Improving turndown ratios and system efficiencies in multiple boiler installations

 The efficiencies of modern modular condensing boilers compared with older atmospheric boilers have improved vastly over the last 40-50 years. By dividing output capacity with greater fuel efficiency, this means that there is often the opportunity to reduce the total number of boilers used in an installation. 

In addition to offering a space-saving alternative, modular boilers offer the benefits of high turndown ratios and load matching for improved efficiencies – delivering the right amount of heat at the right time with little or no wastage. The result of this is lower environmental impact, increased energy efficiency and valuable cost savings.

To demonstrate this, the example below compares non-modulating boilers with newer Wessex and Upton fully modulating, floor-standing models and illustrates the energy efficiency benefits in a multiple modular boiler installation.

Non-modulating boilers 

The turndown ratio is 4:1.
In the first example, we can see that the setup requires four non-modulating boilers that can be turned down from 280kW to 70kW in 70kW increments.

If there is an off-peak demand period, requiring a constant 95kW of power to maintain a heating circuit at a set temperature, this means that two boilers would need to run together to meet this demand. However, the plant would then produce 140kW which is 45kW more than necessary.

Wessex Modumax boiler in three stack presentation

The turndown ratio is 15:1.
In comparison, we can see how three 97kw Wessex Modumax 97/291V units can be installed as an alternative to four non-modulating 70kw models; together, at 291kW, these satisfy the 280kw peak load requirement in the example system and illustrate how the 95kw off-peak load could be met by one single boiler module alone.

However, as boilers run most efficiently at lowest loads, this 95kw requirement can be efficiently achieved by firing two or three Wessex ModuMax boilers and modulating these together to achieve the desired power for example, three boilers, each running at 31.6kw each.

Upton boiler in three-stack presentation

The turndown ration is 15:1.

In the final example illustrated we can see how one 296kW modular boiler system can be achieved by specifying Upton UF300-3 in a stack of three modules to achieve 296kW. Once again, more than satisfying the example peak load of 280kW, this system can be modulated to further enhance efficiency.

Other suggested Wessex and Upton three and two stack options include:


When it comes to using modulating boilers in a modular boiler system, the benefits are clear.  Designed to accurately match the heat load profile of a building, they also provide a valuable solution for plant rooms with limited space.  With the option to be standalone or designed in a horizontal or even as a vertical stack to help reduce footprint, they also simplify installation and maintenance intervals.  Furthermore - if one boiler fails, the remaining boilers in the system can continue to keep a building warm, without causing unplanned downtime to a heating system.

Want to know more? 

Contact Hamworthy for a free site survey to review your current heating and hot water system and see how you could benefit from a more efficient modular boiler system.