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Old 12-05-2014, 02:16 PM   #1
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Do I need the expansion tank for water heater?

I am installing an Atwood 50K Tank-less water heater in my Monaco Dynasty replacing the Suburban 10 gallon tank that was bad.

The old system included a 2 gallon expansion tank on the cold water side that was also defective.

My water system is 1/2 inch PEC with a distribution manifold.

My understanding is that the expansion tank is needed to allow for hot water expansion back toward the input as the water heats. This prevents blowing up the water lines if there is too much expansion of the water.

Since the tank-less system does not contain a great volume of water that can expand with heat, then the need for a place to expand is moot. Thus, I think, no need for the expansion tank.

Is it possible that the expansion tank would be useful in maintaining a smoother flow of water? If the water pump has pressurized my lines to 45 PSI the tank could conceivably provide temporary water pressure and flow until the point where the pump would cycle at 30 PSI. It seems to me that this point would be reached very quickly when a faucet is opened.

I am leaning heavily toward not replacing the expansion tank unless it has a meaningful purpose and I would rather go to a different pump for the purpose of smooth flow.

My pump is fairly new and rated at 3.5 GPM at 45 PSI and I have never experienced any pulsing of the water flow.

When I crawled into the bay I was amazed at the number of 90 degree turns the water lines made just from the filter to the back of the old water tank. From the filter to the input of the tank were 9 of them. From the hot side of the tank to the distributer block were another 7 for a total of 16 bends with each one offering impedance to water flow. My new layout will have at total of 6 bends from the filter all the way to the distribution block.
I am not installing another bypass valve and the 50K model includes the winterization kit.

I am sure many of you have gone through this process before and would be grateful for your comments and suggestions.

Thanks
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Old 12-05-2014, 04:33 PM   #2
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Quote:
Originally Posted by Lemon Twist View Post
Is it possible that the expansion tank would be useful in maintaining a smoother flow of water?
Lemon Twist
In my experience with RVs adding an expansion tank has always resulted in a smoother flow of "RV 12V water pump driven water", both hot and cold, no matter what water heater or pump it is used with.
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Old 12-05-2014, 04:52 PM   #3
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Interesting--most water heater applications [no tank-less] I have seen [RV or residential] use a check-valve on the cold water input to keep warm water from backing up into the cold water side, and by definition should keep any "over-pressure" conditions from back tracking. Heat expansion up the hot side is a different issue--thought the pressure relief valve on the water heater [if operational] was designed to prevent dangerous heat expansion issues.

As for using an expansion tank in general, after many years in a box, I installed the small tank made by Shurflow--seems to even out water flow while on 12v pumping and you do get a hand wash or two without the pump starting every time you open a faucet. Should note that there is a developing thread on this forum suggesting that the makers of variable speed 12v pumps recommend not installing a tank with a VSP??????
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Old 12-05-2014, 07:04 PM   #4
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Home Depot has several sizes of the "expansion" tanks. I've installed the 2 gallon version in my last 3 RVs. You will probably be pleasantly surprised at how it smooths the flow and almost eliminates the hot-cold cycling in the shower.

It's no different than household systems that use a well with a storage tank and pump. If my Endeavor had more room I would have installed the 5 gallon version.

The storage tank for my household well is 400 gallons and the water will run full blast from 1 faucet for 5 minutes before the pump kicks on.

As one of the other posters mentioned though, don't put in a tank if you are using a variable speed pump. The pump will NOT be happy.
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Old 12-05-2014, 07:11 PM   #5
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The variable-speed water pumps eliminate the need for an expansion tank on the cold side to reduce the frequent off/on of standard RV water pumps. The tank-less heater does not need an expansion tank because the water is not hot until you open a faucet and flow triggers water heating. As soon as you close all faucets the tank-less shuts off heating.
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Old 12-05-2014, 10:30 PM   #6
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I have run a VSP with a small expansion tank for almost two years now--no problems! Sure would like to hear "why" VSPs dont like expansion tanks.
PS--VSP doesnt totally eliminate the need for a tank--you lower pressure at a faucet and the pump starts immediately---I like the tank.
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Old 12-05-2014, 10:37 PM   #7
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We recently installed a variable speed water pump in the 5er. Bold text in the installation instructions said not to be used with expansion tank.
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Old 12-05-2014, 10:43 PM   #8
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A typical RV tank water heater is designed to not fill all the way, leaving a pocket of air at the top to be compressed when the water is heated and expands. A tankless system doesn't heat the water until it is flowing, meaning there's a faucet open and no pressure will build up. The nature of a tankless heater doesn't require an expansion tank, a tank water heater doesn't either if you maintain the air pocket in the top of the tank.

The only reason for an expansion tank would be to even out the surge caused by the water pump.
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Old 12-05-2014, 11:16 PM   #9
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as a side note
when i installed the rv water softener it had an added benefit when we are on water pump...it acts as an expansion tank and smoooothes out the flow
sort of weird the pump runs a few seconds after the water is turned off as it build back up the pressure,
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Old 12-06-2014, 08:16 AM   #10
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You don't need an expansion/accumulator tank with a tankless heating system, for the reason Ray described. Actually, you don't even need one with a tank-type heater either, and few Rvs have them as standard equipment. The heater tank itself has air expansion space and a check valve is often used on the tank inlet as well. However, an accumulator tank can help smooth out the water pump action if on the pump outlet side.
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Old 12-07-2014, 11:14 AM   #11
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Expansion Tank

I appreciate all the responses from everyone and have decided that I will put in a T for the expansion tank and cap the pipe towards where the tank will sit. Then I will test the system without the tank for a few days and see what the results are. If I get surging or don't like the way the flow is going I can very easily add the expansion tank back into the system.

The expansion tanks only cost about $40 but I don't like all the clutter in my RV basement.

I will repost what happens and whether or not I had to add the expansion tank.

Thanks everyone!
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Old 05-21-2016, 02:21 PM   #12
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Tankless hot water heater do not need an expansion tank. Reason, the water is only heated when there is a hot water flow demand.
Should I use or add an expansion tank for an RV hot water tank? Yes. This will prevent the safety relief valve from lifting as a result of the hot water expanding. I just purchased a new Winnebago and it did not come with an expansion tank for the hot water. The safety relief valve was weeping water and I told the service advisor that it is not normal. He stated that it is. I told him that the an expansion tank needs to be installed on the hot water line to provide room for expansion. This will prevent the safety relief valve from weeping. Please do not confuse this with the expansion tank for the water pump. If a check valve is installed between the supply line and the hot water tank, you will still need a small expansion tank. Since the RV tanks are typically around 6 gals, the Surflo expansion tank should do the job. I will be ordering one soon and will tell you if my hot water tank safety relief valve is no longer weeping water.
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Old 05-21-2016, 02:48 PM   #13
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Quote:
Originally Posted by BestofMesa View Post
Tankless hot water heater do not need an expansion tank. Reason, the water is only heated when there is a hot water flow demand.
Should I use or add an expansion tank for an RV hot water tank? Yes. This will prevent the safety relief valve from lifting as a result of the hot water expanding. I just purchased a new Winnebago and it did not come with an expansion tank for the hot water. The safety relief valve was weeping water and I told the service advisor that it is not normal. He stated that it is. I told him that the an expansion tank needs to be installed on the hot water line to provide room for expansion. This will prevent the safety relief valve from weeping. Please do not confuse this with the expansion tank for the water pump. If a check valve is installed between the supply line and the hot water tank, you will still need a small expansion tank. Since the RV tanks are typically around 6 gals, the Surflo expansion tank should do the job. I will be ordering one soon and will tell you if my hot water tank safety relief valve is no longer weeping water.


I had to reread this post, I forget the issues since it's almost a year and a half old.

All water heaters have a built in expansion tank. No need to add one after the WH. The relief valves will weep a bit, often allowing them to 'snap' closed will dislodge deposits that caused the valve to stay open a bit.

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Old 05-22-2016, 07:32 AM   #14
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That is fine in theory but the expansion pocket quickly fills with water since it does not have an expansion bladder. Simple physics. If you like going through the process of creating the air pocket frequently which requires you to relieve the water pressure with the hot water tank turned off and opening the relief valve and a faucet to create the air pocket, then have a ball. That is not practical and I am sure that most folks would not want to be bothered with it. If you don't mind hard water spots on the side of your RV from where the hard water is weeping from the relief valve, then that is fine also. I don't want either and the relief valve is not intended to be constantly challenged. This relief valve is the same that is used for home hot water tanks. No different. How often do you see them leaking? Not often. I do have to mention that I had a problem with my home hot water tank relief lifting and dripping on the floor. I tolerated it until I was ready to sell my home. I had a very hot water tank (80 gal) because I had a whirlpool bath in the master bedroom. The builder had installed an expansion tank when the house was built. The problem was that the plumber did not bother to determine if the expansion tank was sized correctly. It was too small. I installed a properly sized expansion tank for the extra large hot water tank and presto, no more problem. It is amazing how easy it is to BS the RV community with a diagram convincing people that the challenging a relief valve is normal. It is not.
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