This page was created after the submission of a question from a site visitor about blowing out water pipes. I felt that the string of responses would be of interest to anyone considering clearing water out of their water pipes before winter freeze-up.

That visitor, Phillip, wrote: “I am planning to buy a compressor to blow out water pipes for winter. The pipe is 3/4 inch and approximately 50 feet long. What size compressor would I need?

What’s on this page about blowing out water pipes?

  • adapter for connecting compressor to water line
  • What you need to blow out water lines
  • safe blow-out pressure
  • procedure
  • how to tell when you are done

Yes, it’s getting to be the time of the year in the Northern Hemisphere where we have to be concerned about water freezing in pipes that are exposed to the frigid wintertime conditions.

Adapter to connect an air line to a water pipe

Adapter to connect an air line to a water pipe
Photo: amazon.com

If water has pooled in the water lines and the temperature drops below freezing, that water will freeze. If there is enough water in the line, as it freezes, the ice expands, and could crack the water line, leading to a leak once the pipes thaw out.

What equipment do you need to blow out water lines?

  • an adapter to allow the hose from the compressor to be attached to the house water line
  • an ample supply of compressed air

What you need to blow out a water lines is a compressor. If there are many water lines, and if those water lines are larger than the typical 1/2″ lines in diameter, the higher the volume of air needed, and the larger the compressor.

In order for the water line to be voided of water, you need to fill it with compressed air fairly quickly. The compressed air, flowing into the water pipe, will drive the water before it and out an open tap at the other end. Preferable the open tap is at a lower level than the air inlet, but if that’s not possible, you can still achieve a fairly water-free pipe.

If the water pipe is metal you could use compressed air at industrial pressures. If if the water lines are plastic or PVC, make sure that the pressure you use doesn’t exceed the burst pressure of the pipe. Best to start with 15 PSI of air or so, and see if that drives the water out. If not, raise the pressure a bit and try again.

If my math is correct (and yes, you do want to check it) a 3/4″ pipe x 50′ will hold about .2 cubic feet of air. If you introduce a few cubic feet of compressed air at, say, 15 PSI, you’ll have plenty to drive the air out of the pipe, as long as the outlet is below the air inlet. Therefore, a small .5 or .75 HP air compressor should do that job OK.

If there’s convoluted piping, add higher pressure and get the flow of compressed air going into the pipe as fast as you can to build the volume of air inside the pipe quickly, to drive the water out before it. That may require a 1.5 – 3 HP powered compressor with a larger tank.

Connect the air line to the house water system

There’s an image above that shows a connector available commercially to attach an air line to a water system. But, that one is designed to connect to the female thread on a sprinkler system.

As noted below, the easiest way to connect air to a house system is at the hot and cold water connections for the washer and dryer. To do that, you will need what’s shown in the image right below.

Air line connector and adapter for connecting an air line to a house line

Since the hose from the washing machine typically connects via a male pipe thread on the water line, I visited a decent hardware store that offered the adapter you see on the right. This one allows the coupler from the air line to connect to it. With this fitting threaded into the adapter on the left, I convert the male thread on the air line to a female thread which allows me to connect the air line to my house water system.

I want to blow out both the cold and hot water lines, so again, this equipment allows me to connect to either where the washing machine lines connect to the house.

Note: Turn the regulator on the compressor down to 10 PSI or so before connecting the air line to the house water lines.

Where to start blowing out water lines?

To blow out the lines in a home a good place to attach the air feed from the compressor is the cold water hose bib for the washing machine. This will blow water back up the line from the washer feed to an open tap elsewhere in the water system.

Open all the cold water taps downstream from there, and as air flows from the compressor through the lines, that should drive all the cold water out.

Next, attach the compressor feed to the washing machine hot water supply hose. Open all the hot water taps elsewhere in the structure, and start adding air. It will help a lot if the hot water tank has been drained first as that’s a lot of water and will take a long time for air from the compressor to blow all that water out.

When all that’s coming out of the other open taps in the house is a little water mist or just air, you should be done.

So, a small compressor that has a tank that holds 2-3 cubic feet of air, one with a regulator to reduce the outflow pressure if need, and an air hose with a high flow blow gun may be all you need for the small line that Phillip has, however, a whole house water system will require a bigger air supply to ensure that compressed air drives all the water out before the air flowing into the water lines.

Hope this helps,

Bill