Modern Building Services

MODERN BUILDING SERVICES SEPTEMBER 2021 25 FEATURE PIPED SERVICES & PUMPS Multiple boiler shunt pumps 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 boiler module is not firing. Individual boiler pumps are wired to and controlled from their respective boiler module and usually provided with a 5 minute overrun period. Again, the rules of flow still apply. Minimumflow for the boiler must be maintained and the pumpmust not be allowed tomodulate. If using a plate exchanger caremust be taken to ensure that the shunt pump(s) can overcome both the resistance created by the boiler, as well as the resistance of the pipework and fittings, and that of the plate exchanger itself. You’ll need to check what boilers you are specifying and if they already come with integral pumps. Some manufacturers sell correctly- sized boiler shunt pumps as part of their boiler pipework kits. Which method should I choose? There is no correct method, as long as minimum flow is always maintained. However, there are some pros and cons to each: Single primary circuit pump pros and cons: • You only need to purchase one pumpmaking it simpler to install, maintain, and can bemore cost effective. However, the physical size of the pump will bemuch larger than individual pumps. This can create installation and maintenance challenges, as often it will require a 3-phase power supply. • By choosing a single pump you could be creating a single point of failure in the system. But this can be negated by installing a twin motor pump or two pumps as duty stand-by. • Pipework can be more complex as you need to configure a reverse return. However, as mentioned previously, this is often built in to manufacturer’s pipework kits • You may get potential heat losses through non-firing boilers Multiple boiler shunt pumps pros and cons: • By installing multiple pumps, you have an Increased system redundancy -should one pump fail you still have other pumps running on other boilers to maintain your heating • The pumps will be physically smaller and thus simpler to install • Modern boiler controls can often control the relevant boiler shunt pump which takes the important pump over-run time away from the external controls • Reverse return configuration is not required in this case • Flow is isolated through non- firing boilers • Cost will be increased as there are more pumps to purchase and this also may result in additional maintenance due to the number of pumps • There could be additional system complexity as there is increased requirements for electrical supplies and controlswiring to each pump Which one should you choose? Ultimately both options allow you to maintain flow rate across your primary circuit with multiple boilers running in cascade. The decision will be based on criticality of building usage and the balance of cost against system reliability and redundancy requirements. For example; a building such as an office block that whilst inconvenient and possibly financially disruptive to be off-line for hours or potentially days but not vital may choose a single primary circuit pump with the possibility of twin head pump or duty / stand-by. However, a system that simply cannot risk any downtime such as a hospital or prison may choose to install individual boiler shunt pumps. It would be considered that the installation and maintenance costs could negate the cost and disruption and potential human suffering that could result if the system were to fail and have downtime. More information can be found at www.hamworthy-heating.com