The issue of return water temperature for condensing boilers.
To achieve higher efficiency, a condensing boiler is designed with an exchanger-condenser that reduces the temperature of flue gases down to 5-15°C above the return water temperature. This allows condensing the water vapour produced by the combustion, delivering thereby an efficiency increase by latent heat recuperation that can theoretically reach 11%.
Clearly, the latent heat gain is achieved by a condensing boiler only if the return water temperature is kept below a limit that brings the exhaust gases below the vapour dew point. This limit is typically of 54-55°C for condensing boilers burning natural gas.
In order to maximize the fraction of the heating season during which condensation occurs, it is fundamental to control supply water temperature in function of the outdoor conditions according to a heating curve, as displayed in figure 1 for a chosen design temperature regime of 80-60°C. It can be observed on this graph that condensation will be obtained (i.e. return water temperature below 55°C) only when the outdoor temperature is above 0°C. Considering for instance the temperatures registered hourly in Greenwich during winter 2010-2011, this tells us that a boiler operating there with 80- 60°C design temperature regime will be condensing during approximately 6192 hours.
The above numbers apply provided that the system works perfectly as intended by design. When flows are not properly distributed in a system, some parts of the plant are in overflow while others will suffer from chronic underflows. Tenants in the unfavoured parts of the building will raise complaints. In reaction, maintenance people will increase the speed of the pump leading to global overflow compared to the real needs and/or lift the heating curve. Both measures will reduce the condensing opportunity. Indeed, a global overflow in the system leads to lower temperature difference and therefore a higher return water temperature. The dashed line in figure 1 illustrates the effect of a 50% overflow on the return water temperature curve resulting in a reduction of the condensing time to 5424 hours (-12.4%) for the considered case.
Fig. Heating curve and resulting return water temperature curves for a 80-60°C temperature regime and a design outdoor temperature of -5°C