On Tue, 01 Mar 2005 13:01:47 +0000, Pete
wrote:

On Mon, 28 Feb 2005 16:25:02 +0000, Mark
wrote:

snip
Do you actually have one or two header tanks in the loft? If you just
have one large tank you may have a Primatic cylinder and you cannot
use inhibitor in case your DHW gets contaminated.

Mark

This is close to my own problem, so I thought I'd ask here.

I have a single tank in the roof. The central heating pump has two
automatic valves below it - one controls the ch and the other is on a
pipe to the hot water cylinder. The whole thing is powered by a
Potterton wall mounted gas boiler.

The central heating alone works fine - all radiators get hot. But the
hot water, when set on the same twice-a-day timer as the central
heating, boils. Atleast, I think it's boiling - there's a sudden rush
of air - pretty alarming - from the cylinder, which then vents into
the roof tank.

Where does the air come from and how can I stop it?

I'm not an expert but did once have a Primatic cylinder. The only
ways I know of to reduce the termperature of the hot water with these
systems is to only have your central heating/hot water on for short
periods (so that the tank does not have time to heat up too much) or
to reduce the boiler thermostat temperature.

The latter, of course, will make your central heating water colder,
but may not be a problem.

When I had a Primatic cylinder I had to keep adjusting the boiler
thermostat. For the first hour of central heating have it on high to
heat the house. After that I turned the boiler stat down to prevent
the hot water overheating. In the summer it's not a problem - you can
just keep the stat on low.

Hope this helps.
Mark