Inconsistent water heat is an indication of water heater problems. Troubleshooting the heating issues takes a little knowledge of your water heater, but it is more inspection than actual disassembly to determine the problem with your inconsistent heat.
Step 1 - Safety First
Always ensure that you have shut off power to your electric water heater, or removed gas input flow to your gas water heater before performing any service or troubleshooting to your water heater. On a gas water heater you can turn your gas pilot control valve to the setting labeled pilot. Also, be sure you have turned off the water supply valve. No smoking around a gas water heater, and be sure to wear clothing that does not hang loosely.
Step 2 - Electric Water Heater
Electric water heaters that have water storage tanks are setup with 2 heating elements. One element is at the top of the unit and the other is on the bottom. If one of these elements is not working then the water will not heat. Putting your hand on the water heater unit will tell you whether an element is working or not. A trip to the breaker box will enable you to ensure that the breakers are both on.
Water heater elements work together on two breakers, one breaker for each element. Verify that both the breakers are set properly. It is not a bad idea to flip the breaker and re-set it. Also check to verify that the fuse in the system has not blown and that the thermostat for each element is working properly. If you notice that a consistently lukewarm water flow occurs when showering then the upper hot water element is likely the cause.
Step 3 - Gas Water Heater
Drain the water from the storage tank in the gas water heater into buckets. Compare the water drained into the buckets with the capacity of the water heater. If you have a 20 gallon tank and you produce 10 gallons of water when drained completely then the problem could be a calcium build-up. Ensure that you have adequate gas flowing through the pilot control valve.
Step 4 - Gas and Electric Water Heater
Hot water storage tanks are insulated. After a few years the insulation may be degrading. Also if your water heater is in an area of your home that is not temperature controlled, such as an external closet or a garage, the insulation may not be adequate to allow the hot water tank to hold temperature of the water. Keeping the heat contained in the water heater instead of dissipating can be determined by checking air temperature around the tank.
Check the water temperature gauge on the hot water heater and ensure that it is adequately set for the water usage in your home. Verify that you do not have any leaks in your fittings and also check all your faucets and water feed lines to ensure that you do not have a leaky faucet, a bad washer in the line, or even a leak into a dishwasher or washing machine.