So, you're actually asking of coal fired central heating systems. In that case, I have the KEYSTOKER Kaa-2 boiler. Very happy with it. It does a Fantastic job of heating my place. That being said, I have seen the tipping point of peak performance at around minus 5, to minus 8, with wind howling. Could not tell by room comfort of 72* - 74*. Could tell by boiler gauge. Boiler was running flat out, around the clock. What would I change in my system? NOTHING! My whole place is at thermostat set point throughout. At the time of purchasing the boiler, I went to buy the next size up, but KEYSTOKER discontinued the K-4, and came out with the smaller Kaa-2. K-6 was way too big for me. Since then, KEYSTOKER had come out with another version of K-4. I'm going to double talk my answer to your question a little. If I were to change anything in my system, I'd step up to the boiler size I originally wanted, the now /new Kaa-4. The only reason would be for the little more capacity. Nothing to do with lack of performance of the Kaa-2. I couldn't be happier with how the Kaa-2 performs. And yes, I'd buy another KEYSTOKER boiler.