Boilers pipes and kilowatts

Is there a resource somewhere for calculating the amount of heat you can transmit through a given size of pipework? Or a rule of thumb for the most common domestic situations?

Reply to
Dave Plowman (News)
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There is an application note and some tables on the Copper Development Association web site. This is all non-linear but begins with the maximum velocity of water through any pipe. Using tables of pipe size and length, it's possible to get to the max. volume and mass of water per second. From there, you can get to the max. amount of heat that can be transferred.

Normally it's worked out the other way round - i.e. you know how much heat that you need to transfer and then determine the minimum pipe size for the required length.

Another factor is the temperature drop. If you are using a condensing boiler as you are and have or can resize radiators, you can work on a calculation of a 20 degree temperature drop and thus reduce the required flow rate.

Reply to
Andy Hall

Only need to ad that a rule of thumb would be "4 or more needs 22". As with all rules of thumb they are toleranced to +/- a piece of string.

Reply to
Ed Sirett

This summer I'm currently about to replace the pipework on our heating system in plastic, as the existing copper-direct-in-screed is now causing too many problems. As we have a bunglow the pipe routes will be in the roof space, with drops in unobtrusive corners. I can runn 22mm in the loft easily enough, but the vertical drops would be much easier in 15mm, with some of the vertical drops then feeding up to three radiators.

Some of the runs are quite long, with the furthest room being around 12m from the central pump position - I was planning these in 22mm anyway to avoid difficulties in getting sensible amounts of heat to this end of the house.

Presumably the composite effect of a 22mm run, with an end section of 2m to

3m in 15mm, would negate some of the pipe friction effects caused by the smaller pipework? Or is any section of 15mm the main restriction?

Charles F

Reply to
CJF

It's definitely length related, which is why the rule of thumb idea is limited. On the basis that Ed describes it assumes a certain number of radiators in a space covered by one trunk run of pipe and typical final 15mm pipe runs of 3-4 metres.

With 22mm pipe there is quite a bit of headroom in domestic situations

- even the main boiler runs are often in this size. With 15mm, you have to be more careful - it's easy to exceed its capacity.

Certainly if you are going to 10-12m there is a considerable effect, which is why there are tables to calculate the sizings properly.

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then download application note P150. This has the explanation of how to calculate the flows according to the heat required and then to determine the pipe sizes required for each run, given the length.

You do need to work in order:

- Heat requirement (calculated for room)

- Size radiators to meet heat requirement. This is already done if you have a working system

- These will allow you to calculate mass of water per second

- You know the pipe lengths that you want. and where you want the joins to be

- This gives you what you need to calculate which pipe size is required for each section

Reply to
Andy Hall

Thanks Andy - I'll download and calculate properly. I have already back calculated for our existing rads and room losses -I was just hoping for enought of a rule of thumb solution for pipework to be lazy!

Charles F

Reply to
CJF

You're welcome.

There is a certain amount of latitude since you have the balancing that you can do from the radiator lockshield valves. With this you can "encourage" the water to go through a longer section of run. However, this won't make up for a completely inadequate flow capacity.

Generally you will need to take the 22mm pipes as close as you can to the radiator drop positions. The challenge is going to be whether you have enough capacity for all that you need to run after the 15mm drops if you are going to run several radiators from one drop.

If there's a problem, your choices would be to do more drops or to run

22mm for the drop. I suppose that more drops in 15mm would be preferable to 22mm unless you are going to enclose the pipes in something.
Reply to
Andy Hall

Also note that plastic has thicker walls and hence thinner water channels. This is going to be most significant on the 15mm pipework. I presume plastic must have different flow tables from copper, but I've never gone in search of it.

Reply to
Andrew Gabriel

Good point.

A quick phone call to the tech dept at John Guest would get that info I'm sure.

Then it's simply a case of substituting that table for the copper one in the CDA note.

Reply to
Andy Hall

And inserts that reduce internal bore.

Reply to
Doctor Drivel

I'd do 2 runs of 15mm, and try and make it 'zone ready'.

Fighting with 22mm coiled pipe in a hot loft is not for the faint hearted :)

cheers, Pete.

Reply to
Pete C

They are short and thin....

Reply to
Andy Hall

That have little effect.

Reply to
dennis

This is really what prompted my question. My old Potterton cast iron boiler had 28mm connections to the heat exchanger. The new Viessmann has

22mm - despite being of slightly higher capacity. I did the original installation myself under guidance from an expert and ran 28mm to the three port valve at the cylinder on the other side of the room and onwards to where the flow split to upstairs/downstairs and similarly with the return. The new boiler is a system one with integral 3 port valve and pump so although I could have modified it to use the existing external pump and valve I've adapted the pipework instead - converting the 28mm side to be heating only and adding a 22mm circuit for the cylinder.
Reply to
Dave Plowman (News)

Again.. "And inserts that reduce internal bore"

Reply to
Doctor Drivel

Was that perhaps that it was also able to work with a gravity HW arrangement? That ususally uses 28mm

Perhaps this is based around a 20 degree temperature drop rather than

12? The pipe size can be reduced and with a condensing boiler it's desirable to have more of a temperature drop anyway.

Makes sense...

Reply to
Andy Hall

Not for the full length of the pipe.

Length is highly important when it comes to overcoming resistance.....

Reply to
Andy Hall

Surprised no-one has yet suggested installing, not just two pipes, but a second boiler...

Reply to
Rod

No - it was designed from the off as fully pumped Y plan open vented. State of the art in 1977. ;-)

Right.

It would have been possible to share the flow - but the 'sides' were reversed so I decided against it.

Other thing is the cylinder had the flow to the top of the heating coil. Viessmann reckon it should go to the bottom.

Reply to
Dave Plowman (News)

I think that the argument there is that you will get a bit more heat transfer since the flow water to cylinder water temperature difference at the bottom will be greater. I've never tested the difference, but doubt if it has a huge effect.

Reply to
Andy Hall

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