Posted by: @mikefl

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My thinking now is that the immersion heater element runs at COP of 1 (at best); and the HP runs at COP of 1 (at the very worst) so it actually makes more sense for the HP to do all of the legionella work i.e. on Mondays set the DHW to 60°C (the maximum it will permit) for 1 hour at 15:00, and no longer use the immersion heater at all.

Does this seem reasonable?  I also think that 60°C is possibly too high, and something like 58°C would be fine. At the moment the HW is at about 58°C by 17:00 on Monday, and stays above 55°C until almost midnight. That seems more than adequate to kill legionella (over 55°C for 7 hours).

 

@MikeFl

When I was looking at legionella cycles a while back, I found a lot of variation in opinion regarding how often to run the cycle. Regs say one thing, others point out that the bottom of traditional cylinders rarely if ever reaches the required temperature. The actual temperature and duration required to kill the bacteria though was pretty much consistent.

If you can reach the required temperature using your heat pump then yes it makes a lot more sense to use it rather than the immersion coil. The only exception I can think of to this is if you have solar panels and a diverter, which essentially will do it for free (or a loss of export)

Posted by: @bontwoody

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If you can reach the required temperature using your heat pump then yes it makes a lot more sense to use it rather than the immersion coil. The only exception I can think of to this is if you have solar panels and a diverter, which essentially will do it for free (or a loss of export)

For what its worth Vaillant R290 heat pumps (I discovered a few weeks ago) are designed to do the legionella cycle in the heat pump only, and appear (unlike many other HPs) not even to be capable of controlling the immersion.  So thats the decision Vaillant made once they got to the point of having a FT high enough.  

I have a Grant ASHP and have the HW output from it set at 60degC for 75 minutes daily. This will charge my 250litre pre-plumb Grant cylinder to 55degC usually within 1hr, and it maintains this temperature for many hours if no hot water is drawn off. It's more thorough at heating the cylinder than the built in 3kW immersion. If I use the immersion the built in immersion thermostat starts to cycle on and off but the tank content thermostat never reaches above 54degC. The immersion stat is set to it's highest setting, supposedly 65degC.

Also, the tank contents cool much more quickly with no hot water drawn off compared to when I heat using the ASHP. That to me proves that the immersion heating is not very thorough. I have more faith in the ASHP providing better Legionella protection daily to 55degC than the 3kW immersion weekly to 54degC! The ASHP heating coil is lower down than the immersion element and a much larger surface area.

The ASHP will give a CoP of 2-2.5 in heating, so is much more energy efficient than the immersion. I have solar PV+battery and a diverter. Initially I thought using the diverter would be most cost effective, but charging the battery using solar, and then using it to power the ASHP is more economical. I'm using less than half kWh than direct immersion, and can export more surplus PV that way at 15p/kWh. 'Every little helps!' 😀