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Old 18-05-2018, 03:28   #1
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Question shot of hot water from cold water taps

Hi,

Since a couple of days we are getting a shot of hot water out of the cold water taps. This happens after running the water a couple of seconds.

We havenít changed anything in the tubing, heater or pump.
We have a Quick water heater operating on shore power or via the engine cooling system and a Sureflow water pump.

I checked if the cold water line is touching the hot water output from the heater and it isnít.
Anyone got a clue?
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Old 18-05-2018, 03:35   #2
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Re: shot of hot water from cold water taps

Quote:
Originally Posted by De.windhoos View Post
Hi,

Since a couple of days we are getting a shot of hot water out of the cold water taps. This happens after running the water a couple of seconds.

We havenít changed anything in the tubing, heater or pump.
We have a Quick water heater operating on shore power or via the engine cooling system and a Sureflow water pump.

I checked if the cold water line is touching the hot water output from the heater and it isnít.
Anyone got a clue?
Same here actually so subscribing. Not a huge problem so haven't looked into it much. I wonder if the hot water tank somehow backflows and supply the cold water as well? Same setup, but with their pressure reservoir as well.
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Old 18-05-2018, 03:54   #3
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Re: shot of hot water from cold water taps

If you can get to the water lines with your hands, especially those near the heater, I'd suggest feeling along them after the water's not been run for a while (an hour or so, say) to find out if heat is leaking into a spot along the cold pipe.
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Old 19-05-2018, 04:40   #4
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Re: shot of hot water from cold water taps

When the water heater heats up, the water expands. If there's no backflow preventer valve, or the valve has failed, it'll forced heated water back out the "cold in" line. It can build up quite a bit of pressure.
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Old 19-05-2018, 05:52   #5
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Re: shot of hot water from cold water taps

Quote:
Originally Posted by CaptTom View Post
When the water heater heats up, the water expands. If there's no backflow preventer valve, or the valve has failed, it'll forced heated water back out the "cold in" line. It can build up quite a bit of pressure.
Indeed.
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Old 19-05-2018, 06:01   #6
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Re: shot of hot water from cold water taps

If your cold water supply feeds into the hot water heater via a T fitting on its way by then local heating by diffusion will heat water resting in the cold branch close to the heater feed. I have the identical problem at my cottage.
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Old 19-05-2018, 08:21   #7
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Re: shot of hot water from cold water taps

The problems already mentioned are real, but they're slow to generate heat and don't start happening overnight.

If you're suddenly getting shots of hot water every time the pump cycles, then you probably have air in the hot water tank. This air gets pressurized by your water pump and then when you open the faucet the pressurized hot water tank pushes hot water out the cold inlet.

The easiest way to get air into your hot water tank is to run out of water and then prime the system with the hot water faucet open. This allows the air to get pushed into the hot water tank where it gets trapped. It is usually only necessary to prime the cold side, which avoids the problem.

Getting the air out probably won't be easy unless you have a bleed valve at the top of your tank. If you don't do anything, the water will slowly absorb the air until it is gone. It could take a while. Absorption rates go up with pressure, but down with temperature.

You also might work around the problem by briefly opening the hot water before you run the cold. That will release the pressurized hot water into the hot water plumbing where it belongs instead of the cold.
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Old 19-05-2018, 08:35   #8
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Re: shot of hot water from cold water taps

Quote:
Originally Posted by CaptTom View Post
When the water heater heats up, the water expands. If there's no backflow preventer valve, or the valve has failed, it'll forced heated water back out the "cold in" line. It can build up quite a bit of pressure.
This. Clean or replace the non-return valve.
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Old 19-05-2018, 09:58   #9
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Re: shot of hot water from cold water taps

I’ve also seen cases where a “water saver” shutoff was installed on a shower head, then the faucet left open and only the saver valve on the shower head closed... bridging the cold and hot sides in the faucet. Drawing “cold” drops the pressure on that side and the hot side makes it up (was so bad at one marina in the BVI that it was impossible to take a shower without being scalded even using only the “cold” side (and the offending valve was on the ladies side, inaccessible to me.
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Old 19-05-2018, 10:19   #10
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Re: shot of hot water from cold water taps

Do you have separate taps or a mixer tap which can cause this when the seals leak hot water back down the cold circuit.
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Old 19-05-2018, 11:58   #11
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Re: shot of hot water from cold water taps

Quote:
Originally Posted by De.windhoos View Post
Hi,

Since a couple of days we are getting a shot of hot water out of the cold water taps. This happens after running the water a couple of seconds.

We havenít changed anything in the tubing, heater or pump.
We have a Quick water heater operating on shore power or via the engine cooling system and a Sureflow water pump.

I checked if the cold water line is touching the hot water output from the heater and it isnít.
Anyone got a clue?
Itís PFM, donít sorry about it and go have a beer.
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Old 19-05-2018, 12:20   #12
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Re: shot of hot water from cold water taps

All the boats Iíve done anything to on the plumbing had a check valve fitting in the cold water line at the water heater. Yours may be leaking.
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Old 19-05-2018, 13:04   #13
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Re: shot of hot water from cold water taps

What color is your hull? I have a dark blue hull which gets very hot in the Caribbean sun....some cold water lines run close to and along the hull, resulting in hotter than ambient water at the cold tap. Took awhile to figure that one out!
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Old 19-05-2018, 16:15   #14
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Re: shot of hot water from cold water taps

Quote:
Originally Posted by River Cruiser View Post
All the boats Iíve done anything to on the plumbing had a check valve fitting in the cold water line at the water heater. Yours may be leaking.
This. There is another thread here where people are saying you do not need a check valve.
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Old 19-05-2018, 22:32   #15
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Re: shot of hot water from cold water taps

I also have a Quick water heater.
It has a pressure relief valve that drains to the bilge to
release the pressure that builds up as the water is heated
It is likely that this valve has failed / jammed / or become blocked by deposits on the valve seat.
Domestic water heaters also have similar pressure relief valves.
Starting from cold, try heating your tank by electric power or engine coolant and observe whether the PRV discharge weeps a little water. If it does not it is probable that the valve is blocked and the pressure in the tank is forcing water into the cold input line

EDIT
Here is a link to the manual

https://www.keoghsmarine.com.au/imag...rB3_Manual.pdf
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