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This dates me, but I remember when you installed or replaced an air conditioning system for a house, you took the square footage of the building, times it by 12, added 6000, and rounded it up to the nearest 1/2 ton (6000 btu/h). For example, a 1200 square foot home = 1200 x 12 + 6000 = 20400 btu/hr. Since 12,000 btu/hr = 1 ton of cooling. The nearest 1/2 ton to 20400 would be 24,000 btu/hr or 2 tons of cooling.

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The formula worked well for the old 8 SEER systems. The only R rating we were concerned with then was at the movies (bad joke about thermal resistance). Most houses were heated by gas or oil. When you added air conditioning, you had to change the blower motor and installed a fan center on the furnace. This was for generating higher fan speeds in the air conditioning mode.

That was a long time ago and changes in HVAC technology have greatly improved since then. Energy efficiency is now a concern. Homes are better insulated and engineering improvements to doors and windows have changed exceedingly.

For example, an older home that had single pane window glass, and little or no insulation in the walls or attic, would be sized for a 3 ton AC system. With upgrades to the windows and more insulation in the walls and attic, a new 2.5 ton air conditioning system could now do the same job as the old 3 ton system using less energy. Today, many factors go into calculating the values to correctly size an HVAC system.

What is the best heating and cooling load calculation for a home?

For residential homes, the best method of calculating heat loss/gain is called a Manual J. This heat and cooling load calculation, developed by the Air Conditioning Contractors of America (ACCA), can be manually or computer calculated. I suggest a computerized version because it provides input values for an easier and faster calculation. Most software versions offer instructions on inputting data. A properly sized HVAC system, combined with a seasonal service check, will provide years of efficient operation.

The vendors' websites, listed below, contains online information on heating and air conditioning software.

HVAC Computer Programs Online

HVAC Calculator

This HVAC Load Calculator (also known as BTU Calculator) provides an accurate real world heat load estimate for both Heating & Cooling. Additionally, it provides equipment recommendation (type heating/cooling system appropriate for your home), & calculates the cost of installing the equipment, including labor & materials!

We use a proprietary BTU calculation algorithm, that DOES NOT over-specify the unit capacity. Most online tools give you a higher heat load estimate than you actually need for your home to sell you more expensive equipment.

Estimate HVAC system load now:

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How to use the heat load calculator

MASSIVE UPDATE (June 24, 2020): We’ve released an extensive update to the calculator, which took over 150 hours of development time and is now over 900 lines of code! This new release introduces price calculations and HVAC Equipment recommendation algorithm, which provides suggestion based on your climate region, size of your house, presence (or absence) of air ducts and/or baseboard radiators in your home.

While BTU heat load calculation was done before this update, many homeowners were unsure which heating and cooling system is best for them. This is where our new algorithm can make an intelligent recommendation, which includes both system capacity (for heating & cooling), appropriate system type, and energy/fuel costs.

We also recommend that IF you plan to use the results of this heat load calculation for making your purchasing decisions, you SHOULD verify the results with this long-form Manual J online estimator.

Multiple heating/cooling systems: Another important new feature is calculating cost of multiple heating / cooling systems being installed in large homes (over 3000 sq. ft.), and specifying largest possible BTU lead HVAC system(s) and then the smallest size system for the remainder of the total BTU load.

For example, if your heat load is 150K BTUs, and maximum residential Central AC size is 60K BTUs (5 Ton) then you need two 60K BTU compressors and a 30K (2.5 Ton) system. The calculator’s algorithm will select the full size system(s) and the smallest needed system to cover the rest of required BTU load, to give you most cost efficient estimate.

Installation Cost Estimating: the tool will estimate total installation price for your new HVAC system, which is based off equipment cost, and national average labor + overhead + profit, that plumbers/HVAC contractors charge for each type of system.

Planned new features: Now that equipment recommendation engine and cost calculations are fully functional, we plan to add 2 final features:

1) Estimating cost of installing new air ducts (if needed).
2) Estimating cost of installing new baseboard or wall mounted radiators OR radiant floors (if needed).

How to calculate HVAC load

It is important that you enter accurate/appropriate details into the BTU calculator. This tool brings you as close to a complicated Manual J estimation as possible. Otherwise, you may end up with a system that is too big or too small.

Step 1 (Climate Region): Select your Climate Region, using the Region Map at the top on the calculator. For example, if you live in NYC or NJ, select Region 3 (yellow). If you live TX, select Region 5 (red), etc.

Step 2 (Area Size): Enter square footage for your house/building or a specific area, for which you are doing the calculations.

This step is Critical for accurate estimation of annualized heating/cooling loads of your HVAC systems! If you leave all settings as default, and only change the Region from 1 to 5 and back, you will see a huge change in Cooling/Heating BTU load.

Step 3 (Rooms / Zones): Enter number of Rooms/Zones where you want a new Heating/Cooling system installed.

Download ASHRAE Heat Load Calculation Excel Sheet XLS

If you plan to use a Central AC + Air Furnace (ducted) system, or a central boiler for heating, the number of zones is not very important in terms of estimating heat load.

This value is most useful for figuring out which type of Ductless Mini-Split system to use.

Additionally, we discuss the PROS & CONS of using a multi-zone vs. installing multiple single-zone Ductless heat-pump systems, in our Mini Split DIY installation guide.

Step 4 (Space Height): Select average Ceiling Height of your house. In most cases this value should be equal to 8 ft. However if you have high ceilings, or cathedral/vaulted ceilings, the VOLUME of your space will be higher.

For cathedral/vaulted ceilings, add the lowest wall height + peak height, and divide by 2, to get the average. For example:

Your exterior wall is 8 ft. high, and the highest point on the ceiling is 12 ft. high. In that case, your average Ceiling Height is 10 ft:
(12 + 8) / 2 = 10

Step 5 (Insulation Grade): Most homes in US built between 1978 and 2000 will have 4″ studs with R-13 wall insulation, and should have R-38 roof / attic insulation. If this matches your home, leave this value as default (Avg. Insulation R-13 walls).

If you have a newer home with 6″ studs, will have R-18 insulation. In this case, select “More than Average” value.

In most cases you should not use “Very Well Insulated” value, unless you have a “Super-insulated” house.

If you have partially insulated home, select “Less than Average” or “Poorly Insulated”.

These two values are most critical in terms of heating, where your heat loss will be the highest. If your primary reason for installing a new HVAC system is cooling, we recommend using “Less than Average” value, to not over-size your cooling equipment.

Step 6 (Windows): Select the average amount of windows in your home. If you have ~1 window or fewer, for every 8 feet of exterior wall length, select “Average Amount”.

If you have more than 1 window, for every 8 feet of exterior wall length, select “More than Average”

Step 6 (Windows/Doors air tightness): Select appropriate window/door insulation level. In most cases, leave this as default “Average” value.

Understanding the results of HVAC load calculation

Unlike other online HVAC calculators, we provide estimated heat load (system size in BTU/h) for both Heating & Cooling as well as recommended HVAC equipment type and size!

You will get TWO results:

1) Cooling & Heating Load in BTUs – this is the actual calculated number of BTUs per hour & TONs needed to heat/cool your space.
2) Best matching Heating / Cooling equipment type for your needs.

1) Estimated Heat Load

You will get an approximate BTU / TONs load for your house, based on the information you entered in the calculator and your region. Both heating and cooling BTU results, are calculated using our optimized BTU calculation algorithm, which is more “conservative” than most HVAC contractors and equipment sellers will give you.

On average these values will be 20-30% lower than “contractor’s guestimate”. However we recommend that you use the lower numbers, for reasons discussed above.

2) HVAC Equipment Recommendation

Our calculator tries to provide the best match / recommendation for equipment to use your particular situation, based on your Climate Region and other inputs.

Equipment Recommendation needs further clarification, as each person’s situation is different. Ideally, this calculator would be perfect for a new construction home, where you have total control of design and specifications of the type of HVAC equipment that will be used. However, most homeowners in US are dealing with existing homes, which brings certain limitations.

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First of all, if you have a Duct System in your home, a Central AC + Hot Air Furnace will be the most cost-efficient system for you. In very hot climates, a furnace can be substituted with an electric heating coil, which would provide warm air on rare cold days/nights.

If you do not have air ducts, and live in climate zones 1, 2 or 3 – the best system for heating is a forced hot water boiler (with baseboards, wall radiators or radiant floor heat), and best cooling system is a multi-zone ductless (mini-split) AC, which are economical and extremely efficient.

In Regions 3, 4 & 5, you rarely have very cold weather. In these areas, winters are very mild, and average low temps are above 0F degrees. Therefore, a high efficiency Ductless (mini-split) Heat Pump system can (and should) be used for both heating & cooling. It is most economical* type of heating/cooling you can get.

Ductless heat pumps can both heat and cool your house, in ambient temperatures as low as -15F degrees, and they are pretty good at doing both. Since they can provide heating, and do so using a fairly low amount of electricity (3-4 times less than electric space heaters), you may not need to install an additional heating system, be it a furnace or boiler, saving yourself about $7,000-12,000+ in installation costs.

However, these should not be your ONLY source of heating in climate zone 1 & 2, where temperatures get very cold in the winter, and power outages are frequent, as ductless heat pumps run on electricity. If you have a backup heating system (such as old boiler or gas/pellet stove, and can last a few days without electricity in case of a power outage, then you can use Heat Pumps for primary source of heating even in colder regions.

A big advantage is that ductless systems are “modular” and operate on zone level. So if you are spending most of the day in the living room, there is no need to cool or heat the entire house! You only need to run 1 zone. At night, you can turn off the living room zone, and turn on the zones in bedroom(s).

Moreover, ductless systems are also about 2x more efficient than even the high efficiency modern Central AC systems, which means your electric bill will be 2x lower! Actually even more than 2x, because of zoning, which is nearly impossible to do with central air conditioning systems.

* While most southern states have very low electricity costs (around $0.10-$0.13 per kWh), places for like California, were electric costs often exceed $0.30 per kWh, and PEAK pricing can go as high as $0.50 per kWh, a ductless AC / Heating system is ideal, as these are often 2 times more efficient than a Central AC, and you can condition only the parts of your house where you actually need cool or warm air, instead of cooling/heating your entire home, while you are sitting in the living room!

Pro Tip: If your house does not currently have air ducts, and your house is single level (ranch/cape), then air ducts and the AC + Furnace could be installed in your attic, using flexible insulated ducts. This is a lot cheaper than traditional sheet-metal ducts that have to be installed from the basement, and extended into all your rooms, especially if your house consists of multiple levels.

In this case, installing Central AIR is significantly cheaper than Ductless Heat Pumps. However, due to massive efficiency difference, a ductless system will quickly cover the initial out of pocket cost difference, by saving an average of 40% in operating costs!

HVAC sizing guide

Getting the right size HVAC system for your home/building is essential to providing enough capacity to heat or cool your living space. If your Heating or Cooling system is too small, you won’t get enough BTUs, and the space won’t be comfortable.

If you get too big of a system, you will be overpaying for the extra capacity: Bigger system = higher cost of installation. You will also be paying too much in operating costs (be it gas, electric or oil) going forward.

Most residential HVAC/Plumbing contractors do not want to spend the time to properly calculate (using Manual J method) the heat load & heat loss of your house (or individual rooms). So instead to cover their “bases”, 99% of pros specify oversized systems (which as explained above, costs more to install and operate).

NOTE: Most contractors and equipment distributors use INFLATED BTU/h values when calculating the heat load and unit size (in tons/BTUs), primarily to cover their backs.
Our calculator uses lower BTU/h values for both Heating & Cooling, to provide a more a “real-world” heat load estimate. However, we strongly recommend that you (or your contractor) do a Manual J heat load calculation of your home or a specific area, before making any purchasing decisions!
This calculator should be used and is intended for Informational Purposes only!

HVAC installation cost

HVAC installation costs vary between regions, depending on the local cost of living. However, equipment prices are about equal across most states. Here are typical prices for Central Air (Central AC + Hot Air Furnace), Hot Water Boilers, or Mini-Split Ductless systems.

Note that Central AC and Hot Air Furnace can be installed together or separately. However if you have only Central AC, you also need a heating system. Since Central Air and Furnace are stackable, they work perfectly in conjunction with each other.

We use a 2300 sq. ft. home size (US average for existing single-family homes) to provide our cost estimates.

• CENTRAL AC COST: A 4-Ton, 14 SEER Central Air costs about $5,595 to $7,837. The system comes with electric heating coil. Includes removal of old Central AC condenser and coil, & reusing existing copper lines and electrical connections. Upgrading to 16 SEER will cost about $800-$1,200 extra.
• CENTRAL AIR (AC + FURNACE): Combination Central Air system costs $7,976 to $11,171 for a 4 Ton, 14 SEER Central AC with an 80K BTU, 96% efficient gas furnace. Includes removal of old Central AC condenser and coil, & reusing existing copper lines and electrical connections.
• BOILER (Radiant Heat): Forced Hot Water boilers start $4,683 – $6,130 for a conventional Gas/Oil boiler OR $6,934 – $10,623 for a Condensing boiler with built-in tankless water heater, such as Navien, Bosch, Viessmann. Includes removal of old boiler, and reusing existing radiators / water lines.
• DUCTLESS HEAT PUMPS: A 4-5 zone whole house mini-split system will cost $13,876 – $18,058. These systems can both heat and cool your house very efficiently. Includes installation of new copper refrigerant lines, and 240V electrical connection, with 1 outside compressor, and 4-5 indoor “wall-mounted units. Floor-mounted, slim ducts, ceiling cassette indoor units will cost $300-$400 extra for each zone. Estimate mini-split cost in your area.

If you want to get HVAC quotes in your area, either call some of the local HVAC installers that you know, or your family/friends can recommend or request free estimates through our referral program.

Choosing the best HVAC system for your home

Use the following guidelines to choose the best heating/cooling system for your home.

As mentioned above, if you live in northern climate regions, we recommend a Gas Boiler for heating and Ductless (mini-split) AC for cooling. If you already have ducts, it will be cheaper in the short term to go with Central AC + Hot Air Furnace.

However, in some cases you will get a Mini Split recommendation for both Cooling and Heating, but the BTU size will be different.

We know this part is confusing. So let’s look at it in depth:

Most Mini-Splits are rated based on their COOLING capacity. A 12000 BTU (1-TON) mini split will have a rated capacity close to 12000 BTU/h. However these same units can also HEAT! And most higher-end Mini Split units will have a much higher heating capacity!

Example: a 9000 BTU Fujitsu RLS3H (single zone) has a 21000 BTU max-heating capacity! Therefore, if you live in zones 3,4 & 5, and plan to get a ductless system for your entire home, use the COOLING size when choosing the equipment. In most cases there will be more than enough heating BTUs available!

In Region 1 & 2, you need to take a closer look at specifications of your unit. However in most cases, in larger systems (2-8 multi-zone installations) the difference in Heating and Cooling BTUs is not as big as in the example above. Therefore, you will either have to oversize a little, or install multiple Single-Zone units throughout your home, to get highest efficiency and available capacity.

If you are unsure which type of heating or cooling system to install in your house, get 3-4 free estimates from local HVAC pros.

Best Hvac Load Calculation Software

Cold climate mini splits: do they heat well?

Many homeowners looking to add an efficient heating system that can be used during the cold months of the year are very skeptical about installing a mini split heat pump. After all, they are primarily used for cooling purposes. However, the reality is that if you get a mini split heat pump DESIGNED for cold weather, it will heat your space in a way that will surprise you – you will be very warm and happy!

Instead of listing all the pros & cons, and potential scenarios, I will give an example. Five years ago, New Brook elementary school in Newfane, Vermont installed Ductless heat pumps + solar panels for HEATING and cooling of the building, with a backup propane boiler (only for days with temps below -4F). This was an unprecedented choice of heating for a school building in this area, and many people were against it. However, the upgrade was finally approved and is working very efficiently to this day.

This means that heat pumps can produce enough heat in cold climates, AND be cost efficient! Pair that with roof-top Solar, and you will have free heating in 5-8 years.

However, if your power goes down, you may be left without heat! Therefore it is important to have a backup plan, if you live in northern climates and want to use mini-split heat pumps for heating!

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