In hot water systems, the part of the reticulation system between the hot water primary loop and point of use is referred to as the "dead leg". Water in dead legs stagnates when not in use and cools down, which means that water must be drawn off before hot water from the primary circuit reaches the outlet. Delivery time of hot water from the primary circuit is affected by distance, pipe size, and flow rate to the fixture.
Circulating hot water in a plumbing system to maintain the design temperature is standard practice in most residential and commercial multi-level buildings with the hot water generally being circulated vertically from the hot water plant down (or up) the building. If hot water needs to be delivered to fixtures far from the central riser, then the delivery time can be substantial and also wastes cold water.
From the vertical riser, it is ideal to branch off and circulate hot water on each floor close to the apartments and fixtures, however, if a hot water meter is installed in a common central cupboard as required by some water authorities or utilities then circulating hot water becomes an issue as any water passing through the meter is charged as consumption, therefore circulating systems cannot be utilised which instead can potentially create very long dead legs and long wait times for hot water delivery.
In addition, adopting a centralised water pressure control station design within the meter cupboard creates a low pressure zone after the valves which also prohibits circulation to the apartments.
The most common solutions include using electric heat trace to maintain temperature in these dead legs, or alternatively increase the number of risers in the building or reducing the size of the piperwork. These options have numerous hinderances with regards to cost, performance or implementation issues which can be improved utilising the H-Box solution.
The new multi zone H-box solution (patent pending) is installed after the water meters on each level to circulate hot water to the apartments on that floor. It is a pre-assembled unit comprising of a heat exchanger, pump, digital timer, expansion tank, new specially designed ‘auto-stop’ valves as well as all other relevant components to ensure the system operates correctly.
The heat exchanger transfers heat from the primary central vertical riser, to a secondary circulating system for each floor serving multiple apartments. Utilising a heat exchanger and pump is not new, however, the issue with using a common circulating system for multiple apartments is that when a user opens a tap fixture, some hot water would inadvertently be drawn from another user’s hot water supply, rather than their own. To prevent cross connection of this type, it is necessary to isolate each individual unit from the rest of the circulating system during the time of hot water consumption. To achieve this, a special new ‘auto-stop’ valve was designed which automatically isolates the relevant circuit when a pressure differential is created from using hot water.