Steam System won't heat unless pressure is at least 3 psi

Bret

I know a steam system should not be running as high as 3 psi, but at lower pressures, the radiators do not heat. The house is very old dating back to the 1700s. Heating system was probably first installed in the first half of the 1900s. The house is about 3500 square feet and there are 32 two-pipe radiators on the system which is split into two zones controled by motorized steam valves connected to two separate thermostats located in the coldest room in their respective zones. The boiler dates from the 1970s and is rated over 400,000 BTUs. There used to be a separate hot water coil on the steam boiler, but it has been removed and hot water is now serviced though a seprately fired water heater. When running at around 1 psi, none of the radiators get hot. At 3 psi, about 40% of the ratiators get hot, about 45% are warm, and 15% are stone cold.

See attached for rough diagram of heating system

MikeC

You can start

by checking the traps in the system,those failed closed wil cause cold radiators.Check over in hot tech topics about zone valves on steam systems.If not piped correctly they turn your system into a real trap smasher.

Unknown

How does the air get out when the zone valve opens?

That would be the key to this. If you have an air vent on the ends of the zone's main supply pipes, make sure it's working. It also must let air back in at the end of the zone's cycle, and/or a vacuum breaker would be near the zone valve, itself. There are several ways it might be, but something has to let air in and out beyond the zone valves and before the pipe drops into a wet return seal. Something....

Looking at your piping and warm radiator locations, I think you have a slow to vent system.

Noel

Bret

2 1/2\" Steam Valve Feeding 3\" Main

Upon further inspection, it seems that the main line serving the larger heating zone is 3" pipe. The zone valve feeding it is only 2 1/2". Is there any good reason why a valve smaller than the main should be used?

Steamhead

Bret, does this system

have traps on the radiator outlets, or shutoff valves on the outlets and air vents on the radiators?

Somehow, the steam isn't making it to one side of the house. I think Noel is right, one side isn't venting the air properly.

If the system has traps on the radiators, it may also have traps piped between the end of the steam main and the dry return. These act as air vents, and if they fail they will cause a similar condition.

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Bret

Traps are on the radiators

No vents on the radiators. In addition to the radiator traps, there is somehting that looks like a trap (or vent?) at the point in the basement where the main meets the pipe going to the up to radiator. This is only presnet on some of the radiator connections.

Steamhead

Are they located

on the last radiator connections on the steam mains?

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Bret

No. They are in the middle.

Steamhead

That's strange

Could it be a trap for a convector that was piped in under the floor? Can you take a picture and post it here?

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Bret

Pictures

Here is a picture of the what looks like a steam traps in the basement. I also included a picure of a typical exposed radiator in the house. There are only 4 steam traps in the basement. The house has bulit-in radiators in addition to the exposed radiators. All of the built-in radiators are on the same side of the house where the basement steam traps are installed.

Steamhead

That explains it

That trap is undoubtedly below a concealed radiator. They put it there so you could get at it without opening the wall.

On your system, the radiator traps let the air and condensate into the dry return. Where does the air go from there? Look for a vent or float trap/air eliminator on that return. It's usually, but not always, at the end of the return near the boiler.

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Bret

Why wouldn't one put steam traps on every radiator?

Attacted pictures show the end of the lines going back to the boiler.

Steamhead

What you see at the end of the steam main

is a "Float & Thermostatic" trap. The float is normally closed, but opens a valve to release condensate to the dry return. The "thermostatic" element is somewhat like a steam trap element- it is normally open to release air, and closes when steam hits it. This is what vents the air from your steam mains into the dry return.

Once the air is in the dry return, it exits the system thru those green vents (Hoffman #75 or 76). I'd first suggest you unscrew those green vents and see if they are working. If one of those vents sticks shut, you won't get any heat in the part of the house it serves.

If the vents are OK, the F&T trap's thermostatic element may be stuck, preventing air from leaving the main. In this case, some air will vent thru the radiators, but those near the end of the main may never get hot.

You can get replacement parts for almost any trap from Barnes & Jones or Tunstall. And the Gorton #2 vent is much better suited to Vapor systems than the green Hoffmans, since it has much greater capacity and therefore requires less pressure to vent.

Web sites are:

www.barnesandjones.com

www.tunstall-inc.com

www.gorton-valves.com

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