Solar Thermal Radiant Flooring
Solar thermal radiant floor heating is an old idea that has been made new again. Many years ago, both the Greeks and Romans heated the floors of baths with wood fires. The concept makes perfect sense - heat rises, after all. Today, solar energy is used to heat water that runs through piping in the flooring. Floors have a thermal mass, so they act as a sort of battery, storing heat for long periods of time.
The following information is adapted from the Energy Efficiency and Renewable Energy fact sheet put out by the US Department of Energy:
In many areas of the country, the amount of solar heat gain from solar thermal radiant flooring is not enough to heat a house by itself, but it can supplement a conventional furnace or boiler and reduce heating fuel consumption from 10 percent to as much as 90 percent.
Types of Solar Radiant Floor Heating
Solar heat collectors can be used with two types of radiant floor heating systems. Both systems use the sun to heat the fluid (air or a liquid) in the collectors. The thermal mass of the floor (the fact that it is 'solid' and 'deep') acts as both a storage battery and a heat sink. The latter function helps keep the room air from overheating.
Solar Radiant Air Floors
In solar radiant air floor systems the sun warms air from the collectors or a sunspace. Air passages might be made out of any of the following materials: plastic drain pipe, concrete block laid on its side, hollow-core concrete panels or layers of corrugated steel deck. In most cases, radiant air floors are not cost-effective because air does not retain much heat. Radiant air floors are used in some cases because they are relatively easy to install.
Solar Hydronic Radiant Floors
Hydronic radiant floor systems pump heated water from a storage tank through tubing laid in a pattern underneath the floor. An individual thermostat regulates the flow of heated water through a solenoid valve and manifold to each room in the house.
There are a number of ways you can use active solar collector systems with hydronic radiant floors.
- The simplest is to pump the heat transfer fluid from the solar collectors directly through the tubing in the floor. However, it can be difficult to control temperature of the water in this type of system. Extreme care must be taken to avoid under or overestimating the system's collector area-to-storage mass ratio. If this ratio is overestimated, there will not be enough heat; if you underestimate it, space can overheat. Most solar systems have the solar loop pump regulated by an electronic pump control.
- Many solar space-heating systems pump the heated liquid to a storage tank. The water in the tank is then pumped through the radiant floor piping. This type of solar radiant floor heating system uses a heat exchanger, similar to a solar thermal glycol-based system. This system provides much greater heat storage capacity and better control over floor temperature.
Back-Up Heating Systems
On cloudy days, when the sun is not bright enough to heat the home by itself, a back-up heating system is necessary. Back-up heating may be one of several heat sources: a wood stove, gas or oil-fired burners, or electric heating elements in the boiler. A conventional domestic water heater may be sufficient as a backup heater.
Cost of Solar Radiant Floor Heating
Whole-house solar heating systems initially cost $14,000 or more. In contrast, a conventional heating system might cost $4,000 to $8,000 for the same size house. The cost of installation often depends on the availability of materials and labor costs. The cost also fluctuates depending on the size, type of house, and amount of solar fraction available.
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