Call MisterHouse to Regulate Your Heat

I am an architect and a software consultant, I am building my own home, and both my wife and I have allergies. Forced-air environmental control systems are not good for people with allergies. I have chosen to use radiant heating. Environmental control systems are one of the areas of home automation into which hobbyists rarely venture. Although the control systems for air conditioners and heat pumps are not particularly complex, a poor control system can wear out an expensive compressor early, if it does not destroy it. This taboo against directly controlling environmental control systems hardware extends to systems that are neither as easy to destroy nor as costly to replace.

This article discusses controlling a home radiant heating system using a home control system running under Linux. I have used several guiding principles in designing and implementing this system. I am a huge proponent of KISS (Keep It Simple), and I am an underutilized software consultant from a community that still believes pennies are money—I am cheap. I think that is a virtue. Simple trumps complex, cheap prevails over costly, software substitutes for hardware, do-it-yourself replaces buying or hiring.

Radiant heating systems are not particularly complex. My house is divided into zones, and each zone has its own controls. To some extent, the control system for the zone interacts with controls for the whole system. In a radiant heating system, when a zone requests heat, water is heated and circulated into something to radiate that heat into the rooms making up that zone.

The first system component is a heat source, and there is a wide variety of possible heating sources. I am using a 144,000 BTU wall-mounted instantaneous natural gas hot water heater with a 95% efficiency rating. One of the features of these units is that they are regulated and controlled internally. Simply drawing water triggers the electronic ignition. The unit itself controls its output to match demand. My unit has some ability to manipulate the preset water temperature as well as monitor its performance. Regardless of whether it is conventionally controlled or controlled by a home automation system, my water heater itself requires no external controls.

Figure 1. Takagi TkJr 144K BTUH Hot Water Heater

The next component is a heat sink—the way heat gets from the distribution system into the zone. In my house, this is PEX tubing, approximately 16" on center inside a concrete slab. There are many other ways of getting heat from the water into the zone, including baseboards, radiators and tubing stapled to the underside of floors. Each method has its own properties, but the most critical of these is the amount of heat it can radiate into the zone. PEX tubing in a concrete slab is one of the more efficient means of transferring heat to a zone. The maximum radiated heat from a concrete floor with radiant tubing is between 50–90K BTUH per 1,000 square feet or 50–90K BTUH for 1,000 square foot of floor—depending on how warm a floor you are willing to tolerate.

Proper design of the heat source and sink are beyond the scope of this article; however, they are not really complex. The heat sink must be capable of supplying more heat than the zone can lose in an hour under the most extreme conditions. The heat source must be able to provide at least as much heat as all zones could demand under the most extreme conditions. Under cold but less than extreme conditions, I do not want the system running most of the time; the norm is for the heat to run about one-third of the time. All heating systems are over-designed.

Another component is the circulator(s). Water must be persuaded to flow through the system. The norm is that at least one pump or circulator is required. Circulation systems can be designed in a number of ways. I have been able to procure small circulators very cost effectively (cheaply), so I have used a circulator for each zone. This significantly simplifies many other aspects of the design of the system. It eliminates the need for zone control solenoids and their controls. It eliminates controls for the circulator separate from those of the zone. It makes providing a zone-by-zone failsafe override easier, and it reduces the need to balance the system.

Figure 2. Radiant Distribution Manifold and Circulators

Another part of the system is a closed-loop water distribution system. Hot water needs to get from the heat source to the heat sink and back. In my home, this is more PEX tubing, as well as manually constructed manifolds.

Finally, there is the control system, and that is where Linux comes in.