Hard Water and Water Heaters: How Much Efficiency Are You Losing?
The key finding: The Water Quality Research Foundation found that a tank water heater operating in 20 GPG water (very hard) loses up to 29% efficiency within just a few years due to scale buildup. A 6mm layer of limescale on a heating element requires about 39% more energy to heat the same amount of water.
How Scale Forms in a Water Heater
When hard water is heated to typical water heater temperatures (120 to 140 degrees Fahrenheit), calcium bicarbonate converts to calcium carbonate, which is insoluble. This precipitates as white, chalky scale that adheres to the heating element and the bottom of the tank. Because calcium carbonate is a poor conductor of heat, scale acts as insulation, forcing the heating element to work harder and longer to heat the same amount of water.
The Real Annual Cost
| Water Hardness | Approximate Efficiency Loss | Annual Energy Cost Impact |
|---|---|---|
| Under 7 GPG (120 PPM) | Minimal (less than 5%) | Under $20/year |
| 7 to 14 GPG (120 to 240 PPM) | 10 to 18% | $40 to $90/year extra |
| 14 to 20 GPG (240 to 340 PPM) | 18 to 29% | $70 to $120/year extra |
| Above 20 GPG (340+ PPM) | Up to 29%+ | $120+/year extra |
Beyond energy cost, heavy scale reduces the effective capacity of the tank (scale takes up space) and significantly shortens the lifespan of both the heating element and the tank itself. A water heater that should last 15 years may fail after 8 to 10 years in a hard water area without proper maintenance.
How to Flush and Descale a Water Heater
How often: Flush annually in moderate hard water areas (120 to 180 PPM). Every 6 months in very hard water areas (above 180 PPM).
Supplies Needed
- Garden hose
- Flathead screwdriver
- Bucket
- Work gloves
Tank Water Heater Flush Steps
- Turn off the heater. For gas: turn the thermostat to Pilot. For electric: turn off the circuit breaker for the water heater.
- Let the water cool. Wait at least 1 to 2 hours. Hot water will scald. Do not skip this step.
- Connect a garden hose to the drain valve at the bottom of the tank. Run the hose to a floor drain or outside.
- Turn off the cold water supply to the water heater (the inlet valve near the top).
- Open a hot water faucet somewhere in the house to allow air into the system as the tank drains.
- Open the drain valve and let the tank drain completely. The water will be discolored and may contain sediment - this is normal.
- Flush the tank by briefly turning the cold water supply back on while the drain is still open. This agitates and flushes remaining sediment. Repeat until the water runs clear.
- Close the drain valve, remove the hose, close the hot water faucet you opened, turn the cold water supply back on, and allow the tank to refill completely.
- Turn the heater back on after the tank is full. For gas, relight the pilot if necessary. For electric, turn the circuit breaker back on.
Before descaling: If your water heater is more than 10 years old and has never been serviced, consult a plumber before attempting a full flush. Very old tanks with heavy sediment can have drain valves that seize or crack. A plumber can assess and handle this safely.
Preventing Future Scale
The best long-term solution for water heater scale is a whole-house water softener. Soft water does not form scale. A family that installs a water softener will extend the lifespan of their water heater, reduce annual energy bills, and eliminate the need for annual descaling. The payback period from reduced energy costs and extended appliance lifespan is typically 5 to 7 years for most US households in hard water areas.
Yes, for significant scale buildup inside the tank, you can use a citric acid or diluted white vinegar flush. After draining the tank (per steps above), close the drain valve, add several gallons of white vinegar through the cold water inlet with the supply valve open just slightly, let sit for several hours, then flush completely with fresh water before refilling. This is more intensive than a regular flush but removes stubborn scale deposits. For very heavy scale, a professional plumber with descaling equipment is more effective.
Tankless water heaters also accumulate scale on the heat exchanger but tend to be easier to descale because they have a smaller and more accessible heat exchange surface. Many tankless manufacturers recommend annual descaling with citric acid solution in hard water areas. The procedure involves running a descaling solution through the heat exchanger using a small pump and bucket - most manufacturers provide instructions. Scale is still a problem but generally more manageable than with tank heaters.
Last updated: April 2026