Rethinking TRVs for Steam Systems

In 1992, I wrote a book called, The Lost Art of Steam Heating. I did okay with that one. People still buy it.

I have a section in there about using thermostatic radiator valves with steam radiators. I'll show you first what I had to say all those years ago, and then I'll share with you what my friend, Paul Shay, had to say recently. Paul owns A Real Good Plumber in New York City and is one of the best steam men I have ever known. He gets his knowledge from studying and by trying things. When those things work, he usually lets me know about it, and it always makes me smile.

So first, let's go back to what I wrote in 1992:           

You can keep a one-pipe radiator from overheating by installing a thermostatic radiator valve (TRV) between the air vent and the radiator.

There are two parts to a TRV. The first is a spring-loaded, normally open valve. You screw this into the radiator in place of the air vent. Then you install the air vent into the valve body.

It’s best to use a straight-shank air vent with a TRV so the condensate can drain more easily. In other words, don’t use the angle-type air vent you found on the radiator. Buy new ones.

The second part of the TRV is an adjustable, non-electric thermostat that fits over the valve stem. The thermostat is adjustable within a range of (usually) 50 to 90 degrees F. The building owner sets it for whatever feels best and then leaves it alone.

The thermostat is filled with a temperature-sensitive liquid or wax. As the air around the radiator gets warm, the liquid or wax in the thermostat expands. The force of expansion gets passed through a bellows and onto the valve’s stem.

The stem closes the valve and keeps the air from escaping the radiator. Since no more air can get out of the radiator, no more steam can get into the radiator, so the room doesn’t overheat.

It’s simplicity itself.

You don’t use one of these valves in the room that has the electric thermostat. That room should be the coldest in the building. It’s the “control” room for the system.

Keep in mind, TRVs don’t make heat; all they do is prevent overheating. They don’t balance systems, either. Pipe sizing and main vents balance systems. But TRVs do make buildings more comfortable, and they save energy because folks are less inclined to open the window when the steam comes up.

For one-pipe steam, I prefer TRVs that have a built-in vacuum breaker. That’s a detail that makes a big difference. Once the TRV closes on temperature, air can’t get back in because the vent is on the other side of the closed TRV. As the steam condenses in the radiator, a partial vacuum forms. Unless you have a way to break that vacuum, more steam will enter the radiator because high pressure goes to low pressure. Always! The result? You get an overheated radiator.  The built-in vacuum breaker solves that problem.

If you use TRVs and find the radiator overheating anyway, check the stem packing on the supply valve. Leaky packing can act as an air vent and bring more steam into the radiator.

No one makes a TRV for the inlet side of a one-pipe radiator (everyone makes them for two-pipe radiators).  That’s because you can’t throttle the inlet valve to a one-pipe radiator without getting water hammer.

If you decide to use TRVs, don’t down-size the boiler to compensate for the reduced demand. You can’t do that because when the system first starts, all the TRVs will be wide open. They’ll also be wide open on a very cold day.

You still have to size the replacement boiler to the connected radiator, plus a pick-up factor – just as you would if the TRVs weren’t there. 

So this is what Paul Shay wrote to me: 

"Dan, I was taking a picture of the wrong and right way to install a second radiator (on page 96 of the Lost Art) to send to a particular man who just does not like physics. I will forward you what I just sent him and you will laugh.

"Then I noticed your section on adding TRVs to a one-pipe steam system. I have not read that section in years, so I read it again.

I know you wrote this a long time ago, and probably have no plans to update that book, but there are a few things I have learned along the way when installing TRVs on one-pipe steam.

"The most important is that they don’t work well if the boiler pressure is over one- or two-psi pressure. I started warning my customers years ago that if the building is sending them five pounds of boiler pressure, the radiators will heat up anyway. The pressure forces its way partially into the radiator, condenses and lets in even more steam.

"In a 120,000 square-foot project we did last summer (NYSERDA financed it), we installed 112 Gorton #2 main vents, 300 TRVs, and 1,200 feet of three-inch-thick fiberglass insulation. That’s the most three-inch insulation I have ever installed, and I was very impressed with how much better the system operated, compared to the buildings with the one-inch-thick insulation we normally install. NYSERDA required that we use three-inch and I learned something.

"We had to go all the way down to 1 psi on the vaporstat before the TRV would actually keep 98% of a radiator from heating. Now we are operating the system at a high pressure of just 14 ounces.

"Another smaller (more picky) point I would make is that TRVs do help balance a system once you have master vented the building. TRVs without master venting would be a disaster, for sure.

"You say in Lost Art that TRVs make buildings more comfortable, but what makes people comfortable is a balanced system, and TRV do help with that.

"This third point is very controversial, I know, and not really related to TRVs, but I thought I'd mention it. At the very end of what you wrote about TRVs and steam systems, you mention the importance of sizing a boiler to the connected load. When you first raised this concept 24 years ago, it put some science into boiler sizing. Compared to the methods used at that time, such as sizing the system using your fingers, or just going one size larger than what was there when you arrived, what you wrote was revolutionary.

"However, the industry seems to have become obsessed with the importance of connected-load boiler sizing. They argue that you must fill all the radiators with steam, but I ask why? If the radiators are larger than you need to heat a room, and collectively, more than you need to heat the whole building, why do you need to fill them with steam?

"In my opinion, the battle is to make sure you don’t fill them with steam, because if you do, you will overheat the building. We do have to fill all of the distribution pipes with steam, and having the correct boiler size, master venting, and proper fiberglass insulation are very important to make that happen. So what I do is compare the connected load with a simple heat-loss calculation and see where it takes me. I get it right every time and often the boiler is smaller than the connected load shows it should be.

"Just my thinking." 

And fine thinking it is, Paul.

I will never have enough years to truly learn all there is to know about these older systems, but the journey is a reward in itself, and one does get to meet some very smart people along the way.