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Operator Error

May 1, 2008
The contractor on the phone wanted to be very careful with this boiler replacement, since it was located near where he lived. People talk, you know. It didn’t sound too complicated, but a jobsite visit can always turn up a few things.

By Patrick Linhardt

The original McLear Air Eliminator.

The Call
The contractor on the phone wanted to be very careful with this boiler replacement, since it was located near where he lived. People talk, you know. It didn’t sound too complicated, but a jobsite visit can always turn up a few things. We made arrangements to meet at the house to measure the installed radiation, take a look at the pipes for improvements, and minimize the surprises for the install crew.

When I pulled up to the address, I realized that I had been through this house once before. It had been some time ago, and with my advanced years, I couldn’t remember exactly why. This happens to heating men as they age. The jobs start to blur.

Once in the basement, it only took a few minutes to get my bearings. It was a typical twopipe steam system from the 1920s, with about a 25-year-old boiler. One supply main fed steam to the rear of the house while the other fed the front. They both ended on the opposite side of the house.

Thermostatic traps above the supply mains act as main air vents, allowing the air to pass to the dry return or closing when steam is present. With this type of system, the air is released back in the boiler room through one opening. That is where we found the first place to improve.

The Troubleshoot
What we found, as you can see in the accompanying photo, was a reduction in the venting rate. Someone, somewhere in the past 80 years, had piped in a main air vent on the outlet of the original McLear Air Eliminator. The opening on that float type vent is -in., while the opening on the new main air vent is much less. Improving the venting rate of a steam system can improve performance and economy.

We decided to propose to remove the old air eliminator and replace it with three main air vents after calculating the amount of air in the mains. The faster the air gets out of the steam’s way, the sooner the burner shuts off.

Another area to improve on this job was the gravity return. After those two supply mains end on the other side of the basement, the condensate from the mains has to make the long journey back to the boiler through the wet return. This was also more than 80 years old, with the accumulation of debris that build up over time.

Choked down pipes will slow the water flow. He decided to propose the installation of cleanouts at each end of the wet return to flush out all that garbage. The faster the water returns to the boiler, the less chance that the automatic feeder can flood the boiler. Overfilled boilers waste energy.

The radiation survey showed that the existing boiler was oversized. This did not surprise either one of us. The majority of installations are like this. Sizing can be an art as much as it is a science, but I could see no compelling reason to apply any finesse to this job. The radiation total was 1,091 sq. ft. The quoted boiler has a net rating of 1,165 sq. ft. That’s a good match.

We figured out how adapt the new boiler to the old supply piping. I showed him how an increase in supply outlet pipe size could decrease steam velocity in the near boiler piping. Slow outlet velocity leads to a steady waterline and dry steam.

It all went in as advertised in the late spring. They gave the boiler a good cleaning in the fall, fired it up, checked it out, and drove away. I got the surprise call about two months later. The contractor told me that water was spewing out of the three new air vents. He was on the way to the job. The homeowner wasn’t too happy.

As he drove, we discussed a few reasons why water would just start coming out of the vents after a couple months of working fine. What had suddenly changed?

The Follow-up
The contractor gleefully reported in an hour later. It turns out that the man of the house is an engineer at a manufacturing plant that uses high pressure steam. He decided to see what would happen if he set his home’s boiler control up to five pounds. More pressure has to be better than less pressure, right? A couple of innocent turns with his screwdriver made a major change to the system.

He failed to grasp the principle of gravity water return and the “B” dimension of a two pipe system (see sidebar). We had checked out that relationship of the water line and its vertical distance to the supply and return mains. It was greater than 56-in., which means the boiler can safely operate up to two pounds of pressure. Above that, the pressure in the boiler is greater than the two pounds of pressure in a vertical column of water 56-in. high. Water can’t get back into the boiler, so it finds it way out through the vents.

Besides the mess on the floor, water hammer in the wet return had displaced a whole load of debris that the flushing in the spring hadn’t. The contractor found the low water cut-off was plugged, along with the pigtail and the boiler drain. The service department had to do three more cleanings before the waterline calmed back down.

After all the problems caused by the man of the house were cleared up, the lady of the house reported that the heating system now works better than ever. The contractor asked me, tongue in cheek, if he may have stumbled upon a new way of cleaning out wet returns? I advised him against trying to duplicate the results.

Patrick Linhardt is the Hydronics Heating Division Manager at Habegger Corp. — Carrier Division in Cincinnati, OH. He often lends his sleuthing skills and technical expertise to local contractors in need. To order his book, Linhardt’s Field Guide to Steam Heating, visit or call 513/703-5347.