"Dave Plowman (News)" wrote:

"riccip" wrote:
It's reasonable to assume a flat would have a small boiler,
otherwise it depends on the household demand for hot water.

Since you're talking about a storage system, an immersion just outputs 3
KW. Any half decent central heating system will input a great deal more
than that to the storage tank.

And therein lies the problem. Using your boiler just to heat HW
alone is a bit like putting a V8 engine into a moped. That's why
combi-boilers were invented (although personally I hate the
wretched things!).

An immersion heater is 100% efficient from the moment it switches
on and requires minimal energy to maintain optimum water
temperature. A gas boiler can be as low as 50% efficient when
heating just the copper cylinder. The system has a warm up period
where no heat is being transferred. Much of the heat produced by
burning gas disappears up the flue and more is lost from the
pipework runs to the HW cylinder, often positioned some distance
from the boiler.

On cold winter mornings any boiler is best left to heat the house ASAP
if "Economy 7" electricity (or it's equivalent) is available.

Then switch on the boiler earlier. And cheap rate electricity isn't
economical for limited use.

It's not that simple an equation. A boiler works hardest when
warming the house from cold. Once the desired temperature has
been reached an efficient CH system with TRVs burns considerably
less gas maintaining those temperatures. So the sooner desired
room temperatures are reached the better and more economical.
Therefore if the HW cylinder has already been heated on Economy 7
the boiler will more rapidly heat the house.

It's expensive to use unless you have cheaper night-time
electricity, in which case you'd set the timer to heat it at
night.

Also during the summer when you don't use your CH it can
be cheaper to turn the boiler off and just heat the water by
electricity.

Again, it would be a very inefficient gas boiler/system where full price
electricity would be cheaper. And running the boiler every day for hot
water keeps the pump in good fettle.

Which is why I said "cheaper night-time electricity". However in
the summer there is no more expensive way to heat water than
leaving a conventional boiler cycling over the HW system alone,
unless you mean combi-boilers?

Why would it cycle? You set the programmer to heat the cylinder then
switch off.

Ask amongst your friends and you will be surprised at the number
of people who leave the boiler on all day to keep the water hot.
Even if it's timed for an hour it will begin to cycle according
to the boiler's stat, thus wasting an incredible amount of gas.
As HW is used, even a minimal amount, the boiler works hard to
reheat that portion of the cylinder.

And if you left an immersion on all the time it would cost even more.

Certainly on peak-time electricity it would. However peak
electricity shouldn't necessarily be considered taboo. Take an
electric shower for example. Heavy on electricity but only for a
few minutes, offering considerable savings over a gas heated
shower arrangement.

riccip