It’s all mixed up. Well at least the water temperature is. Mixing valves have been around for a long time. From what I can find the original mixing valve was manufactured by the Leonard Company in 1911, but I know Honeywell use to make one also, along with the Powers Company. These valves used a bi-metallic bellows to adjust the water temperature. The original design was used for plumbing applications. Most plumbing mixing valves nowadays are either Pressure balanced or thermostatic.
Mixing valves are also used in heating system applications. I first started to see them used in heating when the radiant floor heat “revolution” started back in the 80’s. Yeah, yeah, I know… radiant has been around since the Romans but you know what I mean—when it really started being used in the States as a heating design to lower the temperature of standard atmospheric, high temperature boilers. I even saw some that were used as an early outdoor reset.
European-designed system used a diverting valve while most American companies used mixing valves and yes, there is a difference. There are also three-port and four-port mixing valves. Diverting valves are installed on the system return and have one inlet and two outlets. One outlet is the return back to the boiler and the other mixes with the boiler supply through a standard tee. This used to really confuse me, but now I have it down (I think). Mixing vales are located on the supply and have two inlets and one outlet. The boiler supply is mixed with cooler return water to supply the right temperature to the floor. Motorized head is used to adjust the temperature of the water.
What bothers me (well, along with a lot of other stuff) is that I see many radiant systems using a thermostatic set point mixing valve. So here, then, is the gist of my article (long time getting there, right?). When I started doing radiant I was taught to use a four- or three-port mixing valve. The idea being that since a radiant floor is a low temperature/high mass system you would need to adjust the floor temperature according to the outdoor temperature. A company Stadler (bought out by Viega a few years back) made a nice diverting valve system that used a capillary tube design to allow the three-port diverting valve to adjust the floor temperature. It also had an outdoor sensor. Buderus made a nice packaged mixing station with a built in pump and pressure differential by-pass. The mixing station needs a Tekmar control to operate. All these controls added to the cost of the job and if it was a small bathroom or kitchen with one or two loops of radiant the cost could be prohibitive. Don’t get me wrong, I love Tekmar controls and use them on many of our installs.
As these types of installations became more common the price of all those mixing valves pushed some towards using thermostatic set point mixing valves. These valves were set to deliver the temperature the design demanded, even during the coldest day of the year. But what happens during the warmest days of the year? What we call the shoulder seasons—October and April—you’re delivering more heat to the floor then it needs and you overshoot the thermostat demand. Many times I’ve had customers tell me “we love the floor but we can’t control the room temperature in the warmer seasons.”
So you either design a system with more equipment, or a less complex system gets installed that doesn’t provide comfort during all times of the year.
So along comes my favorite little mixing valve, the Taco I-Radiant mixing valve. I remember being at a trade show when I first saw it. My first thought was, “It’s plastic,” and it was, but it works and it works well. The valve comes with an outdoor sensor, a supply sensor and a boiler sensor. It’s easy to install and is competitive in price comped to other mixing valve stations. Many times when I’ve had a customer complain about the temperature of the floor we’ve replaced the set point valve with an I-Radiant valve and presto, comfort is achieved at a competitive price.
Now, I’m not saying that’s the only way we do radiant floor systems. On larger jobs I still like to use four-way mixing valves. The four-way mixing valve not only provides the correct temperature to the floor but also gives boilers protection by blending some of the return water to the boiler. This protects the boiler from condensation and thermal shock, a very important consideration with cast iron boilers where lower return water temp to the boiler can produce condensation in the boiler and shorten the lifespan.
There is no doubt in my mind that radiant is the best system of delivering heat to a home. But not allowing the system to be adjusted to the demands of the space its heating is just … mixed up.
Scott Milne is the owner of Milne Plumbing and Heating. He and his company have been serving the greater Boston area for nearly 30 years. He specializes in high-efficiency heating systems for custom homes.