Boiler hot water setting for savings

Bendy_House

So, the DHW flow through a Thermal Store is not isolated from the tank?

And, in case anyone is in any doubt, being on this forum right now, carries a risk. You have been warned.

ComicGeek

Bendy_House said:

So, the DHW flow through a Thermal Store is not isolated from the tank?

It is isolated, the only interaction is heat transfer through the plate heat exchanger within the store. I think what Section62 is suggesting that both sides of the system can breed legionella independently, and therefore the risk of both elements need to be considered - however, the whole point of the thermal store is that it allows for differing temperatures so I don't see any issue there.

The issues are actually temperature, time and stagnation, as legionella doesn't establish in moving water. Well used systems where the warm water doesn't sit for extended periods don't have an issue. Most domestic systems have smaller enough pipework and stores that the water can't sit in for extended periods undisturbed. Larger hotel/hospital/commercial systems might have pipework dead legs that sit for weeks unused - in those settings they should run each outlet once a week and check outlet temperatures.

If you are using all sinks, showers and bath outlets at least twice a week then the risk of legionella is extremely low. I keep our hot water cylinder at 55 degrees, set at the lowest I want for washing up - I would probably drop this to 50 if I was just using it for showers. The caveat is that if I'm away for more than 10 days I will always heat the cylinder to 65 for 1 day only, and run the showers (unoccupied!) for 5 mins with the windows open and doors closed to purge the system.

Yes people should be aware of risk of legionella at home, but the 60 degree set point is too simplistic for those who use hot water regularly and are in good general health. The major risk will always be large systems outside of your control, like leisure centres, swimming pools, hospitals, care homes, spa-pools etc. Only about 100 Legionnaires disease cases reported each year in England & Wales, I don't believe any are from domestic settings - I don't believe that's because everyone is diligently setting their boilers/stores to 60 degrees.

With a combi, running the hot water at a lower temperature than 60 degrees shouldn't be an issue if the outlets are used regularly. Again, after being away for more than a week I would increase the water temp for 1 day to purge the system. 

When energy prices were low, a blanket 60 degrees was simple - now it seems crazy to  heat water up to 60, only to then mix with cold water back down to 38 for showers etc. 

Apodemus

Two observations on this discussion:

1)  If you really want to ensure that a hot tank reaches 60C once a week, then the easiest way to automate this would be with a timer on an immersion heater.

2)  The ambient temperatures being quoted elsewhere on this site during the current heatwave are into the area where Legionella growth can happen in any still water body.

Bendy_House

Thanks Comic and Apod. 

Yes, I was aware of what S62 was saying.

Interesting, Apod. For min overall energy consumption, I wonder how effective - and risk-free - a solution could be to charge the hot cylinder to a default usable, say, 45oC, using your gas boiler, with the electric immersion programmed to come on for, say, a couple of one-hour sessions per week at, ooh, 60+oC?

Or is that getting silly and just not worth it?!

Section62

ComicGeek said:

It is isolated, the only interaction is heat transfer through the plate heat exchanger within the store. I think what Section62 is suggesting that both sides of the system can breed legionella independently, and therefore the risk of both elements need to be considered - however, the whole point of the thermal store is that it allows for differing temperatures so I don't see any issue there.

Exactly.

The isolation of one part of the system from another is not a preventative measure alone. The risks applying to each part of the system have to be considered, rather than ignoring one because it is 'isolated' from the other.

The risk in a well designed and operated domestic hot water system is usually low, but never "without risk".

Bendy_House said:

Yes, I was aware of what S62 was saying.

In which case why did you say "So, the DHW flow through a Thermal Store is not isolated from the tank?" ?

At no point did I say that.

ComicGeek

Section62 said:

ComicGeek said:

It is isolated, the only interaction is heat transfer through the plate heat exchanger within the store. I think what Section62 is suggesting that both sides of the system can breed legionella independently, and therefore the risk of both elements need to be considered - however, the whole point of the thermal store is that it allows for differing temperatures so I don't see any issue there.

Exactly.

The isolation of one part of the system from another is not a preventative measure alone. The risks applying to each part of the system have to be considered, rather than ignoring one because it is 'isolated' from the other.

The risk in a well designed and operated domestic hot water system is usually low, but never "without risk".

But there's a point where the risk is so negligible (but not zero) that it can be ignored. Otherwise warnings need to be applied to ditches, rainwater butts, screen wash in cars, water left inside hose reels etc which may have a similar risk level. It's appropriate to ignore a sealed system, we don't worry about legionella in radiator systems over summer.

Even at 60 degrees 100% of legionella won't be killed, I've seen studies where very small levels were found at higher temperatures. Like all things in life we have to set individual acceptable risk levels - if it's a choice between someone being able to afford a healthy meal or keep their hot water at 60 degrees constantly, I know what I would advise.

Apodemus

Bendy_House said:

Thanks Comic and Apod. 

Yes, I was aware of what S62 was saying.

Interesting, Apod. For min overall energy consumption, I wonder how effective - and risk-free - a solution could be to charge the hot cylinder to a default usable, say, 45oC, using your gas boiler, with the electric immersion programmed to come on for, say, a couple of one-hour sessions per week at, ooh, 60+oC?

Or is that getting silly and just not worth it?!

Bendy, I've seen that approach recommended.  Mainly, it has to be said, as an adjunct to a solar thermal system, but I don't see any reason why it shouldn't be used alongside a gas boiler.

If you want a low-frequency high-impact risk to worry about in this weather, do a Google on Naegleria fowleri infection! 

Bendy_House

Section62 said:

Not strictly the case - the water on the stored side may still become contaminated and could potentially pose quite a serious risk to people living in the property and/or neighbours/passers-by.

Also, the water in the HW distribution system can still become contaminated - the isolation of the two parts of the system in terms of water containment doesn't stop the water in the HW distribution system sitting around at temperatures where legionella thrive.

The issues are temperature and time.

Ok, have to admit I missed that you were mainly referring to the stored side possibly becoming 'infected'. Tho' the circumstances where this could "pose quite a serious risk..." must surely be extremely rare?

Do system additives have antibacterial properties for just this eventuality? If not, why not? Surely every stored system water is at risk, especially vented types? The F&E tank must surely be at significant risk, as it'll never, under normal circumstances, reach anywhere near 60oC?

How come folk aren't keeling over every time they go up in their lofts? (Serious question).

rudigarude

I just thought I'd revisit the actual question that was asked, about the eco mode. My boiler is a worcester bosch cdi 30. Actually there are two places that there is an Eco/e on the settings, a button and one on the hot water dial. Bendy mentioned the preheating thing, and on my boiler, that's controlled by the Eco button explained in this video..

https://www.youtube.com/watch?v=UGgXnhYraiY

....and I've not had enabled. I'm such an idiot, that'll teach me for not RTFM. 

Frustratingly, the manual doesn't actually say what the 'e' setting on the dial does, but throughout the manual any of the eco features come with the "only in conjunction with optional diverter valve" caveat. Which I have no idea if we have.

https://www.worcester-bosch.co.uk/support/literature/download/release/6720612454/12539