Why there is not enough hot water is a problem many people face at one time or another. There is nothing worse than finding yourself standing in a shower all lathered up when the water temperature plummets. There are many possible reasons for running out of water ranging from sizing of the water heater to a mechanical or electrical problem.
When faced with not enough hot water you need to first examine whether there is a sufficient capacity of hot water available to meet your needs. A tank storage type water heater can be expected to deliver about 70% of the volume of the storage tank before the incoming cold water dilutes the outgoing hot water dropping the temperature 20-degrees F. Add to this figure the recovery rate at which your water heater can heat the incoming cold water and you will have an idea of your capacity. Tankless coils heat the water as you use it and they are rated in gallons per minute with what they can provide. Tankless coils can deliver their rated capacity endlessly but cannot exceed that capacity without a drop in temperature. Naturally both will be working harder in the winter when incoming water temperatures are much colder. An easy to determine method is that if the system has met your needs in the past and your hot water demand has not changed then something has gone wrong and needs to be fixed. If you are a new home owner then sizing may be part of the problem and should be looked at as well as possible problems that may be mechanical or, electrical. If you have a system which normally meets your needs but you have an unusual situation such as a large number of house guests you will need to determine if you should modify usage during that time or, go to a larger system to cover your needs during the unusually high usage.
Not enough hot water with tank type water heaters can be caused by a number of problems. A thermostat not functioning properly could be causing the temperature of the stored water in the tank to be lower than it should be. Typically the thermostat has a temperature, which it turns on the heating, and a temperature at which it turns off. This is called the differential and sometimes when a thermostat ages and fails when the tank slowly cools the thermostat does not always turn on when it should. People will often figure this problem out when they find if they just jump in the shower they run out of hot water, but if they use water first causing the water heater to reheat then shower they have enough. With electric water heaters a frequent cause of not enough hot water is a burned out lower element due to a build up of sediment in the bottom of the tank, which causes only the top half of the water heater to have hot water. Older gas water heaters may have a similar layer of sediment, which insulates the bottom of the tank from the flame slowing the recovery rate. Another problem that many older water heaters face that were built between August 1993 and October 1996 is they were built with a defective dip tube. Almost all the manufacturers used the dip tubes that had the defect and this resulted in a class action lawsuit, which is now past the settlement date. The dip tube in these water heaters disintegrated often showing up as a white-grey debris, which clogged aerators, the dip tubes function is to direct incoming cold water to the bottom of the tank. With a disintegrated dip tube incoming cold water mixes with and dilutes the outgoing hot water causing the water heater to provide cold water before its storage capacity should have been exhausted.
Tankless coils in boilers may provide not enough hot water as they age and become clogged with minerals from the water. With hard water the buildup is usually faster and you may be able to have the coil cleaned by a plumber who will flush the coil with a acid which breaks down the minerals. Sometimes the coil is beyond repair and in these cases installing an indirect water heater on a separate zone from your boiler is a great idea. Indirect water heaters can provide abundant amounts of hot water sometime several times the amount of a similar sized gas or, electric water heater.
Problems on the distribution side can also make it so you do not have enough hot water. Sometimes water heaters and tankless coils may have a tempering valve on them, which mixes cold water with the outgoing hot water to maintain a safe temperature and prevent scalding. This tempering valve can make it seem as though the hot water has run out. A failure that often happens with the tempering valve is that you have hot water when you run a kitchen or, lavatory faucet but when you use a larger flow fixture like a bath tub or, a clothes washer the water rapidly changes to cold or, never gets hot. Sometimes a defective cartridge on a faucet can cause the hot water and cold water to have a cross connection which can cause temperature problems. Moen and Mixet are two of the better-known valves which can cause a cross connection. Any faucet where both the hot and cold water can be turned on allowing a connection and have the outlet turned off can cause this problem. Sometimes too the problem can be as simple as a hot limit stop on a tub/shower mixer that was set when higher cold-water temperatures were present and with the change of seasons the limit needs to be reset. In homes that have slab construction sometimes there may be a leak under the slab that is always flowing hot water. This constant leakage may make you not have enough hot water. Symptoms of a slab leak can be a water heater running almost constantly or, a hot spot on the floor.
As you can see by the many possible causes this is a problem that requires a careful logical approach to the troubleshooting and often there are symptoms, which you can give the plumber, which can aid in the diagnosis of the problem. Sometimes too you may be able to find the problem yourself. In any case not enough hot water requires careful methodical troubleshooting to determine the exact cause and misdiagnosis can be quite costly. There is a big difference in cost between turning a screw on a hot limit stop and installing a new water heater or, repairing a leak under the slab.