I'm eying up a 1970's house which has microbore heating pipes and rads, conventional heating system with an old Baxi downstairs, HW tank in the airing cupboard, cold and heating header tanks in the loft. Absolutely no idea where the manifold is.
The radiators are ugly as hell and have some kind of valve which has both feed and return pipes going into the same end of the rad. SWMBO will definitely want them changed. If I'm changing all the rads at the same time I might as well change the boiler to a combi to free up some space (or possibly a pressurised HW tank) and get a decent shower.
Is it sensible to try and reuse the microbore, given the lack of access to it, its age, and the fact that there must be some reason people stopped using it 20 years ago, or should I plan to run a whole new set of 15mm? Given the concrete floor, I'd probably end up running pipes in the ceiling void, chased into walls, and behind the skirting,if that's sensible.
The primary problem with microbore is that it can be hard to get enough flow to keep the delta-t low for use with condensing boilers.
If you can't get the flow high enough, then you end up with either some cool radiators, or the combi working at a higher temperature than it'd like to for efficiency.
However. I would be very surprised if you'd get another 5-10% out of the system by replumbing in 22/15mm. Can you have a look and see if the microbore is all the way back to the boiler (very unlikely) or if there is 22/15mm piping to which it's manifolded?
Just as a clue: Our microbore manifold (70s installation in a 30s house) is under a flase bottom in the airing cupboard (together with the pump, and near the 3-position valve).
I looked for replacment "double entry" valves as the OP described, and did find some (though didn't buy them) IIRC the manufacturer was IME Yorkshire.
This can be done quite easily by extending one pipe and hiding it behind the radiator. Then you can put standard valves on each end. There are numerous ways including 8 or 10mm versions of valves or adaptors.
Usual health warning about checking that the mains water flow is adequate, the gas supply is 22mm to the boiler and that a boiler adequate for the smount and rate of HW you need is used.
There's no reason to change it at all - in fact I've seen it used in new houses recently.
If you are changing the boiler as well as the radiators, you can use the opportunity to design the system to improve the efficiency of the condensing boiler. Traditional systems are designed for CH flow and return temperatures of 82 and 70 degrees. A condensing boiler will operate in the same way when needed (in the depths of winter) but will modulate down to a more efficient temperature at other times of the year. However, you can make an improvement by designing around temperatures of 70 and 50 degrees. This would entail choosing radiators with approx 30% higher output ratings.
I did this exercise three years ago and was able to achieve these objectives by moving some radiators to new positions and buying a small number of higher output ones (e.g. twin panel and/or fins).
It's important to work out the outputs of the existing radiators and then do a sanity check on the water carrying capacity of the pipes.
8mm pipe will transport about 1500W over a few metres and 10mm around
2500W. If you are reasonably close to being within these figures, it should be fine.
I found that there was one case where the existing 8mm pipe was not adequate for a new radiator and I swapped that out for 15mm. Apart from that there was no significant change required except as described. I would in any case make sure to flush the system thoroughly. This can easily be done as you switch the system over to sealed operation - you will have a mains supply and can flush out at each radiator position. It's also important to fit a strainer filter on the return to the boiler.
Yep. I've made my offer on the house already, but I'll do a sanity check with a jug and a stopwatch. Can't access the gas pipe to the boiler, but it wouldn't be difficult to run a new one from the meter in any case. Do gas meters ever come in a 15mm variety? I haven't actually checked in the cabinet, but it's a standard external one set into the wall - I'll have a poke in it when I go round with the surveyor.
Thanks, that's good to know.
Do you actively flush the intermediate pipework, or simply check that the water coming out of it looks reasonably clean? When I had my boiler installed a couple of years ago I flushed out all the rads as per the uk.d-i-y guide (written by you, I think?!) and just then just flushed the pipes through with unpressurised water to check it came out clean. Would I need to do more than this for microbore?
Anyway, sounds like I've got less work to do than I though then, plus or minus some work with pen and paper.
No, the standard domestic meter is a U6, I believe, and is able to deliver gas to the equivalent rate of 62kW. You do need to calculate sizes and lengths of pipe to make sure that there is no more than a 1mB pressure drop at the appliance(under full load) relative to the pressure at the meter.
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helpful to do the sums.
For microbore (I have 8mm), I think that it would be better to use pressurised water. I tried initially flushing using the old arrangement of the loft tank. The system had had inhibitor in use from new in 1985 and there was virtually no sludge in the radiators and flow out of the microbore at each radiator outlet was OK but slow.
So, I did an initial flush using the roof tank with water being allowed to exit first of all through the 22mm connections at the boiler and then at radiator connections. I then installed the new boiler with a strainer and isolating valves plus the new filling loop etc.
The next step was to flush out under mains pressure at each radiator valve with the boiler isolated. This completely cleared one side of the system. A little more crud in the form of copper swarf (dark in colour) from the original installation.
I then flushed with exit at the strainer (return side of boiler) so that I know I had clean water at the boiler. Then I replaced the strainer and opened boiler isolating valves and then flushed at each radiator, thus cleaning out the flow side of the system.
So basically it can be done by planning order of work.
Given that you have an unknown system, I hink that it would be sensible to assume that it hasn't been looked after and will need a good flush.
I think if it were me, if the system seems to be working OK now, I would wait until next spring to do the changes. That way, you would know from experience as well as calculation whether there are any rooms that are inadequately heated or marginal.
Otherwise, I'd look at the existing radiators and measure them, find a close equivalent from a manufacturer data sheet and work out what the current outputs are. Then do the heatloss calculations for comparison.
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