Shot of hot water from cold water taps

[De.windhoos]

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 haven’t 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 isn’t.
Anyone got a clue?

 

[johnalison]

I would have thought that somewhere in the line the cold water hose is alongside the hot. I'm not sure from your description if you have checked the whole run.

 

[De.windhoos]

I would have thought that somewhere in the line the cold water hose is alongside the hot. I'm not sure from your description if you have checked the whole run.

Yup, checked the whole run, and I’m not talking about lukewarm but fairly hot water

 

[sarabande]

Does this happen if the water is running slowly AND fast ?

How much cold water comes through before you get the 'shot' of hot water ?

What is the internal diameter of the cold water supply pipe ?

 

[mersey]

Do you have an expansion tank? Still working fine? NRV before the calorifier, that's working?

As its just started happening, I'm wondering if one of those has failed and hot water is making it back into the cold line that feeds the calorifier.

 

[lw395]

Is there a temperature limiting mixer valve anyhwere?
Thermostatic shower valve?

Some of these can allow hot water to flow back up the cold line if the hot is at too high a pressure compared to the cold.
There should be an NRV in their cold feed.

 

[VicS]

^^^^ What they say above... check the NRV in the feed to the hot water system


The expansion tank should maintain the pressure in the hot water system but when you open a cold water tap the pressure in the cold water system will fall until the point is reached when the pump cuts in.

If the NRV is not holding the hot water back there will be a short time when hot water will flow back from the hot water system into cold water system........................ at least that's my explanation

 

[vyv_cox]

It depends just where the T between the cold supply and the supply to the calorifier has been placed. This is commonly at the calorifier itself, i which case expanded hot water can emerge into the cold supply. The usual remedy for this is a NRV at the calorifier inlet and a second accumulator in the hot hose to the tap.

 

[lw395]

Air in the calorifier heats up and pressurises the hot side, above the pressure of the cold side, if there is an NRV on the calorifer input..
The obvious flow path is out of the calorifier inlet, but that should be at the bottom, so cold water 'should' come out.
Any mixer tap can allow hot water to flow across it to the cold side as the pressure equalises.
A NRV on its cold input will stop that.
Making sure your calorifier does not sit with lots of air in it would cure the problem at source, but it's not easy to achieve that on both tacks.

So the thing to do is see if there is an NRV separating hot from cold, and whether it works.

Some boats, the calorifier can get very hot if the engine works hard.

 

[sarabande]

Shirley it's possible to measure the amount of water that comes out before the hot shot. Then with the known internal diameter, find pretty accurately the length of the pipe where the hot water 'slug' is created ?

 

[superheat6k]

Just done this on my boat last weekend. The NRVv to the calorifier was scaled open and the heat in the calorifier caused a veritable steam engine to build up and blast scalding hot water into the cold system. About £5 for a new NRVv or check valve from screwfix fixed it.