Computer Load calculation development aka Unit sizing calculations.
This will be in two parts:
1. The introduction that tells about how the computer program was developed and terminology that it uses, it's support and other related information.
It was 1981 when Don Sleeth wrote his first heat loss and heat gain program using "Manual J" criteria, that was over 28 years ago. There was no such thing as a PC at that time; IBM was still making only main frame computers. It was written on a wonderful little computer, the Sharp PC 1500, which had all of 10K of memory and only a one-line screen! The computer now used has 12,800 times as much memory. Computers have changed but people have not.
People still need computer programs that are simple to use. They need programs that don't require them to use a calculator or look up information in tables. That should be the computer's job.
That fact that Don likes simplicity and the fact that he was an HVAC contractor for many, many years before writing computer programs shows in the design of the program.
What is HVAC-Calc For?
HVAC-Calc Residential 4.0 calculates the heat loss and heat gain of a house in accordance with Manual J criteria. The heat loss is used to size a furnace to heat the home and the heat gain is used to choose the proper size of air conditioner. HVAC-Calc also has tools that you can use to determine the amount of air needed in each room and the size of ductwork necessary or the amount of hot-water or electric baseboards.
Who is the Program For?
The program has been designed for contractors, engineers, architects and homeowners to use. It requires no special HVAC training or skills. There are no training classes or videos necessary to use this program, just follow the manual and you will be totally capable in one evening. We don't sell training; we sell programs that don't require training. If you are stuck on something, don't waste time puzzling over it or getting frustrated. Just pick up the phone and give us a call at 1-888-736-1101 and we'll show you, right over the phone.
What About Support?
Nobody supports their products like they do! They have toll free telephone support from 8 in the morning Eastern Time until 8 at night, 7 days a week. The people answering the phone lines have both heating and air conditioning and computer experience.
If you have a problem understanding something Don would be glad to help you himself. Please try the toll free number first but if you want to talk directly to “The Man” you are very welcome to call. Now how's that for standing behind your products!
The HVAC-Calc programs (both Residential 4.0 and Commercial 4.0) are light, small footprint programs. They will work on any computer that has Windows 95, 98, ( I use Vista), or NT 4.0 and on Macs with Virtual PC or SoftWindows. They were designed to work at the lowest screen resolution of 640 X 480 but look much better at 800 X 600 or higher. If you want to change your screen resolution and don't know how, give us a call.
Reviewing the Heat Loss - Heat Gain Basics
As you will learn below, we are going to calculate the heat loss and heat gain of a house. In order to do that, we have to choose what temperatures we are interested in. For heat loss, for instance, the colder it is outside then the more heat your house is going to lose. What outside winter temperature should you use? Should you choose a furnace big enough to keep you warm when it is 10 F outside? Well, not if you live in Miami, or in Bismarck, ND.
Researchers suggest that you calculate your “Design Heat Loss” at a Design Temperature. The Design Temperature is chosen to be the temperature that your location will be warmer than 97 ½ percent of the heating season. The recommended Design Temperatures for hundreds of cities in the USA and Canada are built in to HVAC-Calc. If you were to choose a furnace that exactly matched your heat loss at the design temperature, it would be able to heat your house just fine 97½ % of the time in an average winter.
Example of a Design temperature
Notice summer for outside 88° with inside 75° / winter outside -5° and 70° inside
The BTU's will be calculated for these outside / inside design temperatures for this specfic area.
Each local will have their own design temperatures and the info will come from a dealers Manual "J" or a computer program in accordance
with Manual "J".
But an average winter means that there are winters colder than average and warmer than average. That is why most contractors would choose a furnace size that is larger than that needed at design conditions by a safety factor. I use a safety factor of 20% to 25%. More is said on this in the sections below.
Weather Bureau Report
Just like there is a Design Temperature for winter, there is also one for summer as well as a Design Humidity Content known as Grains of Moisture. All of these design conditions are built in to HVAC-Calc and available for you to use with the registered version of the program.
What is the Heat Loss of a House?
Heat moves (transfers) from a warmer temperature to a colder temperature. If there is a big temperature difference it moves faster, if there is a small temperature difference, it moves slower. If there is only a little resistance (R-Value) it moves faster, if there is a larger R-value (resistance), it moves slower. If there is a big area separating inside and outside it moves faster, if there is a small area, it moves slower.
Assuming you want to keep the house at a constant temperature, say 72, as the heat moves out you must replace it, using the furnace.
Heat is measured in BTU's (British Thermal Units). As you can see from the paragraphs above, it is not the fact that heat moves out of your house that is important, it is how fast it moves out that matters. It is the rate, or speed, of heat loss that matters. If the house loses 50,000 BTU's each hour then its' heat loss is 50,000 BTU per hour. This is confusingly written 50,000 BTUH even though it should be BTU/H.
When people talk about the Heat Loss of a house, they are usually talking about the Design Heat Loss of the house, the heat loss at the Design Temperature.
There are two sources of heat loss in a house. One source is heat transfer through the envelope of the building, the outside walls, the windows, the ceiling and the floor if it is not above a heated space. The other source of heat loss is infiltration. Infiltration is the amount of cold air that either sneaks into the house through cracks and door openings, or is purposely brought into the house for ventilation.
Here is a summary of the factors affecting heat loss
Reducing the inside temperature and moving to a warmer climate are two ways to reduce heat loss
Area of the building envelope
Smaller houses have lower heat losses than larger ones.
Adding insulation to the walls and ceiling (increasing R-value) reduces heat loss
Energy Conservation measures
Better window frames, sealing cracks particularly around doors reduces infiltration as does better fireplaces
What is Heat Gain
Heat Gain of course is very much like heat loss, except here we are talking about the amount of heat that the house gains in the summer time. In the summer, the temperature difference goes the other way, it is warmer outside and ideally, cooler inside. Heat is transferred from the hot outside and it also is brought in with outside air, infiltration.
The four factors discussed above all affect heat gain also, in exactly the same way they affect heat loss. However there is one additional and very important factor, solar gain through glass. In addition to heat transferring in through the house envelope and sneaking in through infiltration it is also radiated in by sunlight, both direct and indirect, through windows, glass doors and skylights.
There is also an additional unit of measurement that is used to describe the cooling capacity of air conditioners and that is the “Ton”. One Ton = 12,000 BTU per hour (BTUH). It comes from the number of BTU's absorbed by a ton of ice melting in 24 hours. If you have a heat gain of 30,000 BTUH (at the summer design temperature) then you would need to remove 30,000 BTUH in order to keep the house at the indoor design temperature of say 75.
You could remove the 30,000 BTUs each hour by either setting up some fans to blow the inside air over a mountain of ice, being sure to completely melt 2 ½ tons each day, or you can install a 2 ½ ton air conditioner.
Sizing With a Rule of Thumb
Some contractors still feel they can accurately size an air conditioner or furnace with a “Rule of Thumb” such as 800 sq.ft. per ton. Don't put too much value on the number 800, I have heard it all the way from 400 to 1200!. Thumbs can do a lot of things, but they can't size HVAC equipment. Here is an example that blows the “rule of thumb” method right out of the water.
Picture a scenic lake and a lovely custom home on the south shore. The house is 3,200 sq ft with a view of the lake in almost every room. Of course those windows face North. Mr. North who owns the house calls Mr. Size by thumb the A/C contractor to install an air conditioner. Mr. Size by thumb divides the square footage of 3200 by 800 and installs a 4 Ton air conditioner.
Mr. South admires the house from across the lake where he has a building lot and has one built on his lot, with a lovely view of the lake, to the south of course. He too calls Mr. Size by thumb who does the math and installs a 4 ton unit.
Mr. South absolutely cooks in August with the sun streaming in the glass, and must close the blinds too, thus losing his $100,000 view to achieve any relief. Mr. North finds he has to turn his thermostat colder and colder to try to achieve a feeling of comfort. He can never find the right setting for his AC because an oversized Air Conditioner does not maximize it's moisture removal.
Rules of thumb only work sometimes.
Have a load calculation done then you will know for sure.
The first and most important step in sizing a furnace is to do the heat loss calculation. This is what HVAC-Calc does. When you are finished you will have a number that is the Design Heat Loss of the house. Let's say that number is 52,234 BTUH. As discussed above, this means that the house loses 52,234 BTU's each hour when the outdoor temperature is the Winter Outdoor Design temperature for your area.
Obviously, you want a furnace with an output of at least 52,234 BTUH. Most contractors, including myself, would add a safety factor to the requirement. After all, weather data is averaged to come up with the Winter Design Temperature and you want to be warm even on colder than average winters. My own personal recommendation is a 20% to 25% safety factor. I do get some flack from the energy conservation enthusiasts on this recommendation, you have to judge for yourself.
So, 52,234 plus 20% = 62,680 BTUH, this means a furnace with a 65,000 or 70,000 output would do nicely.
Why not add a big safety factor, like 100% to be really safe? There are a number of reasons why not.
Operating costs go up. A furnace that runs only for short bursts uses more fuel, much like city driving compared to highway driving.
Initial costs of the furnace and the larger ductwork go up.
Comfort level may go down due to short bursts of hot air and long off cycles
Furnace life may be shortened due to condensation on the heat exchanger.
Sizing Air Conditioners
Sizing air conditioners is a little more difficult than sizing furnaces. First let's look at the job of an air conditioner. An air conditioner actually has two jobs, job one is to lower the inside temperature (i.e. remove heat) and job two is to lower the inside humidity (i.e. remove moisture).
The tricky part comes in because thermostats, which are devices that know nothing about humidity, control air conditioners. When an air conditioner is running, the warm moist inside air is blown over a cold air conditioning coil called an evaporator. This cools the air and in so doing, humidity in the air condenses to water, and is routed to a drain. This works fine when the air conditioner is running, but, if the air conditioner can cool the air too quickly, it does not run long enough to remove the humidity.
If a home is cooled by an air conditioner that is too large, the occupants tend to keep turning it cooler and cooler because they are not comfortable. The house is cool and damp. So the goal is to match the cooling load of the house (heat gain) quite closely to total cooling capacity of the air conditioner.
There is a further consideration in dry climates. When you run HVAC-Calc, (yes, we will be getting to that part) you will notice that the Total Heat Gain is broken down into two components; sensible heat gain and latent heat gain. Likewise air conditioner capacities are broken down into the same two components, sensible and latent. When you add the two together, you get the total cooling capacity that is used to rate an air conditioner.
The sensible cooling capacity is the capacity of the air conditioner to remove heat from the air, i.e. to lower the temperature. The latent cooling capacity is the capacity of the unit to remove humidity from the air. In a dry climate, the latent cooling capacity is of no interest or value, there is no moisture to remove. In a dry climate, the heat gain of a house is almost entirely sensible heat so you must make sure to choose an air conditioner with a sensible capacity that matches the sensible gain of the house.
For instance, suppose you live in Nevada and your total cooling load is 30,000 BTUH. You may think, great, I need a 2 ½ ton unit. However if you look more closely at the HVAC-Calc printed report, you will notice that the Total Heat Gain = 30,000 and the Sensible Heat Gain = 28,000 and the Latent Heat Gain = 2,000. Now, look at the published capacities of the 2 ½ ton unit. They may be Total Capacity = 30,000, Sensible Capacity = 20,000 and Latent Capacity = 10,000. The unit is too small; it will not remove enough sensible heat. You will need to find a larger unit with a higher sensible cooling capacity.
Sizing Heat Pumps
Heat pumps, in most climates provide heat from the heat pump itself as well as from an auxiliary source such as electric heaters. It is standard practice to size the auxiliary heat as if the heat pump did not exist. One justification for this is that if the heat pump needs repair and it is the worst weather conditions in the winter, the repairs may not be able to be carried out until the weather changes.
The heat pump itself should be sized the same as any air conditioner, however it is standard practice to “round up” with heat pumps. For instance, if the cooling load is over 2 ½ tons but under 3, you would choose a 3 ton unit with a heat pump to reap the benefit of it's larger heating capacity.
Load Calculations & information required to do them ?
Information that will be needed.
Why information is needed ?
Type & size affect the capacity or size needed to maintain your comfort at design temperatures. The correct selection made is very important in sizing and your comfort.
In some states a new residence needs to comply to the energy code.
Examples of a computer program.
Sample reports ( several of many ) that can be given or generated:
(want to save a report, right click on Report or Simple Report then select "save target as" then save to a directory of you choice).
If you don't have the program then click on "free" Adobe Reader above to download.
This program is not just for the "Reports" but any other PDF files as well.
If you would like for me to do a load calculation I will need plans (preferably), a blueprint or some sort of a drawing that will show the footage, elevations, associated R values and type of all components. The previous shown form can be used as a guide. A set of plans will generally have this information.
Graph Paper can be used for a drawing.
You can use the previous examples to help select the type of wall, window, etc.
For the exterior wall each square could represent a foot. It does not need to be totally accurate.
Example: A wall is 11.5 feet, you could use 11 or 12 squares. Windows you can show their dimensions say 4.4 x 5 = 22sq ft.
If you have any questions please email me.
The information in Red is information that should reflect what your home would have.
However pencil is ok.
Windows can show the info above next to their location, ( it's not necessary to draw every window ). The above drawing ,( done with the Smart Draw Program ), is a lot neater than in pencil but is not necessary.
For many years I used graph paper and pencil to draw the customers floor plan, windows, etc on every home I surveyed regardless of what size of unit I was replacing.
The customer & myself were certain of the capacity needed vs the unit to be installed. Of course there were other reasons to do what was done, the amount of air needed for each room, the correct size of pipe or duct that is existing, (is it the correct size & good condition), vs what might be suggested, and utility & agency rebates would require a load calculation.
Dealers / Contractors by doing a room by room load calculation gives the amount of BTU's for air conditioning & heating plus the amount of air to be delivered to each room. That type of information can also help in your decision of zone, multiple systems, or varying speed compressor, and duct size.
The Sample load print out or view does not reflect the above floor plan.
Sample of a print out that can be Emailed to you will be in PDF format.
Again the "free" Adobe Reader to view PDF files clik this but uncheck the McAfee download
For air conditioning keep in mind that air changes should be 4-6 per hour. When less than 4 one can risk stale and stagnet air that might affect the homeowners comfort. [ The homes conditioned cubic volume of air should pass over the indoor coil & it's filter 4 - 6 times per hour ].
Homes that were built with excessive conservation but the load calculation or unit size was ok but the air changes were so low that it caused dis comfort for the home owner. It's not a bad idea to check for the air changes per hour the unit produces during the cooling mode.
Have questions please call or email me.
Every Home should have a calculation. A Heat Gain & Heat Loss. The dealer should be able to do one for you. However if there is a problem getting one done then you can rely on me doing it. Why a load calculation ? Because it tells you what size of unit that will maintain your comfort, otherwise it's a guess. It can also affect your monthly fuel usage or operation bill, and your initial cost of installation.
So do it right.