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Old 01-07-2013, 16:52   #1
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Hot water from cold tap

Hi

I have a raritan hot water tank on our Sabre 402. When any cold tap is run for a minute or so hot water starts to come out. As soon as the pressure pump kicks in the water will go cold again. Is there a malfunctioning one way pressure switch that is allowing this to happen?

Thanks

Allan
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Old 01-07-2013, 16:53   #2
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Re: Hot water from cold tap

maybe you are in Texas?

Ok so that didn't help.. sorry.
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Old 01-07-2013, 17:04   #3
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Re: Hot water from cold tap

Could be that the hot and cold water lines are run for some distance one next to the other. Are they?
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Old 01-07-2013, 17:12   #4
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Re: Hot water from cold tap

Insulate the cold water lines as they pass through the engine room.
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Old 01-07-2013, 17:20   #5
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Re: Hot water from cold tap

I've got the same thing, and it's not just that the cold runs by a hot pipe or through a hot area. I can only guess that there's a check valve missing or stuck open and the pressure in the water heater is greater than the rest of the cold side of the system, at least until the pump kicks on.
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Old 01-07-2013, 17:55   #6
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Re: Hot water from cold tap

As a matter of interest do you have a vent/header tank on your system. It may be it can't vent the expanding hot liquid, having said that it would explode/split a connection if it weren't vented. Unfortunately unless I saw the system I can't make constructive comments. Sorry my friend.
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Old 01-07-2013, 19:13   #7
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Re: Hot water from cold tap

Thanks for the comments so far. It has nothing to do with lines running close to engine. Last year the system worked fine, only started this year once the boat went back in the water. There aren't any leaks but I have to think that maybe something froze over the winter.

Allan
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Old 01-07-2013, 19:19   #8
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I have a check valve on the cold water inlet side of my water heater. Prevents temp creep, which is what it sounds like you have.
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Old 01-07-2013, 19:25   #9
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Re: Hot water from cold tap

Yep. Non-return valve cold water inlet.
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Old 01-07-2013, 19:37   #10
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Re: Hot water from cold tap

Quote:
Originally Posted by Wakuu View Post
I have a check valve on the cold water inlet side of my water heater. Prevents temp creep, which is what it sounds like you have.
I have the same issue and I'm betting dollars to donuts this is what it is. Thanks for the info.

Another trip to the chandlery *sigh*

Alex
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Old 02-07-2013, 15:56   #11
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Re: Hot water from cold tap

Two ways this an be happening. First is from expanding water, where the expanding water is pushed back towards the accumulator / expansion tank.

Second way is if the water heater is below the faucets. Then you can get gravity circulation / thermal siphon in either the hot or cold water lines. This as hot water is less dense then cold water and will tend to rise in either the cold or hot system (or both) depending on configuration of the tank and piping.

For the first a check valve will stop the back flow. However if a check valve is added then you need a thermal expansion tank (same think as an accumulator) installed after the Check valve or your T&P relief will pop off during every heating cycle

If you have a accumulator on the hot water system already, it may be water logged and may need to be drained and re pressurized.

For the second a check valve or heat trap (a u bend about 12" high) will prevent thermal siphon.
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Old 03-07-2013, 09:27   #12
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Re: Hot water from cold tap

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Originally Posted by sailorchic34 View Post
For the first a check valve will stop the back flow. However if a check valve is added then you need a thermal expansion tank (same think as an accumulator) installed after the Check valve or your T&P relief will pop off during every heating cycle
I have a new "on demand" pressure pump. If I install a check valve at the inlet to the water heater, do you know if that going to cause a problem regarding how the pump senses pressure in the line?
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Old 03-07-2013, 10:03   #13
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Re: Hot water from cold tap

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Originally Posted by Suijin View Post
I have a new "on demand" pressure pump. If I install a check valve at the inlet to the water heater, do you know if that going to cause a problem regarding how the pump senses pressure in the line?
A check valve will allow the pump to sense a drop in pressure, but not allow the expanded hot water to feed back into the cold line.
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Old 03-07-2013, 15:17   #14
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Re: Hot water from cold tap

As SM said the check valve will not effect the pump at all. It will prevent thermal expansion, so a expansion tank installed after the check valve will keep the T&P relief from popping off every heat cycle of the water heater.
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Old 04-07-2013, 14:43   #15
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Re: Hot water from cold tap

Wouldn't the check valve be on the cold (inlet) side? I'm thinking the rest of the hot water plumbing would be able to absorb the expansion from the heating cycle, but just a wild guess.
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