Open Vented Heating System Question

Stephen_C

I'm looking to swap out my old hot water cylinder for an insulated one and I've come across what looks like an odd set up with the system that has me confused.

My heating system is an open vented system, i.e I have a fed and expansion tank in the attic which maintains the pressure in the system. However the system seems to have a few additions from a sealed system which don't look right.

In the hot press at the highest point in the heating system, where the expansion pipe should be going back to the tank in the attic, there is an expansion vessel and an air release valve. The top of the air release valve then has a pipe covering/resting on it, this pipe then goes up the tank in the attic as the expansion pipe should.

My question is, if this is an open vented system, do I need this expansion vessel or could the pipe leading to the expansion vessel not just go straight up to the tank in the attic.

If I could get rid of the expansion vessel it would be great as it would free up some space in the hot press.

I've attached a couple of photos of the set up.
The first shows the expansion vessel and air release valve with the top of the valve covered by the pipe leading to the tank in the attic.
The second is the top of the air release valve with the pipe lifted off it.

K.Flyer

Your heating system Was an open vented system until someone fitted the vent valve and the expansion vessel, however if the F and E tank is still filling the system you may have what some would refer to as a "semi-sealed" sytem (or only half the job done)
Why they left the old expansion pipe sitting on top of the vent valve is anybodys guess as it has no function now.
There are a few different reasons that someone may have done this, the most common one being a poor circulation problem which the attempt to rectify was to install a stronger circulating pump. This then probably caused "pitching" at the F and E tank which they "fixed" by fitting the air vent and expansion vessel.

Stephen_C

That makes sense, so it wouldn't be advisable to remove the expansion vessel and go back to the expansion pipe as the pitching problem would more than likely return?

I could however get rid of the left over expansion pipe?

As you can see from the second pic in my last post there seems to be a fair bit of gunk making its way out of the air valve, there even is some spatter on the wall. Would this suggest that the expansion vessel may even be too small?

nmacc

The expansion vessel is probably not too small, but even if it was, it wouldn't cause the spatter you see.

The problem is that the automatic air vent is leaking. They always leak; it needs changing. The only make that I have never found to leak (so far) is Flamco; available from Chadwicks and one or two others.

The expansion vessel does need to permit expansion however, so you need to check that there's plenty of air in it. There's a standard Shrader valve under a cap on the bottom of the vessel. If you depress the valve briefly, air should come out. If water comes out, then the vessel is bunched. Check the pressure in the vessel with a car pressure gauge. This should really be done with no pressure in the systems, so it could be done if the vent is being changed.
In practice I'd expect a reading of around 0.8 - 1 Bar when cold, even without bleeding off the pressure. If you have air in the vessel & this pressure, it's probably sound.