Modern Building Services

24 MODERN BUILDING SERVICES SEPTEMBER 2021 FEATURE PIPED SERVICES & PUMPS 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 that 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 flow rate 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). You’ll 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?’ There’s no correct method, as long as the minimumflow is always maintained. And it will always depend on the project. In this article we’ll look at the different pump options and their pros and cons, as well as suitability to different applications. Single primary circuit pump 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 flow rates specified for the boiler or boilers whilst overcoming the resistance created by the heat exchangers, pipework, 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 flow rate for all boilers, pipe work, and hydraulic breaks must be added together and the highest resistance value used. Pipework should be configured as reverse return to ensure an even flow rate across the entire cascade. Some manufacturers design this concept into their bespoke pipe 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 flow rates. However, boiler controls are not designed to do this and instead monitor flow and return temperatures. Attempting to “force modulate” modern boilers by varying flow rates from the pump will at best not produce the desired result and at worse, over prolonged periods, will damage the boiler heat cells. In order to maximise energy efficiency, reduce electricity usage, and minimise potential heat losses through non-firing boilers, the installation of a 2-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 flow rate pro-rata for each closed valve. Due to the slow opening times of these valves, it’s important to ensure the boiler is set to 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. Barrie Welsh , Technical Sales Manager, Hamworthy Heating looks at different pump options with the pro’s and con’s of each. Which is best – a primary circuit pump or boiler shunt pumps? PIPED SERVICES & PUMPS