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The Challenge of Flushing an HIU
Published:  04 September, 2018
Heatlink, HIU, Heat Interface Units

With over 30 years’ experience in the industry, Alan Barber, Technical Director of Heatlink, has a wealth of experience and is frequently offering advice and support on Heat Interface Units. One of the biggest problems that he sees time and time again is the issue of contaminated water and the difficulty of flushing systems installed with an HIU. But is there a simple fix?

The issue of contaminated water is far from a new topic and I’m sure anyone working in HVAC will be only too familiar with the frustrations it brings with it. Part of the problem relates to iron oxide. Iron oxide is produced when there is iron and oxygen within the heating water system and it eventually blocks pipes and heat exchangers.

The current trend for boilers is to install a magnet filtration device in close proximity. The device will trap and remove iron oxide that would otherwise easily pass through strainers. Heat Interface Units (HIUs), though, are a whole different issue. They’re the district heating equivalent of the boiler and they have water flow on both the mains side and the system side of the heat exchanger. Whilst both systems suffer from the contamination of iron oxide, the mains water with its large steel pipes, cast iron boilers and steel buffer tanks is a larger practical risk.

The cleanliness of water to the long-term performance of the HIU is essential if hot water supplies and heating are to be maintained. Clean water must be achieved before any water is allowed into the HIU. In fact CIBSE, BSRIA and the CSA all promote the need to properly flush the mains pipe work before allowing water into the Heat Interface Unit. The Pre-Commissioning cleaning of pipe work systems published by BSRIA explains clearly about achieving flushing velocities to ensure removal of contaminates.

However, what is not explained is the difficulty of flushing systems where an HIU is specified. The HIU must have a flushing by-pass and drain as you’d expect on a Fan Coil Unit or other terminal device, but there are big differences between the different units. The comparative ease of flushing fifty Fan coil Units compared with fifty HIUs is significant.

Heatlink, HIU, Heat Interface Units

What makes it even worse is that most HIUs are installed in apartments and this in itself brings issues. Apartments are constructed in phases and aren’t as open plan as say an office might be. This means access to the HIUs to reach the flushing by-pass valves and flushing drains is not straightforward. It’s very difficult to check each by-pass is open and the isolating valve to the unit is closed and it is more likely that a number of HIUs will be missed during the mains flush of the system and then each individual flush. Due to the phased approach, areas are also completed across various stages and this results in the water distribution system serving the units not being completed until the last unit is installed.

The good news is that iron oxide can now be fought with a simple magnetic device within the unit, but flushing is still a whole other problem and I’m still working on the simple fix. My strongest recommendation is that all HIUs are installed with flushing by-pass valve arrangements with a suitable size drain that is externally accessible and follows the BSRIA Pre-Commissioning Guide. At least that gives you a fighting chance.