How many times per day/week should our water tank be heated from 55c to 60c?

Dano2k

Hello there.

I have recently moved into a new home and i'm a tad stuck setting up our heating correctly.

Our home is fitted with a 5Kw Mitsubishi Ecodan Air Source Heat Pump.

The Heat pump heats the water tank during 'off-peak' hours twice per day, every day (these are my new settings).

The 'Ecodan' unit is only capable of reaching 55C so an immersion heater is included within the water cylinder to take the water tempature up to 60C and killing any legionella bacteria in the process.

The immersion heater is set using a time clock and separate to all of the other heating functions. The manual suggests only using the immersion heater after the Ecodan has completed a heat cycle to save as much money as possible. As standard, the immersion heater was set to come on in the middle of the day (nowhere near the Ecodan opperation) and stay on for a few hours... (not very 'money saving'!).

I have now switched it off until I know how to set it properly.

My question is -
How many times per day/week should I opperate this immersion heater and heat our water to 60C?

(Please bear in mind the Ecodan will still reach 55C at least twice every day.)

I was considering setting the immersion heater to operate only once or maybe twice per week.

Some people have even told me to leave it off altogether.

If you have any knowledge about legionella bacteria or heating I would be very grateful for your advice.

All the best,

Dan

paceinternet

Many people set their systems to 1 hour once a week.

If the Ecodan really is achieving 55 degC throughout the tank, then the following statement suggests Legionella is less likely to be a problem, so once a week could be ok:

Legionella bacteria are ubiquitous in all water systems and their growth occurs at temperatures in the range of 20 to 45 degC.

But the following is an extract from the same report which says it should be done every day:

LEGIONELLA BACTERIA AND SOLAR PRE-HEATING OF WATER FOR DOMESTIC PURPOSES Report by Dr T Makin1 for the Water Regulations Advisory Scheme

The HSE recommendation to achieve a temperature of 60OC in all parts of hot water storage vessels for a period of one hour daily, has taken into account the higher concentrations of legionella bacteria that may be present at the base of these vessels, and has also made allowance for the more thermotolerant species and for the protection afforded to legionella bacteria by deposits that accumulate in hot water storage vessels. Daily pasteurisation is proposed in L8 in order to control legionella bacteria that may survive this process (e.g. those protected within amoebal cysts or deeply embedded in scale deposits).

albyota

an excerpt from WRAS :-

GROWTH

• Legionella bacteria grow readily at temperatures in the range 20oC to 45oC.

• Legionella bacteria in concentrations of 100,000 colony forming units per litre
(cfu/l) and higher are not uncommon at the base of conventional hot water
storage vessels where temperatures of 20 – 45oC are maintained.
(For comparison HSE Code of Practice L8 recommends review of control
measures where Legionella concentrations are in the range 100 to 1,000 cfu/l
and corrective action where they exceed 1,000 cfu/l.).

• Under optimum conditions, Legionella bacteria can multiply from background
numbers to dangerous concentrations in less than five days.

THERMAL DISINFECTION

• The effectiveness of inactivating Legionella bacteria using raised temperatures
(thermal disinfection) depends upon the temperature and how long the bacteria
are exposed to that temperature.

• Thermal inactivation of Legionella bacteria starts around 50oC but is quicker
at higher temperatures.

• Legionella pneumophila requires on average 3.2 minutes exposure to 60oC to
inactivate 90% of the bacteria.

• Where the water contains 100,000 cfu/l Legionella, the bacteria need to be
held at 60oC for approximately 10 minutes to reduce numbers to below the
HSE action level of 100 cfu/l.

• Hot water storage cylinders that maintain a temperature of 60oC throughout
the whole storage vessel for a period of one hour daily should achieve
satisfactory control of Legionella bacteria, in line with recommendations in
HSE CoP L8.

more info here :- http://www.wras.co.uk/PDF_Files/Preheated_Water_Report.pdf

albyota

pace' beat me to it.

Dano2k

Thank you for that rapid reply 'paceinternet'.

Reading that second part really makes me think it should be every day...

For the time being I have set it to Saturday only. I estimate it will take approx 25 to increase from 55C to 60C.
I think I will keep an eye on the water temperature to check if it is reaching 55C as expected.
I don't really want to change it to every day but at the same time I don't want to cause a risk to my own or anyone else's health.

I am so hesitant to set it every day simply due to running costs (explained below).

A 3Kw immersion heater...
...set to run once per day will run for 9125minutes (6.33days) per year
...set to run once per week will run for 1300minutes (0.90days) per year


I would be interested to know if anyone else has a similar set-up? What they use, or what they think is appropriate?

Thank you!

Dan

Dano2k

"Under optimum conditions, Legionella bacteria can multiply from background
numbers to dangerous concentrations in less than five days."

And I'm starting to change my mind already... ha.

albyota

I too have an Ecodan with a 210 Litre DHW tank, I dont use the immersion heater at all now, but that is my choice, at first, I used to have it come on once per week for one hour on a saturday night.

Dano2k

Ok, well now knowing the hotter the better, maybe I will adjust mine so it still continues to operate once a week but instead of limiting it to the 25 minutes and reaching 60C I will let it run for a full hour (one Ecodan reaches 55C) and hopfully reach 65C+, that way I know it will be over 60C for a good couple of hours.

Dano2k

One last question if I may...

Sorry I don't know much about heating systems. I have read the manual and good 4+ times now and I can't find any mention of how long heat is held within the tank.

I know all tanks/systems are different, I'm really just trying to get a better understanding.

I heat my 180 litre cylinder tank to 55C over an hour and a half period starting at 3:30am. How long would I expect this tank to stay warm for?
I have a second 1.5 hour heat cycle take place in the afternoon (14:30) but I don't know if this is really necessary. I have heard some people only heat the tank once per day.

There are only 2 people living in my home.

Thanks again

Dan

HappyMJ

I wouldn't worry too much about the electric usage it'll probably only take a few minutes maybe 10 for it to increase the temp from 55 to 60 and the thermostat will cut off the immersion heater. The immersion won't be on for the whole hour.

180 litres of water heated to 60 can be warm i.e over 40 for nearly 2 days before the water is too cool to be useful.

Dano2k

I'm not convinced it will switch off as I've seen it above 65C and it doesn't switch off until the end of the timer... not the best system.
I'm going to run it for an hour tomorrow (after heat cycle) and find out.

Thanks for the tank info, didn't know it could stay that hot for that long! Beginning to make me think I could probably loose the afternoon cycle altogether. May trial it one day and keep an eye on temperatures.

Thanks all.

Dan