The examples you quote assume a temp loss in the rad (flow-return) of 20 C, for differing flow temps. Why is that,and is it necessarily the case in practice? In fact, one presumably can change dT by altering flow rate (hence altering temp drop in the rad) without changing flow temp, can't one?
The dT across the system will relate to the type of heat generator you are using. I have assumed (maybe incorrectly) that 20 years after madation you are using a condensing boiler (gas or oil) The controls of which will be attempting to maintain dT20 because that is what the manufacturer will have designed for when sizing all the components.
When small bore wet CH was developed in the 50's by the NCB design temps would typically be 180F flow with a dT20F drop (82C/11C drop), 180F being a sufficient margin of safety for the thermostat tech of the day to prevent boiling and 20F drop being realistic for the single pipe circulation systems and the return temperature being high enough to prevent condensation corrosion of cast iron heat exchangers without more sophisticated back end protection.
With the introduction and subsequent mandation of condensing boilers as energy efficiency moved up the agenda these typical temperatures were altered. but with a nod back to the old paradigm so as to keep existing pipework and emitter systems viable.
75/55 flow return became the norm. 75 not being so very much less than the 80 that the existing rad systems were designed for but spreading the dT so as to achieve a return temperature low enough to gain some latent heat through condensation of flue gases (full condensation does not happen until return temps are iro 30C) Additionally dT20 results in a halving of water flow rate (cf dT11), as you allude to, and therefore a reduction in pump energy requirements (CH pump circulators being a significant fraction of home energy use).
Does it happen in practise? Well it can and it should but...... Balancing a system is a tedious and time consuming task, made VERY much more difficult by 'sizing rads by window cill' rather than heat loss. The introduction of modern electronics to circulators (Grundfos Alpha3 etc) and pressure independent control valves in domestic rad valve form are making the job easier, as they remove some element of interaction between the adjustment of different elements.
The establishment of correct flow and dT is ESSENTIAL to achieving full efficiency and performance from a given design and the common lack is what is chiefly responsible for the performance gap between design/regulatory and reality, where rules of thumb plus a bit for luck, cos that was what my old gaffer said etc etc are more common.
Heat pumps (that the government would like us all to move to) are a totally different kettle of fish to boilers. Thermodynamically they operate in a completely different envelope to fossil fuel boilers and this means the systems that accompany them need to be specced with a completely different starting point. Heat pumps operate well in a very narrow band of conditions typically with a dT5C and commensurately higher flow rates (larger pipe work often needed)
Additionally in order to maintain an acceptable efficiency (COP) flow temperatures need to be substantially lower resulting in a derating of existing rads by two to three times.
Short cycling is not great for any heat generator but is disastrous for a heat pump where it will decimate the efficiency achieved.
Heat pumps do are not a good fit with highly zoned systems, it is readily possible to achieve a situation where turning off/down the heat in one zone with resultant increased heat loss from heated area into unheated causes the HP controls to raise flow temp into heated zone with a resultant drop in COP that means you use more energy to heat less rooms - crazy but true!!
These are SOME of the issues, that mean HP's are rarely a drop in replacement for a boiler, the cost and disruption of the consequential changes to an existing wet system are one of the reasons that HP's are not gaining the traction that might be desirable.
It therefore seems that (in domestic settings) having attempted to undertake a series of fairly accurate calculations and adjustments thereto, one then applies an essentially arbitrary (and potentially massive) 'alteration' dependent upon what flow temp/dT one 'wants' and what is 'practical'. You refer to the 'many who ignore the regs' somewhat pejoratively, but if to do otherwise would be impractical (or just unaffordable), then it would be impractical, and therefore would/could not be done.
While you are not wrong with regard to the result, this just further illustrates my points above that you cannot specify/design a single element (say a replacement rad) in isolation, you need to understand the design rational behind the existing equipment and specc the new equipment on the same basis. Where this cannot be practically achieved (as is often the case with change to HP) you have to start again from scratch - enter cost and disruption, prohibitive to most.
Situation exacerbated by the high chance that the tradesperson you engage will be unaware of any of these issues (or the many others that might be in play).
It's not really something which concerns me very much, but the 'warming up' process needs somewhat more sophisticated modelling,
Absolutely. Something that might be done for a commercial property where the savings might have some chance of offsetting the design costs, in domestic work you are reliant on a best guess which requires both the installer and the client to be aware of the factor.
cannot recall ever having seen any TRVs for sale that would enable that to be done directly - since, whether 'straight', 'angled' or 'corner', the sensor head seems to invariably be at right angles to the spigot that goes into the rad. If one wanted a ho
Most angle TRVs on sale these days have 15mm compression fittings on both ends which enable the sensor to be mounted vertical or horizontal just by rotating the body to the appropriate orientation.
TBOE is the most efficient connection pattern and then TRV head can be both above and the side of the rad, however not generally domestically acceptable. TRV's are such a crude control that the issues of offset, as discussed, are not real. TRV's are really just temperature limiters that are used to compensate for poor (over)sizing of rads and and their response and repeatability is, in any case, very dependant on cost/quality.
