Manual J sizing for Single Family/Multi-Family HVAC Installations!
September 6, 2010
Over the past few years I have reviewed literally 100′s of Manual J calculations. For those not familiar with a Manual J calculation–in common sense terms–it is a program used to size the equipment used in the installation of the HVAC system for a particular home, townhouse, condominium or apartment. Every credible HVAC contractor should have one or access to one. It is not tricky. It is straight forward. It should be the 1st thing calculated when a dwelling unit’s thermal envelope specifications have been established. Isn’t it common sense to size the HVAC system based on the actual conditions that exist?
This following is not all inclusive, but it will give you an idea of what basic information is required to be input into a Manual J program.
Size of dwelling unit, volume of unit.
Orientation-North, South, East, West
Insulation: R value of walls, foundations, slabs, ceilings
Windows: Type, u-value, Solar Heat Gain Coefficient.
Infiltration: Air Changes per Hour, tightness of house.
Approximate Duct Leakage
Indoor Dry Bulb temperatures for winter and summer
There are more than a few other factors including Latitude, Elevation, and Temperature Range, but these parameters are not necessary for you to gain a basic understanding of the concept.
The Manual J is starting to be required in many locations for a building permit. Why is it important? Would you rather have your HVAC system based on a rule of thumb or actual field conditions? Me–If I was building or renovating a structure, especially one I am taking pains to make energy efficient, I want my HVAC based on reality.
Why am I going to build a Sustainable, Energy Efficient dwelling to Energy Star or another verifiable building program and then just have the HVAC contractor size it depending a rule of thumb based on a leaky, minimally insulated dwelling.
I am a Technical Representative for EarthCraft Virginia. When you get an EarthCraft certification, you also get an Energy Star certification. The critical issue in the process is energy efficiency. This means sizing the HVAC correctly and installing it correctly.
I know that when I look at a Manual J calculated for EarthCraft House in Virginia, where Air to Air Heat Pumps are prevalent, if I see a sizing that shows me less than 800 Square Feet per ton, that someone hasn’t inputted the correct data. An EarthCraft or an Energy Star certified house in my part of the world should be between 800-1200 Square Feet of Living Space per ton. That mean if I have a 1600 Square Foot House, the largest HVAC unit should be 2 tons. If I build it tight, then it should be 1.5 tons.
There are standardized inputs for the Manual J. An EarthCraft or Energy Star dwelling unit is not standard. What do I see the most of: Incorrect u values and solar heat gain coefficients(SHGC) for the windows. I see window inputs that do not even meet Energy Star requirements for the location, even though the contractor knows this has to meet Energy Star Thermal Envelope Standards and Energy Star Window specifications for the region.
I see someone who has taken the pain to upgrade their insulation, and yet standard insulation is specified in the Man J. I see infiltration numbers included that are totally excessive. I see duct leakage specified that does not meet Energy Star minimum standards.
I could go on and on. Bottom line: What is the result of all this? Usually an over-sized system, that does not operate efficiently in a supposed Energy Efficient dwelling. Oversizing of HVAC systems means the builder/homeowner/building owner spends more money for less efficiency. In my part of the world it also creates issues with mold, mildew, humidity problems and excessive heating and air conditioning bills. It also means that if my contractor specifies a 3 ton unit and it should be 2 tons if done right, I just spent easily a $1000 for an additional ton I didn’t need. How is that saving me money?
When you are building an energy efficient building, do yourself a favor, specify a manual J calculation and then look at the inputs. If you don’t know what you are looking at, find someone who can tell you. Putting in an HVAC system without proper sizing and input data is a waste of your time and money, whether you are a Home Owner, Builder, Developer or Building Owner. “Build it tight, and size it right.”
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