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:
- Direct observation of what the heating/cooling needs are, which works for existing homes under the right conditions.
- 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.
- 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.
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.
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.
- If you have a single-stage heating appliance, then the "off" vs. "on" time will tell you by what ratio the output of the heating appliance is larger than the actual need. For example, if you had a boiler with a rating of 200,000BTU/hr DoE output and it was firing 75% of the time, then your homes actual heat loss is 75% x 200,000BTU/hr = 150,000BTU/hr.
Almost every oil-fired residential boiler in the US features single-stage operation. A good thing to verify on oil boilers is the actual firing rate, since nozzle size and pump pressure affect boiler input/output dramatically. Gas-fired systems may feature single-stage, multi-stage, or even modulating burners, so review your appliance documentation.
- With multiple-stage or modulating heating appliances, the "firing" time becomes somewhat meaningless, because the boiler could be firing anywhere from the lowest to the highest of output. In fact, if you observe your multi-stage or modulating heating equipment turn "off" in the middle of the coldest night of the year, this is a pretty good clue that it's oversized or installed/programmed incorrectly.
Either way, the only way to tell on those systems what the actual firing rate is at a given time, is to instrument the boiler to tell you how much fuel is flowing into it. That would require observation of the gas meter (while leaving all other gas-consuming appliances off) or the installation of a separate fuel-meter.
Once a meter is in place, observe the flow of fuel through the meter over the course of an hour when the conditions are right. With gas systems, note the number of cubic feet of gas used, then multiply that number by 1,020 BTU/cf for natural gas or 930 BTU/cf for propane at sea level. At higher altitudes, call your fuel provider to find out how much to derate the caloric content of the gas. For example, I hear that the caloric gas content of natural gas in Denver is about 830BTU/cf.
For oil-based systems, each gallon of fuel contains 139,000BTU of energy. The result will be the total fuel consumed in BTUs. Now multiply that number by the AFUE rating of your heating appliance and you should have a pretty good idea of how many BTUs your home lost in the middle of the coldest night of the year.
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.
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.
- If you have a single-stage air conditioning system, then the "off" vs. "on" time will tell you by what ratio the output of the cooling appliance is larger than the actual need. For example, an AC with a cooling rating of 5,000BTU/hr running 75% of the time indicates an actual heat gain of 75% x 5,000BTU/hr = 3,750BTU/hr.
Most entry-level and window-based air conditioning systems features single-stage operation. Theoretically, you could go from room to room in your home over the course of a week and verify the heat gain before installing a central AC, for example. However, you'd have to ensure that the rooms have time to cool ahead of each test.
- With multiple-stage or modulating cooling appliances, the "on" time becomes somewhat meaningless, because the compressor could be running anywhere from the lowest to the highest of output. In fact, if you observe your multi-stage or modulating cooling equipment turn "off" at noon on a design-day, this is a pretty good clue that it's oversized or installed incorrectly.
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 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.
- For heating purposes, add up all the BTUs of the fuel you used and multiply by the AFUE rating of your heating appliance. A gallon of oil has 139,000 BTUs/gallon, a therm of natural gas has 100,000 BTUs/therm, and gallon of propane contains 91,000BTUs/gallon. Some gas utilities measure their consumption in ccf, which is 100 cubic feet, which is 102,000 BTUs of energy (these caloric gas value are averages at sea level, call your utility/gas provider if you live at higher elevations).
Next, divide the total number of BTUs by the number of HDD. The result is a measure of how many BTUs your home lost per Heating-Degree-Day. Now divide that result by 24 to obtain the number of BTUs your home loses per heating-degree-hour. Lastly, multiply the result by the difference between 65°F and the coldest day of the year. For example, if the coldest day you expect to experience is -10°F, then the difference you multiply your heating-degree-hours by is 75°F.
Voilà, you now have a reference point regarding your maximum heat loss. Ideally, spot check it by observing your heating appliance on a cold day and extrapolating the results. For the northern US, the heat loss per square foot of habitable space typically ranges from 15-35 BTUs per degree day. Depending on construction and location details, the number may be higher or lower, however.
- For cooling purposes, you need to keep track of the CDD during the period of observation. Next, multiply the kWh of energy consumption with the Energy Efficiency Ratio (EER) to obtain how many thousands of BTUs (kBTU/s) your cooling system moved out of the house. Now divide that result by the CDD and 24 to obtain how many thousands of BTUs your cooling system needs per cooling-degree-hour. Lastly, multiply that result by the difference between 70°F and the hottest day you expect to encounter. For example, if the hottest projected day is 96°F, then the difference is 26°F.
Now you have the maximum cooling needs for your home expressed in thousands of BTUs per hour. If you want to convert that number into tons of capacity, divide the number by 12. As with all projections, I would spot check the results over the course of an hour, just to be sure.
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.
- If you're interested in calculating just the heating requirements for your home there are a number of inexpensive or free resources. For instance, you can request the DesignPro boiler sizing software from Burnham, a boiler manufacturer. Similarly, Slant/Fin also offers their heat loss and boiler sizing software for free. There is also radiant design kit available from WattsRadiant that offers comprehensive radiant design options.
- For professionals installing a wide range of heating and cooling equipment, a promising suite of integrated program modules is offered by Wrightsoft. The modules allow their users to automate many aspects of running a HVAC business, from heat loss calculations, duct sizing, radiant layout, etc. to quoting. This sort of sophistication doesn't come cheap though. Excpect to pay $400 and up per module. Probably overkill for homeowners.
- Another comprehensive suite of building-related software modules can be purchased from Elite Soft. Their suite also allows you to calculate everything from electrical requirements to Lighting, HVAC, and Plumbing. Price-wise, this software is also out of reach of most homeowners and is clearly aimed at midesize HVAC outfits.
- HVAC-Calc is arguably the best heat-gain and heat loss calculation program a homeowner can license (about $50 for a 2 month license) and I have used it extensively. It only runs on Windows, but even with VirtualPC on a Macintosh, it's reasonably quick and easy to use. You can read my homeowner's review of HVAC-Calc. While I may disagree with the personal conduct of the programmer, I highly recommend the program.
- I have found using Randall C. Wilkinson 's free DuctSize application an acceptable means of deriving actual duct diameters, etc. for Manual-D compliance. However, Mr. Wilkinson presupposes your knowledge of Manual-D concepts and calculations. Thus, if you're serious about verifying your contractors duct calculations, buy Manual-D from the Air Conditioning Contractors of America web site and sit down for an evening of drawing a system diagram, calculating equivalent lengths, and Ductsize to figure out what the duct sizes should be.
Mr. Wilkinson has also written a comprehensive add-on for AutoCAD users called DuctWork. Since many architects use AutoCAD already to design structures, this AutoLISP based add-on is great because it allows ductwork to be drawn right into the structure and it automatically calculates proper duct sizing on the fly (you'll have to provide the CFM requirements though). Unfortunately, AutoCAD LT users need not apply as their version does not have the necessary LISP interpreter. At $150 DuctWork is not cheap but if the program can do what it advertises, it's a intriguing offering for engineering firms.