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

[Luke78]

Hello all!

Seeking opinions/advice on how to handle an idea from a client of mine, I wonder if anyone else has come across this before?

One important caveat: My primary discipline is as a gas safe engineer! I am NOT a "dyed in the wool" plumber of many years experience, so please be gentle & polite with your replies!

To set the scene: I was qualified as a GSRE just over 12 months ago and one of my first clients was a local gentleman who needed a Worcester i Junior combination boiler in a property he had just bought recommissioned. It transpired that the boiler was continually losing heating system pressure and I ended up replacing the PRV and servicing the unit.
The client has since asked me back to undertake some odd bits of pipework as he is, like so many of us, apprehensive about soldering...

Being an enterprising fellow, he has now constructed what he calls "Jean Pain" arrangement for heating water by passing numerous coils of 32mm MDPE pipe through a compost heap. The "system" is fed directly with mains pressure water which he has done (without my involvement!) by simply teeing off from the incoming water main.
I confess to be being mildly impressed- It produces water at 50-60˚ and I actually witnessed him filling a bath with water that was too hot to touch whilst the outdoor temperature was just above freezing one night this week.
Presently, he has a somewhat Heath-Robinson arrangement where the MDPE simply comes in through his bathroom wall with a stop tap on the end; he simply connects a hosepipe and fills the bath as required...

Inevitably, the appeal of completely free hot water has made him ask me whether I would be prepared to tee into the pipework in the property so that the water can be used to feed the kitchen and other outlets.

I am competent enough at pipework to do this for him but there are a few concerns that I would welcome advice on before proceeding.

1) I am thinking it would be necessary to put some sort of shut off valve between the DHW outlet pipework from the Worcester Combi boiler and the rest of the DHW pipework to avoid any damage to the internal workings, flow turbines etc of the boiler from back pressure/pressure in the opposite direction when mains pressure water is introduced into the pipework from a different source.
At the moment I am thinking a quarter turn ball-valve isolator perhaps in series with a double-check valve would suit as it it would be quick and convenient to turn on and off. Would welcome advice from a regular plumber on this.

2) Are there any requirements for installations of this type from a water-regulatory / building regs point of view? (Are they even allowed?!?) I appreciate there can be hygiene concerns, although, ultimately, in this case, the water is coming from the same source (the incoming main) and simply being fed into the pipework at a different point.

3) Are there any other issues I need to consider that have not jumped out at me?

Any constructive/informed comments gratefully received.

 

[snowhead]

The best advice for this, run away as fast as you can and read up on Legionella and requirements for prevention of backfeeding into the incredibly uncontaminated British water supply.
Whilst you're doing that convince the customer to put his water supply back to normal.

 

[Luke78]

Had a feeling that might be the general response! lol.
Thanks Snowhead.

 

[Ric2013]

Could the hot water produced at 50-60°C be used to heat a cylinder, prehaps indirectly, and then the cylinder brought up to an hygienic temperature using gas? I think the manure heat source could potentially be used much as if it were a solar array.

I would be disinclined to connect the manure pipeline directly to the mains. Though arguably the mains could be reasonably protected by a check valve, PRV, and expansion vessel arrangement, I would agree there are safety issues with using the water at the temperatures it has been stored, particularly if the customer has, or ever chooses to install, a shower.

I wish my compost heap got that hot.

 

[oz-plumber]

On the other hand, if the customer want to use his dung heap to heat his hot water, you should install and RPZ valve on the cold supply pipe to the dung heap and leave his current plumbing as is.

I wouldn't connect the hot water from the dung heap to any of the existing plumbing.
If the owner wants to plumb extra outlets to other fixtures, they should be on separate lines.
Do not cross connect anything from the dung heap supply to any of the existing plumbing in the house.

The setup sounds like a breeding ground for bacteria and you don't want any chance of that contaminating the potable water supply.

At least if you install an RPZ valve, you can claim you have done all you can to protect the potable water supply.
The rest of the plumbing is up to him

 

[Ric2013]

On the other hand, if the customer want to use his dung heap to heat his hot water, you should install and RPZ valve on the cold supply pipe to the dung heap and leave his current plumbing as is.

You mean run reduced pressure zone through the compost coil, thus protecting the mains, and allow the customer to have a third tap at the bath for example, and then he has water he is aware might contain Legionella that he can use for select purposes? I agree it seems a cheap and reasonable solution if he doesn't want a cylinder.

 

[oz-plumber]

What I mean is: Install an RPZ valve ( High Hazard Backflow Prevention Valve ) on the supply pipe from the mains to the compost coil - to protect the potable water supply.
The customer can do what he likes with his invention, but as long as the potable water supply in the mains is protected from his invention.

There has to be a separation from his and his neighbours water supply from the compost hot water.

 

[Ric2013]

What I mean is: Install an RPZ valve ( High Hazard Backflow Prevention Valve ) on the supply pipe from the mains to the compost coil - to protect the potable water supply.
The customer can do what he likes with his invention, but as long as the potable water supply in the mains is protected from his invention.

There has to be a separation from his and his neighbours water supply from the compost hot water.

I agree with that, although I think it would be fair that the customer is made aware that even if the mains is protected, the plumbing on his side of the protection still needs to comply with the Water Regulations, though enforcement action is fairly unlikely.

I suspect that the heat given out by the heap will drop over time and this contraption will then become a pain in the backside when he has to keep digging out the coil and re-building the heap without damaging the MDPE, so perhaps a cheap and dirty solution (e.g the one Oz has suggested) that protects the mains until he finally gets bored would be ideal.

 

[Chuck]

There is also the matter of needing an expansion tank and PRV as once there's an RPZ in the mix he's created an an non-approved unvented system complete with an uncontrolled heat source. If the OP doesn't have a G3 ticket he should hand-off to someone who does. Actually, I doubt that anyone who isn't a CEng could design such a system legitimately.

 

[Ric2013]

On an alternative note, why not use the low grade heat from the heap to run a radiator or UFH instead? The customer would still save on gas, but the low temperature would not be an hygiene issue.

 

[oz-plumber]

Sorry Ric,
I didn't see your post, we posted at the same time.
You could use the heat source to heat a cylinder indirectly.
As long as the supply to the compost heap is protected adequately from contaminating the potable water supply.
As you state - like solar hot water.

The problems that arise from this is controlling the system.
You don't want a heat sink that will take heat away from the cylinder, so pumps, sensors, timers and so on - could get obscenely expensive.

Solution: Use the compost heat to heat the cold water entering the tank through a plate heat exchanger. The compost heated water will most probably always be hotter that the cold incoming supply. No controls, no sensors, maybe a timer for the pump to save on electricity when you won't be using hot water.

 

[Ric2013]

There is also the matter of needing an expansion tank and PRV as once there's an RPZ in the mix he's created an an non-approved unvented system complete with an uncontrolled heat source. If the OP doesn't have a G3 ticket he should hand-off to someone who does. Actually, I doubt that anyone who isn't a CEng could design such a system legitimately.

Do you legally need a G3 ticket for a sealed heating system? I thought G3 was only for cylinders?

I'm just trying to understand the law here, so apologies if I seem to be challenging you on this. My thinking is that G3 is to protect people from creating bombs (and applies to vented cylinders too, though most people ignore this) and I can't see that the same level of hazard could be produced by a length of pipe bursting as by a 150l cylinder bursting.

Agreed that a PRV and expansion vessel would be required, as I've already noted above (#4).

 

[snowhead]

Agreed that a PRV and expansion vessel would be required, as I've already noted above (#4).

There's your G3 answer, those components reduce the risk of a bomb.
How big does the PRV need to be, where's it going to discharge, how big does the Exp vessel need to be, what pre-charge. All questions a G3 will know the answers to.

I wouldn't want to be next to any pipe at 3 bar or more when it lets go.

 

[Chuck]

I thought G3 was only for cylinders?

My understanding is that since March 2016 G3 applies to all pressurised hot water systems regardless of volume, not just those with cylinders. AIUI, not all such work needs to be notified but it must be done by a competent person. In practice, if you are trading as a plumber, this means a G3 ticket.

I'm just trying to understand the law here, so apologies if I seem to be challenging you on this.

Not a problem. I try never to claim any authority here, everything I write is just my opinion as it stands today. This is free and probably worth every penny. If I'm mistaken, I'm happy to learn.

My thinking is that G3 is to protect people from creating bombs (and applies to vented cylinders too, though most people ignore this) and I can't see that the same level of hazard could be produced by a length of pipe bursting as by a 150l cylinder bursting.

In normal conditions the centre of compost heaps can, apparently, get to up to ca 75°C, which is hot enough to scald someone. In the summer, in hot sun, they can catch fire, which could create your bomb as the temperature in the heap rises above boiling point.

Personally, I doubt that someone with a G3 ticket would be happy with this system. My point was more that someone who doesn't have one shouldn't go near this.

 

[Pickwickpick]

I'd be really concerned about your customer running baths with water heated directly from the coil. There's no way he can be sure that water is consistently above 60c, especially with a section of the pipe going through the garden into the house.

The only safe way (that's practical) to get hot water from it would be to use it to heat a cylinder, but as others has noted then you're into G3 territory and I know I wouldn't touch that with a barge pole, especially as it sounds like the sort of customer who might then try and customise or alter whatever you set up. If something did then go wrong it could get very messy trying to prove it was nothing to do with you!

 

[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