Learn How to Upgrade to a Combined HVAC System
Combination or “Combi” Systems provide space heating for homes and domestic hot water. A combined HVAC system offers a single, integrated unit that provides hot water for showers, dishwashing, etc. For example, combination HVAC systems work with an air handling unit that provides forced air heating (like a furnace). Also, these systems help provide hydronic heating (like a traditional boiler). Additionally, combined systems provide radiant in-floor heating as well.
Conversely, combination systems also provide domestic water heating. These systems provide an endless hot water supply much like an instantaneous or on-demand water heater. Since combination systems provide all hot water needs for the home (heating and domestic hot water), homeowners must ensure the combination systems provide reliable and energy efficient service.
Combined HVAC System Solutions Info
New residential furnaces and boilers must meet standard efficiency ratings. For home heating, the Annual Fuel Utilization Efficiency (AFUE) provides the efficiency rating. A furnace or boiler that operates at 90% efficiency is 90% AFUE. Conversely, one that operates at 80% efficiency is 80% AFUE. As a result, the higher the AFUE, the higher the heating equipment’s operating efficiency. GreenHomes recommends combination systems maintain a 93% AFUE or higher.
For combination systems that are connected to distribution ductwork, GreenHomes recommends furnace ductwork be sealed where accessible and insulated in all unconditioned spaces or where condensation can occur on ductwork. For combination systems that connect to a series of hydronic heating pipes, ensure proper insulation (where accessible) of the heating pipes.
Combination HVAC System Sizing
The proper size of an HVAC system relates to the amount of heat produced per hour (not the physical size of the unit). Additionally, British Thermal Units (BTUs) express the size of a combination system. BTUs express the size for both home heating and heating domestic hot water. The size of the combination system needed depends on the efficiency level and the square footage of the house and the general climate of the house. For example, a larger, leaker, poorly insulated house in a cold climate needs a higher BTU/hr combination unit. Conversely, a smaller, tighter, well insulated house in a warmer climate requires a lower BTU/hr unit.
Finally, the size of the combi unit also depends on the domestic hot water needs of the home. Typically, the amount of house members, number of bedrooms, plumbing fixtures, etc impact the amount hot water needed. GreenHomes recommends contractors perform a heating load calculation prior to recommending a combi unit size. For example, contactors should use the ACCA Manual J to determine the heating load of the house. Then, they should follow the manufacturer’s sizing procedure for combination systems. It is never acceptable size combination systems based on the size of the older equipment or by using “rules of thumb” to size the new system.