Performing a 'DHW-ectomy' in a Retrofit

ONE OF THE BIGGER challenges in replacing the domestic hot water system on a multifamily housing project is maintaining service to the users while installing the new system. In the case of the older apartment complex that we started to discuss last month (pg. 36), we decided to install three 120-gal. storage tank/heat exchanger assemblies along with three high-efficiency condensing boilers. We were

ONE OF THE BIGGER challenges in replacing the domestic hot water system on a multifamily housing project is maintaining service to the users while installing the new system. In the case of the older apartment complex that we started to discuss last month (pg. 36), we decided to install three 120-gal. storage tank/heat exchanger assemblies along with three high-efficiency condensing boilers.

We were able to perform the installation with a minimal interruption to the end users. In all, we had the hot and cold water shut off for less than four hours while we interfaced the new storage system into the existing distribution system. This allowed us the luxury of performing “dry” pressure tests on our new system, which makes life easier when working in tight quarters.

This first step is critical in the long-term scheme of things because the existing physical plant required some major changes in order to complete the energy-efficiency retrofit. With the boilers being the only source of energy, it was virtually impossible to shut the boilers down for a reasonable period of time to perform the necessary changes.

In this case, one 5-hp space heating circulator pumped the water through the two space-heating boilers whether the backup boiler was firing or not. This negates the full potential of energy savings associated with installing a two- stage boiler control/boiler rotation system. If and when you have weather conditions that would allow the system to operate with just one boiler, if you circulate the water through the “off” boiler, it causes a continuous standby loss in the flue gas section of the “off” boiler. This is the result of the boilers being piped in parallel with only one circulator.

The solution is to set the boilers up with their own circulators and reconfigure the system as a primary/secondary piping system. This is not a major task in and of itself. When the boilers, however, are the sole source of energy for domestic hot water production, it would require that the system be down for days, which would have required a mobile boiler to maintain DHW.

The alternative, which I chose, is the installation of a stand-alone DHW system that would then allow the boilers to be shut down and their piping modified without affecting the DHW production of the system. Hence, what I refer to as a “DHW-ectomy.”

Once the new DHW system was brought online, the other modifications to the space heating side could be performed during the summer without creating any shortages of DHW to the users. The actual DHW physical conversion was virtually seamless. The storage system was set up with the necessary purge valves to allow the new system to be completely filled and purged without having to shut down the old system.

Once the new tanks were filled on the potable side, and the closed loop portions were filled and purged, the new boilers were fired up and the storage tanks were fully heated. Bringing them online was a simple matter of opening the cold-water inlet and hot-water outlet valves of the new system and shutting off the cold-in and hot-out of the old system. The residents didn’t even know it had happened!

Management, however, did notice. The boiler room immediately dropped in temperature by 20°F overnight. The office located immediately above the boiler room also cooled off by about 20°F and the daily consumption of natural gas was reduced by 47% for the first two days the system was online. Not bad for a mechanical room that had previously been viewed as being “as efficient as it’s going to get!”

The condensing boilers used to provide the DHW also provide extremely efficient space heating due to their condensing design. This is what I like to refer to as “base loading.” The long-term goal is to install a control logic that will schedule and rotate boilers based on outdoor air temperatures and indoor response temperatures.

The three high-efficiency condensing boilers will be the first boilers brought online, and the less-efficient atmospheric boilers would be brought online last. This will allow a complete hot-water reset schedule to be used on the space-heating system, thereby reducing the underground standby losses of the system and substantially reducing the energy consumption of the system.

The supply water going to the apartments will start at 120°F when the outdoor air temperature is at about 55°F, and will end up at 170°F when the outside air is at 5°F. The older, inefficient boilers will be phased in someplace between 140°F and 170°F, depending upon the actual loads.

This additional conservation effort should result in additional fuel savings on the space-heating portion of the system of 30% to 50%.

Tune in next month as we address the oversized space-heating circulator and explore our options for controlling bypassing zone valves. Until then, Happy DHW-ectomy Hydronicing!

Mark Eatherton is a Denver-based hydronics contractor. He can be reached via e-mail at [email protected] or by phone at 303/778-7772.

TAGS: Hydronics