Typical gas consumption (cu ft) ?

[vectraguy]

I have recently had my 25 year old boiler replaced with a new condensing conventional one, and decided to monitor the meter readings at 2200 every night for 3 weeks.

The heating and water have two periods a day, namely 0700 to 0900, then 1630 to 2200, there are 7 radiators, and a hot water cylinder.

The average readings so far are 1.5 meter units every 24 hours with actual on time being 7.5 hours.

The meter is still the old type measuring 100's cubic feet per unit (about 31 KWH), and I think that means we are using around 46.5 KWH a day.

Is that a reasonable figure, because I havn't got a clue if it's good or bad.

Thanks vectraguy

 

[JohnD]

I bet you wish you'd kept a record of your old readings.

Are you with BG?

My old mum is, and has a cu ft meter, and I see (fom the BG website) that it used about 180 units in the last month (I record the meter readings on their website to avoid inaccurate estimates)

I see this is about the same amount used in 3 months in the summer (cooking and water heating but before I improved the insulation there.)

I have difficulty in working out kWh billed from cu ft on the meter.

 

[chrishutt]

Interesting question. The gas used by your boiler will vary with the outdoor temperature. For example if the heating system is maintaining an internal temperature of 21°C, at external temp of 10ºC you will consume half the gas that you would at -1°.

Heat losses from the house will also vary with the size and structure of the house. If heat losses from the house are around 10 kW at design temperature of -1°C and the average external temperature during the heating season is 10°C then the average heat loss will be 5 kW.

If the heating system is run for 7.5 hrs then the average daily heating load would be around 37 kWh but we should probably add say 10% to allow for the extra gas consumption in heating the system up from cold twice each day (which heat is given back to the house after the end of the timed heating period), so let's say 40 kWh for heating.

Then we need to add on the hot water load. This will depend on the number of people living in the house and their hygiene habits. If we assume 4 showers a day at 5 minutes duration each and a flow rate of 10 litres / min with water at 40°C and incoming mains at an average of 10°C then the total hot water load for showers will be 200 litres raised 30°C, which will require about 7 kWh. If we add in another 50% or so to cover washing up, hand washing, etc. then we get around 10 kWh for daily hot water load.

Adding our 40 kWh for heating and 10 kWh for hot water, we get an overall daily load of 50 kWh. However in calculating the gas consumption we have to take account of the efficiency of the boiler, so a condensing boiler operating at 90% efficiency would consume 55 kWh worth of gas. To this must be added gas for cooking and gas fires, if any.

These figures are based on a variety of assumptions which may or may not be valid for your particular circumstances, but your figure of 46 kWh a day is clearly comparable.

 

[JohnD]

does 1.5 units daily on the meter mean 150 cu ft? being abouut 3cu ft per kWh? Or have I got the decimal point wrong?

 

[ChrisR]

I have difficulty in working out kWh billed from cu ft on the meter.

Take cubic feet, multiply by the calorific value of the gas (varies) say 1035 to 1040
That gives you Btu.
Divide by 3412 to get kW hrs.

 

[JohnD]

so 150 cu ft = ((150 x 1040) = 156,000 BTU) / 3412) = 45.7 kWh

right? about 3 cu ft per kWh?

and what does 150 cu ft look like on the meter?

 

[chrishutt]

A cubic metre of gas has a calorific value of around 38 MJ. There are 35.3 cubic feet in a cubic metre, so calorific value of cubic foot is 1.08 MJ. To get a value in kWh we divide the MJ by 3.6 to get about 10.5 kWh /m³ or 0.3 kWh / ft³.

 

[JohnD]

Aha!

so 0.3 kWh / ft³.

is same as 3 ft³ per kWh

(nearly!)
3.3 ft³ per kWh actually
but near enough

What units does the cu ft meter read in? e.g. a cyclometer reading of 180, means how many cu ft?

 

[vectraguy]

Interesting question. The gas used by your boiler will vary with the outdoor temperature. For example if the heating system is maintaining an internal temperature of 21°C, at external temp of 10ºC you will consume half the gas that you would at -1°.

Heat losses from the house will also vary with the size and structure of the house. If heat losses from the house are around 10 kW at design temperature of -1°C and the average external temperature during the heating season is 10°C then the average heat loss will be 5 kW.

If the heating system is run for 7.5 hrs then the average daily heating load would be around 37 kWh but we should probably add say 10% to allow for the extra gas consumption in heating the system up from cold twice each day (which heat is given back to the house after the end of the timed heating period), so let's say 40 kWh for heating.

Then we need to add on the hot water load. This will depend on the number of people living in the house and their hygiene habits. If we assume 4 showers a day at 5 minutes duration each and a flow rate of 10 litres / min with water at 40°C and incoming mains at an average of 10°C then the total hot water load for showers will be 200 litres raised 30°C, which will require about 7 kWh. If we add in another 50% or so to cover washing up, hand washing, etc. then we get around 10 kWh for daily hot water load.

Adding our 40 kWh for heating and 10 kWh for hot water, we get an overall daily load of 50 kWh. However in calculating the gas consumption we have to take account of the efficiency of the boiler, so a condensing boiler operating at 90% efficiency would consume 55 kWh worth of gas. To this must be added gas for cooking and gas fires, if any.

These figures are based on a variety of assumptions which may or may not be valid for your particular circumstances, but your figure of 46 kWh a day is clearly comparable.

Many thanks chrishutt for your comprehensive reply, and I live in the middle of a five house terrace, with just the wife and me.

We don't have a shower,so our main daily cleansing is done with one bath each, and we have a living flame gas fire which to be honest is more for appearance than for warming us.

We also cook with a gas cooker, but not a lot due to microwave help, and it seems if from your assessment that the consumption is reasonable which is a relief.

Regards vectraguy

 

[vectraguy]

I bet you wish you'd kept a record of your old readings.

Are you with BG?

Yes JohnD I certainly do, and no I'm with Npower, shortly to change to a better rated supplier.

vectraguy

 

[JohnD]

I get a good deal with Atlantic

You need to stay with them for a year though as they give you the 13th month free (at your average usage)

Good customer service as they're a brand name owned by Scot & South.

 

[vectraguy]

I get a good deal with Atlantic

You need to stay with them for a year though as they give you the 13th month free (at your average usage)

Good customer service as they're a brand name owned by Scot & South.

Thanks John I will take a look at their site.
Cheers vg

 

[chrishutt]

If you switch supplier via certain switching agencies you can get a cut of the commission (£30) for yourself into the bargain. Best place to start is http://www.moneysavingexpert.com/utilities/you-switch-gas-electricity.

 

[vectraguy]

If you switch supplier via certain switching agencies you can get a cut of the commission (£30) for yourself into the bargain. Best place to start is http://www.moneysavingexpert.com/utilities/you-switch-gas-electricity.

Thanks for the link chrishutt, going there now.

vectraguy

 

[vectraguy]

I get a good deal with Atlantic

Hi JohnD just had a look and it seems good, but they have "Standard online" with standing charge (lower cost unit rate) and the same with no standing charge ( higher cost unit rate.

Which do you recommend.

vg