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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Its a cold coil that takes water vapor out of building.   When I started we used only R-12 and air temp supply of just 37-39'  now use R-410A with supply temps of 51-53'   but 1/3 of energy use of with R-410 and ECM motors.

Thanks for your discussion @Kent Browning and @David Bulter. I appreciate the concern and will consider removing the table from the post.  I also like cycle timing as another way to estimate the actual load of an existing home.

Addressing more @Kent’s comments are below.

Yes, the super cooling strategy only works well if the building envelope is in good shape, hence Reason #2 for what homes are ideal for load controllers, "Have Good Insulation and Well Sealed Ductwork." We've seen customers that use super cooling but have poor insulation and leaky ductwork, and no wall insulation and they still have high energy usage.

The table in the blog is dangerous if you don't read the context. I say "This doesn’t mean that a 5 ton unit gets put in place of a 4 ton system. I recommend that if your HVAC system is border line now AND you are considering a load controller for your home, I would recommend you stay with the same size system you have now." It is only for homeowners considering getting a load controller for their home.

Since our cooling bill in Phoenix's is typically double our baseload consumption and the utilities are squeezing homeowners to play the demand/ peak hours game, we need to tackle high energy costs of our cooling load for a bigger impact. Yes pool pump settings and water heater timers help, but the AC makes the biggest impact. How do we do that? With efficiency upgrades like duct sealing, air sealing and shade screens and insulation and don't impact a homeowner's lifestyle. Being good auditors we also need to address a homeowner's lifestyle as well and look at their rate plan, how they use the home and occupancy. If they are good candidates for super cooling and/ or load controllers based on their lifestyle we will advocate for them along with a tight, efficient house to keep the cool air in longer. Load controllers and super cooling are tricky though. Along with lifestyle considerations we need to look at how well they will work and how well homeowner's will actually follow them after we are gone. On homes with properly sized ACs we run into the issue of homes that can’t reach 80 degrees until after 9 pm once the thermostat has been raised for peak hours.  Phoenix nights tend to stay hot longer into the night. Our mornings are a good time to take advantage of super cooling but 8-9 pm are when our kids are going to bed and people will abandon their strategies to keep their families comfortable.

In Phoenix we are going through a perfect storm for load controllers and super cooling to enter into our home performance discussion but I’m we get into gray areas where lifestyle butts heads with efficiency.  Of course anyone can shut their AC off during the summer if they want to save money, but I think load controllers and super cooling can work in the background enough for some homeowners to be useful.

David, please explain your comment: "...utilities are squeezing homeowners to play the demand/ peak hours game"

As an aside... Earlier in my career (1980's) I had the opportunity to work on advanced load control projects, including a very early real-time pricing trial. One thing I learned is that optional time-sensitive pricing programs are largely ineffective because the customers who opt-in and remain in the program are likely to already have a favorable load profile (typically due to lifestyle, e.g., shift workers). Likewise, customers with unfavorable load profiles either don't sign up or they quickly drop out. So the program ends up having much less than the expected impact on the utility's aggregate demand curve. In effect, optional programs reward the good players much more than actually change behavior.

Mandatory TOU would obviously solve the 'self selection' problem, but such a fundamental transformation in electricity pricing is a tough political nut to crack in the residential market (especially 'demand' charges). But as I have written elsewhere on multiple occasions, if and when that were happen, it would fundamentally change the value proposition for various home performance work. Controls and storage (both thermal and electrical) will become relatively more valuable while envelope enhancements and equipment efficiency will become relatively less valuable (but obviously not irrelevant). In other words, the cost trade between storage and controls technology will improve relative to additional R-value beyond what code now requires.

I naively thought this transition would occur decades ago but the political climate for mandatory time-sensitive electricity pricing is worse now than ever before. Also, today's modeling tools aren't up to the challenge. Another problem is that many utilities are shifting more of their rate dollars from kWh charges to fixed charges, just the opposite of what's needed to spur both efficiency and load shifting.

@David Butler - AZ has essentially moved to mandatory TOU.  Conventional rate plans are only available from homes that average under 1000kWh/month

@David Byrnes - I wanted to give you a heads up that APS is working with OptiMiser to add better modeling support for load shifting strategies. This spring we will be rolling out support for the new rates and for DHW timers. This will include the ability to switch rates are part of the recommended measure set. Later in the year we hope to add support for precooling  and other behavioral strategies, as well.  Let us know if there are other ways we can help. I also be presenting an HPC session on these issues along with representatives from APS and NEST. 

@Gamaliel, thanks for the info as I wasn't following APS's most recent rate filing (I live in a different part of the state). I see that the peak energy charge for the non-demand rate plan is 23 cents in winter and 24 cents in summer! Wow.

Since these tariffs have only been in effect a few months, I'll be very interested to see how much push-back they get as customers who are not inclined (or able) to adjust their consumption patterns see their bills soar this summer. Those with heat pumps would already be feeling the pain, especially in the colder areas served by APS.

Setting a minimum threshold for mandatory TOU was smart, although I predict APS will end up having to increase that threshold when customer complaints begin to pile up with the AZ Corporation Commission.

Interestingly, having such a high peak energy rate in winter will clearly discourage heat pumps. Home with gas heat, especially those built to recent energy codes, have a good shot of staying on the conventional rate plan.

Sorry for conflating APS with all of AZ. I agree it will be very interesting to see how it plays out. I think a lot of utilities around the country will be paying attention. 

Luckily the peak period coincides with what will typically be the lowest heat demand period.  It should be easier to work around the peak period in the winter than in the summer.  It would definitely be unfortunate if these rates caused a major move towards natural gas heating. 

Opps, I didn't notice the on-peak period (3PM to 8PM) doesn't change in winter. Scratch my comments about heat pump pain.

In previous years dealing with advanced rate design, I've never seen anything like that before. I wouldn't have thought late afternoon would experience heavy demand in winter, certainly not heavier than early AM. Of course, not everyone heats with electricity but it's not an insignificant market segment. Also, winter lighting loads tend to be highest in the early AM, at least in the studies I've seen.


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