Slow Hot Water

Today I would like to explore the topic of "Slow Hot Water".

First lets get something straight... there is no such thing as "Instant hot water" if you are referring to heating it! Even though you occasionally hear the terms "Instant water heater", when referring to tankless water heaters, technically there is no such thing. It takes time to heat water. More on that later.(maybe even my next blog actually)

Lets define what we mean by "Slow Hot Water". For our purposes slow hot water will refer to the speed at which hot water is delivered to the fixture where you are trying to obtain it, from the time you turn on the faucet.

For some of you the there is no wait... if you have a hot water recirculating pump or system for instance. A recirc pump that keeps the hot water pipe full of hot water eliminating slow hot water from your vocabulary. It's nice but expensive.

Some people will have a short wait... say less than 30 seconds, although if you are waiting for hot water 30 seconds can seem like 30 minutes, all the while running precious drinking water down the drain. Some people will have a moderate wait... say more than 30 seconds and less than 1 minute. An many have a long wait...over 1 minute. Running water down the drain for one minute will result in a waste of between 1 and 3 gallons or more, depending on the fixture flow rate.

Many of our customers have described waits of over 5 minutes! Wow. That is slow hot water!

What are the causes of slow hot water?

The first thing one thinks of is the distance from the water heater to the fixture. That can give you a rough idea, but can be very inaccurate. There are a number of reasons that the straight-line distance from your water heater to your hot water fixture is inaccurate.

Plumbing Layouts

Your actual plumbing layout plays a major role of course in the speed at which you get your hot water. The actual pipe length will depend on whether your plumbing is located in an Attic, under a crawl space, in the walls, or under a slab.

Another factor is what type of piping material is used, soft copper, hard copper, galvanized pipe, or plastic pipe? Since some of the hot water cools off from contact with the pipe as it travels to the fixture, the pipe material and weight are important variables as well in how long it takes.

Your plumbing layout quite possibly does not follow a straight line route. Most copper and galvanized pipe residential plumbing systems follow the joists and beams and make sharp right angles to get from point A to point B.

With these things in mind lets take a look at the plumbing layout for a house with the piping primarily in the crawl space. The water heater is a gas model on a stand 24" off the floor of the garage. The pipe will begin as 3/4 inch diameter hard copper pipe, and transition to smaller branch lines of 1/2 inch dia.

The pipe would exit the top of the water heater, using right angle elbows it would extend up a foot or so, and run down the side of the heater and into the crawl space below the floor. By this time you have already used 6 to 8 feet of pipe and you haven't yet started the journey to the fixture. That journey can consist of a variety of changes in direction through the use of elbows and T fittings, and the pipe diameter can be reduced to the 1/2 inch dia. for branch runs from the major hot water feed to the individual fixtures.

The diameter of the pipe is chosen to provide the minimum flow rates for the various fixtures. The flow rate is different for different types of uses. For instance, a kitchen sink fixture requires a higher flow rate than a bathroom faucet, but not as much as a bathtub fixture.

By not using a plumbing layout with a straight run from the heater to the fixture, 15 or twenty extra feet can be added to the run if it traverses several rooms. So the pipe length may very well be substantially longer than is at first apparent. Every fitting also adds to the virtual length of the pipe. (Elbows and direction changes cause a larger pressure drop than straight pipe.)

Pipe Material and Pipe Insulation and Hot Water Tunneling

As the heated water flows through the pipe, it gives up some of its heat to the pipe material. As much as 50% of the wait time is caused by the hot water being cooled off by the cold pipe as it travels through the plumbing. To make things even more interesting, the actual speed of the water through the pipe affects how much heat gets transferred. When the water is traveling at higher velocities it gives up less than the expected amount of heat because most of the HOT water is traveling through a small channel through the not-flowing cooled off hot water in the pipe.

The hot water "tunnels" through the center. With slower flows the water moves more like a bullet nosed column of hot water pushing the cooled off hot water out ahead of it. Therefore, speeding up the hot water flow rate results in a even faster than expected arrival of hot water due to the tunneling effect.

Other interesting observations are that some plastic piping such as PEX can lose more heat than copper or galvanized pipe to the ambient air space around it; another unexpected result. Insulating the pipe reduces the heat loss from the pipe and as expected this time, can reduce the wait time for that slow hot water.

Pumping the same number of gallons per minute through a smaller diameter pipe causes an increase in the velocity of the water and a shorter wait time. However, since smaller pipes have larger pressure drops, an undersized pump motor could cause an actual increase in wait time if a pumping system is being used.

Saving Water with Low Flow Fixtures or Not!

Saving water is always a priority these days, but low flow fixtures can cause a huge waste of water. Many people have fixtures with flows of down to 1 gallon per minute. With flow rates that low, the water travels in the "bullet" mode and takes longer to reach the fixture, causing more water to be run down the drain.

Electric Tankless Water Heaters (and gas)

Tankless water heaters, especially the electric tankless units, take time to heat water. They are not instant. For the water to reach full temperature it must travel completely through the heat exchanger which takes time. An Australian government study stated that tankless water heaters typically took 10 to 20 seconds longer to get hot water to the fixtures than tank type storage water heaters.

Obviously adding 10 to 20 seconds to the wait time does not help the slow hot water problem.

So there you have it, a fairly comprehensive discussion of why the hot water is slow. You know what to do about it, get a Chilipepper pump of course. A must have accessory for any water heater storage or tankless, electric or gas, or even solar, oil burner, wood burner or other heater as well! Yes, even heat pump water heaters!

Until my next blog... Bill the Hot Water Guy