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Designing the system is not something I'd usually do as a specific
paid job (and I guess that would go for most small installers). I'd
probably discuss the design with you verbally and if you wanted me to do
some of the work (like the boiler installation) I'd do heat loss calcs etc
as required. You could do these yourself easily enough. An installer can
probably get a boiler cheaper than you can so you'd be advised to get
yours to do that.
I would have reservations about a "phased approach" on a system of this age!
At some point (unless you have been meticulous about system cleanliness,
inhibitor etc) you need to clean out the entire system.
Whilst there are advocates of power flushing in this NG, most think it is
overpriced and not necessarily totally effective - depends on system layout,
pipe routing etc to some extent.
If you can arrange at some point before the new boiler is fitted to have all
rads off, I would suggest that cleaning out pipe work with mains pressure
water would be as effective as power flushing if not more so.
Blank off vent/feed pipes. Connect mains water.
Hook up hose to each rad valve in turn and blast water through each valve
and wherever else you can.
Put some kind of filter on discharge end of hose to catch the big bits for
examination (I use net curtain material but something finer would be
This method should give a degree of confidence in the internal state of
However, there are certain points which attract serious crud usually around
the point where vent and feed pipes join the heating circuit.
Would be well worth checking these for obstructions.
Forgot to add - flush out radiators with mains water as well before
For big radiators this needs 2 people as it is important to invert them
frequently while flushing and to shake them to get crud into suspension.
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