Insulating Your Hot Water Pipes can Save
Money and Protect the Environment
From the School of Hard Knocks. Last updated January 11, 2011
Hot Water Heat--The Big Picture
Hot Water Tank Heat Loss
Should You Shut Off Your Hot Water Tank When on Vacation?
How Quickly Does Water Cool in the Pipes?
Newton's Law of Cooling
Rerouting and insulating the hot water pipes in a house can provide multiple benefits, including:
Better quality hot water:
This web page first discusses the rerouting of pipes, then covers the basics of selecting and installing pipe insulation. Additional web pages provide calculations on how fast hot water cools down in pipes, hot water tank heat loss and other topics.
First, decide whether you should reroute any pipes
The plumbers who installed the hot water pipes in our house did not take the time to figure out the most efficient routing. The original routing from the hot water tank in the basement to the kitchen sink took a "U"-shaped path that was 43 feet long, even though the hot water tank is relatively close to the sink!
By rerouting we shortened the pipe to 27 feet, a reduction of 16 feet. At the same time, we downsized much of the pipe from 3/4 inch to 1/2 inch in diameter. Downsizing to 1/2 inch did not cause a noticeable pressure drop at the kitchen faucet, as that pipe only goes to the kitchen faucet and dishwasher.
The combination of a shorter length of pipe and smaller diameter pipe meant that instead of taking three quarters of a gallon of hot water to reach the tap, now it takes only about three-tenths of a gallon. This may not sound like much of an improvement, but with thousands of uses over the course of a year it adds up. It isn’t worth shortening pipe by a couple of feet, but on some houses (like ours) the length can be reduced by much more than that.
Next, decide on the type of insulation
There are five basic types of insulation. The following discussion describes advantages and disadvantages of each. My guiding philosophy has been that as long as I’m investing the labor of installing pipe insulation I might as well spend a couple of extra dollars to get the best possible performance.
In the following discussion I use the term R-value. This is a measure of how well insulation keeps in heat. The higher the R-value, the better. Wall insulation in a typical American home is R-13. The insulation in the walls of our current hot water tank is R-16. In contrast, pipe insulation tends to have a much lower R-value--on the order of 2. I'll talk about how to do much better than R-2.
1) Spiral-wrap fiberglass insulation
Although this is the least expensive option, I would recommend against it because it has the lowest R-value (Frost King lists the R-value as 1.6 on the package) and requires the greatest labor to install.
2) Foam tubing with no sticky-strip
In most hardware stores you can buy foam tube pipe insulation for various sized water pipes. The foam tubes have a slit down the side so you can easily fit them over pipes. Once you install the insulation you may want to tape some or all of the seams, as they tend to pop open. Some pipe insulation has a sticky strip on the slit that allows you to easily seal it. I’ll discuss that as the third option.
Most foam pipe insulation sold at hardware stores has 3/8” thick walls, which yields an R-value of a little over 2. Frost King and Tundra also make a heavy-duty pipe insulation with 5/8” thick walls and an R-value around 4. This is well worth the extra cost. See Table 1.
Table 1: Frost King brand insulation
I've found R-4 insulation for 1/2 inch pipes in some Ace hardware stores under the brand name Tundra. The Tundra brand has tan-colored insulation as shown in the picture above. Check to be sure the walls are 5/8 inch thick.
I have never been able to find R-4 insulation for 3/4 inch pipes in stores, but have ordered
it on the Internet from:
What temperatures will this foam insulation withstand?
The package of Frost King P11 insulation gives a temperature range from -94F to +210F. The temperature of most home hot water is in the range 115F to 140F. (If the temperature is set to 140F or higher, you need to think about turning it down to reduce the risk of scalding.)
3) Self-sealing foam tubing
This type of foam tube pipe insulation is fast to install because there is no need to tape it. Once you push the insulation onto the pipe you can peel off two plastic strips and the pipe insulation seals itself shut with a special glue. This is great for commercial installers where time is money.
I have seen two basic types of self-sealing insulation at hardware stores. One is a less expensive R-2 insulation intended for hot water pipes. The second is a more expensive R-3 insulation that is very flexible. This type is designed for use on the pipes associated with heat pumps and air conditioners, but there is no reason you couldn't use it on hot water pipes as well.
Although some people prefer the self-sealing type, I have avoided it for two reasons. First, I make a lot of mistakes and don't trust myself to do it right the first time. Second, I haven't been able to find a source of R-4 self-sealing insulation available to homeowners.
4) Fiberglass shell pipe covers
Unlike the spiral-wrap fiberglass insulation, these are hard tubular shells of fiberglass with a white paper covering that has a foil reflector on the inside (see picture). They hinge open to go around the pipe and have a sticky strip for easy sealing. I have found these at Home Depot, made by Thermwell.
This type of insulation is intended more for high temperature pipes (like steam pipes) where the heat might damage foam-type insulation. Although this type of insulation can be used for lower temperature pipes it is more expensive than the foam insulation. The R-value was not marked on the store display, so I called Thermwell directly. They said the fiberglass covers with 1/2 inch thick insulation have an R-value of about 2.
I have sometimes put these over foam pipe insulation as a way of boosting the R-value beyond what the foam alone offers.
5) Regular fiberglass attic insulation
The previous four options for pipe insulation have an R-value of at most 4.2. The R-value is actually much lower than this in use, as shown on the web page "How Quickly Does Water Cool Down in the Pipes?" It is possible to jury-rig a higher R-value pipe insulation out of regular attic insulation. This is particularly useful on the vertical section of pipe coming out of the water heater, as this is where much of the heat loss occurs. If you can keep the water toasty in that vertical pipe it is like moving the water heater closer to all your faucets. I discuss the details of how to do this below.
And the Winner is...
So what option(s) should you choose? I feel that the best option is a combination of R-4 foam insulation enhanced by attic insulation where appropriate. Particularly if you are concerned about carbon emissions from burning fossil fuels, you should go for the highest possible R-value.
To install foam pipe insulation you will need:
Knife: The right cutting tool makes this job go much easier, especially if you are insulating more than a few feet of pipe. I recommend against a utility knife--the blade is too short so it makes a ragged cut and I feel that the lack of control makes it dangerous for this application. Try experimenting with kitchen knives on a wooden cutting surface. My favorite knife is a bread knife with small serations, shown below. This knife cuts cleanly with great control. It's perfect for cutting fiberglass insulation as well. (My wife made me buy a separate one for insulation work--it's a poor job that won't support buying at least one new tool.) I made a cardboard scabbard so the knife can't be dulled in my toolchest.
Tape: Never use masking tape or electrical tape to tape the seams. Masking tape becomes brittle and loses its adhesion over time, and electrical tape doesn't stick very well in the first place.
I have used duct tape to seal seams in the past. The key question is how long it lasts. I used duct tape to seal pipe insulation on our old house sometime before 1995, probably around 1993. I recently looked at that pipe insulation and found that the duct tape was still holding very well. No more that an eighth of an inch had released on the edges. That isn't bad for about 15 years installed.
Home supply stores sell different grades of duct tape. My inclination is to buy a better grade for this application so I don't have to come back and do it over later. I would welcome suggestions from readers on this subject.
My experience is exclusively with electric water heaters. I'm not sure how hot the chimney pipe of a natural gas water heater can become. In any insulation project, be sure you are not creating a fire hazard by insulating something that was not meant to be insulated, or by installing flammable insulation near a high heat source.
It is also important not to obstruct the pressure relief valve. On our particular water heater the pressure relief valve is mounted on the top, but it could also be mounted on the side. The pressure relief valve is a couple of inches high, has a small lever on top of the valve, and has an overflow pipe coming out the side and down to the floor.
The pressure relief valve is installed to keep your water heater from exploding. Yes, you read that correctly! If the thermostat inside the tank malfunctioned and failed to shut off the heating elements, then they could heat the water in the tank to boiling, forming steam and enough pressure to explode the tank with considerable force. If you don't believe me (or even if you do), check out the Mythbusters YouTube video on water heaters. It is very impressive. The pressure relief valve is there to relieve pressure before the tank blows.
The pressure relief valve on our tank is designed to release water from the tank if the water temperature exceeds 210F or if the pressure exceeds 150 psi.
Installing pipe insulation is unlikely to obstruct the pressure relief valve, but it is good to understand what the pressure relief valve is and why it is there so you don't inadvertantly compromise it.
Where to Start Insulating:
The vertical section of pipe from the water heater tank is a good place to start insulating. This pipe tends to stay warm as hot water rises by natural convection up the pipe. Insulating this pipe reduces standby heat loss 24 hours a day and keeps the water in this section of pipe hotter, which reduces the time needed to get hot water to the tap.
How much can insulating the vertical pipe help keep the water warm? I tried to get a handle on this by attaching a temperature probe near the top of the vertical pipe coming from our water heater. I then measured the temperature of the outside of the pipe (under the insulation) and compared that to the air temperature. I did this early in the morning before anyone had run any water. R-4 insulation buys an almost 10 degree higher temperature than no insulation even after a night of cooling down.
Figures Above: Two views of R-2 insulation on vertical output pipe. I used masking tape as a temporary seal. Observe where the probe wire goes underneath the insulation on the right-hand picture. The digital thermometer can read both ambient air temperature and the probe temperature.
Here are typical results:
Table 2: Typical Temperatures at Top of Vertical Pipe, Early Morning.
So the outside of the vertical pipe is about 5 degrees warmer than the ambient air with no insulation, 10 degrees warmer with R-2 insulation, and 13 degrees warmer with R-4 insulation. The water inside the pipe should be a little warmer than this.
Insulating 90 Degree Turns:
Insulating 90 degree elbows is relatively easy for copper pipe, at least in principle. First cut a 45 degree angle in the pipe insulation coming from one direction, then a 45 degree angle in the pipe insulation coming from the other direction, then fit them together.
There are two practical problems with this. The first is measuring where to cut. The easiest approach may be to start with a piece of pipe insulation that is too long, cut a 45 degree angle at one end, then fit it over the pipe at the 45 degree end and mark how short to cut it at the other end.
The second problem is cutting a 45 degree angle, which is hard to do freehand. I saw a neat trick for making a jig for 45 degree cuts. This was from the "Handyman Club of America" in a magazine. First find a piece of PVC pipe that the insulation can slip into. Then use a miter box or miter saw to cut a 45 degree angle at the end of the PVC pipe. Then when you want to cut the insulation, slide it into that PVC pipe and slice off the end using the PVC pipe as a guide. I have found that 2" diameter PVC pipe can accomodate R-4 pipe insulation for 1/2" diameter pipes, but is a little snug for R-4 insulation for 3/4" pipes.
Insulating 90 degree elbows is much harder for CPVC pipe because the CPVC elbow connectors are much larger than the pipe itself. Whenever I try a 45 degree cut in the pipe insulation the seam opens up a good bit so I cannot get a good seal. I have tried going to the next size pipe insulation (using 3/4" pipe insulation at the elbows of 1/2" pipes), but those are way too big so are loose. I have reverted to just wrapping the joints with fiberglass as best I can. It's not worth spending too much time fussing over the joints.
Superinsulating the Vertical Pipe from the Water Heater:
At some point I will try augmenting the R-4 pipe insulation on the vertical pipe that comes from the water heater with fiberglass attic insulation. I will use leftover fiberglass insulation for this job, as it isn't worth buying an entire pack of R-30 or R-19 insulation to insulate 5 feet of water pipe. I have some leftover batts of 2'x4' R-19 fiberglass insulation.
The plan is to cut out blocks of say 1 foot by 1 foot, drill a hole in the center of each with a hole saw, then make a slit from the outside to the hole so I can slip the pieces over the pipe. Each piece fits on like a collar and they stack on top of each other. I was skeptical about the hole saw idea at first--I figured the insulation would wrap around the hole saw like cotton candy. But my son compressed the insulation while I drilled and we had no problem. Choose a hole saw that is a little smaller diameter than the outer diameter of the foam pipe insulation. If the hole is too large, then you get a gap between the foam insulation and the fiberglass and this acts like a chimney--hot air will rise up this chimney, taking away heat. So far I have only tried one block.
Hot Water Usage Habits:
Just because you insulate your hot water pipes does not automatically mean you will save money on hot water costs. There is a strong tendency for people to subconsciously think "Gee, my hot water system is better insulated, so now I can use more!"
To really achieve savings it is important for those living in the house to examine their hot water usage habits, and to figure out ways to economize. For example, when doing dishes at the sink, it is important to not leave the hot water running while placing a pot in the drain rack and reaching for another pot to wash. The hot water should be turned off as soon as you are done using it, and then turned back on when you need it again. Even 3 second periods of running the water when not needed will add up to a significant usage over time.
So how much heat is flowing out the faucet when it is turned on? We can compute this with a little high school physics. We will reason this through in stages.
First, how many btus does it take to heat a gallon of water from the temperature of the water coming in (Ti) to the temperature of the water flowing out (To)? The formula is as follows:
Q = mc(To -Ti)
One gallon of water weighs 8.337 pounds.
The specific heat of water is 1.0 btu per pound per degree F.
Assume the water coming into the water heater is 55 F and the water coming out is 120 F.
Plugging in these numbers we get:
Q = 8.337*1.0*(120 -55) = 542 Btu/gal
Next, using a bucket we measured the water flow at the kitchen tap as 2 gallons in 87 seconds. We can compute the flow rate in gallons per second as:
F = 2 gal/(87 sec) = 0.023 gal/sec
Finally, multiplying the number of Btus per gallon times the flow rate we get:
542 Btu/gal*0.023 gal/sec = 12.46 Btu/sec
There are 1055 Joules per Btu, which means that the number of Joules per second is:
12.46 Btu/sec*1055 J/Btu = 13,143 J/sec
But a Joule/second is just a Watt, so this amounts to 13,143 Watts!
This is a huge amount of power. For perspective, this is equivalent to turning on 131 100-watt incandescant lightbulbs at once, or 220 60-watt bulbs. Here is another perspective on just how much power this represents. Do you own a portable electric space heater? Those are typically 1,500 watts. Every time we turn on our hot water at the kitchen sink it is equivalent to turning on almost 9 of those portable electric space heaters at once.
The water heater does not supply this power instantaneously as the water is flowing out, but heats the water more slowly over time and stores it in the tank. The heating element in the tank is only 4,500 watts. So the tank heater must run for about 3 seconds to supply every second worth of water flowing out at the kitchen sink. Nevertheless, 13,143 watts is what we are essentially demanding from the electric utility every time we turn on the hot water at the kitchen sink.
You can e-mail me at support(the @ sign goes here)leaningpinesoftware.com.