Radiant Heating 101: Mixing valves and controls for in-floor radiant heating

A solid control apparatus is the cornerstone of a well-designed in-floor radiant heating system. The chief function of a control system is to modulate and limit water-temperatures to maximize efficiency and to insure against over-heated floors.

There’s two dominant methods for controlling and modulating water-temperatures – mixing valves and injection pumps.

Mixing valves come in two varieties: 3-way and 4-way mixing valves.

A 3-way mixing valve has three pipe connections – hot (from the boiler), cold (from the floors) and a mix port (supply to the floors). The mix port blends hot water from the boiler with cooler water returning from the floors. This is how the mixing valve reduces the temperature of the water as it flows through the floors.A 4-way mixing valve also reduces water temperatures flowing to the floors like a 3-way valve. However, at the same time, it uses one additional port to ensure that hotter water returns back to the boiler. This serves the function of protecting the boiler from “thermal shock,” which can ruin most cast-iron boilers.

A 3-way mixing valve can be used instead of a 4-way valve under the following conditions:

1) When a condensing boiler constructed of stainless steel or aluminum is used. Condensing boilers do not need protection against low water temperatures, and, in fact, increase in efficiency the colder the water is.

2) The system employs large-mass boiler with significant (I mean significant) water-volume. A fine example of a large-mass boiler that does not require protection from low water-temperatures is Viessmann’s Vitola 200.

3) A primary-secondary piping system is employed. The primary loop, also called the “hot loop,” serves the role of protecting the boiler.

4) In a two-temperature system where the proportion of “low-temperature” heating (i.e. radiant floor heat) is small relative to the “high-temperature” (i.e. baseboard convectors, radiators, etc.) zones

Both 3-way and 4-way mixing valves modulate water temperatures by means of an actuator motor that hooks directly to the valve. The actuator opens and closes the valve to raise or lower the water temperatures flowing through the floors. This actuator will get its instructions via some type of electronic control device.

When purchasing a “motorized” mixing valve, bear in mind that actuator motors are designed specifically for a particular mixing valve model. If you get a Honeywell mixing valve, you need the matching Honeywell actuator motor – there’s no mixing and matching among manufacturers.

Furthermore, there are two dominant styles of motorized mixing valves: rotary and diverting.

Diverting-type valves are the simpler and usually less expensive of the two styles, and operates like a gate valve with three positions – open, close and mix. While fully open, the water will be hot. When fully closed, the water will be cold. When in the middle (mix), they’ll do just that. When budget is a concern, or the amount of radiant floor heat is relatively small, I’ll sometimes opt for less-expensive diverting valve, such as those made by Oventrop.

When the radiant load is large and I’m looking for greater accuracy in the water temperatures, a rotary valve will be utilized. A rotary valve utilizes an internal “paddle” that mixes water across a full range of temperatures. Examples include: Viessmann, DISMY (Danfoss), Rehau, Viega and Tekmar.

No matter which type of mixing valve is used – and they all work reasonably well – they’re only as good as their controller. Up until fairly recently, there weren’t too many options for mixing controls. The main player was Tekmar, who still is the major supplier. In addition to producing controls under their own label, they also private-label for other manufacturers including Viega, Taco, Watts and others.

Tekmar controls, such as the 360 Control, work very well in operating a mixing-valve. In addition to modulating the mixing valve, the 360 provides an outdoor boiler-reset function. If multiple mixing valves are to be used and/or you wish the control to perform other functions such as domestic hot water (with an indirect-fired storage tank), more sophisticated Tekmar controls are available. Another advantage of the Tekmar line is that they can be used with most any manufacturer’s mixing valves.

As far as proprietary brands go, I find Viessmann’s HK 1M Universal mixing control useful. The control is integrated within the housing of the actuator motor, and gives the system a very sophisticated look.

What I find attractive is not so much the control (which is very good), but Viessmann’s mixing valves. I find Viessmann’s 3-way and 4-way mixing valves to be among the highest quality, longest-lasting, and dead-nuts accurate. Their strength lies in their elegantly simple design.

For a simple and cheap mixing valve and controller, there’s also the Taco i-Series valves. They are cost-effective solution for smaller radiant applications where super-accurate temperature-modulation isn’t required.

As an alternative to mixing valves, it’s important to mention injection systems as a means of controlling water-temperatures. Instead of using a mixing valve, injection systems use a variable speed circulator pump to “inject” hot water from the boiler into the radiant system.

Injection systems do work well, however they require the use of many additional (unnecessary) pumps – at least three, but usually more -- in order to function.

Injection systems require the designer to employ primary-secondary piping. This means you have a primary pump on the boiler, a secondary pump for the main distribution loop, and typically, additional zone pumps. The actual injection pump itself sits between the primary and secondary loop. Too much for an otherwise simple job, in my opinion. As you would expect, an engineer developed this type of system!

As much as poo-poo injection, there are rare instances where this is the smarter, simpler option. These instances are very rare and usually only occur in larger commercial applications or huge McMansions where the system becomes very sophisticated -- not in basic residential jobs.

If you do choose to go with injection, you’ll need a controller just like you would with a mixing valve. Again, Tekmar is the primary player for these controls.

Finally, it is very common for many contractors, and especially DIY’ers, to substitute a modulating motorized mixing valve with a cheap thermostatic or fixed mixing valve intended for water-heating.

In many applications, it gets the job done, and on a few occasions, have done it myself. While it does serve the function of limiting outgoing supply-water temperatures, it doesn’t modulate, nor does it add any energy-efficiency benefits that a motorized valve brings to the table. Where it works is on a small, isolated radiant loop - when you’re trying to put some heat under a bathroom floor, for example.

I wouldn’t use a cheap fixed mixing valve to do an entire house. Nor would I use one when heating a concrete slab. Concrete slabs hold a tremendous amount of mass. They take a long time to heat up, but more critically, they take just as long to cool off if they become overheated. In my view, modulating mixing valves on an outdoor-reset are a must for slab applications.