# Surface area heat loss calc question

hello,

I am taking the Calculating Envelope Energy Loss Course and when determining the Delta T I am a little confused. The temp inside a house can very greatly which will greatly influence the end result. Is there a standard temperature that the house should be set to? Should these calculations only be performed during specific times during the year? Am I overthinking this?

Thank you

Welcome to our forum, Joseph!..enjoy participating.

In my opinion, this subject can get too many inspectors in trouble. There is so much more to it than just figuring a temp drop or energy loss.

Delta T is simply the temperature differential between two points. I keep my house 65 degrees in the summer. My father-in-law keeps his between 75-77. The difference between the supply and return should be between 16-22 degrees (this can vary depending on who you ask) difference. If I take a measurement right at the furnace/air handler and I have a 22-degree temp drop, do you think my system is being efficient or is there a chance of an airflow problem?

I don’t know this course but I do Building Performance work. Without taking the course to answer your question:

First, if your doing a performance survey, you use the current IA/OA Delta at the time of inspection. If your doing an overall projected loss assessment you select in IA ambient and the yearly weather records. Ie. Heating/Cooling Days for your region.

I can’t tell which assessment your trying to do.

The temp inside a house can very greatly which will greatly influence the end result.

If your talking about room by room differences, you do calculations room by room.

Is there a standard temperature that the house should be set to?

There has to be a Temp difference so set it accordingly.

Should these calculations only be performed during specific times during the year?

I would avoid doing them when the IA/OA is the same temperature (spring and fall). You have to get a delta-t and heating or cooling under those conditions becomes an assessment outside of the normal operating conditions of the building. Will not be as accurate.

This stuff is nice to know, but is something you would progress into beyond Home Inspection. Lots and lots of additional in needed to fully understand and perform. Looks like your new to all of this. Put this on the bottom of your list of things to learn.

Hello Jeff,

Thank you for the reply, however your response seems to be related to HVAC performance. This question is about energy loss. The formula for calculating heat loss through a wall is (Area X Delta T X time )/ R value. Right now it is 66 degrees inside and about 63 degrees outside so the delta t is low and therefore the heat loss is low. If it were 30 out side then the delta t would be higher and it would seem like there would be more heat loss but the house would be exactly the same sealing and insulation wise.

Joseph,

The Delta T doesn’t vary.

Your specific region has a low and high temp that heating and cooling systems are designed around.
The HVAC system will simply run less if your outdoor ambient temp is 40 degrees F and your design low temp is 0 degrees F.

Here in Northern Michigan our low/cold design temperature is -10 F.
However, I always used -20 F (Delta T of 90 ) because the one time its -20 F with a 30 MPH wind and the homeowner can’t get the temp inside their home above 60 F, they become very unhappy.