I did pretty much the same.  The overall conclusion for me is that low and slow makes for a more comfortable living environment and it confirmed that my loss calculations based on actual consumption give a figure which keeps me warm. 

Because my boiler isn't weather compensated I had to manually adjust the ft during the season which means that the trvs were still kicking in, but I don't think that materially affected the experiment.

Cycling is inevitable if your boiler can't modulate down to match your house demand.

In summary I felt it was a useful experiment that will remain in place until I can get an ashp because it results in a greater level of comfort.

I can't really offer any tips beyond what you have done, like me the missing feature is weather compensation but my feeling is that this will further improve comfort.

@jamespa thanks for the reply, we tried this last year and we couldn't keep the living room to the tropical temperature required, which actually led me to investigate more and found some missing insulation, so whatever happens it's helped us.

Posted by: @scalextrix

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@jamespa thanks for the reply, we tried this last year and we couldn't keep the living room to the tropical temperature required, which actually led me to investigate more and found some missing insulation, so whatever happens it's helped us.

Better insulation is best but of course bigger radiators may also be necessary to get tropical room temperature with low flow temperature.  That's part of the design process (t's not much use having an ashp that can kick out 12kW if your rads can only do 8kW at your chosen flow temp).  In a living room consider fancoils, large output, small footprint, can also be used for mild cooling.  Costly for anything well styled (they appear to be most common in Italy, so quite a few are) and of course there is a background hum from the fan, but a useful addition to the low temperature heating retrofit toolbox 

We tried the retrofit fans that stick with magnets to the rads a few years ago just for fun.  While they were silent before putting on the rad, once they were in place the radiator made an irritating buzz/rattle that wasn't acceptable for a living room.  Other people may have different experiences.

We have 2 rads in there, one is a type 22 and can be made a bit wider, the other is a type 21, while it can't be any bigger, we can upgrade to a type 22.  I'm hoping with the improved insulation that should be enough to get what we need.

Posted by: @scalextrix

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type 22.

Can you upgrade to to type 33?

Posted by: @scalextrix

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We have 2 rads in there, one is a type 22 and can be made a bit wider, the other is a type 21, while it can't be any bigger, we can upgrade to a type 22. 

I have a small old leaky listed building with limited wall space. As a very general rule of thumb, if you have correctly sized radiators for a fossil fuel system running at high flow temps, then for an ASHP system running at the upper end of acceptable flow temps, then you will need new rads that have twice the effective area of the fossil fuel rads. This is quite a big jump, something not always appreciated in the early planning stages. Doubling the area of the rad is one option, but so too are different form factors. I ended up with all K3 rads bar two, and some of the K3s needed to be quite a bit larger than the K2s they replaced. Instead of rads that were relatively discrete, I now have rads that no doubt an estate agent would describe as 'a feature'...

1330: Edited to change 'twice the output of the fossil fuel rads' to 'twice the effective area of the fossil fuel rads' as in my experience this is an area in which it is easy to get very confused very quickly... The key point to understand is that the heat transfer to the room depends on the 'delta t' (temperature difference) between the mean radiator temp and the room temp. Halve the mean rad temp, and keep everything else constant, then the heat transfer will be give or take half what it was at the higher temp. To compensate (get the same amount of heat transferred), you have to double the effective size (area) of the radiator.        

Posted by: @cathoderay

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I ended up with all K3 rads bar two, and some of the K3s needed to be quite a bit larger than the K2s they replaced. Instead of rads that were relatively discrete, I now have rads that no doubt an estate agent would describe as 'a feature'...    

This is an area where the industry tendency to oversize can cause a real problem.  I decided to start rad upgrades in advance  so far for half of the house only) and based the sizing initially on the MCS assumptions.  This gave me one K3 in the master bedroom which my wife hates.  Subsequently I have concluded that the MCS assumptions lead to an over estimate of the loss and, had I known this earlier, I could have got away with a much more acceptable K2. 

Sizing doesn't just affect the odu, it affects the whole system, so it's important imho to get it as accurate as possible if wrong sizing is likely to drive difficult to reconcile design decisions 

@cathoderay 🤩 Toodles

Posted by: @jamespa

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Sizing doesn't just affect the odu, it affects the whole system, so it's important imho to get it as accurate as possible if wrong sizing is likely to drive difficult to reconcile design decisions 

I strongly endorse this. Some of the quotes I had from various goons claiming to be heating engineers had hopelessly out of whack rad specs. For radiator sizing, the steps are:

1. Accurate heat loss calcs for each room

2. Select a radiator that will (a) fit in the available space (a sina qua non) and (b) has the right heat output at AHSP flow temps for that room

Follow that procedure, and you won't go far wrong.

I took this ‘simulation approach’ last year.  Based on my own heat loss calculations (including a very wet finger in the air for air change), I suspect that I will need to change many of the radiators. One of my concerns (largely due to it being big concern of my wife) is that the 10mm radiator pipe work downstairs comes out of the wall so the pitch of the valve connections and their distance from the wall are fixed. Replacement radiators will likely need a couple of additional pipe fittings at each end - not an aesthetic solution and additional resistance to the flow. 

Posted by: @ianmk13

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10mm radiator pipe work

10mm pipe is ok for about 1kW at dT5. So really depends how the system is laid out and how you transition to 10mm and the other piping sizes. Lots of consideration other than just replacing pipe tails