Heat treatment

The key factor in deciding whether to instal a condensing boiler is the system return temperature. A new system can be designed to achieve a low return temperature. Modern under-floor heating, air conditioning or large radiator surfaces work with a low return temperature without a problem. But old boilers have a high return temperature, with many older systems set at constant 80°C flow/60°C return temperature. This can create a difficulty if replacing the boiler with a more efficient condensing unit. Yet all is not lost. Simply installing weather compensated controls and a boiler with a modulating burner reduces the flow temperature, which means the system will condense for over 90% of the year. With system temperatures designed to ensure there is sufficient heat for even the coldest days, weather compensated controls automatically reduce the flow temperature on warm days. Even if the system is designed to operate at 90/70°C, weather compensated controls will allow it to condense for over 80% of the year. Weather compensated controls are a must for condensing boilers, whether in new systems or old.

The addition of electronic modulating pumps in the heating circuits pushes efficiency to the maximum as the difference between flow and return temperatures is raised by up to 20°C. Modulating pumps with additional controls (0-10 V signals and temperature sensors) can follow the difference between flow and return temperatures and control the flow rate accordingly. If the difference is not ideal, the pump will control the flow rate or the pressure difference in the system. This increases efficiency, reduces the consumption of electricity and reduces the frequency of burner starts, which reduces flue emissions.

Which condensing boiler?

Once you have established that it makes sense to install a condensing boiler, there are many product solutions to choose from. The table (below) will help you match the right type of condensing boiler to the building type, location of the plant room and the heating system. The table shows a simple example based on a typical installation with a load of 240 kW.

The key issue is often simple access to the heating room. However bad the access, you will always get a cascade of wall hung boilers into place. There is a fairly simple equation to be considered. Compact boilers are small and lightweight and hold little water. This means they respond quickly, have very low energy losses and are generally the easiest to install. The balance for this is that they have high hydraulic resistance, limited maximum flow rates and require care to be taken in the design of a new heating system or in the connection of the boiler to an old system. Boilers of large water content can cope with almost any system - even old ones or those in which the water quality is less than ideal - because the large water volume provides a buffer for the system. Also, hydraulic resistance is low, which allows the use of small pumps. This keeps electricity consumption down and lets the boilers operate at high working pressures. The trade-off is a slow response time, large dimensions and great weight.

Get good advice at the earliest possible stage. Choose the best boiler for the installation and design the system to give the full benefit and to avoid the pitfalls.