So in any water system, it is split into 2 parts. Primary and secondary.
Primary, in a combi boiler system, is the water that runs through your boiler and then into the central heating.
Secondary, in a combi boiler system, is the water that runs through your boiler and then to the taps and showers etc. This hot water is then fairly instant.
Primary systems, as they have no risk of coming into contact with people or being ingested, contain inhibitor. This is basically a anti corrosion anti freeze. As it has anti freeze it won't ice up in cold weather so does not need to be drained down.
Secondary systems don't contain inhibitor so have a risk of freezing up. Hence they need draining down.
So in effect you only really drain down half the system, the bit that comes out the taps and showers. The primary side is left alone.
For context, and the reason they have the lables primary and secondary, in a domestic home, pre combi boiler, with a hot water tank, the primary side of the system heats up the water in the hot water tank by being fed through a coil of pipe. The water in the 2 systems never mixes. But for this system the water that does taps etc never enters the boiler but is heated by water from the primary system, hence it is called secondary. Down side of this, you can run out of hot water if too much is used and it takes a while for the boiler, via the coil, to heat the water tank back up. They are coming back into fashion now due to solar power and people using their excess solar generation to heat the water in the tank via the immersion heater, effectivley using it as a type of energy storage battery.
Our gas is fed centrally on the site so we never turn it off, just turn off and power off the boiler come winter time, in fact any time we leave for an extended period of time.
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