Software helps calculate residential HVAC loads, duct sizes

July 9, 2010
Software that simplifies calculations and look-ups required in Manual J is very helpful, eliminating a lot of the tedium of precise manual calculations. It also reduces a contractor's tendency to oversize equipment to avoid being caught short when ball-parking.

Developers and owners of residential homes are generally keen on keeping heating and cooling costs as streamlined as possible without sacrificing performance. A properly sized system costs less to run and provides the highest level of comfort. Beyond that, in states, counties, cities and townships following the International Residential Code, specifying HVAC equipment now requires that Manual J calculations be performed in order to get a permit.

Software that simplifies calculations and look-ups required in Manual J is very helpful, eliminating a lot of the tedium of precise manual calculations. It also reduces a contractor's tendency to oversize equipment to avoid being caught short when ball-parking.

Oversizing leads to short cycling. Engineering results show it is much better to run an air conditioning system 24 hours a day than to short cycle. It’s more expensive for the end user to run equipment that short cycles; cyclical degradation can wear out the equipment prematurely; and the short cycling does not properly dehumidify the air. To keep the space comfortably dehumidified, the user has to turn the thermostat down below comfort level, an overcompensation that creates more opportunity for mold around the air handler.

Here is a computer program that streamlines estimating heat loss and heat gain for whole house air conditioning systems to determine the proper size of needed equipment. Rhvac byElite Software,, is HVAC load calculation software for residential structures. It performs the Manual J calculations for heating and cooling loads in accordance with the 8th Edition, Version 2, and lets the user design a complete duct system, calculating duct sizes, system losses, and fan static pressure requirement based on ACCA Manual D. The user can enter the entire duct system complete with fittings or only the path(s) with the highest pressure. The duct analysis tool includes a Fitting Selector dialog for easy selection from hundreds of fittings from Manual D.

The program, approved by ACCA, determines building tonnage and room CFM requirements. The user puts in factors that explain how the walls, floor and ceiling are built along with information about the windows and doors and the location and placement of the house on the property, and the software can generate a building rotation report that shows tonnages and room CFM requirements at each rotation.

Rhvac includes a Building Rotation Duct Size Preview window that shows the heating and cooling CFM for each room or duct for each rotation of the building along with the maximum duct size for all rotations.

The solution supports a lot of flexibility in design. It allows up to 20 walls, 20 windows, eight roofs, six floors and six doors per room, and includes a wide selection of spray foam and SIP roofs and walls, and enables custom construction materials and descriptions. It also allows exterior shading (overhangs and offsets).

Rhvac holds lists of "favorite" materials for quick selection and remembers the most recently chosen materials for quick pick the next time. Rooms and zones can be assigned to 15 systems. Up to 1,000 rooms can be grouped into 10 zones per system using drag and drop zoning techniques.

The software, which uses standard Windows data entry techniques, calculates from manually entered data or directly from floor plans created with another Elite software solution, Drawing Board, which can draw both single and double line duct drawings (sold separately), and links to other Elite solutions, including Manual D Ductsize, Bill of Materials, Energy Audit and ECA Ground Loop Sizer.

The user can select equipment in accordance with ACCA Manual S from ARI and GAMA databases. Rhvac is supplied with HVAC model performance data for all the manufacturers listed in AHRI, with data (updated monthly) covering more than 600,000 models of standard air conditioners, heat pumps, ground source heat pumps, furnaces and boilers from more than 250 HVAC manufacturers.

Rhvac also includes a free Quick Sales Proposal option that can be used to create basic sales proposals simply by checking boxes to indicate what information to include.

Here's a specialty solution that could come in handy for contractors working on residential projects where insulated concrete walls are used as an alternative to wood frame walls. HVAC Sizing for Concrete Homes by Portland Cement Association,, provides an effective way to estimate heating and cooling system capacities specifically for single-family concrete homes. Developed as part of a HUD-sponsored project, the software accounts for thermal mass benefits not recognized in traditional residential HVAC design standards. Based on DOE 2.1E and updated to the latest ASHRAE Standards, the program calculates system capacities based on house dimensions, construction materials, air infiltration, locations in the U.S., Mexico and Canada, and thermostat set points. Because the solution calculates yearly energy use, it can be utilized to determine thermal mass benefits of various wall systems, enabling comparison against conventional systems in diverse climates.

Bill and Patti Feldman are freelance writers for magazines, trade associations, building product manufacturers and other companies on a broad range of topics. They can be reached at [email protected].

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