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Worcester

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OK, so I may have this wrong (ducks and waits for the shells to arrive :) ) I'm starting this so that I too can learn...

As far I can can interpret, Domestic Installations are not coverd by the L8 guidelines
Also there is a special exemption from the Health and Safety Regs for renewable energy heated domestic Hot Water (DHW)

Heres' the document / legislation http://www.hse.gov.uk/pubns/priced/l8.pdf
and a recent report from WRAS New Page 2
and Preheated water report Sep 09

It would appear that the MCS standard also condones, not complying with L8

This artilce though rams home the messages and sanctioned loopholes' such as MCS etc with some good advice on how to control legionnella
Solar Video: Solar Legionella exemption. Cut Legionella risk in solar water heating in conventional twin coil solar cylinders. | Solartwin
and here:
Controlling legionella in solar-thermal systems - Modern Building Services


In summary if you decide controlling legionnella in a Domestic Environment is important to you then these would appear to be the solutions on offer:
(ful text of recommendations here:)
http://www.solartwin.com/solartwin-features/technical/legionella-solar-water-systems-with-conventional-twin-coil-solar-cylinders

1 – Zero cost and immediate:

The simplest is to stay clear of hot water spray. Take baths, not showers. ....

2 – Lower cost. (Usually under £100 if you already have an immersion heater in place)

Buy and fit a long immersion heater which is the full length of the cylinder and which will reach to the bottom.
Time this to come on at the end of the day to heat the whole cylinder to 60C.

3 – Medium cost. Requiring a plumber and electrician and keeping the cylinder: (£300-500) Have the twin coil cylinder fitted with a mains powered destratification pump which comes on:


  1. Every day so that it holds the water at 60C all the way down for one hour.
  2. At the same time as the water heating is turned on, whether immersion or other type of water heater.
Why? This will heat the cylinder daily to the bottom and complies with L8 guidance para 158.

4 – Higher cost. Requires a plumber only: replacing the cylinder with a safer one: (£500-£1000)

Replace the twin coil cylinder with another twin coil cylinder which has BOTH heat exchangers (solar and backup) at the bottom. Also time the backup heating to come on in the evening.

Why? Again this will heat the cylinder daily to the bottom and complies with L8 guidance para 158.

5 – Highest cost. Requires a plumber only: replacing the cylinder with a supersafe thermal store: (£1000-£1500)

Replace the twin coil cylinder with a thermal store. With a Solartwin installation, this has one heat exchanger to take heat out only. With conventional solar, some have a second heat-in heat exchanger thus reducing efficiency a bit. You may want to time the backup heating to come on in the evening.

Why? Having fast flowing, frequently replaced water of low heated volume at any one time super-complies with L8.


 

The Solar King

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I'll look at this more tomorrow.
Solartwin ceased trading at the end of July. I am surprised they lasted so long. Barry the guy who ran the outfit was passionate about his business but seemed to be rowing in the opposite direction to everyone else. He is also quite litigious and combative so I need to be careful in expressing opinions. I would take anything published by them with a pinch of salt.
 

Gavin A

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yeah, he once threatened to sue me for pointing out he was talking utter rubbish on the legionella risk from SWH systems, and daring to back it up with academic evidence. I called his bluff, and the court summons obviously got lost in the post.

I'd not be linking to his scaremongering legionella videos though, as he couldn't back up his statements when challenged.

I wasn't aware they'd gone bust though, which is a bit of a shame as the actual design of their system was quite good in a lot of respects, and Barry was obviously very passionate about it all even if he did manage to fall out with most in the industry that he crossed paths with.
 

Gavin A

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As stated in the other thread, L8 is not and never has been intended to apply to standard domestic situations, as it clearly states in the updated guidance
Domestic systems can present a risk, depending on the circumstances of use, but the ACoP will only apply to systems from which the risk arises in relation to any work activity.
[hse]
It's arguable that it could now apply to rented properties, as they've removed the 300l tank size restriction to it.

In general though to be entirely safe from the tank, with a standard tank situation (not thermal store) you either need to operate a hot top situation, where the top part of the tank is always kept heated to around 55-60deg to kill off any legionella bacteria that may be breeding in the cooler bottom of the tank, as in the case with dual coil solar water heating systems, or heat the entire tank to 60 ideally twice a week, though once a week is realistically not likely to be a problem.

So if using an immersun on an immersion heater in a system with a gas / oil boiler feeding a single bottom coil, just ensure the boiler is set to switch on for an hour or so once / twice a week, unless there's a function on the immersun to make sure that happens with the immersion.

tbh though, the killer to Barrys assertion for me was a research paper I found that showed the incidence of legionella was actually significantly lower in twin coil solar tanks than it was on average in the other tanks in the survey, with district heating schemes being far worse. Essentially I think this is because the peak water temperature is more variable all the way to the bottom of the tank, so thermophyllic bacteria can't establish colonies of those bugs that can just survive at the 49 deg temp found on the actual base of the tank in a lot of systems where the coil is further away from the base, and the temperature sensor is positioned higher up and on the outside of the tank, and the max temperature reached is always exactly the same. This really nailed Barry's argument as being baseless for me.

I'll see if I can dig it out.
 
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Worcester

Worcester

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So if using an immersun on an immersion heater in a system with a gas / oil boiler feeding a single bottom coil, just ensure the boiler is set to switch on for an hour or so once / twice a week, unless there's a function on the immersun to make sure that happens with the immersion.
Re the Immersun Yep, the Immersun has a timer function to boost either the immersion or your chosen boiler, we are on oil, so switch the boiler off May-Oct, and have set the immersun to boost hot hater using the immersion for an 1 1/2 hours at 7:30pm each day. (3 adults + 1 kid bath and shower every day)
 

The Solar King

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As with many things it often a matter of interpretation. Here are the two key pieces of regulation/guidance that cover the issue in solar thermal.


From MIS3001 issue 2.1a Feb 2012:


“4.2 Design and installation
The following principles shall be met when designing, specifying and installing a solar heating system. For the principles numbered 4.3.1 to 4.3.13, one means of compliance would be to follow the guidance in Energy Saving Trust publication CE131: Solar water heating systems guidance for professionals, conventional indirect models.


4.34 Incorporate a means to prevent bacterial growth (legionella) at all foreseeable flow rates.
Note: one way of meeting this requirement would be through the use of a secondary means of heating the water to 60[SUP]o[/SUP]C.”


From CE131:
(this document is also referenced in the Domestic Heating Compliance Guide)


“3.11 Risk of bacteria proliferation
The solar pre-heat volume will vary through the year, in temperatures ranging from as low as 5[SUP]o[/SUP]C in winter up to possibly 85[SUP]o[/SUP]C in summer. In this respect, pre-heated water should not be considered as fully treated for DHW use and should not enter the household distribution until it has been further conditioned for comfort and safety. There is an increased risk of bacteria growth at temperatures between 20[SUP]o[/SUP]C to 46[SUP]o[/SUP]C during prolonged periods without DHW draw-off. Special attention should be given to situations where cold water originates from unclean water cisterns (without lids) or where there are porous, suspicious or unknown fittings in contact with the water. The risks can be minimised by:


• Reducing pre-heated storage to below twice the average daily hot water use.
• Using indirect primary circuits.
• Replacing old, poorly insulated DHW stores.
• Using electronic primary pump controls with programming targeted to achieve 60[SUP]o[/SUP]C storage.
• Reducing long lengths of uninsulated secondary pipework.


Irrespective of the above, reliable sterilisation is best achieved by ensuring pre-heated water passes through to another reliable heat source capable of sterilisation of unwanted bacteria. It should be noted that legionella bacteria can be expected to be killed within seconds at 60[SUP]o[/SUP]C.


All solar pre-heated secondary water should be designed to pass through an auxiliary heat source capable of heating to least 60[SUP]o[/SUP]C (at all foreseeable flow rates) before DHW distribution .


It should be noted that the use of de-stratifying secondary pumps or an occasional back-up heat source within the pre-heat store could drastically reduce the solar performance and effectively de-rate the dedicated pre-heat store. To optimise solar efficiency, secondary circulation distribution is best implemented only in the DHW store. However, where a high risk of bacterial proliferation exists, the solar pre-heat store can be designed to be regularly sterilised, ideally at the end of the day to give the best chance for this to be achieved by solar. Such sterilisation should be accurately controlled by time and temperature in order to restore solar storage promptly.”


The belt and braces approach adopted in ST best practice is to raise the tank temperature to 60[SUP]o[/SUP]C on a daily basis. If tanks are sized correctly, there is no energy penalty in doing so (I covered this in the post that caused Worcester to start this one). This may be OTT, but does meet the requirements listed above regardless of actual risk.


Would any of this apply to a PV powered immersion system? I would take cognisance of it is due to potential variability in heat input which is the same as for ST. There is difference in the tank configuration. De-stratification can also reduce tank temperatures and potentially disturb nutrients that can gather in the very bottom of a tank. If a De-stratification pump runs when the top of the tank is hot and then, due to time of day and changes in irradiance there is no further solar input, tank temperatures have dropped. De-stratification could also potentially lead to a situation where some part of the body of water never leaves the tank.


Legionella is a nasty invidious infection, but actually quite hard to contract. It occurs in all water supplies. You can ingest it (drink it) and it will not affect you. You need to inhale infected aerosols. Older people and those with underlying health problems are far more vulnerable than younger healthy people. Those in the latter group may not even know they are infected and think they have a bad cold or flu.


Here is the problem. Reported outbreaks of legionella are mass infections normally caused by poorly maintained cooling towers in industrial sites or similar equipment leading to mass aerosol release. (Edinburgh this year thought to be a distillery, stoke on trent, a hot tub displayed at a distributor). Even in these outbreaks all cases may not be reported. If an individual young healthy person is infected in a domestic property, it may never come to the attention of the public health authorities so true levels of infection in this environment are unknown. You are dealing not only with a perceived but also a real risk.


The next question is the proliferation rate of legionella growth in a suitable environment. On this question I am unable to help. You would need to speak to a bacteriologist who would probably give you a fifty page answer. I would surmise but cannot confirm that it is probable disinfection does not need to be undertaken as frequently as current best practice


I do not have to hand the draft of the revised BS5918. It is maybe it was on line and not downloadable or maybe I simply deleted it. Appendix C is all to do with disinfection. Someone like Ted M may well have a copy and could upload it. This may give greater clarity.


Incidentally, I am pretty certain, but am awaiting confirmation that the hot water daily demand in SAP is based on water stored at 60[SUP]o[/SUP]C.
 

The Solar King

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On the final point in my diatribe, I have had confirmed by someone I know in BRE responsible for SAP that there is an inherent assumption that water is heated to 60deg C and the reason for this is legionella disinfection. (There are standing losses after this point that are accounted for in calculation). For information daily hot water requirement is calculated in litres as Nx25+36 where N is the number of occupants. Knowing BRE this will be calculated from empirical research. In SAP this is then related to assumed occupancy for a given floor area which should not be used for heating system design.

The hot water requirement can be used on its own for system sizing, as long as you take into account the assumptions made.
 

Marvo

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Wouldn't it be easier to send the water through a UV-C tube especially in systems where there's no easy method of periodically raising the temp >60C?
 

Gavin A

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The belt and braces approach adopted in ST best practice is to raise the tank temperature to 60[SUP]o[/SUP]C on a daily basis. If tanks are sized correctly, there is no energy penalty in doing so (I covered this in the post that caused Worcester to start this one). This may be OTT, but does meet the requirements listed above regardless of actual risk.
as I said, using the hot top method also complies with this in a dual coil tank, as long as the top section is kept heated sufficiently by a secondary source.

All solar pre-heated secondary water should be designed to pass through an auxiliary heat source capable of heating to least 60[SUP]o[/SUP]C (at all foreseeable flow rates) before DHW distribution .
The problem is that in my experience, customers (including my dad) insist on switching the gas off entirely to rely on the solar water heating when it's sunny, at which point either method wouldn't work.

When we've done solar water heating though we have mostly installed what would be regarded by some as oversized systems, with significantly bigger panel area to dedicated solar volume than some would recommend, as the controls on the systems provide protection against excessive stagnation temps, and we fit TMV's etc. So, this reduces significantly the chances of the tank spending longer than 3-4 days at below 45 degrees even if the customer does decide to switch off the back up heating supply (we do instruct them not to, but they're a law unto themselves really).

It's difficult to assess how much of an issue this is with immersion heater units, as it depends on their position in the tank, how much of the tank they're heating etc. so will have to be worked out on a case by case basis. As long as the tank gets a full top to bottom heating to 60 or higher every 3-4 days though it's never going to be an issue - once a week would reduce the risk of a problem to a miniscule level, so ok for most situations other than where there's someone at additional risk (lung disease, elderly, immune problems, very young children etc).

This only applies for domestic situations though.
 

solarsavings

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The problem is that in my experience, customers (including my dad) insist on switching the gas off entirely to rely on the solar water heating when it's sunny, at which point either method wouldn't work.


We set our customers heating controls from the usual twice a day to once a day and timed for late afternoon so that if the cylinder hasn't reached 60c(as has happened a lot this summer) then the boiler kicks in.

My own controls are set to off from late march to late september and only switched via the boost button during those periods if the water isn't hot enough (not necessarily 60c in our case).

I did think we would be hearing from Barry Johnson shortly after worcester posted this thread but it's sad to see the reason why he hasn't turned up. I bet his friends in the sta are not too displeased though.
 

langstroth2

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I confess I'm puzzled by the "keep the tank at over 60C" view. Mainly because I'd have thought it would be almost impossible to do so for any length of time likely to be useful for killing nasties. Any tank in typical use - e.g. draw off morning and evening - is likely to be constantly stratified with a layer of sub-60 water at the base, if not the bottom 1/3rd. My tank has 3 sensors top/middle/bottom, and only very very rarely is the whole tank >60C (usually on summer days with decent sun and no one at home). Any smidgen of draw-off and a bottom cooler layer exists. There's also sub60C warm water sitting in pipe work between tank and tap; and warm water in the 'cold' water storage tank in a hot loft during the summer.

I'd have thought it more important to be constantly flushing the system with fresh water - i.e. daily use. Any increased risk in a typical household I'd think would be after the house has stood empty for a few days. Fortunately in the summer this is the very time when the whole tank does go over 65C with a solar system - no HW use, but plenty of sun (hopefully!).

Any how just my thoughts. Up to each person to do what they feel reduces any risk.

e.g (hw tank top = red, bottom = blue. Black is the solar panel)

e.g
 

Gavin A

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there's no guidance to keep a tank at 60.

there is guidance to keep the top of a dual coil tank heated to 60 to ensure that if the base hasn't been getting heated enough, and legionella has been breeding in it that it then passes through the 60 degree layer / get's heated to 60 and get's killed off.

As long as the entire tank is being heated to 55-60 every few days though there's not really any need for the above. The problem comes in devising a system to automatically ensure this happens, which is where the timer function on the Immersun unit will be really helpful.

Personally I dispute that this is an issue at all with a reasonably sized solar water heating system, and we have our gas switched off entirely for months on end in the summer, and only switch it on when we actually run out of hot water. It's only in winter that it's potentially an issue IMO, and by then anyone's going to have the gas on anyway, and tbh I think the SWH heats the base of the tank above 55 often enough in winter not to be an issue on our system anyway (partly because the top half is heated by gas, so there's only half the volume for the SWH to heat up.

btw, this is the report I mentioned earlier, unfortunately it's paywalled, so you can only get the abstract without paying. Somewhere I've got a copy of the full report, but it was on a different laptop... ScienceDirect.com - International Journal of Hygiene and Environmental Health - Occurrence of Legionella in hot water systems of single-family residences in suburbs of two German cities with special reference to solar and district heating
 
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Worcester

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