In a typical household, heating and cooling make up well over half of the total energy use. Insulation and sealing up the "building envelope" from outside air can drastically reduce energy consumption. Next in line should be selecting energy efficient appliances and heating/cooling units.

Many older homes come equipped with radiator heat and an older gas-fired boiler, but no air conditioning. Chances are, you will be either upgrading or replacing the HVAC system(s) in your renovation.

Efficiency

Great strides have been made in HVAC efficiency. For air conditioners, efficiency is measured by a Seasonal Energy Efficiency Ratio (SEER) number. The higher the number, the more efficient the unit is (and usually the more expensive the unit is). Purchase the highest rated unit you can afford.

HVAC Types

Forced Air Furnaces
When it comes to heating, the newest energy efficient forced air furnaces approach the mid 90% range - this means that over 90% of the energy created by the furnace is captured for heating your home. This also means that the exhaust produced is significantly cooler than a traditional furnace - the high efficiency units capture much of the exhaust heat and return it back to help heat your home. High efficiency furnaces can often be vented directly through the wall rather than up a flue through the roof. The cost of a high-efficiency furnace is approximately equal to the cost of a standard furnace plus relining the flue (often a requirement). Gas is significantly more efficient than electricity, though electric furnaces (non-combustion) do not have any venting requirements at all.

Hot Water Heating Systems
If adding central air conditioning is a priority, you will have to make the decision whether to keep your existing hot water system and repair it as required, or to replace it with central heating. The first step in making this decision is verifying the operating condition of your existing system. Some of the key things to check are: the boiler (is the pilot light operational, does it fire properly, is it properly vented), the pump, the pipes (is the system filled properly, are there any leaks, does the insulation contain asbestos), and of course the radiators themselves (are there any leaks, are the valves in working order, are they adequately sized).

All things considered, hot water heat is generally less expensive and more comfortable than forced-air heat. Many people feel that because radiators are designed to heat your rooms through both convection (air movement) and radiation (direct heat), they heat rooms more “evenly”. Also, the interior air in the house is maintained with more humidity. It is important, however, to learn how to maintain your hot water system properly in order to maximize its benefit. An important HVAC investment will be a “radiator key” available for under $2 at many city neighborhood hardware stores. This “key” fits the small valve at the top of each radiator and allows you to release trapped air when filling the system.

If it is in good or repairable condition, I generally recommend keeping the existing hot water heat for general heating needs and adding a forced air system on top for cooling in the summer. For a relatively small cost, your contractor will place an electric heating element in your forced air system as a backup. Removing existing radiators nearly always necessitates patching floors where the pipes enter and often repairing the wall behind the unit.

Multi-Zone HVAC Systems
If you intend to add central air and heat to a building with radiator heat, you will need to find space for the blower unit inside and the condenser outside. In a single-zone system, the blower unit is usually placed in the basement. Some buildings, due to their size or layout, require additional HVAC units to heat and/or cool effectively. Multi-zone systems such as this allow greater temperature control and allow each unit to operate more efficiently. In a three-story home, one unit will typically be located in the basement to serve the first floor, and one unit will be located in the attic to serve the third and second floors.

Radiant Heating
In-floor radiant heating is typically considered as a supplementary system rather than the primary means of providing heat. It consists of a grid of heating elements (electric) or small-diameter tubing (hot water)HVAC Duct running between joists that runs throughout the area to be heated. Though in-floor radiant heating systems can be installed under any type of flooring, they are typically used in areas like bathrooms to heat a cold tile floor and provide continuous, even heat to the room.

Alternative Temperature Control
Alternatives to standard heating and cooling include passive systems such as double-hung windows, shade plantings, and solar heating. Active systems include ceiling fans (a cost effective means of regulating air temperature) and geothermal heat pumps.

HVAC Rough-In

Your HVAC contractor will perform “load” calculations based on the size and layout of rooms, location and size of windows, and the location of the fan and condenser units. These calculations indicate the required capacity of the system and the size of ducts and vents. You can compare the suggested capacities (measured in “tons”) from several contractors. If there are significant disparities, ask them to explain their reasoning. An improperly sized system will cool and heat inefficiently resulting in higher energy costs and more frequent maintenance.

The HVAC rough-in work usually involves installation of the main trunk lines and ductwork for air distribution throughout the house. You will need to have space within the walls for the supply ductwork and the return air – your architect will plan for this in the design stages. Also included in this stage is the installation of flexible duct for bathroom fans and range hoods. Because the HVAC system is large and relatively inflexible this contractor should start work before the others. A “high velocity” system uses smaller, flexible ducts to distribute air. The high velocity vents are generally less visible than the standard floor and wall vents. These systems are generally more expensive.

Your HVAC contractor should be responsible for acquiring all appropriate permits and securing the inspection after work is complete.