curious wrote:

I am thinking of changing from a warmflow bluebird oil boiler to gas but
don't know whether to go for a combi gas boiler or stay with my current
setup of stored hot and cold water. We had a water cut of 3 days in the
new year and stored water was a blessing so what are the main advantages
of a combi. Gas heating is still in the minority in Northern Ireland at
the moment but catchingup.

I'm sure this is a subject on which there will be divided opinions,
but mine is that stored cold and hot water is best, with both at
low pressure of course.

As far as I can see the only disadvantantages of this are that
(1) low pressure means you can't have high pressure showers unless
you either install a special pump or else go for an electrically
heated shower which takes mains pressure water in, and
(2) that you might lose some heat from a poorly insulated hot water
tank to parts of the house which don't benefit from it.

The only advantage of a combi is that it allows you to have high
pressure hot water and does away with the need to have a HW tank.
It takes just as long for water out of the hot tap to get hot,
because water has to travel down the same distance of cold pipe
before it gets from the source (be it the HW tank or the boiler
itself) to the tap.

The disadvantage of a combi is that it costs more, is more complicated,
and there's more in it which can (and does) go wrong, and if you want
a bath it'll probably take longer to fill than from a pre-heated tank,
because the boiler can't heat it up as quickly as you want it to come
out of the tap.

A minor disadvantage is that if your house is already plumbed for
low pressure hot water, and all your pipes, joints, and taps have
been used to it for years/decades, then suddenly changing to higher
pressure may invite latent leaks to read their heads. The risk is
probably low, but not zero.

I was once foolish enough to put one into a flat which previously had
no central heating at all (open fireplaces or gas or electric fires
only, cooking with gas, hot water by immersion heater). The installer
suggested the "obvious" solution was to mount the boiler in the cupboard
which then housed the HW tank, and to remove and discard the HW tank
which would become redundant with a combi.

I've regretted letting the salesman sell me a combi ever since, and
I wouldn't have one now for all the tea in China.