Expansion tank - sizing of All aspects of plumbing questions and answers, help, tips and information

[scody]

Hi all.
I've searched the Internet for a definitive answer to this but no luck so far.
Background
I live in a small cottage in the north of Scotland and am currently ripping out and renewing my central heating (yes, in the Autumn, like an idiot) ahead of receiving a new range cooker/boiler in the next month or so.

I've removed the old open-topped expansion tanks for the heating and hot water systems and will buy new closed-unit ones that don’t fill up with dust, spiders and suicidal mice. The existing hot water cylinder is about 120 litres capacity, and the old expansion tank was 25 gallons so about 114 litres. I know that the conventional rule is to have an expansion tank volume that matches the hot water cylinder so that the replacement water volume is there for when the cylinder is used for showering or running a bath. We don’t require that however since we have an electric shower.

So my question is, can I safely reduce the hot water expansion tank to about half the volume since the only thing we really need the hot water for is kitchen and bathroom sink taps. The volume of hot water needed to fill a washing basin doesn’t require a 114 litre tank in my view – 70 is enough.

Can anyone see any reason why this would be a problem? (Remember – no bath and no shower required.)

Many thanks.

[stoneyboy]

Hi scody,
Provided you have a consistent mains supply at a reasonable pressure you could use a 114 litre feed and expansion tank.
Regards S

[scody]

Many thanks for replying.
I confess I'm probably being a bit slow however; the original cold water tank was 114 litres (25 gallons) so my question was can I safely reduce that? Originally I was thinking 68 litres but am now thinking I'll hedge a little and go for 82 litres. I haven't calculated the size of the hot tank but I believe an 82 litre cold tank would be about 70% of its volume. Given that I have no bath or gravity-fed shower I think that's safe. (Mains pressure is excellent, and it's a bungalow so the loft is low.)

Any views?

[stoneyboy]

Hi scody,
Yes you could go for the smaller tank but the 114litre tanks can be cheaper than the smaller sizes.
Regards S

[scody]

That's great, appreciate the reply - many thanks.
I'm not bothered about the price too much but you're right, it can often seem like there's little logic in the cost of things these days.

Cheers.