Intermittent fuse blow on Central Heating System

[lewisjohn]

I’d appreciate any thoughts/advice anybody might have on
the following.

In 1991 we had a fully pumped central/hot water
heating system installed. The system was wired around
a Randall Mk3, scheme R69 wiring centre. The MPV was a
Smith’s Switchmaster, the pump a Grundfoss-Selectric
Ups15-50, Boiler Baxi Bermuda back boiler, Sunvic
clamp-on cylinder thermostat and a Danfoss Randall
FP975 programmer for the last few years.

This system and set-up has worked without any fault or
problem all these years until two weeks ago. The MPV
packed up and was replaced with a Honeywell V4073A.
After working satisfactorily and as expected for a few
hours the wiring centre fuse “blew” (first time ever)
and has continued to do so intermittently ever since.
As a “solution” the pump was changed for a new version
of the same model. This has proved to make no
difference to the intermittent fuse blowing problem,
though the system works “perfectly” and as required
between fuse blows!

Do you think there any particular wiring issues that
needed to be adjusted /checked for with this
particular Randal wiring centre given the new
equipment?

 

[Agile]

If you are happy with the replacement pump then the motorised valves would be the most likelty suspect.

They often leak at the shaft gland and the water causes arcing at the microswitchs, this does not necessarily immediately blow the fuse as it can depend on service selection and thermostat operation.

Suggest you look VERY closely at the insides!

Tony