DIY Doctor UK DIY Forums

Hi, I wonder if you could help me. The overflow pipe drips intermittently from the large cold water storage tank in the loft. Not the small F and E tank. The ball valve has been replaced and is working. The vent pipe from the hot water cylinder is not venting any water into the tank, and I have raised the level of the F/E tank to the same hight as the cold-water storage tank, which leaves me to believe the only way the tank is filling is by the cold-water storage tank feed pipe from the copper hot water cylinder. This is a gravity fed hot water system from a Rayburn back boiler flow and return to the coil in the cylinder along with a backup emersion heater. Could a blockage or a faulty cylinder coil be the cause? There are no mixer taps in the plumbing system, the shower is a Trevi Therm MK1 built in thermostatic shower valve (cold main in /hot in from cylinder). and the system has never been flushed. I have found some build-up of pipe corrosion due to a slightly low ph in the water. I built the house 23 years ago. So, my real question, is there a conclusive way to determine if it’s the hot water cylinder heating coil, the cylinder, a blockage, or all three, before I rip out the lot at great expense?
Thanks for taking the time to read hopefully respond.

Hi lfstf,
You will firstly need to determine where mains cold and tank fed hot are supplied to the same item. You have already identified the shower valve but do you have a one block kitchen tap? Turn the isolating valves to off and see if the problem persists.
As you have identified the other possible source is the heating coil in the hot water tank. Making the water levels in both tanks should minimise cross feeding from the primary to the secondary systems especially since you have a gravity fed primary system.
If you can determine when the water level rises in the CWT this should help identify the problem.
Regards S