Idiots guiide to best use of ASHP please!

redbuzzard

Ceol said:.

What might actually make a difference in your case is further hot water adjustments. To what temperature do you need the hot water? The lower the better with a heat pump really, although you'll probably want at least 45C. Then you'll need to think about how often your heat pump does a disinfection cycle when it heats the hot water cylinder up higher to kill legionella bacteria. How often does yours do this? You shouldn't need it more than weekly, and some controllers allow fortnightly.

I'm concerned that turning off the "always on" immersion heater will have stopped the legionella cycle.  The immersion heater switch is usually left on to give control to the heating controller, which will run the legionella cycle by using the immersion to heat the water up to 60-65 for a couple of hours once a week.  Mine is actually set to fortnightly at 62C; otherwise the hot water is set to 48C.  

Astria

redbuzzard said:

Ceol said:.

What might actually make a difference in your case is further hot water adjustments. To what temperature do you need the hot water? The lower the better with a heat pump really, although you'll probably want at least 45C. Then you'll need to think about how often your heat pump does a disinfection cycle when it heats the hot water cylinder up higher to kill legionella bacteria. How often does yours do this? You shouldn't need it more than weekly, and some controllers allow fortnightly.

I'm concerned that turning off the "always on" immersion heater will have stopped the legionella cycle.  The immersion heater switch is usually left on to give control to the heating controller, which will run the legionella cycle by using the immersion to heat the water up to 60-65 for a couple of hours once a week.  Mine is actually set to fortnightly at 62C; otherwise the hot water is set to 48C.  

Do also remember that legionella is only typically an issue if the water is stagnant and at the optimal breading temperature, so if you heat a tank and then use that tank full during the day (or even 2 days), you are basically flushing the system anyway and so don't need a higher temperature cycle.

matelodave

How do you kill legionella?

Once hot water temperatures reach 50 degrees, the Legionella bacteria will begin to die off. The speed at which this occurs depends on how high the temperature goes, the hotter the temperature the more effective the kill rate.

A two-hour period where the water is maintained at 50 degrees Celsius will get rid of 90% of the bacteria. If the temperature of the water is raised to 60 degrees Celsius, the same percentage of bacteria will die in just two minutes. Raise that temperature again to hit 70 degrees Celsius and all the Legionella bacteria present will die straightaway.

Also, Chlorine in mains water kills legionella, so assuming that most people with heatpumps or boilers with none-vented hot water cylinders get their water directly from the mains which is chlorinated, there's significantly less chance of having a problem with Legionella than say with an open vented system with a tank in the loft.

As Astria points out, most people dont store their water for long periods nor do they pump it around major buildings like hospitals or care homes, gymnasiums etc where lots of people congregate and hot water can sit in long runs of pipework and is fed from roof mounted storage tanks.

I'm not suggesting that we all become complacent, but IMO the control of legionella has become hyped out of all proportion to the risk in a domestic hot water system but TBH running a heatpum for a couple of hours once a week to get the tank up to 55 which is below the temp that the immersion is required (most can't manage more that 55 degrees and need the immersion to kick in to get the temp above 55)

In my opinion, I stand more chance of getting mown down by a nutty cyclist than I do from getting legionnaires disease from my hot water system. 

Ceol

matelodave said:

How do you kill legionella?

Once hot water temperatures reach 50 degrees, the Legionella bacteria will begin to die off. The speed at which this occurs depends on how high the temperature goes, the hotter the temperature the more effective the kill rate.

A two-hour period where the water is maintained at 50 degrees Celsius will get rid of 90% of the bacteria. If the temperature of the water is raised to 60 degrees Celsius, the same percentage of bacteria will die in just two minutes. Raise that temperature again to hit 70 degrees Celsius and all the Legionella bacteria present will die straightaway.

Whilst I agree with your points for my own purposes, it is worth considering that the point at which the hot water tank thermometer is doesn't necessarily reflect the temperature at all points in the tank, so whilst that point (often about half way up) could be 50C, it could be colder further down and so not lethal to legionella.

As I said, I don't disagree with you and I don't particularly worry about legionella, but having the full information is useful for everyone else to take informed risks.

downshifter

Just wanted to thank everyone for the comments and advice, I have put a chair next to the airing cupboard as I seem to spend so much time there looking at all the different variables and changing a few things - though I do think the settings are probably near enough ok and I certainly understand better.  Incidentally, regarding the immersion heater, it was on all the time previously as I had a woodstove which was supposed to heat the water but didn't enough - and obviously not at all in summer.  Now the immersion clicks on to heat the water about every 10 days or so for legionella purposes, but the ASHP now provides plenty of hot water otherwise.  I've been taking meter readings every 24 hours so will keep a check on how it goes with turning down different radiators etc until I get a point that works for me. 

Windows are open upstairs not just for normal ventilation, after a shower etc, but just for a good 'blow through'.  I know, I sound like my granny but hate to feel airless or that the house smells of dog!!

The other aspect of my new found knowledge is that my neighbours are due to have their pumps fitted soon (social housing) and hopefully I can help them with theirs.  One of them has various disabilities and I can't think how she will cope, and the others have a stove which they are happy with and don't want a ASHP.  (They get free firewood)

Thanks a million again  -  DS