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

[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..