Plumbing – energy conservation (part 1)

In a plumbing system there are two types of energy consumption that come to mind.

  1. Kinetic energy (pumping to move water around), and
  2. Thermal energy (for domestic hot water).

Let’s ignore pumping at this point, because turning cold water into hot water is the real energy consumer we need to worry about.

Material choice matters

Copper tubing is often the chosen and/or required material in a plumbing system. Copper is also known as a pretty good heat conductor, which can lead to inefficiencies. There is an initial energy loss when hot water flows through copper tubing due to the thermal energy transfer from the hot water to the copper pipe.

Using potable water system PEX tubing would result in lower initial heat loss as PEX has inferior heat conducting properties when compared to copper. The problem is that some building codes won’t permit PEX tubing in plumbing systems, as it is the case here in Chicago.

Insulation matters

No matter what the choice of pipe material, all pipes ought to be insulated to deliver as much hot water with as little heat loss as possible to the point of use (such as a faucet).

How much pipe insulation is needed? The Chicago Green Homes Program requires “standard flexible pipe insulation or better [with an] R-3 [insulation value in] conditioned space.” My friend and hot water guru Gary Klein has another helpful rule of thumb.

He recommends that at the minimum the thickness of the pipe insulation equals the thickness of the pipe diameter. For instance, half-inch tubing is insulated with pipe insulation that has at least a half inch wall thickness, three-quarter-inch tubing is insulated with pipe insulation that has at least a three-quarter inch wall thickness, etc.

Managing structural waste

As an introduction into structural waste, I should mention the three different phases that occur when we use hot water:

  1. Phase 1 is the delivery phase, where the hot water is delivered to the point of use.
  2. Phase 2 is the use phase (taking a shower, washing the dishes…), and
  3. Phase 3 is the cool down phase, once we are done using hot water. The hot water sitting in the plumbing system begins to cool down.

I mentioned in the previous post the importance of an efficient plumbing layout and concluded with a description of a structured plumbing system with a trunk, branches and twigs. This does not only contribute to material conservation but also contributes to energy conservation.

Pipe runs are kept short and pipe diameters are kept small in an efficient and compact plumbing system.

  • The shorter the pipe runs, the less heat from the hot water is lost to the copper pipes during the initial hot water delivery phase (as explained above).
  • The shorter the pipe runs, the smaller the hot water volume sitting in the pipe, minimizing structural waste during the cool down phase.
  • The smaller the pipe diameter (within reason), the smaller the hot water volume sitting in the pipe, minimizing structural waste during the cool down phase.
  • The smaller the pipe diameter (again, within reason), the less water and energy is lost to behavioral waste, while waiting for hot water to arrive at the point-of-use.

The last two items mentioned here need some more unpacking. Let’s start with the former:

Smaller is better

Smaller pipe sizes, as laid out above, minimize the volume of water in the plumbing system. This becomes important in energy saving terms, because we lose less thermal energy during the cool down phase.

pipe-sizes

To understand what difference the pipe size, and subsequently the pipe material can make, let’s look at some data from my friend Gary Klein. He looked at various pipe materials and various pipe sizes that are in use today. The table below lists the linear feet for each material at each size that hold one cup of water.

The smallest I can go, as per the current code, is copper type M at a 1/2 inch diameter. 4.7 feet of that copper tubing would hold one cup of water. 3/8 inch tubing would be more proportionate to our low flow fixtures. That means I almost could triple the length of tubing holding one cup of water if I would be allowed to use potable water system PEX tubing.

No such luck! I can dream about the associated energy savings, but I cannot tap into them.

Less is more

We established that low flow fixtures are a good water conservation strategy. The less hot water is used (the less water there is to heat up) the more energy is conserved. Low flow fixtures are thus an excellent energy saver.

Hidden in all this is a potential conflict with our water conservation goals. How so? Low flow fixtures can result in a longer time-to-tap. Because we reduce the flow rate, the hot water will take longer to arrive at the point of use. As a result we waste clean, cold water down the drain, while waiting for the hot water to arrive.

The solution to this problem brings us back to the material conservation strategies – to smaller pipe sizes. We established that we can reduce the pipe sizes in a plumbing system that uses low-flow fixtures. Let’s put it this way:

lower flow = smaller water volume to deliver = smaller pipe sizing

As a result, we reduce the probability of a longer wait for hot water as the pipe size is proportionate to the low flow rate. We also reduce energy loss during the cool down phase, as mentioned above.
Water is only wasted in plumbing systems with low flow fixtures and no decrease in the pipe sizes.

There is another good argument for smaller tubing in efficient plumbing systems: No one that I know likes to wait for hot water, which gets us into what we call behavioral waste.

This will be a nice topic for the next post.

About Marcus de la fleur

Marcus is a Registered Landscape Architect with a horticultural degree from the School of Horticulture at the Royal Botanic Gardens, Kew, and a Masters in Landscape Architecture from the University of Sheffield, UK. He developed a landscape based sustainable pilot project at 168 Elm Ave. in 2002, and has expanded his skill set to building science. Starting in 2009, Marcus applied the newly acquired expertise to the deep energy retrofit of his 100+ year old home in Chicago.

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