Lime Scale vs. Hot Water Temperature & Usage

By Luke Wonnell

Lime scale can be a serious menace to domestic hot water systems.  As lime scale builds up in your system, it will restrict flow through piping, it will reduce the operating efficiency of water heaters, and it can most certainly bring your hot water system to its knees if left unchecked.  For every gallon of hot water that is used in a faucet or shower, one gallon of cold water must enter the system.  This cold water has a certain hardness, which defines the amount of minerals like Calcium entrapped in the water and is expressed as “Grains per Gallon” (gpg).  The higher the gpg, the harder your water.

The image below shows lime scale (Calcium Carbonate CaCO3) under a microscope… notice the jagged, crystalline structure.  Once lime scale starts to form in your system, it’s like a domino-effect, it tends to continue building on itself as shown in the cross-section image of the piping below – yikes!

Fortunately, our friends over at the American Society of Plumbing Engineers (ASPE) compiled an amazing chart which shows the effects of increasing water usage and water temperature on the amount of lime scale that is deposited in your hot water system every year and is based on water hardness of 10 grains per gallon – see below:


As you move along the horizontal x-axis, this shows the daily water usage for a particular building.  Now, as you move up along the different diagonal lines, these represent the temperature of the water circulating through the building.  When you’re at or below 140⁰F, you’re in the safe zone (green) where you have a manageable and treatable amount of lime scale (<30 lbs.) deposited each year.

Once you reach 150⁰F (yellow), you’ll notice there’s an exponential increase in the amount of lime scale for every 10⁰F increase in the water temperature.  For 99% of multi-family buildings, there’s no good reason why you ever need to produce hot water above 140⁰F.  If you find yourself having to crank up your water heater to 160⁰F to make sure your residents get enough hot water, there’s a good chance your domestic water piping is scaling up, which makes it more difficult for the hot water from the heater to flow to the resident’s faucets or showers.

Try to keep your water temperature below 150⁰F to minimize the amount of annual scale buildup in your system.  Contact your Account Manager today to learn about all the solutions The Metro Group can offer for cleaning and de-scaling your domestic hot water system!


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