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Old 05-26-2018, 01:21 PM   #1
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Hot Water Over Pressure – Accumulator or Relief Valve (Both?)

Last week I had the soft line at the water pump blow off and cause a small flood, after a bit of testing I have found that when the hot water heater runs the pressure build up in the system gets VERY high. I suspect it is exceeding 150 psi due to the T&P valve on the water heater dripping during the last 3ed of the heat up cycle. I now have a gauge on the system and have seen the pressure go as high as 120 psi before I got scared and opened a tap to vent the system. I tried to reestablish the “air cushion” in the tank by dumping the T&P valve but it makes no difference. I suspect with fresh hoses and tight clamps the system can handle the 150psi and may have always been this way, the rust in hot water heater below the T&P valve might support this idea.

So I’m looking to improve my system with either a small accumulator or a pressure relief valve and would like input from others that might have address this issue. The pressure relief valve would be eazest to fit but I don’t like the idea of it dripping all the time. The accumulator seems the proper solution but making room for it will be a bit more work. And I still like the idea of the relief valve as a backup safety… the idea of another flood is keeping me up at night.

Gary
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Old 05-26-2018, 03:39 PM   #2
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Gary-

I was unable to maintain an air pocket in the water heater, and so resorted to installing an accumulator. Depending on your water heater piping (i.e., check valves), the accumulator can be placed anywhere in the water system.

I preferred adding an accumulator versus a separate relief valve.

Depending on the size and pre-load pressure in the accumulator, the piping pressure still goes up- but nowhere near what it did before adding the accumulator.

I had to install a 100 psi T&P valve to replace the original, as the accumulator was rated at 125 psi.
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Old 05-26-2018, 06:05 PM   #3
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Mark,

I like that plan, down grading the T&P to the 100 psi version as a safety sounds better than installing a second one. Now I have to figure out how I'm going to modify the shelf under the galley sink to fit the accumulator.

A couple of questions, how big of an accumulator did you install? I'm thinking one of the small (sub one gallon) ones should do. And where did you set your charge pressure? I'm assuming toward the top side of the pump/supply pressure so the normal system pressure doesn't take up all the volume.
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Old 05-26-2018, 07:03 PM   #4
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Gary-

I installed a Jabsco model 30573-0000. It's capacity is 1 liter.

Watts has an online calculator here.

I originally charged the accumulator to 40 psi. However, after it mysteriously lost its charge one day I charged it to 60 psi, as a test. The coach regulator is set to 55 psi. What this means is that the accumulator does not even begin to fill until the water heater has warmed up somewhat. In any case, a pressure between 40 and 60 psi is sufficient. At whatever pressure you set, test it with the water pump and make sure there aren't any odd interactions, too.

It's not easy to find, but Watts makes a 100 psi, 210 deg. F T&P valve. Details below.

UPC PART DESCRIPTION
098268583144 0121341 3/4 LF100XL 100-210
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Old 05-26-2018, 07:18 PM   #5
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Step one :let it cool
Then remove drain plug and open TPR valve manually I like to install a short piece of pipe in the drain to get the water away from the siding.

Then replace plug. close TPR and refill. once full open hot water valve to let it finish filling. You have now re-established air cushion at top of tank (Expansion zone)
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Old 05-26-2018, 07:41 PM   #6
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Don't have to remove drain plug to re-establish air pocket


Turn off water supply
Open closest faucet HOT side to relieve pressure
OPEN T&P Relief valve via lever
LET T&P snap closed when water stops running out
Close faucet
Turn on water supply


Air pocket will establish in top of tank as per design




BUT problems keeping air pocket can occur ....... read 'Marks' post (l1v3fr33ord1)
small accumulator tank installed on cold water line should be enough to control the pressure increase from heating water (it swells when heated...air pocket should control pressure increase but some WH Tanks loose air pockets quickly....no real good reason just happens.??)






Have you measured HOW HOT the water is being heated to???
Normal t-stat should shut down heating when temp reaches 130*F/140*F (depends on BRAND/MODEL)


High temp should shut down ALL heating IF temps reach 170*F/180*F (BRAND/MODEL dependent)
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Old 05-27-2018, 03:24 PM   #7
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On the lower pressure T&P valve, unfortunately I need the 1/2" version. I don't see one at 100 psi but there is a 75 psi version, just not easy to find (Watts 1/2 LF1XL 75-210 - 098268583519 / 0121376).

I wonder if using the drain plug vs. the T&P valve creates a larger air pocket? I guess it would depend weather the hot water outlet pipe is lower that the T&P outlet. But if the T&P is forced open by the system pressure then the water level is going to rise to that level on the first heating cycle anyway. Whatever the case, my system doesn't maintain a large enough pocket to keep the pressure low enough for my liking.

My water temp measures about 140 F at the taps, should be right for my Atwood.
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Old 05-27-2018, 10:54 PM   #8
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75 psi T&P would be blowing shortly after heating cycle began.
Not a solution.....install accumulator tank

Air pocket by design is the area above the highest hot water level....which is where the HOT out dip tube is set
T&P is installed just below the high hot water level

Using the drain plug does not create a larger air pocket and is NOT necessary




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Old 05-27-2018, 11:23 PM   #9
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Installing an accumulator is the plan. The lower PSI T&P would be a secondary backup.

My MH is older and has a fairly low system pressure (35 psi on the pump, 45 psi on city). I'm hoping the accumulator will keep the pressure under 75 psi during the heating cycle and the low T&P would only leak if there was a problem.
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