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Community and Q&A

What effect does home mass have on the peak cooling load?

tedcummings | Posted in GBA Pro Help on

Dr. Bailes presents a graph for his “old and leaky” condo in his blog “How to tell if your air conditioner is oversized”. It demonstrates that his outside design temperature is reached at about noon. It also showed that the hourly run times for his a/c unit remain at lower levels until 5-8 hours later (notably peaking during hours 7 & 8). Presumably his “old & leaky” condo is not super insulated nor air tight. Also presumably, that delay between the onset of design conditions outdoors and the peak operation of the a/c unit is due to not only the conductivity of the insulation but also the mass effect of that home.

I am striving for a “net-zero” home in Louisville, KY (R- 5,15,35,60)( design temps of 93 and 10) and wish to minimize the HVAC equipment needed with air sealing and insulation. I will have significant internal mass as well with ICF and internal brick and stone. It seems to me that the mass effect should also have a significant effect on the peak internal load.

In the case of Dr. Bailes condo, the peak internal load occurs after the outdoor design temperatures have begun to fall, and quickly drops back to normal ruin times within a couple hours. His peak internal load occurs essentially during the 7th and 8th hours after the onset of outside design temperatures while outside temperatures are already falling.

In the case of a home that’s well insulated and air-tight with a large mass effect, would not the peak internal load occur significantly later in the day, perhaps 10 or 11 PM, or even midnight, when the outside temperatures have fallen much further? In which case, what effect would that have on the actual peak load for the a/c unit.
If my ACTUAL peak load were say 10,000 BTUs/hr in my home WITHOUT that mass effect, what would be the ACTUAL peak load in my same house WITH significant mass effect?

It seems that as the outside temperature begins to fall and equals internal air temperature, the heat making it’s way through the walls into the interior will be conducting outward as fast as it is conducting inward, long before that about 10 hour offset (between the onset of the design temperature and the onset of the peak internal load) occurs. Therefore the peak cooling load needed would be significantly diminished. And, the a/c unit would have longer run times over a significantly longer period of time with a more stabilized internal temperature.

Would perhaps a mini split with a peak operating load of say 5,000 BTUs/hr be sufficient to meet the needs of that home when the effects of mass effect are figured in?

Is there definitive research about this?
Can the effect of mass be figured into the peak expected load during peak load calculations?
Does Manual J take mass into consideration?

Ted Cummings

I now live in a 35 year old, leaky, 5400 sq.ft. home with three 15 year old a/c units at a total rating of 8 Tons. This summer I moved one return duct for the smallest, nominal 2 Ton, unit to a central location in an upstairs hall, and used that unit exclusively all summer, including design temperature days. The result was that I was as cool and more comfortable this summer than all those years when all three units were operating. That unit only operated at full capacity for about 8 hours. By comparison, I am convinced that I should be able to operate that super insulated, air-tight home I am going to build (3,000 sq.ft. ranch with a full basement) at 3,000 to 6,000 BTU’s peak load. Which would be one Ton per 6,000-12,000 sq.ft.

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  1. homedesign | | #1

    Hi Ted, I remember seeing AB3's blog about his Air Conditioner run time.
    I don't remember if he described his Condo in much detail....
    1st floor or 2nd floor ? Flat or Studio? or Orientation ..West facing windows? or Neighbors on 2 sides or more? and the habits of his neighbors?
    any way ... it seems to me it would be hard to draw any conclusions about "mass effect" from the graph you posted

  2. charlie_sullivan | | #2

    With enough thermal mass and insulation, the hourly variations in temperature outside don't matter much, and if you can tolerate a small temperature swing inside, the A/C capacity you need is just determined by the average outdoor temperature over the whole 24 hours. You can then choose to use a very low capacity A/C running all the time (on the hottest days), or to use a higher capacity A/C and run it some of the time. It would likely run more in the evening if it was simply on a thermostat, as in the example you show, but you could choose to run it overnight when it will work more efficiently with the lower outdoor temperatures in order to pre-cool the house, and then not need it to run at all during the day. You could do that with a setback thermostat, or by manually turning it on and off.

    The combination of the insulation and the thermal mass decouples the timing of when you run the A/C and when it's hot outside. The thermal mass also helps spread the impact of the solar gain over a longer time. In addition, when you get to a very well insulated house, the heat generated by appliances, electronics, lights, people, and pets inside can be as large as, or even larger than, the heat the comes in through the walls. The timing of that is more spread through the day. Also, the need for A/C can be more for dehumidification than cooling, especially with good insulation. The moisture comes from both of the humid outside air (which is where ERV can help) and of moisture generated by showers, cooking, etc.

  3. GBA Editor
    Martin Holladay | | #3

    Concerning Allison's house: there are several possible explanations of why the air conditioner run time doesn't correspond to the hottest outdoor temperatures. One would be the possible existence of unshaded west-facing windows.

    You are correct that buildings with lots of thermal mass have lower cooling loads, and have cooling loads that are more likely to peak during the evening hours, or even in the middle of the night. Operating an air conditioner in the middle of the night is more efficient than operating it when the outdoor temperature is at its daily peak, so this delay is advantageous.

    These factors are explained and discussed in the following article: All About Thermal Mass.

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