Selecting the correct design conditions for a load calculation is one of the key pillars of an accurate Manual J. Because design conditions are so important, we have tried our best to make it intuitive and automatic to set them correctly.
Manual J design conditions requirements
Use ASHRAE data
We handle this one for you. We use the latest ASHRAE weather data (version 2021) that is available and recommended by Manual J:
"The weather data used for outdoor design temperatures shall be extracted from either Manual J Table 1A or Table 1B, or the latest version of the ASHRAE climatic database."
Source: Manual J8 v2.5, p. 4
Guidance for picking the right weather station
Let Amply pick the closest weather station automatically
Using the iPad's GPS location, the closest weather station is automatically selected for any project you create.
Pick a different weather station to account for water or elevation
On the rare occasion that a weather station, that is farther away from the project location, is more appropriate due to a large body of water or large difference in elevation, we give you the option to pick up to 4 other locations.
The locations are sorted from top to bottom as closest to furthest away. The option at the top of the list (position 1) is selected automatically since it is the closest:
More guidance from Manual J:
"This shall be for the most representative location for the structure, which will be the nearest location of record when elevation differences, and/or proximity to a large body of water (ocean shore, lake shore, river basin) are not an issue.
This may be for a more distant location that has a similar elevation, and/or proximity to a large body of water."
Do NOT modify temperatures unless superseded by local code
The only time you should manually modify values is if you have been told to do so by a legal entity like a code officer per Manual J requirements:
"The design temperatures shall not deviate from Manual J or ASHRAE provided temperatures, unless superseded by local code."
Source: Manual J8 v2.5, p. 5
Indoor Design conditions are not the same thing as thermostat settings
Indoor design conditions should not be confused with homeowner desired thermostat settings. The standard 70F indoor heating temperature and 75F indoor cooling temperature are the required values to avoid mis-sizing equipment. Manual J has a 15-30% factor of safety built into the procedure, so any changes or attempts to add your own factor of safety can lead to massively oversized designs. The built-in factor of safety helps ensure that if actual conditions vary from the design protocol, the design will still hold up.
Outdoor design conditions are 99% and 1%, not 100% and 0%
Manual J specifies the use of the 99% DB outdoor heating and 1% DB cooling temperatures.
Heating 99% design temperature. This is the outdoor temperature that your locations stays above for 99% of all the hours in the year, based on a 30-year average. Turning it around, the outdoor air where you live is going to be colder than this temperature for only 1% of the hours in a year. That happens to be about 88 hours per year.
Cooling 1% design temperature. Your location will go above this temperature only 1% of the hours in a year, based on a 30-year average.
Modifying outdoor design conditions to account for more extreme weather scenarios can result in dramatically oversized systems.
Oversized systems result in discomfort for homeowners for 99% of year - we really want to avoid that. By following the protocol (i.e. by not changing the values) you have a good chance of making homeowners comfortable for 99% of the hours of the year.
How to change design conditions if mandated by a local code requirements
This is worth repeating: Do NOT modify temperatures unless superseded by local code
By changing the values you are acknowledging you have read this rule:
"The practitioner may specify design conditions when local code values are different than the MJ8 defaults. Outdoor design conditions that differ from Manual J 8 v2.5 (MJ8) conditions, ASHRAE conditions, or local code values, are not in compliance with MJ8 procedures. Indoor design conditions that differ from MJ8 default conditions or local code values are not in compliance with MJ8 procedures.
Solutions based on non-compliant data shall never be presented as a Manual J solution."
Tap on Weather Station to reveal the list of available stations and the local code option
Select the Local Code Values option in the list
Read the warning and tap on the Local Code Accepted to acknowledge that you are changing values because a local code requirement dictates that you do so
Modify the design conditions per local code requirements using the "-" and "+"
