Gary Klein, a guru of efficient hot water delivery, has been pushing for change in hot water distribution systems for a long time. In recent years, he and his colleagues have developed a great tool for quantifying the efficiency of a hot water distribution system. It’s called the hot water system rectangle, and the concept is simple.
The hot water rectangle
To find the hot water rectangle, you draw the smallest rectangle possible that includes the water heater and all the hot water fixtures in a house. Then you find the area of that rectangle, divide it by the conditioned floor area, and express it as a percentage. If the hot water rectangle is as big as the conditioned floor area in a one-story house, the ratio would be 100%.
The schematic below shows the conditioned space of a one-story house outlined in black. I’ve also shown the locations of the water heater and all of the hot water fixtures in the diagram. In this simple example, the hot water rectangle is 93%. That’s only slightly less than the conditioned floor area. And believe it or not, as bad as that sounds, it’s not as bad as it could be.
In a one-story house, the number actually can be more than 100%. If the water heater in the house above were in an attached garage, for example, the hot water rectangle would be greater than 100%. The conditioned floor area would be smaller because we’d subtract the area of the garage. But the hot water rectangle would be the same size shown above, making the ratio greater than 100%.
Multi-story houses and the hot water rectangle
When you have a two-story house, you still draw a single hot water rectangle. It has to include the water heater and all the hot water fixtures for both floors. The hypothetical maximum would be 50%, but that’s only if the rectangle doesn’t cross the building enclosure. For a three-story house, the hypothetical maximum would be 33%.
Even though we know the actual hot water rectangle can be larger than those numbers, they’re useful guides as to how efficient a house is. You want the hot water rectangle to be as small as possible and the percentage as low as possible. If you’re close to 100% for a single-story house, 50% for a two-story house, or 33% for a three-story house, that’s not good. If you’ve over those numbers, it’s even worse.
Using the hot water rectangle
The challenge is to reduce that ratio represented by the hot water rectangle. If you’re a home builder or architect looking to improve, this is easy to do. It’s a lot cheaper to move rooms around on a computer screen than it is in a real building. Let’s go back to that one-story house above. The hot water rectangle ratio is often a high number when no one considers the efficiency of the hot water distribution system. And in this house, the ratio starts out at 93%, which is pretty bad.
Without changing the floor plan much, we can reduce that number a lot. The second layout (just below) shows the kitchen rearranged to have the hot water fixtures near the center of the house and the two bathrooms also moved toward the center. The ratio for that configuration is 26%. Most of the rooms are pretty much in the same places, but we moved the kitchen plumbing to the interior and moved the bathrooms a bit.
By making bigger changes to the floor plan, we can get an even bigger reduction in the hot water rectangle. In the third layout (below), the bathroom and kitchen hot water fixtures are even closer together. And the water heater and laundry room have moved to the center of the house. The result is a ratio of 3%. And that’s still not the best we can do.
A real-world example of extreme improvement
Klein worked with Habitat for Humanity builder George Koertzen in Stockton, Calif., and helped him make a dramatic improvement in efficient hot water delivery by reducing the hot water rectangle ratio for the single-story houses he built. Before learning of the importance of clustering the wet rooms, Koertzen was building a floor plan with a 79% ratio. Then he saw the potential and got that down to 15% on his first attempt. Then 4% . . . then 2.5% . . . and finally, an astounding 0.8%!
The way he got down to less than 1% was by first putting the two bathrooms on opposite sides of the same wall. Then he put the kitchen sink and dishwasher at one end and the washing machine at the other end of that wall. He installed the tankless gas water heater over the toilet in one bathroom and the home run manifold over the toilet in the other bathroom. The total length of pipe from the water heater to the farthest fixture is less than 10 ft.
What can you do?
If you’re a remodeler or homeowner trying to figure out how to get efficient hot water delivery in an existing home, you face more challenges. One option is to replace the hot water pipes (assuming they’re accessible). Without changing the floor plan, you may be able to reduce the length and also right-size the diameters.
Another option is to move the water heater to a more central location, if that’s possible. Then reroute and right-size the piping at the same time. Or you could replace one large water heater with two or more smaller ones, closer to the hot water fixtures.
And the massively expensive option, of course, would be to gut the house and completely redesign the floor plan, clustering all the wet rooms in the same part of the house. My hot water rectangle ratio is currently 50% for a two-story house. By switching the laundry room with the foyer and moving the kitchen to the den, I could get that down to 5% or less. But that would be an enormous renovation.
In each of those options, your goal is to reduce the size of the hot water rectangle and right-size the pipes. However, in most existing homes the least expensive option is to install a demand-activated circulation pump under a sink that is at the end of one or more of the trunk lines in the house.
A report full of great info
The hot water rectangle (also called the hot water distribution rectangle or hot water system rectangle) is explained in detail in a 2021 report from the California Energy Commission (CEC-500-2021-043). It’s similar to the wet room rectangle, which may or may not be the same numerical area or ratio.
The authors of the report (Gary Klein, Jim Lutz, Yanda Zhang, and John Koeller) have done us all a great service by doing the research and putting the information out there. And it has a lot more on efficient hot water delivery than just the hot water rectangle. Chapter 5 even presents “two unexpected findings.”
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He also has a book on building science coming out in the fall of 2022. You can follow him on Twitter at @EnergyVanguard.
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