How to Flush a Water Heater
Twenty minutes a year. One of the few maintenance jobs that genuinely pays for itself.
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Flushing a storage tank takes about twenty minutes and should be done annually, or every six months in hard water. You need a garden hose and somewhere for the water to go. The three things that go wrong: forgetting to open a hot tap (the tank will not drain), a seized drain valve, and leaving the heat on while it is empty.
Sediment settles on the bottom of a storage tank. On a gas unit it sits between the burner and the water, so the burner has to drive heat through a layer of mineral deposit — which makes the steel above the burner run hotter than designed and reduces how much hot water you get in the first hour.
Flushing removes it. It is the second most valuable maintenance job on a tank, after checking the anode rod, and the two are worth doing on the same afternoon.
Before you start
- A garden hose that reaches a floor drain, a laundry standpipe, or outside.
- Somewhere for hot water to go. The first water out may be near the tank's set temperature. Do not run it onto a lawn you care about or into a plastic container that will deform.
- A bucket, in case the hose connection weeps.
- Time. Two hours total if you let it cool properly first, twenty minutes of which is actual work.
The procedure
Turn off the heat
Gas: set the control to Pilot or Off. Electric: switch the breaker off. Do not skip this. An element energized in an empty tank is destroyed within seconds.
Close the cold water inlet
The valve on the cold pipe entering the top of the tank. Know where it is before you need it.
Let it cool, at least partly
An hour or two makes the job pleasant rather than hazardous. If you are in a hurry, run a hot tap elsewhere in the house for several minutes to pull some heat out first.
Connect the hose and open a hot tap
Hose onto the drain valve at the bottom, routed to the drain. Then open a hot tap somewhere in the house — this breaks the vacuum. Without it the tank will barely drain and people assume the valve is blocked.
Open the drain valve
Expect discolored water and grit. Let it run until it comes out clear, which on a neglected tank can take several minutes and a lot of water.
Stir up the remaining sediment
With the drain still open, briefly open the cold inlet to let a surge of water swirl the tank bottom, then close it again. Repeat two or three times until the discharge stays clear.
Close the drain, refill, purge air
Close the drain valve, remove the hose, open the cold inlet, and leave the hot tap open until water runs steadily with no spluttering. The tank must be completely full before the heat goes back on.
Restore the heat and check
Breaker on, or the gas control back to its setting. Check the drain valve and the hose connection for weeping over the next hour.
Procedure: Flush a storage tank water heater
Do the anode rod at the same time
You have the heat off, the water off, and a hose connected. Pulling the anode rod now costs you fifteen extra minutes and tells you the single most important thing about your tank's remaining life.
Replace it if it is down to roughly pencil thickness or if the steel core wire is exposed along much of its length. And with the rod out, a tank rinser wand through the opening cleans the bottom far better than draining alone. Anode rods.
The part to have on hand before you start
Pick 1 — Routine sacrificial replacement
Rheem R-Tech magnesium anode rod
A plain magnesium sacrificial rod, which is the right default: magnesium protects better than aluminum and leaves no aluminum residue in the water.
| Material | Magnesium |
|---|---|
| Type | Sacrificial anode rod |
| Service life | Magnesium and aluminum rods deplete; replacement intervals of 3 to 18 months are reported in hard water |
What is good
- Magnesium is the more protective of the two common sacrificial metals
- No aluminum residue in the drinking water
- Tends to produce less odor than aluminum in most water
- Cheap, and replacing it is the single highest-value hour of maintenance on a tank
What is not
- It is consumed — in aggressive water it can need replacing inside a year
- In some water chemistry magnesium causes the sulfur smell instead of curing it
- Removing a seized factory rod can take a breaker bar and a lot of nerve
Skip it if: Skip magnesium if your water already smells of rotten eggs. In water with sulphate-reducing bacteria a magnesium rod feeds the reaction; that is the case for a powered anode instead.
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Specifications above from Rheem SP11526C R-Tech magnesium anode rod listing; Corro-Protec, magnesium versus aluminum anodes. We have not tested this unit ourselves.
Pick 2 — Hard water and smelly water
Corro-Protec powered anode rod
A powered titanium anode that does not deplete, carries a 20-year limited warranty, and is the usual fix for the rotten-egg smell a magnesium rod causes.
| Type | Powered titanium MMO anode with a low-voltage controller |
|---|---|
| Mechanism | Impressed-current cathodic protection |
| Depletion | Does not deplete, unlike magnesium or aluminum |
| Warranty | 20-year limited, with a 90-day money-back guarantee |
| Certification | Certified lead free by WQA under NSF/ANSI/CAN 372 |
What is good
- It does not sacrifice itself, so it does not need replacing every few years
- Usually eliminates the sulfur smell that a magnesium rod produces in some water
- 20-year warranty against a sacrificial rod's 3 to 18 months in hard water
- Lead-free certified for potable use
What is not
- Needs a power outlet near the heater and a controller mounted somewhere
- Costs several times a sacrificial rod
- No protection at all during a power cut
Skip it if: Skip it if your tank is already more than about eight years old. A powered anode cannot undo corrosion that has already happened, and the money is better spent toward the replacement that is coming anyway.
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Specifications above from Corro-Protec product facts; Corro-Protec, magnesium versus aluminum anodes. We have not tested this unit ourselves.
How often
| Water | Interval |
|---|---|
| Soft, 0 to 3 grains per gallon | Every 18 to 24 months |
| Moderately hard, 3 to 7 | Annually |
| Hard, 7 to 10 | Every 9 to 12 months |
| Very hard, 10 to 15 | Every 6 months |
| Extremely hard, 15+ | Every 4 to 6 months, and consider a softener |
The three mistakes
- Leaving the heat on. An electric element fires in an empty tank and is destroyed almost immediately. On gas, the burner can overheat the dry steel. Turn the heat off first, every time.
- Not opening a hot tap. Without an air path the tank cannot drain, and people then force a stuck drain valve that was never stuck.
- Forcing a seized plastic drain valve. If it will not turn, stop. A snapped valve on a full tank is a flood, and the fix is a plumber under pressure.
On electric units: the element option
If you have an electric tank in hard water and the hot water has been getting weaker, the elements may be scaled. Where they are accessible, circulating or soaking them in a descaling solution is cheaper than replacing them, and it is a sensible thing to try while the tank is drained. Descalers.
What goes in with it
A water heater is a system, not a box. These are the parts that go in with it, and the ones people discover they need halfway through the job.
- Expansion tank
Required by code on a closed system, which is most homes with a pressure-reducing valve.
- Drain pan
Cheap insurance anywhere the tank sits above something you care about.
- Anode rod
The part that decides when the tank leaks. Replacing it is the highest-value hour of maintenance there is.