Postby russellfl » Fri May 16, 2008 3:50 pm


I am going to have an oil fired combi boiler installed in my bungalow to replace the open vented system. i need to change all my radiators in the bungalow as they are very old and manky. They are also all fed via 10mm Microbore and go vertically into the roof void between rads. My floors are all floating type. I will be having the boiler fitted by a heating Co. but want to fit the rads myself
My question is should I replace the microbore which is unsightly, with 15mm pipework and route it under the floor boards?

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Simply Build It

Postby plumbbob » Fri May 16, 2008 6:17 pm

15mm copper has to win hands down on every occasion. It will ensure the heat from your new boiler is distributed evenly and will ensure all radiators receive sufficient hot water.

Remember to fit thermostatic valves to all but the radiator nearest to where the room thermostat will be sited.
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Postby russellfl » Mon May 19, 2008 4:08 pm

Thanks Plumbob, thats what I was thinking myself, just wanted to be sure.

Should I lag all the flow/return pipes that will now go under the floor?

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Postby plumbbob » Mon May 19, 2008 6:30 pm

Always lag exposed heating pipes that are outside of the heated zone, ie, under a floor in in a loft.
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Postby htg engineer » Mon May 19, 2008 8:12 pm

15mm or 10mm will work and will both last as long as each other - if looked after.

As for 15mm distributing the water/heat better, it doesn't affect it if designed and installed properly. You could use 15mm,10mm,8mm,6mm or drinking straws, as long as you don't mix and match the sizes the water will be evenly distributed.

If a system is badly designed that is when the distribution will be affected, mixing 15mm and 10mm will affect perforamnce and distribution. Pick a size and stick to it it's either all 15mm or all 10mm.

People tend to be put off by 10mm because of blockages, yes in older systems that weren't flushed, no inhibitors added, connecting a new boiler to old pipework etc. A new pressurised installation with 10mm pipe will last as long as a new installation with 15mm pipe, if it is looked after - adding inhibitors and replacing when the system is drained.

htg engineer
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Postby Perry525 » Wed May 28, 2008 5:44 pm

My final distribution pipes are all 8mm, using a pressurized system at 1.5 bar 8mm is more than capable - it will provide more than enough heat for normal radiators.
If your system has been sized properly there is no reason not to stick with 8mm.
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Postby russellfl » Mon Aug 25, 2008 11:42 am

Thanks all for your inputs, though I have now had a new oil combi installed and the heating engineer is ok with me adding the rads though he said he has to come back when i've got two on at least so he can finally commission it. The thing is he insisted on me using 22mm from the boiler for all the main runs with 15mm locally to the rads, after all you have said do you think he's going over the top? not to mention my extra costs for tubing etc.
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Postby screech » Tue Sep 02, 2008 10:52 pm

no way, 15mm is not over the top, i very much doubt any heating engineer would pipe their own system in 10mm pipe. 15mm means less chance of it blocking up, but the main thing is less chance of it kinking over, i see so many systems where they've bent 8-10mm pipe by hand, kinked it and left it. within a couple of years it's blocked and needs repiping.
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Postby Steve the gas » Wed Sep 03, 2008 5:12 am

I don't like microbore period so Yes
Steve the gas
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Postby russellfl » Mon Sep 22, 2008 9:27 am

Thamks guys. I have done what the heating engineer said and piped all main runs in 22mm and locally to the rads in 15mm and it works a treat.

Thanks again for everybodies input
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