The problem I am attempting to address is that the efficiency of UK gas boilers falls dramatically when they are working far below their rated output.
+ trying to cover points that have previously caused confusion..
The UK 'boiler' has no function other than to heat water.
The water heated by the boiler goes into a loop where it is continuously pumped out hot to water filled 'radiators' and return slightly cooler to be warmed up before carrying on round the loop.
Piccy here:
https://www.scivillage.com/thread-13684-...l#pid56694
Air used by the boiler comes in cold from outside and is vented hot to the outside. It will be heated either by burning fuel gas or just by being passed through an efficient heat exchanger full of hot water.
Any air heated by the boiler is heat wasted. Fuel gas is what comes in from the gas main. Flue gas is what comes out of the flue when gas is being burned - there's mostly nitrogen (80% ish), some CO2 and a few other bits and bobs in it. As will hopefully become clear - a lot of what comes out of the flue is simply hot air.
A typical UK thermostat will call for heat while the room temperature is below the indication on the dial (set-point) and will continue to call for heat until the room temperature rises considerably above the set-point.
When the thermostat calls for heat a typical UK boiler will burn gas in a flow of air to raise the temperature of the water in a heat exchanger to (say) 65C - above 65C the gas turns off but the flow of air does not. The boiler will again burn gas when the temperature of the heat exchanger falls below (say) 62C. The cycle with continue until the room thermostat stops calling for heat.
By law UK boilers are required to be able to work in 'condensing mode' where the temperature of the outgoing air is below the dewpoint of water (55C). To comply with the law some manufacturers retained old designs but tacked on a second water/air heat exchanger so hot exhaust air preheats cool(ish) water returning from the radiators. Obviously the boiler only 'condenses' when the returning water is substantially below 55C which very often it isn't. It is possible the main heat exchanger in these modified boilers would corrode rapidly if condensation occurred in them so advisors are split 50-50 between "Run the boiler as cool as possible" and "Don't go below 65C".
To get a reasonable amount of heat from a radiator it has to be hot (!) but we also want cool water (below 55C) to allow the boiler to condense water out of the flue gas .. the trick is to reduce the flow of water so it comes in hot (say 65C) at the top and leaves cool (below 55C) at the bottom. This can work well with ten radiators but with one .. reducing the flow also reduces the flow through the heat exchanger in the boiler which with (say) a 10kW burner will raise the temperature of the small amount of water it contains by 3C in a matter of seconds - after which time the burner turns off. I don't know how much inefficiency these short burns (blips) add.
I can't make the point without numbers .. I'd foul up if I tried to use American units so its in metric - hopefully the gist of it will still be clear.
1 m^3 of UK gas gives roughly 10kWh when burned.
For combustion the ratio of air to fuel gas is 10 to 1
In what follows the 'air' is not used to heat the building - it has no function other than to allow fuel gas to burn and to transfer heat to the water which is what we actually use to heat the house.
A 10kW boiler theoretically uses 1m^3 of gas per hour and while it does it it heats 10m^3 of air per hour
Assume a rise of 50C .. to heat the air requires 1kJ/C/m^3
10x50 = 500 kJ or 0.14 kWh or just 1.4 % of the claimed output.
Now we attach a 500W radiator to the boiler .. the thermostat calls for heat for an hour so we still blow 10m^3 of air through the hot heat exchanger with the gas burner blipping on and off.
So we have a useful heat of 0.5 kWh and we've lost 0.14kWh in the hot air .. now 28% lost.
Now for the real crunch.. many UK boilers (mine) can run at high power (say 24kW) because they can also provide hot water for showers. They 'modulate' - lower the output when high output isn't needed. So the fan can blow enough air through to burn gas at 24kW .. more than twice what it needs at 10kW. Does the fan blow less air when it isn't needed - can't say I've seen it do it. Maybe with 10 radiators and given time to settle under nice test conditions it might .. but I haven't seen it do it. So maybe I'm burning my gas with 24 m^3 of air for every 1 m^3 of gas. My loss is 2.4 times greater than the 0.14kWh calculated above which gives 0.336 kWh lost .. 60% efficient .. with the gas blipping on and off and probably no condensing I think 50% efficiency would be a fair estimate.