I am about to convert my standard open vented DHW system with indirect tank to a HeatBank system following the wiki @
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- many thanks to John Stumbles for that.
While trying to figure out what specification I require for the plate heat exchanger, I happened upon this article, which suggests hot water should be *distributed* as well as stored at 60degC.
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storage is not an issue with the Heat Bank idea. However, is it really neccessary to distribute water at this temperature? The same article suggests that water >45degC can cause serious scalding; I have been figuring on using a TMV just after the output from the PHE secondary to mix the hot down to a max of 50degC lets say, rather than
60degC.
As I get about 30l/min flow from th rising main, to be able to guarantee my DHW is at 60degC, would mean either a very high Heat Bank/ PHE primary temperature ie probably >80degC, or a really high ouput PHE ie >100kW.
At the end of the day the environment and costs are an issue; the environment because I don't want to heat the store any hotter than necc (have read in another thread that heatbanks work well with the store as low as 70degC); costs because a the higher output PHEs obviously cost more
to be pitched at commercial-type installations. Obviously legionella is possible in domestic ones too but AFAIK the outbreaks that have occurred to date have been in larger ones. I don't know what the incidence of legionella in domestic installations is but if you're concerned about it (perhaps because you have a long dead leg of pipework feeding a shower) there's a fairly elegant engineering solution with a heat bank that is to distribute the primary (from the cylinder) and put a pump + (smaller) PHE + flow switch etc close to the point of use.
No, with a heat bank all the hot is heated from a fresh supply of potable water (unless you live in Gloucester). What is left in the hot pipe afterwards is no different from any domestic hot water system.
On Thu, 26 Jul 2007 23:21:21 -0000 someone who may be Mike Holmes wrote this:-
1) it is impossible to avoid legionella. The little nasties are in the water as it falls from the sky.
2) what should be avoided is a high concentration of the little nasties mixing with people particularly susceptible to them.
3) at low temperatures they are largely dormant. At high temperatures they are killed off. Between those they breed nicely and thus attain a high concentration.
4) people who are not particularly strong are the most susceptible. That is why the well known outbreaks tend to involve hospitals, care homes and the like.
5) in a domestic situation the precautions which are taken in larger buildings are unlikely to be worthwhile.
and articles by chubby, middle-aged, managers in ties, who are usually eager to bring to their readers' attention a major problem and a SAFETY HAZARD which can only be prevented by the ACME Problem Preventer; they usually work for, or own, ACME Inc.. It's not helped by the editorial staff who don't work in the industry and the most relevant text and illustrations can wind up in the editors' bin. The articles are just filling around the paid adverts and are trimmed to suit the available space.
American publishers can usually afford to pay an expert for an informative article.
It's really just presenting the information in the HSE publication L8, about preventing legionella in water systems, which is the definitive Code of Practice on preventing legionella in the UK. L8 is legally binding on building owners and managers who have a "duty of care"; if there's an outbreak traced to a commercial building's water systems and the maintenance regimes have not followed the L8 guidelines, the managers can get charged with criminal offences.
It doesn't apply to houses, because the houseowners don't have a legal "duty of care", BUT it is sensible to follow the same guidelines because legionella will kill you in your own house.
Legionella is in the water supply. Given favourable conditions (nice temperature and nutrients) it will proliferate. An ideal temperature is about blood heat, 37 degC. If you inhale an atomised spray of legionella infested water, you will get infected . If you're elderly or ill, you'll probably die.
You have to keep the cold water COLD (below 20 degC). If you have a tank you have to ensure there is at least one water turn-over per day. Insulate tanks and pipes.
You have to keep the hot water HOT (50 degC or above). Store at 60 degC, 50degC at any hot tap within 60 seconds. If there's a risk of scalding you need a TMV at the point(s) of use. A TMV on the store outlet is no good.
Your system will be a combi in effect. There's little hazard if the water is not allowed to stagnate. I'd distribute at 60 degC since this will sterilize any bugs in the distribution pipes. Then again I wouldn't fit a combi.
I know of someone, a very expert plumber, who managed to get an infection from his own domestic system. He had a second house and used the shower after the house had been vacant for some weeks. The frost protection system had been keeping the house and the legionella warm.
The idea for the TMV just after the PHE is because if your store is at
75degC say, and you open a tap to give a moderate-low flow rate, then you're probably going to get water output from the secondary approaching 70-75degC and - dangerously hot - hence the TMV so you have a steady maximum temperature in your DHW system - unaffected by the store if it is at a higher temperature or flow rate through the PHE. You don't store your DHW, with a heat bank, you store the system water - check the many threads on heat banks or the wiki link above.
I imagine the best option is to set the TMV to distribute the DHW at
55-60degC. My Hans Grohe shower mixer has a TMV built-in under the hood, which you can set with a screwdriver, so will probably tune this down a bit lower.
TMVs at the point if there is a risk of scalding. TMVs at the store outlet are no good at preventing the isk of scalding. You still need to distribute at 60degC ish to avoid legionella in the pipes. The TMVs at the outlets are usually set at a maximum of 43degC (I think) which won't cause scalding to any 'at risk' users.
I know. The TMVs are being used for 2 different purposes.
1)Mix to a safe temperature at the store outlet, whilst keeping above the legionella-friendly temperature.
2) Mix to a non-scalding temperature at the point of use which is below the legionella temperature.
This came up here some months back. A few weeks later, I went to a wedding and found myself sitting next to a bacteriologist or virologist or immunologist or some such. With this fresh on my mind, I asked about it. Answer was that we tend to be immune to the bacteria in our homes as we're continually exposed to them. The problem starts when you eradicate them, and hence lose your immunity. That's fine in your home where they no longer exist, but if you then go into an institution (such as a hospital) where they are still the norm, or walk past a water-cooled air handler with them in, you are now susceptable. He's not an expert on Legionella specifically, but the appearence of it as a disease over the last few decades does tie up with attempts to clean our homes of bacteria, i.e. closing down the training classrooms for our immune systems (as he put it) which is widely recognised as a cause for some previously harmless bacteria to have become recognised illnesses.
What would be interesting would be to see if there's any correlation between those who've caught legionares disease (and had previously been healthy, not immune suppressed), and those who've been eradicating their exposure to bacteria at home (in this case by using water systems which are designed to minimise Legionella, rather than by explicitly spraying disinfectant everywhere).
>>> Seems to be pitched at commercial-type installations. Obviously >> legionella
Tacking onto the end of this thread and directly answering the original question; I converted to a heatbank about a year ago. The DPS system I purchased has two discrete thermostatically controlled loops; one for the 'working fluid' (water) bringing heat from the boiler and a separate one for the HDW. I 've determined from experiment that -for my household- the 'working fluid' is best set to 65 C and the DHW set to 55 C. The 'working fluid' is stored at 65 C - but it's totally contained within the system and in that sense is not distributed. The DHW is -instantly-heated by the plate heat exchanger and goes directly to the tap(s) when-and only when- the tap(s) are opened. I chose 55 C because it's hot enough for washing but no too hot and when mixed with incoming mains water at 9 C produce a bath / shower at 42 C quite nicely
Thanks Brian. Due to the flow rate I get from rising main (c30l/min) I'll probably have to set my store higher than 65degC (or choose a bigger PHE - I've been looking at options which have a heat load of
100 and 104kW) to achieve the necessary output.
On the secondary/output side can anyone help with my remaining query. Here it is: I have decided I will set my TMV (located just after the PHE secondary output) to 55degC. My question is how do I calculate/ estimate what secondary output temperature I need to have to achieve
55degC where > 1 hot tap/shower is open at the same time? Obviously, if there was only ever ONE outlet open, the secondary output could be designed to be 55degC at the designed max flow rate - easy. Equally, if I had an excessive secondary output temp, then there would prob be plenty of "headroom", so the TMV may always be mixing with cold, even if > 1 hot water outlet is open. However, this isn't a very efficient sitn really as you'd either need a large (expensive) PHE or heat your store to a really high temp. I would much prefer to "tune" the system design to be able to design the minimum required PHE secondary output, at the same time ensuring that my shower doesn't cool down if the 2nd shower or the kitchen hot tap is turned on.
There are no recorded cases of Legionnaires disease in domestic premises because they are not investigated. Legionnaires disease is not reportable as such. However, if there's more than two cases it becomes an outbreak and is investigated but only as far as commercial premises are concerned. Private houses are not checked. As always finding out about these cases can be difficult especially if they're not in the same area.
I suggest your ?ologist needs to gen up on Legionella. The disease was discovered in the 1970's. That doesn't mean it didn't exist before then. As medical awareness grew, more cases have been discovered. It's believed that most cases are contracted on holiday where temperatures are higher and water hygiene not is so well established. The victim also returns home to a system that's been sitting at a nice temperature for a few weeks so Legionella has had lots of time to multiply. The first shower releases a lovely aerosol of contaminated water which they breathe in. By the time any symptoms appear 10 days later, the system has been flushed many times and so all traces are gone.
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