Baxi boiler / New heating/water system problems - help please!!!

I have had a loft conversion on a two-storey house, now three storey. As part of the conversion the hot-water system was changed from non- pressurised to pressurised. The new cylinder and associated pressure vessels were re-sited in the loft. The top storey radiators are run off a separate control system.
Since installation of the new system (last August), I have had continual problems with the boiler over-heating and cutting out, usually after around 30 minutes but sometimes even more frequently than that. The builder has had his heating subbies back many times and they have done the following (allegedly):
- replaced the mother board on the boiler - drained and cleaned the radiator system - installed an additional pump in series
None of it has made any difference.
The boiler is a Baxi Solo 3 PFL 70. On the ground floor there are 5 rads, 7 on the first floor and 3 on the second floor.
Finally a different heating engineer came round and diagnosed the following:
- boiler too small for the new environment - flow pipes from boiler to tank should be 28mm, rather than 22mm (replacing these would involve major work ripping out stud walls in the newly finished conversion)
When this information was presented to the builder he indicated that this was rubbish and that only a new boiler was required, as it wasn't powerful enough. However, surely a weak boiler would just take longer to heat up the rads?
On the advice of a friend I de-synchronised the hot water and central heating. This cured the problem for about a week; then the boiler began cutting out again on the hot water phase, and now is cutting out again on the heating phase.
I do not want to have to buy a new boiler if its a simple problem - two suggested faults are a new thermostat and a faulty bypass valve.
Any suggestions as to the likely cause of the problem, or what to do next???
Many thanks!
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snipped-for-privacy@bt.com wrote in

three storey.

from non-

pressure
are run

had
out,
frequently
many times

there are 5

diagnosed the

22mm
walls in

indicated that

it wasn't

take longer

and central

boiler
cutting out

problem -

bypass valve.

what to do

Was the system changed to have all TRV if so was a bypass circuit fitted. Does the overheat LED flash or is a constant on Try turning the boiler thermostat knob to just a quarter on, i.e. water temp lower then about 63c
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John - thanks for the reply.
TRV (Thermostatic radiator valves?) - all the new upstairs radiators have these. The circuit is off at the moment.
On the old bit of the house, 2 of the twelve rads have TRV.
The overheat light is on constantly. Sometimes the bolier on light (next to it) is also on; and sometimes that light flickers.
I will try the boiler thermostat route.
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snipped-for-privacy@bt.com wrote in wrote:

on,
radiators
on light

That doesn't sound good ;(

Baxis realty dont like running hot often the firebox gasket is leaking which trips the overheat stat. When was the boiler properly serviced?
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Recently - we have an annual service plan with British Gas.
I've rung Baxi and they've offered a service visit - 250 including parts and labour. Should I take it up?
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snipped-for-privacy@bt.com wrote in

overheat
Oh dear I did say, When was the boiler properly serviced?

including
Wait and see if it will run reliably on a low heat first.
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On Fri, 23 Nov 2007 08:12:16 -0800, stephen.travis wrote:

Not yet. Till all the more correctable problems have been addressed.
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
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On Fri, 23 Nov 2007 04:57:19 -0800, stephen.travis wrote:

Almost certainly the CAUSE of the over heating is a lack of primary flow. This is not likely to be the boiler, pump(s) or mother board. The buggers should have cleaned the system out when they worked on it, anyway we can assume that has been done.
My guess is that the control system is wrong. You sound like you need something like a 3 zone extended S-plan controls.
If you are electrically competent make a complete wiring diagram of the heating control system. You might also have a pipework routing fault but seeing as the system does work some of the time perhaps not.
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
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