another boiler problem

[andrew]

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

[Roger Mills]

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.

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

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.

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

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.

[andrew]

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

AJH

[Roger Mills]

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.