How To Perform an In-Home Heat Loss – Heat Gain
How To Perform an In-Home Heat Loss – Heat Gain
In this write-up, we will go over how to do a Heat Loss / Heat Gain analysis in a residential replacement using a simple detached home. In a separate write-up, we discuss the value of doing the analysis as a part of the sales process and how to use the results to actually make a sale.
The home comfort technical assessment is an important aspect of the Consultative Sales Process because it allows you to gather the often unchangeable facts about the home. In some cases, but not many, the homeowner may be willing to add ductwork or vents to rooms to improve comfort especially if the costs are not too high. By doing the technical assessment of the home, we can recommend comfort improvements and utilize the powerful tool of gap selling by presenting the facts about their home.
There are three parts to the technical side of home comfort assessment:
- Recording all current HVAC equipment, especially the AC and Furnace’s brand, BTU rating, motor type, and manufacturing year. Also, include any obvious code violations or special notes.
- The current construction of the ductwork coming off the furnace with the measurements and CFM ratings. At the very least, the measurements of both the supply and return ductwork coming into the furnace with the first bends.
- A floor-by-floor, room-by-room Heat Loss / Gain Analysis of the entire home.
While there is some uniformity in house construction after the 90s never assume something and always err on the side of caution/safety in your estimates. There is a 20-30% margin of error that is allowed for while calculating both heat loss and gain of a home. The goal of the calculation is to find equipment that is sized properly for the home because both undersizing and oversizing of the Furnace/AC/Heat Pump can lead to comfort and even safety concerns.
Below is the construction of the house on which this mock analysis is conducted. It is a two-story home with a standard rectangular foundation. The main floor and top floor are uniform with the basement floor. But before we jump into the math and measurements of the Heat Loss/Gain and its calculation, Let’s go through some basic HVAC concepts that will prepare you for this task. We will assume you know remedial HVAC concepts for the most part.
Parts of the home you will measure and/or count:
- Windows
- Return Air Vents
- Supply Air Vents
- Room Dimensions
- Ceiling Height
- Direction of Windows
- Dimension of Outward Doors
- Storm Doors
- Above and Below Grade
- Insulation Type
- Unconditioned spaces
- Foundation Type
- Attic types
- Kitchen Type
Gathering Existing Equipment Details
To access the details about the furnace and AC, you will need to remove the cover of the furnace (top of the AC) to access the rating plate. Also note any addons like HRVs, UV lights, and water heaters they may have. Pay attention to the signs if any equipment is being rented because this complicates the sale of any replacement(s).
Ductwork and Airflow
While you are in the basement your next task is to determine the size of the supply and intake air ducts coming out of the furnace. To achieve this, you will need to measure the dimension of several parts of the existing ductwork. Measure the following items and enter the trunk sizes into the ductulator:
- Supply Plenum
- Supply Air Plenum Take Off
- Supply Air Trunk
- Return Air Drop
- Return Air Trunk
Save the CFMs for after the Heat Loss Gain is performed as a furnace must have enough airflow to operate and provide enough heat/cool to the home. The size of the ductwork determines the airflow in the system which is measured in Cubic Feet per Minute. A worksheet has been provided at the end.
Calculating Total CFM Values
To do this you will need the metal measurements coming off the furnace and a Ductwork Air Duct Calculator often called a Ductulator. For example, a 10×22 In. supply truck provides 1500CFM when the Duculator is set to .10 the Recommend Residential Setting.
Cooling will always need more airflow to achieve comfort within the home than Heating will. So, if there is sufficient CFM for cooling then heating will likely not be an issue. In the case the cooling does not use the furnace then you are measuring the CFMs for heating alone, the values as seen in the chart above separated by cooling and heating. CFMs are the unit used to measure air flow and BTUs are the unit to measure the amount of energy needed to achieve heating or cooling.
Types of Windows:
Depending on the age and make, windows will often tell you what kind they are, and this information is written inside the window frame, if not they are likely old single-pane windows, or something just as bad.
- Single Pane windows are the least efficient and come in a wide range.
- Double Pane windows are the most common window used in homes.
- Triple Pane windows are the most efficient but are not commonly used.
There are many types of windows and glass, but they should fall into one of the three above categories. Sliding glass doors are to be treated as windows in this calculation. Basically, if it is made of glass and is on an outside-facing wall, it is counted as a window because heat is lost/gained through it.
Walls of the Home:
You are measuring walls that face the outside elements, so an interior wall would not be measured and thus not a part of the heat loss/gain calculation. For example, a duplex shares a wall between two living spaces, and this would not be counted. However, a wall connected to an unconditioned space is counted, for example, a house wall that is connected to a garage, crawl space, or whatever it may be, is losing heat and therefore must be accounted for.
Your goal here is to estimate the R-value of the insulation in the walls. For example, no insulation has an R-Value of 0, while the average insulation in walls is R-9. Below, we will guide you through the thought process when determining the wall construction and insulation of a home because you can’t see through the walls in most cases.
Almost all homes will be made of wood, but they could be made from concrete like a unit of a high-rise apartment or possibly a condo unit in a larger walk-up apartment.
In the case of detached homes, they will be made of wood with insulation in the walls and have a concrete foundation (in some rare cases the foundation could be made of brick/cinder blocks). A general rule of thumb is to ask the client if they know what insulation they have and how the home was constructed. If you cannot observe the details yourself and the client does not know then follow these assumptions:
Homes made in the 90s and beyond will have standard above-grade walls with 2×4 wood frames with R9 to R12 insulation inside them These walls then sit on a concrete foundation. In some cases, the basement will be unfinished with naked concrete or be finished and thus insulated (be careful some basements appeared to be finished but merely have interior panels attached to them). Touch the walls of a basement, if they are chilling to the touch chances are the walls are not insulated. Another tip is to look at the windowsills and door jams/framing of the house to see what the dimension of the walls would be on the main/upper floor.
Older homes should get a middle R-Value; use this formula: 0+R11value/2.
Registers and Air Flow
To heat a home with a forced-air furnace you need airflow and there is no way around this. Since furnaces do not provide radiant heat and instead push warm air throughout the house via ductwork, this ductwork must vent into a room and flow between a supply and intake. However, this is only one half of the process, as that same heated and now cooled, must be pulled back into the ductwork and travel back into the furnace to be heated again. This is why rooms need to have both a supply and return air register present, or there will be comfort issues in that lacking room.
Remember as you go through each room to measure its perimeter you are also checking if they have both a supply and return air register.
The most common vent sizes for medium-sized rooms such as bedrooms and dining rooms are 4×10 inches, 4×12 inches, 6×10 inches, or 6×12 inches. In smaller rooms like bathrooms, the vents could be in even smaller sizes. Another important note is that return air vents are often larger than supply vents. Also, keep an eye out for any blocked vents.
Finally, you need to check if the vents are getting enough airflow in them by holding up a facial tissue to the return vents to see if it sticks to them or falls off. If it sticks, then the airflow is sufficient. To get an idea of vent sizes out there, look at the image below.
Step 1: Heat Loss Gain Collecting Measurements Pull out a sheet of grid paper, one is located in your sales Using a laser tape measure makes it much easier and quicker |
3) Measure the perimeter of the entire basement observing any large juts that would impact the total perimeter.
4) Take note of any return or supply registers in the rooms within the basement.
8) Inspect the below-grade portion of the basement, what insulation are they using? If you do not know, ask the homeowner if you cannot tell refer to the note above.
9) You are now ready to move up to the main level. Return to the front corner of the home and measure the walls, doors, and windows on each exterior facing walls (in the case of this example the home is uniform on all levels. This will not always be the case). Label the floors as Base and Main so you do not mix up what calculations belong to what floor.
Below you will find the completed diagram of the home’s measurements. We will now move on to the calculation part of the assessment.
Step 1: Calculate the Total Area of all Exterior Windows
To calculate the sq ft of the windows use the A=LxW formula on each window. Then add each windows amount of glass together. In the case of our example, we arrived at 137sq.ft. of total glass. Note: Red numbers are heating loads and blue numbers are cooling loads. Heating is lost and cooling is heat gained.
(Gross Window(s) Area)xHeating Value=Loss
(6+6+6+6;40+16+9+12+12+12;6+6)x38.08=Loss
137×38.08=5216.96
Step 2: What way does each window face? Calculate the total area of the windows for NSEW.
North Facing Windows:
(Gross Window(s) Area)xCooling Value=Gain
(6+6;12+12+12;6)x17=Gain
54×17=918
South Facing Windows:
(Gross Window(s) Area)xCooling Value=Gain
(6+6;40+16;6)x66=Gain
74×66=4884
East & West Facing Windows:
(Gross Window(s) Area)xCooling Value=Gain
9×56=Gain
Gain=504
Step 3: Calculate the Door Area and Heating and Cooling Separately. Note: Are doors the same size?
((Door Area) x Heating Value= Loss) + ((Door Area) x Heating Value=Loss)
(7×3)x14.28 + (7×3)x14.28=Loss
42×14.28=599.76
((Door Area) x Cooling Value= Gain) + ((Door Area) x Cooling Value=Gain)
(7×3)x9 + (7×3)x9=Gain
42×9=378
Step 4.1: Calculate The exterior walls of the house and the Insulation R-Value of the Siding
(Height of Above Grade Walls Main Floor+Levels) x (Perimeter of Main Floor Wall) = Gross Exposed Wall
(8+8)x(24+30+24+20)=Area
16×108=Area
Area=1728 Sq. Ft.
Step 4.2: Calculate the net wall and determine the heating and cooling values
Area of Grossed Exposed Walls Minus Total Glass & Doors=Net Wall
1728-(137+42)=Net Wall
Net Wall=1549
1549 x Heating R Value=Gain
1549×6.6=Loss
Loss=10223.04
1549 x Cooling R Value=Gain
1549 x2.58=Gain
Gain=3996.42
Step 5: Calculate the Heating and Cooling Values for the above-grade basement wall plate
Basement Above Grade x Perimeter of walls = Area
2.5×108=270 Sq. Ft.
Area x Heating R-Value=Loss
270×11.9=3213
Area x Cooling R-Value=Gain
270×1.4=378
Step 6: Calculate the Heating Value for the below-grade basement wall
Height of Below Grade Wall x Perimeter of Walls = Area
6×108=648 Sq. Ft.
Area x Cooling R-Value=Loss
648×4.9=3175
Step 7: Calculate the Heating and Cooling Values for the Ceiling — Is it with attic/knee wall or Roof Joist?
Width of 2nd-floor walls x length of 2nd-floor walls = Area
24×30=Area
Area=720 Sq. Ft.
Area x Heating R-Value = Loss
720×3.33=2398
Area x Cooling R-Value = Gain
720×2.57=1850
Step 8: Calculate the Heat Value of the Basement Floor — Is it 2 Ft. below grade or slab?
Width of basement wall x Length of basement wall = Area
24×30=Area
Area=720 Sq. Ft.
Area x Heating R Value = Loss
720×1.50=1080
Step 9: Calculate the Infiltration Values — Is the home under or over 2000 Sq. Ft. — Total CFMs?
Ceiling Area x Ceiling Height divided by 60 Mins x Leakage = Heat/Cool Lost per Hour
(720×24.5)/60×0.49=Heat Loss/h
17640/60×0.49=Heat Loss/h
294×0.49=Heat Loss /h
Heat Loss Per Hour= 144
(720×24.5)/60×0.25=Cool Gain/h
17640/60×0.25=Cool Gain/h
294×0.25=Cool Gain /h
Heat Gain Per Hour= 73.5
Leakage Factor x Total CFM Value = Heat Loss/Gain
144×68=9729 HTD
72×8=576 CTD
Step 10: Find out from the homeowner: people living in the home and Kitchen Type to find cooling load.
People x Average Person BTUs = Gain
4×230=gain
Gain=920
Kitchen Allowance = Gain 2400
Gain= 2400
Step 11: Calculate Total Heating and Cool Loads
Calculate the sum of all Heat Loss and Gain figures separately.
Heating Load Subtotal (Red Numbers): 35761
Cooling Load Subtotal (Blue Numbers): 16391
Step 12: Select Average Duct Loss Value
Here, we select the average loss/gain for a home which is 0.09 and we keep in mind the age of the home when you choose your value. We are simply adding +/- 10% to the subtotal.
Leakage x Heating / Cooling Subtotals = Adjusted Gain/Loss
35761×0.9=3218.49
16931×0.9=1523.79
Step 13: Calculate Latent Cooling Losses
The CFM value in this part of the calculation was taken from the infiltration value in Step 9.
Latent Infiltration Gain x Grains x CFM = Loss
0.68x25x73.5=1249.5
Latent for Occupants x Grains = Loss
4×200=800
Step 14: Calculate Total Loads for Heating and Cooling
(Subtotal + Leakage = Loss)
35761+3218= Loss
Final Heating Loss for the home = 38980
Therefore: A furnace rated at 40,000BTUs with ductwork that supports 600-700 CFMs is proper.
Subtotal + Leakage + Infiltration + Laten Occupants = Gain
16391+1523+1250+800=Gain
Final Cooling Gain for the home= 19964
Therefore: An AC rated between 1.5-2 tonnes with ductwork that supports at least 850 CFMs is proper.
This calculation has a margin of error of 30% which is acceptable if all measurements have aired on the side of caution.
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