Integrating the Heating and Cooling of Your Home

Many conventional home heating and cooling appliances perform a single function, such as space heating or cooling (heat pumps supply both space heating and cooling), water heating, or ventilation. Appliances that perform two or more of these functions, however, are becoming more popular. There are many different types of integrated, or multi-function, appliances available that may offer one or more of the following features:

  • Reduced equipment needs.
  • Simplified installation.
  • Lower purchase and installation cost.
  • Compact size for the limited space of small homes, multiple-dwelling units, and apartments.
  • A design that enables the individual components to work together to achieve overall higher efficiency, energy savings, and lower operating costs.

There are several different types of multi-function appliances available. The following is a description of some of those systems and their characteristics.

Combined Space and Domestic Hot Water Heating:

Tankless, combined hydronic (or steam) space heating and water heating systems have been available for years. These systems have a heat exchanger inside the boiler that heats domestic water. This eliminates the need for a conventional water heater. Although this type of system is inexpensive to buy and install, it is, however, more inefficient and expensive to operate than two separate appliances. This is because the boiler must operate continuously even during warm weather in order to heat the domestic water supply.

A boiler with a separate storage tank for domestic water is an increasingly popular alternative to the above system. A pump forces hot boiler water through a heat transfer coil in the tank. A single burner, inside the boiler, supplies heat for one or more space heating zones and a domestic water heating zone. Thermostats control both the operations of the boiler and the solenoid valves that control the flow of water. Once the storage tank water reaches its set temperature, the boiler ceases operation; thus efficiency is higher (30% to 50% or more during warm weather) than for the tankless coil combined units. Since they have a storage tank, these indirect-fired systems cost more than the combined hydronic space and hot water heater systems described above. Unlike direct-fired systems, the tanks usually last longer since they are not subjected to the extreme changes in temperature caused by open flame. Other, less common, indirect-fired systems incorporate such features as an immersed cold coil tank (cold domestic water passes through a heat exchanger inside a tank of boiler water), a "booster tank" (the above system without a heat exchanger the domestic water zone shuts off when the tank is full), and double tank configurations, where the boiler water runs between and/or around internal tanks.

Combined space heating/domestic hot water systems are not limited to hydronic (boiler) systems. Some high-efficiency condensing forced-air furnaces circulate a liquid to cool the furnace's heat exchanger. This heated liquid may either circulate through a liquid-to-air heat exchanger to warm room air, or it may circulate through a coil inside a separate indirect-fired water heater. This type of condensing furnace, when connected to an indirect-fired water heater, provides an improvement in water heating efficiency.

Many new integrated systems also feature sealed-combustion technology. These systems use outdoor air for combustion. Exhaust gases vent to the outside through small diameter plastic pipes. Thus, heated indoor air is not used for combustion purposes. "Power burners" or "power vents" are often used (especially in retrofit situations) to increase the efficiency of the air flow.