Condensing boilers – water produced and energy saved

Domestic boilers sold today are generally required by law to be of the “condensing” type (exceptions being those supplied on a like-for-like replacement basis on shared flue systems). Older boilers still found in many houses burned gas and air, passed the mixture of hot gases over a heat exchanger to recover energy by heating circulating water, and expelled the still-hot gases via a flue.

 

A condensing boiler attempts to recover some of the heat that would be otherwise lost, by condensing water vapour from the flue gases into liquid water.

 

Water vapour is an inevitable consequence of burning natural gas, which consists mostly of methane – complete combustion results in only carbon dioxide, water vapour and heat energy. The chemical reaction is:

 

CH4 + 2O2 >>>> CO2 + 2H2O + energy

methane + oxygen      carbon dioxide + water

 

(In words: one unit of methane, consisting of one carbon atom and four hydrogen atoms, combines with 2 units of oxygen, an oxygen molecule consisting of two oxygen atoms, to produce one unit of carbon dioxide plus two units of water vapour. The chemical reaction gives out energy as heat.)

 

It’s quite straightforward to work out the relative masses involved, from the relative masses of the atoms of the elements:

 

 

Element

 

Symbol

Relative weight

Hydrogen

H

1

Carbon

C

12

Oxygen

O

16

 

 

So we can put the relative weights under the reaction equation:

CH4 + 2O2 >>>> CO2 + 2H2O + energy

16   +   64              44   +  36

 

Straight away we can see that the masses of methane and water vapour are in the proportion 16 to 36, or 4 to 9. So, burning 4kg of methane results in 9kg of water vapour.

 

The amount of heat energy produced by complete combustion is known as calorific value, sometimes abbreviated to CV. From the National Grid website

 

http://www.nationalgrid.com/uk/Gas/Data/misc/reports/description/

 

“Gas passing through the National Grid pipeline system has a CV of 37.5 MJ/m3 to 43.0 MJ/m3” The figure often used by gas engineers is 10.76kW per m 3 per hour, equivalent to 38.736 MJ/m3, which will be used here.

 

If we fully burn a cubic metre of methane, we get 38.736 megajoule (MJ), or 38736 kilojoule (kJ).

 

The density of methane is around 0.66kg per cubic metre. We found above that burning 4kg of methane gives 9kg of water vapour, so burning 0.66kg of methane produces 0.66 x 9/4 = 1.485kg water vapour.

 

When water vapour condenses into liquid water, it gives up heat, known as the latent heat of vaporisation. For water this is 2260 kJ/kg. So burning our 1 cubic metre of methane, producing a total of 38736kJ, could give up to 1.485kg liquid water, releasing 1.485 x 2260 = 3356kJ in the process.

 

If, as in older boilers, we simply let the water vapour go, we only get 38736 – 3356 = 35380kJ of useful heat. If we were able to condense every molecule of water vapour, we would recover an additional 9.5%.

 

[The terms “gross” and “net” calorific value embody this concept: if we let the water vapour go, we only get the net calorific value, but if we capture all its heat, we get the gross value. They differ by a factor of 1.11, or 11% (see reference below). This is greater than my 9.5% figure because additional heat could be extracted by cooling all flue gases to the original starting temperature, usually taken as 25 celsius, and this is how the gross figure is worked out. This doesn’t happen in practice, so perhaps we should take a figure of “about 10%”.]

 

Study of boiler manufacturers’ data reveals that:

  • The amount of condensate produced is about half the theoretical figure derived above. The fact that not all water vapour is condensed into liquid is why we can see it emerging from flues in the right weather conditions.

 

  • A typical modern boiler in non-condensing mode (return water temperature too high to allow the water vapour to condense in the heat exchanger) delivers around 5 to 7% less power than it does when condensing.

 

Those with condensing boilers will have seen from the plume of water vapour on a chilly morning, that they don’t condense fully – indeed at times they don’t condense at all. Those who’ve struggled to thaw frozen condensate pipes on a winter’s night with the boiler locked out may wonder whether it was worth it.

 

Modern boilers boast higher efficiencies for other reasons, including improved heat exchanger thermal transfer from hot gas to circulating water, and more controlled burning. The use of zero governors, which can deliver an optimum gas-to-air ratio over a wide range of boiler heat output, means less excess air (air not taking part in combustion) passing through and removing heat from the heat exchanger. Most (four-fifths) of the air passing through is nitrogen which only serves to waste heat (we don’t want it to react chemically – oxides of nitrogen are regarded as pollutants). Perhaps gas boilers of the future, like some commercial boilers today, will be fed with oxygen rather than air – that is if they aren’t outlawed altogether for producing carbon dioxide. Or perhaps they will burn hydrogen and oxygen and produce only energy and water vapour.

 

Summary:

  • Boilers burn methane to produce water and carbon dioxide.
  • Burning 1m3 of methane yields about 1.48kg of water.
  • If all the latent heat of vaporisation were to be recovered from the water vapour by  condensing it into liquid water, around 10% of all the heat energy produced could be captured.
  • “Condensing” boilers don’t always condense and recover heat in this way – the return heating water temperature has to be cool enough.
  • The difference between a modern boiler in condensing and non-condensing modes is about 5 to 7% – condensing the water vapour recovers 5 to 7% more heat than not doing so.

 

Footnote:

The energy released from burning a cubic metre of hydrogen is a little over three times less than that from burning natural gas. Being the lightest gas with very small molecules, it’s much harder to make things gas-tight for hydrogen. However, apparently it can be usefully introduced in small proportion into the natural gas supply.

 

 

Reference:

http://en.wikipedia.org/wiki/Heat_of_combustion