Andrew Dabin, Product Manager for Hamworthy Heating
Energy-efficient boilers are now the industry standard. Many of these are only compatible with modern pressurised systems. In contrast to this, open vented heating systems were common back in the day. This can be a problem when refurbishing them and that’s why finding a solution to ‘bridge the gap’ between old and new is necessary. An example of good practice is hydraulic separation between the primary circuit (with the boiler(s)) and secondary circuit (existing old pipework). In this article, we take a look at different methods to achieve this.
Assessing primary circuit design
A first point of examination is the condition and size of current pipework. For best performance and efficiency such as enabling the required lower return temperatures for condensing operation, alterations need to be considered. When it comes to the water flow around the system, it is necessary to take into account whether the boiler has an integral pump and if a primary circuit pump is required. Does the current pipework’s flow capacity match the primary/shunt pump minimum requirements?
Possible consequences of an inadequate flow rate are not only annoying for the client but can also be costly: A broken heat cell due to an insufficient water flow costs thousand to repair, a scaled up heat exchanger causes efficiency losses and longer heat-up times as well as a shorter life expectancy. As commercial boilers are a large capital investment, it makes sense considering what they are linked to.
Keeping equipment safe
How are you going to protect your boilers? It is worth considering the installation of 1) a dosing pot for introducing chemicals (to e.g. prevent corrosion) into the system, 2) an air and dirt separator to remove air bubbles and dirt particles and 3) strainers to catch debris. The next step is choosing how to connect to the secondary circuit.
Spoilt for choice
There are several ways to achieve this: via low loss header arrangements, plate heat exchangers or the use of a no flow boiler.
Low loss headers
Low loss headers are also referred to as common headers and are available as different types: horizontal, vertical and some have dual action with combined air and dirt separators.
Using a low loss header in a heating system ensures adequate flow, resistance and temperature around the primary circuit, while flow rates and temperatures in the secondary circuit may vary. Another benefit when using a vertical low loss header is the low flow velocity allowing sludge to sink to the bottom which can then easily be removed from the system via a trap. They are often supplied as part of a package directly from a manufacturer correctly sized to suit the chosen boiler’s connections.
However, alteration costs on an old heating system to include a low loss header or buffer vessel can speak against this choice. Other factors could be space requirements and an insufficient number of ports (depending on how many heating circuits you want to connect to it).
Plate heat exchangers
Plate heat exchangers provide hydraulic separation of heating circuits and protect new boilers from dirt and debris from an existing secondary circuit, as the water does not mix. Several types of heat exchangers are available. While brazed heat exchangers can’t be taken apart, most can be cleaned. Gasketed types can be fully maintained and increased in duty (depending on the frame size) for future extension of the heating circuit. Domestic hot water plate exchangers are solely for the purpose of providing hot water – hence for a different temperature profile and different controls.
Benefits include the protection of boilers through separation which prolong their life. A reduced amount of water in the primary circuit means treatment becomes cheaper (less chemicals used). Additionally, they provide pressure protection.
Downsides are space issues when both heating circuits are pressurised, and two pressurisation units needed. While it is common practice to use several boilers to prevent a single point of failure, only using one plate heat exchanger would reintroduce this risk. The use of high micron filters to catch debris in the system and additional maintenance are also recommended.
No flow boilers
A no flow boiler on the primary circuit is non-dependent on the secondary circuit flow for safe operation. Instead, an internal variable speed circulation circuit ensures water movement when circuit pumps are off or set to low. Furthermore, it uses differential temperature supervision to control the output power for safe operation. The high water content in a no flow boiler equals high thermal mass which allows it to fire without flow and without risk of overheating. Once the control stat stops the boiler, the thermal mass safely absorbs residual heat. These types of boilers often have dedicated return connections for low temperature and high temperature heating circuits to ensure maximum efficiency can be obtained.
The main benefit of installing a no flow boiler is that it removes the need to install it in a dedicated primary circuit as well as the installation of additional equipment such as low loss header, plate heat exchanger and pumps. Thanks to its high water capacity, it can operate with wide differential temperatures and the twin return connection allows maximum operating efficiency. High and low temperature circuits connect to dedicated heat exchanger return connections.
What speaks against choosing a no flow boiler is the required flow isolation through non-firing boilers. This helps the system pumps to modulate which ensures flow through the firing boiler. Isolating any non-firing boiler in any system is good practice. Furthermore, the boiler should not be operating using its own thermostats or integral temperature controls but instead be integrated using a sequence controller or building management system. This improves overall boiler control for many boiler types. Space limitations (access) and weight must be taken into consideration with this option.
The choice is yours
Which method of hydraulic separation is preferable will be mostly determined by available plant room space, time, and budget.
Depending on the choice, other considerations would be low loss header/heat exchanger sizing, type of pump(s) used and if a reverse return is required.
On the one hand, hydraulic separation can be achieved by using low loss header or heat exchanger. This offers flexibility, as this equipment is optional. On the other hand, opting for a no flow boiler means the most integral part of the heating system determines the remaining design considerations.