I've just received my annual gas usage statement. It came as a surprise to me that I'm using almost the average gas consumption in the UK. The average is 16,500kWh and I used 15,000kWh over the year.

The reason this comes as a surprise is because I rarely use the gas central heating. The reason for this is simply because it's expensive (we have a process: clothing, more clothing, thicker clothing, blanket(s), THEN heating). I find it hard to believe that I'm using nearly average when all I'm using my gas boiler for is hot water. There are two of us (my wife and I). We both wash on a morning and evening. We wash the pots regularly. We shower more often than bath so we don't use an unusual amount of hot water.

Being concerned about the cost of gas given my apparent usage, I started to wonder if I'd sprang a leak. I believe disproved this by taking notes of my meter reading before going to bed one evening and after getting up the next morning. During this period of time, we did not use the boiler. The result was good, no gas usage.

I did another exercise to see how much gas my boiler is actually using so I ran the heating for 2 hours and it came out at 3m3. Using a rough calculation of multiplied by 11 and a rough cost of 5p per kWh, it cost me approximately £1.65 to run the heating for two hours.

Although there was no need for blankets that night, I can't help but think this is high consumptions. My boiler is quite old and I expect it has a poor energy rating (probably an E but I don't really know what that means).

So my questions are:

  1. How much gas (m3) would an A rated boiler typically use in a two hour period to run the heating?
  2. How much does an A rated boiler cost (fitted), and would I likely see a saving in the short to medium term?

Average figures are fine as I appreciate there are probably a lot of variables here (e.g. temperature, pressure, number of radiators, etc).

  • If your E rated boiler is running properly and is running at 75% efficiency, then you're losing 25% of the energy. So, even if you had a theoretical 100% efficiency burner, you'd be looking at about £1.32/hour to run it. A 90% A rated boiler should use about £1.45/hour. First thing I'd do is have someone come in to check to see if the boiler is running properly. You can find an explanation of the efficiency bands here
    – Johnny
    Nov 6, 2013 at 2:11
  • @Johnny Thanks for your comments but I don't understand your numbers. You say the E rated (at £1.32/hour) is cheaper than the A rated (at £1.45/hour)? Nov 6, 2013 at 8:54
  • 1
    No, sorry, I meant that even if you had a 100% efficient boiler (which doesn't exist), it would cost around £1.32/hour to run. If you had a 90% efficient A rated boiler, that would cost around £1.45/hour. This is assuming that your current boiler really is running at its rated 75% efficiency, but it might not be....it might be running below that level if it needs maintenance. So probably the first thing to do would be to have a repairman look at your existing boiler to see if it's running properly.
    – Johnny
    Nov 6, 2013 at 15:38

2 Answers 2


I do not know much about the uk boilers, but the principles are the same everywhere. Unless your boiler is ridiculously old and grossly inefficient, then it's always more effective to spend your money on weatherization and insulation first. The price of the new boiler + the installation cost will likely take many years to recoup before you will see much savings.

  • @Vitality. Thanks for your answer. I do have cavity wall insulation the floor in my attic is heavily insulated. I'm doing an attic conversion soon so the roof will be insulated too as a part of that. I'm still single glazed so that's a bit of a problem but we could be looking at £5,000-£8,000 to have good quality double glazing installed. I'm not sure we'd ever recoup that cost, hence looking into the boiler now. Nov 6, 2013 at 8:57
  • @flem: Secondary glazing will have a lower cost. As will thermal curtains. Nov 6, 2013 at 12:08

How much gas (m3) would an A rated boiler typically use in a two hour period to run the heating?

As you noted, it depends on

  • the ambient temperature
  • the room-thermostat settings
  • the number and size of radiators
  • radiator thermostat settings (if fitted)
  • the number and size of rooms
  • the quality of insulation (windows, roof, walls)
  • the age of the boiler.

See table 6, figure 19, etc in this government study: In-situ monitoring of efficiencies of condensing boilers and use of secondary heating

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