@mars  Im going to put a slightly different angle on this post because, whilst I accept its importance in principle and for some properties, Im not so sure it should figure quite so significantly in consumer minds for the vast majority of properties. 

Whilst its certainly the case that pipework size really does matter, its not necessary (or even practical) in many, perhaps most, cases to do extensive calculations or even to be 100% certain, and an attempt to do so would be way over the top.  There is a very real role here for 'engineering judgement' as a first filter, with a contingency plan if the engineering judgement turns out to be wrong. 

Take my house as an example, 200sqm and 7kW loss, fairly typical of a suburban detached house I imagine.  All that was necessary (and the only thing practical without taking up flooring and floorboards) was to note that there appears to be a 22mm primary for upstairs, a 22mm primary for downstairs, that the rads are on 15mm tails and that there is a 22mm flow/return pair for the DHW.  Now of course its possible that the 22mm primaries are sized down to 15mm 'too early', but there is no practical way to discover that without taking up carpet and floorboards upstairs and solid flooring and floorboards downstairs.  Far better to proceed on the most probable assumption ie that it is OK, and then solve a problem if it occurs.  I would suggest that the same logic applies to most houses where both the primaries and the feeds to the rads (ie the bits that you can actually see) are OK, which is likely to be a majority.

A similar approach was taken with the cold water feed to my DHW cylinder which is 15mm only.  The installer checked the garden tap (which is on the end of a long run), noted that the flow was >20l/min, and made a reasonable assumption that the flow to the DHW cylinder would therefore be OK, because the mains pressure was sufficient to overcome loss in the pipe.  There was a fallback option if not, which in practice wasn't needed (and yes the installer did check before leaving!).  That's exactly what I would want any installer to do.  What I definitely didn't want (which some who I firmly rejected proposed) was to run a new pipe without first proving that the old was sufficient, which is realistically only possible by experiment.

So whilst I agree that pipe sizing should be part of the conversation, there is, in many cases, no reason to expect it to be a big deal, provided that the installer has made some simple observations and those 'check out'.  If we don't recognise this there is a risk of going hard the other way, ie forcing a large amount of unnecessary disruption and attendant cost just to be 100% certain that the system will work the first time it is switched on.

@jamespa, that’s a fair and well-argued perspective, and you’re right that in many cases, particularly with 'typical' detached or semi-detached homes, a bit of pragmatic engineering judgement goes a long way. I also agree that no one wants to see homeowners tearing up floors or spending thousands chasing theoretical perfection when, in reality, most systems will run just fine if the primary pipework is adequate and properly balanced. 

Where I think things have gone wrong (and why I wanted to highlight this in the article) is that engineering judgement has too often been replaced by guesswork or wishful thinking. A lot of poor-performing systems I’ve seen (and heard from homeowners about) including our own stem from installers assuming the existing boiler pipework would “probably be fine” for a heat pump, without checking lengths, pressure drops or even the basics like ΔT or pump head. When it isn’t fine, the result can be noisy systems, low flow rates and underperforming emitters, all of which could have been avoided with a few quick flow calculations or at least an informed check. All of this is present in our own system.

So I think we’re saying broadly the same thing: pipework doesn’t automatically need to be ripped out, but it does need to be understood. A decent installer with good judgement can usually make that call without turning it into a forensic exercise, but the key word there is decent. Too many don’t have that level of understanding yet, and that’s where the trouble starts.

Your approach (verify what you can see, make reasonable assumptions where you can’t, have a fallback plan if things don’t behave as expected) is exactly how it should be done. The problem is, not enough installers are doing that middle step at all. They just assume and walk away.

So yes, for most homes it’s not a big deal, but for some, especially larger or older properties, it’s absolutely critical. My aim with the piece was to get homeowners asking the question, not demanding that installers rip everything out. Because as you rightly point out, the balance lies somewhere between blind faith and unnecessary disruption.

@editor 

Agreed.  There is another point to be made here, namely that you cant make the judgement/calculation on pipe size unless you have correctly determined the loss and, as we know, surveys dont necessarily accurately determine the loss.  Since the tendency appears to be to overestimate loss, there is a risk that pipework is unnecessarily replaced based on this.  Personally i would put more effort into loss determination before even thinking about pipework, as this is the foundation of everything else.  I would possibly go so far as to say that a surveyed loss that has not been sense checked by some means shouldn't be used on its own as a reason to replace any pipework.

I've in the process of rectifying my system.  My installer couldn't get the flow rate through the primary pipe work they put in, so I now have 2 pumps just on the primary circuit to get the required flow rate.  I asked them to re-pipe the primary, but no, they won't even admit that wider pipes would help.  We did build a decking structure over the primaries so swapping them would be a pain, but I've now got a 6 pump heating system for an open loop system......  I think the installer thinks I'm being over dramatic but I'm not happy about it. 

Posted by: @davidnolan22

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6 pump heating system for an open loop system

I can understand 2 but what are 6 doing.  Do you have microbore or very long runs?  How big is your house?

@jamespa 

No, the house was back to brick and the installer put in every single heating pipe in my house.  I have 2 on the primary run to the low loss header. One off the header on a rad run, one off the header that then splits and runs to the 2 UFH manifolds, and a pump on each UFH manifold. 

Er...no comment!  

@davidnolan22 Oh my, thats a lot of pumps and is certainly wrong. You should change the primaries, remove the buffer and insist on a better design.

Posted by: @davidnolan22

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Can I go to NAPIT and say: "it works, but it could be better".  From reading these forums, the formal complaints procedures seems a difficult and often fruitless process. 

It’s a complete and utter waste of time and energy, especially with the certification bodies like NAPIT…