Undersink heater, when is expansion vessel needed?

[Norcon]

Agile wrote

The extent of that expansion will be well contained within the volume of the 2.8 m of tube so that the cold water at the tap cannot become heated ( and a legionella risk ! ).

How many have you installed like this? And can confirm that you had no problems?

Will nearly 300cc's of expansion really be accomodated inside less than 3m of plastic pipe?
I'd be expecting joints to pop! (Assuming no relief valve fitted)

 

[TonyW2]

Why is the distance to a draw off point relevant to the amount of total expandable distance and available volume ?
QED

 

[Norcon]

Why is the distance to a draw off point relevant to the amount of total expandable distance and available volume ?
QED

3m of 15mm pipe will hold about 300cc and 10/L will expand by about 300cc when heated thus when expansion occurs upstream into the mains it shouldn't reach the cold tap T off.

 

[TonyW2]

Hence the concern relates to the ability of legionnaires to be emitted at the draw off---not to the availability of expansion space.

Do copper pipes lose heat faster than plastic?
Should the supply pipe be insulated?

 

[hazetimesfive]

legionella...

dont be daft.

 

[Agile]

Its easy to make a daft statement!

Much more difficult to make a reasoned technical argument!

Thats why plumbers have such a poor reputation for knowledge and understanding!

Not helped when many left school at 15 !

 

[Norcon]

Whats your reasoning for this Agile?

I dont think copper pipe is going to absorb any expansion

One would assume you think a 3m coil of plastic will absorb enough expansion to cater for 10/L.

 

[ianmcd]

Whats your reasoning for this Agile?

I dont think copper pipe is going to absorb any expansion

One would assume you think a 3m coil of plastic will absorb enough expansion to cater for 10/L.

I never said the copper tube would absorb the expansion I said that the volume of water contained within the copper tube (or any other material for that matter) would absorb the expansion from the 10L of stored water

 

[Norcon]

ianmcd wrote

I never said the copper tube would absorb the expansion

I never said you did.

I said that the volume of water contained within the copper tube (or any other material for that matter) would absorb the expansion from the 10L of stored water

But you do seem to be assuming the extra volume will be catered for upstream in the mains supply.

What happens when either a stop valve, non return valve ,water meter or similar device is fitted and prevents upstream flow?
What happens when the supply pressure is well above that recommended by the manufacturer?

What you did post earlier was this...

agreed its nothing to do with legionella

Actually I think it has. If the cold tap was left unused for a sustained period and warmed constantly via the volume movement in the pipe then I suspect legionella bacteria could proliferate.

the unit uses the length of pipework to absorb any expansion that takes place

A length of 3m x 15mm pipe either plastic or copper won't absorb the expansion. It will allow the heated volume to be contained within its length thus preventing the heated water reaching the cold tap T off. Assuming no restrictions on the supply pipe.

If it was a 28mm pipe then 1.2m will probably be ample. Again the condition of no restrictions on the supply pipe would have to be met.

hence if the length is too short you need to add some other way to absorb the expansion when heated

A loop of pipe 3m long will do the job.
But again its only catering for a volume movement within the pipe and preventing the warmer water reaching the cold draw off T.
Any restrictions and the relief valve discharges. Provided one has been fitted. If not then something may rupture.

 

[matt1e]

To Gasbanni, the hope is that the 10 litres of water will expand back up the supply pipe. The extent of that expansion will be well contained within the volume of the 2.8 m of tube so that the cold water at the tap cannot become heated ( and a legionella risk ! ).

Tony

tony while I can see what you are getting at,I think you are thinking to hard about the reason for it mate
In my view it is more to do with the fact that it is not very nice to go to the sink for a nice glass of water to savor, then to be presented with warm water coming out. Its as simple as that

basically, cold should be cold, hot should be hot.
The op should be fitting an EV in his case anyway so it wont be a problem

As for legionella, not much of a chance to lose any sleep over, honesty

Matt

 

[Agile]

A heater manufacturer will not get sued for a cup of hot water.

They would if the drinker then dies of legionella.

The BBC had an incident affection 79 of whom three died from legionella.

My boss also caught it and he has been unable to drink beer much ever since ( which was a great loss for him ).

The maintenance manager committed suicide!

The BBC were fined £3000 for criminal negligence and we think my boss got a substantial payout too!

 

[matt1e]

A heater manufacturer will not get sued for a cup of hot water.

They they would if the drinker then dies of legionella.

I'm sure they would, but I doubt they have their lawyers at the ready

The BBC had an incident in which one person died from legionella.

My boss also caught it and he has been unable to drink beer much ever since ( which was a great loss for him ).

And how did they contract it?

I'm willing to bet that an unvented 10 litre water heater was not deemed to be the underlying source

Matt

 

[Agile]

No, it was an aircon wet heat cooling tower heat exchanger on the roof of BH.

The water should have been treated monthly with bactericides but had been "overlooked" by the maintenance manager.

 

[matt1e]

No, it was an aircon wet heat cooling tower heat exchanger on the roof of BH.
.

Now that is belevable

 

[TonyW2]

The joint between the incoming cold supply pipe and the water heater is the most likely place for legionella to develop. (The actual vs theoretical likelyhood of bacterial growth is irrelevant to the manufacturers; if the directions are not followed then any consequences fall to the installer)

When the calorifier contents are heated, the expansion would first occur back along the incoming pipe and at its maximum the volume expanded would finish less than 2.8. away. If the cold draw off is further away than that point, then the crud would not be drawn off.

The distance away of the draw off point would be irrelevant for any pressure rise concerns.Hence the manufacturers are only covering themselves against legionella concerns, not about any operational or pressure issues.

The requirement for a pressure vessel if less than 2.8m away, is to ensure that in this situation the crud is not passed to the draw off point, as the movement caused by any expansion is absorbed by the pressure vessel that is positioned in the shorter pipe distance.

The fact that the water from the cold draw off may be warm, when drunk, would be a warning sympton that contamination may have occurred, but a draw off point does not always have to be used for drinking. (eg cold shower spray)