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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
I want to be prepared when the time comes to replace my 5 ton with a properly sized unit. I heard about an AC guy that put a 2 ton unit in a 2200 sq ft. house and was pleased with the unit. He told people it more economical for the unit to run most of the time (due to start up cost and damage on the unit from starting so often) I understand this logic as long as the smaller unit will get the job done on the very hot days.
if you've heard of this guy than you'd be correct but the guy you've heard of is an hvac specialist. his envelope is tight and I'm sure his home his zoned.
does not matter the size of a unit, the start up is hard on a compressor regardless unit size or square footage.
If the load calculation calls for a 3.5 then you need to install a 3.5. Here is the twist though. A 3-ton, 18-seer is a multi-stage unit and probably rated for 3.5-ton. Anyway, good luck with your decision when you need one!
What would the load rating be for a 3 ton 18 seer unit? 2.1 KW or less? Do the unit comes with the ability to manually set these multi stage units to run on the lower power consumption? Is the lower stage 66% capacity? And what would the load rating be on the lower stage 1.6KW
The usage requirements vary by manufacturer. Typically, I set up the run time for the unit from the thermostat. It starts out on low stage for three minutes and then switches to high stage as long as the temperature differential is greater than three degrees. Once the three degrees barrier is reached it switches to low stage. On most set-ups it rarely gets out of low stage. One thing to note is that in order to actually get 18-seer from an AC/furnace set-up, the furnace has to be variable speed and able to achieve the proper airflow. I doubt that you can achieve a 3-ton airflow with a 5-ton furnace. When you start looking at variable speed furnaces it can get really pricey really quick. Shop around when its time and base your decision on warranty and your comfort level with your contractor.
really all of this talk about rigging the unit so it only runs on low stage is nonsense. Size the equipment based on the load calc. and it is what it is. If your concerned about utilities then spend some time tightening the leaks in your house and ductwork or cut the thing off but I assure you if it calls for a 3.5 then a 3 will not do the job and end up running up your utilities because it will never cut off. multi-stage, variable speed equipment will get you the results your seeking as long as you have a tight duct system and the equipment is properly sized. To give you an idea of the impact equipment selection can achieve, I just installed a new 3-ton, 15-seer packaged heat pump with a new super tight duct system in a customers house. We removed a twenty year old, 3-ton packaged heat pump and standard duct system from the house prior to the installation. We foamed the holes under the house in the floor around the plumbing pipes and a few other places as well. Twenty minutes of foaming and a new system and we cut the customers KW usage in half. I live in eastern NC so our weather is not as extreme as some but the customers savings will still be a huge return on initial investment. Do your research, find a knowledgeable contractor and listen to what he has to say. consultations are usually free.
spend 8 grand to get a new unit, save $50/month average. think about how long the payoff will take. maybe you'll get a grand or two back when you sell the house, but it takes a LONG time to get ROI on a new HVAC system. plus, you have to consider insulation, windows, etc. etc. an older house just uses more juice to cool and heat it, period. if you want efficiency, you really need a new house or you need to burn a ton of money on a bunch of other stuff in addition to the HVAC. in this economy, it just doesn't pay, you'll never see that money back.
but hey, i'm not a real estate agent or an a/c salesman, i'm just a knowledgeable homeowner that does my own work to save money.
I totally agree with you. As I have said often, put the pencil to the paper and you will come up with the proper solution. I agree that the wise thing to do is to run the current unit until the end of its life. I also realize with a 22 year old unit that can be any day so I want to be ready. It's obvious this forum has some wise people on it with a wealth of knowledge for me to utilize. Your logic also applies to 18 seer verses 15 seer units. $1000 or more extra for the 18 seer unit will take a long time for the return in savings. There are lots of thoughts about the two stage units giving more trouble. I've heard some say get a straight 15 seer and not worry with the two stage issues that might arise later. When you figure repairs and initial cost the difference in savings gets much smaller. Comfort and reliability are the top factors to consider. There are lots of things to consider. I appreciate all the feedback. It certainly gives me more info in the decision process.
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