Answers to the questions you've been asking.

Answers To The Questions
You've Been Asking

PM's popular hydronic heating columnist Dan Holohan gets numerous inquiries from readers seeking his help or offering their own solutions to thorny field problems. We have created "Holohan's Hydronics Forum" as a special section to share with you some of these exchanges between Dan and his fans.

Our intent is to promote wide dissemination of technical information and ideas. Opinions expressed by either Holohan or the correspondents about products and techniques are theirs alone, and should not be construed as a PM endorsement.

If you have an inquiry or comment you wish to direct at Dan Holohan, contact him at: Dan Holohan Associates Inc., 63 North Oakdale Avenue, Bethpage, NY 11714. Telephone: 516/796-9276. Fax: 516/579-3046. E-mail:

Flooded-Boiler Problems

I have a low-pressure heating application where I have a small 12-gallon condensate pump and receiver that pumps to the boiler off the float inside the receiver. I'm finding that the boiler floods whenever the heating system shuts down for high-outdoor ambient temperature. The boiler operates between 7 and 10 psi.

At present, the condensate enters the receiver at the same height as the condensate return line. I want to install a 100-gallon feed tank and operate the pump off a pump controller on the boiler. Will I have enough pressure to push the 100-gallon tank full without using the small 12-gallon condensate pump to push condensate to the proposed tank?

Dan replies: A lot depends on the difference in height between the feed pump's receiver and the lowest steam trap in the system. The condensate has to drop out of that trap and flow toward the receiver by gravity. If you're going to be creating a U tube by turning the return line upward to enter the boiler-feed pump's receiver, you'll have to make sure you have enough static pressure on the trap's side of the U tube if you want the water to move.

If you have a master trap right at the inlet to the condensate pump, you need to take a closer look at this entire system. I don't know if it's one-pipe steam or two-pipe steam because you didn't tell me. If it's two-pipe and you have a master trap, you're double-trapping your return lines and that can lead to water hammer problems, as well as slow condensate returns, which may be causing your current problem.

Keep in mind that when you create any sort of U seal in a return line, it's going to fill with condensate, and air will not vent through this seal. (It's like a P-trap under a sink.) So you'll have to find a way for the air to get out of the system, which usually involves main vents. This may be why the building owners are now running such a high pressure on the system.

Steam heating systems, when properly piped, will run on less than 2 psi. I cover all of this (and much more) in my book "The Lost Art of Steam Heating." If you need a copy, you can get it from the Books & More section of Good luck!

Air Vents On Two-Pipe Steam Radiator?

I have a two-pipe steam system with air vents on each radiator.There are steam traps on each radiator as well. Please let me know if this is proper or not. My plumber told me that air vents are not supposed to be on the radiators in this type of steam s

ystem. If this is true, I would like to know why.

Dan replies: Your plumber is right. Two-pipe steam systems, with thermostatic radiator traps at the radiators, don't need air vents because the steam pushes the air through the radiators, through the traps and into the dry returns. From there, the air leaves the system either through main vents or a condensate receiver that's vented to the atmosphere.

If you have air vents on the radiators, the radiator will get hot, even if the steam trap fails. That leads you to believe that all is well, but it's not because the condensate won't drain from the radiators when the traps are bad. You'll also begin to have water level problems back at the boiler, and in most cases, water hammer throughout the system.

Contractor Vs. Engineer

I am a plumber working on the renovation of a steam system. The project involves modification to a two-pipe steam system in an old stone church with two upper levels that are heated with radiators. All of the traps on the existing radiators are working properly at this time.

The condensate from the upper levels is piped back to the condensate receiver. The lower level contains steam coils in ductwork. The condensate from the lower level used to gravity feed directly back to a cross connection on the bottom of the twin steam boilers. This condensate line is physically below the normal waterline of the boilers.

The steam coils and main drips on the lower level do not have steam traps, just check valves. We are changing the condensate receiver to a new one with a larger capacity. The new receiver has two pumps with controls - one pump is dedicated to feed one boiler.

We've removed the cross connection and gravity return connection on the boilers, and now we'll pipe the lower level condensate to the condensate receiver. The project engineer shows a 2-inch steam trap being installed at the connection point between the lower level condensate return and the condensate receiver. This will keep steam out of the receiver, but the lower level condensate return line will now be a steam line, as it accepts lower-level steam-main drips without steam traps.

When the control valves on the lower-level steam coils open, the condensate generated will move through its check valve by gravity into the condensate return line and boil, creating water hammer.

I believe the 16 coils and main drips on the lower level should be re-piped to individual steam traps and that the 2-inch trap on the condensate return line should not be installed.

Can you give me your advice based on the information I have provided?

Dan replies: You are absolutely correct, and the engineer is making a mistake. I hope you're able to convince him. Well done!

Boiler Return Traps?

Who carries parts for old boiler return traps?

Dan replies: Nobody does. But I have a wholesaler buddy who rips out the guts and installs a probe-type low-water cutoff into the body. He wires this through a relay to two 1/2-inch solenoid valves.

One of the valves is on the steam line going to the boiler return trap, and it's normally closed. The other is on the equalizer line between the return trap and the dry return main. This one normally is open.

When the condensate enters the return trap, it wets the probe, which then reverses the action of the solenoid valves. The return trap does its thing and the cycle repeats itself. Pretty neat, eh?