Yes, I am using 70C (67.5) vs 20C to get 2% (1.5 litres), my (oil) boiler has its set point set to 75C and its return is ~ 60C.
I keep my boiler at the above temps as I need my rads to run at their rated output, all modern rads are based on a "50 deg" rating which is the (mean rad temperature) - the required room temperature (often taken as 20C). Older rads, maybe your ones, are based on a "60 deg" rating.
So, in my case, the mean rad temp is (75+60)/2 or 67.5C, (67.5-20) = 47.5C so my rads should produce 93.5% of their 50 deg rating. You can get correction factor tables but I find it far easier by just using excel, the rad output is the (present rating/the "rated" rating)^1.3
in my case above this is (47.5/50)^1.3 or 93.5%. In your case if the rads are 60 deg rated then the output based on a 50C mean rad temperature is only 40.6% (30/60)^1.3 OR if 50 deg rated, 51.4% (30/50)^1.3. That will make your boiler cycle on/off more frequently.
Oil boilers (which cannot modulate) will cycle throughout their life but gas boilers can modulate (and prefer it).
I keep my boiler at the above temps as I need my rads to run at their rated output, all modern rads are based on a "50 deg" rating which is the (mean rad temperature) - the required room temperature (often taken as 20C). Older rads, maybe your ones, are based on a "60 deg" rating.
So, in my case, the mean rad temp is (75+60)/2 or 67.5C, (67.5-20) = 47.5C so my rads should produce 93.5% of their 50 deg rating. You can get correction factor tables but I find it far easier by just using excel, the rad output is the (present rating/the "rated" rating)^1.3
in my case above this is (47.5/50)^1.3 or 93.5%. In your case if the rads are 60 deg rated then the output based on a 50C mean rad temperature is only 40.6% (30/60)^1.3 OR if 50 deg rated, 51.4% (30/50)^1.3. That will make your boiler cycle on/off more frequently.
Oil boilers (which cannot modulate) will cycle throughout their life but gas boilers can modulate (and prefer it).