I needed to do this test before installing a new cylinder, pump and a MagnaClean. Looking at the graphs I'm convinced that the heat is not being transferred from the cylinder coil to the DHW and is causing the boiler to cycle. wiki.diyfaq.org.uk has a good writeup on this subject.
| Time | Gas (ft3) | Delta Temp |
|---|---|---|
| 0 | 1.8 | 0 |
| 10 | 4.6 | 2 |
| 20 | 7.1 | 4 |
| 30 | 10.0 | 6 |
| 40 | 10.7 | 8 |
| 50 | 13.1 | 10 |
| 60 | 14.4 | 12 |
| 70 | 17.5 | 14 |
| 80 | 18.3 | 15 |
| 90 | 19.6 | 17 |
| 100 | 21.8 | 18 |
| 110 | 22.1 | 20 |
| 120 | 23.7 | 21 |
| 130 | 25.2 | 22 |
| 140 | 26.5 | 23 |
| 150 | 28.1 | 24 |
| 160 | 28.2 | 25 |
| Total | 26.4 | 25 |
Time taken to increase temperature by 25 degC was 160 Minutes or 2h 40min
Gas used was 26.4 Cubic Feet which by my maths is about 8kWh
Converting this to Joules gives about 28,800,000 Joules
Now in theory, heating a 120l cylinder by 25 DegC will need 120 * 1000 * 4.2 * 25 = 12,600,000 Joules
This makes the efficiency of the boiler just 45% which is quite a bit less than the 73.1 quoted by SEDBUK.com and is probably due to the the inefficiency of the system together with an old boier
I think you'll agree that this is nothing short of amazing; we get now get hot water in about an hour - Major thanks to John the Scruff