How likely is a frozen copper water line to burst when defrosted?

The bursting happens during the freeze, not the thaw. You find out that the pipe is burst when it thaws, because that’s when the water is free to spray out, but it’s the expansion that happens when the liquid water turns to ice that actually breaks the pipe.

I had this problem in a previous house and I found if you get to it in time you can prevent a burst depending on the severity of the frost. Leave the tap on and warm the pipes as you are doing. If drips start to appear the flow of water will soon flush the ice away. Good luck!

As has been stated, I doubt water thawing would cause any bursting. Water expands when it freezes, and contracts as it thaws. If anything you would be worried about an implosion… but I highly doubt that is going to happen.

It’s not likely to burst if it’s only recently frozen and is just “blocked” for now. The problem occurs as the blockage and the cold conditions persist and the ice continues to build.

If you can get a hair dryer to blow on it, it’s very likely that you can cure the problem yourself, if it’s a recent blockage.

Thanks all. I opened up the cabinets under the kitchen sink and put a small bathroom heater in the cabinet with its warm air flow directed right at the pipes.

This just in. The heater did its job. Water now running. Looks like the pipe didn’t suffer any permanent damage.

First off, it will not suffer damage when it defrosts. Any damage has already been done.
A lot of it depends on whether there is room in the pipe for the water to expand as it freezes. This is exacerbated if there is a 90 degree bend in the pipe. This is where it is most likely to suffer damage because the water will freeze in a straight line and it will burst the end of the 90 out.

@rojo All is well. I’ve left all the kitchen cabinets open for tonight and the water dripping. Tomorrow the plumber will be here to put some thermostat-controlled heater tape on it.