Replacing old steel water main supply pipe

I am in the process of renovating my 150 year old house, which includes fitting a new plumbing system.
A steel water supply pipe ("?) runs in a straight line from the water meter at the boundary of the property, under a garage, up the garden and into the house - a total distance of 35m. The depth of the pipe under the ground is only 25-50cm. The pipe looks old and corroded, although the water from the tap looks clear. There is no evidence of leaking. The water pressure doesn't "feel" high but Cambridge Water measured it at 2.8 bar (mid-morning).
Another point: I plan to install a new, unvented, mains-pressure hot water system.
I am considering whether it is worth replacing the steel water supply pipe with a blue polythene pipe. My options seem to be:
1. Leave the steel pipe where it is and run the new plumbing off that.
2. Replace all the steel pipe with blue MDPE pipe and hire a moling equipment contractor to tunnel the 5.5m distance under the garage.
3. Run blue MDPE pipe from the meter and then around the perimeter of the garage (adds three bends of 2m radius) and then straight up the garden to the house. Total pipe length = 45m.
Option 1 might result in frozen underground pipes, greater probability of leaks and lower pressures. Option 2 looks expensive. Option 3 introduces pipe bends and a longer run.
Which is best, do you think? Are there any other practical options worth considering?
What's "best", 25mm or 32mm blue plastic pipe?
Supplementary question: Is it OK to tee off the supply pipe half way up the garden to a stand pipe to be used for watering the plants? Or should I run a pipe from the house back to garden stand pipe instead?
Thanks in advance for your advice.
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snipped-for-privacy@pandora.be (Pandora) wrote:
Hello Pandora

That would be my choice. Keep the bends as gradual as possible so instead of 2 bends around the garage, one huge bend around it might be better, if you see what I mean. But if there's a problem digging there, probably not worth it for the minimum gain.

45m... Maybe someone else can help with the math side, but I'd go bigger unless the pipe the other side of the main stop was very small and then the gain would be less. For long runs, bigger is best.

Only if you can add a stop cock in such a way that it's 100% frost proof. (Ie, underground). A mini inspection hatch with a T and the short stop ASAP after it, all together and accessible should be ok.
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Simon Avery, Dartmoor, UK
uk.d-i-y FAQ: http://www.diyfaq.org.uk /
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Pandora wrote on Monday (19/01/2004) :

It is not just the pressure which is important, but the flow as well. Old and rusty will also mean clogged up inside.
Your best route will be around the garage even if it does mean an extra few yards of pipe and a little more digging. It will also need to be buried quite deep to avoid both gardening and frost damage.
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Regards,
Harry (M1BYT) (Lap)
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If you have a water meter, I suggest you do everything in your power to avoid leaks. I put in 180 meteres of blue pipe for 600 quid, after having lost 2000 quid worth of water from leaks.
Also consider what you need to dig up, if you are going to do a fancy surface, its probably best to do the pipe first, rather than risk it.
I would guess that the dig round the outside is cheeper, as you are going to have a mini digger on site anyway.
Rick
On 19 Jan 2004 10:03:45 -0800, snipped-for-privacy@pandora.be (Pandora) wrote:

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