I'm working with a 250-year-old house.

The house was unoccupied for several years before I came in, so I don't know if the damage I'm about to describe is the product of freezing.

I'm getting a plumber to install a new boiler, but all the plumbers around me can't come for a couple months to do the installation. I want to close the walls in the meantime (lots of plaster to repair), and for this want to do a pressure test on the heating system pipes to make sure all is ok.

When I put water through the hydronic system, I keep discovering bursted pipes. I am wondering if this is something I'm causing by not controlling the pressure, or if it's just that the previous owners (now dead) did not drain the heating system before dying (ha ha), and I'm now seeing the damage.

The first time I put pressure through the system, I did not control the pressure, and it's possible that I sent through 80-100psi.

enter image description hereIs it possible that I caused multiple pipes to burst? Most of the breaks occured in the connections between two baseboard heaters.

What is the maximum pressure a hydronic heating system can take? The copper in the baseboards seems pretty thin.

  • 1
    It looks like freeze damage, but if the pipes are more than 60 years old, they are very near end of life anyway. Safe pressure is under 80 pounds for pipes usually. A picture of the edge of the pipe with a measuring tape/ruler would help some. If freeze damage suspect all pipes are bad. Using air to test pipes is much neater than water.
    – crip659
    Sep 3, 2023 at 23:36
  • Yes, testing with air is very standard for very good reasons.
    – Ecnerwal
    Sep 4, 2023 at 1:39

2 Answers 2


Bursting between fin tube baseboard sections actually makes a lot of sense for freeze damage.

When a pipe freezes in one spot, it's no big deal. Indeed, plumbers and the water department will use a portable freezing unit to plug pipes that don't have valves when they want to stick a valve in without a huge mess.

However, as more of the pipe freezes, the expansion of water freezing to ice compresses the water in the rest of the pipe. That is what actually bursts the pipe. And it makes a lot of sense that a fin-tube section would freeze more or less in the middle, then the unfrozen water compressed between two ice plugs would burst the pipe in-between the two sections of fin tube.

Type M is the thinnest-wall copper pipe in common use and has a working pressure (should be perfectly safe to use at/below) of 346 PSI for the 3/4" size typical of fin-tube per https://www.engineeringtoolbox.com/copper-tubes-dimensions-pressure-d_84.html (which shows a burst pressure of 4715 PSI, but it does not list a burst pressure for the annealed condition, and joints that have been soldered will cause annealing...) So 100 PSI should not be a problem (but the pressure relief on the boiler should go off at 30 PSI or so...)


I'd suspect ice before overpressure (which could cause the same kind of burst) or age (which I would expect to produce a different kind of damage).

Pipes burst where they happen to freeze, since ice takes up more space than water. If more than one spot freezes, more than one spot may burst, and freezing in one spot may indeed increase risk of freezing in other places along the same pipe since it may block convection that might be bringing warmer water in.

Preventative measures include improving insulation between the pipe and low temperatures outside, and/or adding a pipe heater that you can run during cold weather.

(Most of the pipes in my 125-year-old house don't run through external walls, and those few which do are now inside insulation so they are relatively safe. The kitchen sink and downstairs bathroom may be at risk; the latter has a pipe heater installed by a previous owner that I power up when temperatures get down to ouch degrees.)

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