Expansion tank

[VADROUILLE]

can anybody confirm where the hell an expansion tank is supposed to go. im fitting a new water heater and its telling me that an expansion tank is supposed to go on the cold water side. i bought an expansion tank specifically for hot water and i want to put it immediatly after the water heater.

i allways thought you had an accumaltor on the cold side and an expansion tank on the hot side? im getting conflicting advice and im now stuck untill i get a for sure answear!

cheers

 

[nrbx]

I wouldn't have thought it mattered, as it's job is to absorb the expansion of the hot water as it heats.

Putting it right next to the cold inlet valve would mean you store cold water as it expands as opposed to hot water that will then cool down and result in a cold/hot mix coming out to start with when used.

I could of course be wrong, just how my simple mind is viewing it.

 

[sailorman]

I wouldn't have thought it mattered, as it's job is to absorb the expansion of the hot water as it heats.

Putting it right next to the cold inlet valve would mean you store cold water as it expands as opposed to hot water that will then cool down and result in a cold/hot mix coming out to start with when used.

I could of course be wrong, just how my simple mind is viewing it.

it is fitted between the pump & outlet tap/calorifier.
it gets pressurised by the pump & is designed to stop the pump "pulsing" & give a smooth flow of water

 

[rjcoles]

Have a look at
http://www.surecal.co.uk/Product/AccumulatorTanks.aspx

 

[nrbx]

it is fitted between the pump & outlet tap/calorifier.
it gets pressurised by the pump & is designed to stop the pump "pulsing" & give a smooth flow of water

No what you described is an accumulator tank, and expansion tank is designed to take the expansion of hot water as it heats to avoid having to vent it and waste it.

 

[vyv_cox]

As far as I can see an accumulator tank and an expansion tank are exactly the same thing, with the possible difference that one may have different materials designed for hot water, although I doubt it. Pressure acts equally in all directions, so it doesn't matter in the slightest where your accumulator/expansion tank is sited.

Putting it between the pump and the calorifier will work fine, with the following riders:
If there is a nrv at the tank entry (and if it works!) it will not accommodate expansion.
If the tank was completely full of hot water some might flow back sufficiently to reach the cold supply. Unlikely and not really a problem, except, from another thread, that bacteria just might get into drinking water. Dosing the fresh water tank should prevent this anyway.

My calculation says that increasing the temperature of 25 litres of water from 20 to 80C will result in an expansion of 0.31 litres. These would be very optimistic figures and reality is probably that most calorifiers only reach 60C for no more than 3/4 of their content, for which expansion would be 0.15 litres or less. This should be fine for a moderately-sized accumulator.

 

[pvb]

My calculation says that increasing the temperature of 25 litres of water from 20 to 80C will result in an expansion of 0.31 litres.

Does this allow for the increase in capacity of the calorifier due to expansion?

 

[vyv_cox]

Does this allow for the increase in capacity of the calorifier due to expansion?

I think that would be a calculation too far. The whole thing is so much of a guess anyway.

 

[VADROUILLE]

Well thanks for the advice, i decided not to fit it in the end, it was a 2l tank and i did not have enough spare room/pipe to fit it by the water heater.

i could fit an accumalator tank by the water pump if i can be bothered i suppose, but its not an essential, i switch the water off when i dont use it anyway!

it would be interesting to know if there is any major differance between the two tanks though? (accumalator/expansion)

 

[pvb]

I think that would be a calculation too far. The whole thing is so much of a guess anyway.

So your calculation of "0.31 litres" is, in fact, meaningless?

 

[alan17]

calculation

I guess that Vyv is just making the point that there is in fact very little expansion involved.

 

[daveyw]

As discussed in this thread http://www.ybw.com/forums/showthread.php?t=202338 I just use a fanta bottle tee'd in after the pump.

 

[vyv_cox]

So your calculation of "0.31 litres" is, in fact, meaningless?

I await your revised calculation with interest. Don't forget that calorifiers may be made of copper or stainless steel, so do both, please. Also I think you should allow for various lengths and types of plastic hose, reinforced and unreinforced nylon, rigid pvc and, just in case, copper. perhaps you could also do another to allow for the fact that an immersion heater might be fitted and set at a range of higher figures, say from 60 to 80 C in two degree increments.

After that you might consider whether it is advisable to be such a smartarse.

 

[skomer]

Does this mean that you've fitted the water heater but not the expansion vessel on a sealed system?

 

[VADROUILLE]

Does this mean that you've fitted the water heater but not the expansion vessel on a sealed system?

Depends what you mean by sealed, unless the tev passes water on the water heater or i open a tap, then yes, i suppose it is sealed. but its allways been like that on here, the expansion tank was an extra i was going to fit and now decided against. so theoretically, all i have done now is replaced the water heater, ive not touched the original factory fitted system, that worked before and after i fiited the water heater.

 

[vyv_cox]

Does this mean that you've fitted the water heater but not the expansion vessel on a sealed system?

My Sadler34 came factory fitted with a pressurised water system and a calorifier but no accumulator. AFAIK all the boats wirh calorifiers were the same. I fitted an accumulator when the boat was about 15 years old simply to stop the incessant racket of the pump. Never had any water leakage from the calorifier, which would have been very obvious on my boat.

 

[Dave_Seager]

Ther are two factors to consider here.

The original question referred to the expansion of water as it is heated in the calorifier. Unless some provision is made for this expansion then water will be dumped from the pressure relief valve.

However, you also need to consider the pressure changes caused by the pump. An accumulator will allow some pressure to be stored in the system, allowing the pump to work for longer on and off periods rather than in short bursts.

If you fit the expansion tank/accumulator before or after the calorifier then it will be effective for both purposes. However, the water flow will be different.

If the tank is fitted immediately after the pump then the expansion of water in the calorifier will push a little water back through the pipework into the tank. However, this will come from the bottom of the calorifier which will usually be quite cold anyway.

If the vessel is fitted after the calorifier then it will receive a little hot water as the calorifier heats up. Some heat will be lost but this is probably insignificant. However, when a cold tap is used the pressure will rise and fall as the pump switches on and off. This will push water back and forth through the calorifier to reach the vessel and might cause the hot and cold water to mix enough to reduce the efficiency of the calorifer.

I think that this is why an accumulator is normally fitted after the pump.

 

[Bilgediver]

can anybody confirm where the hell an expansion tank is supposed to go. im fitting a new water heater and its telling me that an expansion tank is supposed to go on the cold water side. i bought an expansion tank specifically for hot water and i want to put it immediatly after the water heater.

i allways thought you had an accumaltor on the cold side and an expansion tank on the hot side? im getting conflicting advice and im now stuck untill i get a for sure answear!

cheers

The expansion tank is just another name for a small accumulator and it goes in the hot water circuit after the calorifier and helps prevent that problem of peeing relief valves on the calorifier when the water goes from cold to hot.

The accumulator goes in the line from the pump to absorb pulses etc.

 

[skomer]

Depends what you mean by sealed, unless the tev passes water on the water heater or i open a tap, then yes, i suppose it is sealed. but its allways been like that on here, the expansion tank was an extra i was going to fit and now decided against. so theoretically, all i have done now is replaced the water heater, ive not touched the original factory fitted system, that worked before and after i fiited the water heater.

It needs some method of taking up the expanding water as it's heated, either in the form of a special tap that will drip, an expansion vessel or an air gap in the calorifier. It may have a pressure relief valve fitted that will release the pressure should it get too high, but this should be a safety device that only activates if the expansion vessel fails. If the pressure relief valve continually opens to relieve pressure every time the calorifier heats up then the prv will eventually fail and drip all the time, as it's not designed to work this way. If your system has no expansion vessel or other way of taking up the expansion and no prv, if the thermostat in the immersion heater fails the water will boil, causing massive pressure to build up in the system with obvious results.