Draining a hot water tank and plumbing with compressed air

Summer is over, so it's time to close the cottages and get them ready for winter. This includes thoroughly draining the plumbing so that freezing water will not cause pipes to crack. In recent years, that's always been my job. The plumbing was designed to drain by gravity alone, but you can't always count on that to clear everything, and there'd be a few leaks to fix the next year. So I came up with a method of using compressed air to drain them, fast.


I start in each of the 9 buildings by closing the main shut-off valve. This isolates the plumbing in the cottage.

I also make sure the hot water tank is turned off!


I use a cheap pancake style air compressor. It's built amazingly cheaply, and it's very loud and not very efficient. But it's light, so easy to take from one cottage to the next.


The first order of business is to make sure the hot water tank is empty. It will drain with gravity, but it takes a long time, so compressed air is very handy for force-flushing it. I open a cold water tap, then inject air in the hot water side of the system through another sink, typically a bathroom sink.

I made a fitting that clamps to the sink for injecting air, though, in a pinch, one could just hold the nozzle against the sink and cup the fingers around it. You wouldn't get a very good seal, but it would be enough to push the water out of the hot water tank.


Forcing compressed air in the hot side will push the water out of the hot water tank back out the cold side. Having nowhere else to go, it comes out the tap.

Very important: Make sure your hot water tank is turned off! An electric hot water tank will burn out its heating elements if it turns on with no water inside.


The reason this works is that cold water is always fed into the bottom of a hot water tank. Even if the cold water connection is at the top, there is a pipe inside the tank to release the cold water at the bottom. Pushing air into the tank from the top (through the hot water side of the plumbing) will push the water out the cold water side.


Once the hot water tank empties out, air and water start sputtering violently out of the faucet.

Once it's mostly air that's coming out, I close the faucet and pump some more air in the hot water tank. I don't know what pressure I typically end up getting in the tank, but I'd say it's probably around 15 psi, or one bar.


Then it's time to open up the taps, one at a time to blow out any water that is still in the pipes.

Last comes the toilet. Flushing it causes the float valve to drop, which in turn opens up the valve. With all the air shooting out into the partially emptied tank, this can cause quite a bit of splatter.

I then typically jam the float back up with a toilet brush and do one more round, pumping up the hot water tank and letting air out of each faucet and tap.


Last I open the main tap of the cottage to let the air push whatever water might still be in the supply line out.

But I did this cottage before the rest of the camp was drained, so I disconnected the main supply hose and let it blast the remaining air out the hose. I didn't want to leave it hooked up, because the main shutoff valves typically leak a bit, and I didn't want to risk leakage pushing water back into the plumbing.


Next it's time to empty the drain traps as much as possible. I use a toilet plunger with a hose in it to push air in the trap.


This pushes the water below the plunger out the drain. I then lift the plunger briefly, lower it again, and repeat until the bowl is mostly empty.

What I realized since though is, with the compressor at 100 psi, by just aiming the nozzle into the drain and letting it blow, the Venturi effect is enough to push the water back as well, emptying the toilet bowl even faster.


I also use the plunger on the sink and shower drains.

I can't get all the water out of the drains this way, so antifreeze still needs to be added, but getting as much water as possible out certainly helps.


After that, it's on to the next building, and the next. With 9 buildings with indoor plumbing at the camp, I don't want to take too much time in each one!

Coincidentally, the cottage I was draining in this article is #7, the construction of which is described here. And the nearest one in the picture at left is the one whose deck railing I repaired in 2013.



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