Not enough hot Water

[jjacobs]

Our Hot Water tank seems to be big enough, but after about an 8 minute shower the hot water diminishes and then only colder water comes out. Is there a valve or something we can adjust to make the hot water last longer?

[LazyPup]

The cause of your cold showers is not so much the fault of the water heater as it is the manner in which most builders meet code compliance.

In an effort to insure that no one can be scalded in the tub or shower the plumbing codes require that the water temperature at the shower may be no more than 125degF therefore, in an effort to meet code where their is no temperature control at the shower mixer the simplest and cheapest solution is to set the water heater at a maximum of 125degF.

(Presently the codes are requiring that all shower mixers used in new construction must have an integral thermostatic temperature limiting feature to insure code compliance.)

Now, while this method will meet code compliance in regard to the maximum temperature it introduces a new problem. While the majority of users typically set their shower temperatures at about 85 to 95degF, a small percentage of users enjoy a "HOT" shower which ranges from 95 to 110degF and there are some people who actually take 120degF showers. The problem here is that while the water heater is full of 125degF water when your shower begins, in order to get the really hot showers you are using nearly pure hot water in your shower mix. This means that for each gallon of water your shower consumes a gallon of cold water is entering the water heater. As a result, the cold water entering the tank is diluting the Hot water stored in the tank at a rate faster than what the burner can make up the heat loss.

The Federal Government requires that all water heaters must have a yellow "Energy Guide" label on the exterior of the tank that shows the "First hour rating" of the water heater.

By Federal standard a water heater must deliver 70% of the tank capacity at the thermostate setting minus 20degF. What this means is that if the tank thermostate is set to the code maximum of 120degF the tank must be capable of delivering 70% of its volume between 120degF and 100degF, which is all well and good if you take 90degF showers, but if you are one of those individuals who enjoy 110degF showers you can easily see why your shower is cold.

Keep in mind that your shower is consuming 2.5gal/min therefore an 8 minute shower would consume 2.5gpm x 8min. = 20gal.

Under the Federal Energy guidelines a 40gal water heater set at 120degF would be required to supply 28gal. between 120degF and 100degF for the first hour.

While it is seldom done in a residential application there is a relatively cheap solution to the problem.

You could have a "Whole House Tempering Valve" installed on the output of the water heater which would limit the outgoing water temperature to the house distribution system to the code maximum of 120degF, then you could turn the water heater temperature up to 140degF or even 180degF. In this configuration the hot water leaving the water heater is blended with cold water to achieve the desired 120degF mix. This means that your 110degF shower is now only taking bout 65 to 70 percent of the demand from the water heater as opposed to the previous 100percent demand. By reducing the rate of demand you reduce the rate of cold water dilution in the tank.

Whole house tempering valves are typically about $25 to $50 and are a simple install at the water heater. This would not be beyond the skills of a moderate DIY'er with copper pipe soldering skills.

Years ago water heaters typically had rather poor insultation blankets and the consumers were encouraged to turn the tank temperatures down to diminish standby losses in the tanks, but the plumbing codes now require that all water heaters must have a minimum of R-12 insulation. Inorder to meet the new insulation standards the water heater manufacturers have discontinued the use of the 1/2" or 3/4" fiberglass blankets and replaced them with cast in foam insulations that are often providing as much as R-50 values, so standby losses are now considered minimal.

In addition, the codes now require that when we install a circulating loop for instant on hot water we must insulate all hot water lines, both in the supply and return loop. In some areas we are required to insulate all hot water lines whether we install the loop or not. (A practice which I personally would highly recommend).