MODELLING OF A DOMESTIC GAS-FIRED CONDENSING BOILER

Within the actual context of energy savings and pollutant emissions reduction, installing
condensing boilers becomes indispensable because of their high efficiency compared to
conventional boilers. A simulation model predicting boiler performances and
condensing rate in the boiler heat exchanger is very valuable.
The model developed within this work is a one-dimension model of a domestic gasfired
condensing boiler. The aim of the model is to know whether or not the boiler
works in good condensing conditions. It gives a better understanding of using and
installation requirements of condensing boilers.
The structure of the model is similar to a conventional boiler model [1] with a
combustion chamber and a dry heat exchanger (HX1), at which a wet heat exchanger
(HX2) is added (Figure 1). The calculation consists of evaluating combustion gas
temperature and gas composition after the dry heat exchanger. Then, retrievable energy
from the flue gas is calculated for a fixed geometry and efficiency of wet heat
exchanger and for a fixed return water temperature. The heat transfer coefficient in the
wet heat exchanger is calculated according to Liang et al. [2] .
The influence of different parameters on condensing performances are analysed: excess
air, return water temperature and flue gas mass flow rate for different heat output power.
The model has been validated with measurements on a test bench equipped with a 24
kW gas-fired condensing boiler. The test bench is represented on Figure 2.