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Hot Water Baseboard off a Steam Boiler


We have a Burnham IN-7, installed by the previous owner about ten years ago, which supplies a circa-1920 one-pipe counterflow steam system. Last summer, we had a hot water baseboard loop piped from the boiler to heat the basement. The general contractor said it was a common and efficient way to heat a basement, and we had no reason to doubt him at the time.

The hot water baseboard loop runs directly from one of the boiler's two bottom return tappings, over an aquastat, and through a pump. Water from the baseboard loop returns into the Hartford loop. All of the tubing, aside from what goes directly into the boiler, is red PEX. I've recently read on several sites that PEX is only rated for 180 degrees Fahrenheit. The pump for the baseboard loop does not shut off when the boiler is making steam for the rest of the house, and so parts of the PEX regularly see temperatures of 212 degrees or more.

Is it safe to subject PEX to such temperatures? If not, what would be the best way of fixing the temperature problem without ripping out all of the PEX and replacing it with silicone tubing or similar?

According to How to run a hot-water zone off a steam boiler by Dan Holohan, it's possible to regulate maximum temperature in a hot water zone by redirecting the zone's return water through a bypass and mixing it with water coming out of the boiler. He recommends using at least a 3/4" bypass. Our hot water zone has two garden hose fittings -- one just before the pump, and one just before the return water enters the Hartford loop. Neither seems useful for anything, and our current plumber does not know why the installer put them in.

Would running a hose between the two fittings work for regulating temperature in the zone?

For reference, attached photos of the supply and return for the baseboard loop.


  • Jamie Hall
    Jamie Hall Member Posts: 20,447
    A hose -- unless it is specially built for the purpose -- will NOT play well with hot water. You won't be a bit happy.

    Better to repipe as Dan shows -- a hard piped bypass with a ball valve for throttling. And when you get it set -- that thermometer he mentions -- take the handle off so no one messes with it.

    PEX of any colour doesn't really like water above 180 either...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Mark N
    Mark N Member Posts: 1,094
    There are tappings on the sides of a Burnham boiler that are for the Alliance indirect hot water tank. This is best place to tap the boiler for water for a hot water loop. The water will fully circulate through the boiler to pick up heat and not just pass through the mud leg of the boiler.
  • JStar
    JStar Member Posts: 2,752
    You'll also need to replace that circulator pump with a Bronze B&G 100. The cast-iron body of the Taco 007 will deteriorate and the water-cooled motor will fail from the acidic boiler water. You should also install a strainer on the outlet of the boiler to protect the pump from debris.