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04-13-2007, 03:44 PM #1
A/C Pulldown Times - What's Good, So-so, and Not So Good?
Assume an outside design temp day, and the system at +4º above the interior design temp. How long should it take a system to satisfy a four degree setback? Give me an idea of what you'd consider terrific for a perfectly sized system with an awesome duct design that's the epitome of the art of HVAC, then what you'd consider typical or average or reasonable, then what you'd consider to be bad.
Just curious. Help me set me expectations a bit. If it factors in, I'm thinking about Phoenix, AZ summer - high sensible, low latent heat load.
Last edited by CottyGee; 04-13-2007 at 05:15 PM. Reason: I'm a dummy. :)
04-13-2007, 04:04 PM #2
4 hours, 6 hours, and 8 hours.Contractor locator map
How many times must one fix something before it is fixed?
04-13-2007, 04:56 PM #3
This has been mentioned before...one thing to bear in mind with a setback strategy is this: you're not only trying to cool the air in a house. You also want all objects around you cool. You can drop the temperature in a hurry with beefy equipment (translation - oversized) but the objects in the rooms will not pull down at the same rate. Why? Heat content of air is .24 btu's per pound. Heat content of the walls, floors, furniture, etc. will likely be a bit above that...in essense it takes longer for these objects to cool down to the room air temperature.
Bottom line: you can sit in a room with the air temperature pulled down to where you want it, but the objects are warmer. While the air will make you feel somewhat comfortable from convection, an object nearby that is warmer will absorb less radiant heat from your body, so you'll feel warmer in spite of the air being as cool as you want. This condition is what is known as mean radiant temperature, or MRT. MRT is one reason to have a good air circulation pattern in a room. Minimal temperature gradiant from floor to six feet up will result in air and objects being at more even temperatures.
This is one reason why I'm not a huge fan of wide setbacks for a/c. Four degrees in Phoenix in summer does not strike me as a good comfort recipe. I understand your electricity billing structure; I think you should get some data regarding how much a setback scheme realistically saves vs. what you spend to pull the house down and then still have questionable comfort.
I would also look at renegotiating the rate structure with my utility, if that's possible."In this house we obey the laws of thermodynamics!"
- Homer Simpson
04-13-2007, 05:14 PM #4
LOL - I wish there was good data on the setback. The only thing I get is what my bill would have been on the standard plan, using electricity the way I did. But I would use electricity differently if I were on the standard plan.
You guys have pretty much convinced me that I need to "right size" my A/C. I'm thinking with now THREE ManJ's in ranging from 5.5T-6.3T of needed A/C, that probably that's about right. (Only actual heat gain # I have is 46,731, which I was told equated to 5.5T of A/C, which I didn't understand, since 5T=60,000 BTU.) Maybe I'll be happier with that and an improved distribution system.
So you've got me thinking right about size. No oversizing. I'm just wondering if my expectations really are out of whack, and (assuming they are) what IS a reasonable expectation.
So, what's a reasonable expectation for a 4 degree drop?
04-13-2007, 05:19 PM #5
In a refrigerator, you are taking heat out of objects. In an air conditioned room where you are going for a four degree pulldown, you have a bunch or warm objects that must give up their heat to the air as well, just a like a warm can of soda placed in the fridge.
Maybe if you could show the utility that it will force you to use more power to achieve the same comfort if you have to avoid a higher daytime rate?
04-13-2007, 06:35 PM #6
CottyGee, you are the customer and I think it's perfectly valid to ask what you *want* the pulldown time to be. Others said 4-8 hours, and I would have hoped for someone to say 2 hours -- better service for what you want.
Although Manual J sizing disrespects your wishes, it does so for reasons that barely exist in your dry climate. The whole catechism I read for Manual J sizing is primarily focused on humidity control -- not your problem. Reason #2 I read for Manual J sizing is to avoid shorter bursts of too-cold air which make people uncomfortable -- you will have to form your own opinion about that. Reason #3 is... allegedly lower energy energy consumption? Is that even valid? If there are more or different reasons for Manual J sizing, I cannot think of them now.
The argument about cooling down the objects in the house and not just the air is logical... but think the problem through the entire 24 hours. That same thermal inertia will *help* in daytime, slowing down the rise in air temperature. It is not at all certain the objects in the room will have fully warmed to the setpoint temperature in the course of the day.
>>Maybe if you could show the utility that it will force you
>>to use more power to achieve the same comfort
>>if you have to avoid a higher daytime rate?
I understand the reasoning the utility has for offering a time-of-day electric rate. The utility has though this through. It is a fallacy to think that "more power" is the yardstick of judgement, since power during peak hours costs much more to make than off-peak energy. If the TOD customer consumes more off-peak power that is perfectly acceptable, since the utility's main goal is to reduce demand at the very peak of the day. Ironically peak demand is a non-issue for the residential customer because he is not charged on KW demand, only on energy (kwh).
Another thing not immediately obvious is that the utility is seldom if ever at liberty to negotiate rates with a customer. A regulated monopoly is tightly bound as to what price to sell at, if it sold to someone a penny higher or lower then certain other customers would have a very legitimate reason to accuse unfairness, even illegality. You can sometimes choose from two or more rate schedules, and that's about it.
The utility has already debated each subject to a fare-thee-well before it ever became law. Representatives of every side argue their case for what is a fair rate, and a judge or PUC panel makes a decision which becomes the legal price.
Therefore CottyGee has chosen the TOD rate and probably saves money using it. Optimizing the TOD cost is different from minimizing energy consumption (and it may not optimize the utility's desire for peak reduction either). His desire for fast pulldown may be difficult for HVAC to serve, but is a fully legitimate customer want. I can see many advantages and little downside to having one or two tons capacity just to serve that pulldown goal. Consider, if it were in the form of a mini-split which only ran a few off-peak hours, then it will not impair humidity issues and energy efficiency need not be as high as the 24/7 system.
Hope this helps -- Pstu
04-13-2007, 07:01 PM #7
Now, depending on what energy is used to make the electricity in Phoenix, there may be no interest whatever in the differential between peak and total energy use.
It almost seems feasible to spent the cheap electricity all night long to freeze water, and use the ice to run chilled water through an air handler all day.
04-13-2007, 07:17 PM #8
>>It almost seems feasible to spent the cheap electricity all night long to
>>freeze water, and use the ice to run chilled water through an air handler all day.
You are exactly right. I have heard of a number of commercial customers doing that in principle. Residential customers would benefit too, except the capital investment to enable this would be prohibitively high in most cases.
The utility may be able to offset or delay building a new power plant, if it can shave the system peak. That is of obvious dollar value. Much smaller concern would be nighttime fuel use, when they can get by with their most efficient baseload plants, and a few pounds of coal or uranium or natural gas won't matter as much. I used to work for a utility in their rate department, and given enough time the principles got into my brain<g>.
Best wishes -- Pstu
04-13-2007, 09:54 PM #9
LOL - I'll tell you all about the power company's goals.
We have the Palo Verde Nuclear Power Plant 50 miles west of downtown Phoenix. Palo Verde power is nearly free. Very cheap! Very, very cheap power. We also have other energy sources, since our share of Palo Verde power isn't sufficient to power the city. (Palo Verde power goes all the way up the west coast, with Calif. having the biggest share - I think bigger than ours.)
During peak power, the utility has to fire up more expensive power sources - coal fired stuff. Their goal is to reduce peak demand through offering an incentive to those willing to shift power usage to off peak hours. Off peak is MOST OF THE TIME. On Peak is M-F from 12Noon to 8:00 pm.
We save something like $1,500/yr being on the time of use program. I don't know how many kWh we would use trying to maintain a steady temp. vs. shifting our power usage.
04-13-2007, 10:29 PM #10Professional Member*
- Join Date
- Jul 2000
- Northern Wisconsin
Have you considered two stage equipment? The larger capacity for bringing you out of setback quikcer and the lower capacity for more comfortable cooling after reaching set point.Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.
04-13-2007, 11:38 PM #11Regular Guest
- Join Date
- Mar 2007
Cotty--I'm in Phx too but power isn't that cheap!
SRP is cheapest which I have but time of use doesn't work well if you have people around all the time and kids. Wife also doesn't like doing the clothes, dishes, and keeping cool till after 8pm! (why spend tens of thousands on a new Trane and sweat all day cause your trying to save a dime but it cost pennies regardless--your new Trane won't pay for itself in power costs for many years)
You should get a medal if your saving $1,500 vs the std residential rates... Flyer I got from SRP stated it was maybe a couple hundred dollars max but maybe you're getting power for free as my total bill in 2006 was $2900 for 32,500kwh of power. (2500sq foot home with two 15 yr old Goodman heatpumps)
What was your annual usage?
04-13-2007, 11:38 PM #12
04-13-2007, 11:49 PM #13Regular Guest
- Join Date
- Mar 2007
Great point... I also live in Phx and can't figure people out with setbacks either.
Spend tens of thousands on a fancy Trane but total power bills is often a few thousand per year for many here in Phx.
Doesn't leaving it at good temp usually work best for people in an extreme climate such as Phx instead of playing temp games trying to save pennies?
Isn't hard on a system over the years to run for hours non-stop because the homeowner cranked the temp up/down?
With my 2 new XL14I ( 3 ton ) I haven't found where it can't cool/heat much faster than my 14 yr old Goodmans! Splits are about the same but VS blower does a much better job keeping all the rooms about the same. ( new larger flex for key runs too )