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ber of occupants/apartment and socioeconomic factors. At Solara and NZV, the populations are quite mixed, although children are a rarity. We designed Solara's hybrid SHW/backup DHW tank/HP system on some basic precepts. Right-size" the solar thermal system; with the favor- able COPs and substantial thermal outputs of the A-W heat pumps, fewer solar thermal and PV panels were needed. Because of the super sized R-35 insulated solar tank (1,200 gallons), peak DHW demand impacts were amelio- rated by the thermal flywheel effect. We sized the DHW backup storage and thermal input capacity to assure that the worst case 3-hour thermal loads would be addressed without resorting to very large tanks and excess resistance heating elements. We chose to install, twin parallel-plumbed 109 gallon internal heat exchanger tanks, each with a single 4,000 watt heating element. That's 218 gallons of storage for 24 apart- ments. And we think we can downsize that for Phase 2! The 3-ton A-W split system "coldish climate" heat pump supplements the capacity of the two heating elements, feed- ing heated antifreeze through the heat exchangers which are located below the heating elements - pre-heating incoming water before the elements raise the temperature to their own temperature settings. Less DHW storage capacity increases water throughputs in these tanks which in turn reduces residence time and risks of bacteriologic propagation and standby losses to the utility room air. We set the electric elements to 140-150 degrees to achieve at least 135 degrees in the upper zones of the tanks. Fewer low COP heating elements minimizes total and peak power draws at the complex. For system sizing, we assumed: 10 minute showers at 1.7 gpm of DHW; Simultaneity of two showers - overlap 5 minutes; three showers per hour maximum; nine showers per three consecutive hours; 10 one-minute kitchen sink draws at 1 gpm per hour; two one-minute lavatory sink draws per shower; and three-hour worst case DHW load = 170-180 gallons. Finally, we designed an East-West DHW distribution sys- tem that relies on R-3 insulated small diameter PEX trunk lines (1" and 3/4" PEX). Small pipes keep DHW residence time short and renter use keeps it hot much of the time. The 3/8" tubing feeds each fixture from one small manifold for each apartment, decreasing delivery time and stand by heat losses. Recirculation is still required, however, because the DHW cools off in the pipes (even with the insulation) renter use is episodic, especially during mornings, afternoons and eve- nings. We chose to use small stainless pumps that pull water from each end of the trunk lines when temperatures in those lines decay below a preset temperature. The pump turns off four minutes later when the trunks have been reprimed with hot water. (We continue to fine tune with pump activation algorithm to minimize energy loss from this necessary sys- tem function.) Where to go from here One day driving back from Boston, I decided what we'd designed and was being installed might be of interest to Gary. I figured he'd have lots of criticisms about the system we designed from scratch. Instead, he was delighted we tried to follow his guidance and came out from California to admire his precepts in place. The experience was delightful. I soon returned the favor by flying to California and spending a couple of days helping to test our shower valve combo and manifold setup along with lots of fittings and tubing using the test setup he and Larry Weingarten had fabricated. We now talk all the time and look forward to the upcoming kitchen and bath show in Las Vegas and the hot water forum in Atlanta. I learned so much from this experience and Gary, I could write a book. Instead, after getting to know and work with Gary in person, we decided to write this series for Plumbing Engineer. We aim to create a dialogue among readers to explore what represents state of the art hot water plumbing and production systems for high performance buildings like Solara. While it feels as if we developed something cool, veterans in this field may have been quietly practicing this approach for a while but just not talking about it. Now, they will get a chance to join in the conversation and we will all do better jobs as expectations for safer and more water, energy and time efficient hot water systems are demanded. So, here we go. A series of articles not meant to be the authoritative restatement of super modern hot water plumb- ing and production design but rather a story of design explo- ration on the fringes of what works in high-performance buildings. Here are several of the issues we will explore: 1. Are pipes in new and existing multifamily buildings too big around? 2. Are DHW design flow rates and volumes too large for high-performance multifamily buildings? 3. Are the published pressure loss values for pipe and fit- tings accurate for modern materials and flow rates? 4. What are the pressure drop versus flow rate curves for different shower valves? 5. What are good relationships between multifamily use patterns and hot water storage volumes? 6. How can circulation loop design and operational con- trols strategies be optimized for use in high-performance multifamily buildings? 7. What can be done to improve pipe insulation installa- tion? 8. Is more research needed into what shower experience renters prefer? This series will be a catalyst for discussions at the upcom- ing Hot Water Forum in March. https://aceee.org/confer- ences/2020/hwf. The convention will be a teaching moment for hundreds of the leading lights in the field of hot water production and distribution. This topic is of special interest for design and device specification for high performance multifamily dwellings. l Peter Skinner, P.E., owner and founder E2G Solar LLC, is a NYS registered professional engineer, educator, and advocate in the environmental field. Gary Klein, president of Gary Klein & Associates, Inc., is intimately involved in energy efficiency and renewable energy, with an emphasis on the water-energy-carbon connection.
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