Just like unruly teenage children, that big copper hot water cylinder in your airing cupboard needs boundaries setting. Yes, and that’s your job too. The boundary in question is how hot the water in your cylinder is allowed to get – for a number of reasons.
Unnecessarily hot cylinders waste energy. All else remaining the same, the rate of heat loss is proportional to the difference in temperature between the water in the cylinder and the air surrounding the cylinder. A cylinder with hot water stored at 45degC – hot enough for most purposes – will lose heat around 40% less quickly than one that stores hot water at 60degC.
Unnecessarily hot cylinders get more scaled up. Those of us in hard water areas are constantly battling limescale – this becomes ever more of a problem the hotter the water in our cylinder is. Amongst other issues, limescale on the heat exchanger coil insulates it, reducing the efficiency of heat transfer into the cylinder.
Unnecessarily hot cylinders can result in scalding water. Hand hot water of 45-50degC is usually perfectly sufficient, although there is sometimes a case for (temporarily) higher temperatures to kill legionella bacteria on a scheduled basis. Hot water over 50degC should be blended with cold water to prevent scalding at the tap.
Boilers that are not running hot enough can also waste energy. Surprise! From time to time we come across a cylinder thermostat that is set too low for the boiler to satisfy. In general, the water leaving the boiler needs to be around 5degC hotter than the setpoint temperature on the thermostat, although this will depend on the pipe run length and the presence of pipe insulation. For example, if the cylinder thermostat is set to 55degC then the boiler ‘flow temperature’ will need to be at least 60degC to ensure the cylinder setpoint is reached. If this is not the case then the boiler and primary circuit to the cylinder can keep endlessly cycling, at least until the programmer calls a halt (if it does at all) – this can result in enormous amounts of energy wastage in the circuit and circulation pump.
So what to do? Cylinder thermostats should be set at a moderate temperature of around 45-50degC unless there are vulnerable people living in the house, in which case 60degC (with the addition of a blending valve) will ensure any lurking bacteria are killed. An appropriate boiler temperature should then be set, i.e. around 5degC higher than the cylinder thermostat setpoint – check to ensure the cylinder thermostat is eventually satisfied after an hour or so of heating up, and tweak the boiler/thermostat if necessary. If you do turn the thermostat down make sure there is enough hot water for everyone still as this effectively reduces the capacity of the cylinder – tweak the hot water timings on the boiler if necessary.
Some cylinder thermostats on pressurised cylinders are buried under a plastic cover which can be removed with a screwdriver – in these cases there is often no marked temperature scale, however you can usually download a manual for the thermostat online to give you an idea of how you should adjust the little dial (do a Google Image search if necessary to find the model).
One or two programmable cylinder thermostats are available – these allow you to run a hot legionella cycle at 60degC for an hour or so each week and then reduce the setpoint the remainder of the time, reducing standing losses. In this case the boiler output temperature must always be set high enough to satisfy the legionella cycle, i.e. at around 65degC.