ASHP - Legionella Cycle

nxdmsandkaskdjaqd

My Legionella cycle is currently set to rum every week at 1am to a temperature of 60C. This setting currently uses about 4.5 kwh.  My normal DHW is set to 45C. Wanted to know what others have their ASHP set to?  

MP1995

nxdmsandkaskdjaqd said:

My Legionella cycle is currently set to rum every week at 1am to a temperature of 60C. This setting currently uses about 4.5 kwh.  My normal DHW is set to 45C. Wanted to know what others have their ASHP set to?  

Cheaper to heat you hot water during the day in summer and winter with your ASHP. But easier in summer no heating running.

45oC and Legionella every couple of weeks in tune with cheap agile prices in summer.

matelodave

I turned mine off and do it manully when I think of it, about once a month.. The water comes direct from the mains so its full of chlorine and the water gets changed regularly so TBH I dont see the need for a weekly cycle.

NedS

We keep our DHW at 45C (although the misses would like it hotter) which is purely by ASHP.

We manually run a legionella cycle as required when rates are cheap - I first heat the DHW as hot as possible by ASHP (ours will get the tank up to ~57C), before allowing the much more inefficient immersion to kick in for the last few degrees. The last cycle was two days ago (10th) and used ~2kW so your 4.5kW seems highish. Is that input energy (used), or generation? Also, how big is your DHW tank? Using the immersion really bumps up the energy usage & cost, so try to run your legionella cycle when the tank is already hot to minimise the amount of work the immersion is required to do.

Also have a read of opinion about whether a legionella cycle is even required. ASHP water tanks are closed pressurised systems - how do you envisage legionella entering the system? Also, having a high turnover of water minimises the risk, so if you are emptying and reheating your tank twice daily then there is little opportunity for legionella bacteria to grow. Something like a Jacuzzi hot tube presents a far greater risk than the DHW cylinder in an ASHP installation. I tend to run our legionella cycle more as a way to heat the DHW tank as a store of energy when prices are free/negative, than from any concern about legionella.

Reed_Richards

I used to heat my water to 50 C but now I get cheap night-rate electricity I take it up to 55 C between 6:00 and 7:00, drop it down to 45 C a few hours later and off overnight.  For reasons I won't go into, my hot water loses heat faster than in many systems but most of the time it doesn't need re-heating during the day.

If it's a sunny day in summer, I might use solar power directed to the immersion heater to raise the temperature to a bit over 60 C.  Last winter I did not bother with a Legionella cycle, hoping the daily 55 C would be enough.      

FreeBear

matelodave said:

I turned mine off and do it manully when I think of it, about once a month.. The water comes direct from the mains so its full of chlorine and the water gets changed regularly so TBH I dont see the need for a weekly cycle.

With my old vented DHW tank & gas fired back boiler, I rarely heated the water above 50°C. As you point out, incoming water is treated, so with an unvented system, the risk is very, very low (not quite zero, but pretty darned close).

Spoonie_Turtle

I can't remember if our water is 50℃ or 45℃, but I'm not at home to check.  Legionella cycle is weekly at 60℃ and I don't know exactly how much it uses but between 2-3kWh in that timeslot for the whole house (varies slightly by ~0.5kWh). 

[I know the risk is miniscule but for various reasons, for us the extra 1-2kWh per cycle is worth eliminating it.]

nxdmsandkaskdjaqd

Thank you the replies above.  I have just realised something based on the above.  The heat pump runs for about 45 minutes, after that, the remaining time comes from the grid.  I now need to change the settings such that I can manually set the  Legionella cycle to run during the day and I can call upon the cycle to run once the DHW cycle has run.

nxdmsandkaskdjaqd

NedS said:

The last cycle was two days ago (10th) and used ~2kW so your 4.5kW seems highish. Is that input energy (used), or generation? Also, how big is your DHW tank?

It's a 200lt tank and the electric usage is taken from the smart meter data.

NedS

nxdmsandkaskdjaqd said:

NedS said:

The last cycle was two days ago (10th) and used ~2kW so your 4.5kW seems highish. Is that input energy (used), or generation? Also, how big is your DHW tank?

It's a 200lt tank and the electric usage is taken from the smart meter data.

Thanks. It's important to understand how the legionella cycle operates for your ASHP. Is the immersion running the whole time or just kicking in later in the cycle? If it's running the whole time, you should see a cost benefit from heating the tank as hot as possible first by ASHP (and preferably during the warmest part of the day), and then running the legionella cycle (immersion) to take the tank temp up to your desired 60C. As I said, our ASHP is capable of heating our DHW tank to ~57C, but some ASHPs can heat hotter depending upon the refrigerant. 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. Some high temp ASHP (R290 based) should be able to hit 60C DHW temps without the need for an immersion. Mine, which is R32 based, cuts out when the LWT hits 60C hence can only heat the tank to a temp 3-5C below this.

MP1995

Ditto to the above our heat pump without the heater element 3kwh can only get to 57oC but if it's kept there at 30 mins that more than enough and not worth employing the 3kwh heater.

Unless it's a plunge price agile event where we get paid for using electricity then the hot water is only heated using the 3kwh heater element.