Reviews:
 ·AT&T U-Verse
| reply to chmod
Re: Another a/c issue. A properly designed and working system will keep any reasonable inside temperature with any extreme but expected outside temperature. And cycle while at it. That means 65F inside with 105F outside if needed.
Humidity can be removed very well even if the unit is "oversized" by running the variable speed air handler in dehumidification mode. I said "properly designed" not "contractor special".
A unit that can barely handle the load isn't more efficient, as a matter of fact it may be unable to take advantage of any setbacks because it will have to run all day long for the house not to be an oven in the afternoon. |
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 Jack_in_VAPremium
join:2007-11-26
Mathews, VA
kudos:1
1 edit | said by cowboyro:A properly designed and working system will keep any reasonable inside temperature with any extreme but expected outside temperature. And cycle while at it. That means 65F inside with 105F outside if needed.
Humidity can be removed very well even if the unit is "oversized" by running the variable speed air handler in dehumidification mode. I said "properly designed" not "contractor special".
A unit that can barely handle the load isn't more efficient, as a matter of fact it may be unable to take advantage of any setbacks because it will have to run all day long for the house not to be an oven in the afternoon.
Can you provide your source of this information about "a properly designed and working system"? I've never seen this described in any technical literature or any real time calculations on new installations. I would be interested in studying the technical data behind the reasoning for this over sizing enough to provide 65 degrees with an outside temperature of 105 degrees.
How many people have a unit with a variable speed air handler? I just got one with my new upgrade system. That mode is only effective "if" the temperature is close to set point and maintaining. De humidification mode is a side effect of the unit running in a slower stage to maintain the set-point. To achieve 65 degrees with outside air of 105 I would need at least a 4 ton unit instead of the 3 ton I have. |
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| reply to cowboyro
said by cowboyro:A properly designed and working system will keep any reasonable inside temperature with any extreme but expected outside temperature. And cycle while at it. That means 65F inside with 105F outside if needed.
Humidity can be removed very well even if the unit is "oversized" by running the variable speed air handler in dehumidification mode. I said "properly designed" not "contractor special".
A unit that can barely handle the load isn't more efficient, as a matter of fact it may be unable to take advantage of any setbacks because it will have to run all day long for the house not to be an oven in the afternoon.
Absurd. A properly designed system, one in which a Manual J Residential Load Calculation, Manual S Residential Equipment Selection, Manual D Residential Duct System Procedure and Manual T Air Distribution Basics for Residential and Small Commercial Buildings are performed, will run as long as possible to address the loads. In addition, a properly designed system will run close to 100% of the time at the outdoor design condition and proportionally less at temperatures closer to the inside design temperatures. Designing a system using peak heating and cooling loads based on extreme weather conditions that occur for only a few hours per season, such as the hottest day on record, will result in an oversized system. Over-sized systems in humid climates may not run long enough under some part load conditions to achieve humidity removal from the air and keep the house within the ASHRAE comfort zone.
An oversized HVAC system will have both a higher initial cost and a higher cost of operation. The frequent starting and stopping of short cycling can lead to premature failure of the equipment. Short cycling limits the total amount of air circulating through each room, and can lead to rooms that do not receive adequate duration of airflow. In the cooling season in humid climates, cold clammy conditions can occur due to reduced dehumidification caused by the short cycling of the equipment. The system must run long enough for the coil to reach the temperature for condensation to occur and an oversized system that short cycles may not run long enough to sufficiently condense moisture from the air. Excess humidity in the conditioned air delivered to a space may lead to mold growth within the house. |
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Reviews:
·AT&T U-Verse
| reply to Jack_in_VA
said by Jack_in_VA:Can you provide your source of this information about "a properly designed and working system"? I've never seen this described in any technical literature or any real time calculations on new installations. I would be interested in studying the technical data behind the reasoning for this over sizing enough to provide 65 degrees with an outside temperature of 105 degrees. Common sense. The system must be designed to keep the user comfortable at all times. Period. There is absolutely no excuse not to. If the outside may reach 105 and people may want 65 then the system must be designed to be able to handle that load.
said by Jack_in_VA:How many people have a unit with a variable speed air handler? All those who have at least a half-decent systems. Contractor specials are *not* included. |
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Reviews:
·AT&T U-Verse
| reply to SayWhat
said by SayWhat :Absurd. A properly designed system, one in which a Manual J Residential Load Calculation, Manual S Residential Equipment Selection, Manual D Residential Duct System Procedure and Manual T Air Distribution Basics for Residential and Small Commercial Buildings are performed, will run as long as possible to address the loads. Don't confuse a system that is *expected* to run morning to evening with a system that is expected to be turned off during day such as a residential setting.
Does it make any sense to keep filling a leaking bucket with a teaspoon or with a cup? Yeah, thought so...
said by SayWhat :An oversized HVAC system will have both a higher initial cost and a higher cost of operation. The frequent starting and stopping of short cycling can lead to premature failure of the equipment. Short cycling limits the total amount of air circulating through each room, and can lead to rooms that do not receive adequate duration of airflow. The difference in a residential system is insignificant. Few hundred on a $20k system. The system starts and stops the same number of times as most thermostats are set for a certain CPH rate. And of course the fan can be set to circulate the air at all times. Again, "contractor specials" are excluded. I have a properly sized system (some may call it way oversized) and it keeps humidity levels in the 50-55% range. In a 70's house that isn't so tight. Near the coast where humidity is a fact of life.
The only thing different from a system designed to the limit is that I don't have to run it all day so that I have under 80F when I get home. It can bring the house temperature to "nice" levels in a matter of 30 minutes. |
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iknowPremium
join:2012-03-25 | reply to SayWhat
said by SayWhat :said by cowboyro:A properly designed and working system will keep any reasonable inside temperature with any extreme but expected outside temperature. And cycle while at it. That means 65F inside with 105F outside if needed.
Humidity can be removed very well even if the unit is "oversized" by running the variable speed air handler in dehumidification mode. I said "properly designed" not "contractor special".
A unit that can barely handle the load isn't more efficient, as a matter of fact it may be unable to take advantage of any setbacks because it will have to run all day long for the house not to be an oven in the afternoon.
Absurd. A properly designed system, one in which a Manual J Residential Load Calculation, Manual S Residential Equipment Selection, Manual D Residential Duct System Procedure and Manual T Air Distribution Basics for Residential and Small Commercial Buildings are performed, will run as long as possible to address the loads. In addition, a properly designed system will run close to 100% of the time at the outdoor design condition and proportionally less at temperatures closer to the inside design temperatures. Designing a system using peak heating and cooling loads based on extreme weather conditions that occur for only a few hours per season, such as the hottest day on record, will result in an oversized system. Over-sized systems in humid climates may not run long enough under some part load conditions to achieve humidity removal from the air and keep the house within the ASHRAE comfort zone. An oversized HVAC system will have both a higher initial cost and a higher cost of operation. The frequent starting and stopping of short cycling can lead to premature failure of the equipment. Short cycling limits the total amount of air circulating through each room, and can lead to rooms that do not receive adequate duration of airflow. In the cooling season in humid climates, cold clammy conditions can occur due to reduced dehumidification caused by the short cycling of the equipment. The system must run long enough for the coil to reach the temperature for condensation to occur and an oversized system that short cycles may not run long enough to sufficiently condense moisture from the air. Excess humidity in the conditioned air delivered to a space may lead to mold growth within the house. those manuals need to be updated to reflect the use of multi speed compressors, multi speed blowers, and thermostatic expansion valves, else it's useless. the maximum cooling can now be much higher, and still be lowered to normal levels and below to remove humidity and ensure proper circulation. |
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| What are you saying? Are you saying we should be installing 5-ton 2-stage systems in 3-ton homes just in case we need the additional BTU cooling capacity for those days when the temperature is above the outdoor dry bulb design temperature? Please elaborate. I'm fascinated by your theory. |
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 alkizmo
join:2007-06-25
Pierrefonds, QC
kudos:1 | Yes that's what he's saying... because why not afford super luxury 
Have you seen the prices on those whole house dehumidifier? Better just splurge that money on pricier fancy AC. |
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iknowPremium
join:2012-03-25 | reply to SayWhat
said by SayWhat :What are you saying? Are you saying we should be installing 5-ton 2-stage systems in 3-ton homes just in case we need the additional BTU cooling capacity for those days when the temperature is above the outdoor dry bulb design temperature? Please elaborate. I'm fascinated by your theory.
i'm saying some would want a constant temperature, and if they can afford it, it's doable. take for instance professional offices, even small one's like doctors and dentists, who keep the temperatures at a constant 72 deg. no matter what the temperature is outside. some homeowners may want that too. the 5 ton capacity would only be used on the hottest days, and it could switch down to 3 ton or less on cooler days. |
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Reviews:
·AT&T U-Verse
| reply to SayWhat
said by SayWhat :What are you saying? Are you saying we should be installing 5-ton 2-stage systems in 3-ton homes just in case we need the additional BTU cooling capacity for those days when the temperature is above the outdoor dry bulb design temperature? Please elaborate. I'm fascinated by your theory.
If you can afford then yes... The extra investment may even pay off soon, if not only through the extra comfort.
If you can't afford then stick with the contractor special pack where you sweat when it's hot and have to keep the A/C running from 12PM just so it's bearable at 7PM. |
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iknowPremium
join:2012-03-25
1 edit | said by cowboyro:said by SayWhat :What are you saying? Are you saying we should be installing 5-ton 2-stage systems in 3-ton homes just in case we need the additional BTU cooling capacity for those days when the temperature is above the outdoor dry bulb design temperature? Please elaborate. I'm fascinated by your theory.
If you can afford then yes... The extra investment may even pay off soon, if not only through the extra comfort. If you can't afford then stick with the contractor special pack where you sweat when it's hot and have to keep the A/C running from 12PM just so it's bearable at 7PM. here's a good example of a variable speed compressor, they DO make them. »www.analogzone.com/grnt0605.pdf
here's a few units using variable speed compressors. »www.energystar.gov/index.cfm?c=m···cac_ashp |
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 mityfowlPremium
join:2000-11-06
Dallas, TX | reply to cowboyro
Re: Another a/c issue. said by cowboyro:said by SayWhat :What are you saying? Are you saying we should be installing 5-ton 2-stage systems in 3-ton homes just in case we need the additional BTU cooling capacity for those days when the temperature is above the outdoor dry bulb design temperature? Please elaborate. I'm fascinated by your theory.
If you can afford then yes... The extra investment may even pay off soon, if not only through the extra comfort. If you can't afford then stick with the contractor special pack where you sweat when it's hot and have to keep the A/C running from 12PM just so it's bearable at 7PM. I think you're being a little severe here.
They probably just stress until it gets real hot.
And that's usually for just 2 or 3 days in these areas. Not like Texas and Florida. |
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