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Mixing of pipework/components' with different internal dimensions - theory vs reality/practical approach

rhh2348 · 2025-01-23T09:52:07Z

Hi all. Consider the following scenarios to support a modest ASHP (with glycol) with a simple setup of single circulation pump through a decent 3-way diverter valve to mid-sized cylinder and two heating circuit runs with mixed emitters operating as one zone and returning through mag filter and flow sensor. 28mm copper from/to ASHP, DN25 pump, 22mm cylinder connection, 2 x 22mm push-fit to 15mm rads and UFH manifold w/ 15mm UFH loops: all remaining components DN25 all remaining components DN20 a mix (e.g. DN25 3-way, DN20 flow sensor) Before I read John Cantor's site, my understanding was that the smallest internal diameter of just one component in a circuit essentially reduced the whole performance everywhere on that circuit to that diameter, however if I've understood his words correctly, even putting a nominal single run of larger-diameter (straight) pipework improves overall performance (by reducing head - thereby increasing flow?). So my initial questions are: should one aim to put the widest pipework *wherever physically* possible, reducing to the smaller diameters at the components that cannot be changed (e.g. rads pipework under floors), OR does wider pipework only make a difference for particular components/usage/minimum distance (e.g. more than 300mm, 2m, etc)? are there any components in particular that would benefit from larger internal diameters - e.g. if water flows through 22mm connections for hot water and space heating, is there merit in using a 28mm 3-way or flow sensor or mag filter? what sort of performance improvements/limitations would one expect based on the answer?

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rhh2348

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Topic starter 24/01/2025 3:09 pm

Also, in terms of positioning of the 22mm-based components, I'd be keen to know others thoughts on where these would go to optimise flow and their performance - examples:

the mag filter - it disturbs flow so should it go before the circulating pump so that the pump can push most effectively round the circuits and back to the outdoor unit, or should it be the last thing before the return pipe to the outdoor unit so that it has most chance of removing debris from getting to the unit
the flow sensor - do we want to know the flow nearest the LWT (so it should go before the circulating pump), the flow that would more likely be going round the heating circuits (so after the pump, or does either position actually make any difference?), or on the return pipework - if the latter, before or after the mag filter - pro's and con's to both!

What do people think?

This post was modified 1 year ago by rhh2348

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RobS

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24/01/2025 9:01 pm

Posted by: @rhh2348

↑

the mag filter - it disturbs flow so should it go before the circulating pump so that the pump can push most effectively round the circuits and back to the outdoor unit, or should it be the last thing before the return pipe to the outdoor unit so that it has most chance of removing debris from getting to the unit

the flow sensor - do we want to know the flow nearest the LWT (so it should go before the circulating pump), the flow that would more likely be going round the heating circuits (so after the pump, or does either position actually make any difference?), or on the return pipework - if the latter, before or after the mag filter - pro's and con's to both!

I would suggest:

Mag filter last on return so it filters out stuff before it gets to the outside unit. Do you also have a strainer in you system?

Before the pump is likely to be less turbulent and so more accurate readings possible. If you decide on the return then after the mag filter so you don't get crap in your flow sensor.

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BobTSkutter

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24/01/2025 9:10 pm

Hello All, remember your designing a loop transferring liquid water. The pump just moves the liquid round in circles. It makes no difference to system pressure drop which order you install the components because it's a loop. However your ASHP vendor will specify what should be installed to protect the plate exchanger inside the heat pump.

I would suggest:

Circuit return > globe valve > mag filter > strainer > flow sensor > ASHP inlet > ASHP outlet > pump > globe valve > Circuit feed.

That way you can close two valves and get at everything to do with the heat provider.

But check the installation drawings for your vendor, they will tell you what must go where.

Regards

Bob

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rhh2348

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Topic starter 25/01/2025 11:19 am

Posted by: @robs

↑

Posted by: @rhh2348

↑

the mag filter - it disturbs flow so should it go before the circulating pump so that the pump can push most effectively round the circuits and back to the outdoor unit, or should it be the last thing before the return pipe to the outdoor unit so that it has most chance of removing debris from getting to the unit

the flow sensor - do we want to know the flow nearest the LWT (so it should go before the circulating pump), the flow that would more likely be going round the heating circuits (so after the pump, or does either position actually make any difference?), or on the return pipework - if the latter, before or after the mag filter - pro's and con's to both!

Mag filter last on return so it filters out stuff before it gets to the outside unit. Do you also have a strainer in you system?

No, I don't - reading https://renewableheatinghub.co.uk/forums/postid/15126, @heacol recommends removing the mag filter and adding a Y strainer! Thoughts??

Before the pump is likely to be less turbulent and so more accurate readings possible.

Agree, however would the DN19 of the flow sensor body not affect performance to the heating circuits?

If you decide on the return then after the mag filter so you don't get crap in your flow sensor.

That's a fair shout as long as the sensor body isn't going to create debris...

From the heat pump perspective, given the flow rate is most pertinent to LWT, input power, and compressor frequency, it's probably best as the final component on the return (with a significant length of straight 28mm pipe before it to smooth turbulence out) for the unit's optimal operation.

This post was modified 1 year ago 3 times by rhh2348

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JohnR

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25/01/2025 11:40 am

It seems to be normal practice to put the filter on the return pipe to the heat pump so it catches any debris from the main heating system before it gets to the heat pump. I would consider putting the flow meter next to the filter so that there's only one section of plumbing with a smaller diameter in the heat pump circuit. Each change in diameter and velocity creates some extra losses.

👍

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Adrian

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26/01/2025 11:08 pm

Maybe the analogy is imagining that cars could just drive faster through those single carriage section. All that acceleration and extra friction would lead to higher energy consumption. If they go too fast they will also damage the road/pipes. Do too much of that and you’d run out of energy (aka petrol, or pump head). Yep also far from perfect…

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