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Hi would really appreciate some advice,

Viesmann heat only boiler, modern condensing , modulating boiler, turned into service mode and left there.

Controller activated hot water heating up and then ran for a long time in service mode at max - peak power without modulating down at any time. The hot water cylinder is a large one so it will have been running in peak gas flow mode for quite some time.

I'm looking to understand the impact to such a daft mistake, in my view this will have used excessive gas at best and at worst the heat generated by funning the boiler full pelt on such a short circuit like the hot water will have passed extreme heat via the pump, three port valve, cylinder heating coil, heat exchanger, boiler internals. As far as I understand it the boiler should not allowed to run in this mode for any period of time except to conduct peak flow gas checks.

Can anyone give me advice about the likely impact to the system. The heating system is now non functional, but I'm looking to get a broader view about what damage could have been done to all the components of the system by running in this mode.

Thanks very much

Depends or doesnt really matter what boiler model it is really!

most modern day boilers have a service mode that self cancells service mode after a pre-set time, say-15mins. The boiler itself and any peripheral equiptment should withstand any heat produced by the boiler as a safety standard, otherwise the ode 'chocolate tea pot' comes to the tip of ones tongue!

is this that your anoyed with a service engineer you had to do the work and it has cost you on fuel above the norm or that something in the system has decided to let go and not function ?

to turn off the service mode manually, just turn to reset and cancel out.

I'm with Red on this. Every boiler I've ever put in service mode drops out of it after roughly 15 minutes. Meaning that you would really only burn a tiny amount more gas than you usually would. It happens. I've forgotten to take boilers out of service mode too.

Not only that but all the safeties - thermostat, limit cut out etc will all over ride service mode.

I would suggest you call your service engineer back, he is entitled 6take a look.