Limit on heat from 15mm pipes?

Hi,
As part of my efforts to get the living room warm enough to actually be habitable over the winter I've done a quick heat-loss calculation and realised that the "professional" who did some work on the heating system a few years back undersized the rads so that the room only has around 60% of the heat going into it that it should have. That could explain a few things.
Anyway, a nice new rad turned up today to go under one of the windows to beef things up. The easiest way to plumb this in is to T off from the flow and return to a nearby rad. If I do this there will be a total of around 8.5kw all coming off one spur of 15mm pipes. Can 15mm pipes carry this much heat, I'm wondering? I could go back and T off from the main 22mm pipes, but frankly that will be a pain in the arse and require a lot of floorboard-lifting and joist-notching (not to mention extortionate copper-tube-buying) and I really don't want to.
Cheers!
Martin
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On Wed, 29 Feb 2012 16:43:17 -0800 (PST), Martin Pentreath

Rough rule of thumb: 28mm - 24kW 22mm - 12kW 15mm - 6kW
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wrote:

Not quite so...
Condensing boilers with a 20C temp differential require smaller pipes, making them more economic to fit.
At 11C temp diff:
15mm - 6.0 kW 22mm - 13.4 kW 28mm - 22.5 kW
At 20C temp diff:
15mm - 9 kW 22mm - 24 kW (81,888 BTU/hr) 28mm - 70 kW
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That maths doesn't add up.
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Dennis, please do not think.
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So you don't see what's wrong then.
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dennis@home wrote:

If I was a child in a playground I would now be shouting "fight fight fight"
FIGHT, FIGHT, FIGHT
--
Adam



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On 01/03/2012 19:20, dennis@home wrote:

The maths does add up to an approximation, but only on the basis of equal pressure loss, and doesn't take into account of noise variation between different sizes of pipes and water velocity.
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A very big approximation.
According to that the heat capacity with a 11C vs. 20C drop is
15 mm 1.5x 22 mm 1.8x 28 mm 3.1x
I.e. its wrong.
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On 02/03/2012 19:22, dennis@home wrote:

I can assure you flow is a function of internal diameter cubed (or to the power 4, I can't recall) assuming constant pressure. I suggest you look up laminar flow in a pipe as a function of diameter and come back here citing references if you believe otherwise.
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Why don't you read what I said as you obviously got it wrong. Its got sod all to do with different flows, etc. Just tell me why you think the flow would be different for two 15 mm pipes or two 28 mm pipes.
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On 02/03/2012 20:53, dennis@home wrote:

So the flow should be the same for 2 different sized pipes? That's a new one. Perhaps we should be building our central heating system with hypodermic sized pipes?
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You are the one that said the 15 mm pipes would have different flows, just as you said the 22 mm ones would be different and the 28 mm ones would be different. If the flows are the same then the ration of the heat capacity would be the same as only the temperature differential has changed. Why don't you read what I said?
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On 02/03/2012 23:17, dennis@home wrote:

How would the temperature differential change if the flow is the same?
If the rad is the same, and the flow is the same, then the temperature differential will also be the same. The only way you are going to change the temperature differential (all other factors being equal) is the reduce the flow rate.
The knock on effect being that you also reduce the average temperature of the rad and hence its heat output.

--
Cheers,

John.

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We are talking about pipes feeding radiators, nothing has been said about the size of the radiators only what the difference between flow and return is. If you fit a bigger rad the differential will be more and the heat the pipe can carry will be higher. However the figures don't make sense. That is there is an error somewhere in the figures.

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On 02/03/2012 23:17, dennis@home wrote:

I did say flows would be different, ie assuming a flow rate would be measured in litres/sec.

I don't understand this fixation with believing flow rate is the same with different pipe sizes?
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God its frustrating when people can't grasp the simplest of things.
Here is the error that was posted..

Now are you seriously saying that you can't see the error?
The difference in heat capacity for the two 15 mm pipes is 9/6 or 1.5 times. for the two 22 mm pipes its 24/13.4 or 1.8 times for the 28 mm pipes it 70/22.5 or 3.1 times.
Now do you see the error? A hint the flow must be the maximum in each case or it wouldn't be the limit of that pipe size. So the flow in the pairs of pipes of the same size is the same.
So why does the 28 mm pipe increase by a factor of 3 when the 15 mm only increases by 1.5?
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On Mon, 5 Mar 2012 20:26:03 -0000, "dennis@home"

What's the internal surface areas, compared?
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On 05/03/2012 20:26, dennis@home wrote:

It most certainly is when such a simple subject of laminar flow in fluid dynamics cannot be grasped by people who think thy know better.

The flow of heat is matched by the flow of water. Flow in a pipe is a higher function of pipe diameter. Next you'll be saying that the stopping distances in the Highway Code aren't consistent with each other. Flow is proportional to pressure, assuming simple laminar flow and a peak velocity below the sound barrier in water. Therefore there is no maximum for all intents and purposes.
When you show your calculations here, it'll be an opportunity to illustrate your erroneous thoughts. No one here who has done Physics A level here or higher can see what you can't see, if you see what I mean? Or perhaps not!!
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They calculations are there for all to see, I even repeated them in case you missed them. I just don't know how to explain it any more simply.
Lets try this..
You have a 15 mm pipe and with an 11c difference it is claimed to have a maximum capacity of 6kW. The same 15 mm pipe at 20C difference is claimed to have a capacity of 9 kW. That is 1.5 times the capacity. The flow doesn't change if its at its maximum as claimed, it can't!
Now explain why the two 22 mm pipes and the two 28 mm pipes aren't also 1.5x?
Maybe you could even do the sums and note that the capacity for the 15 mm pipe should change by 1.8x not 1.5 in the first place.
That is the bloody figures quoted are wrong!
If you really think you can explain it by fluid mechanics feel free to do so!
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