10 Reasons Why Steam Heated Buildings Heat Unevenly
1. The air can’t get out of the system fast enough.
Where there is air, steam won’t go. It is very important to quickly remove the air from the system allowing the steam to travel to the radiators. Main vents are vital to proper distribution. When you are out on a job, make sure to look for steam main vents in the basement.
2. The boiler is piped incorrectly.
A modern steam boiler requires the near boiler piping to help produce “dry” steam for the system. The boiler’s steam chamber is smaller and the supply riser holes are smaller, and this affects the boiler’s ability to separate the water from the steam, so the manufacturer wants to use the header and equalizing line piping to “catch” this water and prevent it from heading out into the system with the steam. When piped incorrectly, the steam is forced to carry water with it as it leaves the boiler. Naturally, the water condenses the steam before it reaches the radiators.
3. The boiler is undersized or underfired.
A steam boiler’s job is to produce enough steam to fill the entire piping system and all the radiation. The job of the cold pipes and cold radiators is to condense this steam, but if the boiler can’t produce enough steam to overcome this mass of cold iron, the steam will not be able to make it out to the furthest radiators. This is why you must size the boiler to the connected load and then make sure the burner is fired to that load.
4. The steam traps have failed.
Two-pipe systems have radiator traps and float and thermostatic traps. Their job is to pass air and condensate into the return piping while preventing the steam from getting past the radiators and ends of the mains. When these traps fail in the closed position, the air can’t get out, so the steam can’t get in. But when they fail in the open position, the steam passes into the return lines. Once there, it pressurizes the returns to the same pressure as the supply lines, and with no difference in pressure, the steam stops moving. You have to make sure the steamtraps are working properly for the system to operate efficiently.
5. The insulation has been removed from the pipes.
Steam mains were insulated so steam could reach all the radiators. When asbestos insulation is removed, the exposed steel piping becomes one very large radiator, and this additional load condenses the steam before it can reach all the radiators. If you see pipes that have had their insulation removed, suggest re-insulating them or make sure the new boiler is sized for this additional load.
6. The steam pipes are pitched incorrectly.
When originally installed, steam mains and horizontal runouts had certain pitches to them that allowed the condensate and steam to co-exist in the same pipe. Over the years, the building settles and pipe hangers loosen up, changing the pitch of the pipes, and allowing condensate to collect in pockets along the piping. These puddles will condense the steam as it passes by, creating uneven heat throughout the building. Make sure the steam mains and runouts maintain proper pitch.
7. The quality of the steam is bad.
If the boiler water is dirty or has a film of oil on its surface, the boiler will make “wet” steam. Because water droplets “rob” the steam of its latent heat, steam is condensed in the piping before it reaches all the radiators. Check the quality of the boiler’s water by looking at the boiler’s gauge glass. When the boiler is making “dry” steam, the top portion of the glass will be dry. While the boiler is operating, raise the water line to within one inch of the top of the gauge glass — if water pours over the top of the gauge glass, the boiler water is dirty and needs to be cleaned.
8. The wet return lines are partially plugged.
If the steam system has wet returns and the complaint is uneven heating, make sure the returns aren’t plugged. If they are, the condensate will back up the return drips trying to overcome the additional pressure drop created by the plugged returns. Condensate will back up into the main vents closing them off before all the air is removed from the mains, and this can create very uneven distribution of the steam throughout the system.
9. Someone has set the pressuretrol too high.
Radiator steam vents have a rating that’s known as “drop-away” pressure. This rating has to do with the maximum system pressure at which the vent’s float can drop down to re-open when the steam condenses in the radiator. If someone raises the pressuretrol setting beyond the vents “drop-away” rating, it is possible to close all the radiator vents in the system, and this leads to uneven distribution of heat throughout the building. Always check the pressuretrol setting on the boiler as well as the “drop-away” rating of the vents in the system.
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