There isn't a single specific "right" flow temperature, but hopefully the stuff below might help.

Generally the return temperature needs to be as low as practicable for the required heat output from the rads. The reason for this is that it is the return temperature to the boiler that determines whether or not it runs in condensing mode - if the return temp is too high the boiler either won't condense or at least won't condense as efficiently. This can make a significant difference - the boiler efficiency rating will have been obtained with a fairly low return temperature.

Return temperature is determined by two things primarily, the flow temperature to the rads and the heat that the rads give out to the rooms. Increase the flow temperature and the heat output from the rooms increases as does the return temperature.

The most efficient way to set the boiler up is to use the lowest flow temperature you can whilst still getting adequate heat output from the rads. For example, at the moment I'm running my boiler at a 48 deg C flow temperature, rather than the 55 deg C I used to run it at, and am finding that the boiler burns for longer each time it fires, but runs with a lower modulation level (less gas being used). The return temperature is down around 35 to 38 deg C, so the boiler should be condensing very well and running at high efficiency.

If your radiators are undersized, then you may find that you have to increase the flow temperature to get the house warm enough. This will hit boiler efficiency quite hard, maybe by around 10% or more.

For your edutainment, I've attached a typical plot of out boiler in(green), out(red), and flue(blue) temperatures. You can see it click in & out - that's the room thermostat which is doing that every hour or so.
I'm pretty sure it's on it's lowest modulation most of the time, on the basis that if I turn down either the temperature knob on the boiler, or turn down the pump speed, then the boiler starts cycling (bad).
You can see that the flue seems to track the boiler return temperature +5C. I think that as the boiler power increases, it will still track the return temp, but with a higher temperature offset.

+piccy of boiler cycling, as I set the pump to posn1, and clearly there isn't enough heat load at that flow temperature

Rob, does your boiler have a helpful data connection or did you insrument it to get that data? Mines got electrickery in it so may well be able to tell me useful data, but no info provided by manufacturer on how to do this.

Interesting graphs. I've been wanting to get some data like that for ages and not got round to it. I'm surprised how low the flue temp is. I guess the whole point of this design is that the cool return water is used to suck heat out of the flue but I kind of assumed that meant it got it down to ~100C, not 43C. I suppose relative specific heat capacities have a lot to do with the effectiveness (water can take the heat out of a lot of gas/air)

Can we assume this is a gas (not an oil) boiler? Also that there isn't a thermal store?

thanks for your advice ,

if you run th boiler at a lower temp doesnt this mean the temp diff to room temp is reduced and so the rad output is lowered?

i have ahot water cyl to consider.

the best way of enhancing boiler efficiency is to install compensation controls, either indoor or outdoor, as they float the flow temperature to match heat required.. my boiler rarely goes above 50c, and the return stays below 45c...indeed with underfloor heating, flow temps can be as low as 25c..with a room temperature of 21c

Almost all boilers accept these, but many Worcester bosch boilers do not.

The benefits of these controls are longer burns, lower temperatures, more comfortable room temperatures, and lower gas bills...

yes, compensation controls enable the boiler to run at two temperatures...one for hot water(high), the other for heating (as low as possible)

If the flow is at 65 deg C and the return is at 55 deg C, then the boiler won't be condensing properly, so will be running at the poorer end of the efficiency rating (probably around 75 to 80%).

The return needs to be below 55 deg C for the boiler to start condensing and the lower you can get it the better, as it probably won't be fully condensing until the return is down at around 40 deg C or so. My return temp is around 38 deg C, with a flow temp of 48 deg C and the boiler efficiency seems to have markedly improved over the old regime I had of running it at about 55 to 60 deg C flow temperature. I get longer boiler burn times, but with a lower flame modulation (the boiler burns noticeably quieter) and the cooler radiators still seem able to keep the house warm, albeit with a slower response time (not a problem for me as the heating is on all the time normally). So far the system has kept the house warm with night temperatures dropping to around -4 deg C outside, so I'm hopeful it will be OK if we get a spell of prolonged cold weather.

all rads bar one have flow over60C at boiler flow of 65C so i'll stopworrying about delta t as system seems reasonably balanced.

i wil reduce boiler flow temp to 60c and see how hw cyl copes

thanks

wireless compensation controls are generally available outside europe and made by all/many manufacturers, including honeywell..

the are not available in the UK except through a boiler manufacturer...

at the end of the day whilst the institutions that write our regulations are wedded to heating appliances that are either on or off there is no stomach for getting these controls to market...