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Radiant Floor Heating

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Ways to keep your floor warm during the winter

A visit to any plumbing and heating trade show will give evidence to the rise in popularity of radiant heat. Not only can it warm the home, it can melt ice off the sidewalk or driveway, or warm seedbeds in a greenhouse. Many factors contribute to its application and promotion. This series of articles will deal with hot water radiant.

Comfort is the prime factor. The human body uses the feet and head to dispose of excess heat. Because of this, warming the floor is not only physically pleasing, but the room will feel warmer than it actually is, allowing for lower temperature settings and fuel conservation. The downside to radiant for fuel conservation is slow recovery. With lower water temperatures in the tubing, setting the thermostat back at night or when the house is vacant for short periods is not possible, unless one wants to live with the discomfort while the temperature slowly rises. An alternative to this will be offered under the heading of "combination radiant and ducted".

Radiant heat can be installed in the ceiling when no alternatives are available, but the comfort factor and efficiency are compromised.

Aesthetics plays an important part in the installation. Baseboard radiators around the perimeter of each room or free standing cast iron units are eliminated. with the piping in the floor or ceiling, the decor is not affected.

With hot water as the median for radiant heat, domestic hot water can be produced by the boiler or water heater, allowing one appliance to do double duty.

The advent of computerized boiler controls that sense indoor and outdoor temperatures and adjust the operating temperature of the boiler to meet demand instead of maintaining a constant high temperature setting increases the overall efficiency of the system. During warm weather, the boiler maintains just enough temperature to satisfy domestic needs.

Radiant tubing comes in a variety of materials: Cpvc, polyethylene, pex (cross-linked polyethylene, and there are many types of pex as well), reinforced rubber, polybutylene, and the original one, copper.

Pex-al-pex and poly-al-poly describe tubing that is layered. A core of pex or polyethylene is wrapped with aluminum, then an outer layer of pex or polyethylene covers the aluminum. The aluminum forms an oxygen barrier to prevent deterioration of the plastic, and helps retain the shape of the tubing during installation. When bent into a curve to make return loops, the aluminum stops the plastic from springing back.

Radiant in Slab

If copper tubing is to be installed in a slab, certain precautions must be taken. If it is slab on grade, soil compaction is crucial. Flexing or cracks in the concrete can result in split tubing and leaks. The copper can be fixed in place with wire or tie straps to the re-enforcing wire or rods, but the steel and the copper must be grounded. The system must be a closed loop. Fresh water (hot water from a domestic hot water heater) will cause an electrolytic reaction between the chemicals in the concrete and the copper, eventually corroding and pitting the copper, creating plenty of leaks. If the slab is on top of wood joists, a double layer of 3/4" plywood is needed to keep the deck as rigid as possible.

Slab on Grade
After soil compaction, 2" of rigid insulation is recommended.The concrete re-enforcing mesh becomes a grid to fasten the tubing to with plastic tie straps or wire. At least 3/4' of concrete should cover the tubing to prevent weak spots that could collapse under heavy objects placed on the finished floor. If extremely heavy objects are to roll or sit on the floor, the deeper and more even the cover, the better.

Slab on Slab
When pouring a new slab over an old one, rigid insulation is recommended even if the old slab has insulation under it. Efficiency and recovery time will be improved. Tubing can be attached to the mesh or to tracks designed to clip the tubing into. At least 3/4' of concrete should cover the tubing for floor strength.

Slab over Wood
There are two ways to insulate the wood deck before pouring the slab over the tubing. Foil faced fiberglass insulation can be fitted into the floor joists directly under the deck, allowing a 2" air space. The tubing is then stapled to the deck or fitted in the tracks manufactured specifically for the tubing. An alternate method is to lay rigid insulation on top of the deck. The fastening devices must reach through the insulation and into the wood to prevent the tubing from floating up as the concrete is poured.

Wood over Slab
If a wood floor is to be placed over a slab, sleepers are used to create a space between the wood and the slab deep enough to fit the tubing on top of rigid insulation. At least 1" of rigid is needed, and 2 is better. Wood has nearly the same R-value as insulation, and if the tubing is sandwiched between two materials of equal insulation values, it will lose heat equally in all directions, including the slab and into the soil below it.

Radiant and Warm Air

There are times when it is impossible or next to impossible to get enough tubing in the floor or baseboard along the walls. Alternatives are floor mounted recessed units or free standing radiators. With central air almost the norm in new construction, ductwork is inevitable. Rather than push the hot water to its limits, using a hydro-air air handler (an air handler with a hot water coil and an evaporator coil for the A/C), the heating demand can be done with a radiant loop and backed up with ducted heat. Hot water is required for the radiant anyway, so adding another circulator and feeding the air handler isn't a big deal. By using a two stage thermostat, the radiant loop can be the first heat call, and the ducted the second. This design keeps the radiant as the priority heat source, and the warm air as an assist.

Air rotation creates the opportunity for a central humidifier and improved air filtration by using a high performance air filter or electronic air cleaner.

This combination gives the homeowner the ultimate comfort system: radiant heat in the floor, central air conditioning, air cleaning and humidification, and the ability to lower the room temperature during absences and retain the ability for fast recovery, which radiant cannot do.

Radiant Controls and Circulation

It is possible to treat a radiant loop the same as any other hydronic system by circulating the hot water as the thermostat calls for heat, but it is not recommended.

Constant circulation through the loops will keep the floor an even temperature and prevent cold spots from forming when there is no call for heat. It also helps keep air out of the system as well as reducing the risk of freeze up in applications where a domestic hot water is used as the heat source and anti-freeze cannot be used.

Sophisticated control units on the market sense indoor and outdoor temperatures and adjust the temperature of the water in the loops to keep the floor warm enough for comfort with minimal cool down. This type of control will also adjust the boiler temperature accordingly for fuel conservation. Another function is injection or mixing hot water into the return water to prevent shocking the boiler with a surge of cold liquid.

Non-electronic controls use a thermostat for the primary heat call that actuates a zone valve and/or circulator to feed hot boiler water to a mechanical control that uses a sensing bulb with a capillary tube to meter the hot water into the distribution manifold.

Many brands of manifolds are available that can be customized to any application. If individual valves for each loop are not built into the manifold, ball valves should be added so that each loop can be flow adjusted to balance the system.

Custom distribution manifolds can be made to order that include the circulator, automatic zone valves, a control panel with indoor and outdoor sensors, supply and return water sensors, and individual adjusting valves, nicely packaged on a frame for ease of installation. These appear expensive at first glance, but if one adds up all the components and the time it takes to construct it from scratch, the price is understandable.

Radiant with Wood Framing

Under Deck
Radiant tubing can be installed under an existing floor with staples, by drilling holes across the floor joists, or by just about any means the installer can design to place the tubing as close to the bottom of the sub floor as possible. Foil faced fiberglass insulation should be positioned with a 2" air gap between the foil and the bottom of the sub floor.

On Top of Insulated Deck
The insulation under the deck must have a 2" air space between the top of the insulation and the bottom of the sub floor. Wood spacers the same thickness of the tubing should support the floor over the tubing. The radiant works best if it is touching the bottom of the floor that is over it.

On Top of Uninsulated Deck
If the existing floor is not insulated and no access exists to do so, sleepers must be placed on top of the floor to create a gap deep enough to place rigid insulation between them that will carry the radiant tubing on top in such a fashion that it will touch the underside of the upper floor.

Masonry Filler
When tubing is installed between sleepers, a masonry filler will increase the thermal mass of the floor as well as act as a sound dampener. Do not fill the space around the tubing with products such as vermiculite or perlite, or any Styrofoam filler that can act as an insulator.

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