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Old 12-25-2015, 03:46 PM   #41
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My trawler only had a check valve at the municipal water connection. All aspects of the freshwater plumbing work as designed without additional check valves. And, my water heater doesn't suffer from incontinence when it's turned on. If you feel you need additional check valves, and the occasional leakage is ok, then there's no problem. Personally, I don't my hot water plumping and WH pressure tested to 150 psi (or whatever pressure the T&P valve opens at) each time I turn the WH on.

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Old 12-26-2015, 01:05 PM   #42
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I think this is a case of "pick your poison".

If you locate the expansion tank on the cold water side and install a check valve between it and the hot water side, you block undesirable hot/cold water mixing, but you leave it to the hot water plumbing to absorb the inevitable expansion when the water goes through a heating cycle. The only expansion space available is stretching of the pipes etc., which of course is limited, then ultimately the relief valve. It would appear this is exactly what you are experiencing. But keep in mind that if your relief valve is opening then your hot water system is reaching 150 psi, which is super high. At that pressure I think you start to risk blowing out plumbing hoses etc.

If you remove the check valve so the hot and cold water can both benefit from the expansion tank, then you get some amount of thermal mixing, but you maintain a steady and civilized pressure level throughout the system.

Personally, I'd pick the latter for certain. There are also ways to create traps that will largely block any thermal mixing.
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Old 12-26-2015, 01:42 PM   #43
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I have no check valve on my heater, and no problem with warm water backing up into the cold side. What would push the hot into the cold side? A mixing faucet? That's got be a minimal effect if at all.
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Old 12-26-2015, 01:49 PM   #44
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I have no check valve on my heater, and no problem with warm water backing up into the cold side. What would push the hot into the cold side? A mixing faucet? That's got be a minimal effect if at all.
1) More pressure in the hot water tank because the water has been heated and expanded.

2) More pressure in the hot water tank because you turned off the water pump or ran out of water.

Really, this thread has degenerated into pure speculation. If the manufacturer of the water heater calls for a check valve, it should not be removed, it's there for a reason.
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Old 12-26-2015, 03:06 PM   #45
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Without the check the water heater tank will be the same pressure as the rest of the system. When heating up, it may move a pint backwards due to expansion, but what problem would that cause.

Mine and probably most wh tanks have the outlet on top, so if pump ventilates it still stays mostly full, at least enough to cover the element.

There are plenty of things spec'd in systems that are there for stupid reasons. Unless someone comes up with a good reason for this check to be where it is, it is in the stupid category.

Allowing a system to pressurize up to lifting a safety relief valve in normal ops is not a good design.
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Old 12-26-2015, 03:35 PM   #46
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Getting ready to leave the boat for a couple of months, but when I get back I think I will remove the check valve and give that a try.

Thanks guys for all your suggestions.

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Old 12-26-2015, 07:13 PM   #47
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Surely those valves are common on HWS tanks, incl domestic ones. My home system has one, you are supposed to exercise(open) it every so often, though I`ve also been told by a plumber to never touch it. In my experience they have a habit of not closing well after being opened, both the one at home and on the boat, and require a couple of opening/closings exercising to seal. I think it might just be doing its job of venting. My pest controller insisted the vent on the externally mounted HWS at home be fitted with a hose leading to the drains to avoid warm water being deposited near the house, it can attract termites.
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Old 12-26-2015, 10:05 PM   #48
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Really, this thread has degenerated into pure speculation. .
No speculation from many of us, merely direct observation. My reasons for removing the check valve were as tree noted, didn't want the 150 psi blowing up boat's hot water plumbing after watching TP valve open continually.
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Old 12-28-2015, 07:26 AM   #49
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"Getting ready to leave the boat for a couple of months, but when I get back I think I will remove the check valve and give that a try. "

Just moving the pick up for the expansion tank to the HW line will solve the problem'
IF the expansion tank is not water filled.
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Old 12-29-2015, 11:26 PM   #50
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What do you mean if the expansion tank is not water filled?
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Old 12-30-2015, 06:36 AM   #51
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Cheap crap expansion tanks like some Jabsco have no diaphragm and will be solid water in time.

Their reccomendation is to drain and vent the unit once a month.

Home style units have a diaphragm and air fill valve.

The pressure is set just below the pumps cut in pressure.

Old diaphragms can leak and the accumulator becomes solid water , no air cushion.

So no place for a bit of expanded hot water to go , except out the safety valve.
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Old 12-30-2015, 09:45 AM   #52
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Thanks FF
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Old 12-30-2015, 11:17 AM   #53
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Quote:
Originally Posted by FF View Post
"Getting ready to leave the boat for a couple of months, but when I get back I think I will remove the check valve and give that a try. "

Just moving the pick up for the expansion tank to the HW line will solve the problem'
IF the expansion tank is not water filled.
Putting the ex tank on the hw line will solve the SRV burp problem, but will create another.

If you are using cold water only, that section of the system is then isolated from the expansion tank by the check valve. The pump will then short cycle.
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Old 12-30-2015, 11:36 AM   #54
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I think some are confusing the thermal expansion tank with the accumalator or pressure tank. Two different animals that are similar in design if not size and function.
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Old 12-30-2015, 01:41 PM   #55
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Most boats use a single expansion tank as it easily serves both purposes.

The larger the expansion tank (20G is good) the less the pump will cycle off and on and the longer the pressure sensor will last , esp on a DC pump.

With hot water heating there will be an individual tank as heating circ water may have anti freeze in it , that will kill if drunk.
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