Plumbing

Telford cylinder - what’s the right thermostat setting for an horizontal unvented indirect cylinder?

Anonymous user 22/10/2026 - 8.00 AM

Hi, I have a Telford unvented indirect water cylinder (horizontal 250l), which causes overheating issues with water dripping in tundish when the boiler kicks in. We tried replacing TPRV and boiler thermostat but still the same issue. We finally decided to take immersion heater out and took a peek inside the tank, which looked very clean and fine. One thing that took my attention though was how short the boiler thermostat probes are. My plumber thought they might struggle to read the right temperature. See photo attached. Then, we called the Telford technical support which claimed that the design is fine and since the thermostat is at the coldest spot of the tank, the official recommendation for the boiler thermostat is 40 degrees. As mine was set to 60, the guy said I was basically overheating the tank. It sounds fine in its own right, but how about legionella? I thought every hot water tank must be set to 60 degrees to avoid legionella, no matter whether its a horizontal or vertical tank, regardless of the manufacturer etc. I can’t see this 40 degree recommendation on Telford’s installation booklet either. Has anyone had such an issue before and do you thinks it’s sensible to set it to 40 degrees?

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Dunstable

Hi, Couple of check: 1. Is there an expansion vessel installed on cold feed to cylinder ? If not, then you should get one installed. If yes, has it been checked? 2. Has the function of the thermostat been checked? Does the water temperature at the nearest faucet match the termostat settings? Bacteria protection can be achieved with what we’re calling “nuke”. Immersion heater on a programmer 65deg C once a week ( for 1-2 hours), paired with tap flushing. This will reduce the energy consumption compared with keeping the temperature at 60 degrees C continuously.

Answered20 October 2026

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Epsom

Sounds like the expansion vessel has failed or incorrectly installed causing the drip. The over heating can be a number of issues faulty motorised valve, faulty thermostat, etc, we generally set our stats to 55 degrees and what what your saying about legionella’s is correct

Answered22 October 2026

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