Are Condensing Water Heaters Suitable for Retrofit of an 80°C/60°C conventional boiler system?

Condensing Boilers for retrofit installations

In previous articles we have considered the heating efficiency gains available through the use of condensing boiler technology and how these efficiencies are achieved. We have also looked at the way the lower temperature set points common to condensing boilers further enhances efficiency due to lower standing losses.

In this article, we’ll be answering a question often encountered in the industry: whether condensing boilers offer a benefit in retrofit 80°C/60°C installations.

Typically, boilers are replaced like for like when replacing an existing boiler that has reached the end of its life. However, performance data shows that even though optimum performance of condensing boilers occurs at temperatures below 60°C, condensing boilers can still provide substantial energy efficiency benefits when retrofitted to a conventional 80°C flow/60°C return hot water system.

The major increases in efficiency are available in times when the heating load is less than the peak design value and can be achieved by varying the boiler operating temperatures from 80/60 at times of peak load to lower temperatures when heating loads are lower.

This is quite simple to achieve using the on-board controls available on most condensing boilers, which permit the supply hot water temperature to be reset by an outside signal. The total efficiency gain achieved is almost as good as if the system was being run at low temperature all the time.

To understand how this is possible, it’s necessary to look at the heating load profile.

Figure 1 shows the annual temperatures for Sydney as a histogram.

Given normal design temperatures, the heating hot water system will be sized to be at 100% capacity in the 4-7°C temperature bracket. However, this comprises only 0.4% of the hours in the year.

If we estimate that heating load decreases linearly from 100% in this temperature bracket to 0% by the time the temperature has reached 18°C, we can estimate the total energy consumption profile for the system, as shown in Figure 2.

It’s pretty clear from this that the vast majority of the energy use occurs at less than 60% of design capacity. This is important, because it is only at the higher loads that the heating coils need the 80°C supply temperature they were originally designed for.

It’s important to remember as a starting point that even in non-condensing conditions (such as 60°C return hot water temperature), a condensing boiler will still provide an efficiency benefit of around 5% relative to a conventional boiler. This is because the additional heat exchanger area of the condensing boiler improves efficiency even when no condensing occurs. However, once we drop the return water temperature below 55°C, the condensing efficiency benefits kick in and we start seeing combustion efficiency impacts of 10% and better – as well as reduced distribution pipework thermal losses.