Don’t pay for an oversized furnace. How to calculate your home’s heat loss and ventilation needs.
- Matt M.
- Apr 28, 2023
- 4 min read
Did you replace your home furnace before, and weren’t too sure about what you were sold?
Most HVAC/R contractors will sell you equipment that’s far off from what your house actually requires, in terms of heating and cooling. Often times, the contractor will try to get the best bargain for themselves and will not practice proper due-diligence when selling equipment to the client. What clients end up getting is an oversized fan motor and heating coil, that’s guzzling the home’s energy consumption, that’s usually overstocked in the supplier’s warehouse.
Homeowners could benefit from this article, especially in times of heating system modifications or furnace replacement. By understanding the method of heat loss calculation, even this basic form of it, you will come to an answer that’s much more suited to your home’s needs, and avoid unnecessary installation of oversized equipment.
The process of calculating heat loss in a house involves several steps: Step 1
Determining the surface area of the house that is exposed to the outside environment, including walls, windows, doors, and ceilings.
Step 2
Identifying the types of materials used for the walls, windows, doors, and ceilings and their respective thermal conductivities.
Step 3
Estimating the temperature difference between the inside and outside of the house, typically assuming indoor temperature of 68-72°F and outdoor temperature of 0-10°F.
Step 4
Calculating the heat loss through each surface using the formula Q = U x A x (Ti - To), where Q is heat loss in BTUs per hour, U is the overall heat transfer coefficient, A is the surface area, Ti is the indoor temperature, and To is the outdoor temperature.
Step 5
Summing up the heat loss values from each surface to get the total heat loss of the house.
Step 6
Adjusting the heat loss calculation for factors such as ventilation, air infiltration, and other sources of heat gain or loss.
Step 7
Using the heat loss calculation to size the heating system, determine insulation requirements, and identify areas for energy efficiency improvements.
Next...
A ventilation system is required in every home, mainly to substitute stale, old air, with fresh air. We must first identify our ACH (Air changes per hour). Older homes tend to have a looser building envelope, which makes them more susceptible to infiltration of air, in which case, balancing of the ventilation system will not occur, and your home will simply require appropriate exhaustion of stale air (through bathroom/laundry vents and hood exhausts in the kitchen).
The ACH of older homes tend to be between 3-11 ACH, and newer homes are recommended to be constructed to a 0.5 ACH tightness level. This low ACH level requires the addition of a balanced ventilation system such as an HRV.
To calculate the Air Changes per Hour (ACH) for a house, follow these steps: Step 1
Determine the volume of the house in cubic feet: Multiply the floor area by the ceiling height of each room. If you have different ceiling heights in the same room, calculate each volume separately and add them together. For example, a 1,200 square feet house with an 8-foot ceiling height would have a volume of 9,600 cubic feet.
Step 2
Calculate the air flow rate required: You need to determine how much fresh air you need to bring into the house to achieve the desired level of ventilation. The recommended ACH rate for a residential property is between 0.5 and 1 ACH. Let's assume we want to achieve 0.7 ACH.
The Canadian air tightness calculator assumes the following values (which you can use for the purpose of your independent calculation) for ACH depending on the construction time of the house:
Loose (Pre-1945): ACH = 10.35
Average (1946 – 1960): ACH = 4.55
Present (1961 - ): ACH = 3.57
Energy Tight (New construction approved by an energy consultant): ACH = 1.5
*If your home is not energy tight, and is older, use a linear interpolation calculator using the values above, given by the Canadian air tightness calculator. Simply Google “interpolation calculator”* ACH = Volume of the house x desired ACH rate (based on year of construction or energy audit) ACH = 9,600 cubic feet x 0.7 = 6,720 cubic feet per hour Step 3
Convert the air flow rate into CFM (cubic feet per minute): Divide the air flow rate calculated in step 2 by 60.
CFM = 6,720/60 = 112 cubic feet per minute Step 4
Determine the appropriate ventilation system for the house: Based on the CFM calculation, you can choose a suitable ventilation system to achieve the desired ventilation rate. There are various types of systems such as balanced ventilation, positive-pressure ventilation, and exhaust-only ventilation.
Keep in mind the Ontario Building Code’s requirement for principal ventilation. Depending on the amount of people residing, your ventilation system may require a higher air flow rate than the ACH calculation would require. The Ontario Building Code prescribes the following ventilation rates for individual rooms:
21.2 CFM for the master bedroom
21.2 CFM for unfinished basement
10.6 CFM for other bedroom
10.6 CFM for habitable room (i.e. kitchen, living, family, mudroom, laundry room, bathroom)
Principal ventilation example – A house has the following rooms: unfinished basement, living room, dining room, kitchen, mudroom, powder room, master bedroom, 2 other bedrooms, master ensuite bath, bathroom, laundry room.
unfinished basement - 21.2
Master bedroom - 21.2
Other bedrooms - 10.6 x 2
Other habitable rooms - 10.6 x 8
Total ventilation must meet - 148.4 CFM
Don’t fall for fraudsters who try to clear the warehouse on HVAC uneducated consumers. Take this information, and secure the right equipment for your home.
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