# How many radiators out of each branch?

Discussion in 'Plumbers' Talk' started by fabro, Jul 15, 2015.

1. ### fabroMember

I am currently running the radiator out of 22mm pipes that branch out to the radiators with either 15mm or 10mm. I am adding one radiator and thinking of adding another T from the already reduced 15mm pipes (1 metre further down it is still 22mm) to have a 4th radiator from that branch. I have heard the rule of thumb is up to 3 radiators. I can wait for a few months until I convert the garage and T from the 22mm but I need to close the ceiling in that room this week and need to make my decision. It is a fairly big radiator if that makes any difference, vertical 2200W.

2. ### Dave does GasWell-Known Member

Your rule of thumb is correct but it does depend upon the output of each radiator a 15mm pipe will carry maximum of 15.1Kw whereas a 22mm pipe will allow 40.2 Kw. So you will need to know the output for each rad and do the maths. Also the flow rate is reduced in a smaller bore pipe, there is also a formula for the length of pipe and heat loss which needs to be considered, but if you work out your 3 rads will exceed 15Kw the go up to 22mm

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3. ### FatHandsWell-Known Member

sorry to hi-jack the thread, but if i understand correctly, 15mm is ok to provide up to 15.1KW, so a tee from 22mm to 15mm is then ok to supply 15kw? which is quite a bit by the look of it. I see the double panel kudox 600 x 600 which are in the rooms in my place are quoted at just over 1kw a piece. any further info about the flow and length?
Cheers

4. ### PlumberbishActive Member

Unless your house is huge run 22mm as your main runs and branch off in 15 to each individual rad...you'll normally get away with a couple off 15 and I've seen whole houses run off 15 but it's not right....so 22mm runs and 15mm branches to each rad

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5. ### FatHandsWell-Known Member

thanks bish, this is what i have - just curious on the run length. essentially i am going to have have 2 600 x 600 double panels off 15mm which going by what has been said will be fine.

I agree with the first part; but the second part is way out.

The carrying capacity of a pipe depends on the maximum acceptable velocity (metres/sec) through the pipe. This is normally about 1 metres/sec as any higher will cause unacceptable noises. The velocity depends on the flow rate (litres/sec) and temperature differential across the radiator. (Some books suggest a maximum velocity of 1.5 metres/sec, but the general view is that 1 metre/sec is more acceptable.)

For a 11°C differential, a 1 metre/sec velocity is equivalent to 6.67 kW through a 15 mm pipe and 14.67 4 kW through a 22 mm. For a 20°C differential it is 12.1 kW though a 15 mm and 26.8 kW though a 22 mm pipe. The higher capacity when running at a 20°C differential is due to the flow rate almost halving compared to a 11°C differential. The velocity therefore reduces by the same proportion.

7. ### fabroMember

My first radiator (1050W) comes straight out of the 22mm with a reducing T, the next one is fed through a 10mm T'd from the 15mm and is 2200W and last one is at the end of the 15mm branch and is 1250W. Planning to have an extra one inserted in the 15mm part for another 2200W. 6700W in total for this branch, well within the 15kW so should be plenty for that 4th radiator then.
I currently have the downstairs ceiling open, is it a good idea to insulate the pipes? I will be filling the ceiling/floorboard space with sound thermal insulation rolls anyway.

8. ### PlumberbishActive Member

If it was my house id do away with the microbore...I'm sure lots of people love it but not me...if it was my house and especially as you can still get to your pipes, id change it so each rad is 15mm coming straight from the 22mm...sam spade seems to know a lot when it comes to the maths side of it-I admit I dont but I was just taught to judge it on experience...

9. ### PlumberbishActive Member

As for the insulation, never hurts to insulate pipes mate, it might not make a massive difference but I suppose the less heat you lose under your floor the more there is ready to come out of the rad

10. ### PlumberbishActive Member

Fat hands if you have the option I'd run 22mm over to them and reduce to 15 as it splits for each rad...like I said it should work with both off the 15mm but the rest of the system might need balancing down a little to force it to go that way a bit more...

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11. ### tom.plumScrewfix Select

I agree with plumberbish, plumbings a hands on job with tools not sliderules, figures can lie and liars can figure, but if the rad gets hot its working, if it does't its not, the prove of the pie's in the heating, as the saying goes,
where i differ from plumerbish is, I've no problem using microbore, its works fine, its been used now for 40 years and is still being used, that speaks volumes tha kneeeers,

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12. ### FatHandsWell-Known Member

hi. I take this to mean turn that the lockshields on the rest of the rads might need to be turned down (and therefore reduce heat output on those) to allow more through to the "spurred" (is that the right word for this scenerio?) rads both off a 15mm feed?
I think i could come down the ceiling in the corner of the wall in 22mm but it might look a bit odd in a living room?!

13. ### RyluerWell-Known Member

IF you use a 2 inch screwed iron ring main you can take 15mm stubs off that.