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I understand that it doesn't need a diverter valve... But I was thinking, couldn't the system be more efficient with one?

For example, the central heating could run at a low temperature (35°-40°) and when the hot water cylinder calls for heat it can go to the necessary 65°-75°.

Why isn't a combi boiler system used that uses the hot water temperature setting for the hot water cylinder and the central heating temperature for the central heating?

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Radiators don't put out a lot of heat at that temperature, so you'd need a lot more area, so the heating loop usually runs warmer than you'd really want the hot water cylinder to get up to, so the boiler thermostat setting is a bit of a compromise with a system with no diverter.

Combi boilers that are either heat or HW but not both, often do have different temperature settings for the two modes, determined electronically using a thermistor sensor on the boiler, but regular systems with CH/CH+HW/HW modes selected through a timer plus room and HW tank thermostats, usually have a single mechanical boiler thermostat setting, since the boiler's control system is usually pretty simple and independent of the external control circuitry. On old pilot light boilers, there may not be any electrical circuitry around the boiler at all, just a mechanical thermostat operated gas valve.

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  • This seems to answer how the system overall works... But I'm not sure it completely answers my question about efficiency. I suppose if I had to rephrase it - why isn't a combi boiler system used that uses the hot water temperature setting for the hot water cylinder and the central heating temperature for the central heating?
    – CurtisJC
    Commented Jul 22, 2019 at 21:46
  • That's exactly what a combi boiler system does. On those, the diverter valve is built into the boiler, and run by the same control system as the boiler temp control (and there's often a timer built in too), so there's the possibility of better integration of the various controls. Regular systems are usually assembled from separate components - pump, diverter, stats, and haven't really changed in decades, opting for the cheapest installed cost. Running the CH loop cooler might be fractionally more efficient, but it'd cost more in installation with bigger radiators plus an additional thermostat.
    – Phil G
    Commented Jul 22, 2019 at 22:02
  • It sounds like I've completely misunderstood how the system works in the first place. Your answers have been very informative and I understand a bit better how it all works now.
    – CurtisJC
    Commented Jul 22, 2019 at 22:16
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That type of water heater doesn't make the hot water on demand. Instead it keeps the tank of water hot at all times.

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  • It still requests heat on a timer right? I've currently got it set up to heat an hour in the morning and an hour in the evening. This means the boiler will go up to the 65° I've set the boiler at to heat the hot water tank. It will also get that hot for the central heating if either zone (in my case the upstairs thermostat or downstairs zoned underfloor-heating system) calls for it.
    – CurtisJC
    Commented Jul 22, 2019 at 20:54

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