Central heating layout, expansion tank overflowing


1 Jun 2018
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United Kingdom
In our new house there is a problem with the central heating system. The header tank regularly overflows when the central heating is on - every few minutes. There is also air in the central heating and the header tank also takes in more water through the valve. The vent from the system into the header tank regularly discharges water into the header tank.

Attached is an image of the layout. I have no idea what the pipe marked ?? is but the pump on it just has a normal 3 pin plug which isn't plugged in.

I suspect the radiators/pipe work are sludged and this causes the pump in the basement to discharge water into the header tank, does that seem feasible or is this layout just incorrect? - i haven't encountered one like this before.

The two motorised 2 port valves are marked with M. The pumps are marked with P


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In our new house
has the system been serviced by some one with some knowledge of dual input systems ? You really need onsite inspection to see whats going on.Hopefully its not a diy install :sick:
The house has been empty for a year or more and we've just moved in, so nobody knows the answer to that - we're waiting for our first visit, inspection and service from someone qualified currently but have been trying to work out what the layout is and whether it can ever work correctly as it is, or needs altering.
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That reply is slightly too cryptic to help unfortunately.

Forgetting the aga side, which is purely driven without pumps using hot/cold water difference and can be turned off and isolated with no change to the problematic behaviour.

The central heating pump in the basement, even on its lowest setting, causes a steady stream of hot water to discharge into the header tank. The layout has this pump, then the boiler, then to a tee between the valve to the radiators and the vent pipe. What is to stop the pump just always pumping through the vent pipe?

Swapping where the feed and vent pipes connect to the rest of the system might help, but then would the CH pump send water the wrong way up the hot water boiler return, through the hot water pump near the cylinder, and again into the header tank? How much restriction does a not running pump provide in the reverse direction to its normal flow direction?

Thanks for any insight anyone can shed.
Assuming I'm reading it rightr igh heating pump on the return is isolated from the expansion tank feed by the motorized valve and hw pump. This is pulling on the rad circuit and if there's anywhere water can enter it would pull it in and push out water through the expansion pipe. In your case it looks like the expansion and feed pipes are so far apart they would serve this all on their own. It may also pull in air if the pressure difference is enough.
There may be a connection on the Aga side that isn't clear though?
Ps disclaimer: I'm not a pro!
It's a Grant Vortex 36/46 Oil Boiler.

The heating engineer will visit later this week, so hopefully get some more insight then!
The unplugged ??? pump is probably a secondary circuit to hot water taps.
Thanks, that might well be it - although the pump direction arrows are pointing into the cylinder (on the highest of the three connections on that right hand side) which seemed the wrong way for this. Everything hot water related seems to work so it's likely now old /unused.
If I'd done it I would have run a vent and cold feed from the F+E tank right down to the oil boiler, and used a galvanized metal F+E tank because of the AGA.;) As well as your illustrated F+E pipes to the AGA.
The feed is on one side of the Grant boiler and the vent on the other. Where is the neutral point?

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