An interesting thing that I was made aware of recently, is that the natural gas piped into our homes produces a surprising amount of water vapour when it combusts.
With modern boilers, this is not a problem since they are well sealed and vented via flues to the outside. However, where there are gas fires, gas hobs or gas ovens, these might contribute to the total amount of moisture in the house, thus raising the humidity & potential for condensation.
I am intrigued to understand this further, to try and quantify this in a way that enables me to visualise the process ‘in the real world’ more clearly.
The same question was asked by someone else & there are responses containing calculations that do shed some light on the matter.
As luck would have it, my father is a retired chemist with a very good brain for this sort of thing, so I ran the calculations past him to double check the numbers before writing this post. He made quick work of clarifying & simplifying a couple of things.
The amount of water produced when gas combusts
There is no water actually present in the gas: the water is formed during combustion by the chemical reaction of methane with oxygen in the air.
1 cubic metre (or 35.32 cubic feet) of natural gas/ methane produces ~1.6 Kg water when it combusts at standard temperature and pressure (STP*). (So it produces approximately 1.6 litres of water).
*STP is a temperature of 1 atmosphere and a temperature of 0 degrees C or 273 degrees K
The chemical equation for the reaction is: CH4 + 2O2 =CO2 + 2H2O + heat
Atomic weights are: C =12; O =16; H = 1
So 1 mole of CH4 (14g) yields 2 moles of water (2×18 = 36g)
Volume of 1 mole of any gas at STP = 22.4 litres.
1 cubic metre = 1000 litres.
So 1 cubic metre of methane would produce 1000/22.4 x 36 = 1604 g water (approx 1.6 Kg water).
How does that apply to my own home?
So for my own house, in the year from Nov 2015-2016 I used 251 ft3 of gas (including supply of a condensing combi boiler). If all of it was used for the hob/oven – (which of course it wasn’t – most of it would have been used for heating via the boiler) :
35.32 cubic feet produces 1.6L of water
1 cubic foot of gas produces ~45.3g or ml (0.043L) of water
So at 251 ft3 per year, 11.37L of water will be produced (so only around 31mL per day)
Considering that in reality the majority of the water vapour generated would actually have been expelled outside via the boiler flue, I cannot imagine that burning gas could currently be contributing that much to the overall moisture levels in the house.
Bear in mind that humans produce around 400ml of water every 24 hours via respiration – which is almost 13 times as much as the average amount per day generated from gas usage in my house. This makes burning gas a relatively small percentage of the total moisture vapour generation, especially when you factor in the huge amounts generated by showering & cooking (moisture released from the food rather than the fuel source).
What about previous usage and heating of the house?
There are signs of gas fires having been installed in the past. Therefore, it is worth considering historically how much moisture may have condensed upon the surface of the wall or even interstitially within the wall structure due to this. Any such process could have been cumulative if subsequently trapped by less vapour permeable plaster, render and wallpapers.
I am currently unable to obtain accurate numbers for the volume of gas combusted by an average gas fire over a certain length of time, although I will attempt to do so in future & update this post to reflect my findings.