Secondary return hot water

[Matt0029]

Is secondary return hot water a form of legionella control?

 

[ShaunCorbs]

No

 

[Matt0029]

Thanks I take it as it doesn't keep the water above 60 degrees c. So doesn't stops its formation.

 

[ShaunCorbs]

Correct will be around 47-55

 

[Knappers]

It can be used to eliminate dead-legs and return doesn't need to be 60 to comply.
Generally a return is more likely to cause a risk than eliminate it but as always its not a one size fits all situation

 

[Knappers]

(Disclaimer: From over a decade old memory)
55 will do and lagging is usually required, I don't think it has to be continuous either and so can be scheduled.

 

[Timmy D]

(Disclaimer: From over a decade old memory)
55 will do and lagging is usually required, I don't think it has to be continuous either and so can be scheduled.

Scheduled is ok. On horizontal unvented cylinders, even without a hot return, a pump is stipulated to circulate the tank and prevent stratification. It’s recommended to run the pump for an hour a day.

 

[Ric2013]

Legionella start to die off at 55°, but (from a decade old memory) it takes an hour.
I'm sure there's a rule about the stored water being at 60°C (why?). Is the 55°C you quote a rule for the secondary return?

 

[Knappers]

Yes I believe it was.
And an hour a day seems relevant too but includes stored water temps (maybe?)

 

[Ric2013]

IIRC the hour was the time it takes to kill the bacteria in stored water at 55 - not a regulation.

 

[Scott_D]

Wouldnt it have to be the same time as the boiler is heating the cylinder?
othetwise you are just shifting water around and not heating it all up to 60

 

[Ric2013]

Wouldnt it have to be the same time as the boiler is heating the cylinder?
othetwise you are just shifting water around and not heating it all up to 60

Well if it's circulating at 55°C for an hour, that is enough to kill all the legionella. Probably if the cylinder is held at 60°C, then 55° is to allow for potential heat losses over the length of the pipe.

But yes, the timed circulation does logically have to take place at a time when the top of the cylinder is hot rather than when the cylinder has cooled (at night, say). In practice, it makes sense to run the secondary at the time when people are using hot water which should be during or soon after the cylinder heating.