Plural of Furnace

What Is the Plural of Furnace?

The Quick Answer

The plural of "furnace" is "furnaces."

The most common fuel source for modern furnaces in the United States is natural gas
Today's furnaces are much more energy-efficient than those from the past.

The noun "furnace" adheres to the standard rules for forming the plurals of nouns in English (shown in the table below).

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The Standard Rules for Forming the Plurals

The table below shows the standard rules for forming the plurals of nouns in English.

Type	Example of Type	Forming the Plural	Plural
Most Nouns	furnace scythe	add s	furnaces scythes
Noun Ending s, sh, ch, x or z	bus dress	add es	buses dresses
Nouns ending [consonant] o	mango zero tomato	add either s or es (There are no rules for this - you have to know.)	mangoes or mangos zeros tomatoes
Nouns ending [vowel] o	patio ratio	add s	patios ratios
Nouns ending [consonant] y	story penny	change the y to an i and add es	stories pennies
Nouns ending [vowel] y	storey () donkey	add s	storeys donkeys
Nouns ending f or fe	dwarf knife	ves and/or s (There are no rules - you have to know.)	dwarfs knives
Exceptions	man louse	some nouns undergo a vowel or letters change	men lice
More exceptions	salmon sheep	some nouns do not change at all	salmon sheep

Why Is There Confusion over the Plural of Furnace?

There is confusion because inexperienced writers often feel compelled to add an apostrophe when a noun ends with a vowel (e.g., piano, tomato, emu). Adding 's is never an option when forming the plural of a noun.