Water conditioning and its control are critical to achieving efficient, reliable operation and longevity of plant, according to Martin Wilkinson.
Historically, schools have often operated with large, inefficient cast-iron boilers. Perhaps the plant was looked after by a caretaker so maintenance was low, but the system was checked fairly regularly.


Due to the low flow rates through these boilers, they became the sacrificial 'sump' of the whole system. The perfect location for corrosion particles to settle out of the mass flow conditions of the water, the buildup of sludge inside the heat exchanger could be immense. Subsequently, they were comparatively expensive to run and environmentally unfriendly. But crucially they still worked: if the pumps were operational they produced heat for the school.

The market trend is to replace these old boilers with low water content, high efficiency condensing, cascade type units where several boiler modules are linked together onto the same flow and return headers (collector pipes).

The idea is to match the system demand by varying the number of modules in operation - avoiding inefficient 'cycling' of the primary heat source. The reality is that this approach needs consideration in both design and procurement of the plant.
Modern boiler heat exchangers are highly sensitive to air and dirt. Unless water quality is closely managed on a regular basis, the heat exchangers will always be the weak link, compromising the integrity of the whole system.

Nowadays, caretakers are no longer given access to the boiler plant, so when the first boiler module goes down, it's often unnoticed until it's too late. With nobody tasked with checking the water quality, inevitably the plant may fail at some point.

We all know that systems must be flushed and cleaned prior to installing new boilers but action needs to be taken to alter the system characteristics that caused the corrosion to occur in the first place. Dosing with chemicals alone isn't the solution, as problems recur - we have to question why?

It's not necessarily a problem with the chemicals; more the application and need for a regular maintenance regime to keep levels at the right concentration. This is particularly true around the school environment where funds are stretched and there's no personnel on site.

A traditional strainer approach can't deal with the microscopic magnetite particles that typically make up the sludge in the boiler. Strainers only collect a relatively small volume of dirt - when they do, the pressure drop increases, as does the energy required to pump the water around the system. The strainers need to be taken offline and cleaned out, a labour-intensive job to remove very little from the system: it's hardly surprising this is frequently overlooked.

Maximising efficiency

Condensing boilers require low return temperatures to allow them to operate in condensing mode, thus maximising efficiency. Typically, this means a temperature difference across the boiler of 20°C (sometimes higher). Radiator systems are required to operate at 11°C hence there is a mismatch to start with.

So boiler manufacturers will specify a hydraulic separator (low loss header) between the primary and secondary circuits. This facilitates the different flow rates and temperature differences around the boilers and the old radiator circuit.

The low loss headers in general circulation tend to be 'empty vessels' or lengths of 'oversized pipework'. The intention is to reduce velocities through the header. The reality is that secondary flow rates may be underestimated at design stage, resulting in the over mixing of the primary and secondary circuits in the low loss header - producing a lower flow temperature to the radiators.

The boiler flow temperature therefore has to be raised to compensate and an installation originally designed to operate at 80°C/60°C may need to run at 85°C/65°C - impairing the boiler's opportunity to condense.

Although efficiencies, the environment and operating costs are important, it's vital that we avoid downtime of the plant and the massive disruption to our children's education.

Modern cascade systems installed onto old steel radiators combined with an inadequate maintenance regime for the circulating fluid is a recurring problem in schools throughout the UK.
Thankfully, the industry is now addressing these problems and the long-term solution is simple.

• Condition the water naturally to reduce oxygen and prevent corrosion: a deaerator removes air content from the whole system through one central point.
• Provide a permanent method of protecting the boilers by removing even microscopic dirt particles from the circulating fluid.
• Employ a method that can handle large volumes of dirt between maintenance periods yet retain a continuously low pressure drop.
• Provide a simple method of cleaning the dirt from the system so that it takes just seconds with minimal water loss - introduce a dirt separator.
• A low loss header that virtually eliminates cross mixing of the primary and secondary water flows allowing the system to operate with the lowest boiler temperatures.
• Select a hydraulic separator.

Of course, I would argue that Spirotech's Spirocross is ideal for school installations since it is a deaerating, dirt separating and hydraulic separator, in a single compact unit.