I want to fit a new combi boiler for my office, and I want to do as much
work as possible myself, just getting the final commissioning done by a
Corgi.

There is a (condemned) floor standing boiler at present, and I want to
replace this with a wall-hung combi immediately above, to minimise the
pipework changes. Hot water is currently from an electric water heater which
will be discarded.

The current boiler is about 20 KW and heating is adequate. New combi needs
only to supply a sink for washing up, but I did think of adding a shower
cubicle some time in the future. Office is ground floor with a handful of
rooms. Cellar underneath.

I originally thought of a Potterton 24, but I am now more inclined to a
Vaillant TurboMax Plus 28, even though this is 25% more expensive.
Obviously, this is overkill for the heating, but I want to leave a bit in
reserve for the shower if it ever gets fitted. Anybody any
suggestions/comments, please?

One reason for the Vaillant is that it takes a 15mm gas supply whereas the
Potterton takes 22mm. The boiler is quite close to the meter but the gas
pipe is only 15mm. Does it matter what the inlet pipe to the boiler is? I
was told by a fitter that I needed a new 22mm pipe for a combi, but he never
bothered to look where the meter was and how close it is. The max gas flow
needed is about 3.5cubic metres/hr for the Potterton and I assume it must be
similar for the Vaillant, so I am a bit surprised that the Vaillant takes a
15 mm gas pipe. Looking at the cda calculator, the 15 mm pipe would need a
pressure differential of 2mbar to deliver this flow over a length of 6m,
whereas only 1 mbar is acceptable. Why is the acceptable pressure drop so
low? The burners operate at between 12 mbar and 2 mbar. If the pressure at
the meter is 20 mbar, at peak flow, the pressure at the boiler reduces to 18
mbar. Why is that not okay? Do I definitely need to rerun the pipework in
22mm?

The present system is gravity fed with a pressure of about 0.25 bar. I would
be right in thinking I need to pressure test the existing pipe-work? What is
the best way of doing this?

I want to flush out the old pipework with Fernox sludge remover/cleaner. Am
I right in thinking that I might as well do that with the old boiler in
place? That avoids washing all the old crud through the new boiler. Do I
need to flush out again with the new boiler fitted, as well? The Fernox is
about 15 quid a bottle, so I don't want to (literally) throw money down the
drain.

What is the best way of getting inhibitor into the new sealed system? Do I
just use the filling loop? Presumably BEFORE pressuring the system with
water? And can you just pour it in with a funnel?

Fitting the new flue sounds interesting. The boiler is mounted on an
exterior wall, so a core cutter bit should do the trick, mounted on a big
drill. How do you fill in around the flue once it is fitted? Just sand and
cement?

Sorry to ask so many questions, but I want to get this completely clear in
my mind before starting.

Thanks

Geoff