Does your fix and flip property need a new or repaired heating system?

When considering an older property for rehab, it is important to remember that many of these houses still have gravity heating systems. The main difference between these and a modern forced-air system is that there is no fan to move the air. This is dependent on the physical property of heated air to rise, basically drawing in unheated air to fill the vacuum.

While gravity heating has its advantages — it is quiet and there are no fan motors to repair — the cons often outweigh the benefits. To say they are big units is an understatement. A gravity heating system fills nearly an entire basement with so many large ducts needed to funnel air to each room in the downstairs of the house. 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 a big consideration in today’s economy. 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 bringing 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 to heat a home. Multiply the square feet of heated floor area by 53; then round up to the next larger furnace size.


_____ sq. feet of heated floor x 53 = _____ 

For example, to size a furnace for a 2,000 square foot home:

2000 sq. feet x 53 = 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 every 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.

When adding a room, enclosing a porch, expanding attic space or converting a garage, the existing furnace may not have the capacity to serve the addition.


Even if the existing furnace has the capacity, a long duct run may reduce the volume of heat delivered at the register to below acceptable levels.

The correct way to serve a remote room addition is to install a new trunk line directly to the furnace plenum. Talk to your heating subcontractor about installing a booster fan in the existing duct run to increase the flow of warm air. If tapping into the existing HVAC system does not make sense, and there are no other practical options, install a floor or wall furnace in the new room addition. Direct-venting thru-wall furnaces is usually an acceptable alternative.

There is a lot that can go wrong when modifying an old steam or hot water heating system. Results may not be what the owner expected. In your contract, include language that limits both the scope of your work and your liability. Expert repair is required when a radiant heating system’s piping develops leaks or becomes air locked. Breaks in ceiling coils can be repaired fairly easily, but repairing breaks in a floor is both difficult and expensive.

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