Water pre-heat with excess solar before combi?

ComicGeek

Section62 said:

I think most of the above ideas would give rise to justifiable concern over the control of Legionella bacteria.

It would break the basic principle of plumbing of keeping cold water cold and hot water hot.  The pre-heat would add an unknown and variable amount of heat to water which is supposed to be cold, without any certainty that it will be heated to a temperature where Legionella would be killed.  The water could easily be heated and maintained for some length of time at a temperature where Legionella thrive.

Some take the view that Legionella poses no significant risk.  But the 'rules' of plumbing are designed to take a systematic approach to Legionella control and to ignore those would be unwise.

This comes up time and time again on here, and is such a low risk in the vast majority of residential environments that more injuries are caused by water that is too hot than legionella itself!! 503 reported cases of legionella in 2019 against 693 reported case of scalds from hot tap water - but 41% of the legionella cases were contracted overseas, so 2.34 cases of hot water scalding for every UK legionella case. Some people try and guess that there are more cases than reported, but the risk in a house/flat is extremely small. 

Copper is less susceptible to the formation of biofilm, which is a key factor in the growth of Legionella bacteria. So use copper for all hot and cold water pipework feeds.

Frequency of use of fittings in the home (remember than legionella is primarily a concern with showers where water droplets are inhaled) means that the systems are flushed on a high number of occasions per week. The hose on a shower from the TMV to a shower head has the same risk as a preheated cold water feed to the shower.

Preheated cold water feeds are kept below a maximum length of 4.75m to minimise risk.

Any thermal store that the heat recovery is connected to would need a legionella heating cycle as per any other hot water cylinder.

After periods of non-use (eg holidays) everyone is advised to run their showers for at least 15 seconds on the cold setting of the TMV to flush through remaining pre-heated water before use. I know at least one legionella expert who does it for 5 minutes with all the windows open and him not in the room, as a full precaution. How many people actually do this in the UK after being on holiday for a week?

I set my hot water cylinder to 50 degrees rather than 60 degrees to save money, no point heating it up and having to mix it down again to 38 degrees for showers/baths. Both showers are used at least once daily and the cylinder is turned over daily, so no stagnant pockets for legionella breeding.

I would ask anyone to measure their cold water pipework within their airing cupboard and tell me that its less than the recommended 20 degrees all year round. 

Section62

ComicGeek said:

Section62 said:

I think most of the above ideas would give rise to justifiable concern over the control of Legionella bacteria.

It would break the basic principle of plumbing of keeping cold water cold and hot water hot.  The pre-heat would add an unknown and variable amount of heat to water which is supposed to be cold, without any certainty that it will be heated to a temperature where Legionella would be killed.  The water could easily be heated and maintained for some length of time at a temperature where Legionella thrive.

Some take the view that Legionella poses no significant risk.  But the 'rules' of plumbing are designed to take a systematic approach to Legionella control and to ignore those would be unwise.

This comes up time and time again on here, and is such a low risk in the vast majority of residential environments that more injuries are caused by water that is too hot than legionella itself!! 503 reported cases of legionella in 2019 against 693 reported case of scalds from hot tap water - but 41% of the legionella cases were contracted overseas, so 2.34 cases of hot water scalding for every UK legionella case. Some people try and guess that there are more cases than reported, but the risk in a house/flat is extremely small. 

....

It does.  And regularly gets dismissed as 'not an issue'.  However, the dismissal tends to ignore the fact we adopt a systematic approach to Legionella control (e.g. things you outlined in the rest of your post) which means people's exposure to risk is minimised (even though they may not be aware of it).

Greater danger comes where people go off-piste and come up with ideas without understanding why things are done a certain way.

A parallel could be drawn with the relatively small number of people who get killed as a result of gas boiler explosions.  The reason for that is primarily because we have a rules-based system for the design and installation of gas boilers, not because gas isn't potentially dangerous.

The essence of my point was that a pre-heat system would need to be designed by someone who understands the risks involved, which includes Legionella control.  Your reply highlights some of the things that need to be considered in the design.

ThisIsWeird

Would it make any difference if the system was low volume and sealed? Eg, the cold water supply is fed to the boiler via a heat exchanger coil sitting in a PV-heated tank. Theoretically no different to the hot water that sits temporarily in a combi with a pre-heat facility.

ComicGeek

Section62 said:r

ComicGeek said:

Section62 said:

I think most of the above ideas would give rise to justifiable concern over the control of Legionella bacteria.

It would break the basic principle of plumbing of keeping cold water cold and hot water hot.  The pre-heat would add an unknown and variable amount of heat to water which is supposed to be cold, without any certainty that it will be heated to a temperature where Legionella would be killed.  The water could easily be heated and maintained for some length of time at a temperature where Legionella thrive.

Some take the view that Legionella poses no significant risk.  But the 'rules' of plumbing are designed to take a systematic approach to Legionella control and to ignore those would be unwise.

This comes up time and time again on here, and is such a low risk in the vast majority of residential environments that more injuries are caused by water that is too hot than legionella itself!! 503 reported cases of legionella in 2019 against 693 reported case of scalds from hot tap water - but 41% of the legionella cases were contracted overseas, so 2.34 cases of hot water scalding for every UK legionella case. Some people try and guess that there are more cases than reported, but the risk in a house/flat is extremely small. 

....

It does.  And regularly gets dismissed as 'not an issue'.  However, the dismissal tends to ignore the fact we adopt a systematic approach to Legionella control (e.g. things you outlined in the rest of your post) which means people's exposure to risk is minimised (even though they may not be aware of it).

Greater danger comes where people go off-piste and come up with ideas without understanding why things are done a certain way.

A parallel could be drawn with the relatively small number of people who get killed as a result of gas boiler explosions.  The reason for that is primarily because we have a rules-based system for the design and installation of gas boilers, not because gas isn't potentially dangerous.

The essence of my point was that a pre-heat system would need to be designed by someone who understands the risks involved, which includes Legionella control.  Your reply highlights some of the things that need to be considered in the design.

I don't think a comparison with gas boiler explosions is at all helpful or relevant. More like eating food when it's slightly past its best before date. 

The largest single reduction in legionella cases was when people were prevented from travelling abroad during lockdown, perhaps we should bring that back to minimise risk. Cases in the community and hospitals remained fairly unchanged.

ComicGeek

ThisIsWeird said:

Would it make any difference if the system was low volume and sealed? Eg, the cold water supply is fed to the boiler via a heat exchanger coil sitting in a PV-heated tank. Theoretically no different to the hot water that sits temporarily in a combi with a pre-heat facility.

It's not the volume, but rather the turnover rates, that is important. Any system that contains water has a risk, but residential systems are short distances of small bore pipework that are quickly and routinely flushed through.

If there is hydraulic separation (ie separate water systems with only heat transfer via a coil) then there is no risk of legionella transfer between the two - but the risk is still there in the final pipework if it sits there for long periods of time at the optimum temperature for bacteria growth. The System B WWHR units do exactly this, but as the heat only transfers when the shower is in use the heat doesn't transfer into stagnant water.

This is the point I'm trying (and obviously failing!) to make. The risk exists in your garden hose sat outside in the sun and connected to the tap with the hose filled but not running (and then run through a sprinkler system to water the grass!). The risk exists in any shower head feed from a TMV. The risk exists in any cold water pipework that passes through a loft space that gets hot in summer, or passes through an airing cupboard. The risk exists in the pipework to the utility room sink that gets used once a week. The risk exists in the communal cold water tank sat on the building roof in full sun. But the number of cases and serious issues are extremely low because the water systems are in use. The major incidents have been due to water quality issues with cooling towers in commercial buildings, remote shower areas that don't get much use, lengthy deadlegs of pipework that you don't really get in residential buildings etc. 

One study calculated there were 2,731 winter deaths in England in winter 2021/22 caused by living in cold damp homes. The priority should be saving energy in other areas so that people can afford to heat their homes. If that means turning down their hot water cylinder to 50 degrees then that should be encouraged. Or using waste heat to preheat their shower.

There's even a much higher death rate from food poisoning each year despite all the campaigns and teaching. There's a lot more out there to be afraid of than some luke warm water.