A lot of people complain that it takes a long time to get hot water at one or more of their hot water fixtures. In my case it’s the kitchen sink. I’ve measured it many times, and it takes two and a half minutes for hot water to arrive there. While I’m waiting for the hot water, 3 gallons of water go down the drain. Then after I turn off the water, another 3 gallons of hot water is left stranded in the pipes between the kitchen and the water heater. So I waste water on the front end, energy on the back end, and spend time waiting that I’d rather use doing something else. Does it really need to be so bad?
The basic problem
When you think it through, it’s easy to see that hot water distribution suffers because of one basic problem. When three gallons of water spill out of my kitchen faucet while I wait for hot water, that tells me something about the volume in the hot water line between the water heater and the kitchen faucet. To waste less water, I need less water to be sitting in that pipe. That’s it. That’s the problem.
The actual amount of wasted water, however, is often greater than the water sitting in the pipes. That’s the structural waste. But when hot water flows through pipes that are too large, it doesn’t just push everything in front of it until the all of the structural waste is out of the pipe and the hot water arrives at the tap. The hot water mixes with the water already in the pipe and sometimes rides on top of the water in the pipe. That lengthens the time to get hot water as well as the amount of water wasted. (See Figure 2 in this Journal of Light Construction article by Gary Klein (pdf) for a visual.)
If you look at some of the plumbing company’s websites, they’ll give you the right answer…partly. But they add several other things, including sediment buildup, undersized water heater, and outdoor temperature, that have nothing to do with how fast the hot water arrives. The real reason it takes so long is the volume of water that has to be flushed from the pipe before hot water flows from the tap. To get less water in the pipe, you need shorter, smaller pipes.
Stay with me as I explain this. Yes, you can get hot water faster without changing out your hot water distribution system. I know that can be difficult and expensive in existing homes. But it’s possible to do some partial measures that can help.
The first part of the problem is excessive pipe lengths. That results from the design of a house and its plumbing system. The big reason is that the water heater and the wet rooms (i.e., the rooms where water is used) are too spread out. When you put a water heater in an attached garage, for example, and a bathroom on the opposite end of the house, there’s a whole lot of pipe for the hot water to traverse on its way to the showerhead. The solution is shorter pipes through designing a floor plan that clusters all the wet rooms in one part of the house…or at least a design that doesn’t spread them out as far as possible.
Another reason for excessive pipe lengths is the design of the water distribution system that connects the water heater to the fixtures. Hot water pipes rarely take the shortest distance from the water heater to a fixture. In my house, a pipe going straight from the water heater to the kitchen faucet would be about 40 feet long. The actual hot water pipe is about 80 feet long.
The second part of the problem is that the pipes are too big in diameter. Why are pipes too big? Because our plumbing fixtures have gotten more efficient while the way we design distribution systems hasn’t kept pace with fixture changes. Flow rates of plumbing fixtures are far lower than they used to be, especially since the 1992 Energy Policy Act went into effect. Showerheads, for example, went from flow rates of 3.5 or more gallons per minute (gpm) to 2.5 gpm, a drop of 37 percent or more. Dishwashers and washing machines cut their flow rates in half. When you cut the flow rate in half, you double the time it takes for hot water to reach the fixture.
The low flow fixtures we use now certainly save water. They even save hot water, but only after the hot water arrives and you’re actually using it. Without an efficient hot water distribution system, you still waste at least as much hot water as the high-flow fixtures while you’re waiting for it to arrive at the tap. We wasted water faster in the old days because that water sitting in the pipes got pushed out faster with higher flow rates.
Putting a low flow fixture at the tap hasn’t changed the volume of water in the line, the structural waste. It simply reduced the flow rate while leaving the same amount of water to push through before hot water can arrive. As mentioned earlier, though, the reduced flow rate may result in even more wasted water.
With low flow, the hot water doesn’t come rushing through the pipe, pushing everything in front of it all at once. That kind of displacement flow, or plug flow, doesn’t happen with low flow rates in larger pipes. The velocity is low enough that mixing occurs, which increases the time it takes to get hot water and the amount of wasted water.
The keys, then, are to decrease the amount of water sitting in the pipes and also size the pipe diameters for good flow patterns.
What can you do?
Most of the websites offering solutions to this problem online are from plumbing companies. Be wary of their advice because many of them want to sell you a tankless water heater or recirculation pump. Some even tell you to install fixtures with higher flow rates. That last one is really annoying because we’ve had low-flow fixtures for nearly 30 years now. Yes, the early low-flow fixtures weren’t as effective, but engineers have solved those problems. My kitchen faucet (shown above) works really well at 1.2 gallons per minute.
Here are a few solutions to the hot water delivery problem, starting with the most comprehensive.
- Reduce the length and diameter of your hot water pipes. This is easy and inexpensive if you’re designing a house. For existing homes, do what you can. If you have an opportunity to change the floor plan during a big renovation, cluster the wet rooms.*
- Move the water heater to a more central location. Again, easier to do in the design phase.
- Use two or more water heaters. If you’ve got one hot water fixture far from the water heater, consider a small water heater just for that use.
- Install a demand hot water recirculation system. Continuous recirculation can add a lot to your water heating costs. Putting a continuous recirculation system on a timer can help. Best is to use the demand-type reciruclation system. Push a button, wait a few minutes, and your hot water is ready to flow from the tap without wasting water in the meantime. The Building America Solutions Center has a good description of how demand hot water recirculation works.
When I remodel my basement next year, I’m going to replace the hot water distribution system. I’m not changing the floor plan, so I’ll be limited in how much I can reduce the pipe lengths, but I can certainly cut the pipe diameter significantly. Right now, I have a 3/4″ copper pipe running all the way to the kitchen. That’s the last fixture and a good 20 feet past the second-to-last fixture. I should be able to cut the pipe diameter to 3/8″ for the last section. Along with the little bit of length reduction I can get, I should be able to cut my wait time from 2.5 minutes to 30 seconds or less. That’s still too long, though, so I’ll also install a demand recirculation system.
Always try to address the real source of any problem. In the case of hot water taking too long to get to the tap, the primary problem is the volume of water in the pipes. Reducing that volume with shorter, smaller pipes is the best way to go. If that still doesn’t get your wait time down enough, use a demand hot water recirculation system.
Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and the founder of Energy Vanguard. He has a PhD in physics and writes the Energy Vanguard Blog. He is also writing a book on building science. You can follow him on Twitter at @EnergyVanguard.
* Caution: I’m giving general advice here. If you want to reduce the diameter of your plumbing pipes, it’s a good idea to pay attention to plumbing codes and, more important, physics. If you’re not a licensed plumber or building professional, find someone who can help you make the right decisions.
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