Heat pump legionella cycles

Home Energy Systems
1

My Vaillant Arotherm+ heat pump has been providing me with heating and hot water for about 3 weeks and on Tuesday we set up an overnight setback period for heating and a legionella cycle for the DHW.

The setback period is 11pm to 5am, and were going to set the legionella cycle to run at 11pm on a Wednesday. Somehow the installer rescheduled the legionella cycle for Monday afternoon instead! I’m still on a fixed tariff, so time of day makes little difference at the moment.

But before I try changing it back, I’ve been looking at the hourly hot water temperatures downloaded from the My Vaillant app, and referring to the HeatGeek diagram on Legionella and Water Temperature: What You Need To Know.

I use very little hot water, and the hot water cylinder has been sized at 200 litres for the potential future occupants rather than current occupants, which I understand.

The normal setpoint DHW temperature is 42 degrees and it’s set to automatically reheat when the tank temperature drops by 10 degrees. So without running a legionella cycle it spends most of its time with the hot water at between 32 and 42 degrees, which provides ‘optimal growth’ for the legionella.

The Vaillant legionella cycle heats water to at least 60 degrees for one hour. It can be configured for any day of the week, but the frequency is fixed at weekly and the temperature is not configurable.

Since I’m an ‘older/more vulnerable’ person, I am advised to store water at a high temperature, while at the same time avoiding scalding!

Routine water heating can be automatic - when the tank temperature drops to a certain number of degrees below the setpoint temperature, or scheduled for certain times/days (haven’t explored parameters for this yet), or there’s a manual setting.

Is anyone else in a similar situation, and if so have you found a solution to keeping the bugs under control/avoiding scalding/minimising cost of hot water?

I’m curious whether anyone else on here is in the same situation? And whether they have found a good way around it.

1 Like
2

My Daikin system allows an “advanced user” to change day, time and temperature of the legionella cycle, or even switch it off. Is there no way for you to do likewise?

I have changed mine to 50°C for a whole night (23:30-05:30, my tariff’s off peak times), so I slow cook the bacteria and have a less dangerous outflow temperature.

I also have a whole house thermostatic mixer adjacent to the cylinder so that household water cannot exceed 45°C. I had that before the heat pump.

3

Thanks Tim, helpful ideas as usual!

I can switch off the legionella cycle, and change the time and day of the week, but it seems that you can’t change the temperature (docs say 60+°C degrees for one hour), or the frequency (fixed at once a week).

I’d wondered whether slow cooking at 50°C overnight was the answer. But how often? Would have thought 2 or 3 times a week, but not clear yet from the docs whether that’s possible.

I first heard about a ‘whole house thermostatic mixer adjacent to the cylinder’ from a Michael dePodesta blog this week. The heat pump designer/installer didn’t suggest having one of those when I raised concerns about low water use when he was carrying out the design. I need to look into them.

4

I’ve had my thermostatic mixer since 2005, and they weren’t new then but more usual in sports changing rooms, for example, where the showers may not have individual temperature control.

I got mine at the time as my late wife’s condition meant that she had reduced sensitivity and longer reaction times. The same reason and time that I got an induction hob.

5

Householders with a high hot water use don’t really need a legionella cycle as the water gets used before potentially dangerous levels of legionella build up. The lower your consumption (or rather the higher the amount of unused hot water) the more frequently the system needs sterilising. You have a larger cylinder than you need (as explained) and that puts you at a disadvantage.

You could abandon the legionella cycle, which probably uses your immersion heater, and heat the water to 50°C as standard using the heat pump. Your draw off probably means that the temperature will remain in the slow kill/dormant range nearly all the time.

I used to simultaneously be Health & Safety Officer and the Environment Officer at my place of work, so I was used to balancing acts.

I have seen it claimed that thermostatic mixer valves prevent really hot water from clearing the inside of your pipes. This is a misunderstanding as if only sterile water passes through them they shouldn’t need sterilising in their own right.

Another advantage of reducing the temperature earlier is a lower Δt between pipe and room and therefore lower heat loss from the pipes. The better your house is insulated the more import this becomes, so as to avoid overheating.

6

I am a long long way from being an expert on this, but if the original question was about frequency of anti-legionnaire cycle, then my understanding is that running the cycle once a week is fine as the bugs are slow growing to get to levels to cause health issues. If there was any risk with once a week, manufacturers would insist on more often than that - they don’t want to end up in the slammer for manslaughter.

7

Thanks again Tim, and Alan.
Having been monitoring DHW tank temperatures for nearly a week, I’ve realised that it’s at legionella optimal growth temperatures of between 42 and 32 for days at a time.
I’m trying to monitor energy consumption of the DHW cycles, via heatpumpmonitor.org, but seeing some weird graphs which people in the OEM forum are suggesting is air in the system interfering with heat meter readings, and also not great for the system long term.
When that’s settled down a bit, I’ll check DHW running costs as best I can and then look at different DHW cycles to optimise cost v hygiene.

8

There’s a slight flaw in the design consultation here in that although the installer rightly asked whether I wanted to enable an anti-legionalla cycle, he didn’t warn about the risk of scalding - I was the one to raise that, and only then did the installer mention thermostatic mixer valves on all hot water outlets, which would potentially be on 6 sets of taps, and I’m guessing quite expensive. Whereas Tim has mentioned a whole house thermostatic mixer adjacent to the cylinder, which future-proofs things, and I’m guessing would be a lot cheaper.

9

You can easily get 22mm mixers at not much more than a 15mm one, and only labour to fit one. If your cylinder has an exit diameter of 28mm, normal only for commercial environments, I believe the price jumps but you still only have to pay labour for fitting one.

1 Like
10

Continuing the theme of thermostatic mixer values, cheaper models require the same pressure for the hot and cold to effect a mix, some more expensive ones are more tolerant. Even so, the pressure can’t be very different. Normally if both hot and cold come from the same source before heating you should be OK. I would avoid the ones that require exact pressure as they will give a shot of inappropriate temperature if one pressure suddenly drops. e.g. if someone flushes the toilet while you are showering.

I have gone belt and braces on that one and the toilets are not flushed from the same water source.

11

Worth watching the heat geek video on legionnaire’s disease. I’d defer to their greater knowledge in this situation!

Briefly, there’s a temperature at which legionnaires grows, a temperature that it’s dormant, and a temp that kills it. Like others have said, the balance of vulnerability, usage volume, and effeciency/cost goals will vary from individual to individual.

For me, I let the heat pump get up to 43C, then use solar every now and again to sterilise the tank. Scalding is definitely a risk after a hot cycle!

12

Thanks James, I watched the video and read the blog earlier this year, and have been x-referring the DHW stored temps from the MyVaillant app against this graphic over the last week or so

image
image1000×579 100 KB
.
I can’t help thinking that as (older) people switch in greater numbers from combi boilers to heat pumps, the number of people being infected is likely to increase.
And conversely scalding risk is also likely to increase - my installer didn’t discuss the need for thermostatic mixer valves on output from tank or all hot water outlets. Only this morning I had a very slight burn from accidentally touching the base of a hot water tap as I went to turn it off after an anti-legionella cycle overnight.
Sadly there isn’t enough flexibility in the Vaillant controls to do what I want to do which is a custom 52 degree DHW heating cycle once a week at maybe midnight, as well as an auto reheat to a lower temperature when tank temp drops too low in between times, due to visitors or other unusual DHW demand.
I don’t think the scalding risk at 52 degrees would be enough to need thermostatic mixer valves, especially as tank would have cooled down a tad by morning.

13

It’s 2 months since starting this thread, and on the advice of the HeatGeek who came to commission the system, I’ve been scheduling a daily DHW cycle in the afternoons. I’m heating to 50 degrees, and am monitoring the tank temperature being reported by Vaillant, to make sure temperatures are maintained at 45+ for several consecutive hours most days, to kill off ‘most’ bacteria. The lower threshold for a DHW cycle is 40, which explains why some days it doesn’t kick in.
The water coming out of the kitchen taps is always hot enough for washing up, showers and washbasin water temp all ok.

1 Like
14

As an aside, the MyVaillant app allows you to download a number of stats files, which all seem to include data from when I set up the app i.e. you can’t just get the last week or month’s data. I email these files to myself so I can analyse them on my laptop, so have over a dozen sets of data so far. This has highlighted a bug in the software, because successive downloads have sometimes reported different temperatures for the same day! Vaillant are investigating… The latest download was a mess, some additional column headings for different time periods, so hopefully they are trying to fix the problem, although why on a live system? I haven’t had the heart to check if the same problem exists for other sets of data.

15

That is a common sense approach. The sheepish following of the 60°C rule is a waste of energy and money in many cases.