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The last three articles outlined pump selection in domestic water recirculation systems. In general, these systems have low flow and low head pumps. What happens in a high rise with pressure reducing valves (PRV) in the hot water supply? The pressure reducing valve introduces a fixed pressure drop which must be added to the calculations. Figure 1 shows a simple representation of a multistory system with a pressure reducing valve. Figure 2 shows the same system with the addition of the PRV to the system.
In order for this system to operate correctly in figure 1, the pump would need 2 PSIG or about 5 feet of head. When we add the pressure reducing valve in figure 2 there are two things that change. First, the 20 PSIG drop across the PRV must be added to the pump head. Our pump which required 2 PSIG or 5 feet now requires 2 PSIG plus 20 PSIG or 51 feet of head. Second, the balancing at the upper floors becomes very critical. If the upper floor balance valve is not throttled severely, most of the flow will bypass the floor with the PRV. Next week we will look at a problem in a domestic water recirculating system and how the service contractor and engineer can solve it.
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Disclaimer: R. L. Deppmann and it’s affiliates can not be held liable for issues caused by use of the information on this page. While the information comes from many years of experience and can be a valuable tool, it may not take into account special circumstances in your system and we therefore can not take responsibility for actions that result from this information. Please feel free to contact us if you do have any questions.