Accumulator for Hot Water System

[Transaxial]

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.

I think you are on the right track but just overthinking the expansion of the hot water from either electric heat or from the engine heat loop. The hot water in the system expands when heated. That is normal and is a very small volume. Liquid is not compressible though. The hot water tank should be full so there is no air anywhere in the system to compress so a very small amount of heat expansion causes considerable pressure rise, which causes the relief valve to weep. By adding a $30-40 bladder tank between the cold inlet to the hot water tank and pressure supply pump you accomplish two things. 1/ You provide an outlet for a very small volume of water caused from heat expansion at a negligible change in pressure in the bladder tank. Any hot water moving from the hot water tank to the bladder would be insignificant but prevent a pressure spike on the otherwise closed system. 2/ The expansion tank greatly reduces pump cycling for both hot and cold water usage, which is a very nice feature.

I also plumbed a temperature regulating valve for domestic water coming off the hot water tank because that water is dangerously hot at engine temp of 180-195 degrees when the engine has been running. These can be set to 125-135 degrees F so as not to scald an unsuspecting water user. With 180 degree water in your 10 gallon hot water tank, you get considerably more shower temp water when mixed. That little bladder in my system has made all the difference in many ways, for the better.

 

[JRO]

My hot water tank relief valve discharges quite a bit.

How much is quite a bit? If it really isn’t that much you are worrying about nothing. If it’s really a lot... You should fix the system you have rather than compensate by adding another system.

 

[Antares]

I had the same problem. My relief valve was discharging into the bilge. One I was sure it was the source I replaced it. They get old and tired and start to dscharge at, in my case, at a lower temperature. The marine releif valves have hotter set points then the ones for domestic use. I believe this is to accomodate engine coolent temps. A new Raritan valve took care of the problem for me.

 

[JRO]

5 Oz. Per day... That’s a slow drip. When heating water 24/7, expect the T&P valve to do its job.

 

[Lepke]

Hot water can flow backwards into the cold. In another life I owned film processors that had to run at a specific temp. One aspect of controlling the temp was a check valve on the hot water tank input. Otherwise as the tank heat came on, the hot would backup into the cold. Cold was part of controlling the temp, so with the hot backing up every time the heating elements came on, the temp would go high and ruin film.
A 1 gallon accumulator should be fine. It doesn't need to be marine. About $50 online.

 

[Shay]

Put a 1 gal. expansion tank anywhere in the a cold line. It doesn't need to be near the water heater. look at the fittings in the cold supply line to the water heater and make sure you don't have a check valve in the supply. If you have a check valve remove it and with the expansion tank in the cold line the whole boats water system will work better.

Shay Glass

 

[catalinajack]

I was the guy who posed the question: can the expansion tank be safely installed on the hot side of the water heater? First, thanks to all who took the time to provide their thoughts!

What I have learned. Putting the tank on the hot side is as safe and effective in absorbing the expansion of heated water as putting it on the cold side. A review of manufacturers' literature documents this fact although their preferred method is the cold side. None explained why or provided any pros and cons. My sense is that, if there was ANY danger in doing so, the corporate lawyers would have none of it. Thinking logically, we are talking about a tank installed in the SAME line, just in a different place. Of course, the expansion will be absorbed. Shay recommends removing the check valve. I, and likely others, do not like the idea of having hot water back up into the cold water supply line. That is the very reason for the check valve in the first place.

There were some comments that an expansion/accumulator tank on the cold side has the dual benefit of mitigating the annoying pulsating (pressure and volume variations) of many 12-volt water pumps. Some pumps vary in delivery pressure as much as 20 PSI (cut-in, cut-out pressure), very annoying in the shower stall. However, not all of us have that problem. I do not. I use a Grundfos 110VAC water system booster pump - think farms and cottages that use springs or lakes for a water supply. Unless, a faucet is just cracked open, the Grundfos comes on and stays on delivering constant pressure and the ability to deliver volume at pressure far in excess of any boat demand imaginable. The Grundfos has its own one gallon internal absorbing tank. The Grundfos is large, however, and is not a practical solution on smaller boats. Just another example of there are very "shoulds/musts" when it come to boat issues.

Bottom line is I will be installing a one-gallon expansion tank on the hot side keeping the cold water check valve, when I get around to it. This heater has been in use weeping at the T&P valve for about a year now as was the previous identical heater for 11 years with no ill affects.

Put a 1 gal. expansion tank anywhere in the a cold line. It doesn't need to be near the water heater. look at the fittings in the cold supply line to the water heater and make sure you don't have a check valve in the supply. If you have a check valve remove it and with the expansion tank in the cold line the whole boats water system will work better.

Shay Glass

 

[GoneFarrell]

The "leaky" check valve just downstream of the pump has been working fine for me. Its an all-metal flapper style check valve, PEX lines/fittings throughout.

 

[hughespat57]

If you want to install after water heater this tank I used can be, it is FDA for potable water, 150 PSI operating pressure and 200F rated, with stainless fittings and poly lined. $41



I removed my check valve at the heater and installed this tank after my pump to act as both an expansion and accumulator tank.

[FONT=&quot][FONT=&quot]https://www.amazon.com/dp/B01M7SWXMT/ref=cm_sw_em_r_mt_dp_G9L4Fb9PBCXJ6?_encoding=UTF8&psc=1[/FONT][/FONT]


but for reference:
water at 180F has a coefficient of expansion of 0.000640
water going from 70 to 180 (worst case) would increase by 110 degrees F

for a typical 6 gallon marine tank the formula is
dV = 6 gallon * 0.000640 * 110 which is 0.4224 gallons

So a half gallon greater volume after heating

So only a half gallon maximum is going to "back flow" from the heater to the plumbing and tank. I don't consider that significant enough to worry about the heat loss. And it occurs so slowly it is not going to affect the plumbing.



In a constant volume system (no expansion tank) the pressure will exceed the safety valve after approx 20 degree rise (give or take the tank and plumbing expansion). After that the valve is going to slowly release approximately 0.4 gallons as the temperature increases.


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My hot water tank relief valve discharges quite a bit.

Anyone ever use this Groco Accumulator? Was thinking of the PST-1/.5 gal.

 

[Nick F]

Late contribution (at least, I hope it is a contribution!)

My calculation differs somewhat from hughespat57. If I am right, the volume change is rather less:
The coefficient of volumetric expansion of water varies quite a bit with the temperature, so its safer to look at the difference between the densities of water at the two temperatures. From 70F to 180F there is a 2.8% difference. For a 6 gallon tank this would give a 0.17 USgal volume increase (about 2 fluid ounces).
A 1 pint accumulator would absorb this with minimal pressure rise, and larger accumulators would obviously work too, but take up more space.

You may want to play with the accumulator's gas pre-charge pressure. Choose anything below, say, 50 psi to avoid relief valve weeping (by a substantial margin). If you want to reduce the pressure pump cycling the pre-charge pressure will need to be below the pump cut-off pressure. Probably best to select the pump cut-in pressure.

Nick

 

[catalinajack]

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.

Problem solved. Here is what I found. Although some responders suggested that the "proper" way is to install an expansion tank/accumulator on the cold water side, the fact is that it doesn't matter a wit where it goes. In fact, some manufacturers state in their instructions that it can be placed anywhere in the system. It will function just as well whether on the cold water supply or the hot water side after the water heater. That includes acting as an accumulator for certain water pumps.

You will see in the picture that I installed a pressure gauge. I can report that, when I turn the cold water on, the pressure decreases to the cut-in pressure of my pump, the pumps pumps until it reaches the cut-out pressure, exactly what one seeks with an accumulator on the cold water side. And why would it not? It is all part of the same system. The tank will absorb pressure regardless of where it is placed, cold water line or hot water line cuz, well, the hot water line is just a continuation of the cold water line.

In my case, I needed to decrease the line pressure in the hot water heater to below the 100psi of the temp and pressure relief valve. Now, the pressure does not exceed 60psi, the cut-out pressure of my pump.

My heater is located far from the expansion tank, underneath the bed in our cabin. Doesn't matter. Pressure is pressure which will seek relief and is relieved anywhere in a closed system. I chose the spot you see in the picture to use the capped off "tee" where the hot water heater used to feed the hot water lines. Quite convenient.

So, the bottom line is for those who are having a problem with a weeping T&P valve is to install an expansion tank somewhere AFTER the water heater. And for those who have variable speed pumps that do not tolerate accumulators, simply install a check valve on the cold water inlet to the water heater.