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Old 03-08-2016, 11:43   #1
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Odd pressure water accumulator configuration, an accumulator on both hot and cold

In the process of reworking the freshwater pump and verifying the pressure on the two large accumulators on the system I was amused to find that one is painted blue (from Jabsco) and is connected to the 22mm primary freshwater manifold of the boat, the other is painted red (from Jabsco, not owner painting) and is connected to the 22mm hot water main line running down the middle of the boat.

The red tank had no air pressure and so I filled it to 35psi (nominal system pressure with the new pump is 55psi, prior system pressure was about 40 psi).

The blue tank had very little pressure and I also filled it to 35psi.

The original system had a check valve between the cold water side and the hot water heater inlet so the accumulator on the hot side might have just been there to take up expansion as the hot water heater warmed up.

I recently replaced the hot water heater with a new Isotherm with the mixing valve on the hot water heater. Nothing in the instructions suggests that a check valve between the hot water heater and the cold water side is recommended.

Looking at various water system company diagrams, some show a check valve on the inlet to the hot water heater, others do not.

So, the questions:

1) is there any good reason for an accumulator on the hot water side of the system or should I put it in parallel with the one on the cold water side.

2) is there a good reason for a check valve in the cold water inlet to the hot water heater?
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Old 03-08-2016, 12:58   #2
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Re: Odd pressure water accumulator configuration, an accumulator on both hot and cold

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Originally Posted by botanybay View Post

1) is there any good reason for an accumulator on the hot water side of the system or should I put it in parallel with the one on the cold water side.

2) is there a good reason for a check valve in the cold water inlet to the hot water heater?
1) Accumulator tank on hot side: Is the tank before or after the hotwater heater? I can't see a lot of benefit. I would assume the tank would cool around the same rate as the hot water lines. Forcing you to empty the tank first, then fill it, then fill the line with hotwater before it reachd the tap. That's a lot of wasted water waiting for delivery of hot.

If it's before the hotwater tank, then what would be the difference as having a single accumulator tank for both sides? (I have a single tank directly after the pressure water pump (before the hotwater heater).

2) So a siphon on the cold side doesn't draw hot water out of the tank when a cold faucet is run?
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Old 03-08-2016, 14:24   #3
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Re: Odd pressure water accumulator configuration, an accumulator on both hot and cold

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Originally Posted by botanybay View Post
...
So, the questions:

1) is there any good reason for an accumulator on the hot water side of the system or should I put it in parallel with the one on the cold water side.

2) is there a good reason for a check valve in the cold water inlet to the hot water heater?
I'll combine the answers as they may be cause and effect related.

As you mentioned, the blue tank on the cold side is an accumulator tank. It is installed to help smooth out the pressure [and prevent the pump from rapid cycling in low flow situations] if you don't have a variable speed pump. [Its pressure should be set 2-3 PSI below the cut-in pressure of your water pump for optimal performance.]

The check valve in the cold feed to the water heater will help prevent a thermal syphon of hot to cold if your plumbing is so inclined... But it also prevents expansion in the water heater from going into the cold side accumulator tank... That may be why they installed an expansion tank on the hot side...

One test would be to bypass the hot expansion tank and see if your water heater needs to occasionally bleed excess pressure from its pressure relief valve with the expansion tank unavailable... [I'll wager it does- especially when heated by the engine coolant loop...]

Another test, with the hot expansion tank still bypassed/removed, would be to bypass/remove the cold-in check valve and see if you get any hot water back-flushing or siphoning into the cold feed line. [Feel it near the water heater... a few inches or a foot or two may be due to expansion in the water heater. More than that could be a syphon... and the check valve may be needed... more below...]

If not, keep both the check valve and hot expansion tank removed and you are finished (and have a spare expansion/accumulator tank and check valve...)

If there is some thermal syphoning going some distance into the cold feed line [again, the check valve is bypassed/removed right now...] I'll bet the cold water line comes from above to the water heater. In that case, try rerouting it so it comes from below before it enters the water heater to help break the thermal syphon... [Think convection cycle...]

In case this is helpful.

Best wishes optimizing your plumbing.

Cheers!

Bill
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Old 03-08-2016, 16:14   #4
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Re: Odd pressure water accumulator configuration, an accumulator on both hot and cold

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Originally Posted by Shrew View Post
1) Accumulator tank on hot side: Is the tank before or after the hotwater heater? I can't see a lot of benefit. I would assume the tank would cool around the same rate as the hot water lines. Forcing you to empty the tank first, then fill it, then fill the line with hotwater before it reachd the tap. That's a lot of wasted water waiting for delivery of hot.

If it's before the hotwater tank, then what would be the difference as having a single accumulator tank for both sides? (I have a single tank directly after the pressure water pump (before the hotwater heater).

2) So a siphon on the cold side doesn't draw hot water out of the tank when a cold faucet is run?
The second accumulator is at the far end of the hot water line going forward. Definitely on the output side of the hot water heater. The only reason I can think of for it would be to keep the pressure relief valve on the hot water heater from releasing a bit of hot water when the tank expands (assuming there is a check valve in the system). The hot water tank had a 2.5 Bar pressure relief valve on it and so it would definitely release some water while the tank was heating. I suspect that if the accumulator pressure was set below the pressure relief but higher than normal it would be able to take the excess water from the hot water heater.

However, all of the above is speculation, trying to explain a configuration which I can't yet justify.
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Old 03-08-2016, 16:26   #5
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Re: Odd pressure water accumulator configuration, an accumulator on both hot and cold

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Originally Posted by wrwakefield View Post
I'll combine the answers as they may be cause and effect related.

As you mentioned, the blue tank on the cold side is an accumulator tank. It is installed to help smooth out the pressure [and prevent the pump from rapid cycling in low flow situations] if you don't have a variable speed pump. [Its pressure should be set 2-3 PSI below the cut-in pressure of your water pump for optimal performance.]

The check valve in the cold feed to the water heater will help prevent a thermal syphon of hot to cold if your plumbing is so inclined... But it also prevents expansion in the water heater from going into the cold side accumulator tank... That may be why they installed an expansion tank on the hot side...

One test would be to bypass the hot expansion tank and see if your water heater needs to occasionally bleed excess pressure from its pressure relief valve with the expansion tank unavailable... [I'll wager it does- especially when heated by the engine coolant loop...]

Another test, with the hot expansion tank still bypassed/removed, would be to bypass/remove the cold-in check valve and see if you get any hot water back-flushing or siphoning into the cold feed line. [Feel it near the water heater... a few inches or a foot or two may be due to expansion in the water heater. More than that could be a syphon... and the check valve may be needed... more below...]

If not, keep both the check valve and hot expansion tank removed and you are finished (and have a spare expansion/accumulator tank and check valve...)

If there is some thermal syphoning going some distance into the cold feed line [again, the check valve is bypassed/removed right now...] I'll bet the cold water line comes from above to the water heater. In that case, try rerouting it so it comes from below before it enters the water heater to help break the thermal syphon... [Think convection cycle...]

In case this is helpful.

Best wishes optimizing your plumbing.

Cheers!

Bill
Thanks Bill!

With the old hot water heater the pressure relief was just above nominal system pressure (hot water heater limit, I used a somewhat higher one in a pinch and the old hot water heater sprung a leak!) It turned out that the hot side accumulator had no pressure in it so was not acting as an accumulator at all.

At the moment I don't have a check valve in the system and only have extremely minor thermal cycling (or potentially back flow from the hot side accumulator when opening the cold water sink valve) I get a slug of 10F higher temp water about 20 seconds after opening the tap full on which lasts for perhaps 10 seconds. Might just be that the hot side runs beside the cold side in some places and when the line is hot it warms the cold line a bit. Not a significant concern.

Because of the configuration of the plumbing it would be easy to block off the hot side accumulator and add it's capacity to the cold side accumulator. It would also be trivial to install a check valve in the hot water line.

It sounds like it may be partially trial and error to see what is an "optimal" configuration.

I am currently spending a week away from the dock with the system running with the hot side accumulator and no check valve. Might try adding a check valve when I get home and see if it makes a noticeable difference (liveaboard so lots of time to try slightly different configurations).

Thank you again for the feedback!

David
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Old 03-08-2016, 17:41   #6
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Re: Odd pressure water accumulator configuration, an accumulator on both hot and cold

With a 2.5 bar pressure relief I'm thinking the original water heater was a British clorifier, perhaps with a copper tank. The check valve was to prevent thermoshiphon, IE hot water flowing out the cold inlet. Which can happen in Certain If there is a check valve in the cold water inlet then a British clorifier would require the second expansion tank.

With the new Isotherm, I would leave the check valve off, unless you get lots of hot water backflowing into the cold side. A u bend in the inlet piping would also prevent thermoshiphion flow in the cold side.

If you add a check valve to the cold side then proper design and plumbing codes would require a expansion tank on the water heater. I'm not wild about using the T&P for thermal expansion as sometimes they don't reset tight and you could loose water to the bilge.

Back in the 80's drilling a small 1/8" hole in the clapper of the check valve was an approved method for relieving pressure. Current codes don't allow that, but then they don't apply to boats either.
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Old 03-08-2016, 22:20   #7
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Re: Odd pressure water accumulator configuration, an accumulator on both hot and cold

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Originally Posted by sailorchic34 View Post
With a 2.5 bar pressure relief I'm thinking the original water heater was a British clorifier, perhaps with a copper tank. The check valve was to prevent thermoshiphon, IE hot water flowing out the cold inlet. Which can happen in Certain If there is a check valve in the cold water inlet then a British clorifier would require the second expansion tank.

With the new Isotherm, I would leave the check valve off, unless you get lots of hot water backflowing into the cold side. A u bend in the inlet piping would also prevent thermoshiphion flow in the cold side.

If you add a check valve to the cold side then proper design and plumbing codes would require a expansion tank on the water heater. I'm not wild about using the T&P for thermal expansion as sometimes they don't reset tight and you could loose water to the bilge.

Back in the 80's drilling a small 1/8" hole in the clapper of the check valve was an approved method for relieving pressure. Current codes don't allow that, but then they don't apply to boats either.
Yes, that is exactly the unit you are describing. The unit was a welded copper tank which sprung a leak on one of the straight welds. Interestingly I was able to limp along for a few weeks by cutting away the insulation, finding the leak, and repairing with Gflex epoxy from West Systems and a couple layers of fiberglass cloth. Sanded hard to bright copper, applied epoxy wet, then sanded with 80 grit through the wet epoxy, then built up a couple of layers of glass over a 1 foot square area. Held nicely for 6 weeks while I waited for a new water heater.

Definitely a British unit from 1990 when the boat was built in England.

I also don't like using the T&P valve for releasing expansion pressure so if I find I need the check valve I will definitely keep the unit. I have also seen these valves not fully reseat and dribble into the bilge.

As the plumbing of the isotherm (with the balancing valve) has the water heater inlet open to fresh water, I can't see any reason that the hot side would build up excess pressure (temperature is only going down as the system cools).

As a curiosity I will check if water can flow back through the mixing valve if I let pressure off of the cold side is there effectively a check valve built into the mixing valve.

Thank you for the confirmation!

David
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Old 04-08-2016, 20:45   #8
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Re: Odd pressure water accumulator configuration, an accumulator on both hot and cold

I have two accumulator tanks. None is worth the cost. they have one liter capacity, after turn on a tap a little (Much less than one litter) and close, after a while, the water pump turns on. It means trat the benefit of having a accumulator is irrelevant.
I allready refilled it them with air and the time between turn on the water Tap and the water pump begin to work is too short to make it worth.
Thats my Experience...sorry beeing negative
Regards and good luck
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