With commercial heating and hot water plant refurbishment we are seeing a shift from traditional atmospheric boilers and water heaters to modern condensing models.

We take a look at the critical factors that affect this type of project and why it is vital to assess the current system along with the proposed new scheme.


Understanding the clients’ philosophy and justification for their approach can identify any aspirations for building excellence such as BREEAM ratings or particular performance criteria.

In existing buildings a thorough plant room survey will identify factors that, if ignored, could be to the detriment of the project outcome. This must not be explored in isolation, but extended to review the building and entire system:

Has the building had a change of use or occupancy that may affect the demand or is it an old dirty system that will require specific water treatment prior to any equipment, such as commercial boilers, being installed?

Are there measures that have been, or could be, put in place for increased efficiency and reduced wastage such as splitting the system into zones, installing external temperature sensors for weather compensation and thermostatic valves on some of the radiators?

There are many factors to consider, and these will vary on each project.

Coping with condensate

It is crucial to recognise early on the type and condition of the existing flue system and its route, as many boiler refurbishment projects today may require a flue system upgrade or full replacement.

Updates to the Building Regulations Part L and European Directives such as the Energy-related Products Directive are driving the change from atmospheric or pressure jet boilers and water heaters to condensing or high efficiency models in order to comply with the outlined efficiencies. However, the existing flues on this type of system are not able to deal with a modern pressurised and wet system where condensate will form.

The flue system must be water tight and pressure tight, and designed to drain the condensate from the flue and prevent flow back into the boiler. Where possible you may be able to make use of the existing chimney with a liner thus enabling it to cope with condensing operation.

A provision must also be made to remove condensate safely from the boiler itself. A 100kW commercial boiler will generate around 13 litres of condensate per hour. The condensate is typically 3.5pH, so slightly acidic but can be disposed of normally through the drainage system. If the boiler or water heater is not located next to a suitable outlet, the condensate trap/drain must be connected to a drainage system using corrosion resistant material. It may be beneficial to install a condensate pump to assist with effective removal, particularly important in basement plant rooms.

The new flue.


Modern boilers with low water content are less tolerant to fluctuating flow conditions in the heating circuit so a low loss header is recommended to safeguard the condensing boilers from low flow conditions. The provision of a low loss header ensures that a steady flow of water is maintained through the boilers protecting them from potential damage as a result of firing when there is insufficient or no water flow.


Only part of the benefit of a boiler or water heater upgrade can be realised unless a review and improvement to the control system is implemented.

For multiple boiler installations a good boiler sequencer, such as Hamworthy's Merley controller, will help maximise the benefits of the new boiler plant and since older atmospheric boilers only operated on/off or high/low, compared to the fully modulating operation of condensing boilers, it is unlikely the previous controls will be sophisticated enough to implement effective sequencing strategies.

Ultimately a commercial heating and hot water plant refurbishment project should always be viewed in the wider context; not just the boilers and water heaters in isolation. This approach will ensure that a best practice solution is achieved and the customer gets best value for money, high efficiency compliant products with full system integration.

Heating and hot water system upgrade at a nursing home

With a thorough site survey and objective review of the existing heating and hot water system the number of boilers in this installation were reduced from eight atmospheric to three Purewell VariHeat condensing boilers creating more space in the plant room for servicing and maintenance.

An air and dirt separator and dosing pot was installed to clean and protect the system as well as safeguard the boilers from any dirt in the existing heating circuit.

Why not attend one of our free CIBSE-accredited Continuing Professional Development seminars to learn more about heating plant refurbishment? Learn more here.