Has anyone had a combi conversion with microbore?

Looking at converting our system to a combi, but stressed about costly leaks given our pipework is 8mm microbore and 30+ years old.

Has anyone had this done successfully?

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  • grumbler
    grumbler Posts: 58,629
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    I'm no expert, but do you mean that microbore is more prone to leaking than bigger diameter?
    Yes, the pressure will be higher, that can cause leaks with any pipe diameter.
  • FreeBear
    FreeBear Posts: 14,272
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    edited 3 January at 2:17PM
    grumbler said:
    I'm no expert, but do you mean that microbore is more prone to leaking than bigger diameter?
    Yes, the pressure will be higher, that can cause leaks with any pipe diameter.
    If the current CH is a sealed system, then a combi will be running at a similar pressure. So I doubt it would be any more prone to developing leaks than a conventional large bore system. The only issue might be the pipes are too small to allow a decent flow if the OP wants to run at a low flow/return temperature (40°C or less)

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  • DullGreyGuy
    DullGreyGuy Posts: 9,202
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    Our property is about 20 years old and has microbore with a combi-boiler as it has had since day 1. 
  • kittennose
    kittennose Posts: 142
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    FreeBear said:
    grumbler said:
    I'm no expert, but do you mean that microbore is more prone to leaking than bigger diameter?
    Yes, the pressure will be higher, that can cause leaks with any pipe diameter.
    If the current CH is a sealed system, then a combi will be running at a similar pressure. So I doubt it would be any more prone to developing leaks than a conventional large bore system. The only issue might be the pipes are too small to allow a decent flow if the OP wants to run at a low flow/return temperature (40°C or less)

    It is open vented. Circa 1985.
  • FreeBear
    FreeBear Posts: 14,272
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    FreeBear said:
    grumbler said:
    I'm no expert, but do you mean that microbore is more prone to leaking than bigger diameter?
    Yes, the pressure will be higher, that can cause leaks with any pipe diameter.
    If the current CH is a sealed system, then a combi will be running at a similar pressure. So I doubt it would be any more prone to developing leaks than a conventional large bore system. The only issue might be the pipes are too small to allow a decent flow if the OP wants to run at a low flow/return temperature (40°C or less)

    It is open vented. Circa 1985.
    Ah. In that case, I would strongly recommend doing a hydraulic pressure test after the plumbing has been modified, but before connecting the boiler. If you can get the system pressurised to say 5 bar and leave it for at least 24 hours, any leaks should show up.
    I pressure tested my system at 6 bar for over a week before a new boiler was installed. Had one leak appear after an hour or so (poor solder joint), but once that was fixed, no more appeared.

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  • ThisIsWeird
    ThisIsWeird Posts: 4,458
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    As said above, microbore is no more likely to leak than any other bore. And there's more than one leaky bore on here already - drrrr-cheeesh.
    In the majority of cases, the system that was run under only gravitational-pressure will be ok with the increased pressure of a sealed system, as the pipes and rads are all designed for 10+ bar, and the joints should have been made to the same standard. It's only if there was a faulty or poor joint in the first place, or a rare corroded pipe, that an issue would appear when changing to the new system. You'd have to be unlucky.
    If you could carry out a pressure test as FreeBear suggests, then that would be reassuring, but I'm not sure how many plumbers would bother, or offer that service.
    Chances are you'll be fine.

  • FreeBear
    FreeBear Posts: 14,272
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    ThisIsWeird said: If you could carry out a pressure test as FreeBear suggests, then that would be reassuring, but I'm not sure how many plumbers would bother, or offer that service.
    The other problem is most plumbers won't want to wait a minimum of 24 hours - Some just want to get in and out as quickly as possible.. And of course, doing a boiler swap this time of year means an extra day or two without heating.
    Her courage will change the world.

    Treasure the moments that you have. Savour them for as long as you can for they will never come back again.
  • molerat
    molerat Posts: 31,577
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    edited 3 January at 4:44PM
    My son had it done last year on his 1980s? installation with no problems, the only leak was a pipe they left open, and has managed to reduce the flow temp without issue.
  • ThisIsWeird
    ThisIsWeird Posts: 4,458
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    Ditto.
    Fitted a combi to an ancient one-pipe system that had been powered ('kettling') by a floor-mounted Mexico, and no issues.
  • Vortigern
    Vortigern Posts: 3,234
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    Looking at converting our system to a combi, but stressed about costly leaks given our pipework is 8mm microbore and 30+ years old.

    Has anyone had this done successfully?

    Yes, original 1988 system changed from open vent cylinder with feed & expansion, to combi.  No issues with leakage.

    How much of your pipework is microbore? Mine was 22mm flow & return from boiler to cylinder, then 15mm distribution and finally a short length of coiled 8mm to each radiators. You don't see the 15mm pipe until you take some floorboards up.
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