The example below compares old UR atmospheric boilers with newer Purewell VariHeat condensing boilers demonstrating the real benefits of these efficiency improvements in a multiple modular boiler installation.

A system designed for a peak demand of 280kW using four 70kW boilers.

Comparing the efficiency of atmospheric boiler systems with condensing boiler systems.

On/Off atmospheric boilers – Hamworthy UR 

Turndown ratio is 4:1 
The plant can be turned down from 280kW to 70kW in 70kW increments.

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

When the required temperature is attained, one boiler would turn off, the other would continue alone at 70kW until the circuit temperature dropped to a point which triggered the second boiler to turn on again.

High/Low atmospheric boilers – Hamworthy Purewell Classic 

Turndown ratio increases to 8:1. 
For a demand of 95kW, one boiler could fire at full load (70kW) and another at part load (35 kW) producing 105kW.

This arrangement gets closer to the required 95kW than with the UR boilers but still uses 10kW more power than necessary. With both On/Off and High/Low boiler examples, the plant's output power switches between levels above and below the ideal power required, reducing the overall plant efficiency and increasing wear and tear on the second boiler as it would need to cycle on and off.

Fully modulating condensing boilers – Hamworthy Purewell VariHeat 

Turndown ratio increases to around 13:1. 
Moreover, due to infinite fan speed burner control, the incremental adjustment of each boiler's output power between part and full load settings is far more precise. 

The example 95kW requirement could be achieved by firing two, three or four Purewell VariHeat boilers and modulating these together to achieve 95kW. Four boilers running at 23.75kW each would be the most fuel efficient as boilers run most efficiently at lowest loads. Alternatively, one boiler could be run at 70kW (full load) and a second boiler could be run at 25kW (near fully modulated).

Control strategies

With a multiple boiler programmable controller, such as the Merley sequence controller,  the appropriate number of boilers can be set up to fire and modulate together to accurately respond to variations in demand in the most efficient manner, and so increase overall plant efficiency with improved load matching of the entire boiler plant to the building’s heating load profile.

Choosing the right control strategies for multiple boiler plant e.g. using unison or cascade operation, boiler firing order, etc., will depend on the application priorities and the planned maintenance regime.

Reducing number of boilers required

Improved turn-down ratios and typically higher outputs of single boilers, as well as better insulated buildings, means that when refurbishing a plant room there is often the opportunity to reduce the total number of boilers used and make savings in the overall cost of purchase and installation.

With fewer units to maintain, reductions in annual maintenance can also be realised.

In the example, 3 x 95kW Purewell VariHeat models are shown as an alternative to 4 x 75 kW models; together these satisfy the 280kW peak load requirement in the example system, and the 95kW off-peak load could be met by a single boiler module.


See how this was achieved at the Hallmark Carlton Hotel who were able halve the number of installed boilers when they upgraded their heating plant.


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