Heat capacity of central heating pipes

Can anyone remember the rule of thumb capacity of central heating pipes (15mm,22mm and 28mm)? I'm intending 70 degree flows. Just need to work out if I need 28mm to the up/down zone valves. I'm sure 22mm is fine after these.
Christian.
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See: http://tinyurl.com/k0rx Pipe Sizing and Flow Rates
It can be calculated, with a flow rate of 1.5m/s as recommended in BS4449, that a 15mm pipe is able to carry 4.25kw of sensible heat a 22mm 12kW and a 28mm 35kW.
Due to the higher flow and return temperature differentials in a system designed for condensing boilers (approximately twice that of traditional systems) an equivalent size pipe would carry the same amount of heat.
Therefore it can be calculated that a 15mm pipe is able to carry 9kw, a 22mm pipe 24kW and 28mm pipe 70kW.
When designing an installation, with radiators sized for a flow temperature of 70oC with a return differential of 20oC, you may permit the use of smaller pipe diameters for the heating circuits.
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In any case 22mm will be quite enough. There won't be any saving to had downsizing to 15mm and I'm happy with 22mm rather than 28mm as it will fit a pipe bender. I haven't calculated the radiator powers yet, so don't know if 70/50 will be doable in reality. They look to be on the small side to my eye.
Christian.
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On Thu, 14 Aug 2003 17:12:44 +0100, "Christian McArdle"

Christian
The one to watch out for is not to drop from 22mm to 15mm too early, in the sense of having too many radiators (actually too much capacity) on 15mm runs.
If you are going to derate for 70/50 operation vs. 82/70, in general it means dividing the required power level for the room by 0.6 rather than 0.89 for most radiators, and then referring to the main table in the manufacturer's data sheet to choose the radiators.
I found that when I refurbished my system, in some cases the radiators were way oversized anyway so nothing to do, in some cases I moved radiators from one room to another and in some cases I fitted new radiators. However, there was only one case where I had to use a radiator that had a larger footprint of wall space (if you follow my meaning). For the others, it was possible to use a double panel or a finned radiator where a lesser one had previously been.
.andy
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It must be right then.....

.andy
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The rads are already in (but need TRVs and zoning). I calculate my total requirement as 12kW, even after allowing 50% extra for extra rapid warm up. Looks like 22mm is fine. I may get a loft conversion in the future, but the boiler will actually be sited in the loft, so won't go through the run I'm considering here.
Christian.
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The controls will be S-plan, with (eventually) four zones. Upstairs, downstairs, loft conversion and conservatory.
The cylinder being next to the boiler should result in it getting some priority, but I'll probably leave it at that. With the cylinder on 24/7, the increased recovery times when the heating is on shouldn't be too bad.
Christian.
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Christian,
Are you installing a quick recovery cylinder? If so, no need to keep it on 24/7. One advantage is that they heat so quick you can have a cold cylinder, think I want a bath, switch it on, and by the time you have phaffed about preparing soap and getting undressed you have a bath full of water ready. They are approx 15-40% cheaper to run than simple part L cylinders. You can have a simple diverter valve and your zone valves on the CH circuit.
It is a far better setup. BTW, the Drayton 2 port valves with an end switch, on sale at B&Q for approx 32, can have the plug screwed out the bottom and a BSP to copper compression fitting screwed in to make it a 3-port.
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I'm probably going for something like a DPS heatbank with pressurised primary coil, provided I can persuade them to make a 150L or 180L version with the double plate heat exchanger (they only list the 300L with this as standard). I need the double heat exchanger as I think the projected 30lpm to 42C of the single is too low. I need pressuried DHW and heating circuits, as I will be converting the loft and putting in a shower room and radiators. Whilst in general, I'd prefer an unvented cylinder, the location in the loft with no obvious safe and visible location for the pressurised vent makes the heat bank more sense, despite the fact that it will reduce the opportunities for the boiler to condense.

I'm more likely to wire it up conventionally and put in relays if I find a problem in practice, which I don't anticipate.
Christian.
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Good move. They will probably fit a quick recovery coil anyhow as standard, but check. But!! go for a single plate heat exchanger with a 150/180L. If it does not cope then insert another plate heat exchanger in parallel yourself. This is a great advantage. DPS use push-fit brass fittings. This would only take about an hour to fit. Try uprating the coil of an unvented cylinder as you go along. Heat banks have that great flexibility.

Heat banks are far better than unvented cylinders and you can DIY them.

The difference between condensing while heating an unvented cylinder or heat bank is minimal. In fact greater overall efficiencies will be had from the heat banks as the primary blending valve will be set approx to 55C. You also have the rapid heat up of the heat bank and within a few minutes from cold you will have usable hot water. Another advantage is that you don't have to reduce the mains pressure, so if you have 6.5 bar, as I do, then you can have this at your taps.

famous last words.
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I've just had a thought (just rambling, not thought through). Would it be a good idea for the cylinder primary coil return pipe to go through a plate exhanger with the incoming mains cold? This would pre-heat the incoming mains a little and reduce the primary return line temperature quite considerably, giving good condensing and something for the boiler to "push" against. I can see a problem that you might unnecessarily heat a short section of DHW piping when the cylinder is recovering but no DHW flowing, but this could be dealt with by insulating the section to reduce losses and this 82C heated section would be sorted out by the thermostatic output valve, before it hit the taps, probably.
Opinions?
Christian.
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On Thu, 14 Aug 2003 16:10:02 +0100, "Christian McArdle"

There is a technical note published by the Copper Development Association which covers how to easily calculate the pipe sizing requirements using a simple set of tables to allow for fittings, pipe lengths etc.
http://www.cda.org.uk/megab2/build/Pub150%20UKCB.pdf
The example given covers the conventional 82/70 degree system. It's easy enough to substitute in the required mass flow rates for 70/50 operation
.andy
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