Summary: This article is about trouble shooting hot water recirc systems for hotels. For many different reasons, hotel hot water recirc systems can be a major challenge to commission at the opening of a hotel. My goal here is catalog all the problems and associated solutions to hot water recirc systems that I or anyone else knows. I encourage readers to share their examples. This will be an article that continues to grow over time.
Related Article: Hotel Hot Water Recirculation Systems
Problem: Cold water entering the hot water system: If you are observing cold water entering the hot water system, it is likely that cold water pressure is higher than the hot water pressure. When this pressure difference exists, there will be a natural tendency for cold water to migrate through mixing valves into the hot water system. This problem often occurs when a water softener is installed serving only the hot water. This problem is easily fixed with a pressure regulating valve for each of the hot and cold water systems. Locate the hot water pressure regulator on the outlet side of the water softener. Then set the pressure of the hot water regulator a few pounds higher than the cold water. This will assure any migration of water through the mixing valves will be toward the cold water, which is a forgivable situation in most cases.
Problem: Plumber installs more risers than originally designed. Sometimes a plumber will decide that it is easier to install a separate riser for each stack of fixtures instead of combining risers to serve several fixtures via horizontal branch connections. This change is OK, but it creates more risers than included in the original design. With more risers to serve, the recirc system must be upsized to maintain the original design flow in each riser. Generally this will involve upsizing the recirc pump and the return piping.
Problem: Recirc lines leaking after several years of operation: Recirc lines need to be sized for continuous flow or they will erode in a few years and develop pin-hole leaks. The sizing criteria for recirc lines should be a maximum velocity of about 3 feet per second. This is much slower than the criteria used for domestic water lines, because water lines operate very intermittently.
Problem: Oversized Recirc Pump: We all know it is better to oversize a recirc pump than undersize it. But this is only true if balancing valves are installed. If the oversized pump is allowed to operate without balancing, the circ lines will erode from excessive water flow velocity. I recommend a conservatively sized recirc pump (i.e. oversized), and the installation of a balancing valve at the outlet side of the pump. Of course, someone must actually perform the balancing, which often is the greatest challenge of all.
Problem: Hot water entering the cold water system: There are many ways this can happen, but one example I encountered was a high rise hotel with two zones of domestic water. Since PRVs were involved to create the two pressure zones, the recirc loop was required to circulate through a PRV. This required the pump to have a very high head to overcome the PRV pressure drop. The result was a back-pressure that forced hot water into the cold water system system in the mechanical room. This problem only occurred under low flow conditions, since at that time there was no other release for the hot water system. The solution was to install a check valve in the cold water pipe serving the boiler. This eliminated any reverse flow caused by the recirc pump.
Aside: Although we solved the problem, I was never fully satisfied I understood the physics. Here is why. The mystery was how could the recirc pump force hot water back against the cold water source? If you “count” molocules of water coming and going, there is no way that a hot water recirc pump can create a net increase in system water that would force hot water back into the cold water. My only guess is that there really was no perfect separation of the hot and cold water systems due to the mixing valves throughout the hotel. And what was really happening was that the high pressure recirc pump was recirculating hot water through the mixing valves with the resulting “appearance” of hot water being forced back into the cold water. Regardless, the check valve solved the problem. Howver, I felt very unsatisfied not really understanding why the fix worked. I would be interested in hearing about similar cases.