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Calculating your Home's Heat Gain and Loss

Heat loss calculations are fairly straightforward, because they only involve sensible heat, i.e. the heat that you and I can feel. Complicating matters, the cooling load is made up of two components, the latent vs. the sensible heat. The higher the average humidity in your area of the country, the higher the latent load, which refers to the water vapor that must be condensed out of the air stream in your home to lower humidity.

The lower the humidity in your home, the cooler it will feel. This is the main reason to size your air conditioning system to match the load as best you can. Air conditioning systems are typically set up for a 70/30 split, i.e. for every ton of nominal capacity, 70% of the capacity is sensible heat removal, the rest is latent heat removal. However, the exact ratio differs by model and manufacturer.

I can think of three different ways to calculate the heat loss or gain that your home is experiencing:

  1. Direct observation of what the heating/cooling needs are, which works for existing homes under the right conditions.
  2. Indirect observation, using utility bills, climatic data, etc. to estimate the energy your home has used to be heated or cooled. This approach works best deep within a heating or cooling season.
  3. Using a heat loss/gain calculation package to calculate the theoretical loss of your home, based on the construction details of the structure. This is the only approach that works for new construction and it has the added benefit of determining the latent vs. the sensible heat gain.

Direct Observation

This approach works well if you can be patient and wait for the right conditions to materialize in your home. I have broken it down into two sections, by heating and cooling appliances to keep it simple.

Heating Appliances

For the purpose sizing heating systems, you need to wait for the coldest day of the year you expect the house to maintain its temperature in. Make the thermostat hold a comfortable temperature setting (i.e. do not let it setback during the night), then wake up an hour before sunrise (ideally, the wind would be blowing at 15MPH also...) If you have an indirect water heater (i.e. a water heater that is powered by your boiler), ensure that no-one uses hot water during the observation period.

Now clock the fuel consumption over the course of an hour or more. Unfortunately, the complexity of this approach depends on your heating equipment, so please bear with me.

What makes direct observation so powerful is that it captures your home "as is". Given that heating contractors hate getting "no heat" calls and similar complaints, many tend to oversize appliances, which ensures more than enough heat but which also ensures higher operational costs for their customers. However, as the contractor is not footing your annual heating bills, he/she doesn't mind.

Thus, if you have the opportunity to "right-size" your equipment, do. For example, many oil-fired boiler manufacturers allow a wide range of firing rates. A smaller nozzle, a lower pump pressure may be all it takes to downfire your boiler to the right input rate. The cost of doing so is minimal. Similarly, when the time comes to install new equipment, you'll have a better idea of what is needed and can back up your assertions with hard data. A good contractor will listen to you and consider your point of view.

Cooling appliances

The approach to directly observing your homes' maximum heat gain is similar to the approach of calculating the maximum heat loss. However, the environmental conditions differ! Instead of waiting for the coldest night of the year, you'll want to measure your home cooling needs on the hottest day of the year, around noon.

For cooling, right-sized equipment is even more essential for comfort because of the complex interplay between temperature and humidity. Oversized AC systems will not be able to run long enough to allow them to remove humidity from inside your home. As a result, you will feel cold and clammy at the same time.

Indirect Observation

Indirect observation is a less accurate approach than direct observation because it is spread out over the course of a day, a week, or a month and may be subject to other appliances skewing results. First, you have to decide over what period of time to observe your system. If your utility bills are measured on a monthly basis, this can be a good starting point. Note the first the and the last day on a statement, along with total energy (fuel) consumption.

For those among us that use oil or propane for heating and thus may not have a month-to-month billing cycle to compare to, you can either lenghten the period under observation (i.e. to conicide with fillings), or install separate meters for each appliance. Similarly, if your electrical or gas bill comes in one month with an "Estimated" consumption, lengthen the observation period to include actual numbers only.

Another thing to account for is the consumption of other appliances in your home. For example, you may have a gas-fired water heater and a gas-fired heating system running off of city gas. In such cases, I subtract the energy consumption from months where there is no heating load (i.e. the middle of summer) from the month under observation. If your cooling or heating system uses electricity to heat your home (i.e. heat pump, ground-source heat pump, electric-heat, air conditioning, etc.) a separate watt-hour meter for them will be needed for the indirect approach to work.

Your next challenge is to find good heating-degree-day (HDD) and/or cooling-degree-day data (CDD) for your local area for the days under consideration. This should be actual data (not averages). For this, I have found the Weather Underground site to be quite useful. Simply type your zip into the location bar at the top left, wait for the current conditions to pop up, then scroll down to the History and Almanac section to get a more detailed history for each day. Now add degree days for heating and/or cooling and you'll have the total number HDD and CDD for your chosen period of observation.

Armed with HDD, CDD, and fuel consumption data, you go about determining how many BTUs your home needs to stay comfortable for a given HDD or CDD.

Heat Loss and Heat Gain Calculators

There are a number of programs that allow you to calculate heat gain and heat loss in your house. By their very nature, such programs are probably for the more technically minded among us. Wether you have the time or inclination to do all that work up front or not, always go with a contractor who does a "Manual-J compliant" heat-gain/loss calculation. The reputable ones will do it as a matter of course, the other folks you wouldn't want in your home anyway. I did a Manual-J to compare it to the loss/gain calculations of my contractor. As with medicine, its good to have a second opinion before making a potentially life-altering decision.

All heat-gain/loss calculators require you to enter a lot of data. As buildings get bigger, it really pays to have a well-organized and quick program. Once you're done, all that work is rewarded when you can make informed decisions regarding upgrading your house via insulation, refenestration, energy appliances, etc. For example, by installing better insulation, you may be able to install a smaller AC system.

If you're going to install a new air-based heating or cooling system, be sure that the installer uses "Manual-D compliant" duct sizing to ensure that your ducts are large enough to carry all that air quietly throughout the house. As with heat gain/loss calculations, duct sizing requirements are a science, so there is no excuse not to get a duct system that will adequately heat or cool your home.

The below programs are but a small sample of the large universe of programs available for heating and cooling-related calculations. Some are limited to heat gain/loss calculations, others branch into related areas, such as duct sizing, radiant loop design, etc. Most vendors offer free demos to try before you buy. This is an offer I would take advantage of, as all the data entry required for heating and cooling system calculations is a lot easier if the program seems intuitive to you.