Optimising hot water schedule

[NormanB]

Sounds like a faff to me.

I've been swayed towards heating the HW just before it's needed, e.g., daily showers. If heated to 60c it has a lot less time to lose the heat and as it gets used the temp comes down anyway.

If I don't have a regular shower schedule then a 30 min boost before we need it is probably even more efficient. remaining water should be hot enough for other uses the rest of the time.

Why did you say that you do not understand and then contribute something of real value.

 

[NormanB]

It strikes me that the risk of legionella in domestic premises is vanishingly small. The regulations apply to places like schools, hospitals and nursing homes where the consequences could be serious because of the numbers at risk, and things like cooling towers or plant where many people passing by might breathe in vapours.

There was a PHE report in 2016 covering the 3 previous years, total cases sub 400 each year of which between 40 and 50% were overseas travel related. Of those cases, there were few deaths mainly in older people - overall case death rate 7%. So - we can expect about 350 cases each year in the UK, of which 200 are 'home grown; infections of which 14 might die. I think with those case numbers we have bigger things to worry about in a population of 66 million. Take care but don't obsess about it perhaps.

The WHO guidance repeats what has been stated above: 50 degrees at the outlet, 60 at the boiler, and recommends weekly running of otherwise unused taps to prevent stagnation. I doubt many of us have hot taps which we don't use. Cold to be sub 20 degrees, which isn't likely to be a problem in the UK. Its a respiratory disease, you get it by breathing in sray or mist, not from washing your hands in it. I always wonder about the guidance because shower mixers bring the temperature down, but the water entering the mixer should be above 50 in any event. Still though, it could 'breed' in the shower head which never gets above 50 unless you enjoy coming out of the shower very pink. Maybe if we have been away for a few days - stagnant water in shower hose and head - we should run it as hot as hot can be for a few moments before we use it. Care home sthough have one set of regulations requring high temps to avoid legionella and another one requring mixers to be set below a threshold to avoid boiling the residents. Must be hard to reconcile.

Back to the original question, there is no single answer because its lifestyle related and we all live differently. Combi boilers and conventional HW tank systems have wholly different characteristics. We had new boiler, hw tank and so on 18 months ago. The tank is much better insulated than the old one. The hot water flow to the bathroom/shower is pretty efficient, to the downstairs handbasin its OK, to the kitchen it is awful. This dry summer we were collecting water in a washing up bowl to put on plant pots, typically we would run 5 litres before the water got hot at the tap. There are just 2 of us in the house now (what follows all changes if we have family or visitors to stay).

I used to lazily and unthinkingly have it coming on early am until after we had showered, then again late afternoon for a top up. Calling for heat up to tank thermostat temperature for 2 hours a day. Then I thought, when do we need hot (as in proper-hot) water? Really only to shower each morning plus a bit to wash up things that didn't or couldn't go in the overnight dishwasher run. I'm still unsure about running hot water to the kitchen if its just a small amount to do, a litre in the kettle warmed to 70 and added to a bowl of cold might be quicker and avoids the energy and water wasted in that long pipe run - must do some sums soon.

During the day and evening we only need warm-ish water. I have been pleased to find one 30 minute 'on' cycle just before we get up each day is plenty, and the boiler may only be firing for half of that time. No gas here so its oil, summer usage about 1 litre a day. If I have had a hard-work day in the garden or similar and want an evening shower I just press override and wait 20 minutes or so. No point setting that every day because I don't need it every day. But, we all vary in what we do and what we want so my one half hour cycle a day won't suit all.

As a general principle though, I now think about things a bit more and will set water and heating (coming soon with cool evenings) to the minimum timings and work up from there if I have to rather than my old "just leave it on when we are in" approach. On water, with a decently insulated tank you might be surprised how little you need the boiler to run.

You are absolutely spot on.
The only caveat is a domestic setting with infrequently used showers.

 

[NormanB]

If you are storing the water at 53C bacteria will grow.
If you are heating the water and then using it straight away no problem.

The lenght of time that the water is stored hot will determine how many bacteria will grow.

Not as simple as that.
If you use the water no problem

 

[Glitch]

I believe I have an answer to the unexplained gas usage.

The Nest system has a Bacteria prevention feature. It will kick in if the HW has not been heating continuous for 2 hours in the past 48 hours.
I have played around with the HW schedule to use short bursts (less than 2 hours) so the feature will kick in.

It can only be disabled in settings on the thermostat not in the Nest app.

https://support.google.com/googlene...-GB#zippy=,turn-bacteria-prevention-on-or-off

 

[Richard_C]

The Nest system has a Bacteria prevention feature. It will kick in if the HW has not been heating continuous for 2 hours in the past 48 hours.

Well found and explained. Looks like it will render many people's energy reduction attempts pointless unless they realise that nest-knows-best. Seems wasteful to me as chances of infection are low. Even those without second homes (most nest users?) might not run their boiler for 2 consecutive hours except in winter.

 

[Glitch]

Well found and explained. Looks like it will render many people's energy reduction attempts pointless unless they realise that nest-knows-best. Seems wasteful to me as chances of infection are low. Even those without second homes (most nest users?) might not run their boiler for 2 consecutive hours except in winter.

Nest information seems fairly hard to find. The bacteria prevention feature is not mentioned in the App so people are unlikely to know about it.

I wouldn't have spotted it without the data from the Smart meter.

Now I need to decide whether to disable it or not. Didn't have it before Nest so doubt I need it now, although HW was on longer cycles

The HW schedule is now 0700 - 08:00 and 14:30 - 15:30 plus ad-hoc boosts when necessary.

Still need to work out how to optimise the heating and boiler efficiency.

 

[johna.clements]

Not as simple as that.
If you use the water no problem

Not sure what is wrong with my statement.

"If you are storing the water at 53C bacteria will grow.
If you are heating the water and then using it straight away no problem.
The length of time that the water is stored hot will determine how many bacteria will grow"

 

[davethebb]

The wrong approach, condensing boilers work much better when the complete system has been designed around that type of boiler and not as a retofit in an old system. In a new design the radiators are sized larger to handle the lower flow temperature, with the old boilers with high flow temperatures you would aim for an 11° C temperature drop across a radiator. With a condensing boiler you need a 55° return temperature and using a 75° flow temperature gives a 20°C temperature difference across each radiator. To get this increased temperature difference you need larger radiators to deliver the same heat output because of the lower flow temperatures. Just turning down the boiler and reducing the flow temperature will mean less heat output and it will take longer to heat up a room so no savings. It is like a kettle, to boil a litre of water will require a given amount of energy, whether your kettle is 1Kw or 3Kw the energy used will be the same, only the time changes. This is over simplifying it because you need to take into account heat loss, as you slowly increase the temperature over a period of time your heat losses increase, heat it up fast over a short period of time and more of the energy performs useful work and is not lost.

Absolutely correct. You must have a balanced system as far as possible. However, with TRV's this is not always 100% possible because the flow in the system changes - the new learning pumps are good because they compensate for this and reduce electricity consumption at the same time. Don't reduce the boiler output to obtain the reduced return temperature, that is not the correct approach, the key is to achieve the temperature differential the higher the better.

 

[Glitch]

I get the need for a condensing boiler to run at a lower temp, the need for the temp difference on the return and to use bigger rads.

Sounds like the smart pumps are a recent attempt to give this type of system a chance to work efficiently.

I have 10 radiators and an area of underfloor heating.
Most radiators have TRVs
Most radiators are turned off because it's an empty nest.

I might be able to improve it a bit but I've got little or no chance of getting the best out of the boiler. I'd be surprised if 1 in a 1000 similar systems work optimally.

 

[Pedronicus]

I certainly would not be turning rads off in unused rooms as the heat from the adjoining rooms will transfer through the walls/ceiling to the colder rooms thereby making the heated rooms colder. If TRVs are fitted then turn them down to a lower setting but not off. If TRV's not fitted then fit some to the 'nest' rooms. You certainly won't be able to correctly balance a system with rads turned off.

 

[mikej460]

It's worth pointing out that the legionella risk only applies to direct systems i.e. where the hot water is stored in a tank and is used as needed. In a modern indirect tank in an unvented system (no header tank) the heated water never leaves the tank, simply circulating through the boiler to maintain the set temperature, it then heats the cold water via a heat exchanger once a tap or shower is switched on. You also get power showers at mains pressure.

 

[Glitch]

I certainly would not be turning rads off in unused rooms as the heat from the adjoining rooms will transfer through the walls/ceiling to the colder rooms thereby making the heated rooms colder. If TRVs are fitted then turn them down to a lower setting but not off. If TRV's not fitted then fit some to the 'nest' rooms. You certainly won't be able to correctly balance a system with rads turned off.

That seems counterintuitive when trying to cut heating bills.
Surely the TRVs will shut off at low temps and you're back to square one. Unless you have a smart pump?

I should add that I have a wood burner that I'll be using to heat the downstairs on cold evenings and sometimes during the day. I know it probably won't save anything but I prefer a live fire when the opportunity arises.

 

[Just4Fun]

You certainly won't be able to correctly balance a system with rads turned off.

I believe I could correctly balance my system if I had fewer rads. How are rads that are turned off any different?

 

[johnny]

I always subscribed to the idea the Hot Water Setpoint should be 60°C.

I’m beginning to think the old 60°C stemmed from when energy was dirt cheap the planet didn’t matter!

60 degrees c is the minimum temperature that will kill Legionella any lower and you could contract Legionella which is a potential killer . HSE - Legionnaires' disease - Hot and cold water systems - Things to consider

 

[Phil Pascoe]

It's worth pointing out that the legionella risk only applies to direct systems i.e. where the hot water is stored in a tank and is used as needed. In a modern indirect tank in an unvented system (no header tank) the heated water never leaves the tank, simply circulating through the boiler to maintain the set temperature, it then heats the cold water via a heat exchanger once a tap or shower is switched on. You also get power showers at mains pressure.

I queried this before and it was pointed out that legionella can get in other ways such as mains repairs.

 

[Pedronicus]

I believe I could correctly balance my system if I had fewer rads. How are rads that are turned off any different?

The whole system with all rads on is balanced correctly to start with. Once this is done then rads can be turned off as required. If the system is balanced with rads turned off then, should those rads needs to be turned on again the whole system will be thrown out of balance. At least that is the way I was taught to do it and that has served me well in 40+ years in the heating industry designing/installing in industrial, commercial & domestic applications.

 

[PDW125]

As per the front page of that report, it is :
This guidance is for dutyholders, which includes employers, those in control of premises and those with health and safety responsibilities for others, to help them comply with their legal duties.

As of December 2021 (last reported data) there are no records of legionella in the U.K. from a domestic hot water source. If you have a sealed unvented system then there is no entry point for bacteria, and the chlorine in the water will disinfect the system. Storing at 48-53°C which is the reasonable maximum with a non R32 refrigerant based ASHP. Some newer ones can still reach CoP 2 (ie twice as better as direct electric) at 62°C but the efficiency diminishes rapidly.

If you have gas then an unvented cylinder with a mixing valve can easily go to 75°C and give around 20% additional theoretical tank capacity. Do not do that with a standard cylinder or one without a tank blending valve as the risk of scalding is significant.

 

[furnace]

to the downstairs handbasin its OK, to the kitchen it is awful.

I have a large, old gaff with long pipe runs. The kitchen tap needed running for ever to get hot water so I installed and undersink water heater that does the job perfectly. I think it's the 15 litre capacity version and provides all the kitchen sink water I ever need, and should it run out, it takes only a few mins to reheat. It was about £100.

 

[johna.clements]

As of December 2021 (last reported data) there are no records of legionella in the U.K. from a domestic hot water source. If you have a sealed unvented system then there is no entry point for bacteria, and the chlorine in the water will disinfect the system. Storing at 48-53°C which is the reasonable maximum with a non R32 refrigerant based ASHP. Some newer ones can still reach CoP 2 (ie twice as better as direct electric) at 62°C but the efficiency diminishes rapidly.

If people change their actions their risks will change.

There are bacteria in the mains water. Chlorine keeps the numbers in check but you still get requests to boil the water.

Legionella is found in the soil and freestanding water. A certain amount of soil gets into the mains when they carry out works. They put a large dose of chlorine in the pipe they are working on and flush pipes but people do make mistakes. Everybody has seen brown water at times.

Peoples immune systems will fight off bacteria without you noticing if the number is low. If the number is large you have a problem.

The longer water is stored the more bacteria multiply.

If water is stored above room temperature and is not heated to kill bacteria they will multiply.


Bacteria is in the water system so we either have to use it quick enough so that they do not grow to dangerous levels or periodically heat the water. Save money by using less hot water and do not heat it as much will allow bacteria to grow, you can not have both.

If domestic premises change their behavior to be like commercial premises they will have the risks of commercial premises with similar systems.

 

[Chris70]

If you are storing the water at 53C bacteria will grow.
If you are heating the water and then using it straight away no problem.

The lenght of time that the water is stored hot will determine how many bacteria will grow.

We have a Legionella Routine programmed. The temperature is raised at 02:00 each Wednesday for 2 hours to 60°C. I appreciate your concern, but I think we’ve got that covered.