"Jean pain" heated water input to combi domestic hot water system

[Luke78]

Hi guys; to be clear, what I have been asked to do is to connect directly to the hot water pipework without it passing through the boiler...

Interesting idea to feed pre-heated water into the boiler, however...

 

[oz-plumber]

I don't think the combi will have any issues with water coming in temperatures up to 60c. The thermister / sensor will let the boiler know not to fire up the gas.

Re: Legionella
The bacteria multiply where temperatures are between 20-45°C and nutrients are available. The bacteria are dormant below 20°C and do not survive above 60°C. Legionnaires' disease is a potentially fatal type of pneumonia, contracted by inhaling airborne water droplets containing viable Legionella bacteria.
This was copied from: hse.gov.uk.

I would hate to know how many cold water pipes in houses are storing cold water at temperatures of between 20c & 45c for extended periods - particularly in summer.

 

[Ric2013]

I think my concern is, like Rob Foster's that the water is being stored in the danger band for Legionnaire's. If it were always at 60°C then I would be happy with it, but if it sometimes sits at 40°C, as it well may, then precautions are needed.

Feeding a combi boiler is all well and good (I think solar systems often do this and the pre-heat cylinder is heated and held above 60° once a week to kill germs), but if the water simply passes through the compost pre-heat then the water would only be heated to 60°C briefly (if at all) as it passes through the boiler heat exchanger, and I think it takes an hour to kill it off at 60°C.

Possibly the compost coil water could be pumped through a plate heat exchanger that could be used either directly to heat the mains water feeding the bath, or to pre-heat the boiler. In this way, none of the water that is being stored at a dubious temperature ever comes through a tap.

FWIW, if this hippy system works and can be incorporated safely into the domestic system, then I for one think it's a great idea.

 

[oz-plumber]

Here are the temperatures and kill times.

This does not allow for water being replenished with fresh cold water at under 20c.

• Above 70 °C (158 °F) – Legionella dies almost instantly
• At 60 °C (140 °F) – 90% die in 2 minutes (Decimal reduction time (D) = 2 minutes)
• At 50 °C (122 °F) – 90% die in 80–124 minutes, depending on strain (D = 80–124 minutes)
• 48 to 50 °C (118 to 122 °F) – can survive but do not multiply
• 32 to 42 °C (90 to 108 °F) – ideal growth range
• 25 to 45 °C (77 to 113 °F) – growth range
• Below 20 °C (68 °F) – can survive, even below freezing, but are dormant

Other temperature sensitivity[29][30]

• 60 to 70 °C (140 to 158 °F) to 80 °C (176 °F) – Disinfection range
• 66 °C (151 °F) – Legionella dies within 2 minutes
• 60 °C (140 °F) – Legionella dies within 32 minutes
• 55 °C (131 °F) – Legionella dies within 5 to 6 hours