pumping up cold feed

[rbanx]

Gravity DHW (Dumball) with feed and expansion coming off flow and return at cylinder. Heating is controlled by room stat and pump on return to boiler.
DHW only does work OK, when heating is called hot water is pumped up DHW return past cylinder and up cold feed into F & E Tank. Suspected sticking Dumball but checked and is OK, now suspect pump. Ideas would be appreciated.
Heating appears to be a one pipe system and at one point has a 3inch iron pipe running through the loft this empties when the pump is on. It did work for last 10 years.

 

[seco services]

you say you have a restriction or a blockage sludge etc in the system.

 

[rbanx]

Seco Services, thanks for the reply, no I don't think there is a blockage. When I was draining the system to check the Dumball, which is on the heating flow above the boiler, with water still in the system it was coming from both connections to the Dumball. All pipework near the boiler is 28mm copper it then goes into 2.5 inch and 3inch iron, I can't see the iron being blocked. I still suspect the pump, is it possible for the pump not to have enough force to lift the Dumball, then pump up the DHW (cylinder) return.

 

[seco services]

i don't suspect the dumball and if i did i would be looking for it to pump up the vent not the cold feed, unless vent is blocked.

 

[rbanx]

Checked Dumball, that is OK. do you feel pump does not have power to lift it and is then pumping up both cylinder flow and return.

 

[seco services]

no i don't suspect pump either.

i'm with crud in system.
have you got a drawing of the system pipework ?