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!