Josi wrote:

I need your help.

We need a bit more information...

I've got a conventional oil fired boiler. The central heating is

How is the hot water heated: Convected or "fully pumped"?

(i.e. the boiler will have a flow and return pipe for the central
heating, but does it also have an additional two separate pipes for the
hot water circuit? (if just the former it is a fully pumped setup, and
if the latter it is convected of what commonly known as a "gravity"
circulation)

working fine, but I have two problems, which I don't know
-incidentally- whether they are related or not:

a. I have no hot water at all. The flow is fine, but the water is cold
-rarely, if I have kept the central heating on for a long while, it
might come off lukewarm if at all.

Again hard to say for sure, but I would guess you have a fully pumped
system and a valve that would normally direct water through the heating
coild in the hot water cylinder has failed in some way.

The pictures here may help you identify how your system works:

http://content.honeywell.com/uk/homes/systems.htm

b. The expansion tank overflows every now and then.

Does it ever refil from its ball valve, or does it seem to just acquire
water anyway?


--
Cheers,

John.

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