I've got what I hope is a straightforward central heating question:

My house has a pressurised hot water system, based on a Heatrae Megaflo
unit. I've only lived with vented systems before, so I'm not familiar
with how closed systems work.

Above the Megaflo, connected at the top of the boiler/radiators/tank hot
water circuit, there's an expansion vessel. This has a pressure gauge on
the bottom that's currently reading approx 1 bar. There are two other
connections to this expansion vessel. One is to an overflow pipe, and
the other is to a flexible hose that's connected to a cold water pipe
with a tap on it. The tap is currently closed.

So it looks to me like this tap is for topping up the system. What I'm
not sure about is under what circumstances I'd need to do that ? Also,
what is the pressure gauge on the expansion vessel for ?

Any advice gratefully received,

--
Trevor

On Sun, 23 Nov 2003 21:36:39 +0000, me wrote:


I've got what I hope is a straightforward central heating question:

My house has a pressurised hot water system, based on a Heatrae Megaflo
unit. I've only lived with vented systems before, so I'm not familiar
with how closed systems work.

Above the Megaflo, connected at the top of the boiler/radiators/tank hot
water circuit, there's an expansion vessel. This has a pressure gauge on
the bottom that's currently reading approx 1 bar. There are two other
connections to this expansion vessel. One is to an overflow pipe, and
the other is to a flexible hose that's connected to a cold water pipe
with a tap on it. The tap is currently closed.

So it looks to me like this tap is for topping up the system. What I'm
not sure about is under what circumstances I'd need to do that ? Also,
what is the pressure gauge on the expansion vessel for ?

Any advice gratefully received,

The expansion vessel etc will be for topping up the primary heating
circuit - see FAQ below. 1 bar is fine and will not need to be topped up
for now.

The Megaflow contains its own expansion space for the HW - this is in the
form of a trapped air volume inside the tank. The air has a habit of
dissolving into the water over a period (typically a year or so).
There is likely instructions on how to renew the air pocket on the side
of the cylinder.

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html

there's an expansion vessel. This has a pressure gauge on the bottom
that's currently reading approx 1 bar.

The parts you describe are unrelated to the Megaflo. They would exist on any
modern wet central heating system, even with a vented cylinder or no
cylinder at all. On a combi or system boiler, they would likely be within
the boiler casing.

They are used to top up the primary circulating water.

What I'm not sure about is under what circumstances I'd need
to do that ? Also, what is the pressure gauge on the expansion
vessel for ?

Look at the gauge regularly when the system is cold and the system turned
off (heat in the system or the pump being on will give false readings). If
it drops below 1 bar, then:

1. connect up the flexible hose
2. turn the tap on until the pressure rises to 1 bar.
3. turn the tap off
4. disconnect the flexible hose
5. bleed all the radiators
6. next day, bleed all the radiators again
7. check pressure again, particularly if much air was bled

If you need to top up regularly you may have a leak, either to the outside
world, or within the hot water cylinder heat exchanger. (But not in your
case, as your cylinder should be at higher pressure than the central
heating, so you are more likely to get rising pressure, not falling).

Christian.