During the most recent round of Department of Energy funded energy conservation efforts for low income multifamily buildings, numerous older buildings (circa 1970) received new physical plants to replace the older inefficient systems that were installed during the cheap energy era. Most of these buildings contained the typical cast iron natural gas fired boilers. They were operated at one set temperature, typically 180°F. In most cases, the boilers also provided Domestic Hot Water (DHW) thru either a side arm heat exchanger or an immersed coil inside of the boiler. The potential for energy conservation when going from these cast iron behemoths to the state of the art modulating/condensing appliance is typically around a 30% minimum reduction. It can also be as high as 50% depending upon all of the existing conditions and operating parameters. This makes these projects very attractive to the funding agencies that are looking for considerable energy and water conservation efforts.
On one of the projects, a senior housing project owned by a non-profit organization, the retrofit contractor had replaced two large cast iron boilers with immersed coil DHW generators that were coupled to a remote storage tank. The flow path of the DHW was to go into the storage tank, out of the tank to a thermostatic anti-scald mixing valve, then on to the building via supply main run through the ceiling of the main floor, with 13 individual risers going up 12 floors to the beginning of the circulation return main located in the ceiling of the 12th floor, then back down to the storage tank for additional heat. The tub and shower valves were a pressure balanced valve that was typical of installations done during the 1970s. There was a 2-horse power pump moving water between the immersed coil in the boiler and the large storage tank.
The replacement system consisted of two large modulating/condensing boilers and two reverse indirect DHW tanks piped parallel reverse return, and two anti-scald temperature/pressure compensating mixing valves and then on to the distribution system.
Shortly after converting over to the new system, the residents began complaining about shortages of hot water in certain risers. The installing contractor investigated the possibility of failed shower valves, and in fact replaced all of the shower valves in one of the affected risers in an effort to correct the situation to no avail. This particular riser was directly over the mechanical room, and the shortages made no sense what so ever. That is when I was asked by the funding agency to go to the property and determine why the building was experiencing the shortages and fluctuations. I began my investigation by deploying data logging equipment in the mechanical room, as well as at the top of the system on the circulation return line main before it made its drop back down to the mechanical room. After five days, I pulled the data loggers and reviewed the information obtained. I had considered the possibility of failed check valves on the circulation return line. The data loggers showed me that my suspicions of cold water back flowing through the system from the incoming cold water main were in fact not occurring. In fact, nothing polled from the data loggers indicated that anything abnormal was occurring in the mechanical room. Supply water temperatures remained relatively stable. The circulation return was somewhat erratic, but didn’t really show me anything that would cause me concern. I then turned my attention to making certain that there were no inadvertent cross connections between the hot and cold water lines anywhere within the building. I checked all mop sinks, automatic clothes washers and a sink in the beauty shop. My research didn’t find any cross connections between the potable hot and potable cold, other than the known and wanted cross connection at the anti-scald mixing valve, and I had already determined that it was working correctly with the data logging I had performed.
Tune in next month as we continue looking for the source of fluctuating and non-existent hot water in a multi-story mid-rise building. Until then, keep your hot water hot, and your cold water cold, and happy potable water hydroncing.
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