Dhw expansion vessel combi boilers

[Matt0029]

Most manufacturers ask for a small dhw expansion vessel on combi boiler installs if a water meter is fitted as this stops the expansion of the heated hot water from expanding in to the mains. But if there is a large distance between the boiler and the water meter is one not needed? Like when installing a undersink heater you can allow the expansion to go back in to the mains if there is a larger enough distance (I think 1.4meters for a 5litre under sink heater) to the nearest cold water draw off point.

 

[Scott_D]

I dont think there is a set distance.
I fit them but dont check them on an annual service so its just to tick a box

 

[EvilDrPorkChop]

Most manufacturers ask for a small dhw expansion vessel on combi boiler installs if a water meter is fitted as this stops the expansion of the heated hot water from expanding in to the mains. But if there is a large distance between the boiler and the water meter is one not needed? Like when installing a undersink heater you can allow the expansion to go back in to the mains if there is a larger enough distance (I think 1.4meters for a 5litre under sink heater) to the nearest cold water draw off point.

Think it's more for water hammer on components rather than expansion from any heat.

Vaillant don't ask for them, but I have started fitting them where there's a meter or high water pressure now.

 

[Matt0029]

I dont think there is a set distance.
I fit them but dont check them on an annual service so its just to tick a box

Thanks but if the water meter is say in the street I'd guess there would be enough pipework to absorb any expansion?