Can Someone Do A Calculation For Me?

[Jay]

Hi,

I have had ongoing problems with my gas usage. I thought my old system was inefficient so I had a new Worcester Bosch 28i combi boiler fitted several weeks ago, however it seems that I am still gobbling gas like it's going out of fashion.

I had BG out to reset my meter as I am on a prepay meter and it was on the wrong settings, but it still is eating gas. I put £10 credit in yesterday and it has already used £5, so at this rate I am spending £150 a month which is nuts.

I know that there are lots of factors such as insulation etc etc but I only have a two bedroom semi and it has double glazing and loft insulation.

Firstly I need to know if I am getting ripped off by BG i.e. is my meter metering the gas correctly (BG cannot help me on this so far as they have no records of my payments!)

So as a starting point can someone work out for me if firstly it is even possible for a 28i to burn that amount of gas in a 24 hour period? It's not like it is even on full! The boiler clicks on and off so it certainly is not burning all the time, but if it were burning all the time at full blast, what would the consumption of a 28i be in a day? And what would that cost?

I know it can be worked out but I wouldn't know where to start :-)

Any help gratefully received,

Jason

[Andy Hall]

The startting point is to read the meter at the start and end of 24hrs and provide the figures. That way it is possible to calculate whether the meter is giving impossible figures.

Secondly, which model are you talking about? Is it the condensing Greenstar 28i or the conventional Junior or??

Thirdly, measure what the gas consumption is later tonight when it gets colder by reading the meter at hourly intervals without the hot water being used. This will give you a rough idea of gas use for CH if you multiply by the number of hours that the CH is on. You can then subtract that from the day's total to get an idea of what is used for CH.

If you look on the web site of your gas supplier, you should be able to find a figure to convert cubic metres of gas used to kWh and then relate that to the cost. have a look at their tariffs and you should be able to work out what is happening.

Also from the kWh figures you can work out whether it is likely to be correct for the boiler. Go to the Worcester Bosch site and installer section. Find the boiler and then the intallation manual. It will give you the maximum input rate for the boiler in kWh and cu.m/hr of gas.

[Roger Mills (aka Set Square)]

It would help if you told us what tariff you are on. How much do you pay for a kilowatt-hour of energy. Without that information, no-one has a clue how much gas your £5 will buy.

On the assumption that the 28i is a 28 kilowatt boiler, if it ran full chat, continuously, it would use 28 x 24 = 672 kilowatt hours in a day. [That assumes that the 28 kW is the *input* power. If it's the output, the input will be upwards of 10% higher when efficiency is taken into account].

I currently pay Scottish Power 1.326p per kW-hr for gas plus a standing charge of about 15.5p per day - so 672 kW-Hrs would cost me just over £9, including standing charge. But my prices are capped for 3 years - so BG will be a lot dearer than that - particularly when using a pre-payment meter.

Having said that, I wouldn't expect a 28kW boiler to have a duty cycle of more than 25% to heat a 2 bedroom semi - unless you leave all the doors and windows open!

FWIW, I have a 4-bedroom detached house, and we have a 96-year-old living with us - so the heating is on every day from early morning 'til late evening - plus a gas fire for a lot of the time. I have used 5624 kW-Hrs of gas for the whole of February - which works out at just about 80 quid, including standing charge.

[Marcus Fox]

Is your meter using ft^3 of gas and calculating it as a m^3?

Marcus

[Roger Mills (aka Set Square)]

It would actually be worse the other way round - because imperial meters work in *hundreds* of ft^3 - and 100 ft^3 is in the region of 3 m^3.

[Jay]

Further info:

My supposed tariff is 4.633p per Kwh for the first 1143 kwh per quarter and then 2.617p per kwh for the rest.

[Roger Mills (aka Set Square)]

OK, a lot depends on how the meter actually implements this dual-rate tariff. [My experience of pre-payment meters is limited to the shilling-in-the-slot jobby which my parents had about 50 years ago!]. IF the meter charges you at the dear rate for all of the first 1143 kWHrs each quarter before letting you have any cheap ones, you could still be on the dear ones - which might explain the high rate of money consumption. In this case, you'll only be getting about 108 kWHrs for each £5 until you reach

1143.

If, on the other hand, it does it on a daily basis, it should charge you

4.633 for the first 12.7 (or thereabouts) kWHrs per day, and 2.617 for the rest. In this case, you would get about 181 kWHrs for £5. For a 28kW (input) boiler, this would represent a duty cycle of about 27% averaged over a 24 hour period - which is at least *possible*.

As Andy H has suggested, you need to keep reading the meter frequently, and calculating the actual consumption, in order to see whether the meter is charging you correctly or not.

[Marcus Fox]

Yeah, got up much earlier than I was accustomed to and tired after a long day.

Marcus

[Sponix]

Well, we are in a 3 bedroom mid terrace, 30+ year old boiler, paying by direct debit, adequate levels of insulation but a draughty front door and we pay on average around £25 per month.

£150 *is* nuts.

sponix

[Ben Blaukopf]

27% of 28kW @ 90% efficiency is 6.8kW of output, which is a *hell* of a lot for a 2 bed flat.

This is a 2 bed flat, right? If it's a big 2 bed flat with say 1.5kW in the living room, 1.5 kitchen/dining, 1kW each bed, 500W hall, 1kW bathroom, then that comes to 5500W.

So make the rads slightly bigger and they still have to be on *all* the time.

I don't see how on those figures the meter can be correct.

Ben

[Jay]

Right, I've done some further calcs, which seem to suggest that the meter is not at fault...

I checked my meter reading (credit in pence left,) and waited for it to clock down, it jumped down by 3pence (standard increment) so I noted the cubic metre reading, which was 3602.796, I strated my stopwatch and turned the hot water tap on full and ran it until the credit jumped down another

3pence, it went down at 3602.897, this was 120 seconds later.

So, can you check I have done this correct please -

Gas consumed 0.101 cubic metre convert to kwh 0.101 x 11.06 = 1.112 kwh convert to per hour - 1.112 x 30 (because it took 2 minutes) = 33.36 kwh

The output of my boiler on the hot water is supposed to be 28kwh, so an input of 33.36 kwh might be about right?

Have I worked this out correctly?

If I HAVE worked it out correctly I what else could I look at next - just checked the metre and I only have used £4.20 in gas since 3:30pm yesterday. We have not used any hot water (apart from a couple of hand wash basins full) and the heating has been set to 16degrees overnight. I am wondering if there might be a problem with the gas fire which is in the lounge?

[Andy Hall]

It's in the ballpark, and considering you only measured for two minutes, is reasonable.

You didn't say whether you had the 28i Junior non-condensing or the Greenstar condensing boiler.

The specification of the Junior for an output of 27.5kW is an input of

30kW and a gas rate of 3.17 cu.m/hr. You were talking about 3.03 which is pretty close.

The boiler might be adjusted slighly high on burn rate, but it isn't substantially over based on gas consumption rate.

Assuming your 3p scales up and paid for 1.112 gets you to approx the

2.6p/unit you mentioned before. This would imply that you had used 162kWh since yesterday (say 22hrs) and gives an average usage rate, if it were linear (which it isn't) of 7kW.

Usage depends on heat loss of course. This is proportional to the difference between the inside and outside temperatures. It becomes hard to calculate unless you were able to record the outside temperature say hourly during the day and night and thus calculate apportioning the usage. Even so, it doesn't tell you a great deal. With setting back the temperature to 16 degrees over night you reduce the heat lost compared with if you had left the thermostat at 20 or 21 degrees. On the other hand, the outside temperature is lower at night anyway.

This seems doubtful. You could repeat the meter exercise with just the fire and see what it's using. However, they are rated at typically 3-5kW so it is not likely to be a big factor - plus it's heat into the room anyway.

Based on what you've said, it doesn't sound as though the boiler is firing incorrectly or that the meter is reading incorrectly in terms of measurement of gas volume.

This really leaves two things:

- Insulation of property is such that a lot of energy is being used because of heatloss. This depends on size and construction of property. What is it? How old? Are the walls solid brick or cavity? Double glazing?

- Charging of usage. I think you mentioned that the tariff is at one price for the first N units and a lower price for the rest. Could it be that it charges at the high rate always and you get a rebate when they empty it or subsequently? I can remember many years ago a relative having a coin meter. The gas board guy would come and empty it and give them some money back based on the tariff. Do you have the option to switch to a quarterly meter and a better tariff?