Comments on: Cooling Load Variation With Changing Temperatures

By: Travis Jones

Travis Jones — Thu, 29 Jun 2023 11:14:09 +0000

Bruce!
I’m in Dallas as well would love to connect. I’m in roofing and solar—and I’m learning about home performance. I bought a blower door because I want to help my clients who want solar to understand their home’s performance and how we can improve it before or in conjunction with adding solar.

I need some knowledgeable HVAC partners!

Feel free to call/text if want to connect: 214-893-5959

Best,
Travis Jones

By: Robby Schwarz

Robby Schwarz — Wed, 28 Jun 2023 21:36:32 +0000

Allison,

Can you tell me if the 30 year design temps are created on a rolling average or where those numbers derived years ago looking back 30 years from a specific date. I guess I am wondering if we continue to use the 1 % design temperatures that code requires us to are those design temperatures, especially the outdoor design temperature, beginning to take into account climate change?

Also the indoor design temp is not realistic for what clients actually want the temp to be especially in the summer. Why not use 70 degrees for both winter and summer indoor design temp.

I find HVAC contractors blaming code and required design temps for poor design.

Thanks

By: Allison Bailes

Allison Bailes — Wed, 28 Jun 2023 19:45:47 +0000

In reply to Cindi. Cindi: Yes, Manual J does overstate the loads. See the comment by John Proctor. We do change the indoor and outdoor design temperatures occasionally, but you have to be careful with how far you go. Some programs and codes require certain numbers or ranges. And keeping the thermostat set really low in a hot humid climate can cause problems, like, for example, totaling your house. 10 Consequences of Keeping Your Home Really Cold in Summer

By: Allison Bailes

Allison Bailes — Wed, 28 Jun 2023 16:52:10 +0000

In reply to Bruce Breckenfeld. Bruce: Yes, measuring runtime is the best way to determine the actual load. Well, actually it's the best way to determine how closely your AC capacity matches the load. If you measure the capacity of the AC, then you know the load.

By: Bruce Breckenfeld

Bruce Breckenfeld — Wed, 28 Jun 2023 16:49:02 +0000

I’ve been using my ecobee thermostat logging feature to show the actual runtimes. I look at a “design day” of 102 degF (for me this was yesterday in Dallas) and see the runtime during 8 hours of the afternoon is 6.0 hours runtime or 75%. So my cooling load is 75% of my estimate of the A/C output and I include a 20% for the flex ducts in the attic. Could this be more accurate than any ManJ calc?

By: Cindi

Cindi — Wed, 28 Jun 2023 16:17:07 +0000

To be clear, changing your design point isn’t always cheating, but smart.
1. Many have reported that Manual J highly overestimates the loads, which results in equipment short cycling. Changing the design point is one solution for that.
2. Some people want their house to be like a frigid icebox, others (like me) hate the feeling of AC and will never run it until it’s absolutely miserable (85-90F). Some people run hot and some run cold. Some are willing to manage temperatures by using shades, move to other areas of the home which are cooler, etc. in an effort to avoid that cold feeling and to save energy.
You are allowed to have zero AC so if you do get it, why shouldn’t you be allowed to select the level that you want?

By: Allison Bailes

Allison Bailes — Wed, 28 Jun 2023 15:36:05 +0000

In reply to john proctor. John: There's definitely some significant buffer built into the calculations. Thanks for bringing that into the conversation here.

By: john proctor

john proctor — Wed, 28 Jun 2023 15:25:58 +0000

Allison — You struck an item in our industry that is quite misunderstood. As you ans I have discussed before, Manual J does not calculate heating and cooling loads. It ESTIMATES heating and cooling loads. When people take the results of those calculations as the true load, they are severely tempted to go to higher capacity equipment. That equipment is then oversized, under performs efficiency wise, and in moist climates underperforms in humidity removal.
We have proven in numerous tests in the field that the actual SENSIBLE cooling load is less than Manual J estimated load. Not just by a little bit, but usually the true sensible cooling load at design is 2/3rds of MJ. Let it also be noted that the MJ latent load is often lower than the actual latent load in damp climates. Thank you Allison

By: Allison Bailes

Allison Bailes — Wed, 28 Jun 2023 15:17:11 +0000

In reply to Neal. Neal: How the cooling changes throughout the cooling season would be interesting to look at. Also, the amount of cooling load variation would depend on the particulars of the house. As you mention, a house with a lot of direct solar gain from windows would be different from a house with less window area, better windows, and overhangs. Also, small houses where the internal loads form a bigger part of the cooling load would show less change in the cooling load with temperature.

By: Allison Bailes

Allison Bailes — Wed, 28 Jun 2023 15:12:08 +0000

In reply to Bill Hullsiek. Bill: Yeah, balance point diagrams definitely help when looking at heating loads versus capacity. I've never heard of anyone doing that for cooling, but the same principle would apply. As the outdoor air gets hotter and hotter, the cooling load increases and the air conditioner capacity decreases.