22mm/15mm pipe central heating

[Eddm87]

Hi all.
I’m putting in a total of 4 new rads in an extension. They will emit close to 10000btu.
Problem I have is that the old side of the house is run in 15mm the whole way. I don’t want to replace all these pipes to 22mm obviously.
Should I run the new rads in 22, dropping to 15 down the walls obv, or just run it all in 15 as the flow rate is already limited from the 15mm it’s being teed off from anyway?
I know that ch is usually in 22 as it can distribute more heat, but does it just mean that all the rads will take longer to heat up? Or that they will just not ever get hot enough?
Thanks

Edd

 

[TeaAndBiscuits]

There's also an issue of flow rate - too low and you can get sludge build up, too high and you get noisy pipes.

But more intuitively, there's not a lot of point stepping up to 22mm from a 15mm pipe.

 

[sirocosm]

Normally you would run 22mm main flow and return, and then have 15mm circuits off that feeding 2 or 3 rads each. Is that possible? You wouldn't have to replace all the 15mm, just enough to have a path back to the boiler from the last spur.

 

[oldbuffer]

1. No point going 15 - 22 - 15. The 15 will be the limiting factor.
2. 10,000 BTU is about 4,000 BTU below the limit for heat carrying capacity for 15 mm pipe.
3. If you already have radiators running off the 15 mm pipes you expect to use for your extension, it is highly likely the pipework won't be able to carry the heat.
4. If so, your new radiators won't get warm.
5. You really need to re-pipe the backbone in 22 mm at least to the point where only around 14,000 BTU (about 4 kWh) remains on the 15 mm.

 

[MeldrewsMate]

There's also an issue of flow rate - too low and you can get sludge build up, too high and you get noisy pipes.

Indeed, anything above 2.5m per second will give noticeable noise. As for too low, I disagree; if there is no sludge then it cannot be deposited, and we are not in the business of making sludge. The argument around too large a pipe is simply the capital cost of their installation, and the extra water content they hold.

Using that figure of 2.5m/sec maximum velocity, I calculate that the volume passing a single point will be (using 15mm od pipe, 13.6mm id) 250 x PI x 1.36 x 1.36 / 4 = 363 cm3 per second. With a temperature drop of 15C and a SHC for water of 4.2 J/gC, I get about 22 kJ per second, or 22kW, or about 77,000 btu/hr....now it's a long time since I did that calc, and I'm sure I got a figure more like 30,000 btu/hr last time...would anyone care to corroborate or challenge my calcs - that a pair of 15mm copper pipes with a water velocity of 250 cm/s and a 15C temp drop will be capable of delivering 22kW of heating?

In any case, the pumping effort to achieve that flow velocity in 15mm pipe in many domestic systems would be too high for a domestic pump, so I would always design a system with a target of 0.5 to 1 metres per second flow velocity.

 

[Eddm87]

Thanks guys. More than a good overview! Knew it was something along those lines but wanted a clearer understanding. Where I plan to tee off from is about 3 metres from the boiler so I will see if I can upgrade those 3m to 22mm otherwise, as you all insinuate, there are likely to be issues.
Thanks again

edd