What's the best control strategy? Two industry leaders take opposing views.

POINT: Simpler Controls Are Better Controls

by Mike Chiles, vice president and general manager, Watts Radiant

When prehistoric people discovered fire, they controlled the heat by throwing more logs in the fire. Today we let electrons regulate the heat. We stay warm and comfortable while automating both simple and complex tasks.

The room thermostat is a control. For a radiant heat system, the device directs heat to a specific space. When that space reaches the desired temperature, the thermostat may reduce or stop the heat delivery. More complicated and expensive controls provide indoor room temperature feedback, data logging, telephone and modem connections, and many other options.

Snowmelting controls may be as simple as a remote switch to activate the system manually, or as sophisticated as a system that interfaces many boilers, building automation, peak-load shaving, and thermal storage.

In a heating system, the initial system design and the controls selected by the designer determine the level of comfort. There are instances where complex controls are required, such as a building automation system. A sophisticated system may be warranted in special situations, such as heating low mass structures with large solar gain. But in our years of experience of designing, building and troubleshooting radiant heating and snowmelting systems, simpler is usually better.

The choice of a complex control strategy should be deliberate and only undertaken after sober reflection of the occupant’s needs, the budget and the capabilities of the local service technicians.

Without question, controls allow systems to operate more efficiently. Rather than using resources where they’re not needed, controls prevent excessive use of energy, allowing optimization of system design and comfort. More fundamentally, system design plays a key role in selecting controls that will accomplish the desired task, without excessive cost or complexity. Improper choice of controls can result in systems that are not cost effective, and may exceed the capabilities of local service people to maintain the system in good working order.

The heart of a good system starts with system design, beginning with room-by-room analysis and proper assignment of the rooms to appropriately grouped control zones. Zoning, or decentralizing rooms into areas of similar use, thermal load, always provides for more comfort, energy savings and a more elegant and stable system.

Simpler controls, combined with a sensible control strategy, offer advantages of comfort and reliability. Properly designed and installed, a system with simple controls will usually offer no less comfort than if more expensive controls were used; and may offer a much greater degree of reliability and customer satisfaction.

A broken, improperly programmed or improperly installed control is of no benefit whatsoever. A control that only a master technician can service is of no benefit if that technician leaves his employer or is away at another job when trouble hits.

Simple controls are also less expensive to purchase, install and maintain. With fewer parts in the system, overall cost and installation time are reduced. There’s also less wiring, fewer boxes to hang and fewer adjustments to make. With fewer parts, reliability is enhanced and the risk of improper programming is reduced. Customer satisfaction is higher and referrals are more likely.

One of the more popular arguments against simple controls is that radiant floors are difficult to control and require complex and costly controls. Nothing could be further from the truth for at least 90 percent of the radiant systems installed in the United States. Radiant floor heating systems are usually high mass systems that store energy in the floors, walls and ceilings, not in the air. As the air temperature inside drops, the radiant surfaces automatically release more heat to the building interior. As the air temperature increases, less heat is transferred. This elegant form of temperature control is accomplished solely through the laws of physics and actually requires very little guidance from complex controls.

There are many types of controls to regulate the incoming water temperature -- from mixing valves (either modulating or manual), to variable-speed injection pump controllers.

Manual mixing valves aren’t as accurate as automatic modulating mixing valves. But, in many high-mass radiant floor systems with plenty of concrete, exact control of water temperature is a nonissue at least to the occupants. When combined with a standard room thermostat, a manual mix valve offers reliable control in most instances.

Mix valves have been around for quite a while and have a record of reliability. They generally range between manual or automatic; three-way or four-way design. Three-way mixing valves vary flow between a hot port and a cold port, maintaining a constant flow and constant temperature from a mixed port. These systems are inexpensive and operate quite well. A thermostat turns on the circulators and heat source, while the mix valve provides the required output temperature to the hydronic zone.

An automatic, nonelectric mixing valve offers greater control than a manual-mixing valve. Unlike manual mixing valves, automatic mixing valves maintain a more or less constant output temperature despite varying input temperatures from the hot and cold mix-valve ports. And in most instances, automatic mixing valves do an admirable job of assuring comfort, efficiency and control for radiant floor heating systems. These mix valves are long lived, with simple to replace working elements.

Other modulating mix valves offer true automatic control that varies with the heating requirements of the space. Their controls measure heating requirements of a space by either measuring outdoor temperature, indoor temperature, or the supply and return temperatures for the system. These controls are simple in execution and operation and especially well-suited for lower-mass radiant floor systems.

Variable-speed injection pumps offer the same benefits of the modulating mix valve control, and in our experience may offer lower first cost advantages, as well as being more reliable than motorized valves. But the old adage, “Use the right tool for the right job,” holds true. What we’ve found in many instances is that radiant heat and snowmelt systems can be controlled in a simple fashion. They’re easier to install and maintain, perform well, deliver value and efficiency, and exceed customer expectations.

There is a time and place in the industry for a wide variety of control options. But a well-trained and experienced system designer will carefully design a system and thoughtfully select the components to meet the best interests of the occupants and building owners. By making systems unduly complex, especially at the residential level, we do the industry a disservice and price systems out of the range of the average homeowner.

The benefits of comfort and energy savings should not be restricted to those favored few who live in luxury homes. As the market matures we will see a growing acceptance that there are many suitable ways to control radiant and snowmelting systems, and that the simplest way to do the job is usually the best.

COUNTERPOINT: Performance Determines Control Choice

by Steffen Knuever, vice president, tekmar Controls Systems Ltd.

I agree that a properly designed and installed system is essential for proper operation of any heating system. However, part of that proper design must include consideration of all functions that the controls must accomplish.

Although manual mixing valves or tempering valves may be inexpensive at best they provide a constant year-round mixed water temperature out to the heating system. But that is not what the system needs! When the heating load is at its maximum or the system is recovering from night setback, that high supply water temperature may be appropriate. But, for most of the year, the heating load is smaller and a lower supply water temperature will provide the necessary heat transfer, improve energy efficiency, and also improve control of the indoor (room) temperature.

In order to adjust the supply water temperature to match the heating system’s minute-by-minute requirements, either mixing valves or variable speed injection pumps can be used as the mixing device. The mixing device is most cost-effectively operated by an electronic control. This electronic control can range from a simple “black box” with a few dials and/or switches to a sophisticated (or bewildering) array of displays and options. Usually, within this range of electronic controls, one can find the “right tool” for the job. There is no benefit to making the job more complex than it needs to be, but our goal is to make sure that the job is done right.

The choice of control is determined by the functions the control is expected to perform. For example, in addition to determining what supply water temperature the heating system requires, the control could also ensure that the boiler doesn’t receive such cool return water that its flue gases condense. Or, the control may exercise pumps and valves during the summer so they are not stuck and inoperative at the beginning of the next heating season. Or, the electronic control could coordinate domestic hot water generation and operation of the boiler with operation of the mixing device in order to improve the system’s operating efficiency.

When we buy a car we consider the various features and functions that are available such as A/C, cruise control, ABS, air bags, etc. They all add complexity to the vehicle and, typically, involve electronic controls, but they are useful functions. In fact, we now consider some of these technologies necessary equipment and won’t buy a car without them. In the same way, every hydronic radiant floor system should have controls and mixing devices that provide all the functions needed for occupant comfort, equipment durability and energy efficiency. It is very unlikely that all those features and functions can be provided by a manual mixing valve and a few basic thermostats.

It is the HRF system manufacturers, designers and installers’ responsibilities to provide the end-user with HRF systems that are able to provide the superior system performance promised to them by our industry. Any system can be made cheaper by putting simpler or no controls in it. The real question is how well will it work.