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Old 06-15-2013, 06:16 PM   #1
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Making 5 Ton AC unit 4 Ton unit


I have a split 5 Ton 10 Seer Lennox AC with a separate 120K Natural Gas unit. They share the same fan. The Lennox was manufactured in 1991 and has never been touched. It's a great unit and does a wonderful job, but I think the unit is too big for my 2000 sq ft house. It only cycles for 12 minutes and really cools the house off, especially while it is running. Can the unit be reset to out put as a 4 ton unit to make it cycle longer? If this is possible and the big question, will it use less electricity when it's set at 4 tons? My objective is a longer cycle but I don't want to burn the same amount of electricity with my unit running longer.

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Old 06-15-2013, 06:18 PM   #2
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Making 5 Ton AC unit 4 Ton unit


No, there is no way to take a five ton and make it a four ton, or vice versa. You could replace the compressor with a smaller sized unit, but then you also have to replace the a-coil

At 22 years old, those units are meeting their end of life span, and really are good for nothing but scrap at this point. Maybe get a few hundred dollars for the scrap.

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Old 06-16-2013, 02:56 AM   #3
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Making 5 Ton AC unit 4 Ton unit


When that system was installed,energy weas a LOT cheaper and oversizing everything was the l;azy way to make sure you had enough.
You are in a position right now to plan for their replacement and because you have time you can do it right.
Gone are the days when you could guess on sizing HVAC equipment.You can now have a proper manual J done to find out just what size equipment your house needs.Any contractor that won't do a proper manual J is lazy and probably would be lazy with his install.With new equipment and after a proper manual J is done,a manual D could be done easily to figure out if your duct system will allow you to get the effiency that your new equipment will give you to each room of your house.
Good luck on doing things properly.
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Old 06-16-2013, 05:40 AM   #4
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Making 5 Ton AC unit 4 Ton unit


There is a way to make it work as only a 4 ton unit. Cost a lot to do it though.

Rawal valve.
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Old 06-16-2013, 06:14 AM   #5
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Making 5 Ton AC unit 4 Ton unit


With a system of that age I wouldn't do anything as long as it's running with no problems. I would consider doing the calculations already referenced and then start thinking about a high eff replacement.
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Old 06-16-2013, 08:37 AM   #6
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Making 5 Ton AC unit 4 Ton unit


a new higher efficiency unit will save lots of elec $$ and a proper sized one will save even more as it will dehumidify much better and allow you to keep your house temp higher while feeling the same level of comfort. chances are the compressor valves are worn and it has lost some capacity and pretty soon the condenser fan motor will die so big repairs/failure is imminent. better to plan for a replacement when it is not so hot then when you really need it. every yr you keep it costs more to run in elec $$ plus the increased cost of a new unit due to inflation. a heat load calc is necessary

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Old 06-18-2013, 09:40 AM   #7
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Making 5 Ton AC unit 4 Ton unit


A 4-ton might use 20% less kW, but it will run 20% longer to move the same number of BTUs so you will not save any money. The savings comes from a higher SEER, which is independent of tons/size.
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Old 06-18-2013, 10:55 AM   #8
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Making 5 Ton AC unit 4 Ton unit


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A 4-ton might use 20% less kW, but it will run 20% longer to move the same number of BTUs so you will not save any money. The savings comes from a higher SEER, which is independent of tons/size.
Not really true. If his 5 ton was 13 SEER. A proper sized. 4 ton 13 SEER would save him money.
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Old 06-18-2013, 11:05 AM   #9
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Making 5 Ton AC unit 4 Ton unit


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Not really true. If his 5 ton was 13 SEER. A proper sized. 4 ton 13 SEER would save him money.
Doesn't the 4 ton 13 SEER have to cycle on for longer (say, 15 minutes) than the 5 ton 13 SEER (say, 12 minutes)?
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Old 06-18-2013, 02:56 PM   #10
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Making 5 Ton AC unit 4 Ton unit


It will run longer. But not proportionally longer. As in not 20% longer run time. Also it won't come back on nearly as quick.

The 5 ton would quickly cool the air. But the furniture and other objects in the room won't cool quickly, so they heat up the air quickly, which brings the 5 ton back on quicker.
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Old 06-18-2013, 03:14 PM   #11
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Making 5 Ton AC unit 4 Ton unit


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It will run longer. But not proportionally longer. As in not 20% longer run time.
I don't see how that is possible. 5-ton = 60,000 BTU/h. If you have a house with 48,000 BTU/h heat gain, the 5-ton must run on a 80% duty cycle. The same house with the same 48,000BTU/h heat gain, the 4-ton must run on a 100% duty cycle... exactly 20% longer?
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Old 06-18-2013, 04:16 PM   #12
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Making 5 Ton AC unit 4 Ton unit


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I don't see how that is possible. 5-ton = 60,000 BTU/h. If you have a house with 48,000 BTU/h heat gain, the 5-ton must run on a 80% duty cycle. The same house with the same 48,000BTU/h heat gain, the 4-ton must run on a 100% duty cycle... exactly 20% longer?
5 tons is a nominal rating. Doesn't mean it actually has 60,000 BTUs of capacity. Same for the 4 ton.

Next, only on the hottest days would the 4 ton run 24/7. The 5 ton having to restart, would use more electric restarting. Remember, a compressor draws Locked Rotor Amp every time it starts.

Now consider all the mid temp temps, when the 4 ton keeps the house feeling cooler because its keeping the humidity down. But a 5 ton would need to have the thermostat set 2 or more degrees colder to make the house feel the same way. So its actually running almost as much as the 4 ton.

I tore a lot of 10 SEERs out, and put smaller 10 SEERs back in. And people saved money.

I've torn out 13 SEERs and put smaller 13 SEERs back in, and those people are saving money. Humidity control makes a big difference.

Unless you include all the variables(latent heat makes a big difference, someone being able to set the stat at 73 or 74 instead of 72, fewer start up using less wattage), you will come up with the same power consumption. And you will be incorrect.
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Old 06-19-2013, 12:22 AM   #13
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Making 5 Ton AC unit 4 Ton unit


Quote:
Originally Posted by beenthere

5 tons is a nominal rating. Doesn't mean it actually has 60,000 BTUs of capacity. Same for the 4 ton.

Next, only on the hottest days would the 4 ton run 24/7. The 5 ton having to restart, would use more electric restarting. Remember, a compressor draws Locked Rotor Amp every time it starts.

Now consider all the mid temp temps, when the 4 ton keeps the house feeling cooler because its keeping the humidity down. But a 5 ton would need to have the thermostat set 2 or more degrees colder to make the house feel the same way. So its actually running almost as much as the 4 ton.

I tore a lot of 10 SEERs out, and put smaller 10 SEERs back in. And people saved money.

I've torn out 13 SEERs and put smaller 13 SEERs back in, and those people are saving money. Humidity control makes a big difference.

Unless you include all the variables(latent heat makes a big difference, someone being able to set the stat at 73 or 74 instead of 72, fewer start up using less wattage), you will come up with the same power consumption. And you will be incorrect.
BT is right. The 5 ton would put out the cooled air quickly and the thermostat would become satisfied but all of the objects and any solids in the structure still have quite a lot of thermal mass and are slow to reject it into the air. Then causing the thermostat to cycle the system back on. AC systems aren't necessarily meant to be turned on in the middle of the day and cool down the house in ten minutes. That how car systems are made for. Residential ones should be set to their set point in the morning and then be able to keep up with the heat gain through out the day through a few cycles or, on the hottest day, a constant run 24/7.
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Old 06-19-2013, 05:56 AM   #14
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Making 5 Ton AC unit 4 Ton unit


A note on cost and size. There are many parts to a new installation , not just cost. The calculations need to be done and accurate, the size of the equipment must be accurate according to the calculations, the seer should be taken into consideration, and the cost of the unit.

Now what do we have? A house that's comfortable, a unit with good life expectancy operating at a reasonable cost. That sounds like a goal I would shoot for not just one of those things.
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Old 06-19-2013, 07:18 AM   #15
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Making 5 Ton AC unit 4 Ton unit


I would ride that sucker til the wheels fell off then replace it with a properly sized piece of equipment. With a load calc. you may find that a 3.5-ton AC would be sufficient.

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