ASHP - Legionella Cycle

FreeBear

NedS said: The aim is to minimise use of the immersion heater which will only ever be 100% efficient and rely as much as possible on the ASHP which may be 200-300% efficient.

With the weather as warm as it has been today, 400% or even 500% efficiency might be achievable if the heat pump is run midday.

matelodave

I usually have my water to heat once a day to 45 degrees. On the day that I run a legionella cycle, the water heats to around 50-55 degrees using the heat pump and its only the last bit that uses the immersion heater to get it up to 60.

However the advantage is that the water doesn't need heating the next day as it usually stays around 45 degrees so I doubt that it costs much more unless you are a heavy user of hot water.

Strummer22

It's probably somewhere in legislation that it should be run weekly as the default but in practice that's much more often than is necessary. I wish the controllers allowed you to set a monthly cycle; as it is, I have it turned off and run it every so often when I remember. I've got a GSHP and not on a variable tariff, so doesn't really matter when I run it (so long as I remember the water will be hotter than normal afterwards and don't scald my kids in the bath or something). 

DHW set to 47C otherwise. 

Reed_Richards

In 1998 I bought an Atag gas boiler.  My OH doesn't like scalding hot water coming out the hot taps so we set the tank temperature to 50 C (this was controllable).  But once a week, early on a Sunday, the boiler would run an automatic Legionella cycle and take the cylinder temperature up to 60 C.  I think this was fully automatic, if I could control it I never found how.  I mention this just to illustrate that Legionella cycles are not new and not confined to heat pumps.