Posted by: @jamespa

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Yes for sure, but large radiators dont change the mass flow calculations and thus the viability or not of the microbore.

Right, JamesPa, they don't, but me being nosy .... wanted to see if there's a chance of getting away without radiator swaps .... optimistically assuming that the microbores will do.

Posted by: @lejamaiscontent

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The mock-up run will clarify if the pipework is fit for a heat pump, but not which nominal power output you will need. If you put gas consumption figures to the flow and outdoor temperatures tracking, you will get nice statistics helping you to design the system. This is something no professional can obtain without producing a quite crazy bill.

In principle you are almost certainly right, but in practice its going to need a lot of measurement and interpretation unless its a slam-dunk case which sods law says it wont be.  The experiment and its interpretation needs careful thought with reference to the physics.  Im not saying it shouldn't be done, but interpreting it meaningfully if it turns out to be marginal is non-trivial. 

My advice is think through the experimental method, interpretation and possible outcomes first otherwise you will miss some vital measurement.  I, for one, don't immediately know how to interpret several possible outcomes without measurements you are unlikely to make unless planned in advance, and I have a degree in physics

Posted by: @lejamaiscontent

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Right, JamesPa, they don't, but me being nosy .... wanted to see if there's a chance of getting away without radiator swaps .... optimistically assuming that the microbores will do.

Maybe, but a separate question. 

I find it helps to thing of heating systems in 4 stages:

(a) Heat demand (set by the fabric, desired room temperature and OAT),

(b) Heat output (set by average surface temperature of the emitters relative to the desired room temperature and their effective surface area).  

(c) heat transport (set by the mass flow rate in the pipework and deltaT across the emitters - individually and collectively) and 

(d) heat production (set by the boiler/heat pump capacity at the design OAT and FT). 

For satisfactory operation the specifications for b-d must be capable of being at least as much as a.  All four will, in operation, end up in balance one way or another, possibly averaged over time if the heat source is forced to cycle because it cant modulate far enough down to match the demand.

If you divide the problem this way you can partition it so that each stage can be individually assessed.

For most efficient operation and greatest comfort you want the lowest FT possible.  This leads to large emitters and low DT to satisfy (b).  This in turn leads to high flow rate to satisfy (c).  This in turn is why pipe diameter matters, because the mass flow rate is limited, in principle by pump capacity, but in practice in most cases by the maximum velocity of the water in the pipes before erosion and/or noise becomes a problem

@jamespa 
Sorry I'm that late, I didn't get a reminder of having been cited, dunno how

Bravo! for the strikingly structured approach of yours. This is a remarkable piece which surely will be remembered and referred to.

Only problem is, when you distrust measurement (being in good company then, as it is done by many, quoting idioms like 'measurement means mistake') while determining the 'Heat demand (set by the fabric [....]', is it a good idea then to replace it with estimation mostly, since for most building fabrics there is no documentation available? And build the following three steps on that estimation?

I myself do not very much trust heat demand calculations even in new builts (from a full set of planning documentation, nota bene) since EN 12831 more than once has been commented on delivering overestimated results even then (and when no doc is available and guessing takes place, surely the results get no better).
Similar train of thought I found in aitchtetepees://protonsforbreakfast.wordpress.com/2023/11/01/heat-pumps-cop-envy-is-pointless/ where a Joe Wentworth commented 'It is also not helped that i have found MSC required heating calcs to be a significant overestimate. [....] It feels measurement may be a good tool in the future rather than calculation leading to oversized systems.'
Just these days I discovered aitchtetepees://heatpumpmonitor.org/ with its ample documentation of existing setups -- their 'summaries' contain a 'Heat demand tool' where the discrepancies between 'Heat loss value on form' and reality (without quotation marks) are nicely noted; I think at least half of the setups run on oversized plants. Not that this is a general disaster, but when it's already on the brink if the hydronic system can transport and put out the heat, with big money at risk, it's surely a good plan to prevent overestimating.

When you say 'otherwise you will miss some vital measurement' -- which one do you have in mind?

Would it be useful to shift such basic considerations of ours into the general thread about microbores, whose existence I only just noticed?

Posted by: @lejamaiscontent

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When you say 'otherwise you will miss some vital measurement' -- which one do you have in mind?

When I wrote the piece I didn't have anything in mind, I was just conscious that unless it's a spam dunk conclusion there wasn't enough information to be certain.

If you want to determine the load at the room level (which is what is needed I think) you are going to need radiator size/type, flow temp and deltaT across the radiator.  You will also need know the duty cycle.  Probably this is needed (given the context, ie determining whether microbore has sufficient capacity) for the most lossy 2-3 rooms only, the others will probably be ok, but some back of the envelope calcs would be necessary to determine this.