How to Do Manual J Calculation? (A Step-by-Step Guide)

Manual J calculation is often required for authorities to approve the construction of new homes in the United States. However, many homeowners struggle to get it done. So, this post will provide a step-by-step guide on how to do Manual J calculation based on the 7th edition.

In summary, the Manual J procedure requires you to measure the square footage of your walls, windows, doors, roofs and floors, and multiply them by their corresponding Heat Transfer Multiplier (HTM) to calculate the amount of heat loss and heat gain in your home.

Compared to other load calculation methods, Manual J is relatively straightforward. After I break down each element of the calculation procedure, you’ll have a better understanding of how it is done.

What is Manual J?

Before we start the calculation, let’s recall what is Manual J.

Manual J is a residential heating and cooling load calculation handbook produced by the Air Conditioning Contractors of America (ACCA) that outlines the procedures and the building data necessary for the calculation.

The procedure is built on top of the ASHRAE CLF / CLTD method which is a more commonly used method among HVAC engineers. Manual J is tailored to typical residential houses such as single-family-detached homes, small multi-unit structures, condominiums, townhouses, and manufactured homes (mobile homes).

So, Manual J should not be used for commercial and industrial buildings.

The latest version of Manual J is the 8th edition which has updated weather data, construction materials, accounted for diversity in multi-zone systems and more. For complex buildings, the 8th edition updates can significantly impact the results.

How to Do Manual J Calculation?

Manual J considered the following factors that will affect how much heat is lost or gained in your house:

• Outdoor Design Conditions – Different locations have different winter and summer temperatures.
• Window Type – Single or double pane windows and different window frames.
• Door Type – Glass door, wood door or metal door.
• Wall Type – Wall type (wood siding or brick), wall layers, material thickness and insulation grade.
• Roof Type – Roof color, insulation grade, ventilated or non-ventilated, ceiling or no ceiling.
• Floor Type – Insulated or uninsulated, above crawl space or basement.
• Building Orientation – Window facing direction, room primary exposure direction.
• Infiltration – Good or bad air tightness.
• Ventilation – Any outdoor air or ERV system.
• Duct Loss/Gain – Heat lost or gained due to ductwork.
• People, Lighting and Equipment

There is a vast number of possible combinations in each factor. For example, there are more than 200 locations where you can refer to and use the outdoor design conditions. For windows alone, there are over 50 combinations comprised of single pane, double pane, wood frame, metal frame and skylight.

The key to a successful Manual J calculation is to pick the right construction type (for wall, window, roof, etc.) as per your house specifications and apply the corresponding HTM value.

All of the data and HTM values are provided in the Manual J handbook. You likely need to purchase the book (7th edition) if you want to do it for your house as the data I’ll be showing here is limited.

Nevertheless, I’ll use an example and show you how the procedure is done. You can decide if you want to buy the book and do it by yourself. If you’re a first-timer, you likely need about 4 to 6 hours to complete the Manual J calculation depending on the size of your house.

Step 1: The Indoor Design Conditions

The first step to do the Manual J calculation is to establish the indoor design conditions. The indoor design conditions refer to the temperature you want in your house in winter, and the temperature and relative humidity you want in summer.

Fortunately, Manual J fixes this design parameter so that we have one less thing to worry about. The indoor design conditions as per Manual J is:

• Winter 70°FDB
• Summer 75°FDB

The °FDB means degree Fahrenheit dry bulb temperature which is the room temperature we’re referring to. For summer, you can choose to design based on either 55% or 50% relative humidity. This will affect the grain difference which in turn affects the latent load later in the calculation.

Step 2: Location and Outdoor Design Conditions

Manual J provides a long list of states and cities to which you can refer to the designated outdoor design conditions. The outdoor design conditions refer to the dry-bulb temperature in winter and the dry-bulb temperature and grain difference in summer.

Let me give you an example.

Say we’re building a new home in Lexington, South Carolina, United States. Referring to Table 1 of the Manual J 7th edition, the nearest reference location is Columbia AP. With that, we get the outdoor design conditions as follows:

From the above table, the location is Columbia AP. The winter design dry-bulb temperature is 24°F and the summer design dry-bulb temperature is 95°F. The grain difference is selected based on either 55% or 50% relative humidity as I mentioned earlier.

Assuming we design based on 50% relative humidity, the grain difference is 35.

The daily range is 22 M. What’s important here is the M word. It means medium. Depending on the location, you may have L, M or H which stands for low, medium or high. This will decide which HTM value to use later.

Step 3: Calculate for the Exterior Walls

Now that we’ve established the design conditions, we proceed to calculate the square footage of various house components, starting with the exterior wall.

To help you visualize and understand, I’ll use an example for the entire process. So, let’s first go through the sample house that we’re going to use the Manual J to calculate its heating and cooling load:

For simplification, we’ll only focus on calculating for the bedroom. The bedroom size is 14 ft by 10 ft so the floor area is 140 sqft. Two sides of the wall are exposed to the outside weather. So, both walls are considered exterior walls.

The exterior wall that has a 3636 double glass window is in the north direction as shown by the compass in the plan view drawing. So, we’ll call that the north wall. The other exterior wall is the east wall.

The north wall is 14 ft wide and 8 ft tall. The height of the wall is based on the ceiling height. So, the surface area of the north wall is 112 sqft.

The 3636 double glass window is 36 inches by 36 inches which translates to 9 sqft.

For the exterior wall calculation, we want only the net surface area of the wall. So, the north wall is 112 sqft minus the 3636 window equals 103 sqft.

We do the same for the east wall and we get 75.5 sqft.

Therefore, the total surface area for the exterior wall is 178.5 sqft.

Now, the surface area multiplied by the HTM value is the heating and cooling load of the exterior wall. So, we need to find the corresponding HTM value from Table 2 and Table 4 of the Manual J 7th edition.

For heating load, refer to Table 2 of the Manual J 7th edition:

From the above table, No. 12 Wood Frame Exterior Walls with Sheathing and Siding or Brick, or Other Exterior Finish is the construction material and layer of the exterior wall. This matches the construction material used in our sample house.

The winter TD is the temperature difference between the outside and inside in winter. As worked out earlier, our winter design dry-bulb temperature based on Columbia AP is 24°F. The indoor dry-bulb temperature is fixed at 70°F in winter. So, the winter TD is 70 – 24 = 46°F.

Since 45 is the nearest winter TD, our HTM value is 3.6 Btuh per sqft.

Now, we calculate the heating load for the exterior wall as follows:

• Sensible Heat Loss _{exterior wall} = 178.5 sqft x 3.6 Btuh per sqft = 642.6 Btuh

Moving on to cooling load, we refer to Table 4 of the Manual J 7th edition:

From the above table, we also need to calculate the summer temperature difference by taking the summer outdoor design dry-bulb temperature of 95°F and minus the indoor design dry-bulb temperature of 75°F. The result is summer TD 20°F.

Now, under each summer TD, there are two or three HTM values. These HTM values are tied to L, M or H. This refers to the daily range low, medium or high that I’ve mentioned earlier.

Based on the location of our sample house, Columbia AP, the daily range is M, medium.

So, the HTM value is 1.9.

Now, we calculate the cooling load for the exterior wall as follows:

• Sensible Heat Gain _{exterior wall} = 178.5 sqft x 1.9 Btuh per sqft = 339.2 Btuh

If you use a different insulation grade, for example, R-11 or R-19, then you’ll be referring to a different row in the table, getting a different HTM value, and thus, a different sensible heat loss and gain value.

The 7th edition of the Manual J only provides the HTM values for one type of exterior wall which is the wood frame with sheathing and siding or brick. This is a very typical type of exterior wall for homes in the U.S.

However, if you have a completely different exterior wall, then you may want to consider the updated 8th edition of the Manual J.

Step 4: Calculate for Partition Walls

Exterior walls and partition walls are calculated differently. Exterior walls typically have a greater temperature difference than partition walls. So, the heating and cooling loads should be lower for partition walls.

In the bedroom of our sample house, assuming that the hallway and bathroom are heated and cooled the same as the bedroom. So, there will be no temperature difference between these rooms. Hence, the only partition wall is the wall between the bedroom and the storage area.

The surface area of the partition wall is 10 x 8 = 80 sqft.

From Table 2 of the Manual J 7th edition:

Since the adjacent room is not outside, we can’t use the winter outdoor design dry-bulb temperature of 24°F to calculate the winter TD. Instead, we estimate the average room temperature in the storage area.

Assume that the storage area will have an average room temperature of 45°F in winter. The winter TD is 70°F – 45°F = 25°F. Hence, the HTM value is 6.8.

Therefore:

• Sensible Heat Loss _{partition wall} = 80 sqft x 6.8 Btuh per sqft = 544 Btuh

For cooling load:

For cooling, the HTM values for partition walls are provided in No. 13. Again, we estimate the average room temperature of the storage area in summer.

Assume the average room temperature of the storage area in summer is 86°F, the summer TD is 95°F – 86°F = 9°F. The nearest summer TD is 10 and since the daily range is medium, the HTM value is 1.4.

Therefore:

• Sensible Heat Gain _{partition wall} = 80 sqft x 1.4 Btuh per sqft = 112 Btuh

The HTM values for insulated partition walls are also provided in Table 4 of the Manual J 7th edition.

Step 5: Calculate for Windows

For windows, we need to know how many layers of glass are used. Is it single, double or triple glazing? Storm windows and skylights also have their specific HTM values. Calculations for windows are slightly more complicated than for walls.

In our sample house, both windows are double-glazing mounted in a wood frame. Assuming the glass is clear, not tinted with any color, from Table 2:

We use winter TD 45 since the temperature difference is between the outdoor design dry bulb and the indoor design dry bulb. The HTM value is 24.8.

As calculated earlier, the surface area of the 3636 window is 9 sqft and 4.5 sqft for the 1836 window.

Therefore, the sensible heat loss due to these two windows:

• Sensible Heat Loss _{3636 window} = 9 sqft x 24.8 Btuh per sqft = 223.2 Btuh
• Sensible Heat Loss _{1836 window} = 4.5 sqft x 24.8 Btuh per sqft = 111.6 Btuh

For cooling load, the orientation of the window matters. The 3636 window is facing north and the 1836 window is facing east. Referring to Table 3A:

From the above table, window facing directions are given as N, NE, NW, E, W… It stands for north, northeast, northwest, east, west… Every direction has its own HTM value.

To locate the HTM value, we also need to know if the window has any shading. The HTM values shown in the above table assume there is no internal and external shading. If you plan to shade the window using draperies or venetian blinds, you need to use the corresponding HTM value. The value will be lower.

So, since our summer TD is 20, the HTM value for the 3636 north window is 23 and 72 for the 1836 east window.

Therefore:

• Sensible Heat Gain _{3636 window} = 9 sqft x 23 Btuh per sqft = 207 Btuh
• Sensible Heat Gain _{1836 window} = 4.5 sqft x 72 Btuh per sqft = 324 Btuh

If you have heard people mention before, east and west windows are regarded as the highest source of heat gain in a house. This reflects on the higher HTM value shown in the above table which results in a much higher heat gain.

For sliding glass doors, the procedure is similar to windows. So, I’ll skip it here. For non-glass doors, the process is simpler. You just need to find the corresponding door material in Table 2 for winter and Table 4 for summer.

Step 6: Calculate for the Roof and Floor

The roof and floor can cause significant heat loss or gain if it is not well insulated. That’s why many people advise to insulate the attic to conserve energy.

Assume that the roof has the same area as the floor, both the roof and the floor are 140 sqft.

The roof is made of asphalt shingle and sits on a naturally-ventilated attic with R-19 insulation. The floor is hardwood and sits on top of an open crawl space with R11 insulation. From Table 2:

The HTM values for the roof and floor are 2.4 and 3.6 respectively. Therefore:

• Sensible Heat Loss _roof = 140 sqft x 2.4 Btuh per sqft = 336 Btuh
• Sensible Heat Loss _floor = 140 sqft x 3.6 Btuh per sqft = 504 Btuh

For cooling load, the color of the roof matters. The roof in our sample house is dark-colored. So, we refer to Table 4:

Again, our daily range is medium. So, the HTM value for the cooling load of the roof and floor is 2.3 and 1.3 respectively. Therefore:

• Sensible Heat Gain _roof = 140 sqft x 2.3 Btuh per sqft = 322 Btuh
• Sensible Heat Gain _floor = 140 sqft x 1.3 Btuh per sqft = 182 Btuh

Now, we’ve completed what is known as the external heat loss/gain calculation. This is the hardest and most time-consuming part. Let’s recap our results so far:

Now, let’s move on to other components that might add more heating and cooling load to our result.

Step 7: Infiltration Load

Infiltration means when there is unwanted outside air coming into the house. In winter, cold outside air will increase our heating load. In summer, warm outside air will increase our cooling load.

To calculate the additional heating and cooling load due to infiltration, we must first know how much outside air is coming into the house. Based on the Manual J procedure, we have to know what airtightness level our house falls into; poor, average or best.

For each airtightness level, we then based on our house’s total floor area to determine the air change per hour in winter and summer. For example:

Assume our sample house has a total floor area of 1200 sqft. This includes the bedroom, living room, kitchen, dining room and other rooms that I have not shown in the plan drawing.

Next, we also assume that the air tightness of our sample house is average. According to Manual J, average airtightness refers to tested leakage of windows and doors between 0.25 and 0.50 CFM per running foot of crack.

With that, we refer to Table 5 (winter) of the Manual J 7th edition:

From the above table, the winter air change per hour (ACH) for our sample house is 1.0. Assume our sample house has a uniform ceiling height of 8 ft. The total volume is 1200 x 8 = 9600 cu.ft.

In addition, fireplace will increase the winter air changes per hour. For every fireplace, add 0.1 (best), 0.2 (average) or 0.6 (poor) based on the airtightness level of the house. Since our sample house is average airtightness, assuming we have one fireplace in the living room, the additional air change per hour is 0.2.

So, the calculation for the heating load due to infiltration is as follows:

• Winter infiltration CFM = (1.0+0.2) ACH x 9600 cu.ft x 0.0167 = 192 CFM
• Winter infiltration Btuh = 1.1 x 192 CFM x 46 winter TD = 9715.2 Btuh

Notice that the winter TD is 46°F instead of 45°F because 46°F is the exact value. Earlier, we used 45°F due to the fixed reference in the Manual J tables.

Now, the winter infiltration Btuh of 8096 is for the whole house. To proportionally share it with the bedroom, we take the windows + doors area of the bedroom, divide it by the total windows + doors area of the whole house, and multiply it by the winter infiltration Btuh.

Assume that the total windows + doors area in our sample house is 100 sqft. The total window + door area in the bedroom as calculated earlier is 9 + 4.5 = 13.5 sqft. Hence, the heating load due to infiltration is:

• Sensible Heat Loss _infiltration = 13.5 sqft / 100 sqft x 9715.2 Btuh = 1311.6 Btuh

For cooling load, it is slightly different as we need to include the additional moisture as well.

From Table 5 (summer) of the Manual J 7th edition:

Since our sample house has a total floor area of 1200 sqft and the air tightness level is average. The summer air change per hour is 0.5. So, for summer infiltration:

• Summer infiltration CFM = 0.5 ACH x 9600 cu.ft x 0.0167 = 80 CFM
• Summer infiltration Btuh = 1.1 x 80 CFM x 20 summer TD = 1760 Btuh

Similarly, the total volume is 9600 cu.ft. The total windows + doors area is 100 sqft. And, for the bedroom, the total window + door area is 13.5 sqft. Therefore:

• Sensible Heat Gain _infiltration = 13.5 sqft / 100 sqft x 1760 Btuh = 237.6 Btuh

Now, based on the summer infiltration CFM and the grain difference we know from the outdoor design conditions which is 35 at 50% RH, we calculate the latent cooling load as follows:

• Latent Heat Gain _{infiltration (whole-house)} = 0.68 x 35 gr. diff. x 80 CFM = 1904 Btuh
• Latent Heat Gain _{infiltration (bedroom)} = 13.5 sqft / 100 sqft x 1904 Btuh = 257 Btuh

From the above, we can see that the better the air tightness level, the lower the infiltration CFM and thus, the lower the heating and cooling load due to infiltration.

Step 8: People, Lighting & Appliances

People, lighting and appliances are regarded as internal heat gains. This section is only for cooling load. The heat emitted by people, lighting and appliances is traditionally not accounted for reducing the HVAC heating capacity requirement.

Internal heat gains are largely depending on how many people you use in the calculation. Generally, the number of people is 2 times the number of bedrooms in the house. So, if you have two bedrooms, use 4 people for the calculation.

Furthermore, this heat gain should be added to the living room or dining room as that’s where the people will be staying at during peak cooling hours (12 to 4 pm).

For the sake of our example, we assume 2 people in the bedroom during peak cooling hours. For each person, add 300 Btuh of sensible heat and 230 Btuh of latent heat. The sensible heat had already included the lighting. For the kitchen, add 1200 Btuh of sensible heat.

So, for our sample house:

• Sensible Heat Gain _internal = 2 people x 300 Btuh = 600 Btuh
• Latent Heat Gain _internal = 2 people x 230 Btuh = 460 Btuh

Nowadays, it is very likely that people are staying in the bedroom during peak cooling hours. So, we need to calculate based on the actual behavior of the people.

Final Step: Equipment Sizing

Now that we’ve calculated both the internal and external heat gain/loss, we finally can sum them up and size the equipment. In summary:

So, the total heating and cooling load (sensible + latent) in our sample house, and bedroom is 3673 Btuh and 3040.8 Btuh respectively.

Given that the bedroom floor area is 140 sqft, the heating load is 26 Btuh per sqft and the cooling load is 22 Btuh per sqft which are well within the typical range.

The airflow required for the bedroom can then be calculated based on the sensible heat:

• Airflow _heating = 3673 Btuh / [ 1.1 x (95°F supply temp. – 70°F indoor temp.) ] = 134 CFM
• Airflow _cooling = 3040.8 Btuh / [ 1.1 x (75°F indoor temp. – 57°F supply temp.) ] = 154 CFM

The supply temp. is the temperature of the air coming out of the vent. Heat pumps are usually about 90-110°F in heating mode and cooling is about 52-58°F.

Nonetheless, once you have the heating and cooling load, airflow can follow the furnace, mini split or heat pump CFM. However, you may need to know the duct static pressure if you’re using a ducted system so that you can set the airflow correctly.

This is for our sample house and it is just one room. If you use an excel sheet and form a template, it’ll be faster for other rooms. Once you’ve done for the entire house, you can get something like this report:

The above is one of the pages of my calculation report for one of my clients. As you can see, the HTM values are shown and the sensible heat losses/gains are calculated accordingly, and I use the Btuh per sqft as a check figure to verify my calculations.

Additional Loads Based on HVAC System

At this stage, we have successfully calculated the heating and cooling load present in the bedroom. Depending on what type of HVAC system we want to go for, we need to account for a few more factors.

The steps that I’ve yet to include are:

• Ventilation Load
• Duct Loss/Gain
• Multi-Zone System

In the following, we’ll account for the above loads as necessary based on the type of HVAC systems:

Ductless Zoned System

From our sample house bedroom Manual J calculation, the heating load is 3673 Btuh and the cooling load is 3040.8 Btuh. With that, we can go ahead and use a 6000 BTU ductless system like a wall-mounted mini split, portable or window AC to heat and cool the bedroom.

If we’re not bringing in the outside air via the mini split or any other ventilation devices, we’ll not be having the ventilation load. However, if we’re bringing in the outside air that induces a temperature difference, then we need to include the ventilation load.

The calculation for the ventilation load is the same as the infiltration load.

First, we need to know the ventilation CFM being supplied to the room at what temperature. Then, use the same formula to calculate the ventilation Btuh.

Since ductless mini splits don’t use any duct, the heat gain/loss due to duct is not applicable. However, we need to account for the multi-zone factor. From Table A2-1 of the Manual J 7th edition:

From the above table, the bedroom in our sample house is mainly exposed to the sun in the northeast direction. The total windows area is 13.5 sqft and the total exterior wall area is 178.5 sqft. So, the percentage of glass on the wall is 13.5 ÷ 178.5 x 100 = 7.56%.

So, we use 10% glass.

The multi-zone correction factor for our bedroom is 1.00. Hence, no change to our sensible heat gain.

But, if the bedroom is exposed in the southwest direction, at 10% glass, we need to multiply the sensible heat gain by 1.20 which means the new sensible heat gain is 2323.8 x 1.2 = 2788.56 Btuh.

For each room that has a head unit (indoor unit), we need to multiply a multi-zone correction factor as per the Manual J procedure. The same applies to window and portable air conditioners.

Ducted Central Air System

Similarly, if we have a ventilation load, we need to add it. But since central air systems are not considered multi-zone, we don’t have to account for the correction factor. However, we have duct loss/gain.

When a long stretch of duct is involved, we need to account for duct heat loss in winter and duct heat gain in summer, depending on where we run the duct.

For our sample house, assume we run the duct in the attic and the duct has R6 insulated. From Table 7A:

Since our winter outdoor design dry-bulb temperature is 24°F, the duct loss multiplier is 0.05.

From there, we multiply the sensible heat loss by the multiplier to obtain the duct loss Btuh. So, 3673 Btuh x 0.05 = 183.65 Btuh of duct heat loss.

Therefore, the new sensible heat loss is 3673 Btuh + 183.65 Btuh = 3856.65 Btuh.

For cooling load, we refer to Table 7B:

Based on the above, our sensible heat gain of 2323.8 Btuh times 0.10 equals 232.38 Btuh of duct heat gain. Therefore, the new sensible heat gain is 2556.18 Btuh.

Summary

There you have it, the full Manual J 7th edition calculation procedure. The new 8th edition has additional steps for better accuracy in certain applications but in general, this is what it takes to calculate the heating and cooling load as per the ACCA Manual J.

So, let’s recap:

1. We first set the indoor and outdoor design conditions.
2. Then, we calculate the area of the wall, window, roof and floor.
3. Based on winter and summer TD and the construction material, we find the HTM value.
4. Next, we multiply the area by the HTM value to get the heat gain/loss.
5. If we have ventilation, we need to include the load as well.
6. Depending on the type of HVAC system we use, we apply a correction factor or a duct multiplier.

The Manual J procedure is quite simplified compared to the RTS or HBM method used in the commercial sectors. Though it is complex for non-professionals, it is not impossible to do.

Hope this post gives you an insight into how Manual J is done.

If you have trouble or need me to help you do the Manual J calculation and other HVAC design work, feel free to contact me for a quote.