I have noticed that there is only about 2 degrees temperature drop between the water from my boiler going into the hot water cylinder primary coil at about 72degC and coming back at just under 70degC. This is when the boiler has just fired up, the circulating pump is on minimum speed (1) and the water in the hot water cylinder is at about 40degC. Is this small temperature drop through the primary coil normal, or is this a sign that the coil is caked with limescale or something? The hot water cylinder does eventually heat up to 60degC (the set point), but it takes at least an hour. The boiler is a powerful one (40kW) and is on hot-water only mode at the moment. It fires for just 60secs or so, then off for 5-6 minutes, and repeats this cycle until the hot water cylinder is up to temperature.
Unless the coil is in cold water you will not get a large difference between the flow and return temperatures. If you want to check for limescale round the coil, turn off the water into the tank, partly drain down, remove the immersion heater and have a look in the tank.
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