I’m a solar engineer – not a mechanical one. That’s one reason that I found out the hard way that designing domestic hot water (DHW) production and distribution systems for multifamily buildings is a lot harder than designing a single-family residential system.  Here are two main reasons:

1. Renters expect hot water on-demand – no interruption
2. Renters expect a good shower experience – no excuses

How do I know? Last year, I designed from scratch and guided the installation of a hybrid solar/heat pump DHW production system married to a hybridized 3/8” cross-linked polyethylene pipe (PEX) home run distribution system at Solara. Solara is an upscale all-electric netzero multifamily 24-apartment four-building (soon to total 11 buildings) complex near Schenectady, NY. 

While this was an arduous experience, these systems have been working well. I reaped many realizations and got the chance to work closely with the inimitable Gary Klein (Mr. Tiny Twig) – what’s not to like? Well, we had issues. Read on. 

We decided that our Solara experiences have been unique enough to justify a series of articles in Plumbing Engineer about the design, specification and operational factors that are critical to a successful and cost effective netzero cold climate multifamily hot water system. This article sets the stage for articles to follow.

Background

Before Solara, I had been a solar thermal guy for a decade or so – designing and installing dozens of custom commercial and single-family residential solar hot water (SHW) systems, wrote a textbook, taught courses – I kinda did it all.  

When I got a chance to design and install the SHW systems at the new Netzero Village complex (26, six-apartment duplexes) near Schenectady, NY, I was delighted. While working alongside the plumbing crew one day, I suggested that instead of the default ¾” to 1⁄2” PEX DWH trunk/branch/twig distribution systems, they consider home runs from the utility room and the use of 3/8” PEX tubing – just like Gary had been preaching about for years. (Twigs are the “last mile” of tubing to fixtures in a distribution system.)

The GC’s interest was piqued and asked for some pressure loss and delivery time calculations. I obliged him and my home run and 3/8” PEX design was adopted – just like that. The shower twig remained ½” PEX to be on the safe side. We all knew that 3/8” tubing and the reducers at the valve and manifold for each apartment could add pressure loss and we knew that a good shower was king. None of us wanted any surprises there.

I went back to finishing my solar systems. The home run/3/8” tubing design was a big success and that’s how I started an unlikely mini-career in hot water plumbing design. 

The Solara Development Arises and Our Shower Surprise!

Months passed and my solar client itched to build a more plush development.  He finally procured funding and some land for another second netzero development. This one had 24 apartments per building, instead of six, and was just down the road from netZero Village.  

“Would you design the hot water distribution and production system for us,” he asked.  

“Sure,” I said. 

I was thinking, “How hard can it be to design a multifamily hot water system? Just scale up that netZero Village system and add a small trunk and recirculation line.”  

What ensued was not a simple exercise at all! Rather, there were days of number-crunching. There were weeks of pressure drop and delivery time calculations, bench testing, and multiple design iterations and refinements. Turns out what seemed simple was not.

The plumber voiced concerns about how different the design was from his previous years’ of experience with trunks and branches but with the GC’s support, a very good approximation of our design got installed – 1 ¼” PEX feed line to 1” and ¾” PEX east and west ‘trunkettes,’ (extensions from the larger trunk line feeds). We finished off the system with 3/8” PEX home run lines to every fixture group (shower, washing machine, lavatory sink and kitchen sink/dishwasher.) We also installed in each building a 14-panel rooftop SHW system feeding a 1,200 gallon R-35 insulated solar tank with dual electric hot water back up tanks. We even broke the mold by installing “coldish” climate 3 ton air to water split heat pumps (outdoor compressor units with antifreeze heat transfer fluid).  

The most challenging aspect of our design was the downsizing of the DHW distribution piping. This critical element was needed to reduce standby energy losses, speed up hot water delivery and save money on piping, insulation, fittings and labor. Doing so comes with a price, however. Smaller pipes increase pressure losses and low pressure can jeopardize renter satisfaction – especially important for showering. We knew that the cold water pressures in the town were not high – roughly 50+- psi at the riser. Did I mention that a good shower is the kingpost of any hot water system?

So, to make sure the smaller pipes would really work in these buildings, we confirmed our pressure loss estimates for the trunk/trunkette segments as soon as the pipes were hung on the ceiling of the second floor corridor. Check! We set up a testing rig for 10-40 foot segments of 3/8” PEX home run lengths (plus the manifold and reducer couplings).  The testing determined pressure losses and delivery times at various flow rates through 3 gpm. For flows up to 2 gpm, the losses were not insignificant but acceptable for good showerhead sprays. Check! But, we forgot to test the shower valve/cartridge combo plus the twigs.

As soon as the construction debris was out of the tubs a few days before move in day, the plumber tried the showers. Quelle horreur! The showerhead spray was just too weak to satisfy renters. I checked the pressure at the showerhead spigot and found very low pressure – no wonder the spray was so flaccid. I reconfirmed my pressure drop bench testing numbers for the twigs – all was OK. Reluctantly, I had to conclude that the shower valve/cartridge combo had stolen too much pressure. And, we couldn’t test them in-situ – they were now buried in the walls. Some valve combos reputedly don’t rob pressure, but not this model! 

We fixed the problem in the first four buildings without changing out the shower valve combos. But, not without a lot of consternation. Stay tuned for the next article in our series for the rest of the story.

Almost a year, including half of the winter, have passed since Solara opened. The overall DHW system has worked very well – no complaints about the modified showers. The solar thermal systems provided the bulk of heat needed from late spring until early fall. After that, the electric elements and air-to-water (A-W) heat pumps have taken over progressively more of the thermal backup loads during periods of extended grey weather.  

Welcome to the real Solara

The developer designed Solara as a logical extension of his earlier very successful netZero Village complex. That earlier 13-duplex building complex has been fully occupied since it opened. The solar thermal systems have operated very effectively and the 3/8” home run tubing has proved itself as a viable last mile for fast hot water delivery with low heat losses. 

Solara has been marketed as “Ecoluxury Living” with all utilities included in the monthly rental price. Four of the 24 apartments have two bedrooms and the upper two floors are served by an elevator. Heating and cooling for each apartment are provided by cold-climate air-to-air heat pumps. Energy Recovery Ventilation (ERV) air exchangers provide the ventilation. Energy Star appliances include heat pump clothes dryers. The overall building achieved less than 0.5 air changes per hour at negative 50 pascals. The Energy Use Index (EUI) is below 40 kBTU/ft2/yr! I think that it is safe to say that Solara defines the state of the art for high performance buildings. The very efficient hot water distribution and production system is one reason that the EUI values are so low.

In order to estimate DHW demands for the production and distribution system design, I monitored the DHW use at netZero Village and other recently renovated multifamily buildings in upstate New York. Clearly, high-efficiency fixtures and appliances have lowered per capita water use, often below 20 gallons per capita per day (gpcd). That demand depends, however, on the demographics, including age, number 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 favorable 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 ameliorated 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 apartments.  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, feeding 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 system 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 evenings.  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 system 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 plumbing and production design but rather a story of design exploration 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 fittings 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 controls strategies be optimized for use in high-performance multifamily buildings?
7. What can be done to improve pipe insulation installation?
8. Is more research needed into what shower experience renters prefer?

This series will be a catalyst for discussions at the upcoming Hot Water Forum in March. https://aceee.org/conferences/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.