How to Confirm and Locate Hot-Cold Water Crossover

[walkup bldg guy]

I manage a five story 16 unit building in New York City, and for months now, had resident reports of hot water that is only lukewarm much of the time, or takes very long to get hot. Even in off-peak (early hours ~ 4 AM) where there should be plenty of stored hot water, the water is initially hot enough, gets cooler after 10 mins (mid-shower), then takes a while to get hot again, if at all. I thought this situation might be improved by have a recirculating pump installed, but that doesn't seem to have helped either, leading me to suspect there is a crossed line introducing cold water into the hot water supply lines.

The building has 3 units on each floor. Some units have kitchen & bathroom sinks, shower faucets with separate hot & cold water handles. Others have single handle kitchen, bathroom sink, shower faucets & also dishwashers, washers in unit. In the basement is an AO Smith HW420 water heater connected to a 200 gallon AO smith water tank with a grundfos alpha2 15-55sf SS circulator pump. HW flows up to apartments via 1" copper riser to top floor, 1/2" laterals branch out from riser to fixtures in each unit. Recently added a dedicated 1/2" return line from the top of the HW riser back down to HW tank/heater line with a Taco SmartPlus-e ECM circulating pump.

This is definitely a building wide issue. Despite the thermometer on the HW line from tank reading 135 degrees, the lukewarm water calls keep coming. I am wondering if the new recirculating line has just helped to more quickly spread the cold water? I have read other posts that state you should shut cold water supply to heater, and then open hw at farthest fixture. If you still have flow after HW is drained, then there is a crossover. Since, I would rather not drain an entire 200 gallon HW tank, I am hoping my piping is configured such that I can temporarily bypass the tank to do this test.

With my setup (separate tank, hot water heater), couldn't I just shut off supply going from the tank to the HW riser? I don't think there is piping directly from the HW heater to the riser. Then if I open hw at fixtures in units, if there is water flow, wouldn't that also confirm there is a crossover? I presume I should shut off the recirculating pump in either scenario.

There has been a lot of work done and new fixtures added in the last few years so locating the crossover feels daunting. I would appreciate all thoughts on the best approach to fixing this problem.

 

[Twowaxhack]

Turn the pump off.

Close the cold water valve at the inlet of the water heater

Bleed the pressure to zero.

If the flow doesn’t stop and the valve is holding tight at the cold water inlet of the water heater, then you have a cross connection.

You don’t have to drain any tanks......just the initial pressure.

 

[Geofd]

We had a similar issue at one of our high rise dorms, had to check all janitors faucets for soap mixing stations, have a special hose thread connection made by eco- labs, not a big fan of this set up but it works, also had to check all the shower valves, if the diverter had seperate piping the students used that to shut the water of and left the mixing valve on check for shower heads with on off buttons, we also had bad check valves at the mixing stations the single handle faucets did not contribute to hot/cold cross over ..... But can are there other biz in the bld besides the apts???? Also check your laundries