Just installed my first Nest learning thermostat.  Really easy and affordable.  Check it out and let me know what you think:



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On the cooling side:

1: It takes 10 minutes of run time for pressures to stabilize in most air conditioner systems.

2: Rated efficiency doesn't happen until pressures stabilize

3: Oversized systems rarely run longer than 10 minutes at a time, therefore the A/C system never reaches peak efficiency.

4: Oversized systems often  don't have the larger ductwork required to operate properly.

5: Oversized system don't remove humidity as well, therefore occupants turn down the thermostat to be comfortable.

On the heating side I don't think it's nearly as bad from an energy wasting prospective, but the "hot blasts followed by freezing" sure wrecks havoc with comfort...


Does the oversizing study assume replacing the ductwork when a system is oversized or is the 5 ton system trying to push through 3 tons of ductwork? Did the system in the study have a TXV or orifice as the expansion device. TXV systems are a smaller oversize penalty because they don't have to rebuild pressure each cycle.

It doesn't pay to replace an otherwise properly functioning HVAC equipment just for energy savings, that's why my oversized systems is still in my house. When it gets finally gets replaced, yes proper size equipment will be going in.

there are actually HVAC contractors than match the equipment with the ductwork. if the unit is a 4 or 5 ton with 3 ton ductwork, they just stick a 3 ton in there w/o doing any load calculations. Yes it's haphazard but the results are better than you would think.

I'm with you on this one PJ (for once!). All else being equal (or equally bad), setback has the potential to save more than downsizing source equipment, although if the AC is grossly oversized, say, 200% or more, who knows. It's true that shorter cycles mean the inefficient start-up phase represents a larger portion of overall run-time. But newer more efficient systems reach peak efficiency quicker than they used to. I routinely oversize heat pumps by 20% to 30% to reduce supplemental heat usage. In that case, the efficiency trade-off is a no-brainer, especially for air source, as long as the system can still handle the latent load. What's killing so many high performance homes is gross oversizing.

The impact of undersized ducts is a separate issue. There may be causlity, but that's besides the point. Also, just because a system is properly sized to begin with doesn't mean the duct system isn't undersized. Poorly designed and installed ducts are an epidemic, and this has even greater impact on efficiency and performance than equipment sizing.

David Butler

I finally wrote up why thermostat setbacks work. If you're interested, check it out here:

If You Think Thermostat Setbacks Don't Save Energy, You're Wrong!

I figured Chris might come in on my side:

"The elephant in the room we are not looking at:   The building shell and how good it is. How do we normalize the data based on that?   Whatever, nothing against set backs - if you dont want to tackle the shell. "  
Allison, not sure what house you have that will drop 8f in 2 hours, or how you are able to leave it 8f for the remaining 6 then magically have it back to comfort levels in a split second, but the size of the equipment, and thus the ductwork, must be enormous, and your house pretty crappy.  A decent structure might see that backslide 2 days a year.   
My clients homes don't drop that fast, and when they do let that occur (vacation) they take a LONG time to recover at anywhere close to design.   
So change your delta loss savings assumption significantly on BOTH sides of the 8 hours please.   

Actually, that number is given too much relevance.  Everyone seems to only be looking at the loss side, not the replacement side.   "How can we reduce loss by gaming thermostats?"

Methinks you are missing the forest for the trees.   Step further back.  Look at more of the picture.  The bigger "elephant in the room" is the incredibly broken assumption that heat replacement always comes at the same efficiency. That this equipment is like some plug and play toaster, that rated efficiency is what is magically achieved under all and any conditions.   x btu burned does NOT always = y btu delivered. 
I don't think you truly understand how important it is to maximize heat replacement effectiveness.   
This is accomplished by banging hard on the condensing side of combustion equipment as well as refrigeration equipment. Low, slow, cold, maximum surface area for btu transfer. (Sorry if that looses anyone.) Think load matching. Think maintaining momentum, not accelerating and decelerating.   
Long recovery used to increase efficiency (circa 1970, single stage non-communicating, leaky shell).  Now hard recovery costs efficiency.   
The saving of efficient delivery way overtrumps delta savings by colder spaces. And you don't deliver efficiently at full throttle. Look at the efficiency curve on an ECM motor, and apply that to how you look at this whole equation.   
Sorry if you don't like car analogies, but If you put the petal to the metal on your car it simply is not going to deliver power as efficiently as if you drive it gently.   
Sure setback may show a tiny reduction in heat loss, but if you open the carburetor getting back up to speed you've just given up more than you gained. And now you have to buy a v8 instead of a v4.   
You guys can argue theory all you want. Are any of you tracking results - besides Chris?   
Thought not.   
There are simply too many cases where this prescriptive recommendation fails, and I am seeing it. It fails often enough that we need to abandon the assumption and tell people they need proper diagnostics before a recommendation can be made.   

Maybe I need to simplify further. 

Do outdoor reset's save money?




If you optimize the reset, what happens to recovery?  


So, if you setback and want recover, what happens to the efficiency of your mod/con?


 Boiler guys will get this, the rest go read Dan Holohan's stuff: http://www.heatinghelp.com/



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