Return Water Temperature Challenge in Condensing Boilers

Generally, efficiency of boilers depends on the return water temperature and the boiler's thermal load. Combustion gases start to figure 1 boilercondense the water vapor available in all boilers including condensing and fire-tube types at the dew point and a temperature of about 130°F (54.5°C), As the reduction in temperature of output combustion gases continues, more water vapor is condensed. However, if the return water temperature in the heating system does not reach 60 °F, the boiler will not reach the maximum claimed efficiency (98%) (See Figure 1, provided by manufacturers of condensing boilers). In the catalogs provided by manufacturers of condensing boilers, it is stated that the maximum claimed efficiency of 97% is obtained only when the return water temperature is 80 °F. (Refer to the legend of Figure 2)

Here, the important point is that the 80 °F temperature (of return water) in condensing boilers could be used only in underfloor heating systems. However, to produce hot water, another boiler with an inlet and return temperature of 180 and 160 °F is required.

Moreover, if the heating system used in the project is not of underfloor heating type, the condensing function is obtained only at initial startup when the boiler's temperature is not that high. Yet, as the temperature of return water increases, a significant drop in efficiency will be observed.

In addition, to calculate the thermal load of heat exchangers, coiled devices, radiators, unit heaters, fan coil units, etc. in the heating of buildings, the inlet hot water temperature should be 180 °F and the return water temperature should be 160 °F. As shown in Figure 1, if the return temperature is about 160 °F or 57 °C, maximum efficiency in condensing boilers will be below 86%. In other words, in condensing boilers with an inlet temperature of 180 °F and a return temperature of 160 °F, the maximum efficiency will be 86%, which is fairly equal to the efficiency of other three-pass full wetback fire-tube boilers with fully corrugated primary and secondary furnace and spiral fire tubes.

Figure 2 shows why condensing boilers are not appropriate for heating systems with return water temperatures of over 90 °F. But, steel boilers cost one-third of a condensing boiler and according to various studies, over 90% of condensing boilers in the United Kingdom have never functioned in their condensing phase.

Therefore, using condensing boilers is limited to low capacities for detached houses and in case of using underfloor heating system, an electric water heater is required to supply hot water (due to the need to lower hot water temperature to increase efficiency).


Basically, condensing boilers face the challenge of low return water temperature and never reach their high efficiency in higher return water temperatures. Therefore, the use of condensing boilers at high initial prices has no economic and technical justification. According to Figure 1, to obtain a 96% efficiency in condensing boilers, the return water temperature should be 90 °F (about 33 °C). With a figure 2 boilerhigh return water temperature of 55 °C, (about 130 °F), the efficiency of condensing boiler reaches 87%, which is equal to the efficiency of other three-pass full wetback fire-tube boilers with fully corrugated primary and secondary furnace and spiral fire tubes.

Maintenance Problems of Condensing Boilers

Another noteworthy point is the high frequency of reported defects of condensing boilers (up to twice) which has converted them into a nightmare for users according to the Guardian. In addition, the high maintenance cost of condensing boilers has challenged their application. According to the Guardian, even in countries such as the United Kingdom, despite the high fuel prices, condensing boilers have not been able to economically justify their high prices, and consumers complain about the lack of cost reductions despite their high initial cost. Some other problems of condensing boilers are:

  • The burner of condensing boiler should be a premix radiation gas burner, which is not manufactured in Iran. Thus, along with other parts, its maintenance is difficult and its high cost does not make it economical.
  • In condensing boilers, water flows in the coils, so the boiler is a water-tube boiler with high incidence of sediment and sludge formation inside the coil and it quickly burns out due to its lack of water. Water flow discharge is of critical importance in condensing boilers. Boilers should not work without water flow; otherwise, they will quickly burn out even if the most advanced control tools are used. In addition, it is impossible to visit heat surfaces in condensing boilers to check sediment formation and other defects.
  • Due to the drop of flue gas temperature in the condensate boiler, as CO2 and SO2 condense in the outlet and turn into acids, the flue should be made of stainless steel, which is not economical due to its high price. In addition, polyethylene flues are not recommended due to reasons such as high return water temperature (160 °F) relative to the condensing boilers (40 °C), and if the burner is not operating properly, the flue gas temperature will rise and the polyethylene flue will be irreparably damaged.

Maintenance problems and very high cost of condensing boilers are just to increase their efficiency to over 88%. However, using three-pass full wetback fire-tube steel hot water boilers with fully corrugated primary and secondary (accordion) furnace and spiral fire tubes, higher efficiency is attainable by installing an economizer in the flue. However, it is not economical due to maintenance problems (requiring separate cold water piping to cool down the flue gas) and the need for stainless steel flue. Currently, the highest efficiency of 89% is obtained in three-pass full wetback fire-tube boilers with fully corrugated primary and secondary (accordion) furnace and spiral fire tubes, with gas, diesel or dual-fuel burners made in Iran with a reasonable price, return water temperature of <160 °F< and inlet temperature of 180 °F.