You live in a third-floor apartment and notice that there is low water pressure in your shower. Why is this happening? We often conclude that our plumbing might have a leak, a partially open valve, air bubbles or simply that the apartment’s height is causing the problem. But this is rarely the only explanation.

According to the Régie du bâtiment du Québec, residential buildings with more than 8 units and more than 2 storeys must be protected by a backflow prevention device (BPD) installed at the water inlet. This requirement may therefore apply to your building.

Most people do not really know how a backflow prevention device works and do not realize that it can sometimes contribute to a drop in pressure in their home. The two most common models, the Reduced Pressure (RP) and the Double Check Valve (DCVA), are equipped with spring-loaded valves.

As water flows through the device, it must pass through these valves, creating resistance and inevitably resulting in a pressure drop. Simply put, the valve allows water to flow in one direction, but as soon as there is a back pressure attempting to force the water back in the opposite direction, it closes. The result is that the water is blocked and cannot "back up" into the drinking water network.

The pressure loss is caused by these valves, as they create resistance when water flows through the device. By definition, any spring-loaded backflow prevention device therefore causes a certain pressure loss. A total pressure loss of approximately 8 to 12 PSI for RP and 3 to 8 PSI for DCVA! These values are already quite significant. However, the losses increase with the water flow rate. They could therefore be even greater depending on the water demand.

These devices must be checked and certified annually to confirm that the valves, springs and other components are functioning correctly and still provide the minimum resistance required by the standard to ensure adequate protection against backflow.

Let's take the example of the apartment on the 3rd floor. If the shower pressure was normal one year, then problems appeared the following year, the BPD could be the cause. A defective valve can offer less resistance, allowing the water to flow more easily. During repairs, the valves and springs are replaced with new compliant parts, restoring the device’s normal resistance and potentially making the pressure loss more noticeable. In short, a properly functioning backflow prevention device always results in some pressure loss.

How to fix pressure loss?

References

1. Calcady. (2026). RPZ backflow pressure loss calculator. https://calcady.com/trade/plumbing/rpz-backflow-pressure-loss/

2. Toolgrit. (2026). Backflow preventer sizing calculator. https://www.toolgrit.com/tools/backflow-preventer-calc

3. Piping Technology System. (n. d.). Check valve pressure drop calculation formula. https://pipingtechs.com/check-valve-pressure-drop-calculation-formula/

4. EZ Virtual Tools. (n. d.). Check valve pressure drop calculator. https://hydraulics.ezvirtualtools.com/calc/check-valve.html

5. Municipality of Sainte-Brigitte-de-Laval. (n.d.). L’installation d’un régulateur de pression d’eau potable. https://sbdl.net/capsules-municipales/linstallation-dun-regulateur-de-pression-deau-potable/

6. Atlas Backflow Services. (2025). Can a backflow preventer cause low water pressure in my building? https://www.atlasbackflow.com/post/can-a-backflow-preventer-cause-low-water-pressure-in-my-building-1

Loss of pressure: an unsuspected culprit