No heat for half the main run

Steamhead (in transit)

If the air can't get out, the steam can't get in. I'd check these first. Dunham Vapor systems are pretty straightforward, and a good steam man should be able to get it going.

Where are you located?



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Douglas Sanborn

I recently purchased a Georgean house built in 1925 with a Dunham steam system. A new boiler had been installed last February and appears to be properly installed with respect to the 'A' and 'B' dimensions. however the system does not seem to build pressure. I have replaced the thermostatic valves on all the rads with new Mepco units but still cannot build pressure. If I open one of the steam traps on the second floor rad at the end of the problem supply main I can eventually get steam, but it is very slow to arrive and seems to have little velocity but all the cold rads are getting heat. The pressuretrol is set to cut in at 1/2 lb with a 1 lb differential. I even took the main vent off of the dry return near the boiler but with the system "closed" there are 4 or 5 rads that won't heat. I am now thinking that maybe the supply and returns should be flushed as I am sure that this has never been performed! What is the recommended proceedure and should any chemicals be used?

Christian Egli_2

Ah, it's the first day of the winter

Two pipe atmospheric return Dunham systems were never meant to work with their return air hole plugged with an air vent.

What is called an air eliminator is not a steam stopping air vent, in spite of the thread, keep the hole above it free and clear. Open it as much as you can. This is where the system breathes and it breathes optimally with only this one nostril.

In fact, install a bit of pipe in there that is not threaded on the exit - to make it openly clear.

For the rest, you'll need to observe any steam leakage through this hole, if it ever occurs, it will mean you have a faulty radiator trap somewhere - suspect also the crossover traps.

Dunham stuff is very durable and taking care of the traps is very important. Good for you for doing it.

The steam main itself, like in any two pipe system is very well vented through the open radiator traps on start up. With good traps like you have, you do not need to ever worry about two pipe main venting.

Measure your B dimension, and make sure your system never ever goes past, like, 1.5 PSI. These systems will not, repeat, will not work above the B dimension. If you want to work at higher pressures, you'll have to install a condensate pump. I'm sure you don't want to do that.

So, simply keep your return hole open - no air vents. Easy.