Most often the building's hot water recirculation loop has stopped working well: a failed pump, a loop that is out of balance, or a mixing valve or heater problem. The upper floors sit at the far end of the piping, so they lose hot water first. Confirm the loop is running before you replace anything.
How the loop is supposed to work
Many apartment buildings heat water in one central plant. The US Department of Energy's Building America program says these central systems "generally feature a pumped recirculation loop to more quickly deliver hot water to individual dwelling units." A pump keeps hot water moving through a supply pipe and back to the heater, so hot water stays close to the fixtures. A DOE case study describes these loops as commonly used in multifamily buildings "to reduce wait time for hot water at faucets."
Hot water reaches the top floors last, so a weak loop shows up there first.
What usually goes wrong
The pump is off or failing. No pump, no loop. Without circulation, the pipes cool and the far end waits for hot water to be pulled all the way from the plant. Check whether the pump is running, and whether its controls are working. Some pumps run on timers or on a temperature signal. The DOE case study describes demand controls that "run the pump when there is demand and the pipe has cooled below 100°F." If those controls fail or are set wrong, the pump may not run when it should.
The loop is out of balance. Multi-riser loops use balancing valves to send flow to each branch. Caleffi, a valve maker, says that in a multi-circuit recirculation system with traditional mechanical balancing valves, "significant fluctuation of water temperature can occur" as demand and supply temperature change. When one riser gets too little flow, its floors go cold. A balancing valve that has been closed or moved during other work can do the same thing.
Unwanted flow in the loop. Caleffi's product literature says an optional check valve protects against "circuit thermo-syphoning," which is flow driven by heat alone. A missing or stuck check valve is worth ruling out.
The heater or mixing valve can't keep up. If hot water is weak building-wide at peak times, the plant may not be producing enough or may be set too low. A thermostatic mixing valve that is failing or set wrong can also restrict hot water downstream. Ask a plumber to log supply and return temperatures at the plant and at the top of each riser. The DOE case study stresses commissioning after loop controls are installed, and suggests logging supply and return temperatures for a few days to confirm they work.
Don't fix it by turning up the temperature blindly
The CDC's Legionella guidance sets the temperatures to hold. It says to store hot water "above 140°F (60°C)" and to ensure circulating hot water does not fall below 120°F (49°C). To manage scalding, it says to install thermostatic mixing valves "as close as possible to fixtures." If your building lacks a water management plan, see whether your building needs a Legionella plan.
What to do next
- 1Confirm the pump is running and moving water.
- 2Feel the return pipe near the heater: cool means little or no circulation.
- 3Note which floors and risers are affected.
- 4Have a plumber measure temperatures at the heater, each riser and the far fixtures.
For related problems, see why a commercial water heater isn't making enough hot water and whether a recirculation pump is worth it.
HQ diagnoses hot water loops in commercial and multifamily buildings. We are licensed and insured, offer 24/7 emergency service, and give free estimates with upfront pricing.