Although a single appliance is the most frequently chosen option, large dwellings or commercial premises often benefit from having two or more smaller appliances, rather than a single appliance, to meet the building’s maximum heat requirements. So, when it comes to replacing a single large appliance, it’s well-worth discussing this option with your customer.
To maximise the potential benefits of this type of installation a suitable control system can be fitted to enable the appliance to be installed in a “cascade” system (where boilers can be sequenced to come into and out of service when required). This arrangement has a number of benefits:
- Reliability – if one appliance breaks down, other appliances continue to supply heat. Similarly, it may be possible to service one appliance at a time whilst the heating continues to run.
- Efficiency – individual boilers can be turned on and off according to demand which improves efficiency. Outside temperatures may only be low enough to require an installation to run at the maximum appliance output required by heat calculations on a few days each year. A cascade system will ensure the system operates at maximum efficiency as it responds rapidly to changes in demand, switching on only the appliances needed to meet demand.
- Longevity – throughout much of the year it is likely that one or more appliances will not be used. Reduced use equals reduced wear.
- Flexibility – a boiler room may not be able to accommodate a large commercial appliance, or it may be impossible to manoeuvre a large appliance into place. The installation of multiple smaller appliances can suit plant rooms of an irregular size or that have difficult access.
- Parts – if identical appliances are installed this makes spare part management simpler.
The idea of linking multiple appliances together may seem too complicated for a technician who is used to the domestic market. The good news is that appliance manufacturers often provide helpful advice on how this can be done, and many offer a specification/design service.
Low loss header
Usually, multiple boiler connections are made via a piece of equipment called a ‘low loss header’. In simple terms, a low loss header is a vessel installed between the appliances (primary circuits) and the heating circuits (secondary circuits – see illustration). This is not just a matter of having a convenient way to join the various parts of the system, it also helps us overcome a common design problem – the required pressure, flow and temperature for the secondary circuits are likely to be very different from those required for the appliances.
How does a low loss header overcome this problem?
The heating fluid is shared between the appliances, the low loss header and all the connected heating circuits; they are not physically separated. However, the fluid in the primary and secondary circuits is “hydraulically separated” because the low loss header is specifically designed to be a “neutral point” in the heating system. This allows the circuits connected to it to operate independently of each other.
Separating the primary and secondary circuits in this way allows boilers to operate at a constant flow rate in the primary circuit, whilst temperatures and flow rates may vary significantly in the secondary circuits.
To ensure the system works effectively, OFTEC would recommend that technicians employ a specialist company to design and manufacture the low loss header, so that correct flow rates are achieved and correct pipe connection sizes are fitted. Your appliance manufacturer may well design and manufacture this for you.
Finally, careful consideration needs to be given to flueing arrangements. Specialist advice should be sought if you plan to connect multiple appliance flues together. However, it is often possible to utilise standard flueing arrangements for each individual appliance.
In the next issue of Oil Installer we will look at linkup systems. These enable us to combine two or more heating technologies in one combined system, for example a solid fuel appliance alongside an oil-fired appliance