VWART Calculations

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The following table shows the VWART figures from the Swedish tests.

The following VWART figures are adjusted to provide figures based on a keep warm temperature of 25C, and a return temperature limit of 55C.

VWART Calculation Nomenclature VWART (°C) Volume(m3 pa)
DHW VWART DHW 19.5 22.79
Space Heating VWART SH 48.7 50.75
Standby / Keep-hot VWART SBY 24.4 3.83
Heating Period (including space heating) VWART HEAT 47.1 11%
Non-Heating Period (not including space heating) VWART NO HEAT 20.2 89%
Overall   23.2 77.37

The settings for this test were based on a keep warm temperature of 55C, and a return temperature limit of 55C.

VWART Calculation Nomenclature VWART (°C) Volume(m3 pa)
DHW VWART DHW 21.8 27.6
Space Heating VWART SH 55.0 70.8
Standby / Keep-hot VWART SBY 55.0 108.7
Heating Period (including space heating) VWART HEAT 54.0 82.3
Non-Heating Period (not including space heating) VWART NO HEAT 48.3 124.8
Overall   48.8 207.1


One can see that by setting the keep warm temperatures lower it has a direct impact on the resulting VWART figures. The key point is that with the DATA you have the ability to set these temperature limits as the installation allows to gain the best efficiencies.

The Swedish tests do not allow for such functionality and made no specification as to keep warm temperatures. Some were set high and some lower, making the published VWART pretty meaningless for comparison. It is more a representation of what the keep warm temperature was set to for the test.

An important point is that of return temperature limitation during heating mode. In the real world, radiators are rarely balanced, and as a result the return temperatures are higher than they should be. Under these conditions, most HIUs will simply pass these higher return temperatures to the plant room and the VWART figures go out the window. The return limiting functions of the DATA will guarantee that the intended VWART figures hold, however poorly radiators are balanced.

A final point covers the real world problems that occur when primary temperatures are dropped on a site to overcome unintended issues such as overheating corridors. With HIUs targeted at a set (and fixed) central heating temperature, the effect of dropping the primary flow temperature is usually that the HIU tries harder to achieve its target - in other words, the valves open further, and return temperatures fly uphill. In the worst instances, the primary flow is dropped as low as the setpoint heating temperature, making it impossible for the HIU to achieve full target. In this case the valves fully open and the return temperature can be approaching that of the flow. All VWART calculations again, go out the window. It is worth knowing therefore that the DATA HIU tracks the primary temperature and adjusts its targets accordingly. The return temperatures will NEVER rise above the figures needed to maintain VWART figures as intended.

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VWART Calculation Methodology