HIU Differential Pressure Self-Learning Function

The DIGI, DATA and SLIM electronic HIUs, all come as standard with a function to learn the differential pressure of the primary system during operation, and adjust their characteristics to suit.

Differential pressures (DP) can vary significantly across a network. The properties nearest to the plant room will see the maximum pressures generated at the plant, where as the index flat, furthest from the plant, will see much lower pressures - enough to work at full network load. The typical range of pressures is from 0.5 bar up to 1.5 bar, but can be higher on some networks.

The traditional approach is to fit a Differential Pressure Control Valve (DPCV), that knocks out excess pressure so the HIU only sees a constant DP. This is a perfectly adequate solution, however it has three disadvantages compared to using an electronic self-learning system:

• They add cost, taking up more space and hence increasing HIU size and heat loss.
• They introduce pressure loss of their own, so that the index flat will in fact need a higher DP to achieve maximum load.
• One more component to go wrong.

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  A Typical DPCV

The self learning feature works by examining the effect of a valve movement compared to known characteristics, and hence determining what the DP actually is. The self learning feature takes three seconds to perform, and is done every time a tap is opened and temperatures stabilise.

The following graph shows how the self learning feature works during an independent test. The test shows the commissioning process where the unit is turned on for the first time. The preset DP value is 0.6 bar. The actual DP of the system is 2.5 bar. The first test draw-off shows how the system would react without self-learning in place. The hot water temperature overshoots the target on first draw-off, and a spike in DH flow (red dotted line) can clearly be seen. The second draw off starts with heat in the system, allowing advanced functionality to start and enables the system to learn the new DP value. From there on you see no spikes in DH flow or significant overshoot of temperatures when taps are opened.

DP values will vary with load for any one property, so once the initial commissioning has been performed (by running hot taps a few times) then the system is setup to go, and will continue to fine tune itself every draw off.