I would infer that it is to dump heat in the event of a power outage stopping the circulating pump. It is described as mounted high-up (presumably for gravity circulation) and might not provide useful heating if left operational under normal conditions, hence the use of the normally-open valve held shut electrically.
Yep,it's a familiar setting,with all described,as making sense....but it cannot be signed off as suitable for use,by a Hetas inspection,if required for any reason.
There are fundamental failures,in the described system. The Hetas accreditation has an extra module for wet systems,to allow installations to be checked as compliant.
The heat dump emitter is a safety addition,and whether or not it provides any functional heat,is secondary to it's inclusion,as a fail-safe part of the system. It is not valved,either by addition of manual or electric valves,deliberately,to prevent accidental or intentional non-operation.
The reason for all the above panic? ......is a solid fuelled boiler is most generally,an uncontrolled heat source,that is,whether it is 5,12 or 25Kw,it cannot be turned off instantly,as can a gas or electric device.
It just disregards any external faults,failures,power cuts or pumping,and continues inputting that heat in to the system,which by design,needs a means of dumping that heat,should all the other sinks (radiators,stored DHW,etc) be satisfied.
The means of dumping this excess heat,should never rely on valving or electrical power,as it needs to be permanently failsafe. The very fact that this thread begins with possible sticking electrical valving,is proof enough of the above reason.
The no valving,although much ignored,is there for a good reason. When these systems run in to flow issues,although open vented,the slowed or stalled water in the source,will kettle. This can be very dramatic,and sound like the end of the world,with vent pipes blowing the water out of header tanks,and such,but the banging and vibration will loosen all or any of the debris and scale,within the boiler and pipework.
This,with luck,will still make it through the pipework,but can form a blockage at any restriction,with any valve,even if open,providing an opportunity to choke.
The last part is even more dramatic,and has levelled many properties in the past.
It goes like this....blockage = steam build up = pressure increases/temp rises = standard copper/olives let go = release of pressure = heated water instantly flashes to steam 1600 times the volume,and kaboom.
The one and only reason i have painfully rattled on,is the thought that someone may trawl the web for solid fuelled info,and take a brief description of a system,as a blueprint for their own?
