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Hot water cylinder - legionnaires

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Hey all,


On my final stages of sorting a new boat deal. The boat in question had a small hot water cylinder mounted above the engine. (Volvo MD2-55) Apparently the tank blew up or leaked and they removed it.


I’ve searched around and found things like the Isotemp 30L cylinders which can mount in either direction. Uses both heat exchanger coils and 220v which is great.


However, this got me wondering how the heck do you deal with legionnaires when water needs to be kept above 60c? Sure if you’re on shore power that is not hard. But when sailing (not motoring) how do you ensure you tank water doesn’t grow bad things?   And just getting hot for short duration doesn't sound safe either, would reckon it needs to be kept hot for a while to be really safe?


Thoughts comments?





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Legionnaires grows between 25 and 45 degrees. In NZ storage hot water cylinders are required to maintain a temperature of 60 degrees to ensure they don't sit in the danger zone.

I would have thought on a boat not being used the water will be too cold so not a problem then when you heat it you want to make sure the water gets above 60 degrees before you use it.

What you dont want to do is keep the water just warm as that is prime breeding conditions.

What I dont know is how long the water should be at 60 degrees to be sure of killing the bug but it probably isnt all that long. Most of the time your water is either going to be too hot or too cold for the bug to breeding. If it does happen to breed every time you heat above 60 degrees you will kill it again. 

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From my research, chlorine needs to be 2ppm (basically shock treatment) for 24 hours then returned to 0.5ppm to be safe.  Doubt that is practical on a boat. 


As for numbers I see sites saying 90% of legionella dies at 60c after 2 minutes.  But then another one said 60c at 24minutes. 


Granted most people are not at as high a risk even if they contract it.  However, due to my health problems and medications suppressing my inflammation/immune system, I am one of those unfortunate few who have to bear factors like this in mind. 


Funny thing is that I doubt this would have crossed my mind if I didn't have to consider refitting a hot water cylinder and just used an existing system.  :think:

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 Isotemp 30L cylinders which can mount in either direction. Uses both heat exchanger coils and 220v which is great.



Those Isotemps are set for 80 degrees C internally and have a mixing valve at the outlet that mixes with the cold water to produce 55-60 centigrade hot water.  Its the equivalent of a 50litre cylinder with the 80 degree temp and it matches the coolant temp of 95% of marine diesel engines.

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I would think that by the very fact you are thinking about legionaries, it wont be a risk for you. That and a HW tank will be either too hot or too cold for growth.

Legionaries is a problem in poorly maintained air con systems, being ponding condensate water (which is naturally warm) then spread around by air flow, of compost, which is fairly self explanatory (medium temp, lots of nutrients, spores from here to Africa...) Compost is not normally a problem on a boat.


If you get the Isotemp unit at 80 deg C, and a tempering valve (mixer valve so it comes out at less than scolding) then you will well and truly kill all of the bugs, and have an efficient hot water system. The key bit is storing the water at well over 60 deg C, and mixing it down to a safe and usable temp.

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I recently put in a 15l Isotemp.... absolutely brilliant system. Heats up so fast. 

Very impressed.

The boat has a compressor driven fridge/freezer... so running the engine each day charges the batteries, heats the water and cools the fridge/freezer all at the same time. As the engine needs to be run regardless it made sense to maximise what it does when running.

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