Thermostatic radiator valves


Constructed in 1914, this building in Nanaimo, B.C. still has much of its original hydronic heating system.

A quick and easy way to provide hydronic zoning

By Roy Collver

Some hydronic products are so well designed and practical that they persist without change for many decades. Seventy-five years ago, Mads Clausen, Danish inventor and founder of Danfoss, developed the first thermostatic radiator valve (TRV). The product took off like a rocket. The company has since manufactured over 350 million of the devices in a wide array of configurations, styles and sizes.

Other manufacturers have created their own designs, also shipping them by hundreds of millions all over the world. In Europe, you see them everywhere you go. What need in the market did Danfoss fill with this product, and do we still have that need today?

Back in 1943, hydronic and steam heating systems were, at best, controlled by a single thermostat. Many were controlled only by the occupants turning the boiler on when it got too cold, and off when it got too hot. Rooms were often too hot, some were too cold, and it didn’t take modern energy audit software for people to realize that there was a lot of waste going on.

The TRV provided a simple way to zone these early systems room-by-room and radiator by radiator. Removing the existing manual hand-wheel flow control /shut off valve and replacing it with a TRV was an easy fix. It still is. There are countless older (and not so old) systems in operation today that do not even have manual flow control valves on each radiator. Unscrew the existing hot water supply elbow, replace with an angle TRV valve and provide your customer with instant zone control.

These devices are an inexpensive, reliable, accurate and stand-alone product that is easy to install. They don’t use energy to operate, don’t need to be upgraded three years after you install them and don’t need software updates. How do they work, where can they most effectively be applied and why are they only popular in certain areas

The building still has the chute for the original coal-fired boiler, which was later updated to oil.

Simple but sophisticated

The hardware for these valves is elegant in simplicity but the technology is quite sophisticated nonetheless.

Some new gee-whizz electronic radiator valves have started to appear, but we will talk about those some other time. Most TRV’s are made up of two basic components – a valve body with integral throttling device and a mechanical thermostatic actuator that attaches to the body.

Valve bodies come in many shapes and sizes, but designers need to be careful to ensure a given manufacturer has North American distribution and support. International catalogues often show a dazzling array of choices, but they may be incompatible with our piping and fitting sizes.

Thermostatic actuators generally attach to valve bodies with a threaded securing ring and can be installed or removed without draining the system. The throttling valve in the body is spring loaded to open the valve to full flow/heat if the actuator is removed. Installing the actuator pushes the valve stem into the body towards a valve seat, which reduces the flow.

Thermostatic valves will replace the manual valves on the building’s cast iron radiators.

The most popular configuration is a rotating knob with numbers indicating higher or lower temperatures. The lowest number (usually a 0) pushes the valve stem to the fully closed position. As the knob is turned to higher numbers, the valve stem is gradually retracted, allowing more flow and heat. The thermostatic element inside the knob is filled with a substance (liquid, wax, or gas) that expands at a precise rate when heated, and as the air around the actuator heats up, the element expands and throttles the hot fluid flow down to cool off the radiator. Once set, the actuator will modulate the flow through the radiator to maintain the room at a very precise temperature.

Hydronic upgrade

I recently poked around a 1914 vintage building in Nanaimo, B.C. to have a look at their still functioning hydronic gear with an eye to upgrading the system. The original design had been messed with here and there but was basically in its original state with most of the cast-iron column radiators still in place and connected to the still liquid-tight 104-year-old steel and cast-iron piping system.

Someone had told the building owners that it was a bad idea to replace the existing oil-fired boiler with a gas-fired condensing boiler – an assessment with which I strongly disagreed. These hundred-plus-year-old cast-iron and steel hydronic systems often present a great retrofit opportunity for gas-fired condensing boilers as long as you pay attention to the details and respect the original system design.

Danfoss developed the radiator thermostat in 1943 and, since then, approximately 350 million have rolled off the production line.

One of the most cost-effective and comfortable ways to marry these systems to a new condensing boiler is by using the boiler’s built-in outdoor reset functions to reduce water temperature and provide constant flow through one of the new “smart” pumps. This will provide a nice even heat output from the radiators at all load conditions. Utilizing TRV’s for zone control and temperature limitation of individual radiators in hot zones further refines the system. Many TRV models are specifically designed for these retrofit applications with valve body configurations that will fit exactly into the space left after the old manual valves have been removed.

A good idea

Here are a few reasons why TRV’s are a great idea:

  • Simple user interface – (engineer speak for uncomplicated) – it doesn’t get much easier to understand than a knob with numbers on it. 0 means no heat; 10 means a lot. That’s valid in all languages. It’s easy to troubleshoot over the phone – you may only need to explain what the snowflake symbol means (“away” freeze protection setting).
  • No wires required – the thermostat is built right into the valve.
  • Simplify new installations – These devices are great for new construction as well because they can simplify the design. Many TRV’s include integral isolation valves and air bleed valves and are configured to eliminate other bits and pieces.

And the downsides?

Here are some reasons TRV’s are not as popular as they should be many places:

  • Too simple of a user interface – many people can’t help but over-think everything. They want to know precisely what the exact temperature is, they want to see the actual degrees, in the scale of their choice. For some reason, they seem to want to be constantly fiddling and checking things. It’s hard for these folks to deal with “set and forget”. Happy TRV users, however, find the number makes them comfortable, and then embrace the fact they have one less thing to worry about.
  • You have to bend over to adjust them. This is a deal breaker for more people than you can imagine, but there are TRV’s on the market with long capillaries and wall mounted temperature dials to satisfy these folks.
  • No landlord override – Using night setback functions and outdoor reset control of boiler supply temperature does a good job of restricting excess heat to apartments, but that might not be enough control for those landlords or building owners who want to micromanage their tenants, gather data on energy use, or be able to monitor and override from a remote location.
  • No integration with other system controls: This can be a problem if it isn’t accounted for in the overall system design. The answer can be as simple as operating the system from a thermostat in a designated master zone. If the coldest zone is the one calling for heat, the remaining zone temperatures can be individually trimmed by the stand-alone TRVs – a strategy I am going to recommend for the 1914 building mentioned above.
  • Lack of local supply and support: In some areas of the country, the local wholesalers have no experience or expertise with these products and do not carry stock and may not even have access to most TRV product lines. All I can say is be persistent or go online and look – you should be able to find what you need and locate a reasonably close source of supply. Plan ahead.

TRVs are under-utilized technology. We have become so enamoured with the newer digital stuff that we fail to appreciate the almost Zen-like simplicity of the TRV. They work really well and leave you be. If the building occupant is a bit chilly, they simply turn up the dial to the next number. Why sit in your living room lounge chair, right next to your radiator – and have to stop texting or watching videos on your smartphone in order to fire up your thermostat App so you can check the temperature and turn it up a degree or two (provided your App doesn’t need to be updated and you can remember your username and password)? Just lean over and tweak the TRV knob.


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