10mm droppers to large radiator.

dhutch · 19 Mar 2019

We need to run pipework down to the ground floor rads as part of a full heating system replacement. Edwardian property with good thick plaster and good bit rooms. Modern Viessmann boiler.

Rads in one room are expected to be fairly large, something like 700x2000 K2 Stelrads, at just under 4kw a pop. Maybe slightly smaller. But keen to get return temps down to a sensible level to make the most of condensing boilers.

The suggestion from our plumber is to plumb them in 10mm coated copper coil, up the wall, and then directly into a 22mm copper backbone. Seams reasonable to me, and running 15mm down would be a royal pita for various reasons. But just wanted to put my mind at rest before it happens.

Daniel

oldbuffer · 19 Mar 2019

1. Assuming temperature drop of 20 degrees across radiator, and a system water speed of around 0.8 to 1.0 metres / second then you should be OK
2. If temperature drop is 11 degrees (typical of older, non-condensing boiler systems) then you would be right on the limit for heat and would require a water speed of 1.5 metres/sec, which is right on the limit for noise.
3. If there is any chance at all, better run under ground floor in 22 / 15 mm.

ianmcd · 19 Mar 2019

I would use 15mm even if it is a lot more work

Madrab · 19 Mar 2019

Yup, I think you might struggle with that size of rad on 10mm. 15mm if it's at all possible IMO.

D_Hailsham · 19 Mar 2019

Systems need to be designed with a 20C differential for two reasons: it brings the return temperature down to the dew point where condensing starts, and the slower flow rate reduces the friction loss, which is high in modern heat exchangers. There is however the disadvantage that radiator outputs are reduced. 75C/55C reduces the output to 85% of the nominal value, and 70C/50C reduces it to 73%. So your 4kW rad will produce 3.4kW (75/55) or 2.9kW (70/50). If, however you actually need 4kW you will have to install a rad with a nominal output of 4.7kW (75/55) or 5.5kW (70/50).

Assuming that you need 4kW, the flow rate is 4/(4.18x20) = 0.048 litres/sec. If the rad is fed by a 10mm pipe (inside 8.6mm), the water speed is approx 0.83 metres/sec, which is well below the suggested maximum of 1m/sec (Too high and the water flow can be heard; too low and sludge can settle in horizontal pipes.).

If a 20C differential is used, meaning a lower flow rate, the pressure loss due friction in the pipes is a quarter of that if a 10C differential is used, meaning that a lower output pump can be used.

dhutch · 19 Mar 2019

Ok, so all in all it sounds like we should be all good, which is nice to know. Although suspended floor, the crawl void is only a few inches and the whole floor is period oak tiles which we are not lifting.

The plumber confirmed this evening that the calcs come out at 21000btu which seems to 6kw or 3kw each. So is suggesting a pair of 600x1800 rads.
Presuming there is no harm in oversizing slightly I might suggest going a bit larger, but will also somewhere take his lead.
The room is about 6x9m with 9ft ceiling, solid walls external on two sides, the shorter of which is mainly bay double glased window. Bedroom above, suspended wooden floor below.

There will be about 18 rads and a couple of towel rails off the existing 5yo vitodens 100 35kw system boiler. About 6-7 of which will be this sort of size, with the rest smaller down to the little one in the downstairs loo.

Thanks

Daniel

durhamplumber · 20 Mar 2019

D_Hailsham said:
Systems need to be designed with a 20C differential for two reasons: it brings the return temperature down to the dew point where condensing starts, and the slower flow rate reduces the friction loss, which is high in modern heat exchangers. There is however the disadvantage that radiator outputs are reduced. 75C/55C reduces the output to 85% of the nominal value, and 70C/50C reduces it to 73%. So your 4kW rad will produce 3.4kW (75/55) or 2.9kW (70/50). If, however you actually need 4kW you will have to install a rad with a nominal output of 4.7kW (75/55) or 5.5kW (70/50).

Assuming that you need 4kW, the flow rate is 4/(4.18x20) = 0.048 litres/sec. If the rad is fed by a 10mm pipe (inside 8.6mm), the water speed is approx 0.83 metres/sec, which is well below the suggested maximum of 1m/sec (Too high and the water flow can be heard; too low and sludge can settle in horizontal pipes.).

If a 20C differential is used, meaning a lower flow rate, the pressure loss due friction in the pipes is a quarter of that if a 10C differential is used, meaning that a lower output pump can be used.

And in reality,how much difference does that all that guff make?

dhutch · 20 Mar 2019

If it reduces the output to 85%, presumably about 15% difference, if you take it down to 70/50 nearly 30% difference. But point taken!

Measuring up the space 2000 would be tight in one of the places, so we are looking at one 1800 and one 2000.
Plumber wants to do 600 because the calcs say its ok, they mainly fit 600 and it will look nicer. I am still angling for 700 which is another 20% output on the basis you might as well given they are all non-stock items anyway.
The other main reception has two 600 x 1800 and in s smaller room with less external walls. I also want to have good heat-up times as it will be out second (so semi-occasional) room.

I am sure either is fine, but if your running lower temps for good condensing, why wouldnt you go a bit bigger on the rad.

Daniel

Madrab · 20 Mar 2019

TBH .... I find that things can always look fine on paper but doesn't always replicate in practice.

I am having problems running anything larger than a 1400x600mm on 10mm drops from a 22mm main F&R. The system isn't new though and it's pretty large at 16 bigger rads over 3 floors and may be suffering from restrictions (pipe size/lengths/no of bends/etc) and/or a lazy supplementary pump, which will all be getting swapped out during the summer, but it does show how easy it is to run into problems when you run up to the limit of a system. Building in head room is always good practice IMO.

I'd also be careful when mixing pipe sizes to the rads, as balancing then becomes a much more sensitive exercise.

dhutch · 21 Mar 2019

Who knows, looks like it will be one 600x1800 and one 600x2000 for the big room. 6x9m with a big window, but it's still a lot of radiator.

Dan Robinson · 21 Mar 2019

There is big difference between dt11 and dt20.

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10mm droppers to large radiator.

durhamplumber

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