Slow Hot Water – What to do?

I got this email regarding slow hot water and thought I would use it and my answer as my blog for today, so here it is.

Hello,

My Dad wants to install new copper piping throughout his 2200 square-foot 2-story house. He is very concerned about 'slow hot water' and has decided he needs 1-1/2" insulation on the copper pipes instead of the standard 1/2" insulation.

He is calling around the country looking for that 1-1/2" insulation - and to Canada! He's found a plumber willing to install it for a higher price.
Do you have any studies that compare 1/2" insulation on copper pipes versus 1-1/2" insulation, and what the cost/benefits would be?

And also, I think your pump solution sounds great, so could you show a comparison between your pump solution with the standard 1/2" copper pipe insulation versus no pump and 1-1/2" copper pipe insulation?

He has a regular tank water heater. Does he need to swap out his water heater?

Thank you for your help.

My slow hot water answer

Typically, pipe insulation is used for energy conservation purposes although adding insulation will reduce the amount of time it takes to get your hot water slightly.

The speed of the hot water is most affected by the flow rate of the fixtures and the diameter of the piping.

The speed or velocity of the water is dependent on the flow in gallons per minute and the pipe diameter. The larger diameter the pipe the slower the velocity and the longer it takes to get hot water.

Generally speaking it takes about 45 feet of ¾” inch diameter pipe to hold a gallon of water and about 70 feet for ½” diameter pipe. With a flow rate of 1 gallon per minute, the water in the ¾” pipe will flow 45 feet in about 60 seconds, and for the ½” diameter pipe the water will flow 70 feet in that same 60 seconds.

Since the flow is restricted by the fixtures, it will take longer to get hot water with the larger diameter pipes.

To get hot water to the shower fast, turn on the water in the bathtub full blast, it has the highest flow rate of any bathroom fixtures!
The second most important factor is the amount of heat that is pulled out of the piping material as the water travels to the fixture. That is where the pipe insulation comes into play.

If you had perfect pipe insulation and no heat could leak from the pipes, you would always have instant hot water. But there is no such thing as perfect insulation, and the heat will leak out fairly rapidly no matter how much insulation you use.

Pipe insulation is normally only used when a continuous hot water circulating system is being incorporated. It doesn’t really save energy, since the hot water will cool off very rapidly even with heavy amounts of pipe insulation. The reason 1-1/2” insulation is so hard to find is that it is a waste of money.

I don’t know of any studies that involve 1-1/2” thick pipe insulation.

Slow hot water delivery time is also related to the water velocity in another way. When water velocities are slow, the hot water traveling through the pipe travels much like a bullet would through the pipe.

Higher velocities cause laminar flow where a smaller diameter flow of water travels through the center of the pipe and the water in contact with the pipe walls doesn’t flow, getting the hot water to its destination even more quickly.

Plumbing layout is very important.

If you are planning on using a pumping system then the plumbing layout should be looped from fixture to fixture instead of a layout with T connections and lots of branch runs. That way a single pump can supply fast hot water to all of the fixtures.

About the water heater

I presume you mean should he replace is tank water heater with a tankless water heater? If so then the answer is no. Tankless water heaters take longer to deliver hot water since they have to turn on and start heating the water while the tank type water heater has already hot water to start the journey.

Tankless water heaters typically take 10 to 20 seconds longer to deliver hot water than tank type heaters.

So in conclusion, unless you live in a very cold area using pipe insulation will have a very small affect on how slow or fast your hot water delivery is. Increasing the pipe insulation beyond ¾” thick will most likely have no measurable effect.

Smaller pipes will deliver the hot water faster than large diameter pipes for a given flow rate.

Demand pumps pump at a faster rate than typical flow rates available from the hot water fixtures.

For true "instant hot water" use a full time hot water recirculation system and insulate the pipes with at least 3/4" thick insulation.