Electrify Everything: What is a Manual J?

As laid out in the Climate Action Plan, the County strives to cut greenhouse gas (GHG) emissions 80% by 2027 and 100% by 2035 – one way that we are trying to get there is by helping residents transition from fossil fuels in their homes. Electrify MC is a pilot program that provides residents with information on how to make this switch more affordably.  

Photo Credit: ACCA

When you’re ready to switch away from oil, gas, or propane, it’s a good idea to have the new equipment sized to the needs of your particular home. Replacing heating and cooling equipment is an expensive proposition, so it’s also a good time to consider the insulation levels and airtightness of your home. An insulation upgrade may allow a smaller system, potentially saving money up-front and over time with lower bills. An energy audit can be scheduled through your utility’s Home Performance with ENERGY STAR audit (Pepco, BG&E, Potomac Edison). The Manual J is the next step to right sizing the equipment in the home.  

What is Manual J?  

Manual J is a book containing a procedure for calculating the heating and cooling requirements for a residence. In other words, an HVAC load calculation. It’s a standard from the Air Conditioning Contractors of America (ACCA).  

The term “Manual J” is often used in the building trades to refer to the report that’s generated from using Manual J to assist in sizing HVAC equipment. There is also a Manual S for “system” and Manual D for “duct,” among others. 

Why would I need a Manual J? 

Photo Credit: Jeremy Good

An accurate load calculation is important in new construction because it helps determine both the size of the HVAC equipment as well as the duct system. In existing construction, it’s particularly useful when upgrading a heating system. Natural gas furnaces (and other fossil-fueled heating appliances) typically come in a wide range of capacities. Heat pumps are reversible air conditioners and are available in a smaller range of capacities. That means some care and planning is often needed to ensure good results when switching from a furnace to a heat pump. 

One challenge is balancing the heating and cooling loads for a given house. Since Montgomery County is in a heating-dominated climate, the heat pump – including any auxiliary heat – must be capable of maintaining comfort on the coldest days. Choosing a larger heat pump than needed provides extra insurance against a cool house during a cold snap—but it’s bad news in the summer months. An oversized heat pump (operating as an air conditioner) won’t run enough to adequately dehumidify the air, leading to potential discomfort on medium-hot days. Oversized equipment cycles on and off more frequently, leading to extra wear and tear on the compressor and other components. 

How is a Manual J Performed? 

An accurate load calculation takes all aspects of the home’s construction into account, from the walls to the windows to attic insulation to the building orientation and surrounding or adjacent buildings. A blower door test – often performed as part of an energy audit – provides valuable information about air leakage, which can be a big contributor to heat loss. Blower door test results will typically make a load calculation much more accurate. A typical load calculation includes floor plan drawings of the home, with details for all six sides of the structure, including the roof and floor. A report can include which components of the building are contributing to the heating or cooling load, such as windows or the roof.

What’s the typical cost of a Manual J? 

A Manual J load calculation for a typical house with a single heating and cooling system often falls in the $200 to $500 range. That’s usually less than the cost of a heat pump that’s one size larger. So, properly sizing a heat pump, based on a Manual J, can work out to be less expensive. 

Ready to Electrify? 

Montgomery County DEP is encouraging a switch to heat pumps through the Electrify MC pilot program, which launched in Summer 2023. Residents can access an Electrification Help Desk for answers to nearly any question about electrifying your home. Incentives are available for those who want to electrify their homes through our competitively chosen contractor, Elysian Energy. They will complete a Manual J load calculation as part of the electrification process, of course! More information is available here.