Thermostatic radiator valves are perhaps one of the most underappreciated hydronic components in today’s market. And yet they are so simple, inexpensive and useful.

It seems everyone is focused on electronic controls for system zoning. The majority of systems I encounter in the field are zoned with zone valves or zone pumps. There will be a low-voltage thermostat placed in a location that best represents that zone’s temperature.

The thermostat is wired to a zone control panel. This panel receives the call for heat from the thermostat and opens a zone valve or energizes a pump to send heated water to the respective zone. The zone control panel also sends a signal to the boiler or heat source at the same time to provide the heated water for the zone. 

Here’s how a TRV is different. It is a nonelectrical, mechanical device that controls flow based on ambient room temperature. As the room warms up, the TRV starts slowing down the flow, and when the room cools down, the TRV allows the flow to increase. 

A TRV can be used to control a single heat emitter or it can be used to control multiple emitters in a zone. Many people seem to think they are only for use with radiators, as the name suggests. Nothing could be further from the truth.

I suppose TRVs were given an appropriate name when they were invented by Danfoss back in 1943. Almost everyone had those stately cast-iron radiators emanating warmth in every room of their house. And that’s what TRVs were originally designed for – controlling the heat output from the radiators to help balance the system and provide room-by-room zoning. Hence, the name. 

I would argue in today’s world, they should be renamed, “Nonelectric Zone Valves.” Not only to gain more market recognition, but also to make the name more accurate with modern-day applications. 

Pic A

Pic B

Probably most of the TRVs you have encountered were mounted on a radiator and looked something like the one in Picture A.

Historically, that has been one of the most common configurations. Yet, today, they come in all kinds of configurations suitable for many different applications. Everything from straight-bodied valves to three-way diverting valves to valves with remote sensors and valves with remote control bodies.

Picture B shows a wall-mount control head with a 16-foot capillary tube connecting the actuator. It’s easy to see how a control like this, coupled with a straight-bodied valve, might be used to control a heating zone with multiple emitters. 

How do they work?

Let’s take them apart, and see how they work. To get a visual, look at Picture C.

Inside the sensor head is the sensor element. It is a small container that is filled with an expandable liquid or wax. The liquid/wax will expand with an increase in temperature and contract with a decrease in temperature. When this occurs, it will open and close the valve disk respective to the increase or decrease in ambient temperature. 

The control head in Picture B works much the same way. However, instead of the control head mounting directly to the valve body and actuating the valve directly, it uses a capillary tube filled with an incompressible liquid to open and close the valve.

As the thermal element in the sensor head warms up, it expands, pushing on a diaphragm. On the other side of the diaphragm is the incompressible liquid, which gets pushed through the capillary tube and pushes on the diaphragm in the actuator. This diaphragm, in turn, will push down on the stem of the valve body, pushing the valve disk toward the valve seat and closing the valve.

The exact opposite happens when the sensor head cools off. Typically, there are springs to force the valve open as the thermal element cools down. 

This is how the TRV controls and modulates flow. 

Pic C

As simple as they are, there are still plenty of folks who want to make them complicated. I remember spending no less than an hour arguing with one of my sales guys about whether the TRVs close when they reach set point or whether they are partially open at that point. 

I said: “They modulate. They never close completely during heating season unless the room temperature goes above set point from some other source.” 

He said: “Not true! They open and close the whole way. If the room is at set point, the valve is closed. When the room drops in temperature it opens. That’s how they work." 

Neither of us was conceding, and I wasn’t making any money standing there arguing. So, I left. I contacted a TRV manufacturer. They should know the answer better than anyone else I assumed. 

Here’s what I was told: When the room reaches the set point temperature, the valve is still open by a fraction, allowing just a little bit of flow. They also said the valve typically operates within the last millimeter of its stroke, modulating the flow with minute temperature changes in the room. 

Why would I use them?

Why wouldn’t you? There are many benefits. They are inexpensive for starters. You don’t have to run any wires. You can easily provide room-by-room zone control without a cluster of thermostats, zone control panels and zone valves. They provide excellent comfort control in a hydronic heating system as well.

And they don’t use any electricity either!

How should I set up my TRV system?

This depends on what type of system we are talking about. Let’s go through a few of them. 

High temperature radiator system.

In this type of system, you will typically have a noncondensing, high-temperature heat source. It may use a limited outdoor reset curve, or it may be a fixed water temperature. You will want to install a TRV on each radiator, except the one in the coldest room. In this room, you will install a thermostat to turn the boiler and pump on and off. 

If you are controlling the boiler with an ODR (outdoor reset control) and it has a WWSD (warm weather shutdown) function, you can put a TRV on every radiator and place a jumper on the TT terminals of the boiler. The boiler will then turn on and off based on the high limit setting differential and add heat as needed. When it gets warm outside, the boiler will shut down. 

You should also use a variable speed pump for this setup. One that operates on proportional pressure or “Auto Adapt” technology. 

It should also be noted, this type of setup is best suited for a high-temperature boiler that has sufficient thermal mass, such as a cast-iron boiler. Some noncondensing boilers, such as copper fin-tube, are low mass and have a minimum flow rate that must be strictly adhered to. 

High-temperature baseboard system.

In this type of system, let’s look at having the same type of boiler and pump setup as the first example. However, instead of radiators, we have fin-tube baseboard as the heat emitters. TRVs can work just as well here as with radiators, but we install them a bit differently. 

The majority of fin-tube baseboard in my area is installed in a series loop. This means there are a number of baseboard units connected in series, spanning multiple rooms. Typically there will be a thermostat in a central location to those rooms to control that zone.

Pic D

We can still use TRVs to gain individual room temperature control, but they will be a different kind than you are used to seeing. For this we have to use a TRV with a three-way diverting valve body shown in Picture D.

The valve has to be installed at the supply of each baseboard heater. Then you have to install a bypass pipe connected to the bypass port on the TRV and teed into the return pipe of the baseboard.

There are a few different control head options available for this. One type requires you to drill a hole through the face or end of the baseboard end cap. The TRV sensor head will protrude through the hole to allow temperature sensing and adjustment. 

If you have multiple baseboard heaters in one room, you may want to install a three-way TRV with a remote wall-mount control head. The bypass pipe would run from the TRV valve to the return of the last baseboard in the room. 

The centrally located thermostat can still be used to turn the boiler on and off in this setup. The TRVs will serve as dynamic balancing devices and distribute the BTUs through the home proportionally to the temperature setting in each room.

There are also no flow concerns from the boiler side, since the water flow is being diverted rather than slowed down. This increases your flexibility of boiler choice, allowing good results with low-mass boilers as well as high-mass boilers. 

Micro zones.

TRVs are one of the greatest tools to eliminate the problems associated with micro zones. 

What is a micro zone? A micro zone is exactly that. It is a miniature zone when you compare the heat load of that zone to the heat load of the entire structure.

Since the boiler is sized to meet the heating demand for the whole structure, it is incredibly oversized for the micro zone. If the micro zone calls for heat while none of the other zones are calling, the boiler will short-cycle. This wears the boiler out faster and reduces efficiency. 

Here’s how to set it up: Use a TRV that best meets the application, and use it to control the flow going to the micro zone. The piping going to the heat emitters in the micro zone should be tied into the boiler distribution piping in such a way that it gets flow whenever any of the other zones turn on.

This will allow the room to receive heat anytime there is a call for heat in the building and the TRV will prevent the room from getting overheated. The micro zone cannot send a call for heat to the boiler, and so, will not cause any short-cycling problems. 

One of my favorite uses for this application is in bathrooms in homes with radiant floor heat. Bathrooms often don’t have a lot of floor space, and it seems they always have a window and considerable exterior wall. This drives the heat load up and the corresponding BTU/hour per square foot requirements, as compared to the rest of the house.

Instead of spacing the tubing at 8 inches, I might space it at 4 inches to increase the heat output and make the floor a more uniform temperature. The TRV controls the flow going through the tubing and prevents the room from getting to warm while keeping the floor warm more of the time. 

A few years ago, I was involved in modernizing a heating system in a house that was getting restored. This was an older historic house with tall ceilings, brick walls and giant old cast-iron radiators.

The house had been split into two apartments at some point, and after going through years of tenants, it had lost much of its original grandeur. The house got sold, and the new owner set out to restore the house back to its original beauty.

He hired me to put together a plan for the heating system. He wanted to keep the radiators, but get rid of the giant steel piping mains and the behemoth of a boiler sitting in the basement. This boiler had pumps on it so big that I just stood there and looked for a while. I wasn’t used to seeing this size of anything in a residential home.

In the next issue, we’ll go through the calculations, system design, piping, and control strategies that we used to bring this system back to life. 

Harvey Ramer is the owner of Ramer Mechanical (RM) LLC. RM specializes in radiant heating and hydronic heating systems. The company also provides other mechanical services to the residential and light commercial market. Ramer also provides heating system design services and consultation across the country. Contact him at hrvramer@yahoo.com.