Active solar heating systems transmit the heated fluid—either liquid or air—directly to the interior area or to a storage system for later use. An auxiliary or backup system delivers the extra heat if the solar system is unable to offer sufficient space heating. 

When storage is present, liquid systems are more frequently employed and are ideal for absorption heat pumps, coolers, and boilers with hot water radiators. Systems that use liquid or air can be added to forced air systems.

Active Solar Liquid Heating

The best solar liquid collectors for central heating. They are identical to those found in household solar water heating systems. Although concentrating and evacuated tube collectors are also available, flat-plate collectors are the most popular. 

The solar heat is absorbed in the collector by a heat transfer or “working” fluid, such as water, antifreeze (often non-toxic propylene glycol), or another kind of liquid. A circulating pump is turned on by a controller when it’s time to flow the fluid through the collector. Since the liquid goes quickly through the collector, its temperature only rises by 10° to 20°F (5.6° to 11°C) during this time. 

The efficiency of the system is reduced, and heat loss from the collector is increased when a smaller volume of liquid is heated to a greater temperature. The liquid moves to a heat exchanger or a storage tank for immediate use. Piping, pumps, valves, an expansion tank, a heat exchanger, a storage tank, and controls are further system parts.

The heat transfer fluid affects the flow rate. See solar water heating for further information on the several types of liquid solar collectors, their sizes, upkeep, and other concerns.

Heat Storage in Liquid Systems

In tanks of water or the brick mass of a radiant slab system, liquid systems store solar heat. The heat from the working fluid moves to a distribution fluid through a heat exchanger outside or within the tank in tank-type storage systems.

Depending on the overall system architecture, tanks might be pressurized or not. Before selecting a storage tank, take into account its price, size, durability, location (in the basement or outside), and installation requirements. If a tank of the required size cannot go through the entrances that are currently in place, you might need to build a tank on the spot. 

Additionally, tanks must abide by regional construction, plumbing, and mechanical requirements, as well as temperature and pressure restrictions. You should also take notice of the amount of insulation required to prevent excessive heat loss and the type of protective coating or sealing required to prevent leaks or corrosion.

In systems with exceptionally high storage needs, specialty or specialized tanks could be required. They are often made of high-temperature plastic, fiberglass, or stainless steel. There are additional tanks made of concrete and wood (for hot tubs). Each style of the tank has benefits and drawbacks, and because of their size and weight, all require careful installation.

Instead of using one huge tank, it could be more feasible to employ numerous smaller ones. Using regular residential water heaters is the most straightforward storage system choice. They are simple to install, lined to prevent corrosion, and comply with building codes’ requirements for pressure vessels.

Heat Transfer in Liquid Systems

To disperse the solar heat, you can utilize a radiant floor, hot water baseboards or radiators, or a central forced-air system. In a radiant floor system, liquid that has been heated by the sun is pumped via pipes set into a thin concrete slab floor, which radiates heat throughout the space. 

Liquid solar systems benefit greatly from radiant floor heating because of how effectively it operates at low temperatures. Even though most systems include them for temperature management, a correctly constructed system might not require a separate heat storage tank. Back-up heat can be generated using a regular boiler or even a residential water heater.

Usually, tile is used to complete the slab. Compared to other kinds of heat distribution systems, radiant slab systems take longer to “warm up” the house from a chilly start. But once they start working, they deliver a steady stream of heat. Rugs and carpeting will lessen the system’s efficiency. For further details, see radiant heating.

To efficiently heat a space, hot-water baseboards and radiators need water that is between 160° and 180°F (71° and 82°C) in temperature. The transfer and distribution fluids are typically heated to temperatures between 90° and 120°F (32° and 49°C) using flat-plate liquid collectors. 

In order to use baseboards or radiators with a solar heating system, it is necessary to increase their surface area, raise the temperature of the solar-heated liquid using a backup system, or switch to a medium-temperature solar collector in place of a flat-plate collector. A liquid system can be integrated into a forced-air heating system in a number of ways.

Before the primary room-air return duct reaches the furnace, the basic design calls for the installation of a liquid-to-air heat exchanger, also known as a heating coil. As it passes through the liquid in the heat exchanger that has been heated by solar energy, air returning from the living space is warm. The furnace supplies additional heat as needed. At the lowest working temperature of the collector, the coil must be big enough to transmit enough heat to the air.

Preheating Ventilation

Air serves as the working fluid in solar air heating systems, absorbing and transmitting solar energy. Solar air collectors can either pre-heat the air entering a heat recovery ventilator or flowing through the air coil of an air-source heat pump, or they can directly heat specific rooms.

Air collectors may create more useful energy throughout a heating season than a liquid system of the same size since they produce heat sooner and later in the day than liquid systems. Air systems do not freeze, unlike liquid systems, and slight leaks in the collection or distribution ducts won’t have a big impact on the system’s operation, though they will reduce performance. Solar air collectors, however, function at lower efficiency than liquid solar collectors because air is a less effective heat transfer medium than liquid.

Despite the fact that some early systems stored energy by passing solar-heated air through a layer of rocks, this method is not advised due to the inefficiencies involved, the risk of condensation and mold growth in the rock layer, and the negative effects that moisture and mold have on indoor air quality.

To conceal their appearance, solar air collectors are frequently built behind walls or rooftops. For instance, airflow pathways might be included in a tile roof to utilize the heat that is collected by the tiles.

Space Heaters

For heating one or more rooms, air collectors can be mounted on a roof or an outside (south-facing) wall. Despite the availability of factory-built collectors for on-site installation, DIYers may prefer to construct and install their own air collectors. For a few hundred dollars, a straightforward window air heater collector may be constructed.

The collector has a black metal heat-absorbing plate with glass in front of it, as well as an airtight and insulated metal frame. The plate is heated by solar radiation, which then heats the air in the collector. Air is drawn out of the area, passed through the collector, and then blown back into the space using an electric fan or blower. Ducts are necessary to transport air from the room to roof-mounted collectors. Holes are cut into the wall for the collector air input and outputs, and wall-mounted collectors are then positioned immediately on a south-facing wall.

Simple “window box collectors” fit in a window opening that already exists. They may be passive or active (using a fan). Air enters passive collectors from the bottom, rises as it is heated, and then enters the chamber. When the sun is not shining, a baffle or damper prevents the room air from the thermosiphon back into the panel. Due to the relatively tiny collector surface, these systems only produce a modest quantity of heat.

Air Exhaled Collectors

Buildings are heated using a straightforward method called transpired air collectors. The collectors are mounted over a building’s south-facing wall and are made of black, perforated metal panels. The old wall and the new facade are separated by an air gap. Even when the outside air is frigid, the black outside façade absorbs solar energy and quickly warms up on sunny days.

Through tiny openings in the collectors and up through the air gap between the collectors and the south wall, a fan or blower introduces ventilation air into the structure. The air passing through the collectors is warmed by up to 40°F by the solar energy they have collected. Transpired air collectors don’t need pricey glass, in contrast to conventional space heating solutions.

Transpired air collectors are often inappropriate for today’s securely sealed dwellings since they are best suited for big structures with substantial ventilation loads. Small transpired air collectors, on the other hand, might be used to warm the air coil on an air source heat pump or to pre-heat the air entering a heat recovery ventilator, enhancing their effectiveness and comfort level on chilly days. The cost-effectiveness of employing a transpired air collector in this manner, however, is not yet known.

Aspects of Active Solar Heating Systems That Are Economical

In cold climates with good sun resources, active solar heating systems are most cost-effective when they are replacing more expensive heating fuels like electricity, propane, and oil. For solar energy systems, several states provide sales tax exemptions, income tax credits or deductions, and property tax exemptions or deductions. Could insert the following here: At DSIRE, you may find a list of incentives for renewable energy sources, including active solar thermal.

An operational solar heating system will range in price. Commercially available collectors have at least a ten-year warranty and ought to endure for many more years. By using a collector that would otherwise be idle to heat water in the summer, an active space heating system may operate more economically.

Using an active solar energy system to heat your house can drastically save your winter heating costs. A solar heating system will help lessen the number of greenhouse gases and air pollution that are produced when you heat your home or generate energy using fossil fuels.

Making a Solar Heating System Selection and Size

The location, architecture, and heating requirements of your home are all important considerations when choosing the best solar energy system. Your options may be limited by local covenants; for instance, homeowner organizations may forbid the installation of solar collectors on specific housing components (although many homeowners have been successful in challenging such covenants).

The amount of heat that a solar heating system may produce depends on the local environment, the kind and efficiency of the collector(s), and the collection area. Designing an active system to provide 40% to 80% of the heating requirements of the home is typically the most cost-effective option. 

Except when employing solar air heater collectors that heat one or two rooms and require no heat storage, systems that provide less than 40% of a home’s heat are rarely economically viable. A well-built, well-insulated home using passive solar heating techniques will need a smaller, less expensive heating system of any kind, and it may not even need much additional heat from sources other than the sun.

In addition to the fact that it is typically impractical and expensive to design an active system to provide enough heat 100% of the time, most construction codes and mortgage lenders need a backup heating system. When the solar system is unable to satisfy the required heating load, supplemental or backup systems provide heat. A wood fire or a traditional central heating system are both acceptable backups.

Solar Heating: Building Codes, Covenants, and Regulations

You should research local construction codes, zoning laws, subdivision covenants, and any unique rules governing the location before installing a solar energy system. In order to install a solar energy system on an existing structure, you most likely need a building permit.

While the majority of towns and communities support home renewable energy installations, there is a handful where such installations are more of a novelty and, as such, may not have been covered by their laws. To install your system, you must abide by current building and regulatory regulations.

Building code and zoning compliance for the installation of solar panels is normally a municipal concern. Even if there is a statewide construction code, it is often enforced by your city, county, or parish. The following are typical issues that homeowners have had with construction codes:

  • overly heavy for the roof
  • Insufficient heat exchangers
  • incorrect wiring
  • unauthorized interference with sources of drinkable water.

Possible zoning problems include the following:

  • Interfering with side yards
  • putting up unauthorized protrusions on roofs
  • placing the system too close to property lines, such as roadways.

Compliance is also necessary with special area laws, such as covenants imposed by the homeowner’s association, subdivision, or local community. It is simple to ignore these covenants, historic district restrictions, and floodplain rules. Contact the zoning and building enforcement departments of your local government as well as any relevant homeowner, subdivision, neighborhood, and/or community associations to learn what is required for local compliance (s).

Solar Heating System Controls

Because they must evaluate more signals and manage more components, controls for solar heating systems are typically more complicated than those for traditional heating systems (including the conventional back-up heating system). Sensors, switches, and/or motors are used in solar controls to regulate the system. Other controls in the system stop the collectors from freezing or getting too hot.

A differential thermostat, the brain of the control system, gauges the temperature difference between the storage unit and the collectors. The thermostat activates a pump or fan to circulate water or air through the collector to heat the storage medium or the home when the collectors are 10° to 20°F (5.6° to 11°C) warmer than the storage unit.

These controls operate, perform, and cost differently. To assess the system’s performance, certain control systems keep tabs on the temperatures of various system components. The most advanced systems employ microprocessors to regulate and optimize the flow of heat to storage areas and different parts of the house.

Solar panels can be used to run low voltage, direct current (DC) pumps or blowers (for air collectors) (for liquid collectors). The solar collector receives the same amount of heat from the sun as the solar panels can produce. The blower or pump speed is carefully set to maximize the amount of solar gain to the working fluid. The blower or pump runs slowly in low sun situations and quickly in high solar gain conditions.

It might not be essential to utilize separate controllers when used with a room air collection. Additionally, this guarantees that the system will function in the case of a utility power interruption. Although it is costly for big installations, a solar power system with battery storage may also supply power to run a central heating system.

How to Install and Care for Your Solar Heating System

Effective siting, system design, installation, component quality, and longevity all affect how effectively an active solar energy system functions. Although the collectors and controllers of today are of a high caliber, it might be difficult to get a qualified contractor who can correctly design and install the system.

A system must be properly maintained after installation in order to maximize performance and prevent failures. You should create a calendar outlining the maintenance chores that the component makers and installers prescribe for your installation since different systems require various kinds of maintenance.

Your homeowner’s insurance policy already provides coverage for the majority of solar water heaters. Damage from freezing, though, usually isn’t. To learn more about the policy of your insurance company, contact them. It is essential to notify your provider in writing that you have a new system, even if they will cover it.

If you are interested in more articles like this, here’s one about the uses of mini solar panels for home.


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