Overflow water pouring out from the loft to the garden

[purpledog]

Hi,

This is a cold winter and it seems we have pushed our gravity system to its limit...
today, after 4 hours of non-stop heating (thermostat at the maximum), a lot of water suddenly poured out on the patio of my back garden (I'd say 5 pints maybe).
I rushed upstairs and stop the heating. It made some very weird sounds (cannot describe it, sorry!) but the water stopped pouring out...
I had a look in the loft and it seems that the water is coming from the small tank. I opened it and the water was very hot, is that normal?
Note that the water could also come from the bigger tank (filled with cold water) because there's also a vent pipe here.

I know started the system again (lower thermostat) but I'm not exactly feeling confident about this...
The pump is working (I can sense it with my fingers) and it is set on "low speed".

Well, that's it... So far so good... but if you have any idea of whwt went on please tell me!!

Thanks,
Frank




Here is the system description:
The boiler is an "ideal w2000"

This is the pump:
MYSON unit fove
240v 1-50hz 0-49A 115W
code 890 U5C252


There's also another device around:
MYSON AERJEC II 0290

There's two valves attached to the hot water tank:
1) SUNVIC SZ1301
2) DANTOSS RANDHILL LTD
"auto/man" switch
MK429ER TYPE HPA2

 

[twgas]

quite possibly a build up of muck in or around where the cold feed joins the system