DIY Doctor UK DIY Forums

can anyone advise why my small water tank in the loft keeps going to overflow - checked ball valve operation and it closed about 4 inches below the overflow.

What else can cause that ?

s70rds,
If you have a mixer tap in the kitchen the casting may have perforated allowing mains water to feed into the hot water system OR the coil in your hot water cylinder has perforated. Turn off the mains water and see if the tank still overflows, if yes its most likely to be the cylinder.
end

When you say the small water tank in the loft, do you mean the smaller of two cold water tanks with ball valves? If so, another possible cause might be "overpumping" from your central heating system If your pump has variable speeds, try turning it down.