COMMERCIAL HEATING | PUMP SPECIFICATION PROS AND CONS: PRIMARY CIRCUIT VS BOILER SHUNT PUMPS Opting for primary circuit pumps or boiler shunt pumps in a commercial heating system will depend on system and balance of cost, and system criticality. Hamworthys Ian Roe explains A s discussed in CIBSE Guide B1, any heating system, regardless of size or complexity, consists of three sections: the primary circuit, which contains the heat generators; the secondary circuits, which contain the heat emitters; and the hydraulic connection between the primary and secondary circuits. Primary circulation is important in maintaining correct system temperature output and ensuring boilers have the constant minimum flowrate as specified by the manufacturer. Your system flow will be managed by installing a pump or pumps into your primary circuit (unless your boilers have integral pumps). Youll need to choose whether you want a single primary circuit pump or multiple boiler shunt pumps. A question we get asked a lot at our CPD seminars is: Which is best boiler shunt pumps or a primary circulation pump? The answer is either, as long as the minimum flow is always maintained. And it will always depend on the project. In this article, well look at the pump options, and suitability to different applications. A single primary circuit pump application is where the entire primary circuit flow is dependent on a single large pump. This pump needs to be capable of circulating the entire primary circuit at the minimum flowrates specified for the boiler or boilers while overcoming the resistance created by the heat exchangers, pipework and fittings, and any resistance created by hydraulic breaks, such as plate heat exchangers. When a single primary circuit pump is installed providing circulation for a group (cascade) of boilers, the combined flowrate for all boilers, pipework, and hydraulic breaks must be added together and the highest resistance value used. Pipework should be configured as reverse return to ensure an even flowrate across the cascade. We design this concept into our bespoke pipework kits. The primary pump must be set to a fixed flow. It is a common industry misconception that boilers can be forced to modulate by varying flowrates. However, boiler controls are not designed to do this and instead monitor flow and return temperatures. Attempting to force modulate modern boilers by varying flowrates from the Single primary circuit pump Boiler shunt pumps pump will not produce the desired result and at worse, over prolonged periods, damage the boiler heat cells. To maximise energy efficiency, reduce electricity usage and minimise potential heat losses through non-firing boilers, the installation of a two-port valve on the flow or return of each boiler to serve as flow prevention is recommended. The individual valve would close when the boiler is not firing, stopping flow through the heat exchanger. The single primary circuit pump could then be controlled so that it reduces flowrate pro rata for each closed valve. Because of the slow opening times of these valves, its important to ensure the boiler is set so that it does not fire before correct flow is established. This arrangement is beneficial when there is insufficient space to install dedicated boiler module shunt pumps and if the project is a refurbishment where an existing pump might be suitable for re-use. Another method to maintain correct flow in the primary circuit is to circulate the water through each boiler using a dedicated shunt pump per boiler. The pump is installed in the return connection to each boiler module to isolate flow when the module is not firing. Individual boiler pumps are wired to and controlled from their respective boiler module and usually given a five-minute overrun period. Again, the rules of flow still apply. Minimum flow for the boiler must be maintained and the pump must not be allowed to modulate. If using a plate exchanger, care must be taken to ensure that the shunt pump(s) can overcome both the resistance created by the boiler, and www.cibsejournal.com February 2023 57