Expansion Vessel

[Keeping Up]

The pressure will never be near to zero, and there is nothing on the 'hot' side to drive the water backwards. I repeat my challenge to anyone to explain how hot water can be 'drawn off' into the cold side. I'll state it again:

 

"Let us assume that the system initially has no NRV and no hot-water expansion vessel, but has a pump (operated by a pressure switch) and an accumulator. When a cold tap is opened, the pressure is slightly reduced; some water flows out of the accumulator and some later flows back in; this water is all supplied from the cold tank by the pump. I cannot see any mechanism here which causes hot water to "fall" into the cold"

 

Can anyone state, rather than guess, how the lack of an NRV could reduce the amount of hot water available.

[chris w]

Let us assume that the system initially has no NRV and no hot-water expansion vessel, but has a pump (operated by a pressure switch) and an accumulator. When a cold tap is opened, the pressure is slightly reduced; some water flows out of the accumulator and some later flows back in; this water is all supplied from the cold tank by the pump. I cannot see any mechanism here which causes hot water to "fall" into the cold.

Allan

 

When a cold tap is opened, the pressure will drop to zero (well zero barg) over the first few seconds. It takes 5 or 6 seconds on mine for the cold water pressure gauge to fall to zero when a hot ot cold water tap is opened. As the pressure drops and passes the water pump's cut-in pressure (typically around 20psig) the pump will turn on. So the pressure won't be "slightly reduced" it will be fully reduced.

 

I added an NRV to one of the boat's in my marina because the guy was getting hot water out of his cold tap for a couple of seconds. That fixed the problem immediately.

 

Chris

Edited December 14, 2008 by chris w

[frangar]

Blimey!!

 

Need to go for a sit down because for one me and Chris W agree on something.......

[Natalie Graham]

Blimey!

 

Reading this lot I think there's much to be said for keeping the water in a brightly painted jug on the cabin roof.

 

Natalie.

[Big COL]

The pressure will never be near to zero, and there is nothing on the 'hot' side to drive the water backwards. I repeat my challenge to anyone to explain how hot water can be 'drawn off' into the cold side. I'll state it again:

 

"Let us assume that the system initially has no NRV and no hot-water expansion vessel, but has a pump (operated by a pressure switch) and an accumulator. When a cold tap is opened, the pressure is slightly reduced; some water flows out of the accumulator and some later flows back in; this water is all supplied from the cold tank by the pump. I cannot see any mechanism here which causes hot water to "fall" into the cold"

 

Can anyone state, rather than guess, how the lack of an NRV could reduce the amount of hot water available.

 

Alan

I'm with you on this one. Your description is the system I have on my boat.

I also fail to see just how water can fall back, on reducing the pressure on the cold side there will be a small amount of water falling back, this action will create a vacuum in the space created in the top of the calorifier by the action of the water falling back. This vacuum will then stop any further possibility of water falling back.

The same principle as while you hold your thumb on the end of a pipette no fluid can leave the pipette, removing your thumb allows the fluid to empty. Therefor for water to run back from a calorifier there must be some ingress of air on the hot side, which is unlikely, as this would show itself as a leak when the system is pressurised for normal use. Unless I have the physics wrong, water falling back just can not happen, or shouldn't.

[David Schweizer]

This is all very interesting, but way over the top, we are talking about boats not Nucler Power stations. I do not have an accumulator, I do not have a NRV, and guess what? the water still comes out of the taps.

 

I really sometimes wonder whether some people just like to make things over complicated in order to increase the chance of getting mucky mending them when they go wrong.

 

P.S I think Natalie has got it just about right.

[RLWP]

Blimey!

 

Reading this lot I think there's much to be said for keeping the water in a brightly painted jug on the cabin roof.

 

Natalie.

 

Except as the water heats up in the sun it will drip from the lip onto the cabin roof. Perhaps you should arrange a guttering system for this..

 

Richard

 

And the hot mixes with the cold!

[frangar]

This is all very interesting, but way over the top, we are talking about boats not Nucler Power stations. I do not have an accumulator, I do not have a NRV, and guess what? the water still comes out of the taps.

 

I really sometimes wonder whether some people just like to make things over complicated in order to increase the chance of getting mucky mending them when they go wrong.

 

P.S I think Natalie has got it just about right.

 

 

I do agree with you David...its just I want hot water for a bit longer!!!

 

Oh and when I have been blacking my water tank I have used my can....you get funny looks at water points these days I tell you!!

 

As I said before I will fit the valve and see what happens....then let you know!

 

 

Cheers

 

Gareth

[Keeping Up]

When a cold tap is opened, the pressure will drop to zero (well zero barg) over the first few seconds.

OK, if you've observed it then I stand corrected. But I still don't understand how any hot water can flow back to a cold tap, unless there is a significant volume of air in some form of expansion vessel on the hot side.

 

Unless it's all in the comment that someone made somewhere, that a horizontal calorifier will always have some air-space at the top whereas a vertical one won't. So maybe you don't need a NRV is you have a vertical calorifier and no expansion vessel? I may just try removing mine and seeing if it makes any difference; as far as I can see its only function is to be a nuisance when I'm trying to drain the system (I have to open a drain tap on the top of the calorifier, to let some air in)

[chris w]

.............. its only function is to be a nuisance when I'm trying to drain the system (I have to open a drain tap on the top of the calorifier, to let some air in)

Put the NRV in the cold line just before the calorifier and you won't have this issue anymore.

 

Chris

[Keeping Up]

Put the NRV in the cold line just before the calorifier and you won't have this issue anymore.

Ah but then I'd need an expansion vessel (isn't this where we came in?)

[denboy]

As of about 1o minutes ago i maybe tempted to put an expansion tank in as my calorifier went BANGDoh

[frangar]

As of about 1o minutes ago i maybe tempted to put an expansion tank in as my calorifier went BANGDoh

 

 

What really bang??....with water everwhere.....OOPPS!!!

 

Was it a marine grade one??? I have been told that they have a life expectancy of 10 years (according to Cleghorn Warning) which means I'm on borrowed time!!!

 

Gareth

[denboy]

What really bang??....with water everwhere.....OOPPS!!!

 

Was it a marine grade one??? I have been told that they have a life expectancy of 10 years (according to Cleghorn Warning) which means I'm on borrowed time!!!

 

Gareth

Yes BIG bang 4 years old marine grade me forget turn immersion off silly me ah well just searching interwebby for a new one Doh £220 cheapest so far

[Ray]

OK, if you've observed it then I stand corrected. But I still don't understand how any hot water can flow back to a cold tap, unless there is a significant volume of air in some form of expansion vessel on the hot side.

 

Unless it's all in the comment that someone made somewhere, that a horizontal calorifier will always have some air-space at the top whereas a vertical one won't. So maybe you don't need a NRV is you have a vertical calorifier and no expansion vessel? I may just try removing mine and seeing if it makes any difference; as far as I can see its only function is to be a nuisance when I'm trying to drain the system (I have to open a drain tap on the top of the calorifier, to let some air in)

 

 

Is it a case of the water in the calorifier expanding as it heats up. Because it can't get out of the calorifier the way it is meant to, the pressure builds up until a tap is opened, whereupon the pressure in the calorifier forces water back out of the bottom and into the main supply pipe. If the open tap is close to the calorifier it is probable that the water from the calorifier, being under pressure, will endeavour to escape through the tap. Because the pressure from the calorifier keeps the supply pipe "charged" the pump won't cut in until (virtually) all the calorifier's pressure has been exhausted.

[RLWP]

Is it a case of the water in the calorifier expanding as it heats up. Because it can't get out of the calorifier the way it is meant to, the pressure builds up until a tap is opened, whereupon the pressure in the calorifier forces water back out of the bottom and into the main supply pipe. If the open tap is close to the calorifier it is probable that the water from the calorifier, being under pressure, will endeavour to escape through the tap. Because the pressure from the calorifier keeps the supply pipe "charged" the pump won't cut in until (virtually) all the calorifier's pressure has been exhausted.

 

And if you have any air trapped in the calorifier it will make this effect more pronounced.

 

Richard

[frangar]

Yes BIG bang 4 years old marine grade me forget turn immersion off silly me ah well just searching interwebby for a new one Doh £220 cheapest so far

 

Sorry to ask this but did you have a pressure relief valve fitted??? - cue another long argument!!

 

Gareth

[denboy]

Sorry to ask this but did you have a pressure relief valve fitted??? - cue another long argument!!

 

Gareth

Yes failed miserably

[Keeping Up]

Is it a case of the water in the calorifier expanding as it heats up. Because it can't get out of the calorifier the way it is meant to, the pressure builds up until a tap is opened, whereupon the pressure in the calorifier forces water back out of the bottom and into the main supply pipe. If the open tap is close to the calorifier it is probable that the water from the calorifier, being under pressure, will endeavour to escape through the tap. Because the pressure from the calorifier keeps the supply pipe "charged" the pump won't cut in until (virtually) all the calorifier's pressure has been exhausted.

And if you have any air trapped in the calorifier it will make this effect more pronounced.

 

If you don't have any air trapped in the calorifier, or an expansion vessel, the pressure build-up would be enormous unless it was released through a PRV or a tap. My whole point is that water is not compressible, the calorifier and pipework cannot expand (well maybe the tiniest bit, but not enough that you coule measure!). Even if the hot part of the system is all at full pressure, the release of just a few cc of water back down the pipework would be enough to eliminate the pressure totally.

 

If you have a vertical calorifier, you cannot get air trapped in it. The take-off is at the very top.

 

I still cannot see any way, without an expansion vessel or trapped air (such as may be the case in a horizontal calorifier) in which the NRV could play any part in keeping the hot water in its rightful place.

[David Schweizer]

What really bang??....with water everwhere.....OOPPS!!!

 

Was it a marine grade one??? I have been told that they have a life expectancy of 10 years (according to Cleghorn Warning) which means I'm on borrowed time!!!

 

Gareth

Ten Years??? the calorifier in my boat is 22 years old, and showing no signs of leaking or mishaping.

 

Another "Theory" gone wrong, or perhaps things were made better in the past.

Edited December 14, 2008 by David Schweizer

[bottle]

Yes BIG bang 4 years old marine grade me forget turn immersion off silly me ah well just searching interwebby for a new one Doh £220 cheapest so far

 

Did the thermostatic control also fail or do you not have one?

[frangar]

Ten Years??? the calorifier in my boat is 22 years old, and showing no signs of leaking or mishaping.

 

Another "Theory" gone wrong, or perhaps things were made better in the past.

 

Well I did wonder myself and you have to bear in mind they sell new ones.....so they have a vested interest!! you can hear the conversation now..... "it went bang sir? Well it was 11 years old and the only last 10!"

 

And ones in houses last for years and years.........apart from frost damage and maybe operating at a higher than spec'ed pressure (eg using one rated for a gravity feed eg a house in a boat) then I cant really see what there is to fail.....mind you like you say if modern steel is anything to go by perhaps the copper corrodes now........

 

Did the thermostatic control also fail or do you not have one?

 

 

The immersion heater I have has 2 stats....one you adjust for the water temp...set to the recommended 60C and a high temp cut out set at 95c which you have to manually reset if it trips..this is a safeguard against the 1st one failing.....I have noticed that the ones sold by swindlers dont seem to have the high temp one fitted.......might make them cheaper but obviously not safer!.....Think I got mine from a either Cleghorn Warning or a yacht chandlers in Southend!

 

Cheers

 

Gareth

Edited December 14, 2008 by frangar