Ok. Having read
https://www.heatgeek.com/what-is-boiler-modulation/ I can say that my 24year old combi gas boiler was made with the right parts in it to be capable of modulating with 2:1 ratio but its controller as shipped had a fairly dumb algorithm observed as follows: Its central heating behaviour since new was constant temperature non-modulating (on-off) boiler at fixed 50% gas consumption of 15 kW. When a microswitch on the diverter saw flow through the hot water pipes, it would within six seconds modulate gas consumption upward to 100% 30kW and ignite flame if necessary unless overtemperature was exceeded.
so the observed algorithm is
WHILE hot water, OPEN gas modulation DC solenoid to 100%
ELSE gas modulation DC solenoid = 50% (most of the time, for central heating)
ALWAYS close gas mains solenoid while temperature > setpoints on front panel
I never did get a sensible test done to show how a measured heat output compares to 15kW, but it gets all the radiators too hot within an hour.
50%|0% ON|OFF constant CH temperature behavior was usual within the ON hours selected by a mechanical segmented timer.
In the year 2001, I typically had that set to be ON for a short period around breakfast time and a few hours in the evenings.
The vast bulk of gas saving which I got was from changing the control algorithm to "Mostly OFF". In the past 24 hours the gas boiler flame has been going at 50% = 15kW for six minutes.
On the chart in the link, that is to the extreme left of the yellow and orange chart at boiler load = 0.2%, for which what the chart says cannot be right.
I rate mine as getting efficiency of "about 90%" of the gas burned resulting in "useful" heating of the bulk of the house, much of that as unmeasureable warming in the pipes and walls. (Efficiency of delivering hot water to the sink is atrocious unless I let the washing up heap pile up until bath day.)
The "not useful" heat is in the non-condensing boiler exhaust, and during a six minute burn that won't be much. I've set mine up so that the central heating pump is ON if the flame is on and stays on for an extra 3 minutes after a burn, by electronic timer countdown. I don't expect use of the central heating pump for a single 9 minute run yesterday could wear it out more by comparison to on-all-day, but the proper way to decide would be for a Vaillant qualified engineer who can access the MTBF data for their usual CH pump to comment. A common way that I operate it on coldest days this winter has used gas burn = 3 minutes ON every 30 minutes, with the CH pump on for 6 minutes each time, so boiler load averages 5% (ON at 50% modulation for 10% of the time) and the CH pump is on for 20% of the time with a dozen starts per day.
I claim that by using it as little as this, I'm into savings not accessable to a DC solenoid partly closing down gas consumption. An extended chimney flue condensing heat exchanger might have been a nice-to-have, but not worth replacing the whole boiler for.
This year CH circulation temperature on leaving the boiler has been left below 50C and in mild weather 35C by using these short burns. Tepid to warm radiators and the air con heat pump can add up to about 4kW of heating on average, and for this mid-terraced house in this mild winter of 2024, that has been enough. I could run the gas combi boiler more but have not needed to. Today, 20th March, the air con heat pump is running on mainly solar electricity and providing about 2kW downstairs, which by leaving open some doors partly spreads to upstairs.
What would the professionals say ?
