# How to Read Manual J Reports

### Image Credits:

1. Energy Vanguard

1.
May 23, 2018 12:04 PM ET

by Jeremy Good

Reports from a smart thermostat can also help sanity-check a Manual J report when replacing an existing system when there aren't other major changes to the house.

I recently got a Man J performed prior to choosing a replacement A/C unit. It was very helpful in terms of considering part-load dehumidification and, of course, confirming the system size. But the other big reason I got a load calc was for future boiler replacement. I know that my current one is oversized, but by how much? The report put heating load at about 60% of the net output of my current boiler. I used the Ecobee data from the coldest stretch this past winter to calculate the actual gas use. There's some error in this method since I wasn't measuring actual heat output, but it should be in the ballpark. (For single-stage appliances this is easy math.) There was good agreement between my calculated heat input and the design-day load for similar outdoor temps.

Point being, the duty cycle info from a smart thermostat can be valuable in baselining a load calculation report or for a rough gauge of system performance.

2.
May 23, 2018 12:24 PM ET

Response to Jeremy Good

Jeremy,
For more information on using historic fuel use data (or historic run-time data) to help size new equipment, see Dana Dorsett's article on the topic, Out With The Old, In With The New.

3.
May 23, 2018 12:50 PM ET

That's a good article,
by Jeremy Good

That's a good article, thanks. I've been in my house less than a year, so thermostat data gave me a much more precise estimate and in a target outdoor temp range. I have 12 months of the previous owner's utility info, though I have no idea what their habits were.

4.
May 23, 2018 1:56 PM ET

equation typo
by Cramer Silkworth

the equation should be AC capacity = CFA divided by sf/ton, not multiplied, no?

5.
May 23, 2018 2:06 PM ET

Response to Cramer Silkworth