Solar Hot Water, Heating and Cooling Systems


  1. Introduction
  2. Active Solar Domestic Water Heating
  3. Active Solar Space Heating
  4. Active Solar Space Cooling
  5. Passive Solar Water Heating

CSI Numbers:

15424 Domestic Water Heaters
15050 Basic Materials & Methods
15175 Tanks
15180 Insulation


The solar systems that will be discussed in this section are not a part of a building’s structure. The function of the solar energy equipment is to convert sunlight to heat that can be used for: (a) space heating; (b) space cooling; (c) domestic hot water.


Solar systems should be employed only after extensive conservation strategies have been implemented. Solar energy systems typically have a high initial cost and extremely low operating costs. To reduce the high initial costs, reduce the size of the required system by the load that the solar system will need to provide. In space heating and cooling applications, the home should be weatherized and insulated to very high standards. In water heating applications, hot water piping should be insulated and water conserving fixtures should be used.

The goal of the solar system should not be to accomplish 100% of the home’s heating, cooling, or water heating needs under all conditions. The system should be sized to reflect seasonal variations in demand and in the sun’s heating characteristics. Additionally, by combining systems to perform multiple functions (i.e. space heating and water heating), the solar system investment can provide a return all year.

The City of Austin will provide a rebate under the Appliance Efficiency Program for solar domestic hot water systems if installed in an all-electric home. There are no tax incentives currently available to assist in the first costs of solar systems, but there is an exemption for solar energy devices from being appraised for property tax. There has been a dramatic reduction in the number of businesses and equipment relating to solar systems since federal tax incentives were eliminated in 1985. Only a few businesses in our area provide solar systems.

Solar installations are governed by City Ordinance 900104-J. This ordinance follows the 1988 Uniform Solar Energy Code established by IAPMO (International Association of Plumbing and Mechanical Officials). Building, plumbing, and mechanical (when the system provides space conditioning) permits are required for solar installations. There are several types of solar systems in each of the categories of space heating, water heating, and space cooling. Of the three general categories, space cooling by solar energy is the least cost effective except in passive applications, which are discussed in the Passive Solar Design section.

Active Solar Heat Satisfactory Satisfactory in Limited Conditions Satisfactory in Limited Conditions Satisfactory in Limited Conditions Satisfactory in Limited Conditions Satisfactory in most conditions
Active Solar Cooling Satisfactory in Limited Conditions Unsatisfactory or Difficult Unsatisfactory or Difficult Unsatisfactory or Difficult Satisfactory in Limited Conditions Satisfactory in most conditions
Passive Solar Water Heating Satisfactory Satisfactory in most conditions Satisfactory Satisfactory Satisfactory Satisfactory
Active Solar Water Heating Satisfactory Satisfactory in most conditions Satisfactory in most conditions Satisfactory Satisfactory in most conditions Satisfactory
Satisfactory Satisfactory
Satisfactory in most conditions Satisfactory in most conditions
Satisfactory in Limited Conditions Satisfactory in Limited Conditions
Unsatisfactory or Difficult Unsatisfactory or Difficult



Active and passive solar space heating and water heating, are well-developed technologies. Active solar space cooling is marginally developed.


There are adequate suppliers on a national basis for all solar equipment except space cooling. There are few local suppliers.


Solar domestic water heaters are reasonably priced ($1000-$3500) and can show pay backs of four to seven years depending upon the fuel displaced (electric or gas). Space heating systems can vary from inexpensive wall heaters ($800) to costly large central systems ($4000+). Space cooling systems are not currently competitive. Reducing demand to keep systems small helps control costs.



Most lenders are not knowledgeable of solar systems.


There are problem areas associated with the general public perception of solar systems: solar may be considered futuristic; some may believe new technological breakthroughs are needed to make solar viable; solar systems are considered uneconomic; and, the business instability of solar system providers during the early 1980s. A primary concern for owners of a solar system is whether it can be maintained by conventional means (the owner does not have to assume extraordinary responsibilities).


City Ordinance 900104-J adopted the 1988 Uniform Solar Energy Code of IAPMO (International Association of Plumbing and Mechanical Officials). The “Solar Energy Code” is found in Chapter 13-8-500 of the Land Development Code in Article VII. The Solar Energy Code presents equipment and installation standards. Building, plumbing, and mechanical permits are required for space conditioning.


1.0 Introduction

Solar energy can be captured for use in a home in several ways. This section will look at using solar energy to heat water and/or air. The hot water created by a solar system can be used for domestic hot water or space heating. Hot air solar systems are primarily used for space heating.

The fundamental requirement for a solar system is to have a sunny location where the solar collectors can be located.

The collectors should have full sun from 9 AM to 3 PM.The collectors should face south at approximately the same angle as our latitude (30 degrees).

Collectors can be oriented as much as 30 degrees off of south and still function well. Similarly, the slope of the collectors can vary by plus or minus 15 degrees without significantly harming the performance of the system.

2.0 Active Solar Domestic Water Heating

The active water systems that can be used to heat domestic hot water are the same as the ones that provide space heat. A space heat application will require a larger system and additional connecting hardware to a space heat distribution system.

2.1. There are five major components in active solar water heating systems:

  • Collector(s) to capture solar energy.
  • Circulation system to move a fluid between the collectors to a storage tank
  • Storage tank
  • Backup heating system
  • Control system to regulate the overall system operation

2.2 There are two basic categories of active solar water heating systems – direct or open loop systems and indirect or closed loop systems.

2.2.1 Direct Systems

The water that will be used as domestic hot water is circulated directly into the collectors from the storage tank (typically a hot water heater which will back up the solar heating).

There are two types of direct systems – draindown and recirculating. In both systems, a controller will activate a pump when the temperature in the collectors is higher than the temperature in the storage tank.

The draindown system includes a valve that will purge the water in the collectors when the outdoor temperature reaches 38 degrees. When the temperature is higher than 38 degrees and the collectors are hotter than the storage tank, the valve allows the system collectors to refill and the heating operation resumes.

The recirculating system will pump heated water from the storage tank through the collectors when the temperature drops to 38 degrees.

These two systems have serious drawbacks. The draindown valves can fail in a draindown system and the result can be the expensive breakage of the solar collectors. The draindown valve will typically sit unused for a very long time and then will need to work the first time without failing. The cycling of air and water in a draindown system collectors as a result of periodically draining down (thereby emptying the collectors) can cause a buildup of mineral deposits in the collectors and reduce their efficiency. The recirculating system circulates buildup from potable water heated from the storage tank through collectors during potential freeze conditions and effectively cools the water (wasting energy).

2.2.2 Indirect Systems

Systems that use antifreeze fluids need regular inspection (at least every 2 years) of the antifreeze solution to verify its viability. Oil or refrigerant circulating fluids are sealed into the system and will not require maintenance. A refrigerant system is generally more costly and must be handled with care to prevent leaking any refrigerant.

An indirect system that exhibits effectiveness, reliability, and low maintenance is the drainback system (see Figure 1 on next page).

The drainback system typically uses distilled water as the collector circulating fluid.

The collectors in this system will only have water in them when the pump is operating. This means that in case of power failure as well as each night, there will be no fluids in the collector that could possibly freeze or cool down and delay the startup of the system when the sun is shining.

This system is very reliable and widely used. It requires that the collectors are mounted higher than the drainback tank/heat exchanger. This may be impossible to do in a situation where the collectors must be mounted on the ground.

An indirect or direct system can be used for heating swimming pools and spas. Lower cost unglazed (no glass cover) collectors are available for this purpose.

Drainback Hot Water System

Figure. 1
Drainback Hot Water System

The fluids that are circulated into the collectors are separated from the heated water that will be used in the home by a double-walled heat exchanger.

A heat exchanger is used to transfer the heat from the fluids circulating through the collectors to the water used in the home. The fluids that are used in the collectors can be water, oil, an antifreeze solution, or refrigerant.

The heat exchangers should be double-walled to prevent contamination of the household water.

The controller in these systems will activate the pumps to the collectors and heat exchanger when design temperature differences are reached.

The heat exchanger may be separate from the storage tank or built into it.

2.4 Guidelines summary for solar domestic water heating systems:

A well designed system will provide 50-80% of a home’s hot water needs (less in winter, more in summer).

There should be 10-15 square feet of solar collector area for each person in the household.

The storage tank should hold 20-30 gallons per person.

There should be no shade on the collectors during the hours from 9:00 AM to 3:00 PM.

The collectors should face south and be tilted at a 30 degree angle (slight variations noted above will not significantly harm performance).

The collectors and storage tank should be in close proximity to the backup system and house distribution system to avoid excessive pipe losses. The pipes need to be well insulated.

Mixing valves or thermal shutoff devices should be employed to protect from excessively high temperatures.

Select systems that are tested and certified by the Solar Rating and Certification Corporation (SRCC).

3.0 Active Solar Space Heating

The active solar space heating system can use the same operational components as the domestic water heating systems, but ties into a heating distribution system that can use heated fluids as a heat source. The distribution system includes hydronic radiator and floor coil systems, and forced air systems.

Solar collectors are also constructed that heat air. The hot air developed in such collectors can be used directly in the home during the daytime or stored in massive materials (rock or water).

3.1 Water Heating Collectors

3.1.1 The tilt of space heating collectors is generally the latitude plus 15 degrees (45 degrees in Austin).

The purpose is to align the collectors perpendicular to the sun’s rays in the heating season when the optimal performance is needed.

3.1.2 The number of collectors used in a space heating application is based on the heat load of the house.

Average heat load / collector rated heat output = number of collectors needed.

By basing the size of the collectors on the average heat load of the home during the heating season, the system will not provide enough heat during the colder part of the heating season. Since the heat load of the house is dependent upon the extent of its energy conserving features, the greatest energy efficiency the home can have, the smaller the solar system will have to be.

3.1.3 The space heating system, like the domestic water heating system, must be backed up by an auxiliary heating system.

It is not practical to size a solar system to provide all of a home’s heat requirement under the worst conditions. The system would become too large, too costly, and oversized for most of the time.

3.1.4 The storage system should be sized to approximately 1.5 gallons of storage for each square foot of collector area. The fluid that is heated and stored (typically water) can be distributed into the house heating system in the following ways:Air distribution system – The heated water in the storage tank is pumped into a coil located in the return air duct whenever the thermostat calls for heat. The controller for the solar system will allow the pumping to occur if the temperature in the solar heated water is above a minimum amount needed to make a positive contribution to heating the home. An auxiliary heater can be used in two ways. It can add heat to the solar storage tank to maintain a minimum operating temperature in the storage tank at all times. In this case, the coil from the solar system will be located at the air handler supply plenum rather than in the return air duct. The auxiliary heater can also be a conventional furnace that will operate less often due to the warm air entering the air handler from the solar coil in the return duct.


Figure 2
Space Heating System

Hydronic system with radiators – The heated water is circulated in series with a boiler into radiators located in the living spaces. Modern baseboard radiators operate effectively at 140 degrees. Solar heating systems can very often reach that temperature. Using the solar system’s heated water as the source of water for the boiler will reduce the boiler’s energy use particularly if it senses the incoming temperature and will not operate when that temperature is above the required distribution temperature.Hydronic system with in-slab heat – The solar heated water is pumped through distribution piping located in the floor of the home. Lower temperatures are used in this type of system (the slab is not heated above 80 degrees in most cases). The auxiliary heat can be connected in series with the solar system’s heated output water or it can be connected to the solar tank to provide a minimum temperature.

In the Austin area, most homes use an air distribution system that can provide air conditioning as well as heating. The hydronic systems are much less common but are considered highly effective in terms of comfort, efficiency, and health impact (no blowing air to stir up dust). The air distribution method described above can work quite well with a conventional gas water heater as a backup. (This is discussed further in the Gas Water Heating Section.)

3.2 Air Collectors for Heating

Appear similar to a water collector.

Usually a black metal absorber in an insulated box with a glazed cover (glass or plastic).

Air from inside the house is drawn by a fan into a series of channels in a space behind the absorber where it is heated by the hot absorber plate. The heated air then enters the home directly or enters a storage medium (such as rocks) so the heat will be available during the night.

A simple controller is used to turn on the fan(s) in this type of system. The controller uses sensors in the collector to activate the system when it is hotter in the collector than in the house interior or storage medium.

Air collectors can be mounted vertically on the south wall of a building if used for space heating only. In that location, properly designed overhang will prevent them from heating up in the summer.

For a year-round application of air heating collectors, it is necessary to use an air-to-water heat exchanger. This is not a very efficient system for heating water compared to fluid circulating collectors, since heat (and thus efficiency) is lost at each transfer point.

Air collectors are more practical in climates with longer and colder winters than in Austin. The investment in storage systems for air collectors is substantial in time, money, and materials. The use of air collectors to put heat into the house directly can be readily achieved with properly oriented windows in our area. Daytime temperatures in the winter can be relatively high; the additional hot air from an air collector can overheat a home that does not have extra thermal mass to absorb the heat.

4.0 Active Solar Space Cooling

Solar space cooling is quite costly to implement. If the solar system is used for space cooling only, installed costs can run $4,000-$8,000 per ton. It is best to use a solar system that serves more than just the cooling needs of a house to maximize the return on investment and not leave the system idle when cooling is not required. Significant space heating and/or water heating can be accomplished with the same equipment used for the solar cooling system.


Figure 3
Schematic of Solar Absorption Cooling System
T = system flow sequence

4.1 Dessicant and Absorption Systems

The technologies that are being developed for gas cooling systems are the same ones being developed for active solar space cooling systems. Desiccant cooling systems and advanced absorption systems are the primary technologies that are used. High temperature liquid collectors are typically used in these systems.

4.1.1 Desiccant system

A moisture absorbing material (desiccant) is located in the air stream going into the living space. As the air passes through the desiccant, which is usually located on a wheel that slowly rotates into the air stream, moisture is removed from the air, dropping the humidity level in the air stream to the point that an evaporative cooler can then cool the air. The desiccant is dried by the heat generated by the solar collectors as it rotates out of the air stream.

4.1.2 Absorption air conditioning

Heat from solar collectors separates a low boiling refrigerant in a generator which receives the pressurized refrigerant from an absorber. Solar heat can also be used in the evaporation stage of the cycle.

5.0 Passive Solar Water Heating

A passive solar water heating system uses natural convection or household water pressure to circulate water through a solar collector to a storage tank or to the point of use. Active systems employ pumps and controllers to regulate and circulate water. Although passive system are generally less efficient than active systems, the passive approach is simple and economical.

Passive water heating systems must follow the same parameters for installations as active systems – south facing unshaded location with the collector tilted at the angle of our latitude. Since the storage tank and collector are combined or in very close proximity, roof structural capacities must accommodate the extra weight of a passive system which can be 300 pounds or more.

5.1 There are two types of passive water heaters: batch and thermosiphon

5.1.1 Batch System

The batch system is the simplest of all solar water heating systems.


Figure 3
Schematic for Ground-Mount Batch Domestic Water System

It consists of one or more metal water tanks painted with a heat absorbing black coating and placed in an insulating box or container with a glass or plastic cover that admits sunlight to strike the tank directly. The batch system’s storage tank is the collector as well. These systems will use the existing house pressure to move water through the system. Each time a hot water tap is opened, heated water from the batch system tank is removed and replaced by incoming cold water.The piping that connects to and from the batch heater needs to be highly insulated. On a cold night when no one is drawing hot water, the water in the pipes is standing still and vulnerable to freezing. In many applications, insulated polybutylene piping is used because the pipe can expand if frozen. The water in the batch heater itself will not freeze because there is adequate mass to keep it from freezing.

Since the tank that is storing the heated water is sitting outside, there will be heat loss from the tank during the night. This can be minimized by an insulating cover placed on the heater in the evening.

The most effective use of a batch water heater is to use hot water predominantly in the afternoon and evenings when the temperature in the tank will be highest.

Manufactured batch heaters have a “selective surface” coating on the tanks that will absorb heat most readily yet permits very little heat loss. This feature is very valuable in these type of systems as it helps insulate the tank.

5.1.2 Thermosiphon Systems


Figure 4
Thermosiphon System

The thermosiphon system uses a flat plate collector and a separate storage tank that must be located higher than the collector. The collector is similar to those used in active systems.

The storage tank, located above the collector receives heated water coming from the top of the collector into the top of the storage tank. Colder water from the bottom of the storage tank will be drawn into the lower entry of the solar collector to replace the heated water that was thermosiphoned upward. The storage tank may or may not use a heat exchanger. The thermosiphon system is more costly and complex than the batch system. In our area, it is best to use an indirect system (one that employs a heat exchanger). In that case, antifreeze can be used in the system eliminating freeze ups.

5.1.3 Sizing

The sizing of a batch system and thermosiphon system are both based on a usage figure of 20 gallons of hot water per person per day. For example, if the storage tank in these systems is 40 gallons, that would equal the requirement for two people. The collector area in the thermosiphon system should equal approximately 20 square feet per person.

The system is not sized for 100% of the energy requirement. A backup source is needed.