A Heating System Can Not Be "Properly" Sized By Square Footage!

NO "Heating Professional" would ever design or size a heating system using square footage!

Sizing the System - Rough Square Foot Method If You Must:
If you really insist on a "really rough" rule of thumb is 50 Btu per square foot.
i.e.: 225,000 Btu Output / 50 Btu would heat approximately 4500 square feet.
This should never be considered an accurate estimate of your heating needs!

If an appliance is rated by square footage, it is usually for marketing purposes and
should not be used to size the appliance for your needs.

The products literature is usually worded:
This appliance will heat "up to XXXX square feet" or a range of square footage.
They say it will heat "up to" because the manufacturer has no idea as to whether you are
heating a super efficient building or a tent. An inefficient building will require approximately
50 Btu per square foot (or even more) where as a new efficient building can require
25 Btu per square foot or even less if it is super efficient. If you are building new it pays to
build as efficient as you can afford, as long term it will save you money!

There is a huge difference in heating a 2000 square foot house built in 1960 and a
2000 square foot modern styrofoam insulated concrete block house and all the different
types of buildings that fall in between these two categories!

Then consider green houses which can require as much as 100 Btu per square foot!

There is also a huge difference in heating requirements between Arizona and Alberta!

To specify that a specific heating appliance will heat "up to XXXX square feet" means they
likely have never tested the actual performance and output of that heating appliance!

The manufacturer and sales people who sell heating products cannot tell you realistically
that this appliance will heat XXXX square footage, as they have no idea as to:
a) the efficiency of the building being heated
b) the climate conditions the building is exposed to (wind, maximum cold temperature, etc.)
c) the actual Btu heat output of the fuel source being used (coal, grain, pellets, wood)
as it varies by product, product conditions such as moisture content, etc.

See Output Factors.

You cannot size an outdoor boiler by the square foot rating that the outdoor boiler
manufacturer provides, as if you can find it in the fine print of the manufacturers
literature they all state: "if you are close to 50% of the square footage the
outdoor boiler is rated for we recommend that you should go to the next size bigger boiler"

As you can see using square footage info to size a heating appliance is highly speculative
since leaving lots of room for under sizing the heating appliance for the actual building demand.

You should obviously determine your heating requirements and properly size your alternative heating appliance to at least 80% of your conventional heating systems capacity, or you will find
yourself loading your alternative heating with fuel more often then you would like,
or burning more conventional fuel (fuel oil, propane, natural gas, etc.) then you would like.

FYI - Square footage sizing is never seen on the rating plate of any conventional
heat source (fuel oil, propane, natural gas heaters, stoves, furnaces, boilers etc.),
they all have Btu input & Btu output ratings, bonnet capacity, kWh, etc.