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Old 12-14-2020, 08:38 PM   #1
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Accumulator for Hot Water System

My hot water tank relief valve discharges quite a bit.

Anyone ever use this Groco Accumulator? Was thinking of the PST-1/.5 gal.
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Old 12-14-2020, 08:43 PM   #2
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Ours discharges as we are underway as the coolant temp causes the expansion.

Our solution was simply to put a hose on the relief output into a plastic pint glass. The water that is expelled is collected and evaporates. Simple solution that keeps the water out of the bilge.
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Old 12-14-2020, 08:44 PM   #3
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Hmm, not sure accumulators were designed or meant for post hot water systems. Only ones I am aware of are for the fresh water source, feeding (before) the hot water tank.

Where were you thinking of putting it to resolve your issue?

Might be worth giving Groco a call.
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Old 12-14-2020, 10:16 PM   #4
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Too answer some questions:

The expansion tank goes on the cold water intake. The pressure in the bladder should be set at the cutoff pressure of the pump so that water only goes in the tank during expansion.

Groco makes good stuff for a princely sum. The same product can be had for a lot less. My preference is the Watts PLT line.

https://www.watts.com/products/plumb...sion-tanks/plt

The PLT-5 should be more than enough for a marine water heater. Under $40 plus shipping.

Home Depot is showing a similar size different brand for around $25.

https://www.homedepot.com/p/Everbilt...E&gclsrc=aw.ds

Ted
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Old 12-15-2020, 07:33 AM   #5
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Some accumulators are made for furnace heating systems where the heated water is never consumed.

I would select an accumulator (any make) that was built for use with potable water.
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Old 12-15-2020, 08:03 AM   #6
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The exact item you are looking for is a water heater expansion tank. Required on residential water heater installs in most locations now. Home Depot, Lowes etc have them.

This is assuming that there is nothing wrong with your T&P valve. Which there may be. Also if you use engine heat to heat the water you could be going over temperature for the valve.
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Old 12-15-2020, 09:14 AM   #7
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Ted, putting the expansion tank on the cold water side being the correct and only method is most confusing to me. The tank is installed in-line. I just don't see that installing it on the output side makes a bit of difference in terms to limiting the internal pressure of the water heater especially if a directional (in only) cold water supply valve is used as some installations recommend. What am I missing? My relief valve weeps ever so slightly, not enough to justify adding an expansion tank, but I do want to understand just in case.
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Originally Posted by O C Diver View Post
Too answer some questions:

The expansion tank goes on the cold water intake. The pressure in the bladder should be set at the cutoff pressure of the pump so that water only goes in the tank during expansion.

Groco makes good stuff for a princely sum. The same product can be had for a lot less. My preference is the Watts PLT line.

https://www.watts.com/products/plumb...sion-tanks/plt

The PLT-5 should be more than enough for a marine water heater. Under $40 plus shipping.

Home Depot is showing a similar size different brand for around $25.

https://www.homedepot.com/p/Everbilt...E&gclsrc=aw.ds

Ted
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Old 12-15-2020, 09:33 AM   #8
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I had the same issue with the expansion causing the relief valve to overflow. I traced the problem to my boat having PEX for water lines instead of flexible hose. The PEX doesn't expand and contract like typical boat water hoses do, so there was nothing there to absorb the minimal expansion.

I put in a one gallon big-box store accumulator on the cold water side (back-pressure from hot water expansion works, too). Solved the problem entirely.
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Old 12-15-2020, 09:55 AM   #9
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Originally Posted by jhall767 View Post
The exact item you are looking for is a water heater expansion tank. Required on residential water heater installs in most locations now. Home Depot, Lowes etc have them.

This is assuming that there is nothing wrong with your T&P valve. Which there may be. Also if you use engine heat to heat the water you could be going over temperature for the valve.
If you have engine coolant loop in the how water heater that is probably the case. 180 or so hot water is above the valve rating.
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Old 12-15-2020, 10:20 AM   #10
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Originally Posted by catalinajack View Post
Ted, putting the expansion tank on the cold water side being the correct and only method is most confusing to me. The tank is installed in-line. I just don't see that installing it on the output side makes a bit of difference in terms to limiting the internal pressure of the water heater especially if a directional (in only) cold water supply valve is used as some installations recommend. What am I missing? My relief valve weeps ever so slightly, not enough to justify adding an expansion tank, but I do want to understand just in case.
First, the reason you want an expansion tank is to keep from over pressurizing your water system. The T&P valve on your water heater is activated by Temperature or Pressure. Temperature is designed to prevent components from failing by excessive heat (a failed electric thermostat continuing to heat the water). Just as with a car radiator, liquid expands from heat. Without the radiator cap venting the excess liquid, pressure would rise until something failed to relieve the pressure. With your water heater, the T&P valve protects the system by relieving pressure. But at what pressure? Go read the tag on your T&P valve to see what pressure it is supposed to open at. Each time it vents, you have hydrostaticly shocked or pressure tested the water heater and all the plumbing after it. Why would you intentionally do that?

The reason for teeing the cold water inlet for the expansion tank is to have the lowest temperature going into the bladder tank and not the hottest. Second, depending on location, the expanded water in that tank will cool. If it was on the outlet side of the water heater, the cooler water would come out first, be cool,and you would wait longer and waste water waiting for hot water.

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Old 12-15-2020, 10:25 AM   #11
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Temperature -- Pressure / T&P

The T&P valve on a water heater is not something to be trifled with.
1st off, your T&P relief valve is not an operating control. IOW, it was not designed to open/close repeatedly. Echo OC's post. It is a SAFETY device, designed to be a sentinel to protect the pressure vessel from overpressure/temperature conditions that transform the pressure vessel to a bomb.

DO NOT allow conditions where a pressure/temp device is constantly leaking or discharging to persist. It compromises the integrity and function of the valve. Correct the underlying condition that's causing the valve to operate, then replace the valve.

A thermal expansion tank can be used to provide a buffer for the expansion, it's intended to be installed in the cold supply to the system. In a domestic application, thermal expansion becomes an issue in municipal water systems that utilize a double check that protects the public supply at the house connection. It allows water in, but prevents ANY flow in the opposite direction. Thermal expansion created by the water heater causes an increase in pressure enough to exceed the T&P valve's typical 150PSI rating, (many marine WH use 75PSI) and the valve opens to protect the tank. Water's not compressible. A thermal expansion tank plumbed in the piping downstream of the dual check allows the expansion to compress the air in the tank's bladder. Problem solved.

In our marine plumbing, a check valve is typically located in the cold water inlet to the water heater. This check valve serves a different purpose than the dual check in a domestic application, it's intended to prevent backfeed of hot water into the cold system. It's function is more comfort than safety. Still, it can create conditions where the thermal expansion overpressures the relief valve setting. Installation of a thermal expansion tank in the cold water piping is ineffective because the check valve prevents flow back into the tank's bladder. The solution is not to plumb the tank into the hot water piping since the expansion tank isn't rated for hot water.

The solution lies in the water heater inlet check. Since that check valve is NOT a safety device, the simplest solution is to allow minimal flow back through the check valve. Scoring the seat of the valve or drilling a tiny hole in the flapper will allow minimal backflow while retaining the function. If there's an accumulator tank on the plumbing system, it will likely have the capacity to absorb the thermal expansion, or if not, a larger or additional tank can be installed- in the cold water piping where it's intended.

If the T&P is opening in response to excessive temps created by engine heating, that's a different safety issue that a thermal expansion tank may not resolve. In my own case, my engine runs a 193F thermostat, and that can approach the 210F opening point of the T&P. It's a dangerous scald hazard, so I implemented temperature control on the engine heating coil plumbing to prevent overheating the water heater. It fails to closed, so failure of the control system simply prevents the engine from heating the water heater.
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Old 12-15-2020, 11:07 AM   #12
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My T&P (Relief Valve - RV) has leaked excessively since day 1. These RV’s need time to seat, so the manual says some leakage in the beginning is normal. Didn’t stop, and by that I mean it will fill up a 16 oz container in about 3 days – too much.

I therefore replaced the relief valve about 1.5 years ago thinking it may be bad. No improvement.

There is no way to turn my heat input down.

My engine coolant is plumbed to the hot water heater on a loop, but that doesn’t cause the issue. It’s when its supplied by the electrical side (shore or Gen) when the RV lifts the most.

I was told (second hand from the hot water heater manufacturer) one method to prevent the RV from constantly lifting is by plumbing in an accumulator on the hot side. But, as others have noted, and I learned this myself on some websites yesterday after my initial post, it does not appear these accumulators are designed for hot water and should be plumbed in to the cold side. Installing an accumulator helps with other things like constant pump cycling for small faucet draws, but it would not solve my problem I now know, which is also confirmed by the replies on this thread.

One more thing – it appears code criteria for boats concerning hot water heater relief valves has changed. These heaters are now required to have 100 psi relief valves, where on shoreside (similar rated T/P heater units) they are 150 psi. The manufacturer doesn’t note the MAWP on my heater plate, by design I assume because less is more. I think I just figured out the solution.
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Old 12-15-2020, 11:38 AM   #13
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Originally Posted by Fletcher500 View Post

Snip

. But, as others have noted, and I learned this myself on some websites yesterday after my initial post, it does not appear these accumulators are designed for hot water and should be plumbed in to the cold side. Installing an accumulator helps with other things like constant pump cycling for small faucet draws, but it would not solve my problem I now know, which is also confirmed by the replies on this thread.
Except for the part that they do make thermal expansion tanks for your specific issue and that they are required on domestic installs. And will solve your problem.

https://www.hotwater.com/lit/im/tanks/332845-000.pdf
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Old 12-15-2020, 12:56 PM   #14
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I'm repeating what others have said, but here goes.


A T&P valve is an emergency safety release. If it activates, something is wrong that needs to be fixed. In normal, correct operation they should NEVER activate.


So something is wrong with your plumbing system, and what's amazing is that many, if not most boat and RV water systems are incorrectly built. Your T&P valve is telling you that, and it's right.


You MUST have some way for the water to expand in any sort of heated water system. All these cheap-ass water system just leave it to the pipes, hoses, and T&P valve to so it. Even the most lax building code would flunk such a system, yet there is no such code for boats and RVs.


So yes, you need an expansion tank. It doesn't have to be very big, but you need one. Locating it on the cold water system is preferable PROVIDED there isn't a check valve where the cold water feeds the hot water heater. That check valve will block the hot water's ability to expand into the expansion tank. In this case you will either need a second expansion tank, or you can remove the check valve. Maerin had an interesting idea too which is to create an imperfect check valve that intentionally leaks back a little bit to equalize pressure.
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Old 12-15-2020, 12:59 PM   #15
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Originally Posted by Fletcher500 View Post

I was told (second hand from the hot water heater manufacturer) one method to prevent the RV from constantly lifting is by plumbing in an accumulator on the hot side. But, as others have noted, and I learned this myself on some websites yesterday after my initial post, it does not appear these accumulators are designed for hot water and should be plumbed in to the cold side. Installing an accumulator helps with other things like constant pump cycling for small faucet draws, but it would not solve my problem I now know, which is also confirmed by the replies on this thread.



If you have a pump accumulator tank, that will do the job just fine PROVIDED there is no check valve where the cold water feeds the HW heater. One option is to just remove the check valve. You will probably get some thermal mixing, but it's better than running your plumbing system and the explosion release pressure.
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Old 12-15-2020, 01:05 PM   #16
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One more thing – it appears code criteria for boats concerning hot water heater relief valves has changed. These heaters are now required to have 100 psi relief valves, where on shoreside (similar rated T/P heater units) they are 150 psi. The manufacturer doesn’t note the MAWP on my heater plate, by design I assume because less is more. I think I just figured out the solution.

Even 100 psi is a LOT of pressure for a water system. It will stress things at a minimum, and will exceed the allowed pressure for others. Whale quick connect fittings, for example, are only rated to 90 psi for hot water. That's why the 100 PSI relief pressure is an emergency release pressure not an operating pressure. But too many of these systems are designed to use the T&P valve as an operational pressure regulator rather than it's intended use as an emergency release.
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Old 12-15-2020, 01:32 PM   #17
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The T&P valve on a water heater is not something to be trifled with.

In our marine plumbing, a check valve is typically located in the cold water inlet to the water heater. This check valve serves a different purpose than the dual check in a domestic application, it's intended to prevent backfeed of hot water into the cold system. It's function is more comfort than safety. Still, it can create conditions where the thermal expansion overpressures the relief valve setting. Installation of a thermal expansion tank in the cold water piping is ineffective because the check valve prevents flow back into the tank's bladder. The solution is not to plumb the tank into the hot water piping since the expansion tank isn't rated for hot water.

The solution lies in the water heater inlet check. Since that check valve is NOT a safety device, the simplest solution is to allow minimal flow back through the check valve. Scoring the seat of the valve or drilling a tiny hole in the flapper will allow minimal backflow while retaining the function. If there's an accumulator tank on the plumbing system, it will likely have the capacity to absorb the thermal expansion, or if not, a larger or additional tank can be installed- in the cold water piping where it's intended.
Creating a "leaky" check valve can certainly solve the problem. I think the point Ted is making that some are missing is if you place the expansion tank on the water heater inlet side between the check valve and the WH the heated & expanding water can back up into the expansion tank but NOT into the cold water system (which is the reason for the check valve. It may mean moving the check valve a short ways if it is currently immediately at the WH inlet. Move it to the inlet side of a "T" for the expansion tank.
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Old 12-15-2020, 01:46 PM   #18
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We used a small expansion tank from the local Home Depot. It's very simple and works perfectly. Plumbed on the cold side of the hot water tank, with the inlet check valve moved upstream of the pressure tank. Thus cold water flows through the check valve, through the expansion tank, then into the hot water tank. When not flowing water, the super-hot water can expand "upstream" against the expansion tank. Remember that the expansion tank is already full of cold water at this point. Expanding back into it does NOT fill it with 180 degree water! There's some expansion going on as the engine transfers heat into the water heater tank, but there's essentially no FLOW happening; the expansion tank doesn't really get hot.
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Old 12-15-2020, 02:21 PM   #19
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Buy your expansion tank from an irrigation shop, the Groco is 5 or 6 times as expensive and it's the same thing with a Groco decal.

Install it on the cold side, after the pressure pump. Put a valve on the intake and the exhaust so you don't have to wait when you shut off the water for maintenance.
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Old 12-15-2020, 02:31 PM   #20
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One option is to just remove the check valve. You will probably get some thermal mixing, but it's better than running your plumbing system and the explosion release pressure.
Good advice. This is what we did - remove the check valve - on our new hot water tank when the relief valve was popping open. The existing Groco fresh water pressure tank served as the modulator. It was an easy fix and no space or extra $ were required.
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