Combined (Integrated) Space- and Water-Heating Systems
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In many houses that have been well insulated and air sealed, the space-heating load is so low that it is hard to justify paying for a high-efficiency furnace. If it's time to replace the water heater as well as the space-heating system, think about integrating both functions in a single appliance. This can increase efficiency and reduce costs, if you choose the right equipment. For more on water heaters, see chapter 10 (Hot Water).
A combined—or integrated—condensing gas-fired space- and water-heating system can have an efficiency of over 90% for both applications. Space heating can be hydronic or forced-air. The overall purchase and installation cost will be lower than it would be for individual appliances.
Different models of tanks will have different materials and designs. Use ones made with stainless steel or copper coils and stainless steel or plastic tanks, rather than less-expensive materials. The heat exchanger coils should be accessible so they can be cleaned.
You might be tempted to use an old conventional gas-fired water heater as the basic energy generator, connecting it to a fan coil for warm air heating. But don't. The system won't be efficient. Integrated systems based on heating boilers work much better, although they can be expensive.
Mid-efficiency gas-fired combined systems are not as efficient, overall, as condensing systems.
Combined Gas Space and Water Heater
- Warm air to house
- Heat exchanger coil
- Water circulating pump (draws in hot water from the heating unit)
- Return air from house
- Air filter housing
- Duct for combustion air from outside
- Hot water to house
- Cold water inlet
- Heat exchanger coil (connects to 13)
- Sealed combustion chamber
- Water storage tank
- PVC piping vents for exhausting gases outdoors
Excerpted with permission from No-Regrets Remodeling by Home Energy (1997)