When gas burns in air, as well as producing heat, there is a chemical reaction producing carbon dioxide, water vapour and nitrogen. A gas water heater burning 30MJ/h will produce more than one litre of water, if the water vapour is allowed to condensate. Condensate is the result of air borne water vapour being chilled below the dew point.
The dew point is the temperature at which water vapour turns into liquid. Condensation may occur in the primary flue of a gas water heater during the initial heat up period, and when the water heater is called upon to heat a high proportion of its total capacity – such as occurs after a heavy draw off of hot water. In areas where the water supply is very cold, the condensing of water vapour in the primary flue of a water heater may be of a sufficient quantity to cause it to drip onto the top of the main burner. This will continue until the water inside the water heater reaches approximately 50°C. Normally the condensate re-evaporates upon striking the hot burner. This may result in a slight “sizzling” noise being made.
This condensation may be misinterpreted. It’s not unusual for a householder or building maintenance engineer to think they have a leaking water heater cylinder. It is not the case of the cylinder leaking. It is interesting to note condensation is more prevalent during winter months when cold water temperatures are at their lowest.