On 25/04/2012 23:05, kent wrote:

Thanks John for such a detailed and considered response. Our old boiler will
have to replaced soon and I have been thinking about options. Combi appealed
because without stored water losing heat I figured it would be more efficient.

I would not worry about that aspect much... firstly since the hot water
provision (however its done) is in reality only a fairly small part of
your total cost of heating. (one of the reasons why most of the solar
hot water systems never make financial sense)

If you hot water system loses heat, its into your house anyway in most
cases, so just less for the heating to do. An extra cylinder jacket will
also reduce it low levels.

However if 2 (or even 3) people wanted to shower at the same time I didn't think
that one combi boiler would be able to deliver sufficient hot water. This is a
fairly standard 4 bedroomed house with gas central heating and a gas fired
stove, but all cooking appliances are electric.

Before deciding on a solution, you need to make an assessment of the
capability of your cold main. If it can only supply 10 lpm (litres per
minute) or has very poor pressure, then any system that relies on
heating the incoming cold main on the fly, could be disappointing.

You can do some tests timing filling of a bucket of known volume from
your "best" tap. You can also get pressure gauges from toolstation etc
that will measure your actual supply pressure.

Assuming the mains can deliver a good flow rate, then you can look at
the options. Combis are not the only way to get mains pressure hot water
(unvented cylinders, heat banks, and thermal stores being others).

As to multiple showers, much depends on the shower in question and its
consumption. At a previous place I installed a 35kW combi when
converting the loft and losing the space for the cylinder etc. That
could (just) manage two showers at a time if there were no other demands
for water in the house while they were going on (there was a detectable
loss of "power" to each though).

Combis can be handy if you particularly need the space the cylinder
takes up, or not so good if you want a warm airing cupboard (although
there are ways round that)


--
Cheers,

John.

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