provided you have a typical modern indirect hot water cylinder, having a small feed and expansion tank in the loft to feed the boiler and heating, it is OK to run the immersion heater with the boiler drained.
Post a pic of your cylinder showing all the pipes that go into the side of it if you are unsure.
Yes, it's a modern (?) indirect hot water cylinder, having a small feed and expansion tank in the loft to feed the boiler and heating. Boiler is a Potterton Netaheat 10-16 Mk11 F.
System is over 20 years old hence I'm currently replacing radiators and plan to have a new boiler installed next summer.
Just wondered if there was anything I'd missed in my thinking about running the immersion heater.
the immersion heater dips into the water in the cylinder. The water you have drained out of the boiler passes through a coil in your cylinder, it does not mix with the water you use in your taps, it is a separate sealed circuit, so draining the water from the boiler does not drain the immersion, so it is OK.
If you do not use the immersion heater often, check the temperature of the hot water. Sometimes old thermostats fail and the water can get dangerously hot.
Modern thermostats have an extra overheat thermostat, but yours probably doesn't. However if it is not too old, you can replace the main immersion thermostat quite easily (no need to drain the cylinder)
there are some old cylinders that work differently, but they are increasingly rare.
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