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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    Two heating questions: 1. I've got two pumps on my system, one seems to be at the zone valve end and the other near the return end. Does it matter if my rads return before or after the second pump?
    2. How much difference will be noted if one rad is fed with 10mm pipe compared to 15mm? Bearing in mind I'm running a low 35c flow temp....
    • CommentAuthortony
    • CommentTimeOct 12th 2017
    1 they should all do the same
    2 makes it very difficult to balance the system and 10mm rad will likely not get sufficent flow if it is a double rad.
    It's a single rad, I have two in the hallway to finally plumb in.
    • CommentAuthortony
    • CommentTimeOct 12th 2017
    Try it and see, I would always use the same size pipes all the same to all rads
    • CommentAuthorJeff B
    • CommentTimeOct 12th 2017
    Can I ask why the system is running at only 35C? ASHP with oversized rads perhaps?

    Why do you want to use 10mm pipe for these two rads instead of 15mm? If some very good reason why you can only use 10mm then I would say go for it. I had a mixture of 10mm and 15mm in my previous house and for single rads I don't think it'll make any difference.
    • CommentAuthordimengineer
    • CommentTimeOct 12th 2017 edited
    If you are running a low flow temperature, you need twice the flow (roughly) this means for a 10mm vs a 15mm pipe you'll need 4 times the velocity. Pressure drop is proportional to Velocity squred, so the PD to the 10mm rad will be 16 times what it would be for a 15mm rad in a high temp system.

    All other things being equal, for your 10mm vs 15mm rads, you'll still have 4 times the PD. So dont do it.
    I'm running an ASHP with oversize rads hence low flow temps.

    I'm unable to get pipes directly to these two rads as the floor joists are right next to the wall so cannot 'hide' the runs.

    I don't really want 15mm pipe on show in the hallway so I was going to use 10mm along the Skirting.

    Falling that I can use 15mm along the Skirting in the other room hidden behind the sofa and bring it through the wall in 10mm? Would that be okay? No different really to throttling the flow with a trv surely?
    Just work it out. You know the power rating (kW) of your rads, and the flow and return temps (35/30?), so you can calculate the flowrate you need. Then back calculate the velocity.

    If the velocity is much above 3m/s, think again!
    • CommentAuthortony
    • CommentTimeOct 14th 2017
    How about hiding 15mm pipes behind the skirtings?
    • CommentTimeOct 14th 2017
    Or even skirting heating?
    We have one radiator fed by 10mm plastic coated pipe that was buried in a screed, the rest of the system is 15mm. It only gets warm near the top, doesnt get enough flow to heat the whole rad. Fortunately we insulated the room and now it doesn't need much heat.
    We were thinking about getting one of those tall vertical radiators, we could run new 15mm pipe through the ceiling and drop down to it, wouldn't look too bad with the pipes boxed in or maybe exposed chrome pipes.
    • CommentAuthorcjard
    • CommentTimeOct 15th 2017
    Chase out the wall or come through it behind the rad so when the pipe merges its behind the radiator?
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