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Residential Heat Loss and Heat GainBy
HVAC Computer Systems Ltd.
What is Heat Loss?
The typical home owner would like the inside of their house to be around 72º on the inside in the winter. This is called the Winter Inside Design Temperature. However, because it is cold outside, heat travels through the building envelope, the walls, windows and ceilings to the outside. This heat is lost by conduction. Also, cold winter air leaks into the house and warm air leaks out. This is called infiltration.
There is a continuous movement of heat from the inside to the outside, which is measured in units called BTUs (British Thermal Units). The speed of the movement of heat is called the Heat Loss and is measured in BTUH, which means BTUs per Hour.
If it is 72º inside the house and 52º outside then the 20º temperature differential will cause a certain number of BTUs to leave the house each hour, let’s say that that number is 9,768 BTUH. The heat loss of this house at 52º is 9,768 BTUH. This means that your heating system needs to produce 9,768 BTUs each hour to keep the house at 72º, when it is 52º outside.
If it is even colder outside, then the house will lose more heat each hour, the heat loss will be higher. When selecting a heating system, at what outside temperature do you need to know the heat loss? Well, this of course depends on where you live, how cold your winters are. The temperature to use as an outside temperature is called the Winter Outside Design Temperature. This is the temperature, say 10º for instance, at which only 2 ½% of the time is colder than 10º. The heat loss of the house when calculated with an outside temperature of the Winter Outside Design Temperature is called the Design Heat Loss. Because the heat loss at any temperature other than the design temperature is not really a relevant number, we usually just say Heat Loss, rather than Design Heat Loss.
So, to recap, the Heat Loss of the house is the number of BTUs lost each hour when the house is at the Inside Design temperature inside and the outside is at the Winter Outside Design Temperature.
Here is a summary of the factors affecting heat loss
1. Temperature difference
Reducing the inside temperature and moving to a warmer climate are two ways to reduce heat loss
2. Area of the building envelope
Smaller houses have lower heat losses than larger ones.
3. Thermal Resistance
Adding insulation to the walls and ceiling (increasing R-value) slows the movement of heat, thus reducing heat loss.
Better window frames, sealing cracks particularly around doors reduces infiltration as does better fireplaces
What is Heat Gain?
Heat loss is made up of the heat lost by conduction through the building envelope and infiltration. Heat Gain occurs in the summer time. Heat Gain is made up of
- heat gained by conduction (through walls, windows, ceilings etc)
- heat gained by infiltration (warm outside air coming in, cool inside air leaking out)
- moisture gained by infiltration (moist outside air coming in, dryer air leaving)
- radiation from the sun, either direct or indirect, through windows, glass doors and skylights.
- heat and moisture given off by people.
- heat given off by appliances
So you can see that heat gain is a little more complex. Notice that items 1 and 2 are directly related to the temperature of the outside air, just like their counterparts in winter heat loss calculations, but items 3, 4, 5 and 6 occur no matter what the outside temperature is.
To make things a little more complex, heat gain calculations take moisture into account as part of the Design Heat Gain. Fortunately, a computer program like HVAC-Calc handles this complexity for you.
Sensible Gain and Latent Gain
The heat gain associated with the temperature of the air is called the Sensible Heat Gain. The heat gain associated with the water in the air that leaks in due to infiltration and the water that evaporates from peoples skin as well as the moisture in their breath is called the Latent Heat Gain. If you add up the Sensible Gain and the Latent Gain you get the Total Heat Gain.
There is a Total Heat Gain at every outside design condition however the one of interest is the Total Design Heat Gain at the outside Summer Design Conditions.
The Summer Design Conditions consist of more than just the outside temperature. They consist of the Summer Design Temperature (only 2½ % of time warmer than this) and Summer Moisture Content (measured in grains of water per pound of air, typical Houston 113, New York 98), Daily Temperature Range (High, Medium or Low). The daily range is a measurement of how the temperature varies during the day. A high daily range means temperatures start cool in the morning, hot in midday and cool down at night. A high daily range will result in a lower heat gain than a low daily range where it starts out hot and stays hot all day.
Fortunately, with a computer program such as HVAC-Calc, the Summer Design Conditions and Winter Design Conditions for hundreds of cities are built in to the program. You select them once and then forget it.
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 is 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 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 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. Due to the difficulty of obtaining ice these days and the problems associated with drinking two and a half tons of ice water each day, most people will choose the 2 ½ ton air conditioner.
Sizing Air Conditioners and Furnaces
HVAC-Calc will provide you with the Design Heat Loss and Design Heat Gain of the house. You will need to use these numbers and some judgement to choose from the available sizes of furnaces and air conditioners.
But first, what about the short cuts?
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, in my 33 years of load calcs, I have heard it all the way from 250 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. Sizebythumb, the A/C contractor, to install an air conditioner. Mr. Sizebythumb, who’s been doin’ this for a long time, 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. Sizebythumb who, trusting his thumb, 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 to achieve any relief, thus losing his $100,000 view of the lake. 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 AC does not remove humidity.
Rules of thumb can’t work.
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 15% to 25% safety factor. I do get some flack from the energy conservation enthusiasts on this recommendation, you have to judge for yourself.
Please note: There is no safety factor built into the program. The results are as accurate as possible. Just in case you missed that, there is no safety factor built into the program.
So, 52,234 plus 20% = 62,680 BTUH, this means a furnace with a 60,000 to 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.
Life of furnace may be shortened due to overheating heat exchanger.
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. You do not add a safety factor to the Design Heat Gain when choosing the size of the air conditioner. You choose one that is as close as possible to the actual load.
If you are in a very dry climate
There is a further consideration in dry climates. When you run HVAC-Calc, 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.
Heat Loss and Heat Gain are calculated by measuring the dimensions and thermal characteristics of a building and, at the inside and outside design conditions, calculating the number of BTU’s lost and gained.
Furnaces are sized to meet, usually with a safety factor, the heat loss. Air conditioners are sized to meet the heat gained, with no buffer or cushion as too large of an air conditioner will do a poor job of removing humidity.
While the calculations can be done by hand, the advantages of using a computer program such as HVAC-Calc are speed, ease of use, factors built in and the capability of making changes to factors to see their effect.
I hope this explanation has been clear and helpful and I welcome you to try our heat loss heat gain program, HVAC-Calc, we have both contractor and home owner versions.
HVAC Computer Systems Ltd.
HVAC Computer Systems