How The Europeans Do Hydronics

Some buildings just live on and on in my mind. Some are old and some are new, but each began as someone’s idea and then became real only by the power of imagination and engineering.

I watched men and women construct such a building in Frankfurt, Germany during March of 1997. They were getting ready to turn the place over to the owners when I visited there. I was with some friends and we were getting a tour of the building, courtesy of Mr. Grohal and Mr. Thaler, who are proud engineers.

They took the time to show me their work, and for that, I will always remember them. As I said, some places (and some people!) just live on in the mind.

They called it the Kastor and Pollux building. It is 426 feet tall, a very good size for this bustling commercial city. Three independent hydronic-heating systems served this building.

They built it that way to avoid high static pressures. These guys wouldn’t dream of heating a building directly with steam, as we’ve done in so many of our big American cities. In Germany, they consider steam heating to be a technology of the 19th Century.

But they did have steam in the basement. In fact, Mr. Grohal and Mr. Thaler were gushing over how unusual it was to use steam, albeit it indirectly, to heat a big building.

They walked up to this impressive plate-and-frame heat exchanger and started to explain to us how they were taking 60-psig steam from under the city streets and moving massive amounts of BTUs into the water that coursed up through the building. I let my eyes go wide and never mentioned that, in New York City, this was like white bread in a supermarket.

No big deal. But they were so eager to explain it all to me that I kept my mouth shut. I found their enthusiasm delightful. I have such a deep appreciation for passionate engineers. And I don’t care where they live.

When we finished marveling over the steam in the basement, we all went upstairs to wander around. About 80% of this place has direct hydronic radiators. Hydronic coils in the ventilation system pick up the other 20% of the load. The best part for me, though, was the atrium lobby where they were using a radiant floor heating system. That’s something I don’t see in New York City office buildings.

In Germany, many of the engineers have also been specifying hydronic radiant heat in the lower part of the walls, below any expanses of glass. This is to counteract the downward convective flow of the colder air that’s falling from the windows. They don’t bury the tubing too high up a wall, though. Someone just might hammer a nail through a tube.

They’re also aware that a radiantly heated wall will set up convective air currents in a room, and that’s something they try to avoid, especially in rooms with high ceilings.

In this building, they placed decorative hydronic convectors at several levels across the lobby’s glass wall. Here, too, their goal was to keep cold air from falling, and it seemed to work beautifully. The space was very comfortable. Near the lobby’s revolving doors, they placed high-output convectors to counteract the cold air that came in with the people.

If you used your imagination, you could “see” the air currents in this space trying to rise, trying to fall, and then finding a point of equilibrium. A lot of thought went into those invisible air currents. And this, by the way, is the sort of thing that annoys the spouses of Wet Heads. We can stand in one place for the longest time, staring at invisible air currents.

“What are you looking at, honey?”


Outside, they had the sloping driveway leading to the underground-parking garage piped for snowmelt. They also piped for snowmelt on the decorative ironwork, up near the top of the building. They didn’t want ice forming on that iron. They were concerned that a stiff wind might knock the ice off the ironwork and send it cascading down onto the people in the streets.

Good foresight, eh? I remember when the Citicorp building first opened in New York. Citicorp has a roof that looks like something you’d use to prop open the gates of Heaven. During the first winter, a sheet of ice that was about the size of the outfield at Yankee Stadium came down from that roof like a guillotine blade.

It made the New Yorkers move a tad faster than they usually more, which is pretty fast to begin with! The cops had to close off about ten square miles of Manhattan until some engineer could figure out what to do until spring arrived. The Germans must have read about this in the newspaper, and then shuddered. You live and learn, right?

As I wandered from floor to floor in the Kastor and Pollux building, I noticed a bodacious amount of Danfoss thermostatic radiator valves. I asked about them and learned that there were 4,500 of them sprinkled throughout the place. This is so typical of European construction. In Germany, the law requires that any room larger than eight square meters must have a TRV on the radiator.

I remembered a job I once visited in New York City. This was back in the mid-Seventies when the price of fuel was soaring, and TRVs looked like a neat way to keep people from opening their windows during the winter.

An engineer specified about a thousand Danfoss valves for this hydronically heated apartment building.

The contractor installed the valves, but then, during that first winter, many of those valves seemed to be bypassing. It turned out that the big pumps in the basement were the culprits.

Those pumps were there long before the Danfoss valves went in. They were so oversized that when the TRVs began to close, the pumps just drove them back open again. We wound up trimming the pump impellers to solve the problem.

In this German office building, they used Danfoss’ ASV differential pressure regulators to avoid this problem. They piped one between each two-pipe, direct-return riser to maintain a uniform differential pressure on all the riser groups in the building.

As the TRVs throttled the flow through the radiators, the differential pressure valves bypassed some of the flow across the risers.

The head-pressure problems we had in New York City weren’t a concern in Frankfort. You live and learn, right? Besides, the circulators in the Frankfurt building weren’t that large to begin with. They don’t oversize pumps in Europe, as we so often do in the US. The pumps in this building were all inline and each ran on variable speed. Grundfoss made those pumps.

We wandered some more and came to a loading dock. This space was also heated with a radiant floor. Mr. Grohal and Mr. Thaler explained that they didn’t like to use unit heaters as the primary source of heat for loading docks or garages because the heated air drifts toward the ceiling and escapes whenever someone opens the large overhead doors.

They sometimes use unit heaters as a secondary source of heat in these areas, but radiant is always their first choice.

They bury the tubing about eight inches deep in areas such as these because they’re concerned that someone may drive anchors into the concrete at some point to support equipment. Because of the depth of the radiant tubing, they don’t cycle these systems. Rather, they use a floor sensor to maintain the floor’s surface temperature at a constant 68 degrees F.

“It must be expensive to build this way,” I commented.

“Yes,” both men agreed. “But much better for the environment. And for the owners of the building as well. It costs them less to operate the system in the long term.”

I made a note of that statement, and the way they ordered their priorities. It’s one of the things that has helped this job live on in my memory. The environment first, and then the client.

How’s that for a difference?

Dan Holohan

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