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Old 03-13-2011, 02:29 PM   #16
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Central Air Conditioning running all day


Well I have been in the business for quite some time and I have also sized many units I can't do a load calculation obviously but from what I've read it sounds too small

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Old 03-13-2011, 02:54 PM   #17
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Central Air Conditioning running all day


Thank you to everyone who has offered their suggestions. I thought I would provide a bit more info because of them.

My home has R40 walls (10 inch) and R60 in the attic. The central AC unit was a new unit installed last June into my existing electric furnace. (Before that, we didn't have central AC, we were using two portable AC units only.) The unit is a 2-ton Kenmore 77410 Series with a SEER of 14.5 and 2-speed fan. I was told that my Nortron electric furnace and ducts might not be able to handle a 2.5-ton unit since the coil in the furnace might freeze up. The furnace is not new but has hardly been used since I heat with wood. The electric furnace was our back-up. We have a prevailing north-west wind so our windows were usually open whenever possible during the summer. I have noticed in some of my recent readings that the outside unit should preferably be installed in a shady area. However, my unit is installed in a full sun area on the south side of the house.

The unit would typically run continuously whenever the temperature was 28 degrees celcius or above outside. However, by suppertime, the house was usually comfortable and cool.

A Sears service technician came out last July and measured the air temperature difference between the in and return sides of the unit - it was about 11 degrees Celcius. His only other suggestion was to stuff T-shirts in my basement ducts so that all the cold air was sent upstairs. In August, the salesman came out and measured all the rooms and windows and later confirmed that my AC unit was the proper size. Sears then sent their most senior tech and he found the unit was a bit low on freon and topped it up. He also measured the temperature difference and I believe it was around 12 or 13 degrees Celcius difference. The furnace fan is also running on its' fastest speed. When the service manager told me that the manufacturer had told him they were meant to run continuously in hot weather, I asked for his number and called him directly. He repeated what the service manager had told me. Apparently today's AC units have smaller compressors to save on energy although they do work much longer.

By suppertime, as long as the outside temperature wasn't in the mid-30's, the house would be cool but the unit kept running. I would typically turn it off in the evening.

I've tried to compare my electricity bills but it hasn't been easy since I was using portable AC units and fans at times before and the price of electricity has increased quite a bit here. We live outside of Ottawa.

I have a Hunter Energy Monitor II programmable thermostat which I've had for about 15 years. Someone told me that some AC's have problems with some of the older programmable thermostats. Maybe I'll try a new one in the Spring and see if it makes a difference.

Someone suggested I try setting the unit a bit higher. This summer, I am going to try setting the unit to, let's say 21 degrees, so it'll start earlier in the morning, then increase the setting to say, 23 or 24 degrees at 10:00 AM to see if that makes any difference in the run time.

Thanks again to all for the comments and suggestions.
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Old 03-13-2011, 03:28 PM   #18
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Central Air Conditioning running all day


Ottawa can be pretty humid so that adds to the work load. Part of your problem has to do with the way you are running the unit. I would NOT use a setback program in Canada. I tell all my customers to set it at 22 and let it run all the time unless the weather changes and you know it is going to be cool (under 20 C for a few days). The units are designed to safely run continuosly and if you were getting 11 C supply which is 50-55 for the US guys then that sounds reasonably OK to me. If you let it start in the morning it will progressively remove the humidity (which you have more than me) and it will work better. As this is your first AC you have nothing to compare to. I think it is the right size and you do need to keep NO cold air going into the basement. Your Nortron should have a max 3 ton blower so it should be OK. The trouble with internet troubleshooting is we don`t always get all the necessary info in the beginning. I would still get another checkup from a experienced tech to make sure. Not all the big box subs are poor quality but a lot are so a second opinion is a good idea. A new Honeywell Focus Pro thermostat from HDepot or Rona will work better than the old one you have now and save you energy $$ by being more sensitive and accurate. Hopefully you have drapes or blinds for your windows. I have low e-argons but the best one is around a R4 so drapes and blinds are still required.
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Old 03-13-2011, 05:54 PM   #19
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Central Air Conditioning running all day


okay, now we're getting somewhere. The 2 ton is officially equipped with a 1.5 ton compressor so that is your system size, not 2 ton. It is true that newer systems are built like that, not all but most. So in essence the sales guy said that your ducts are spec'd for 1.5 ton and a 2 ton compressor (in a 2.5 ton unit) not will work as per your ducts even though blower speeds can be manually set..? Now we need to know duct size to calculate cfm to see how far you chain is being pulled and I'd have to still think it's quite a bit, from this end only though as I'm not where you are or familiar with systems running all day. never heard of that before.

you should be able go a bit bigger and add a duct to or two or replace existing (if any duct needs work/replace/upsize to begin with hence cfm and size of ducts)/upsize and for not too, too much cost so there are many ways around that.

The main thing here is your comfort.
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Old 03-13-2011, 06:35 PM   #20
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Central Air Conditioning running all day


Losing a 1/2 ton of cooling. Afraid not. Lots of them are 22-23-24,000 BTU compressors but never a 1/2 ton lower. Quaildog here is some engineering specs for more info. see p.5 KBthu = 1000s of BTUs /hr and a 2 ton unit is 24,000 BTU/hr

It varies according to the outdoor temp and humidity but the compressor is always within 2000 BTUs of the manufacturers rating. Copeland does not make them exactly 2 ton and in the model # there may be a 22 meaning 22,000 BTUs. A 2 ton compressor is a nominal rating which can be between 22-24,000 BTUs depending on the manufacturer.
http://tech.lennoxintl.com/PDFs/ehb_13acx_rtgs_0904.pdf
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Old 03-13-2011, 06:45 PM   #21
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While the compressor may be rated at only 18,000 BTUs at 130F condensing temp, at 105F condensing temp is will have a rating of 24,000 to 27,0000 and some odd.

My A/C will run for 6 hours straight when its 98F(36F) outside(about 6 degrees above our outdoor temp design). My A/C is 8 years old.

Your furnace may not have a strong enough blower, if it was designed without an internal evap coil.
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Old 03-13-2011, 06:51 PM   #22
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Central Air Conditioning running all day


Probably true about the blower size/capacity. 14 yrs ago Nortron would probably have put a 2 ton blower in his unit which probably is a 15 kw. The newer ones come equipped for 3 tons of cooling. If he is getting a 55 supply temp his Delta T is 15-20 deg and good. It has not froze up so it is safe to use that way. No way for us to know for sure what is going on w/o actually being there and seeing the duct size etc etc. That is an expensive custom home and I highly doubt the contractor would have skimped on the duct sizing. They don't want problems or to lose their reputation and to be a R2000 certified builder everything is done to high standards. In the cheap cookie cutter homes yes, but not in that grade of home.
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Old 03-13-2011, 08:04 PM   #23
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Quote:
Originally Posted by beenthere View Post
While the compressor may be rated at only 18,000 BTUs at 130F condensing temp, at 105F condensing temp is will have a rating of 24,000 to 27,0000 and some odd.

My A/C will run for 6 hours straight when its 98F(36F) outside(about 6 degrees above our outdoor temp design). My A/C is 8 years old.

Your furnace may not have a strong enough blower, if it was designed without an internal evap coil.

Can I ask how we figure out the btu's from the condensing temp, the equation? Thanks.
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Old 03-13-2011, 08:22 PM   #24
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Central Air Conditioning running all day


Quote:
Originally Posted by Doc Holliday View Post
Can I ask how we figure out the btu's from the condensing temp, the equation? Thanks.
I have info on it somewhere. But no idea where. Its basically how many pounds of refrigerant being moved per minute and how much wattage the compressor uses to move that much refrigerant. Along with the condensing temp.

However, Copeland publishes the capacity of their compressors at the different condensing temps.

A 2 ton unit using a compressor that is rated at a 105 degree condensing temp may have a total capacity of 23,000 BTUs at 95 outdoor temp, 80 degree indoor temp, and a total capacity of 24,800 BTUs at 85 outdoor temp and 80 degree indoor temp.

While the compressors are smaller in displacement/pumping capacity, the condenser coils are larger/more efficient at rejecting heat and do reject more heat from the lower volume of refrigerant, so the larger compressor is not needed.
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Old 03-13-2011, 08:29 PM   #25
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Central Air Conditioning running all day


Is this something I'll find in my Refrigeration and A/C Technologies book?
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Old 03-13-2011, 10:15 PM   #26
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Quote:
Originally Posted by Doc Holliday View Post
Is this something I'll find in my Refrigeration and A/C Technologies book?
Should be in there.
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Old 03-14-2011, 05:02 AM   #27
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Central Air Conditioning running all day


Hi Ive read your problem and I can gurantee you there was a step missed in replacing your system.I agree with doc holiday .If a system is designed correctly it should be shutting off.First off you said 1500 sq ft per floor.Is this home multi level. when this system was changed out,was the system done as a whole both indoor and outdoor unit.Im kinda turning towards the system sizing.Its good that you have a home with good windows and insulation.From the sounds of it you have multi levels on home and air distribution can be a factor on system performance.Id like to get some more info on this .being a system set up for an weatherization program ,was there a recognized heat load calculation performed ...manual j or other approved program?Is the air flow set up properly,how about a superheat/subcool measurement performed.theres a lot of things that make a diiference on the performance ,effeciency and the economics of a systems performance.A system to run top notch everythings designed evenly and for proper sizes to enable your system to produce the proper comfort throuhout the home with out any hot/cold spots .The home should be even temp for proper comfort level

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