Posted by: @scalextrix

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What if thats not possible, and room A is sized with 100% of power output to heat loss, and room B is sized to 158% of power output to heat loss; I assume room B is going to get much too hot?

The modern advice is not to have TRVs and to go as "open circuit" as possible, so how do we mitigate the oversized radiator in room B?  Should we make it smaller?  What % of size deviation can be handled without adding flow restrictions and controls?

The deviations can be handled by 

a) the lockshield valve.  this restricts the flow rate through the rad in question which increases the deltaT for that rad which reduces the average temperature and hence the output.

b) the fact that rooms tend to share heat anyway because the thermal separation is poor.

I had mismatches of 150% plus, no problem

Posted by: @scalextrix

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Bonus question; if you open up all the radiators in a reasonably balanced system, and let the heating come to equielibreum with OT, and measure the temperature within each room; is that a better way to calculate heat loss?  

No, you wont leave it like that for long enough to overcome all the other variations that mother nature throws at us.

Posted by: @scalextrix

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Is there a method for that?

yes, (a) measure it over several weeks monitoring individual radiators and the heat pump, Adia Thermal make the kit to do this or (b) plot your gas/oil usage vs average OAT for several months or preferably a whole season, estimate the efficiency of your boiler, and work from there (gives you whole house only).

@jamespa thanks for the reply, so really in this case we have to reduce the flow to the radiator to achieve it.

In my specific case the oversize radiators are upstairs which isn't really the best from a heat sharing perspective, but I get your point.

Posted by: @scalextrix

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@jamespa thanks for the reply, so really in this case we have to reduce the flow to the radiator to achieve it.

In my specific case the oversize radiators are upstairs which isn't really the best from a heat sharing perspective, but I get your point.

Mine are also upstairs because I upgraded them before I realised how much the heat pump industry overestimates house losses.  Still not a problem although the lack of 'valve authority' with the very old LSVs on a couple of the rads in question makes for a slightly tricky, but not impossible, adjustment .  Of course that is my house, yours may be different.  The 'excess' upstairs capacity helped my installer to be happy to leave the bathroom well under-provided for, thus I could retain my exiting towel rail rather than replace it.

That said if you can make the downstairs ones bigger go for it.  Lower flow temperature = lower cost and greater comfort, as you clearly know!