Tour & Andersson's Peter Rees talks balancing valves

All HVAC systems should achieve two objectives: delivering the occupancy comfort specified at the design stage and using as little energy as possible.

Variable speed drives match duty against building load, so only power required at a given time is used. For more than 85% of the heating and 72% of the cooling season, the load on an HVAC system is typically 50% of design condition, so there are big energy savings to be achieved. However, variable flow systems introduce variable differential pressure on the circuit, which can cause a degradation of control quality.

Control valve authority is constant when the system is working at 100% flow mode; at all other times authority is variable. This is where many problems start. The authority of a two-port control valve is the ratio between the pressure drop across the control valve at design flow over the total pressure drop across the valve when fully closed. Flow in system sub-circuits is dependent on the setting of other control valves in the system, as pressure across the system is not constant at non-design flows.

The efficient operation of a terminal unit depends on the supply water temperature and water flow; when the space or flow temperature is reached, the control valve starts to close to reduce the output. The authority of the control valve to maintain design condition is dependent on the pressure drop across the control valve remaining constant as it modulates. In a variable flow system the pressures across the control valves are affected by other valves closing and pump speed changes. So, the differential pressure can increase or decrease, resulting in variable control valve authorities and too much, or not enough, flow.

This leads to occupants altering the system set point to counteract these negative effects. To meet this new demand, the pump head must be increased, consuming more energy, and negating any energy savings the variable speed pump had achieved.

To maintain a properly functioning variable flow HVAC system, a method of stabilising pressure must be introduced. A differential pressure control valve (DPCV) or a pressure independent control valve (PICV) can be used to solve this problem, by diverting excess pressure, where there is an unwanted pressure drop, or opening to increase flow where there is insuffient pressure.

Two-way control valves are designed to modulate system flow to the required level. This would appear to negate the need for hydronic balancing, as it should be obtained automatically. However, as optimum specification for control varies between systems, installing perfect control valves is practically impossible – consequently, most are oversized.

In order to counteract these problems in constant flow systems, manual balancing valves are used. In variable flow systems, DPCVs replace manual balancing valves. After commissioning, the DPCV prevents the flow and balance in the sub-circuit being affected by pressure changes caused by other sub-circuits opening and closing or pump speed changes. DPCVs are designed to alter their position in response to changes in pressure, to maintain pressure equilibrium.

There are caveats regarding installation of DPCVs. They must be installed in the manufacturer’s recommended orientation, as some are only suited to horizontal pipes. Also, it’s best to install pressure test points across DPCVs so their operation can be checked during commissioning and fault finding. Because of this, the ‘fit and forget’ solutions to hydronic balancing mooted by some suppliers are not particularly effective.

The position of the DPCV also affects the authority of the terminal control valve; the closer the DPCV is to the terminals, the better the terminal control valve authority is maintained.

PICVs can achieve the same effect, but have the advantage of being suitable for installation at each terminal unit, replacing the terminal control valve and balancing valve. However, when deciding what type of pressure control to use, designers should be aware that choosing PICVs will increase costs.

Building services designers and installing engineers using a system that employs DPCVs or PICVs can see benefits beyond occupant comfort. The projected energy savings from a variable speed pump will be achieved, with a reduction in commissioning times.

Modern HVAC systems provide excellent environmental controls, without the need for excessive energy consumption. However, the absence of an appropriate means of measuring the final balanced system could result in time lost identifying the cause of poor distribution due to pressure drops in system terminals. Installing either DPCVs or PICVs should never lead to removing balancing valves, leaving a system with no means of troubleshooting balance problems.