How accurate are stated boiler efficiencies?

[stem]

Today, once again the car manufacturers (mainly German it seems) are being accused of providing false fuel consumption figures that are 50% or so better than what is actually achievable driving in the real world.

This got me thinking. Could the same be true of the efficiency figures given for boilers?

My mother had a small 2 bed bungalow with 6 radiators and a perfectly good, but 25 year old non condensing, wall mounted boiler, with cast iron heat exchanger, and SEDBUK E rated, exchanged for a condensing Combi SEDBUK B rated, hoping to save on gas, but it made no detectable difference in gas consumption year on year.

 

[Dan Robinson]

Boiler efficiency is about right normally. But that has nothing to do with system efficiency.

 

[Nige F]

Thanks stem and Dan - that confirms my suspicions about leaving my Potterton Kingfisher 11 in situ for a few more years. Then it'll be an Intergas for me.

 

[Razor900]

I think gas boilers all get tested at Appledore don't they so it's an independent rating?

Can't wait to see what the Extreme rates at over here. It's been tested at 122% efficient in the Netherlands

 

[muggles]

Today, once again the car manufacturers (mainly German it seems) are being accused of providing false fuel consumption figures that are 50% or so better than what is actually achievable driving in the real world.

This got me thinking. Could the same be true of the efficiency figures given for boilers?

My mother had a small 2 bed bungalow with 6 radiators and a perfectly good, but 25 year old non condensing, wall mounted boiler, with cast iron heat exchanger, and SEDBUK E rated, exchanged for a condensing Combi SEDBUK B rated, hoping to save on gas, but it made no detectable difference in gas consumption year on year.

Real-world efficiency is very different to test lab efficiency, and can be massively affected by the controls used

 

[D_Hailsham]

Can't wait to see what the Extreme rates at over here. It's been tested at 122% efficient in the Netherlands

Is that net or gross?

 

[Razor900]

Not sure if I'm honest but I've seen the certificate and it's waaaay in front of anything else I've seen.

The boiler gets a unique advantage because it condenses on hot water as well

 

[Brigade77]

I passed a test once . . . & I'm 110% efficient !

 

[Steelmasons]

Is that net or gross?

....I would imagine the figure would be gross...

 

[Steelmasons]

It's been tested at 122% efficient in the Netherlands

.....you can't fit a quart into a pint pot...

 

[D_Hailsham]

....I would imagine the figure would be gross...

I thought the theoretical maximum gross efficiency for natural gas was 111%, i.e an extra 11% was provided by condensing.

 

[Steelmasons]

I thought the theoretical maximum gross efficiency for natural gas was 111%, i.e an extra 11% was provided by condensing.

So you're saying a boiler with an input rating of 100 kW/h would yield an output rating of 111kw/h???

 

[vulcancontinental]

So you're saying a boiler with an input rating of 100 kW/h would yield an output rating of 111kw/h???

Must have been reading a viessmann catalogue.

You have your answers right there then, 100% nett is sensible heat potential. 111% gross, sensible and latent potential measured traditionally.

Nothing is perfect of course.

At 122% it's magic, the boiler must be creating heat from nothing. Maybe the flue products are exiting the boiler cooler than the combustion air going in. Is the Extreme more than a gas boiler?

I think that tells the original poster all he needs to know about manufactures efficiency figures.

Some manufacturers have their own Kiwa (Gas Tec) certified and calibrated equipment others outsource. Coincidentally Volkswagen run a test facility.

One manufacturer in the UK is currently advertising knowingly incorrect figures to enable their boilers and control to acheive an A+ ErP package rating.

 

[bernardgreen]

Applying basic physics might produce actual efficiency figures that are less attractive than those produced by manufacturer's sales literature.

But as the manufacturer's do not state what the efficiency figure relates to they can rightly claim their results are accurate for type of efficiency they are measuring / calculating

burning 1 cubic foot of natural gas will create 1000 British Thermal Units ( Btu )

1000 Btu is equivalent to 0.293 kWh

Read the gas meter

Raise the temperature of 200 litres of water by 30°C, (10°C to 40°C ) this will require 7 kWh

Read the gas meter

From the gas meter readings determine how much gas was needed to heat the water.

7 kWh of heat will be produced by burning 24 cubic feet of gas. ( 24 * 0.293 = 7.032 )

Do the experiment and calculate

( 24 / Gas used ( cubic feet )) * 100

to get the overall efficiency of the boiler when heating water. 10-40

It is acceptable to include in the 200 litres the water that is used to carry heat from main heat exchanger to the secondary heat exchanger, ( not applicable to boilers that do not use a secondary heat exchanger )

immediately repeat the experiment raising the 200 litres by a further 15°C (40° to 55°C ) this will require 3.5 kWh

Calculate

( 12 / Gas used ( cubic feet )) * 100

to get the overall efficiency of boiler when heating water. 40-55

 

[Dan Robinson]

I have a copy of the certificate here