Published Saturday January 19, 2013:  Updated 9/30/2013

Hot Water Pipe Insulation

Wouldn’t it be nice to have instant hot-water available at every tap in the house?

In our home, I want to have the security, affordability and convenience of a conventional hot-water tank but still be able to have instant-hot water at each tap.  

As hot water from the water heater travels through the plumbing, some of its heat and energy is lost into the pipe itself and the surrounding space. After running the water for a while, the hot water heats up the plumbing, the ambient water is purged and eventually, hot water is available for use at the faucet.  

Immediately after using the faucet, the water still in the pipes begins to cool. This is caused by thermal conduction from the pipe into the much cooler surroundings.
Regardless of the type of water heater, there will always be some heat wasted in the pipes.  

By adding pipe insulation, the energy wasted by the cooling of the water still in the pipe can be slowed down.  The inconvenience of waiting for more hot water is reduced.  

With perfect pipe insulation, the hot water in a pipe could remain hot indefinitely. If money were not an issue, using Aerogel insulation would keep the water in the pipes hot for weeks. But let’s be realistic. While my labor (at home) is cheap, my budget for this endeavor is only $50. Let’s see how far I can take this with conventional pipe insulation.

 Insulation MaterialEnergy lost (English)  Energy Lost (metric)
 Bare copper Pipe 200 BTU/hr-ft 192.3 watts/hr-meter
 Foam insulated pipe  20 BTU/hr-ft 19.3 watts/hr-meter
 Dynamic Vacuum Pipe  4.0 BTU/hr-ft 3.9 watts/hr-meter
 Cryogenic VJ Pipe  0.45 BTU/hr-ft  0.43 watts/hr-meter
 Aerogel   (R-105) 0.004 watts/hr-meter

Insulating Pipes (Single layer)

The short answer: Pipe insulation is a worthwhile venture. I spent $18 in materials.

Our water heater is located in the center of the basement and the plumbing branches out in either direction. The kitchen on one side and bathrooms on the other. I added pipe insulation to just the hot water pipes on the bathroom side of the house. The plumbing is no longer accessible on the kitchen side.  

I should have done measurements before the insulation was added but I forgot. Instead I compared the performance of the insulated bathroom section against the un-insulated kitchen section. Both runs are nearly identical in length and water volume.  

Before, insulating the pipes, the water would get uncomfortably cold (for washing your face) within 30 minutes. After that, purging the pipes with new hot water was necessary.  To make matters worse, most of the master bathroom plumbing runs along an exterior wall. The cool-down time is very short in the winter time.  

After insulating the hot-water pipe, the water stays warm enough to comfortably wash your face with it, (even 1 hour 20 minutes later).

Insulating the hot-water pipes made a big difference in comfort. If adding insulation made a good improvement, then perhaps adding more insulation will be even better.  

Double Insulating Plumbing pipes: The results are in, sort of.

On January 1, 2013, I added a 2nd layer of pipe insulation to our hot-water pipes. This 2nd layer was made out of 2 pieces of ½” foam pipe insulation wrapped around the single 1st layer of 3/4" inner diameter foam insulation. Each piece formed a half circle around the 1st layer of insulation. One of the foam pieces had adhesive edges and that would stick to the 2nd piece, forming an air seal.  

I also added metal foil tape every foot to keep it all together. They sell larger diameter insulation for pipes but buying small stuff and doubling it up is super cheap. 

It was kind of labor intensive and I wasn’t able to directly access as much pipe as I had hoped.  But at the cost of not watching TV that night, I got it all done.  There is a 6’ section of pipe that runs along an outside wall that is completely blocked by a HVAC duct. On the first layer, I was able to slide a piece of insulation over the pipe and shove it along the pipe behind the duct work. On the 2nd layer, I couldn’t do this since there wasn’t enough space and the 2nd layer of insulation wouldn’t slide over the first layer of insulation.  I was able to reach all the other sections of the hot-water pipes. 

This 2nd layer (green plot) definitely improved things but not as much as I had hoped. Granted it was 12 degrees F outside at the time I conducted this 2nd test. Colder temperatures probably made it perform much worse than when I ran tests late at night on November 14th when it was around 39 degrees outside. Even after running the faucet for a few minutes, the water wasn’t as hot as it was the first time I conducted the test. I added a corrected plot (purple) normalizing the data sets.  It shows there was further improvement.  

In any case, the cool-down time has more than doubled from the original, un-insulated pipe.  

By The Way:  

Here’s a bit of info about Kaysville city tap-water temperatures (most, if not all of the culinary water comes from snow pack in the Wasatch Mountains).  It never crossed my mind before but it is obvious if you think about it - the temperature of tap water will vary greatly throughout the year.

 Date Water Temperature (at our house)
 July 24, 2012 66 degrees F
 November 14, 2012 61 degrees F
 January 18, 2013 48 degrees F

Tap water is 18 degrees F cooler in mid January than in July.  It takes more energy in the winter time to heat water to 120 degrees F than in the summer time.  Not only that, people (myself included) tend to take longer showers in the winter because it’s colder outside and you don’t want to get out as readily. 

In a 100% efficient water heater (gas ones are only around 55%), it would take 131.6 watt-hours to heat up 1 gallon of summer tap water. It would take 175.2 watt-hours to heat up 1 gallon of winter tap water.

 Water Heating Summer Winter
 1BTU will raise 1 lb of water by 1 degree F theory  theory 
 1 gallon = 8.3 lbs Temp F Temp F
 cold water 66 48
 hot water 120 120
 delta (degrees F) 54 72
 BTU to heat 1 gallon 448.2 597.6
 Watt-hours to heat 1 gallon 131.36 175.15

That’s a 25% increase in energy usage in the winter time.

I could drive a mile in the Geo EV on just over a gallon of winter hot water energy.  Hmmmm, I need to fix that drip in the shower.  

Water Heater insulation

While we are on the topic of plumbing insulation, another improvement that is very easy and inexpensive to do is add an insulating blanket around your water heater.  Just make sure you don't block off the intake and exhaust air paths on gas water heaters.  
My gas water-heater is 6-years old.  Back in April 2012, I bought a $20 water heater insulation blanket kit from Home Depot.  

Depending on the season of the year, it has saved between 8 and 28 therms of natural gas each month.  Even with Utah having some of the cheapest natural gas in the nation, this wrap still paid for itself in under 3 months. 

Water Heater Standby Losses
Out of curiosity one night, I took a couple pictures of my gas meter exactly 1-hour apart.  

According to the lower 2 dials, (1/2 cubic foot and 2 cubic feet), I used 0.54 * 2 = 1.08 cubic feet of gas. The only gas appliance that was running this whole time was the pilot light on the water heater. 
In 1 month's time, that pilot light alone will burn through 777.6 cubic feet or 7.09 therms of gas. 
That is the equivalent energy to using 207kWh per month (6.9kWh per day) of electricity. Or put another way, it's like 289 watts running continuously. That's not very energy efficient.  This is one of the advantages of on-demand water heaters, no stand-by losses.  

Electric water heaters can benefit even more by adding insulation. Because you don't have to worry about blocking off the combustion air flow, more insulation can be wrapped around them.  
A friend of mine who like me, obsesses over such things, wrapped his electric water heater with 3-layers of 1/2" foam insulation followed by a layer of Reflectix.  His water-heater standby losses went from 1.4kWh per day to 0.6 kWh per day.  That's only the equivalent energy of a 25 watt light bulb running continuously, or 11 times less stand-by losses than a gas pilot light.