Valve Sizing Tool

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The hidden cost of valve oversizing

Too often we have customers telling us that, ‘just to be sure', they will opt for our HORNE 25, a 25mm thermostatic mixing valve (TMV) when a HORNE 20 (22mm) or, perhaps, even a HORNE 15 (15mm) TMV will be sufficient for a specific group mixing application such as a group of Duŝo shower columns or a row of basins.

The decision to oversize adds unnecessary cost in a number of ways:

  • The cost of the valve itself. A far greater weight of brass is needed for the Horne 25 compared to the Horne 15. The larger valves are also manufactured in significantly smaller production runs, therefore both these factors add significant, yet completely avoidable cost.
  • A single HORNE 25 costs the same as two equivalent specification HORNE 20s, but the HORNE 20s are DO8 (Type 3 TMV) approved (healthcare standard).  Using two valves rather than one also gives a better turndown ratio and enhanced thermostatic performance.
  • There’s also a need to purchase extra couplings to reduce pipe size from 28mm / 22mm down to 15mm toward the point of use.
  • The volume of the mixed water dead-leg is greatly increased, which, during periods of low usage, has implications for the control and management of Legionella bacteria.
    • 15mm pipe: 145ml per metre of deadleg
    • 22mm pipe: 320ml per metre of deadleg (increased by a factor of 2.22!)
    • 28mm pipe: 539ml per metre of deadleg (increased by a factor of 1.68 or 3.72!)
  • The increased deadleg also wastes water and energy (from heating the water initially). Cooling to the ambient temperature between shower usage, the volume of the deadleg is likely to be purged to drain before showering actually commences. Note that the current Legionella control recommendations (L8 and HTM 04-01) recommend the deadleg be as short as practicable and not more than 2m in length.
  • Environmentally, with a bigger carbon footprint, oversizing costs us all valuable resources in terms of ore minerals, water and fuel.

A smaller valve will do the job just fine: increasingly, new installations are being designed with pressure boosted systems and this means that our HORNE 15, despite the pressure drop across the valve, can easily manage to achieve flow rates of 24 Litres per minute...i.e. 3 shower heads running concurrently at 8 Litres per minute.

Also consider, when sizing the installation, if all the outlets will ever be in use at the same time. This diversity factor is important. If there’s a row of 5 basins in a primary school toilet, it might be that only 2 or maximum 3 taps would ever be running at once. Therefore, size the valve for the peak demand flow rate: 3 x 6 L/min = 18 Litres and well within the capabilities of a single HORNE 15 TMV.

Our Valve Sizing Tool accommodates for site parameters such as water supply pressures and temperatures and also caters for a variety of appliances at different flow rates. It also reminds the user of the diversity factor. Use it to properly size your next group mixing installation and save yourself, or your client, a lot of money!