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Any chance you can get a model # of the outdoor unit or if you already know the capacity in tonnage that would be helpful. I also revised the above post so you can understand why it's so important.
The model no. for the outdoor unit is Luxaire HBBC-F042SA. I don't know what the capacity is. We just moved into the house.
I did a bit of an experiment last night. I ran the A/C for several hours with all the attic vents open. In doing so the system seemed to be turning on a lot more often and running longer. I then closed all the attic vents and found that the system was turning on less often and was running for a shorter period of time. This supports my original theory that the hotter air from the attic constantly being pulled into the system is making it work harder.
Okie dokie then. Again shorter run times doesn't always mean things are better and you're saving money but to each his own. You should look up how a refrigeration cycle works it would be beneficial.
No, you choked the system out most liekly. How big is the system? How big are the returns and what is the spacing between the slats and are the return vents filter grilles? Meaning is that where you replace your filters if and when you do.
You don't know that, you failed to ask how big the house is, what percentage the attic represents, etc.
Yes if you close off 50 - 60% of the house, it can be a problem, but closing off a single room in a normal house is not an issue.
Try closing off 1/4 of your mouth and see how easy it is for you to breathe. This all negates that you change the amount of heat being added to the evaporator coil which also changes the point in the evaporator where the latent heat of vaporization happens. If you choke it off enough, like say they use pleated filters you could make your evaporator coil a useless piece of equipment that does nothing but removes latent heat. How big the house is has very little to do with what I'm referring to thanks for trying though.
Quote:
This represents a savings of 10% for the customer. Most of the cooling season the cooling loads are well below the capacity of properly sized air conditioners, and for oversized units the short cycling is a substantial problem. Because of the short cycles, Bill's high-efficiency air conditioner is less efficient.
Interesting article. So it sounds like longer cycles are actually better for the unit. With the attic vents open my a/c was cycling about every 10 minutes. (on 10 minutes, off 10 minutes). Does that sound reasonable? When the attic vents were closed, the cycle was more like 8 minutes on, 20 minutes off. I'm just having a hard time grasping how the 8/20 scenario is not saving electricity since the unit is running less.
Just an FYI A/C system are designed to run @ 400 CFM/ton. So a 3.5 ton system should be moving around 1400 CFM. You can lower it to 350 CFM/ton if everything in the system is verified. What that does is: You'll lose some sensible capacity and gain latent capacity. If all conditions are as they should be and AHRI standard conditions are present you'll have a capacity of:
As you can see with just a loss of 100 cfm you lost around 3,000 btu/hr of capacity or about a .25 ton. May not seem like a lot but cutting of one return and using pleated filters you can lose much more than that. Let's look at 350CFM/ton:
As you can see you're now down to a 3 ton system. Of course that doesn't change the compressor fan or anything else except the amount of cooling your system can provide for the same energy usage.
Interesting article. So it sounds like longer cycles are actually better for the unit. With the attic vents open my a/c was cycling about every 10 minutes. (on 10 minutes, off 10 minutes). Does that sound reasonable? When the attic vents were closed, the cycle was more like 8 minutes on, 20 minutes off. I'm just having a hard time grasping how the 8/20 scenario is not saving electricity since the unit is running less.
When your unit starts that the point it draws the most energy. As it equalizes, after around 15 minutes or so, that's where it's most efficient. For the first second or so as it starts it's in a condition know as locked rotor amperage. Now it's only like that for a brief moment but I would expect at that point it might be using somewhere around 14,000 watts or 14 kw/hr as far as the energy provider is concerned. It's not as dramatic as it sounds on the surface of what I'm saying because it quickly drops. The less heat it needs to remove as long as all conditions are equal the lower the power usage is. But don't confuse that with altering the refrigeration cycle. That latent heat of vaporization is critical which is why expansion valves are a very good idea. They guarantee the refrigerant always boils off in the evaporator before leaving it and it throttles the valve to keep it that way in almost all conditions barring low charge or bad fan motor or clogged filters or the wrong filters.
Though it's in metrics the point is the same. You see how long that line is at 100°C. What that is telling you is that know sensible (readable?) temperature change is happening but heat is still being adding. It's approximately 971 times more heat transferred than just adding 1°C from say 2°C to 3°C. The amount of heat ice take to heat up is half as much as when it's a liquid.
When your unit starts that the point it draws the most energy. As it equalizes, after around 15 minutes or so, that's where it's most efficient.
Definitely confusing. But bottom line, you're saying that even though the unit is drawing air in directly from the attic (which is likely at least 10-15 deg hotter than the rest of the house), this is not causing the unit to run less efficiently? That's the answer I'm trying to get at. Cold air is being pumped into the attic, but the sun beats down on the roof all day so it's always significantly hotter up there and I was thinking that would cause less efficiency. What should a normal on/off cycle be in minutes?
Unless the attic is insulated from the lower floors, which if it's finished I'm "guessing" it's not. Heat transfer faster with the greater the TD (temperature difference). My suggestive solution to you would be to foam the underside of the roof. It's not all that terribly expensive and will solve the larger TD the attic and second floors. Also making sure, if you have can lights, that they are well sealed. In the winter another great thing to do is check where the inside walls are located in the attic and make sure they are sealed. The higher temps. in the home have a stack effect, like a chimney, by allowing the conditioned warmer air to rise right through any cracks and crevices that lead into that attic. If it's covered with carper and drywall this obviously is not as easy. You can also achieve a slight bit more efficiency by, however you have possible, shading the outdoor unit. The unit you have is a 12 SEER system so it's not to terribly inefficient in the first place. I would look to see if you have a flow rater or piston type metering device. If you do something called a thermal expansion valve can add as a much as 1 extra SEER rating to your system. The envelopoe is the most important part to achieving energy savings, then next would be buying the smallest unit that achieves what you need it to which can be found through having a load calculation done. Remember that when a proper load calculation is done the goal is not to have a system that can cool the house on the very hottest of days but one that is designed for about 80% of the hottest of days you'll see in your area. It's also always a good idea to put, even if it's $50/month, money up for repairs and/or replacement time. It'll make things much less of a headache if it clunks out on you. Agaian this is all my opinion through my experience from schooling and being in the trenches. I hope this is helpful.
Last edited by BigJon3475; 08-06-2009 at 12:47 PM..
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