1. Don't use a contractor who wants to size solely on the square foot method or replacing with same size warm air furnace that is already existing. These units were more then likely oversized when installed due to the common sizing practices then. Would you buy clothing or carpeting without know what size you needed. Today most homes have been upgraded with new windows, doors and/or insulation updates. The proper way of sizing a new warm air furnace is to use Manual J. (Manual J is the manual which explains the way a heat loss should be done. Originally it was done by measuring the room sizes, window sizes, determining the direction the exposed wall and windows were facing, insulation values and other information. The contractor would mathematically calculate all the values and use charts to determine the heat gain for the structure. Today the measurement still need to be taken but all the calculations are done by computer. This takes the human error out as long as the measuring was done properly). This is called a Heat Loss Calculation. This is the proper and most accurate to determine the size of the warm air furnace to achieve the most comfort and lowest operational costs. When buying a warm air furnace bigger is not better (Fuel oil and gas boilers do not start at their peak efficiency. If the warm air furnace is oversized it may never run long enough to reach peak efficiency, which will only escalate your fuel bills! Oversized furnaces will have larger maintenace bills and a shorter life do to short cycling and heat exchanger condensation. This is a condition of many cycles on and off instead of longer more efficient cycles. The numerous starting of the unit will shorten motor and control life, and increase electric bills because it takes 2-3 times more electric to start a unit the to run a unit).

2. Don't be tempted by the lowest bidder. Ask the contractor for references from satisfied customers. Be willing to pay for the time and material to have the hot water boiler installed properly. Don't have the job sized properly and get a poor installation.

3. Check the manufacturers AFUE ratings (This stands for Annual Fuel Utilization Efficiency. This rating is used for gas and oil hot water boilers and warm air furnaces. This is a figure lower then the combustion efficiency due to taking into account the chimney losses and jacket losses). The higher the rating the more efficient the boiler will be. When making comparisons make sure you are comparing AFUE to AFUE and not AFUE to Burner efficiency or Combustion Efficiency.

4. After you have Comfort-Calc do your Heat Loss Calculation ust these figures to determine which furnace size you need. After choosing a manufacturer look at their NET BTU ratings. (The net Btu rating is the actual btu's that can be to heat your home) Caution do not use the Input Ratings, this will make the unit to small. If the NET BTU is between two furnace sizes go to the up to the next size. Never go down to the lower size. Example: You require 90,000 Btu's. the manufacturer offers a unit with a net rating of 87,000 and 105,000. You would use the 105,000

5. All manufactures have a temperature rise figure they want the furnace adjusted to. This figure is the amount of heat being picked up by the air passing through the furnace. If the blower speed is to slow not enough air will be passing through the furnace and it will get hotter then designed. If the air is moving to fast through the furnace will cool down too much and cause condensation in the furnace causing it to rust out faster. Make sure the contractor adjusts the blower speed to meet the specification.

6. If the warm air furnace has central air conditioning on it, the blower will need adjusted to a different speed then for heating.

7. If the warm air furnace is oil fired, make sure the burner is combustion tested for the highest efficiency. This is done by testing the combustion flue gasses and fine tuning the oil burner. Make sure all the safety's are tested.

8. If the warm air furnace is gas fired make sure the gas pressures are tested and adjusted. Make sure all the safety's are tested.

9. Make sure the thermostat anticipator is set to coincide with the control requirements for a more accurate thermostat operation and avoid temperature override. This figure will be in the manufacturers manuals or can be tested with a meter to determine actual settings.

Reference :
Website http://www.comfortcalc.150m.com/Tips.html#WA Manual J

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