Dealing with a Frozen Drain Pipe

I've had a recent success in dealing with a frozen pipe and if you've got the same problem you might be interested. The pipe in question is the PVC drain pipe from our kitchen sink. It also handles the output of the dishwasher. With all that hot dishwasher water you'd think it wouldn't freeze but that just wasn't the case. The pipe runs against the outside wall for a stretch of about two feet.

I tried a manual snake without luck. As I later learned, one of the things to look for to determine that you're dealing with a frozen pipe instead of a routine clog is what comes back when you retrieve the snake. If it's clear, you're probably dealing with a frozen pipe instead of a hair, food, or grease clog.

As you might guess we called a plumber, who tried his power snake. It's pretty much the same as the manual one but with a power drill doing the turning. The snake came back clean so he packed up and went away leaving us to wait for the pipe to break so he could come back and charge a bundle for opening up the wall and replacing it. But he did make two reasonable suggestions:
- Pour hot salt water down the drain.
- Put a space heater under the kitchen sink cabinet.
I had been thinking about the salt water. It would lower the melting point of the water in the drain and might slowly melt the ice. So I tried. And while I was at it I put a small electric space heater under the sink and aimed it at the drain pipe. These suggestions might work in some circumstances but didn't help us. The heater might work much better on copper pipe than on PVC but who has copper anymore?

That evening an idea struck. If you can't heat it up from the inside, why not try and heat it up from the outside. Eureka!

I thought about various forms of insulation that might insulate the outside wall and slow the heat loss. A blanket? Leftover foam insulation? My wife suggested cardboard, which sounded like a pretty good idea so I retrieved the shipping carton from an old DELL server? What else?

I settled on making a tent by leaning a 4x8 sheet of plywood against the exterior wall of the house. Over this I stapled a leftover plastic drop cloth. The idea was to capture the heat from the wall in a sort of tent around the wall. I was even planning to run an extension cord outside and put the space heater in the tent. If I didn't set the house on fire, that was pretty sure to work.

It turned out that I didn't need the heater because there was an exhaust vent for the kitchen counter range that exited the house right near the window over the sink and thus near the drain pipe. I extended the plastic sheet over the vent so that the warm exhaust would be captured in the tent. Then I went inside and turned on the fan so that it would pull warm air from the inside of the house through the vent and heat the wall. I returned to work and hoped for the best.

Forty-five minutes later I went to get some coffee and the water in the sink was gone. The outside thermometer that is inside the plastic tent read 40 degrees (F). The thermometer a few feet way that wasn't in the tent read 21 degrees (F). The tent and the exhaust fan did the job.