Energy Vanguard offers HVAC design services in the Greater Atlanta area serving the USA and all of North America.

As a homeowner you've heard "When replacing a system your contractor should perform a Manual J." This is good advice, but is not always necessary.

Manual J

A Manual J based HVAC load calculation determines how much heat the house loses in winter and gains in summer. Manual J is both a whole house and a room-by-room calculation; which allows the system designer to determine how much conditioned air each room needs for both heating and cooling. These caclulations are is essential for new homes, homes undergoing renovation or considering improvements like new windows, air-sealing or added insualtion, where there is no 'history' to review. What about other circumstances?

Evaluate the Present System

In an existing house, with an operating HVAC system you can use observation to determine if your system is correctly sized and the distribution system is working well.

Since your house is built and you've had an operational HVAC system, it is easy to see if the present system is reasonably close to the correct size, installed well and balanced appropriately.

When it was working correctly, think about the hottest afternoons and coldest nights.

  •     On those peak events did it run for less that 1/2 an hour and cycle off?
  •     Did it run for 60 minutes an hour (continuously) at peak occasions for the full hour AND lose more than 1 degree at the thermostat?
  •     Do any of the registers and grilles exhibit wind noise?
  •     Were any rooms noticeably hotter or colder than other rooms?


If you answered NO to all questions then your system is about the right size for your house and reasonably well balanced. Replace the present system with the SAME size.


If you answered YES to any of the questions - it would be worthwhile have a Manual J (to figure the right-size system) and Manual D (to determine if any ducts need to be replaced/resized).

Why not just add more capacity?

One of the issues confronting owners with duct problems who are wanting to replace the sytem and fix the ducts is limited access to the ducts, so large scale changes to the duct system may not be possible. Those are opportunites where onsite troubleshooting and problem-solving from a skilled technician will be important. Knowing how many cfms should be delivered to a room is a key data point which tranforms guess-work into directed balancing.

When operating correctly, if your system was not able to maintian the thermostat and a contractor suggests increasing system size without adding any ducts, (or showing why your present ducts are oversized) should be met with skepticism. It is much more common for ducts to be undersize - adding capacity will only make this worse.

Get What You Pay For

This questionaire provides a good overview of what questions to ask and what you should expect from a contractor who will be replacing your central AC, furnace or heat pump: ACCA Contractor Score Card [download link].

Energy Vanguard can help you with a complete HVAC design. Fill in the form above right to find out more.

If you would like to really dig into the details of how HVAC loads are calculated, how the right-sized system is choosen, how ducts are sized, how a ventilation system works, evaluate contractor quotes, compare efficiency levels and more!

Consider our on-line course -coming soon-. This in-depth course is suitable for the homeowner, builder, or contractor who wants a thorough understanding of how to get the right system for your existing house, new house or renovations project.

After completion of the course, if you decide that a complete HVAC design is the right decision for you, 50% of the course fees are applied to your down payment.

Energy Vanguard can help you with a complete HVAC design. Fill in the form above right to find out more.

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