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Long story short Im in custody of an old Stelrad Ideal Sprint combi boiler - it cannot be replaced it has to be repaired. The problem is due to its age I'm struggling to find a gas engineer that will tinker with it rather than just say it's obsolete and needs replacing.
Anyway - I've had my Fluke scope on the Honeywell Modureg valve as it isn't modulating on the CH side when the circuit is upto temp which results in the main gas valve noisely cycling on and off. Voltage tends to stay around ~13.5vdc
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DHW looks okay and shows modulation as the hot water flow is altered. Maximum voltage on full flow is 28vdc.
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It's a simple boiler with a single thermistor on the primary loop which has been changed to no effect and which works fine with DHW.
The diverter valve has been replaced with an OEM unit as the diaphragm failed.
Any ideas?
The basic firing sequence is probably something like, fan on first, pressure sensor then detects pressure drop / rise and engages the gas valve, ignition strikes, gas burner temp sensor will keep gas valve open (else it closes). Then you have a couple of feedback loops, there will be a temp sensor in the boiler which cuts off the gas when the water in the boiler gets too hot.
I've been keeping my Potterton Nettaheat going myself (35 ish years old), lots of OEM spares available.
I haven’t seen one of those boilers in a while!
Going by your description above the gas valve and solenoid are working correctly as you have seen a voltage increase/decrease and the corresponding change in burner pressure when it’s working on hot water.
It would imply that that the PCB is faulty.
I think my email is on my profile if you want to discuss ( I am a commercial/domestic gas safe engineer ).
Found a book last night on Google that describes the operation of the Sprint 75.
The modulation only works for DHW and is triggered by the diverter switch - I was under the impression that modulation worked on CH as well.
CH gas flow is by a fixed ~10vdc to the Modureg.
The CH water temperature maintenance is by a 4minute hysteresis on/off cycle - however mine is cycling consistently every 18seconds.
Looking like the circuit board.
Worth checking you have a constant flow through the pump into the CH circuit as insufficient flow would possibly lead to the fluctuating that you are getting.
Thanks for the reply.
CH sounds okay you can hear the water flowing after it's been drained/refilled and has air in the system and also with a screwdriver in the ear on the rad and the pump.
The CH circuit also takes ~20 mins to get upto temp from cold. The load is two large cast iron rads and a medium sized standard single radiator.
They are nice and warm too touch and well bled.