In an earlier contribution to this discussion,
wrote:

Hi

I need some advice on a problem I'm having having with my central
heating.

First off, my existing setup is a Baxi Solo 70/4 PF in an S-PLan+
arrangement. It has/had a bypass that consisted of 1' of 15mm pipe
bridging the flow and return with a Gate valve to control the flow.

I tried to turn the thermostat on the boiler up to increase the
temperature of the radiators in an attempt to get the house up to
temperature. This works fine until the room thermostat clicks off (or
the ch timer cuts off) then all hell breaks loose. Obviously the water
is kettling in the boiler and this results in all the banging. Easy
enough so far. Assuming there was a pump overrun on the boiler to cope
with this I started messing about with the gate valve to no great
effect. I then decided that the bypass wasn't dumping enough heat so
re-routed a towel radiator (with no TRV) in the bathroom to act a
bypass. Still no joy. I then realised that the pump wasn't actually
overrunning at all! A quick google and I had convinced myself that
this boiler has a timed overrun of 7mins (how I came up with that,
don't ask). So I decided this was knackered and not wanting to bugger
about with things too much at this time of year, decided pipe
thermostat wired in parallel to the pump would be the answer.

Duly fitted and everything seemed great, cranked up the boiler
thermostat, set the pipe thermostat and left it too it. Result was a
nice roasty-toasty house and no banging/thumping when the room
thermostat clicked off.

Now you've probably guessed by now that I'm not writing this to tell
you that I fixed it!

When the room thermostat clicks off, the 2-port valve closes and the
boiler shuts down. The pipe thermostat keeps the pump running until
the temperature drops. However, before the pipe thermostat cuts off,
the boiler springs back to life heating the water back up again and
so it goes on. SWMBO likes the bathroom to be warm but we've got an
impromptu sauna now So I've put the boiler thermostat back down to
position 2 and turned the pipe thermostat setting up and am back to
where I started.

A further search resulted in finding the manual for the boiler (Baxis
spares partner site, doh) It turns out the boiler thermostat is what
controls the overrun and following the fault finding diagram would
suggest that replacing it should solve all my ills. I've no problem
doing this but wonder if this will just re-introduce the cycling
problem and there is something more fundemental going on.

Should the boiler not have some sort of interlock that stops it from
firing when there is no actual heating load. I'd noticed it coming on
every now and again in the summer but thought nothing of it at the
time.
Sorry for the long post but its a slow day at work!

Cheers,

Mark.

If your Baxi Solo is like mine, the pump over-run is controlled by the main
thermostat - with an extra set of change-over contacts. When the boiler is
below stat temperature, the pump is connected to switched live - so only
runs when there is a demand from the room stat/zone valve or whatever. When
the boiler is above stat temperature, the pump is connected to permanent
live - so it runs for as long as it takes to cool the boiler down - rather
than for a fixed time.

It sounds to me as if your system has never been wired correctly. The boiler
needs a permanent live from the heating system FCU, and a switched live from
volt-free contacts in the zone valves, (which are all connected in
parallel). The pump must be connected ONLY to the boiler's pump connection.

It also sounds as if you have compounded the problem by fitting a pipe stat.
If you implement a pump over-run with a pipe stat, it *must* have
changeover-contacts - with COM connected to the pump (and no other pump
connection) NC connected to boiler switched live and NO to permanent live.
Any other arrangement will probably bring the boiler on when the pump is
running in over-run mode - which is what you appear to have.
--
Cheers,
Set Square
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