Steam Distancing

To start, my sympathies to anyone who lost someone during the pandemic. Many thanks to the medical community that risked their own lives to save another life. Truly heroic efforts in the face of uncertainty.

Things might be back to normal when you read this column. Hopefully you were able to adapt to our new normal as our new normal changed just about every day. It seems that the virus was the most severe in the areas of the country that have the most boiler heat, New York City metro, Boston, Philly, Detroit and Chicago. Think about how bad it could have been if those areas had forced air furnaces blowing contagious air borne particles around.

Steam is an air borne particle, always has been. However, we like to keep steam contained inside pipes and radiators, not allowing it to escape into the spaces we heat. As a side note, there was a one pipe steam job at an apartment building where one of the tenants, a retired fireman, took the automatic vents off his radiators. He liked the high humidity level that the direct steam provided. Can’t make this stuff up. People do funny things.

When steam is confined inside the pipes and radiators, it maintains a certain distance from any air that is in the pipes and radiators. Steam and air don’t mix. They are both vapors. Scotch and water do mix. They are both liquids and at the right proportion are quite tasty and therapeutic. Since steam and air don’t mix, it can cause problems in our heating systems. Where air is trapped inside a pipe or radiator, steam won’t go and we have problems. Time for some scotch or a closer inspection of the venting.

Steam moves from the boiler through the pipes to the open automatic air vents on the end of the pipe mains or any one pipe radiators. This is because high pressure moves to low pressure. If you blow up a balloon to create a high pressure inside, when you let go, the higher pressure in the balloon moves back to the lower pressure outside the balloon. A steam heating system works the same way. Higher pressure builds up in the boiler and starts to flow out towards the lower pressure that is in the piping and radiators.

What we can’t see inside the pipes and radiators is air, but it is there. For the steam to flow to all the radiators and provide even heating, all the air that is inside the pipes and radiators must flow out. That is the job of the automatic air vents. They have to be in the right places and sized properly to keep the air flowing out so the steam can flow in. The higher pressure of the steam, and it can be as low as a few ounces, makes it flow towards the open air vents that are at the lower pressure that is outside the pipes and radiation.

When the steam reaches the automatic vent, the thermostatic element inside reacts to the 213 degrees temperature and closes to keep the steam inside. At that point, the slight natural vacuum created as steam condenses keeps the steam moving to the radiators where the cold temperature of the room keeps the steam condensing. Nothing could be simpler.

Well, something always has to go wrong and that something could be steam and air practicing distancing. Where the air is, steam won’t go. Steam can compress the air, but not eliminate it. The symptoms of air problems in a steam system are generally uneven heat and rapid cycling. Uneven heat is defined as areas or radiators that don’t get as warm as the other areas or radiators in the system. The rate at which the air is removed is the rate at which the steam enters.

In a two pipe system, without any vents on the radiators, the air passes through the radiator trap or vapor device to be vented at the end of the return main. If that main vent is stuck closed or has been removed, then any radiator on that main would suffer.  Another common problem on two pipe systems is steam passing through a radiator trap that won’t close. Steam then gets to the automatic air vent at the end of the main and shuts off the flow of air before all the radiators get a chance to heat up. This can also happen on the vapor systems, systems where the operating pressure is less than a pound of pressure. They don’t have a thermostatic element on the return side of the radiator to stop the flow of steam like a regular trap. They work on the principle of metering the amount of steam that can enter the radiator with the radiator valve.

Internal to the original valve was an adjustable or fixed orifice that only allowed enough steam into the radiator that the radiator could condense. If the radiator is rated at 50 square feet EDR, then the valve only let that much steam in, with nothing left to pass to the return.  When an original valve is replaced, the new valve lets too much steam to flow into the radiator, which then gets to the main air vent prematurely, just like a trap that is stuck open. The solution is to close the new valve down until just enough steam gets through to heat the radiator, but not too much that steam gets into the return line. Another bad thing that could happen is water hammer, which will be another subject someday.

In a one pipe system, with vents on the radiators, the air passes through that radiator vent. If one radiator heats better than another, the first thing to check is the venting rate of the automatic air vents. Faster venting means more heat while slower venting means less heat. Some vents are adjustable to help balance the system. Slow down venting of the radiators that are too hot and speed up venting of the radiators that are too cold. Remember, the one pipe radiator valve cannot be used to balance the system, since it has to be completely open or completely closed.

Just like the two pipe systems, the end of steam mains need to be operating and properly sized. Air needs to flow out of the system fast enough so that steam reaches each riser at about the same time. At an apartment building with long mains, we finally convinced a reluctant landlord to increase the end of main venting to get more even heat. That worked like a charm and decreased his energy cost by 30%.

If the venting isn’t up to speed, the steam pressure builds quickly and shuts off the burner prematurely on high pressure. The ideal burner run cycle is not interrupted by the high pressure limit. Increasing the venting rate can generally solve that rapid cycling issue. All of this is more fully explained in Linhardt’s Field Guide to Steam Heating, only available as a free electronic download at steamupairoutwaterback.com.

Be safe out there.

Patrick Linhardt is a thirty-five-year veteran of the wholesale side of the hydronic industry who has been designing and troubleshooting steam and hot water heating systems, pumps and controls on an almost daily basis. An educator and author, he is currently Hydronic Manager at the Corken Steel Products Co.