An idea is slowly dawning but let's double check the facts first:
1) The hot water is gravity driven, ie no pump required.
2) The central heating has never worked properly.
3) When the CH goes off, radiators get hot - but only upstairs.
4) Water spews from the vent pipe when the pump runs.
Gravity HW systems work fine on their own but when you try to run CH off the same boiler the HW cylinder overheats. Problem: how do you stop the CH water from bringing your cylinder up to radiator temperature?
Answer 1: turn down the boiler thermostat. This is a cheap and nasty fix.
Answer 2: put a zone valve in the HW return and close it when the pump starts. It's (usually) in the return because the vent (usually) comes off the top of the cylinder coil so that flow must always be open.
I believe the following theory fits all the known facts though, as Karl Popper would have said, that doesn't prove it's right:
Some total plonker has put a zone valve in the combined HW/CH return!
Think about it. HW on its own works perfectly. You switch on CH, the pump runs and that valve starts to close. (That's already wrong; a switch on the valve should start the pump after it closes but that's a minor detail.) Some hot water flows round the CH pipework but not for long because the valve soon blocks its return path. Result - hot water gets to radiators but returns stay cold. Does this sound familiar? There's more.
The pumped water has nowhere to go except backwards through the cylinder heating coil or up the vent where it spews out into the tank. The constant flow of water through the tank makes it dirty and the oxygen it picks up speeds corrosion. While all this is going on, the boiler is full of static water and is doing its little dinger. This may also send water up the vent pipe, especially if the feed is on the boiler side of that valve.
You switch off HW and the valve opens. The boiler badly needs to let off steam, perhaps literally, and so, with much gurgling, near boiling water rushes into the upstairs radiators. Sounds familiar?
To confirm this theory you'll have to find that insidious valve. Follow your pipes from the boiler all the way up to the cylinder coil and, most importantly, back again. If it's free flow all the way round, maybe the valve is in the CH circuit so follow this instead. If you still can't find a valve then my theory goes out the window and it's back to the drawing board.
1) The hot water is gravity driven, ie no pump required.
2) The central heating has never worked properly.
3) When the CH goes off, radiators get hot - but only upstairs.
4) Water spews from the vent pipe when the pump runs.
Gravity HW systems work fine on their own but when you try to run CH off the same boiler the HW cylinder overheats. Problem: how do you stop the CH water from bringing your cylinder up to radiator temperature?
Answer 1: turn down the boiler thermostat. This is a cheap and nasty fix.
Answer 2: put a zone valve in the HW return and close it when the pump starts. It's (usually) in the return because the vent (usually) comes off the top of the cylinder coil so that flow must always be open.
I believe the following theory fits all the known facts though, as Karl Popper would have said, that doesn't prove it's right:
Some total plonker has put a zone valve in the combined HW/CH return!
Think about it. HW on its own works perfectly. You switch on CH, the pump runs and that valve starts to close. (That's already wrong; a switch on the valve should start the pump after it closes but that's a minor detail.) Some hot water flows round the CH pipework but not for long because the valve soon blocks its return path. Result - hot water gets to radiators but returns stay cold. Does this sound familiar? There's more.
The pumped water has nowhere to go except backwards through the cylinder heating coil or up the vent where it spews out into the tank. The constant flow of water through the tank makes it dirty and the oxygen it picks up speeds corrosion. While all this is going on, the boiler is full of static water and is doing its little dinger. This may also send water up the vent pipe, especially if the feed is on the boiler side of that valve.
You switch off HW and the valve opens. The boiler badly needs to let off steam, perhaps literally, and so, with much gurgling, near boiling water rushes into the upstairs radiators. Sounds familiar?
To confirm this theory you'll have to find that insidious valve. Follow your pipes from the boiler all the way up to the cylinder coil and, most importantly, back again. If it's free flow all the way round, maybe the valve is in the CH circuit so follow this instead. If you still can't find a valve then my theory goes out the window and it's back to the drawing board.
The water only goes into the header tank when the pump is turned on, after 5 or 10 seconds the flow finshes. It does come into the tank from both the downpipe and the overflow pipe. Would this still fit into your theory?
