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No insulation creates heat and no matter how much of it you use without a heat source the pipes will freeze.

I know just what your thinking Craig. (dam plumbers):o John

To repair the leak will require removing the check valve. At that point I would replace the check valve along with the two males. John

We would use a winged test plug with a hose fitting attached to the end. John

Its a Watts 009 RPZ. John

You will be far better off with the recirc loop connecting to the bottom of the heater with the circulator pulling the water back to the bottom of...

A bad valve or obstructed line on the discharge of the pump will cause the problem you're having. John

There is nothing made in between the two sizes. John

Don't know what to tell you. Pipe sizes are, and have always been 3/8"-1/2" and 3/4". Nothing in-between. Have you tried a 1/2" shower arm? John

There has never been 7/16" pipe. Pipe threads are tapered. What you are measuring is what is remaining of the threads of the broken shower arm. ...

As far back as I can remember shower arms have always been 1/2". It sounds like you have some pieces of the old shower arm in there. John

A combo is the same as a Y and 45. Perfectly fine. John

From the age of the home you may have some lead drain lines serving the tub and toilet that need replacing. John