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IRC M1401.3 against the 30 to 60 BTU per square foot rule

A leading price guide tells homeowners to size a furnace at 30 to 60 BTU per square foot, which lets a 2,000 square foot house have anything from 60,000 to 120,000 BTU. The residential code does not accept floor area at all: section M1401.3 sizes heating equipment by ACCA Manual S from a Manual J load calculation. This page shows how wide the rule is for your house, what the code asks for instead, and what a replacement costs.

The floor area rule, the code, and the price

sq ft

Heated floor area only. This does not size a furnace; it shows what the floor area rule allows and prices the job.

Specification summary

Generated from on . Reopen that address to reproduce these figures exactly.

Entered

Result

This band is drawn from the floor area you enter above.
The floor area rule for this house

Furnace prices

Installed, to replace
ItemLowHighUnit
Electric furnace$2,000$7,000to replace
Gas furnace$3,800$10,000to replace
Oil furnace$6,750$10,000to replace
Mini-split$5,000$15,000to replace
Old furnace removal and disposal$60$330added

Source: HomeAdvisor, how much does a furnace replacement cost. Last price check: September 13, 2026.

The source gives $2,823 to $6,896 for a replacement, $4,814 on average. It also prints a table of installed cost by house size at exactly $1.20 to $2.00 a square foot, which puts a 2,000 sq ft house at $2,400 to $4,000, below the low end of its own gas furnace row; that table is left out. Its headline reads 2025 on a page dated June 2026.

Whole-house humidifier prices

As each row states
ItemLowHighUnit
Whole-house humidifier, installed$392$768total
Drum (bypass) unit$100$300unit
Flow-through unit$200$600unit
Steam unit$300$1,100unit
Steam humidifier running cost$150$500per year

Source: HomeAdvisor, how much does it cost to install a whole-house humidifier. Last price check: September 13, 2026.

A steam unit alone reaches $1,100 in the source, above the $768 top of its installed range.

Heat exchanger replacement prices

Installed
ItemLowHighUnit
Heat exchanger replacement$1,250$3,000total
Standard furnace$1,000$3,000total

Source: HomeAdvisor, how much does heat exchanger replacement cost. Last price check: September 13, 2026.

The source's headline starts at $1,250, its standard furnace row at $1,000, and its text at $380.

Furnace repair prices

Per repair
ItemLowHighUnit
Furnace repair, typical job$133$505total
Ignitor$150$500total
Gas valve$200$600total
Control or circuit board$200$600total
Draft inducer motor$200$1,500total
Blower motor$150$2,000total
Heat exchanger$100$1,500total
Technician$50$150per hour

Source: HomeAdvisor, how much does it cost to repair a furnace. Last price check: September 13, 2026.

This guide prices heat exchanger work at $100 to $1,500; the same publisher's heat exchanger replacement guide gives $1,250 to $3,000.

Outdoor wood furnace prices

As each row states
ItemLowHighUnit
Outdoor wood furnace, installed, typical$9,000$16,000total
1,500 to 2,000 sq ft home, unit$6,500$10,000unit only
1,500 to 2,000 sq ft home, installed$8,500$14,000total
2,500 to 3,000 sq ft home, unit$7,500$12,000unit only
2,500 to 3,000 sq ft home, installed$9,500$16,000total
Wood gasification boiler, installed$10,000$20,000total
Firewood to run it$800$3,200per year

Source: HomeGuide, how much does an outdoor wood furnace or boiler cost. Last price check: September 15, 2026.

The source prices the fuel separately: $800 to $3,200 a year of wood on top of the installed price.

Pellet furnace prices

As each row states
ItemLowHighUnit
Pellet furnace, installed$4,000$17,000total
Pellet furnace$2,500$12,000unit only
Installation labor$1,500$5,000total
New ductwork, if none exists$2,400$6,600total
Wood pellets$600$1,200per year

Source: HomeGuide, how much does a pellet furnace cost. Last price check: September 15, 2026.

The source adds $2,400 to $6,600 for ductwork in a house that has none.

Geothermal heat pump prices

As each row states
ItemLowHighUnit
Geothermal heat pump, typical project$4,541$26,537total
Capacity$2,500$8,000per ton
1,000 sq ft home, 1.5 tons$2,500$8,000total
1,500 sq ft home, 2 tons$5,000$16,000total
2,000 sq ft home, 3 tons$7,500$24,000total
2,500 sq ft home, 4 tons$10,000$32,000total
3,000 sq ft home, 5 tons$12,500$40,000total
Excavation$1,550$6,300total
Trenching$5$12per linear ft
Ductwork$1,400$5,600total

Source: HomeAdvisor, how much does a geothermal heat pump cost. Last price check: September 16, 2026.

The table prices 1.5 tons at the source's one-ton rate, and its text says homes need 1 to 1.25 tons per 500 sq ft, which is 4 to 5 tons for 2,000 sq ft against the 3 tons in its table.

Worked example

A 2,000 square foot house replacing a gas furnace.

  1. Rule of thumb. 2,000 × 30 to 60 = 60,000 to 120,000 BTU.
  2. Width. The high end is twice the low; any furnace between passes the rule.
  3. Code. M1401.3: sized by Manual S from a Manual J load.
  4. Price. Gas furnace $3,800 to $10,000 installed, plus $60 to $330 to remove the old one.

The same guide's size table puts this house's furnace at $2,400 to $4,000 installed, below its own gas furnace row. The type table is the one used here.

The formula

The rule, for what it is worth:

BTU = floor area × 30 to 60
30 to 60
HomeAdvisor's range, June 2026
Manual J
the load calculation the IRC refers to
Manual S
the equipment selection method that follows it

A factor of two in the rule covers the whole difference between a mild, tight house and a cold, leaky one, which is what a load calculation is for.

Why this page does not give a furnace size

Furnace size calculators multiply floor area by a number. The code a furnace is installed under does not size equipment that way.

The load calculation it names, ACCA Manual J, is not published free, so this page shows the width of the rule, quotes the code, and prices the job from sourced figures.

A rule of thumb twice as wide as it is useful, and a code that does not allow one

The price guide sizes a furnace by floor area:

To figure out what size you need, estimate about 30 to 60 BTUs per square foot. So, for a 2,000-square-foot home, you'd need a furnace rated between 60,000 and 120,000 BTUs.

HomeAdvisor furnace size and installed cost by house size, updated June 17, 2026, against its own gas furnace row
HouseGuide's BTU rangeHigh against lowGuide's installed costPer sq ftGuide's gas furnace row
1,000 sq ft30,000 to 60,000 BTU2 times$1,200 to $2,000$1.20 to $2.00$3,800 to $10,000
2,000 sq ft60,000 to 120,000 BTU2 times$2,400 to $4,000$1.20 to $2.00$3,800 to $10,000
3,000 sq ft90,000 to 180,000 BTU2 times$3,600 to $6,000$1.20 to $2.00$3,800 to $10,000
4,000 sq ft120,000 to 240,000 BTU2 times$4,800 to $8,000$1.20 to $2.00$3,800 to $10,000
5,000 sq ft150,000 to 300,000 BTU2 times$6,000 to $10,000$1.20 to $2.00$3,800 to $10,000

Every row spans a factor of two in BTU. And every installed cost is exactly $1.20 to $2.00 a square foot, so a 2,000 sq ft house is priced at $2,400 to $4,000 while the same guide starts a gas furnace at $3,800; for houses up to 3,000 sq ft the low end is below that.

The residential code sizes equipment another way, in Virginia's adoption of the 2021 IRC:

Heating and cooling equipment and appliances shall be sized in accordance with ACCA Manual S or other approved sizing methodologies based on building loads calculated in accordance with ACCA Manual J or other approved heating and cooling calculation methodologies.

Utah's adoption goes further and says who may do the calculation:

An individual performing load calculations shall be qualified by completing HVAC training from one of the following: HVAC load calculation education from ACCA; A recognized educational institution; HVAC equipment manufacturer's training; or Other recognized industry certification.

This page does not size a furnace. ACCA's Manual J is not published free, and a floor area rule is not what the code asks for. The calculator shows the rule's range for your house so you can see how wide it is, and prices the job.

What to ask for with a quote

The Manual J report. It lists the design heating load in BTU per hour, from the house's walls, windows, insulation, air leakage and the local design temperature. A furnace's output should be close to it.

Output, not input. A furnace's rating plate gives input BTU; its output depends on efficiency. Compare the load with output capacity.

A replacement sized to the old furnace is not a load calculation either. Houses change: insulation, windows and air sealing added since the old one was installed all reduce the load.

The sources, and what a real sizing needs

Read 2026-09-13.

Virginia Residential Code 2021, Chapter 14 Heating and Cooling Equipment and Appliances, International Residential Code 2021 as adopted with amendments in Virginia, read through UpCodes with the jurisdiction and edition stated on the page. Section M1401.3, equipment sizing by ACCA Manual S from Manual J loads, with its exceptions.

Utah Residential Code 2021, Chapter 14 Heating and Cooling Equipment and Appliances, International Residential Code 2021 as adopted with amendments in Utah, read through UpCodes with the jurisdiction and edition stated on the page. Utah's amended M1401.3 and its added M1401.3.1, which requires HVAC training of whoever performs the load calculation.

How Much Does a Furnace Replacement Cost?, HomeAdvisor staff writer, reviewed by After Hours Home Improvement, LLC, updated June 17, 2026. The 30 to 60 BTU per square foot rule, the table by furnace type and the table by house size.

A load calculation needs the house, not its area: wall, ceiling and window areas and their insulation, air leakage, duct location, orientation and the local design temperature. Ask the installer for the Manual J report with the quote; it is the document the code refers to.

Frequently asked questions

What size furnace do I need for 2,000 square feet?
By HomeAdvisor's 30 to 60 BTU per square foot rule, anywhere from 60,000 to 120,000 BTU. The residential code, IRC M1401.3, requires sizing by ACCA Manual S from a Manual J load calculation instead, which narrows it to a figure for your house.
How many BTU per square foot for a furnace?
HomeAdvisor gives 30 to 60. The spread is a factor of two, which is why the code does not use floor area to size equipment.
Does the code require a Manual J?
IRC M1401.3 requires heating and cooling equipment to be sized by ACCA Manual S or other approved methods, based on loads from ACCA Manual J or other approved methods. Utah's adoption also requires the person doing the calculation to have HVAC load calculation training.
How much does a new furnace cost?
HomeAdvisor's June 2026 guide gives $3,800 to $10,000 for a natural gas furnace, $2,000 to $7,000 electric and $6,750 to $10,000 for an oil-burning furnace, installed, plus $60 to $330 to remove the old one.
Is a bigger furnace better?
IRC M1401.3 lets equipment exceed the Manual S capacity only in listed cases, such as multistage equipment whose range covers the load, a load no unit meets on both total and sensible heat where the next larger size is chosen, or where the specified unit is the manufacturer's smallest. Oversizing is not otherwise part of the method.
Can I size a furnace myself?
The rule of thumb is not a sizing method under the code. A Manual J calculation needs the house's construction details and, in Utah, someone with HVAC load calculation training.
Does the same sizing rule apply to a boiler or a geothermal system?
Yes. IRC M1401.3 covers heating and cooling equipment generally, so a boiler or a geothermal heat pump is sized by ACCA Manual S from a Manual J load, the same as a furnace. A BTU per square foot rule is no better for a boiler than for a furnace. The prices on this page cover furnaces and mini-splits.
How much does a furnace control board cost to replace?
HomeAdvisor's furnace repair guide lists a control or circuit board, the part often called the furnace motherboard, at $200 to $600. The same guide gives $133 to $505 for a typical repair and $50 to $150 per hour for a technician. A new board does not change the furnace's output, so it is not a sizing question.
How much does a furnace humidifier cost?
HomeAdvisor's June 2026 guide gives $392 to $768 for a whole-house humidifier installed on the furnace, with a bypass unit at $100 to $300 and a steam unit at $300 to $1,100.
How much does a geothermal HVAC system cost?
HomeAdvisor's June 2026 guide gives $4,541 to $26,537 for a typical system and $2,500 to $8,000 per ton of capacity. Its table gives $7,500 to $24,000 for a 2,000 sq ft home. The capacity a house needs is the load this page calculates, divided by 12,000 BTU per ton.
How much does it cost to install a natural gas furnace?
HomeAdvisor's June 2026 guide gives $3,800 to $10,000 to replace a gas furnace, with removing the old furnace adding $60 to $330.
How much does it cost to replace a furnace gas valve, ignitor or blower motor?
HomeAdvisor's June 2026 guide gives $200 to $600 for a gas valve, $150 to $500 for an ignitor, $150 to $2,000 for a blower motor and $200 to $1,500 for a draft inducer motor, with a technician at $50 to $150 an hour.
How much does a pellet furnace cost?
HomeGuide's June 2025 guide gives $4,000 to $17,000 for a pellet furnace installed, $2,500 to $12,000 for the unit and $1,500 to $5,000 for installation, with new ductwork at $2,400 to $6,600 where the house has none.
How much should a new high efficiency gas furnace cost installed?
HomeAdvisor's June 2026 guide gives $3,800 to $10,000 to replace a gas furnace, with removal of the old one at $60 to $330. It does not split high efficiency models out. A furnace and AC together are $5,634 to $14,686 in This Old House's September 2026 article.
How much does a whole-house furnace humidifier cost to install?
HomeAdvisor's June 2026 guide gives $392 to $768 for a whole-house humidifier installed on the furnace. A drum or bypass unit is $100 to $300, a flow-through unit $200 to $600 and a steam unit $300 to $1,100, and a steam humidifier costs $150 to $500 a year to run.

Check these numbers yourself

How every figure here is verified: Sources & Method. Who builds this: , and the method is on About. Found something wrong? Tell us and it gets fixed or removed.

Figures on this page last checked against the source documents on 2026-09-13. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

This page does not size a furnace. The code sections are quoted from Virginia's and Utah's 2021 adoptions, read on 13 September 2026; your jurisdiction may differ. Prices are HomeAdvisor's, not this site's. Gas and electrical work on a furnace is commonly permit work.