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. And it will always depend on the project.
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. 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. 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 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. 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, as well as the resistance of the pipework and fittings, and that of the plate exchanger itself. Some manufacturers sell correctly-sized boiler shunt pumps as part of their boiler pipework kits. Boilers such as the Stratton mk2 boiler and Upton boiler have shunt pumps available along with Hamworthy pipework kits.
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 flow rates. However, boiler controls are not designed to do this and instead monitor flow and return temperatures. 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.
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 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. 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, as well as the resistance of the pipework and fittings, and that of the plate exchanger itself.
We sell correctly-sized boiler shunt pumps as part of the boiler pipework kits on our Stratton mk2 wall hung boiler and Upton boiler. 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. 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. Home Knowledge Articles Primary circuit or boiler shunt pumps in commercial heating.
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