I'm about to relocate a couple of radiators, and plan to loop some 10mm plastic pipe behind the skirting boards into the 15mm main run.
Is there a calculation/formula that determines what losses are at play? Or if the resistance to flow might become a problem?
For example, the 10mm-15mm reducers look incredibly unlikely - especially where the pipe strengtheners reduce the internal diameter to about 5mm at joins. It obviously works, though, and on the 5 radiators (of 14) that I've done, there doesn't seem to be any disadvantage.
8mm carries 1500W, 15mm-6kW, 22mm-13kW. The first limiting factor is the velocity of the water in the pipes. If it exceeds 1.5m/s which is the maximum velocity specified in BS 5449 you will start to get noise problems.
The second limiting factor is the pump head and pipe length/resistance.
In theory perhaps, but not in reality. You are limited in max temp by the boiler's upper limit (often mid 80's for modern condensors), and the flow rate limited by the amount of noise you can accept in the pipework.
On noise in general, far and away the noisiest part of my home's system is the 15mm copper pipe. In the attic rooms, plumbed with 10mm and 15mm plastic, the heating cannot be heard.
Well that was kind of the question - I have seen the same figures (e.g.
6kW for 15mm pipe) quoted for decades - with not much updating going on... Now in one sense - the heat delivery rate of a pipe is not affected by technology - its dictated by the fluid in it, and how fast it moves.
The 1.5m/s flow rate lets us calculate a maximum volume of water that can be delivered per second for a given size of pipe. That suggests:
15mm^2 pipe area = 3.14 x 0.0075^2 = 0.0001767 m^2 Volume of water in 1.5m of pipe = 1.5 x area = 0.000265 m^3 Total delivery rate of water is thus 0.27 litres/sec
So that could shift = 0.27 x 4200 = 1.134 kW for every degree drop in temperature at that flow rate and pipe size.
So if those sums are right, that suggests a (surprisingly high) maximum heat delivery rate of just over 12kW for a 15mm pipe at a flow velocity of 1.5 m/s. This assumes you have enough radiator surface area to actually drop a full 11 degrees. Obviously the oft quoted 6kW figure is based on different assumptions - perhaps a flow rate of 0.75 m/s
Also the ability to actually dissipate heat will be heavily influenced by the relative flow temperature - a given rad will dissipate less at 70 deg than at 80. If you want to increase the temperature drop on the system to get the the return below 54 degrees, then you need slower flow rates.
In the end, it's all about what's considered acceptable losses ie equivalent to the electricity supplied to the pump.
Domestic central heating for the masses only became /economic/feasible with the development of the "high pressure" circulating pump. A pioneer was Angier March Perkins.
Prior to that it was only for the wealthy with gravity systems and large diameter pipework.
The resistance increases quite a bit as the 15mm goes to 10mm. But only for a short run (about 1m), where it goes into the rad via 15mm. This implies the flow, and the rad outlet, is now at this reduced level. Is this correct?
I'd say anecdotally there's very little difference in heating up times between a 10mm and 15mm-fed radiator. But the figures suggest otherwise
- I'd have expected at least a doubling of warm up times.
The 10mm pipe strengtheners are about 10mm long, with an internal diameter of 5mm. I'm not sure how to account for these in the calculations.
And overall, I don't see how these calculations need to be factored in to boiler design in terms of pumping the water around. I can't see any reference to 'flow resistance' in the installation manual for example. I take it I need to be looking at the pump spec as well as the boiler output?
Of course - but my posting a link to complicated, impressive looking stuff doesn't mean I understand it all - or that it's accurate ;)
The flow rate is determined by the total path from the pump and back again. So inserting 1 m of 10 mm pipe doesn't /determine/ the flow. It /reduces/ it but only to the same extent as inserting a (much) longer length of 15 mm pipe. And bear in mind you may have room to compensate with the radiator valve.
If it helps, think of the pipes like cables. Thin cable will have a higher resistance per m than thick cable. But you have to add up the whole circuit to work out total resistance and the current that flows for a given voltage.
They'll be equivalent to a much longer length of pipe but I've no idea how much. The manufacturer's specs might say. What make are they?
That's field where I've never delved beyond checking the pump will handle the head and flow rate - which was easy as I've only done a very few for small homes.
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