Flows must be compatible at system interfaces

Unfortunately, in some cases, budget constraints do not allow the jobs to be made in the best way. In these cases, the installation is not optimised and can create a costly uncomfortable indoor climate. On the hydronic side, some major problems can be avoided if three fundamental conditions are fulfilled:

• The design flow must be available at all terminals.     
• The differential pressure across the control valves must not vary too much.
• Flows must be compatible at system interfaces

This article deals essentially with the third condition.

Production units, pumps, pipes and terminal units are designed to provide a certain maximum load even if a diversity factor has been considered. If this maximum load cannot be obtained because the plant is hydraulically unbalanced, the investments made are not valorised.

If the system never requires the maximum power installed, it means that the chillers, pumps ... are oversized and the plant is not correctly designed. When the plant is well balanced, it's not necessary to oversize, which reduces the investment and the running costs.

It is quite obvious that overflows in some parts of the plant create underflows in other parts. Unfavoured circuits are not able to provide their full load when required. However another problem will occur. At full load, the supply water temperature will be lower than expected in heating and higher in cooling due to incompatibility between production and distribution water flows.

Figure 1 concerns a heating plant with three boilers working in sequence. The distribution loop has a low resistance in order to avoid hydraulic interference between the boilers and between the circuits. For this reason any hydraulic resistance has to be avoided in the bypass "DE". A check valve between D and E, for instance, will put the secondary pumps in series​ with the primary pumps, disturbing heavily the function of the three way mixing valves.

If the two circuits are identical, they have each to take 50% of the total flow. Assume that they take 75% instead. On point "A", the first circuit takes 75% of the total flow. It remains 25% for the second circuit. The second circuit takes 75% flow and receives only 25%. It will take 50% from its own return. On "C", 25% of hot water are mixed with 50% of the return water of the circuit 2. For this circuit, the maximum supply water temperature is 69°C. In design conditions, with an outdoor temperature of -10°C, as long as the first circuit takes its maximum flow, the room temperatures in circuit 2 cannot exceed 14°C. When the room set point of circuit 1 is reached, its three way control valve starts to shut. The supply water temperature of the second circuit increases to a maximum of 80°C with an available power 10% below the design value. In these conditions, the maximum room temperature will be of 17°C for the second circuit. Increasing the pump head of the second circuit to "solve" the problem will make it worse.

Start up is much longer than expected and the power installed is not completely transmittable. To avoid this problem, the total maximum flow absorbed by the circuits must be equal or lower than the maximum flow provided by the production units.

We might think that it would be sufficient to reduce the secondary pump head, in one way or another, to limit the flows. Attempting to avoid overflows this way will simply make the underflows in unfavoured units more significant. Consequently it remains necessary to balance the terminal units between themselves. If the overflow in the circuit is the result of no balancing, we can imagine that some circuits receive only 50% of their design flow. For these circuits, the situation is worse. The supply water temperature is 10°C lower than design and the flow is also reduced.

Balancing investment represents typically less than one percent of the total HVAC costs, allowing the maximum power installed to be transmittable, valorising all the investments made.

The concept of flow compatibility also involves special working conditions at interfaces.​

In a floorheating plant, the supply water temperature may be 45°C, for example, with a return water temperature of 40°C.

The boiler must be protected against fume condensation and its water inlet temperature must be at a minimum of 55°C.

In order to obtain correct operation of this plant, all flows must be adjusted so as to obtain the necessary temperatures.

Since the boiler is supplied at 55°C, and for a design ΔT of 20°C, its outlet at full load is 75°C. If the flow through the floorheating circuit is 100% for a ΔT of 5K, it will then be 100 x 5/20 = 25% flow in the boiler.

In order to obtain 45°C at the supply to the heating circuit whereas 75°C are available at the open 3-way valve, a flow qb has to be recycled so that:

        qb x 40 + (100 - qb) x 75 = 100 x 45, which gives qb = 86%.

The difference 100 - 86 = 14% therefore circulates through the pipes between the floorheating circuit and the boiler. The boiler receives a flow of 14%. As the flow through the boiler must be 25%, a circulation flow qgb = 11% is necessary.

As we can see, flows are not arbitrary and will not be obtained by chance. They must be adjusted with balancing valves.​

Fig 3a represents a chiller water plant with four chillers. If the distribution circuit is not balanced, the maximum flow qs may be higher than the production flow qg. In this case, the flow qb in the bypass reverses from B to A, creating a mixing point at A. The supply water temperature ts is then higher than design and the maximum power installed is not transmittable.

Fig 3b represents a terminal unit working at constant flow with a 2-way valve in injection. If the flow in the terminal unit is too high, the flow qb is always in the direction B to A. The supply water temperature ts is always higher than design and the maximum design capacity is never obtained in the terminal unit.

For both examples, an overflow of 50% in the distribution or through the coil will increase the supply water temperature from 6°C to 8°C.

If the balancing valves are well adjusted, they just take away the local overpressures, due to the nonhomogeneity of the plant, to obtain the design flow in all coils in design conditions. If afterwards the balancing valves are fully open, the control valves are obliged to shut furthermore. The friction energy can not be saved that way, it will just be transferred from the balancing valves to the control valves. It is then quite obvious that balancing valves do not create supplementary pressure drops.

Manual balancing valves have to be adjusted, which is a beneficial constraint, because the balancing procedure gives the possibility to detect most of the hydronic anomalies during the commissioning operation. However, the old proportional balancing method does not guarantee the lowest pumping energy as the pressure drops in balancing valves are not minimised. This is done by using the Compensated or TA Balance methods1 which reports all the excess pump head on the main balancing valve close to the pump. The pump head is than corrected to obtain the design total flow with the main balancing valve reopened. This operation allows the minimum pumping costs in the distribution.

There is no discussion about the necessity to balance a plant working with constant flow distribution. It is very well known that an overflow somewhere creates permanent underflows in other places.

In variable flow distribution, some believe that two way control valves can solve the problem as they automatically should provide the required flow in each terminal unit. This is correct if the control valves are well sized, if the control loop is stable, if the set point of thermostat is not at an extreme value, if the terminal units correspond with the maximum power required,.... a lot of "if".

In practise, the correct sizing of a two way control valve is already problematic. The pressure drop in a control valve fully open and for design flow must be equal to the local available differential pressure on the circuit minus the design pressure drop in the terminal unit and accessories. Who knows the available differential pressure on each circuit ? What is the pressure drop in the coil when this coil selection will depend on the contractor not yet chosen at the design stage ? And, if we know these values, we cannot find  the control valve calculated as the commercially available Kvs values vary by step of 60%. The pressure drop depends on the square of the Kvs; if a control valve creates a pressure drop of 25 kPa for design flow, a Kvs just below determines a pressure drop of 64 kPa. There is nothing between. In some exceptional cases,  it's possible to find control valves with adjustable Kvs, but the problem is to adjust the Kvs at the correct value. This is impossible if the flow is not measurable. A balancing valve is then required anyway to measure the flow and provide a shut off function ! Moreover, if the pump is oversized, the control valves will create overflows when fully open and take away this overpressure when operating. The pump oversizing will never be detected that way while a balancing procedure will reveal the overpressure, which can be compensated by set-up correctly, the variable speed pump for example.​

A HVAC plant is designed for a certain maximum load. If full load cannot be obtained because the plant is not balanced for design condition, all the investments made for the whole plant are not valorised. Control valves cannot manage this situation as they are fully open when maximum load is required. Sizing the two way control valves is difficult and the valves calculated are generally not available on the market. Consequently, they are generally oversized. Hydronic balancing is then essential and represents typically less than one percent of the total HVAC investments.

Each morning, after a night set back, the full power is necessary to recover the comfort as soon as possible. A well balanced plant does it quickly. Earning 30 minutes for start up, related to 8 hours working time, saves approximately 6% of the energy consumption per day, that means more than all distribution pumping costs.

It is important to compensate for a pump oversizing. Balancing valves settled with the TA balance method reveals this oversizing. All the overpressure is reported on the balancing valve close to the pump. The correct action on the pump being made, this balancing valve is just reopen.

Hydronic balancing requires the correct tools, up to date procedures and efficient measuring units. Manual balancing valve remains the most reliable and simple product to obtain the correct flows in design conditions, giving always the possibility to check the flows for diagnostic purposes.

Reference: Total Hydronic balancing - R. Petitjean

Edition TA HYDRONICS - 530 pages - 1997.​