Going on from SFK's link, I'd guess you mean the water dew point, the temperature (at a given pressure) at which water vapour present in a gas mixture starts to condense out of the gas.
I've never heard of a problem of natural gas pipes pipes freezing up, so the dew point should be below lowest expected ambient temperature. In compressed air systems, dew point -40° is often specified.
Dew point of natural gas is more relevant to Off Shore Gas platforms where the gas may condense its carried water in the pipes as it passes through cold sections of pipe. Not relevant to UK boilers.
Condensing boilers improve efficiency by additionally taking heat from the hot waste exhaust air from the burnt gas and putting it into the water. This process causes condensation to form when the waste exhaust air reaches its dew point as it is cooled.
Full details taken from a website here...
A condensing boiler has two main heat exchangers. This time the water enters the secondary (and condensing) chamber first. Picking up latent heat from the hot flue gases as they cool. The moisture in the gases condenses into droplets that form at the bottom of the heat exchanger. The condense is mildly acidic (about the same pH as tomato juice) and is expelled to drain. From there the water enters the primary heat exchanger to collect more heat before flowing to the radiators and hot water tanks. The flue temperature will be around 55 degrees celsius. That means up to 200 degrees of heat is being transferred to your heating water, not expelled into atmosphere. This means you save loads of money.
This, also, might be of some assistance, its important to remember that condensing (dew point) only begins at ~ 55C so the boiler return temp needs to be 35C/40C to get really meaningful condensing effect.
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