I've always been a strong proponent for properly sizing A/C's together with home performance upgrades but with utility companies moving towards more demand based rate plans we've come across some situations where I find myself having a change of heart.  I wrote about it on my blog here http://www.greenintegrateddesign.com/blog/the-ideal-home-for-a-load.... Interested in what other seasoned professionals think.

Tags: HVAC, based, controller, demand, load, oversized, plans, rate, systems

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Well you are in an interesting market as humidity levels there are almost non-existent thus you don't need the long run time to help pull that "heat" from the air. With that set the temp lower just before on-peak & pre-cool the house. The other factor is not how long the fan run but the start-up, as I recall that draws the most power. Your chart is way off, I have seen plenty of houses easily hitting 1000 sf & better per ton so it doesn't help anyone see if their houses unit is just at the cusp. Better to get your ducts in order is the biggest issue there - 10% leakage equals 25% of loss in the attics while most loose close to 50% of the cooling from what I recall.  

When it is the high end of a multi-speed/variable speed system just sitting there waiting for climate change to catch up


I feel this ia dangerous train of thought. And i would consider the examples in there of sizing of systems are more on the very oversized scale. What climate are these referring to?  But there is no talk of moisture removal in here. The number one issue with oversizing equipment is moisture removal. I would maybe give this article more thought if you had a dedicated dehumidifier. 

FYI - APS / SRP basically means Phoenix, AZ with no humidity except for after a monsoon blows through

Thanks for the clarification. I was in PHX last week at a Construction Instruction class (those guys know their stuff, denf worth it!). Without the humidity factor I can see a possibility for this working in this specific application. It would be devastating in my area (Iowa) where we are going for moisture removal in the summer. I would be interested in the added cost of running and repeatedly starting a larger blower motor compared to keeping a smaller one going, as it requires far less energy to keep the fan going compared to starting it. Agreed, look to the ducts first!

Own stock in ECM fan motor companies by any chance?

The reasons to size an air conditioner properly:


People in the industry used to think you'd save money on your air conditioning bills with a properly sized air conditioner, but that thinking has changed.

Of course, you want to do this while your AC is still in good shape, not when it's on its last legs.

If it runs for only 5 to 10 minutes before shutting off, it's definitely oversized. If your runtimes are over half an hour at design conditions, it's probably sized close to the actual cooling load.

Keep in mind that for all but the hottest hours, a tightly sized system will have excess capacity. Even at design conditions, it doesn't take much extra capacity to recover from a demand response interruption.

Many of my clients use setback (actually 'set up', but who says that!) in cooling mode. What I've found is an extra 10 to 15% capacity is more than enough to recover from several degrees of setback. Also keep in mind that a carefully crafted Manual J model already has at least a 10% buffer, assuming there's site verification of envelope specs.

More importantly, when considering the impact of demand response, almost by definition, recovery from interruptions will be in cooler hours of the day. Likewise, pre-cooling strategies.

Thanks for the feedback David

There are many problems occur when you have an oversized air conditioners. It lowers the temperature of the air and remove the moisture of the air and many more things. Some of the common problems associated with an oversized air conditioner are listed below:

  • Comfort
  • Air Quality
  • Noise
  • Money
  • Dust Mites & Mold
  • Structural damages
  • Wear and tear

These above are some of the common issues associated with the oversized ac unit. There are fixes that you can make to solve the problem of an oversized air conditioner but the main one is quite expensive: Replace the unit with a smaller one that provides more efficient cooling and moisture control.

This post and the link to the blog have a number of problems in addition to some valid points. I am only going to address a few of the worst IMHO -

'A properly sized air conditioner cannot "super cool" the house'. (Note the single quotes because I am paraphrasing.) A properly sized AC is sized based on a temperature close to the high temperature of a summer day. Most climates have cooler parts of that same day. We call them night time and early morning. If you want to "super cool" your house, even a somewhat undersized AC should have the capacity available. Since the whole idea behind this approach is over cooling at one time and then 'coasting' thru the peak demand times - why not do the same then? The demand on the power grid is lower and best of all, the efficiency of the AC is much better at low outdoor temperatures.

The 'coast thru' approach to maintaining indoor conditions is based on a lot of assumptions about thermal mass, insulation levels, shading, glazing, air sealing, etc etc etc. Are any of those considered? Maybe I missed those in the blog.

'Over sized AC is based on square feet per ton (see table in blog)' - Wow, it is so much easier to size the AC now that I have that table, why would I ever bother with a Manual J again? Or better yet, I can pull out my old 'stand at the curb and sight thru the holes template' method of AC sizing. 

Turn off the pool pump or water heater during high demand periods? Great idea. Most of those 'coast thru' just fine. Oversize your AC in order to be able to do that with your AC? That is not such a good idea.

@D.Byrne: After reading Kent's spot-on post today, I referred back to your blog linked in the original post (which admittedly, I didn't look at prior to my earlier comment).

I hate to throw you under the bus (really!) but there's a huge disconnect between your comment "I've always been a strong proponent of properly sizing A/C's", and that table Kent correctly called you out on!

Notwithstanding your thoughtful premise that an A/C may need to be oversized a bit to take full advantage of time-of-use pricing, that table represents exactly the sort of mindset got us into such a mess in residential HVAC to begin with!

I often (always?) look at square feet per ton for my projects, but only after having done a proper load. Your table purports to guide readers to judge the capacity of their A/C when considering load control strategies. Yes, you add the caveat that the only way to know the true load is to do a Man J calc (which is actually not true, more on that in a minute), but that doesn't excuse the propagation of such wrong-headed advice.

In reality, the 'true' cooling load normalized by conditioned floor area varies all over the map, depending on numerous factors, not the least of which are vintage of home and glazing exposure. Generalizations such as the values given in your table are worse than useless because they given credence to the idea that there's a at least a reasonable correlation between floor area and load for the entire housing inventory, old and new.

As an aside, Man J is not the only way to determine the true load... Actually, there's an easier and more accurate way to estimate the actual load of an existing home... it's called cycle-timing. In brief, you simply time the A/C cycles during late afternoon on a peak day, just after the daily high (~3 to 5 PM) to see what the run-time percentage is over 2 or 3 complete cycles. It's important that the indoor temp be stable at the desired setpoint for a couple of hours prior to the test. For example, if an existing 4-ton system runs 75% of the time, then the actual load is 0.75 x 4 = 3 tons.

There's a bit more to translating the results of cycle timing to sizing a replacement system, but this approach is more accurate than a Manual J model.



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