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A riser is slow to heat

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Published
July 10, 2009 Subscribe to RSS feed

There's too much radiation attached to the riser.
A steam riser can carry just about any load if you get the pressure high enough, and if you don't care about velocity noise. The challenge with steam heating is to deliver the right load to the radiators using low pressure, usually not more than two-psi pressure at the boiler.
The Dead Men sized the radiators to heat the space on the coldest day of the year with about one-psi pressure at the radiator. They used low pressure steam so the radiators wouldn't overheat and the fuel bills wouldn't soar. They used pipe-sizing charts that showed them the load limits for steam heating (See The Golden Rules of Hydronic Heating). If you connect too much radiation to a steam riser, you won't be able to heat it all unless you raise the pressure to an abnormally high level. And when you raise the pressure, you create other problems: high fuel bills, water level problems at the boiler, and noise.
Check the radiation against the carrying capacity of the pipe. Try closing some radiators to see if that helps. If it doesn't, you may have to repipe.

There's sludge in the horizontal runout to the riser.
This often happens after a one-pipe steam system floods. Water works its way up into the radiators, and the sludge washes down into the horizontal runout to the riser. A puddle of condensate gathers around the sludge, causing the steam to condense. The radiators won't heat properly because the steam can't make it past the sludge and the trapped condensate. If you remove a radiator and shine a light down the riser, you'll see a reflection. That's the trapped water.
Break the riser at its base and flush it from the top under pressure from a garden hose.

The riser needs to be dripped.
In a one-pipe system, all the condensate from the riser and the radiators returns through the horizontal runout to the riser. If too much condensate falls down that riser, the steam won't be able to move toward the radiators.
Drip the line if you can. If you're dripping into a dry return, use either a steam trap or a loop seal.
If you can't drip the riser, increase the size of the horizontal runout to the riser by one size over normal, and pitch it at least one inch per foot back toward the main (see The Golden Rules of Hydronic Heating for pipe sizes based on connected load).