Someone ("The Todal") on the legal forum has just made an interesting energy-saving suggestion that I hadn't seen before - has anyone here done it? The suggestion is to set the DHW temp lower than would be needed to prevent legionella and to use the immersion heater, on a timer, to boost the temperature to 60+ degrees once a week (or so). It's a long way from the DHW tank to the furthest taps so I have a circulation loop and pump to reduce the time and wastage for hot water to reach the taps, but this means heat loss from the pipework. Running at a lower temperature would seem to be advantageous and the cost+pain of making the change is trivial. Some questions are: how often does the water temp need to exceed 60 degrees? for how long? and are the legs from the loop to the taps a concern with lower temperature?
It's standard fit on all heat pump installs, where the flow temp from the heat pump never exceeds 60C.
(some can do higher, but efficiency suffers so you would much rather not run the heat pump that high).
Mine has a separate DIN-rail timer and contactor to do it - the heat pump itself has a setting and relay contact for it, but they decided not to use that for whatever reason.
You ideally want to arrange a cycle where the boiler heats the hot water up to whatever its native flow temp is (say 50C) and the immersion only kicks in once the boiler has got there. That means you're only using electric heat for the last 10C. My setup has a tank stat that only enables the immersion when the temp is above that threshold.
If your boiler speaks OpenTherm or a similar control protocol, a smart thermostat can tell it to raise its flow temp using gas alone, avoiding needing the electric heat.
The average is once a week, but it depends on the susceptibility of the occupants to airborne pathogens. For healthy people once a fortnight is OK, for eg elderly or pregnant twice a week might be better. (We currently have it off and haven't died).
According to this site, legionella dies at:
2 hours at 50C
2 minutes at 60C 'Straightaway' at 70C
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So a simple bringing up to temp should be enough.
I'm not sure about the pipework runs, but I suppose if you've nuked the pathogens in the tank then any hot water drawn off will nuke them in the pipes. Especially with your circulation system, once it's reached temp it'll nuke anything in the pipes as it goes round. Copper pipework is also anti-microbial. I suspect legionella is mostly going to be a risk in legs that are rarely used, have air contact and don't get properly heated - eg shower hoses that always get mixed-down water.
Are your pipes insulated, BTW? Would seem wasteful to be circulating heat around especially on days when you don't want heating.
The once a week sanitisation thing is standard on some boilers - I have mine set to do a hotter reheat once a week to 70, rather than the normal set temp (which I have at 60)
You could do it with an immersion instead.
Insulating the pipes would be good as well if you have a circulation loop.
However there is a downside to consider, with a lower cylinder temperature you have less total stored energy - so you will run out of hot water faster as you will be unable to mix as much cold in at the point of use.
once a week is ok. If you let it hit 70 just once it will then stay above long enough.
Not really - they will get to flow temp fairly quickly when you draw off water.
Also, the above suggests that a flow temp of 50C might be acceptable if you can ensure the water stays at 50C for 2 hours. For example, if you set the boiler to heat the water in the middle of the night, chances are nobody's going to be drawing much water from the tank for hours and that would be enough to kill it.
I've probably been looking in the wrong place or using the wrong search terms, but I couldn't find a single reference where Legionnaires Disease has been confirmed as linked to a *domestic* hot water supply. Not a commercial or care home, etc, supply, but purely domestic. There are loads of hits about what water temperatures should be to avoid the theoretical risk, how to test, etc, but I couldn't find a single case of actual disease. I couldn't even find a single report where the Legionella bacterium had been found in a domestic HW supply.
Can anyone point to a site(s) where this information appears?
I thought, in the main this nasty only occurs in water exposed to the air for some time, ie its more likely to be water trapped in pipes, in air conditioning ponds where the wind can make droplets etc, than in a really sealed system. Brian
I have temperature sensors on the outside of my hot water tank which are in contact with the copper tank and covered with the thick insulation jacket. One at the top and one approx half way down. The difference in temperature is around 10C when solely heated by the boiler and 15+C when heated by the immersion heater.
On a "traditional" system with a hot water tank the thermostat attached to the tank controls the hot water system so if it is set to say 50C it will stop the boiler when that temperature is reached irrespective of the water temperature supplied by the boiler. There will also be a temperature gradient top to bottom of the tank so even when set to 60C much of the water in the tank will be below that temperature.
More modern control systems that actively monitors the actual temperature of the water in the hot water tank will be able to raise the temperature above normal but for systems installed before modern "smart" controls I doubt that most of the Legionella preventative measures being discussed here would be effective.
Well, from my perspective that was a Rumsfeldian unknown unknown.
Thanks, interesting stuff!
Well, some are and some aren't. It's not a small house and when the heating and plumbing systems were replaced about 5 years ago (it was too big a job for moi so I paid someone who spent about 6 weeks with 4 to 6 people doing it) to my regret I sort of left them to it. Since then I've been gradually lifting floorboards to reduce noise and insulate.
The boiler is reasonably new (5 years) but is a W-B dumb oil boiler. The controls are Honeywell Evohome (which is great) but do not allow for timed changes of DHW temperature.
If I'd done it myself (or realised what the installer was doing!) they would all be insulated, but about half to two thirds of the loop is insulated.
It's quite a large cylinder (300 litres, I think) so that shouldn't be a big problem
The bedrooms have basins and most of them aren't used unless we have family or friends staying, but the leg from them to the loop is quite short.
IME Most traditional tanks with a stat, have the stat reasonably low on the cylinder (say bottom quarter - although in some cases that will depend on the preference of the installer). So they will stop heating when that bit is 60. If most of the water above will be higher than that due to stratification the average cylinder temp should be above the set point.
If heating with a immersion, then you may need to account for the placement when selecting the sanitisation temp. So if it can only heat the top half, heat it hotter than if it can do more of the cylinder.
It tends to be boilers that support smarter controls that offer these types of feature. On mine there is no cylinder stat as such, but there is a NTC thermister that connects directly back to the boiler. The controls connect back vie eBus, so the boiler knows the actual cylinder temp, the set point temp, the sanitisation temp, the date and time, and also knows if it is running the rads or reheating the cylinder. So on so on the required day it will run the cylinder reheat to a higher temp, and also use a higher boiler flow temp when doing it.
You could cobble together a solution with a second cylinder stat set to a higher temperature. Connect both the wiring via a changeover relay, and have 7 day timer controlling the relay. Then one day a week it could switch in the stat set to the sanitisation temp.
I have always ignored the legionella risk for just me. I heat the hot water cylinder to about 35-40C, and never had a problem. Legionella is everywhere (including in rain). Regular exposure confers immunity. Legionella only started to become a problem when we started trying to super-sanitise water systems, and that may be because people stopped being regularly exposed to it.
Having said that, when my parents used to come over, I cranked up the hot water temperature up (and that also means I was storing more of it which was also necessary). I might do the same if my immune system was compromised for some reason.
That would be a jolly good idea BUT it's a W-B Greenstar with what seems like a metal jacket and a port for the temp sensor to slide into. It would need some butchery to be able to get an additional stat in contact with the tank. As an aside, I'm extremely impressed with how well the tank is insulated
- the metal(?) jacket is never more than very slightly warm.
My largish tank has a foam jacket, and that also never feels warm to the touch. However, looking at the numbers on the label, the declared heat loss is equivalent to about 350 W, so simply maintaining its temperature will use up to about 8.4 kWh per day.
I hadn't thought to look for the info ... until now. There's nothing on the tank itself but the spec for a W-B Greenstore SC-300 says "Standing heat loss 2.17kWh per 24 hours" ... something I didn't consider when it was installed. I DID consider (and worry about) the weight of 346kg, especially as it's sat next to a socking-great oil boiler (nearly
300kg), but the slab hasn't done anything nasty after 4-5 years so I guess it isn't going to move now.
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