How to include regulators in a ventilation system project? Szymon Płonczyński, CAV and VAV system specialist at SMAY, writes about the installation of differential pressure regulators and the installation of CAV and VAV regulators in series or in parallel within a project.

Parallel installation of CAV and VAV regulators

CAV and VAV regulators can be installed in parallel. Both VAV and CAV regulators maintain a set flow level, regardless of the pressure changes upstream and downstream of the device. Figure 1 shows three rooms side by side. Each room is operated by a separate CAV regulator.

If we open a door between any two rooms (Figure 2), we will join two environments. It will be similar to a situation in which two variable or constant air volume regulators control one room. The flow in those devices will not change.

Installing regulators in open space type rooms

Regulators are used in parallel in open space environments when it is not possible to supply the required amount of air with only one diffuser. In general, each of them — individually or within groups — will be equipped with VAV or CAV regulators. In case of variable air volume regulators, we often use zoning. We virtually divide a large space into several smaller ones and assign a sensor (i.e. a CO2 sensor) to each of them.

Ventilation system zoning when using VAV regulators

Every zone is operated independently by one or more VAV regulators that supply the required amount of air (Figure 3). If only one regulator was used for the entire space — with the average CO2 measurement in an extraction duct — we could end up having the set parameter considerably exceeded in one part, whereas the amount of fresh air in another part would be larger than required. We can solve this problem by means of zoning.

Duplicating CAV regulators in parallel installations

We also install regulators in parallel when the largest regulator of the series is too small to supply the required amount of air. In such situations, we duplicate the devices. How? We divide one channel into two separate ones. Then, we put a constant air volume regulator in each of the channels and connect them once again. Figure 4 shows an example of such a set-up. Naturally, we should consider the required straight section upstream and downstream of the regulator, following device instructions.

Can differential pressure regulators be installed in parallel?

No. Differential pressure regulators cannot be installed in parallel in one room. This applies to the same type of installation (i.e. a supply or an exhaust system) as well as to the supply and exhaust installation. If we use the same set point, it is not possible for the two to work at the same time, i.e. closing and opening simultaneously.

Risk of differential pressure regulator oscillation

It is known that pressure difference measurements are subject to error. If one of the regulators decides to close a damper, the other one will open it. The devices will thus oscillate, performing the opening and closing actions one after another. In case of different pressures, regardless of whether we are dealing with under or overpressure, one of the devices — mainly, the one with a lower (absolute) set point — will entirely close or open, depending on the configuration.

Figure 5 shows an incorrect installation with the use of differential pressure regulators, whereas Figure 6 shows a correct configuration.

Can differential pressure regulators be installed in series?

Yes, differential pressure regulators can be installed in series, but only in one configuration: the first regulator should maintain pressure in a duct, while the other one should maintain pressure in e.g. a room. In this configuration, every regulator performs different tasks that will affect each other to some extent, but they will not cause the devices to work against each other.

Combining differential pressure regulators with air volume regulators

By following these principles, we can freely combine VAV, CAV and differential pressure regulators. Figure 7 shows a combined system with all of the discussed devices used correctly.