split/double/size of cylinder if our water needs vary?

StephenStephen · 8 Jan 2019

Hi all,

We're about to have a new heating system installed
(previous thread for a bit of history: https://www.diynot.com/diy/threads/too-many-boilers-for-the-22mm-pipe.511647/ )

Most of the time we don't use a lot of hot water, and our 117 litre cylinder is more than big enough.
Sometimes we have visitors and need more hot water (at present we put on a second boiler with it's own tank).

The new system will be 1 boiler with a 300 litre tank

I (foolishly) didn't tell the heating engineers before getting the quote that most of the time our hot water need is a lot less than that.

I'm concerned that most of the time we'll be heating a lot more water than we need.
Should I get back to them and ask if we have some kind of 2 smaller tank system or something? Or does it not make a lot of difference to energy usage?

(Here's what we're having installed if we go for the current quote:
Worcester 35CDI System boiler
300 litre Worcester Greenstore unvented indirect hot water cylinder.)

Thanks for reading, and any thoughts, much appreciated, Stephen

flameport · 8 Jan 2019

StephenStephen said:
I'm concerned that most of the time we'll be heating a lot more water than we need.

Not an issue - heat losses from modern cylinders are tiny, and the difference between heat loss on a large cylinder and a smaller one is a few pence a day at most.

StephenStephen · 8 Jan 2019

flameport said:
Not an issue - heat losses from modern cylinders are tiny, and the difference between heat loss on a large cylinder and a smaller one is a few pence a day at most.

Thank you, very much appreciated.

flameport · 8 Jan 2019

For your specific cylinder, details here: https://www.worcester-bosch.co.uk/products/cylinders/directory/greenstore-unvented-cylinders
standing losses are 76W for the 300 litre, and 40W for the 120 litre, a difference of 36 watts, or 0.864kWh per day, 315kWh per year.

With gas at about 4p per kWh, and using 5p to cover the fact that no boiler is 100% efficient, that's £16 per year - and that also assumes the cylinder is at maximum temperature 100% of the time and the heat loss in the winter isn't actually contributing to heating the building either.

The majority of losses occur in the pipework, so make sure all of it is properly insulated.