The way heating systems are designed will result in roughly equal dts across each radiator.

However design and reality may well differ so if it turns out that in practice radiators exhibit different DTs after balancing, that is not a problem so long as the house heating is satisfactory.

You can design for higher DT than the default 5 but there is an efficiency penalty of approximately 1.5% for each degree the system dt in increased above 5.  This efficiency penalty may well be worth suffering to avoid major disruption.

If you want to design some rads for a DT of 8 and some for a DT of 5 you will need to do very detailed calculations of each circuit.  This is probably not worthwhile and also requires details about your plumbing that cannot be readily obtained without taking up all the floorboards and calculations that few will be able to carry out.

Far simpler than trying to adjust for varying DT is to design the whole system for DT8 and adjust at commission time by balancing.  This will result in marginally larger radiators than necessary but is the practical solution to the problem you describe.

@jamespa Thanks for getting back to me.

I know my pipework lengths and diameters (a combination of lifting floorboards, a borescope, and a thermal camera) so can have a good go at calculating pressure drops for each radiator. 

On the subject of higher dT for the whole system rather than for a single radiator, I've often wondered what the efficiency penalty is so thanks for providing a number - can you point me towards a source with some more detail on that? Do you know if a Vaillant aroTHERM plus 5kW varies pump speed to maintain dT5, or runs a fixed pump speed so allows dT to vary?

Posted by: @medwardz

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On the subject of higher dT for the whole system rather than for a single radiator, I've often wondered what the efficiency penalty is so thanks for providing a number - can you point me towards a source with some more detail on that?

I cant sorry.  Its based on some calculations I did from Mitsubishi's R32 COP VS FT tables, which roughly gives a penalty of 3% for each degree increase in the DT at design OAT.  Its a bit less, perhaps only 2.5%/C, for very low design FTs (eg <40) and a bit more for relative high design FTs (eg >50).  So its rough and ready at best.  Other commentators seem to say similar, but it maybe that I started it off! 

I then assume that, if DT is changed, you need to raise/lower the FT by half the change in DT to keep the average surface temp (and thus rad output) the same.  So if FT of 42.5 works at DT5, you will need FT of 45 at DT10.

Posted by: @medwardz

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Do you know if a Vaillant aroTHERM plus 5kW varies pump speed to maintain dT5, or runs a fixed pump speed so allows dT to vary?

I have the 7kW Arotherm and frankly I cant work out exactly what it does as I don't have a way to monitor pump speed aside from the Mk1 eyeball.  It certainly does vary pump speed a bit, but there is no evidence from the little data I have collected that it maintains a fixed DT.  The guys on the Vaillant thread on openenergymonitor may know.