On Sun, 7 Sep 2003 12:52:18 +0100, "GB"
wrote:

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?

How about a condensing version such as the EcoMax?

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?

The issue is the flow rate that the boiler requires and the pressure
drop allowed. As you deduced from the CDA site, these can easily be
worked out.

It is quite common, for convenience to run the last metre or so of
pipe at the boiler in 15mm even if most of the run is 22mm. It's the
overall behaviour that matters. Often boilers are fitted with an
isolating cock with 1/2" BSP inlet and this goes more naturally with
15mm tube for a short run.

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.

So he's flying by the seat of his pants and assuming characteristics
of the installation. It depends on the required gas rate and the run
length of pipe.

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 CDA site information is based on the British Standard for doing
this. From what you are saying, you probably do need to run in 22mm
apart from possibly the last short piece to the boiler.


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?

It wouldn't hurt, although remember that you are only going up to 3bar
maximum which is the same or less than the cold water mains.

The most likely problems are going to be that the radiator valves will
weep at the seals. I had one which did this and decided for the
sake of completeness to change all of them. The easiest test is
simply to do the conversion to sealed operation, capping off the
boiler connections and check for pressure loss on the gauge.
If you think that there might be problems, run the system after
installing for a few weeks before adding the inhibitor in case you
need to drain and fix.



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's what I did a couple of weeks before ripping out the old boiler.

That avoids washing all the old crud through the new boiler.

It's a good idea to put a strainer on the return to the boiler anyway.

Do I
need to flush out again with the new boiler fitted, as well?

I did an initial flush round with remover and the old boiler, then
switched boilers putting lever ball valves on either side of it (there
is a pressure relief valve in the boiler for safety). I added in the
sealed system components and then using the filling loop flushed at
each radiator. THis drives the crud out at each radiator point and
I flushed through both valves and took the radiator outside for a good
pressure washing.
I filled and used sludge remover again before finally flushing and
filling with added inhibitor. Possibly the second go with remover
isn't necessary but £15 in the context of the system is nothing.


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?

You can get sludge remover and inhibitor chemicals from Fernox in gel
form which can be injected through a radiator vent - I use an empty
one, not a full one as the instructions suggest. Alternatively you
can put in a chemical introduction point by using a valve and short
piece of vertical pipe and then a funnel to introduce chemicals.



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?

Generally unless the manufacturer says otherwise.



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

Thanks

Geoff


..andy

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