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?

Oversized air conditioners.

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


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