Undersink heater, when is expansion vessel needed?

[matt1e]

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)

where is the legionella going to be coming from?

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.

What calorifier? there isn't one

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.

they are not,as legionella poses no threat
they are being so tight as to not supply a vessel by hopefully using the mains to absorb the back pressure and as such are covering against the result
ie the chance of some of the heated water coming out the cold

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.

nope nowt to do with crud,all about expansion relief (although legionella do indeed feed off crud/sludge etc)
oh and a check valve is also req for that to work

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)

warning of contamination of hot water into the cold - YES
warning of possible legionella - NO

You are scare mongering a bit now
one last time, for a number of reasons

a mains fed directly heated 10 litre unvented water heater is not deemed to be a legionella risk

 

[TonyW2]

"additional accessories may be needed to comply with the Water Regulations: If your nearest cold water draw off is close to the heater, you will need to fit a check valve and expansion vessel" ( M/f instructions)

Are the Water Regulations concerned with the sensory niceties of a cold water tap supply or bacterial contamination?


(Calorifier---sorry, meant container.)

 

[matt1e]

"additional accessories may be needed to comply with the Water Regulations: If your nearest cold water draw off is close to the heater, you will need to fit a check valve and expansion vessel" ( M/f instructions)

your point with the above statement tony?

Are the Water Regulations concerned with the sensory niceties of a cold water tap supply or bacterial contamination?

both