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

[WC1]

It's a really interesting idea but I can't see how the risk of explosion and contamination (and potentially both at once) are worth getting involved to save the customer a few quid on their energy bills.

 

[Ric2013]

I was suggesting a vented cylinder. Then there is an open vent and no risk of explosion?

Would I be right to suspect the customer is interested in Alternative Technology and that this isn't about saving money? Bet he won't want to use lots of high tech sensors though (although sounds like he is happy to use yards of plastic pipe and the pumped mains water).

This is sounding like the sort of thing you'd have read about in "Mother Earth News" back in the day.

 

[Luke78]

Thanks everyone for the replies.

As I suspected, this looks like a complex set of issues and thanks to your contributions I will respectfully advise my customer that he needs to take advice from someone better qualified to to give it. I'll stick to servicing his boiler!

Incidentally, this IS primarily about alternative technology, as suggested by Ric2013, rather than a purely monetary consideration...

 

[oz-plumber]

What would be the issue of running the cold feed to the combi boiler through the compost pile.

The pipe will always be flushed whenever hot water is required.
I really can't see an issue with it.

We have thousands of kilometers of water pipe underground, above ground and they are not affected with bacteria that is harmful.

If the homeowner wants to run the cold feed to his combi, through a compost pile to pre heat the water - let him.

It may be unusual, but on consideration, I don't think there will be a regulation that could condemn the installation.

 

[Pickwickpick]

What would be the issue of running the cold feed to the combi boiler through the compost pile.

The pipe will always be flushed whenever hot water is required.
I really can't see an issue with it.

We have thousands of kilometers of water pipe underground, above ground and they are not affected with bacteria that is harmful.

If the homeowner wants to run the cold feed to his combi, through a compost pile to pre heat the water - let him.

It may be unusual, but on consideration, I don't think there will be a regulation that could condemn the installation.

Won't the combi have issues with its cold feed coming in at 60c when the compost heap is putting out its max heat?

I don't think bacteria ingress from the compost heap itself is an issue, its more a problem of Legionella multiplying if/when the temp drops to 40-50c and the water is stagnant in the pipe for extended periods..

 

[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