How to Estimate a Furnace Size

Regardless of whether you are a property owner or a contractor, finding a furnace of the right size to fit a specific space can be quite a challenge, especially if the sizes of the rooms are odd. Furnaces are quite reliable and long-lasting but even the best furnaces will require replacement at some time or the other.

While a good furnace specialist will give you all the information you need and provide a suitable option, it’s a good idea to have some basic understanding about what kind of furnace would work best for your application.  The information here will help you estimate the furnace size and this will help you pick an installation that works best for you. Even before we start estimating the size of the furnace, there are certain aspects, which have to be taken into account as they have bearing on the type of furnace you eventually pick.

1. Power of the Furnace

The furnace you choose has to be powerful enough to effectively heat the space it’s meant to be installed for. And it goes without saying that when you have a smaller home, you will need a much smaller furnace and vice versa. As a matter of fact, if you opt for a furnace of the incorrect size, you will eventually end up paying more in the long run. You require just the appropriate amount of power so that the heating levels and distribution are right. Care has to be taken to ensure that it doesn’t have excessive power as that will only make it cycle on & off all day, which will waste energy and money.

2. AFUE Rating

The Annual Fuel Utilization Efficiency (AFUE) rating is a crucial aspect of furnace size estimation. This rating refers to the gas heating equipment’s efficiency; this is the amount of heating that is delivered by the equipment, for every dollar that’s spent on fuel; typically the higher the rating, greater the efficiency. With the modern energy-efficient models that are now available in the market, you are able to get far more value for money as well. The information related to this rating should be mentioned in the paperwork or on the furnace itself; it is something you should check out and use in the calculations.

3. Exposure to the Sun’s Rays

If you have a large number of windows that are exposed to the sunlight for a major portion of the day, the indoor spaces will be naturally warmer. This means the furnace installation won’t have to work that hard to maintain a higher temperature. This is when the sunlight works for you. Insulation is the other factor that comes into play at this point. If there is a lot of good-quality insulation in your home, the indoor spaces will be able to better retain the heat your furnace generates.

BTUs/ Square Footage

The square footage is one of the main aspects of estimating the furnace size. If your furnace is oversized, it will unnecessarily use excessive power while one that’s smaller than it should be, will work overtime; and you will end up spending more money on your energy bills.  BTUs are the other major consideration; British Thermal Units measures the total amount of heat your furnace produces, which is usually in the range of 30-35 output BTUs/square foot. You would have to first multiply the furnace’s input BTU by the efficiency; the next thing to do is to compare the output BTU of the furnace to the actual amount of BTU required in that space.

Estimating the Size of a Gas Furnace

Since there are a number of variables to sizing a furnace, each unit is available in a variety of BTU outputs to meet the requirements of different spaces. This is why it is important to use all the criteria mentioned above, and experience to ensure that all this information is put into a package that works. While there is software which helps make the job easier, the professional that installs the furnace has to make the final determination.

Some Additional Considerations:

As mentioned earlier, the size of the space is the most important factor in estimating the furnace size. Aside from this and the points we just discussed, the other considerations that have to be kept in view and questions that would have to be asked are:

•    Is the home open and does it have a number of large windows and doors?

•    What are the size of the rooms?

•    Are there many smaller rooms, or fewer larger ones?

•    The orientation of the structure as a home with living areas facing southward will end up getting more heat from the winter sun.

Regional Climate and House Construction

•    The climate in the area your home is located in would also have to be used while estimating the size of the furnace. For instance a home located in Northern California would have to face much milder winters and the amount of heating power it requires will be less compared to a home in the Midwest.

•    The construction of the house is another factor that matters. For instance, if a home is made of brick, it is more naturally-insulated than a structure that has timber siding.

•    To a certain extent, the type of landscaping you have also has an impact on the amount of heat required to keep your home warm. If you have large trees surrounding your home, these provide protection from the winter winds and your home will require less heat to maintain more comfortable indoor temperatures.

Efficiency of the Furnace

Over the last few decades, there has been quite a lot of advancement in furnace technology. A high-grade furnace that has 93% rating and generates 100,000 BTUs, will produce far more heat compared to a less –expensive furnace with a rating of 80%, but of the same size. When you opt for a furnace that has a higher efficiency, you can get the same amount of heat from a smaller unit.

Estimating With the Use of Software

Since excessively large furnaces waste energy, resources and money, most municipalities now require that the contractors use relevant software to estimate the best furnace size for your new home. There are a variety of software packages, but most work very similarly. The square footage of the space that has to be cooled will be input into the system, which will compute the answer. If homeowners were happy with the service their older furnace provided, the replacement unit is then sized to match it.

Drawing a Working Estimate

A rough/working estimate will also be drawn based on the square footage and your climate, the efficiency and the size of the furnace. The standard that HVAC and Heating equipment companies use:

•    They calculate a minimum of 25-30 BTUs of heat per sq. in a home located in a moderate- warm climate.

•    A home that’s around 2,000 square feet in size would require about 50,000 to 60,000 BTUs to ensure it’s heated properly.

•    If you have a less-efficient furnace that operates only at 80% efficiency, a furnace of about 60,000- to 72,000-Btu would be required.

•    If a high-efficiency furnace is being installed, your home would require a furnace of 52,500- to 63,000-Btu furnace.

Estimating the Size of an Electric Furnace

Many homeowners, who are considering replacing their old furnaces, prefer electric units. While there are a number of benefits to opting for an electric furnace, these too have to be sized correctly.  While sizing a unit is a very important point, this is never a standalone aspect; it always works in tandem with aspects such as insulation, power and efficiency.

Electric Furnace Size

It is crucial that you have a furnace that functions well with reference to the size of the house. If you opt for a very small furnace, it would have to strain a lot to heat your home and you will find that you are saddled with high utility bills. Contrastingly, if you buy an oversized furnace, it will continue cycling at shorter intervals, which will have an impact on its energy-efficiency.

Just as in the case of a gas furnace, the size of an electric furnace is measured in BTUs. In order to get a fair estimate, you would have to determine the square footage for the area that’s to be heated, or the total square footage of the home. If you don’t have access to this information, your County Assessor would be able to provide you the accurate total. The steps in this table will help you determine what the accurate BTU requirement is. These are the BTUs required per sq. for the different geographic regions in the US:

•    South, Southeast and Southwest – 20-35 BTUs per sq. ft.

•    East, West, and Midwest – 30-40 BTUs per sq. ft.

•    North, Northeast, Northwest & the Higher Mountain Regions- 40 to 50 BTUs per sq. ft.

In order to determine which the optimal number in each of these categories is, you would have to take into consideration factors such as the climate in comparison to the surrounding areas, and the level of insulation in the home. If your home has very good insulation or you live in a region with a warmer climate, you should preferably use the lower figures on the scale. On the other hand, if you live in a home with less insulation or in a region with cooler climate, its best to go with the higher figures in the specific scale.

Comparing the Efficiency of the Electric Furnace

The energy rating shows how much of the heating unit’s energy is released as warmth. It’s always better to choose one with a higher efficiency number. For instance, if the furnace has an 80% rating that much of energy is directed towards heating the home while the remaining 20 percent gets burned off. These types of ratings are very common for gas furnaces.

In comparison, electricity always gets converted into heat completely and this is why the units typically rank at 100%. It’s also why many people prefer electric units over gas units. When other ratings are being compared, ensure you are making the comparison against the same sources, AFUE and Steady State Efficiency being the two primary ones.

Other Important Checks

It is crucial that you verify whether the unit is compatible with the duct systems that already exist on your property. If there is any damage to the duct or if it has holes/leaks, the system output would not meet your expectations. Make it a point to inspect the ducts and ensure that they are in a working condition before you actually order the furnace.

As discussed at the outset, sizing is never a standalone aspect. It works in conjunction with factors such as insulation. If you upgrade your insulation, it may be possible to slightly reduce the amount of power required to heat your home and you may be able to get the desired results with a smaller size unit. This will also reduce your electricity bills.  It must be kept in view that adding more insulation may not always be an option in every home. However, certain steps can go a long way in reducing your energy bills. If you need in excess of 68,000 BTUs, you can either opt for 2 furnaces that have their individual ducting systems, or a gas heating installation. The thing to keep in mind is that a larger heating element consumes more power. Most electrical furnaces are available in increments of 7,000-10,000 BTU and when it comes to sizing, you only need to get close.

If you choose a furnace that’s more than ten percent below the heating requirement of your property, it’s better to opt for the next size. While it’s alright to either oversize or undersize by a small margin, don’t oversize by more than twenty percent as that can result in short cycling which reduces your comfort and results in energy wastage.

Tips For Furnace Duct Sizing

While it is very important to choose a furnace of the right size, you would also need to ensure that the heating ducts that transfer the heated air throughout your home are also sized correctly. That will provide your home with the appropriate heating requirements. If the ducts are too small, the heater would have to operate more frequently to ensure the correct temperature is maintained as the ducts aren’t able to carry the heat efficiently.

If the ducts are excessively large, that will lead to an increase in your utility bills. The one way to add to the effectiveness of your ductwork is to ensure that its size has been calculated correctly as well. These are the few things you would have to do to size the furnace ducts correctly:

1. Measure the Rooms

One of the most important figures to have when you are estimating the size of the ducting installation is the square footage of every room that’s to be heated. Ensure that the measurements are accurate.

2. Know How Much CFM is Required

The cubic feet per minute (CFM) is the other number you require and this depends in the size of the furnace to be installed. You would have to do detailed load calculations and before estimating the duct sizing and finding the right furnace size for your home. This is quite a specialized task and its best that professionals handle the job for you.

3. Combine the Heating Requirements Of Various Rooms

If more than one room has to be heated and not the entire home, you would only have to add all the requirements of additional rooms while calculating the size. Based on its size, every room will have a different CFM and you would have to accurately measure the complete area of every room.

If one room requires 200 CFM while another requires 100CFM, the total of the 2 rooms would be 300 CFM; the ducting’s diameter would be based on that CFM amount.

4. Installing the Right Duct Size Connections

Once the duct size calculations have been done with all the calculations in mind, you would need to increase the duct size proportionately. The right size would have to be run from the furnace, and the installation would have to be handled expertly. If the ducting is installed incorrectly, that will impact the furnace’s heating capacityas much as improper sizing would.

If you want your furnace to perform optimally, you would have to ensure that the joints are fitted optimally and that the insulation is up to the mark. All your efforts of sizing your furnace correctly will be futile if you don’t size the ducting and its connections correctly; and ensure that the insulation is up standard.

As you can see, it can be very difficult for a layperson to understand all the nitty-gritty of estimating a furnace size for your home. This is why it’s best to hire the services of expert professionals from a reputed company for the job.