The answer: Probably less than you think and likely a LOT less than the current size of your oil or gas furnace or boiler.

A professional contractor can do a “Heat Load” calculation on any room or house and give a good idea of what is needed. However, you might already have a good estimate based on your past fuel use.

We recently had a new gas furnace installed (2400 sf- New England) which was much smaller BTU than the one it replaced. The new unit is 60,000 BTU (input), which puts about 55K BTU into our ductwork. It is also 2 stage, so it has a low setting which allows it to run quieter for most of the heating season.

Here is how you can work backwards from an oil or gas bill and get some idea of how much heat your house actually needed in the coldest weather. The small calculator below is only for oil and gas central heat.

Method: Find your highest monthly (or 30 day period) heating bill – even though Oil is not metered monthly it’s likely you’ve some idea of the number of gallons you used in the coldest weather.
For Oil – Enter Gallons For Nat Gas – Enter Units (therm, CCF)
We don’t need price for this calculation – we are only trying to determine rough heat load.

Using my home as an example, the highest monthly bill was 180 therms.
The Calculator shows 43,000 BTU/hr should suffice.
Our 60K BTU new Gas Furnance should surely do the job!

Heating Systems should not be oversized – bigger is not better.

Space Heaters

The above example shows us why Space Heaters are so effective. A plug-in electric space heater is usually 5500 BTU maximum. In the normal winter season, a single such heater might produce 1/3rd of the BTU’s we need for the entire house! Kerosene or Gas Spacers are often larger – in the range of 9,000 to 20,000 BTU per hour. When your neighbor remarks “Our efficient gas fireplace insert heats the entire house”, they are likely being truthful – abeit talking about more moderate weather and not keeping the far ends of the home as comfortable.

The same applies with Pellet Stoves and Wood Burning Stoves – these appliances can provide the primary heat in many cases. Many homeowners feel more secure having more than one type of fuel providing their heating needs. Many space heaters don’t need electric to operate – a great backup when the power goes out.

Summary

In a new home or with a replacement furnance the contractor will perform a more technical inspection and calculation to determine furnance or boiler size. However, past fuel use can be a window into what to expect. Most homes have oversized heating systems so it is likely a replacement unit may be of lesser BTU rating than your exist setup.