Early morning hot water overflow...

[Supersonic]

I've an Economy 7 powered boiler which is only on for 4hrs overnight.

I'd went on holiday for a week in August and turned the power off to save it boiling away to itself whilst away and on my return, re-setup the timings, but every morning for a week or two, I woke to hear the hot water overflow spitting out considerable amounts, possibly for an hour or more. it resolved itself after about 7-8 mornings of this until I went away again at New Year (there is an open section of pipework so i can see the water...)

Same again, I turned if off for a week, and on return, did the same, and again, its firing out hot water for an hour or more but so far, hasn't gone off yet after nearly 10 days.

I'm scratching my head? Any ideas?

 

[oldbuffer]

Given that is "has an open section of pipework..." I'd guess its an unvented hot water cylinder with a lower immersion heater connected to Economy 7.

These cylinders have the potential to be very dangerous, and should be serviced by a "G3" qualified engineer annually.

One of the most popular models is the Heatrae Sadia "Megaflo". This has an internal air bubble to accommodate the expansion when the water is heated. This bubble needs to be replenished from time to time, and the instructions on how to do so are printed on the side of the cylinder. You don't need an engineer to replenish the bubble.

 

[flameport]

but every morning for a week or two, I woke to hear the hot water overflow spitting out considerable amounts

When that occurs, something is wrong.
It needs attending to on day 1, not leaving for weeks in the hope it might fix itself.