Properly Sizing Furnaces
When replacing a furnace you need to make sure that the replacement is sized correctly. It is all too easy to fall into the pattern of just looking at the BTU input rating of the old furnace and replace it with a new one with the same input. The key in terms of sizing is to determine the output (Btu’s – British Thermal Units) needed to heat the home not the input. As furnaces get more efficient the input rating is less in order to get the same output (Btu’s) needed to heat your home. Older furnaces range from 50% to 70% efficient and new furnaces now start at 80% and go up to 95+%.
Example:
100,000 Btu input of a 60% efficient furnace will give you 60,000 Btu output. 100,000 Btu 80% efficient furnaces will give you 80,000 Btu output.
So if you need a Btu output of 60,000 to heat your home, you would down size your input rating of the new furnace to 80,000 Btu’s. Depending on the manufacture of the new furnace, they will increase in size by 20,000 Btu’s each time.
Factors in correctly sizing your new furnace:
- Square footage of your home
- Ceiling height
- Windows
- Insulation in walls & attic
- Size of your existing duct work
Make sure your technician measures your home and does a load calculation and measures the duct work to ensure they allow enough air to move through the house for the new furnace. Newer furnace models rely on more air flow to heat your home then older models, so duct sizing is very important.
MADMAX
This is so true new furnaces must be the proper size or you will be wasting gas and have to replace the furnace sooner then you should .We live in chicago and are furnace never shuts off and you will have alot of furnace repairs and need to tune the furnace and clean the ducts.