On Thu, 01 Jun 2006 20:38:36 +0100, thescullster wrote:

Looking at a replacement central heating/HW boiler and reckon I will need
something like W/Bosch 24Ri. This states a requirement of 2.67cu.m/hr of
gas.

I have a 22mm gas pipe running right across the house to the location of the
current boiler.

22mm copper, not 3/4" outside diameter steel, right? The latter is 1/2"
bore.

Assessment of length and addition of effective length for bends etc gives
23m equivalent length of pipe.
From R D Treloar, this looks to be right on the limit for 22mm dia pipe.

Also we have a gas fire tapped off this main (approx 4 m from gas meter).
This is an occasional use decorative affair claiming 6.5Kw gross heat input
required.

If it's 4m from the meter I'd guess (without doing the calcs for you,
which you seem capable of doing yourself, with Treloar's help) that it
won't make too much difference.

Does anyone know enough to comment on the viability of this install?
What would the effect be if the boiler were running and the fire turned on?

In practice, none.

As we have concrete floors throughout ...

Ahem! you say 22mm pipe, suggesting copper rather than steel. Now you say
concrete floors. Errr, it _is_ adequately protected where it's laid in the
concrete, isn't it?

.... and the boiler location is as far from
the meter as geographically possible, what are the alternatives (please
don't suggest moving)?

If you replaced accessible lengths with 28mm and/or 22mm elbow bends with
swept bends you could reduce the effective length by perhaps a few metres.

Also I note that previously it would have been likely that this main would
serve:
Valor Homeflame gas fire
Landing gas heater
Gas cooker
CH boiler 50,000 Btu

Surely this lot would put as much if not more load on the supply?

Maybe. Maybe it was under-specced for that lot. Quite possibly they added
appliances without calculating & checking (shouldn't have, of course).

Final question - if the gas supply pressure as measured is high, can I
take this into account in calculation?

No. It has to be 1mBar drop in the pipework. (If the meter regulator is
high when you install the appliance and subsequently gets re-set correctly
are you going to shut down the appliance because the pressure is now below
spec?)

In practice one sees a lot of installations in which the drop is
1mBar. If I came across one I wouldn't be worried as long as the pressure
is still reasonable, and far enough above the boiler's required burner
pressure not to affect operation. However in installing an appliance one
can't take such an accomodating attitude but should do it by the book.