On Wed, 21 Dec 2005 15:07:14 +0000, danw
wrote:

I'm seeking advice.

We're currently looking to replace our system boiler and hot water tank.
The boiler and tank are too small for our house (detached, 30s, 3 beds,
one bath, 3 downstairs rooms with small kitchen extension).

We've had a couple of heating engineers in and are receiving conflicting
advice; as I am sure I will get here!

First chap said that a combi would be no good for our house. He said he
would never use them and that they are no good for a house of our size.
He also said that we would need to replace our old iron gas supply, even
though it appears to be large enough.

Second man said that a combi would be fine, he recommended a
floor-standing, Worcester Greenflow. Said we wouldn't need to replace
the gas supply it would be fine. Said a wall mounted boiler would not be
big enough. Having a floor standing boiler is less than ideal as it
would take up much needed floor space in our small kitchen.

Third man says we would be fine with a wall mounted combi but said we'd
need a new gas supply as old iron pipes a likely to rust and this can
get inside the boiler. He also said they are prone to cracking and leaking.

What we would ideally like is: a combi, wall-mounted, with no new gas
supply if possible. We are a small family (2 little kids), with not a
lot of spare cash, and not a lot of room in our kitchen for the boiler.

I know that, ultimately, the decision is ours but if anyone has any
advice for us it would be much appreciated.

thanks in advance, dan

There are a whole bunch of things he

- If you are thinking about any form of direct mains fed hot water,
then measuring the cold water rate at the cold tap in the kitchen is
very important. If the supply is less than about 20 litres/min, you
will be quite disappointed by the results in comparison with the
existing storage system in terms of time to fill the bath. With two
small kids, this will matter increasingly.

- An instantaneous boiler, be it a simple combi or one with storage
like the floor standing WB Highflow (was that what you meant?) need a
substantial gas supply. Normally in a run of copper tube, 22mm is
used and on longer runs, 28mm. Remember that the iron pipe will have
thicker walls and the inner diameter of something that looks to be
around 22mm or more on the outside is not a lot larger than 15mm on
the inside. The fitter should do the proper calculations, but it is
possible that the pipe is inadequate. If you don't want to change
the supply, this may rule out combi type boilers.

- The house size is not typically that important. With either a combi
or a system boiler, in modern models the burner modulates, meaning
reduces output to match required load. In a house of your size, it
would be unlikely that the heating load would exceed the requirement
of water heating unless you went for a peewilly little combi of
11-13lpm which you probably wouldn't want anyway.

- Bear in mind that combis have a flow rate specified in litres/min
for a 35 degree temperature rise normally. If you consider that in
the winter, cold mains temperature is often around 5 degrees, the
effective bath water temperature being 40 degrees means that the
specified flow rate becomes the total flow rate.
For a 150 litre bath, the filling time can be frustrating if the
boiler is too small.

- CPSU boilers like the WB Highflow help with this to a point, but
only to the extent of the store. Once that runs out, the rate is
reduced to that of the unaided boiler - around 12lpm. The
manufacturer quotes 20lpm, but that includes the store behaviour and
they don't specify how they made the measurement. Either way, it's
possible to calculate how long it will take to deliver a certain
volume of water at a certain temperature.
Nevertheless, with the caveats, this might be a good choice. Have you
thought about siting it in the airing cupboard instead of the
cylinder? Is recovering the cylinder space important?

- There are larger wall mounted combi boilers with storage like the
Alpha. However, at dimensions of 900x600x450, it is not going to look
petite on your kitchen wall. Of course it needs a substantial gas
supply.

- If you wanted the minimum impact solution, a replacement system
boiler (condensing model) and a fast recovery perhaps slightly larger
cylinder could well be a good option. You could choose a boiler that
wouldn't require a supply upgrade and the HW performance could be
improved substantially.



--

..andy