Restoration is easy.

[Yorkie]

Gary’s description of how the diverter valve works is spot on, as is his statement that as often as not, the valve itself isn’t faulty, but the fault is in the actuator.

The valve is simply a little hard rubber ball about 22mm diameter which - in a three post valve - is biased in one direction by the spring return on the actuator when at rest. Most systems are wired to give ‘hot water priority’, that is that when starting from cold, the valve will send hot water from the boiler to the hot water cylinder heating coil only, then when the cylinder is up to temperature, the cylinder thermostat puts voltage onto the diverter valve motor to switch to heating, and provided the programmer and room thermostat (often now in one combined wireless unit) are calling for heat, the little synchronous motor of the diverter valve move the lever on the valve which operates the rubber ball across to the heating position, blocking off the hot water port. The synchronous motor stalls in that position until voltage is removed from the motor, at which point the spring returns the ball to open the hot water port.

As Gary says, there arev two micro-siwtches in the valve actuator. The Achilles heel isn’t the brassware part of the valve which contains the ball – it’s the gearing arrangement, which consists of a small cog on the synchronous motor, which operates a quadrant made of mild steel with teeth stamped (rather than 'cut') on the edge. With the passage of time, and especially when the valve isn’t operated through the summer months, the valve starts to seize up, making it harder for the motor to move the quadrant, and the teeth start to wear and eventually slip. Sometimes a new synchronous motor with its new brass cog will suffice, sometimes a new operator will be needed, which – provided the lever which operates the ball on the valve can be moved back and forth with finger pressure - will suffice, in which case the operator can be removed by taking out a couple of screws without the need to drain down the system. But if (as is often the case), the valve is stiff; it makes more sense to change the whole valve, which calls for a partial drain down of the system. Service engineers tend to be reluctant to change the whole valve as they’re usually in a very awkward place in the airing cupboard and a devil to get at, so the engineer often hopes for the best and just changes the operator, only then changinig the rest of the valve if that doesn't do the trick.

In the first 19 years of living in this house since new, I’ve had to fit two new operators, and three years ago, a whole valve as the ball was totally seized.

A complete valve is about £100 retail. EG:

http://www.screwfix.com/p/honeywell-v407...1626654437

Actuator, typically £50.

http://www.ebay.co.uk/sch/i.html?_fln=1&_itemId=220824648137&_nkw=HONEYWELL+V4073A+ACTUATOR+40003916+003+NEW

BG prices are at the top end, but from past experience having served my time with the then ‘Gas Board’ and spent 40 years in service management at every level with what later became BG, I’d say there are three things that work in the customers' favour with BG. Firstly, because BG service so many appliances, that engineers soon become familiar with particular faults on particular boilers. Secondly, certainly throughout my career, their training was second to none. Lastly, they have a huge spares backup due to the sheer volume of work they do. The engineer will have a laptop and if (unlikely) he doesn’t have the part on his van, he’ll be able to say when he can return.

I don’t think Colin’s bill for the work done was over the top and I doubt a local ‘Gas-Safe’ plumber would have charged much less. If he rings BG, he'll talk right away to someone in a call centre. If he rings a local plumber, he may well get a message on a mobile phone saying 'sorry, please leave a message and I'll get back to you'. (sometime, whenever). Low overheads, lower prices perhaps, but no back-up infrastructure.