Techy question

[bster]

how can a boiler output be higher than its input

Talking about a Worcester Ri15

max heat output net 40/30* = 16.06kW
output 50/30 = 15.92kW
output 80/60 = 15.00kW
max rated heat input net = 15.40kW.

 

[gas4you]

If you look at the water temp figures you will note that the lower the water return the higher the boiler output. This is the characteristic of a condensing boiler, whereby the lower the return temp the more heat output to radiators due to the boiler being able to condense more at a lower temp. I think it is something to do with dew point.

I'm sure one of the other more technical able posters on here could explain the exact terminology in great detail

 

[ChrisR]

"Latent heat of condensation" of the flue gas, adds to the calorific value of the fuel measured by the traditional method.

 

[bster]

cheers lads

and why is it a higher efficency with the lower flow and return tems?

 

[gas4you]

Cooler water condenses easy on thr return to the boiler therefore letting more heat be taken out of the water.

I'm afraid I now the principles but not the exact science

 

[seanfhear]

You will notice that the maximum rated INPUT is given in kW NET. This does not take into account the Latent (Hidden) Heat of Vaporisation of the water vapour in the products of combustion when an amount of gas is burnt to release this figure. This means that a certain quantity of heat is lost passing to atmosphere as a water vapour.(2.26 joules /g of vapour), hence the poor efficiencies of traditional boilers.
The maximum OUTPUT is also given in kW NET. but in order to enhance the description of the "quality" of their equipment they show their output by now including the Latent Heat of Vaporisation which of course can only be considered in the condensing mode. If these figures were to be stated in output / input kW GROSS X 100. you would then get the true maximum efficiency % of the boiler, but only when operating in the the condensing mode. Obviously some water vapour is lost in the products as they leave the flue because these products are still relatively "hot" thus you will not get 100% efficiency!
Brian.

 

[bster]

nice one seanhear but..............

output / input kW GROSS X 100

still equals 100% efficency though

I get it its output NET/input gross x 100.

cheers mate

 

[gas4you]

Although I have trouble sometimes understanding and remembering these equations, i do now that this 'NET' business is how manufactures can quote efficiencys higher that 100% sometimes to make their product appear better!

 

[seanfhear]

Actually, I put the GROSS in capitals to indicate gross heat output (into water) and gross heat input of the gas, sorry if I caused some confusion, my apologies, but I used the basic energy formula for any % efficiency calculation. mechanical, gas burning appliances, electrical equipment etc. It would appear that gross heat output would include flue losses and adventitious heat gains (into the surrounding atmosphere) from the boiler unit itself giving 100% efficiency, but as you know, these losses cannot be taken into consideration. We are only concerned with the heat transferred into the water so we use the output of the boiler, so as you quite rightly say bster, the output is NET as far as heat generators are concerned, being as we cannot use the heat losses from the products of combustion or as mentioned before heat gained from boiler unit, although Potterton many years ago, put ventilation slots in the sides of one of their free standing boilers and thus increased the STATED output by approx.5%.. Some boiler manufacturers for years have advertised their products efficiencies using NET heat input figures. I seem to remember many years ago, oil boiler manufacturers used this method during the competition with the gas industry. The calculated % efficiency looks wonderful using net figures.
Strictly speaking, as you know, these energy losses that we get from a boiler have somewhat been recovered, to a certain extent, by extracting the Latent heat from the flue gases. Hence the Condensing Unit. But again the efficiencies can only be elevated when in the condensing mode. We will never get 100% efficiencies, as to achieve that, we would have to cool the products of combustion down to the temperature of the incoming gas.


Brian.

 

[gas4you]

Glad someone can put into proper words what I know but can't describe