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2018 National Home Improvement Estimator, All Rights Reserved Page 391

Chapter 14, Plumbing and HVAC

Not every older home needs an extra bathroom or a larger kitchen, but nearly every older home needs
plumbing, heating and cooling systems brought up to modern standards. No home is truly comfortable
without effective plumbing, heating and cooling. In a very old house, the entire mechanical system may
have to be replaced.

Warm Air Heating
Many old houses still have gravity heating systems. The main difference between these and a modern
forced-air system is that there’s no fan to move the air. It depends on the physical property of heated air
to rise, drawing in unheated air to fill the vacuum. This simple circulation system moves the warm air
through the house. Gravity heating has its advantages; it’s quiet, and there are no fan motors to repair.
But they are very large units, with so many large ducts needed to funnel air to each room in the
downstairs of the house that they almost fill the basement. They also tend to be heavy gas users. That
may not have been a problem 50 years ago when gas was cheap, but fuel cost is something we have to
consider today.

A new high-efficiency forced air furnace will recoup the replacement cost in just a few years. And since
modern furnaces are small, and use smaller ducts, eliminating the gravity system’s octopus-like duct
arms leaves most of the basement space available for conversion into living space. Forced air heating
has the added advantage of being able to bring the house up to a comfortable temperature in a matter of
minutes, rather than the hour or two a gravity system takes.

The Btu capacity of a residential heating system depends on climate, window size and orientation,
insulation and square footage to be heated. For cost estimating purposes, there’s an easy way to
calculate the Btu capacity of the furnace needed. Multiply the square feet of heated floor area by 53;
then round up to the next larger furnace size. For example, to size a furnace for a 2,000 square foot
home:

2,000 times 53 equals 106,000. The next larger furnace size is 125,000 Btu.

Altitude also affects the size of the furnace needed. Reduce the stated Btu rating of a furnace by 4
percent for each 1,000 feet above sea level. For example, the capacity of a 100,000 Btu furnace
installed 5,000 feet above sea level would be 20 percent less, or 80,000 Btu.