I’m glad I am not a heating engineer as the whole subject is too complicated for my feeble brain and I really appreciate everyone’s time to educate this simpleton
So to recap, when it is b****y cold (below -1C), due to the heat loss in the house the boiler is going all the time if the right sized radiators have been fitted. When it is say + 5C the boiler switches on and off or modulates (what ever that means), as the room thermostat reaches temperature.
Heating systems are designed for -1C outside and the boiler and radiators sized to suit this temperature. When sized right the boiler will be on 100%. Modulating mean raising or lower the burner, like on a gas hob when you tune it up and down with your hand. Older, and simpler, boilers. had set gas rate burners, that were either off or on. Modulation automatically lowers and raises the gas rate of the burner.
Modern boilers tend to modulate and are most makers try to go for a one size fit all (in most cases).
Now here is the bit I still don’t understand. If it is off then it is burning no gas and I assume if it is modulating it is burning less gas so why it that a bad thing in relation to condensing when it is burning gas even though it is doing it efficiently? Surely using no gas means a lower gas bill?
Modern boilers, depending on the boilers design, lower the boiler temperature to enhance condensing.
So without a weather compensator, running the radiators at 60C is better than running the radiators at 80C in terms of efficiency as there is more chance of the returning water being below 53C so it will condense but to maintain efficiency on cold days I should turn the boiler up a notch or two.
A weather compensator raises and lowers the radiator temperatures depending on the outside weather temperature. At 20C outside it will give 20C radiator temperatures, which is off. At -1C outside it will give 80C, at 10C outside it will give 50C. It is easy to draw a graph from this and calculate the approx radiator temperatures. . The return temperature could be 10 to 20C below this flow (radiator) temperature as the radiators give off heat. It is easy to see the advantages of compensation with condensing boilers. The "curve", or graph line, of the weather compensation can be usually set. The max temperature of the rad could be 60C (you have fitted larger rads) with 0C outside, so the graph will adjust to suit down to 20C rad temperatures when 20C outside.
Many weather compensators sense the inside temperature, say set to 21C, and adjust the rad temperatures down again to suit when 21C is neared. When 21C is reached the boiler is then off. The idea is to keep the boiler operating as low as possible matching input to output and enhances condensing efficiency.
Many boilers come with weather compensators as standard, or an option. You should always go for them as they really do work and drop gas bills and keep the house at a nice stable temperature. Unfortunately most plumbers don't understand them and few outside weather sensors are fitted. Most put on crude room stats. A waste of a good boiler when they do this. Best get men who specialise in heating to give advice.