My boss's house is large and heated by a 60kW boiler in an outhouse, the
system is antiquated but the boiler ( gas, non condensing, recently
replaced a free standing oil boiler). It seems to be traditional S plan but
with large commercial valves on the ch circuit. I noticed a strangeness
some while back in that if the single thermostat ( one zone only) called
for the ch valve to be open the dhw solenoid was inhibited.

The boiler is constantly on and controlled only by its internal water
temperature. It has a manual valve controlled bypass between hot out and
return, so there is constant blending of return and hot into the boiler. I
understand this was to keep the oil boiler from condensing but why would
the gas boiler have retained this backend protection? On the wood boilers I
deal with this blending is done with a thermostatic valve.

I would have expected the thermostat to have controlled the boiler and the
ch solenoid valve. As it is this valve has failed and of course the system
now doesn't react to the room stat at all.

AJH

On 15/01/2011 22:41, andrew wrote:
My boss's house is large and heated by a 60kW boiler in an outhouse, the
system is antiquated but the boiler ( gas, non condensing, recently
replaced a free standing oil boiler). It seems to be traditional S plan but
with large commercial valves on the ch circuit. I noticed a strangeness
some while back in that if the single thermostat ( one zone only) called
for the ch valve to be open the dhw solenoid was inhibited.

Sounds like whoever "designed" it intended only one circuit to be
operational at any one time. Maybe the room stat has change-over
contacts, with the feed to the HW valve being taken from the Normally
Open contact - which closes when the CH demand is satisfied.

The boiler is constantly on and controlled only by its internal water
temperature.

In that case, it AIN'T S-Plan. If it were, the boiler would be
controlled by the auxilliary contacts (are there any?) on the zone
valve(s) and would be turned off when all demands are satisfied.

It has a manual valve controlled bypass between hot out and
return, so there is constant blending of return and hot into the boiler. I
understand this was to keep the oil boiler from condensing but why would
the gas boiler have retained this backend protection? On the wood boilers I
deal with this blending is done with a thermostatic valve.


It may be simply to maintain some flow through the boiler (if it needs
it) when both zone valves are closed. Or it could be that whoever
changed the boiler didn't know what it for, and decided to leave it alone.

I would have expected the thermostat to have controlled the boiler and the
ch solenoid valve. As it is this valve has failed and of course the system
now doesn't react to the room stat at all.


If it were S-Plan, the boiler wouldn't be firing under these circumstances.

AJH

I'm not sure whether you're asking a question, or just getting it off
your chest.

If you're looking for advice, mine would be to convert it into a
*proper* S-Plan system[1] and, if it needs a by-pass, to fit an
automatic by-pass valve.

[1] This may just be a re-wiring job or you may need to replace the zone
valves - or at least their actuators - if the current ones don't have
auxiliary contacts.
--
Cheers,
Roger
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Roger Mills wrote:

I'm not sure whether you're asking a question, or just getting it off
your chest.

If you're looking for advice, mine would be to convert it into a
proper S-Plan system[1] and, if it needs a by-pass, to fit an
automatic by-pass valve.

[1] This may just be a re-wiring job or you may need to replace the zone
valves - or at least their actuators - if the current ones don't have
auxiliary contacts.

Thanks Roger, it was advice I was looking for. I'll suggest he has it
rewired as S plan and shut the bypass.

AJH

On 16/01/2011 21:50, andrew wrote:
Roger Mills wrote:

I'm not sure whether you're asking a question, or just getting it off
your chest.

If you're looking for advice, mine would be to convert it into a
proper S-Plan system[1] and, if it needs a by-pass, to fit an
automatic by-pass valve.

[1] This may just be a re-wiring job or you may need to replace the zone
valves - or at least their actuators - if the current ones don't have
auxiliary contacts.

Thanks Roger, it was advice I was looking for. I'll suggest he has it
rewired as S plan and shut the bypass.

AJH

But note from my previous post that you may need to replace the zone
valves if the current ones don't have auxiliary contacts which close
when the valve is fully open.

As far as the by-pass is concerned, you need to consult the boiler
installation manual to see whether it needs to keep the pump running for
a bit (pump over-run) after it stops firing. If it *does*, you *will*
need a by-pass - and can't just close off the existing one. In that
case, it would be better to replace it with an automatic one which just
opens when the pressure increases as a result of all zone valves being
closed.
--
Cheers,
Roger
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