When heat interface units meet a system

Space heating, heat interface unit, HIU, PICV, Altecnic
Heat interface units and central plant in harmony — Neil Parry

It is standard practice to modulate the gas input to a boiler so why not modulate the heat input to a heat interface unit? Neil Parry of Altecnic examines the benefits of such an approach.

In the UK, we are now gaining more and more experience of central plant/district heating systems, and the use of heat interface units (HIUs) is increasing. As the market grows, so it seems does the amount of different HIUs available. A lot of them seem the same and have similar heat outputs, but are they?

The definite answer here is no. While some of the differences can be small and of subjective benefit, the method of controlling the HIU is certainly not and can have significant implications for the tenant and the landlord.

The simplest way to control the output of an HIU is with a mixing valve on the secondary side of the plate heat exchanger and a constant-flow regulator on the primary. The problem with this method is simply that the flow through the primary side of the plate heat exchanger is always the same, regardless of demand. This inevitably leads to rapid scaling within the domestic plate and reduced ∆Ts on the primary.

Small primary ∆Ts are the nemesis of district heating. They reduce the benefit of renewable-energy sources, and increase pump speed and primary flow rates — making the system very inefficient and costly to run.

Controlling on the primary therefore is a better methodology, but the method of control is vitally important.

Most cheap 2-port valves can withstand only a little differential pressure. As the plant-room pumps adjust to system demand, this simple 2-port valve is often swamped with flow due to the increased differential pressure, and a number of problems occur.

The temperature of the DHW from the taps increases beyond the set point, the DHW temperature within the plate heat exchanger increases, causing scaling, plant-room pumps increase speed to compensate, which makes matters worse, and the whole system goes out of ‘balance’, leading to starvation at some of the HIU points.

One way around these problems is to utilise a 3-pipe reverse-return primary system, with the associated design risk and additional cost.

Space heating, heat interface unit, HIU, PICV, Altecnic

Alternatively, a large number of differential-pressure-control valves (DPCVs) will need to be installed at various points around the primary system, greatly increasing the capital and installation costs.

It is not uncommon to see these DPCVs listed as accessories by some HIU suppliers, leading the belief that they may be optional. They are only optional if you are not concerned about whether the system works or not!

By far the most-effective method of controlling HIUs is with pressure-independent control valves (PICVs). As the name suggests, this type of control valve operates independently of pressure. The valve will then ‘focus’ on providing the stable DHW temperature while ‘ignoring’ any changes in differential pressure on the primary side of the HIU.

This ensures stable water temperatures for DHW and heating for the tenant, reduced scaling within the DHW plate heat exchanger and, importantly, the HIU only taking the amount of primary water needed to ensuring the system stays in balance and the system ∆Ts remain as high as possible, thereby dramatically increasing the efficiency of the system.

For the installer PICVs simplify installation work because no further commissioning valves, flow-balancing valves (automatic or otherwise) or DPCVs are required.

All our SmartSat HIUs utilise PICVs for control. These valves are controlled automatically and motor driven to ensure fast reaction to changes in primary pressures, system demand and tenant requirements.

The overall control of these valves is carried out by the sophisticated electronic controller. This allows us to further increase the efficiency of the HIU with features such as return-temperature compensation, intelligently controlled domestic bypass and digital commissioning.

Neil Parry is specification manager with Altecnic.

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