When Tom O'Brien, owner of Thomas Heating and Cooling, Havertown, Pa., first got called in for a 9,500 sq. ft. custom home, the customer was ready to settle for a hydro-air job.

"But when I looked over the plans," O'Brien says, "I made a simple suggestion: Why not put a radiant floor in the children's playroom? Everybody knows kids love to play on the floor."

Since the playroom was located above a three-car garage, O'Brien said it would be easy to install a staple-up system.

The owner was more than intrigued - as an allergy and immunology specialist, the homeowner recognized that draft-free, dust-free radiant floor heating had major comfort and health benefits. O'Brien went on to suggest radiant tubing to warm the master bathroom's tile floor. To explain the concept, Tom showed some photos from other jobs.

The homeowner liked the idea. Then he gave O'Brien a suggestion of his own: Why not install radiant heating in the whole house?

And that's how O'Brien ended up installing 18,000 feet of PEX in the home's 16-zone radiant system, which encompassed the first and second floor.

Each zone uses an average of four to five loops comprised of 1/2-inch piping. Piping from the main manifold board by the boiler supplies the kitchen, foyer and laundry room tile floors. For other areas, piping is stapled up under the subfloor with hardwood flooring or carpeting.

But since the house is 115-feet long, a 250-foot branch using 1-inch PEX piping runs from the boiler to a remote manifold that feeds the living room, sun room, office and hallways in the northwest wing. Another branch using approximately 350 feet of 3/4-inch piping runs off this remote manifold to the second floor to supply the master bath, master bedroom and adjoining home office. All the circulators, relays and mixing valves for the second-floor circuit are located on the first-floor remote manifold to maintain quiet operation.

"Basically, I used the remote manifold to split the house in two halves," O'Brien says. "The larger diameter PEX piping was much easier to use for long branching runs than copper pipe. I could run one continuous length without soldering sections together. Plus, having a remote manifold allowed me to shorten the leaders to the zone loops."

For the radiant floor slab on the first floor, a thermostat maintains a supply water temperature of 115 degrees F for hardwood and tiled floors, and 140 degrees F for carpeted floors.

Six circulators supply water to 16 thermostatically controlled zones. Five mixing valves are used to maintain required supply temperatures, with individual zone valves opening up to supply a given zone. One boiler supplies the heating system, and another fires an 80-gallon indirect water heater.