmm.... Yes, I too would be worried about weight.
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Old 05-22-2014, 09:58 AM   #13
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Originally Posted by Bob Miller View Post
Weight like that really isn't an issue. One would be hard pressed to detect any difference in economy with or without 50 lbs. of water on board.
While not a issue in regards to fuel economy it sure can be an issue in regards to the trailers towing stability as normally the water tank is right at the back end. Especially if the OP has a standard Scamp with side bath and only one propane tank and battery on the tongue.....
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Old 05-22-2014, 10:10 AM   #14
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Originally Posted by JulianK View Post
Thanks for the input. That sucks. It seems that it is not very easy to purge the water that remains in the water heater. Not only it is unusable, but I will have to carry it around. Do most people have water left in the heater the whole camping season?

.
It depends on my camping plans or where I got the water from as to whether or not I leave the water in the tank. If the trailer is going to be sitting unused for some time or in really hot weather I drain it.... simple to do ... open the pressure relief valve and open up the drain plug. The reason for draining it is that some water - particularly from wells etc will smell to high heaven if left sitting in the tank in hot weather. If I am going to be using the trailer again in a few days and I believe the water in it is fine I don't drain it . The hot water tank is small and sits pretty close to the axle so its not really an issue traveling with it partly full.

You should drain and clean out the hot water tank regularly regardless - due to the design of the hot water tank it doesnt drain really well and sediment will build up on the bottom of it and it is unable to get out unless you use a Hot Water tank Rinser to clean it out.
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