Central Heating Gravity system (again)

[John Cross]

You *must* have a clear route from the boiler to the expansion pipe with

no valves in the way. You must therefore put the valve in the short bit of pipe between the T-piece and the connection to the top of the cylinder coil.

This would mean that the 2port *would have* to be fitted in the main loft on the cylinder which would be a pain in the a**. I was already going to use a RF tank stat to get from the boiler to the main loft.

It would be very much harder to go for a Y-plan because this would require > the 3-port valve to be installed in your downstairs loft - where the CH & HW > circuits split - and you would have to re-pipe your overflow all the way > from the boiler side of this junction.

Whilst not strictly correct I reckon that it you add a 2 port valve on the HW return it *should* work ok. Others seem to have done it. Do a search on google and have a look at:

have a similar system (gravity HW and pumped heating without controls) - but in my case I have a strange expansion tank setup. In my system the Gravity HW pipes are also feed/vent, except that my vent pipe extends *into* the water in the expansion pipe. If the water in the expansion tank drops so that the vent pipe is not underwater then I get no hot water (after having a good look) the vent pipe has been extended at some time by 3-4 inches to keep it below the water line. It therefore looks to me that what looks like the vent pipe (top of cylinder coil to vent) is actually also a feed pipe and both pipes could act as either a vent or a feed as required. If a valve was added into either pipe - then the system could still vent though either the HW flow or HW return (which are also feed/vent pipes). If the valve is closed in the feed - then the system could feed water from the vent pipe (which is below water level) if required.

Has anyone else seen this before?

[Christian McArdle]

Indeed, it is often recommended to do this, as the valve runs a good 10C cooler.

Yes. It is called a dangerous bodge. If your boiler thermostat fails, the boiler may explode. It was done because some cretin didn't understand how to fix a pumping over problem. I would fix this at your earliest convenience, hopefully whilst the house is still standing.

Christian.

[John Cross]

Indeed, it is often recommended to do this, as the valve runs a good 10C cooler.

Is it OK to fit an Zone Valve on the HW return pipe? where this pipe is also the header tank feed? From my thinking it should be ok as the vent is unrestricted - however you would have to make sure that there was a bypass fitted (i.e if the system uses the bathroom rad as bypass the valve may close this route)

Never realised that this was a bodge, as it been like this for the last 25+ years.

How should the problem be fixed (if the vent pipe is higher than the water level then there is no HW). The system is currently gravity HW.

[Christian McArdle]

No. The water feed must be unrestricted also. For safety, you must use one of the two following systems:

1. Vented:

This requires a short unvalved unrestricted vent arrangement to release steam pressure. It also requires either (a) a boiler with additional overheat cutout or (b) an unrestricted path from the header tank into the boiler for quenching purposes.

1. Unvented:

This requires a boiler with an additional overheat cutout. It also requires a pressure relief valve for steam but should not be connected to a permanent water supply.

You appear to have a vented system without a vent. This is very dangerous.

If you have a vented system and want to install an additional zone valve, you mustn't put it on the vent line as you know. However, you may only put it on the feed line if the boiler has an overtemperature cutout and so doesn't rely on quenching water in an emergency.

Furthermore, some ancient boilers (and more recent solid fuel types) require the gravity circulated circuit to be unrestricted as a safety measure and to not have any valves at all.

Christian.