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How often should you flush a water heater with Cambridge's hard water?
Manufacturers say to flush a tank water heater at least once a year, and more often in hard water. Cambridge water is among the hardest in Ontario, so for most homes here once a year is the minimum, not the target.
Last updated September 25, 2026 · Reviewed by the Cambridge Plumbers team
How hard is Cambridge water?
Most of the Region of Waterloo's drinking water comes from groundwater wells, with some supply from the Grand River. Groundwater picks up calcium and magnesium on its way through the ground, which is why the Region describes its water as hard and notes that it causes scale in pipes and appliances.
The Region's water softener information site lists Cambridge at roughly 20 to 36 grains per gallon depending on the part of the city, and Ayr at about 21. At those levels a tank water heater collects a layer of mineral sediment on the bottom of the tank every year.
What sediment does to a water heater
When water is heated, dissolved minerals drop out and settle. In a gas tank, that layer sits between the burner flame and the water, so the heater burns longer to deliver the same hot water. In an electric tank, scale builds on the lower element until it overheats and fails. Common signs:
- Rumbling, popping or crackling from the tank as it heats. That is water trapped under sediment flashing to steam.
- Hot water running out sooner than it used to.
- Gritty white or tan flakes in faucet aerators and showerheads.
- A drain valve that trickles or clogs when you open it.
- Repeated electric element failures.
How often to flush in Cambridge
A.O. Smith, a major tank manufacturer, says most tank water heaters should be flushed at least once a year and more often in hard water areas. For Cambridge:
- No softener: plan on every six months, especially in the harder parts of the city.
- Softener feeding the heater: once a year is usually enough, provided the softener is working and set to the right hardness.
- Tank already older than about eight years and never flushed: talk to a plumber before starting. See below.
Tankless heaters are different. They have no tank to flush, but the heat exchanger needs descaling with a cleaning solution circulated through isolation valves. That is a separate service from a tank flush.
How a flush is done safely
Following the manufacturer's steps:
- Turn off the heat source. On a gas tank, set the control to pilot or off. On an electric tank, switch off the breaker. Never run an electric heater with the tank empty; the elements burn out in seconds.
- Let the water cool. Tank water can be hot enough to scald. Wear thick gloves and eye protection.
- Close the cold water supply to the heater.
- Attach a hose to the drain valve and run it to a floor drain or outside.
- Open a hot tap upstairs so air can get in and the tank drains.
- Open the drain valve. When the tank is nearly empty, briefly open the cold supply to stir up sediment, and repeat until the water runs clear.
- Close the drain, refill the tank with the hot tap open until water flows steadily from it, then restore gas or power.
A.O. Smith also recommends checking the anode rod once a year and lifting the temperature and pressure relief valve lever annually to make sure it opens and reseats. In hard water the anode is consumed faster, and once it is gone the tank itself starts to corrode.
When not to flush it yourself
- The drain valve is plastic and the tank has never been flushed. Old plastic valves on heavily scaled tanks often clog open or break, leaving you with a tank that will not stop draining.
- The relief valve has not been opened in years. It may not reseat and will drip afterwards.
- The heater is a rental. Rental agreements usually restrict service to the rental company's technicians. Check your contract.
- It is a gas unit and you are unsure how to relight it. Gas work beyond the manufacturer's lighting instructions belongs to a licensed gas technician.
The plumber behind this site handles water heater repair and replacement, including power-vent and tankless units, with gas work done by a licensed gas technician. If a flush turns up a failed valve, a leaking tank or an anode that is gone, you get a straight answer on repair versus replacement. For planning numbers, see the Cambridge plumbing cost guide.
Signs the heater is past flushing
A flush will not fix everything. Water pooling under the tank, rust-coloured hot water, or a tank that still rumbles loudly after a thorough flush usually means the tank lining or bottom is failing. At that point the useful conversation is replacement: whether to stay with a power-vent tank, move to tankless, or go electric, and how to set the new heater up so Cambridge water does not shorten its life the same way.
Slowing the scale down
Flushing removes sediment that has already formed. To slow the build-up, make sure any softener is set to the hardness where you live. In Galt and Hespeler houses with older galvanized supply lines, rust flakes can add to the sediment, which is another reason to flush more often. If you are not sure where to start, book a visit and the plumber will look at the heater before naming a price.