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Central A/C tonnage questions

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  1. May 29, 2015 #1

    Borg

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    The HVAC system on my home is going to have to be replaced this year. I've been getting estimates for replacing the system but, I have some confusion about some of the things that I'm being told by various contractors. I have a lot of questions/confusions but I'll try to list them as succinctly as possible. My home is a 3 story townhouse with approx. 2080 sq. ft. of living space and currently has a 20 year old 3.5 tonne A/C unit. The lowest level has about 20% of the total sq. footage.

    Questions:

    Most contractors have included replacement of the A/C lines as part of the job. Contractor A (with very good Yelp reviews) has told me that it isn't necessary if the lines are good because line leaks are almost always due to corrosion at the coils. He did open the furnance and there is significant corrosion there. He said that he would pressure test the lines and save me about $1500 if they don't need to be replaced. This seems reasonable to me since I could always replace the lines later if they fail. So, is he cutting corners or are the other contractors just not ?

    Most contractors have just looked at my current 3.5 tonne system and given me estimates for 3.5 or 4.0 tonne units. Contractor A measured the size of my home, did a load calculation and said that I only need 3.0 tonnes on the A/C. I've looked at some calculators online and we seem to be at the top end of the 3.0 range but I know that we also don't want an oversized system. I think that if there were cooling issues later, I could always have an attic fan installed to decrease the loading on the A/C. Does it seem reasonable to put a 3 tonne unit in 2100 sq. ft with an attic fan as a backup plan?

    RE the corrosion issue on the lines. Contractor A has stated that the main location of corrosion is usually on the coils where two different metals are coming into contact with each other. Seems reasonable given what I've seen. He stated that the Trane system that he wants to install doesn't suffer from that since all parts and components are made of aluminum. Of course there will be a single metal difference where the A/C line connects to it but that seems easy to fix if it corrodes. My question is whether aluminum works well for an A/C coil or if there is anything else I should be wary of with respect to aluminum.

    Thanks for any answers. :woot:
     
    Last edited: May 29, 2015
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  3. May 29, 2015 #2

    russ_watters

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    What is the hottest temp you see in the summer? Humid or dry climate? Well insulated house? The best way to verify your current sizing though is based on how long your current unit runs on a very hot day, if you've ever taken note.

    I think it is reasonable to keep the lines, as long as they are sized correctly for the new AC unit's refrigerant type (GW/ozone restrictions are resulting in changes).

    Aluminum is fine/standard.
     
  4. May 29, 2015 #3

    Borg

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    Thanks for the response Russ. I was hoping that you would see this. :oldsmile:

    I'm just south of you in northern VA. We get one or two 100 degree days and around 20 90+ degree days a year on average. The house is reasonably well insulated with about 24 dual pane windows and doors. Climate can get humid at times but doesn't usually last more than 5 days. The current unit was cooling OK but it's been on its last legs for more than 5 years. We've had to have the freon recharged for about 7 of the last 10 years. At first it lasted about 2 years, then only one and last year it didn't make it through the entire summer. So it's a bit difficult to really say how well the unit was handling the heat vs. not functioning. If I remember correctly, it did cycle on and off quite a bit with short periods for the off cycles (especially if someone walked past the thermostat). The electric bills were rarely over $125 if that helps.
     
  5. May 29, 2015 #4

    jim hardy

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    Does the old unit still cool ?
    You might consider replacing the motor run capacitors for both of the fans and the compressor, oiling both fan motors, cleaning the coils and replacing the evaporator drip pan and drain pipe.
    There's chemicals that professionals use for that nasty job - i've used this one, my local hardware store carries it... http://dwdavies.com/product/koil-kleen .
    "Electric Motor Oil" is slightly different than automobile motor oil, your A/C man or a better hardware store will have it.

    That should take about a day's labor plus some parts. You'll want to hire it done first time for there's technique involved in that cleaning process and possibly some sheet metal work in the drip pan..

    My unit is at least thirty years old, 100% copper coils inside and out, though the fins are aluminum. It survives because it's simple - no "computer controlled three phase field vector oriented control " motors.

    SO, you might take a look at what is your energy cost per year to run the old one. Probably a new one will cut that by 1/3 (maybe increase from EER ~12 to ~16 ?) so payback in North Dakota would be longer than in South Texas.. Inquire about extended warranty on the computer boards in new units.

    I had a 2000 sq ft house in Florida that a two ton unit cooled nicely.


    Those are my thoughts.
     
  6. May 29, 2015 #5

    jim hardy

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    oops your post appeared while i was typing.

    ignore my prior post. I assume you've put soapsuds on all the fittings looking for that leak....
     
  7. May 29, 2015 #6

    Borg

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    No I haven't done that. There's probably nothing in the system right now anyway. The coils are pretty corroded but I'll get a camera and post a pic in a few minutes.
     
  8. May 29, 2015 #7

    Borg

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    Picture of the coils. Quite a bit of corrosion and there is a dent on a coil (second from bottom left). If the lines test out OK, I'm pretty confident that the coils are what's been leaking. An actual leak test from a contractor would be about $300 to $500 which is probably a waste of money at this point. I'm really tired of trying to keep this dinosaur living each year.

    AC_coils.jpg
     
    Last edited: May 29, 2015
  9. May 29, 2015 #8

    jim hardy

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    Indoor coils.. copper with aluminum fins.... get some Mr Bubble kids' bubble-bath and apply to the fittings. And around bottom where copper comes through that rust.

    Especially on outside unit where the pressure ports are .

    Good luck !
     
  10. May 29, 2015 #9

    Borg

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    I put some on the fittings and ran the system for a bit. I didn't see anything.
     
  11. May 29, 2015 #10

    jim hardy

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    a hidden leak.. They can be hideously difficult to find.

    sorry about that..
     
  12. May 29, 2015 #11

    russ_watters

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    You're welcome.

    3.5 Tons sounds about right for you. If you had no complaints about capacity before, there is certainly no reason to go up to 4 Tons. But losing its charge like that is pretty bad.
    You might consider a 3.5 ton evaporator and 3 ton condenser. I have a mismatched 3 ton evaporator with a 2.5 ton condenser. It allows you to squeeze a little more capacity and efficiency out of the condenser, ending up somewhere between the two.

    An attic exhaust fan can help a bit, but I'm not sure how much - and unless it is really hot out, they use more energy than they save.
     
  13. May 30, 2015 #12

    Borg

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    The loss of the freon every year has been a recurring pain. If the contractor's suggestion that similar metals throughout the condensor would minimize potential loss is true, that would be great.
    I sent an email to the contractor to see if he would consider it. I won't know if a 3 ton was too small until after it's installed so this would help to remove some of the worry. Plus, it would be fun to see the look on future contractor's faces when they try to understand the system. :woot:
     
  14. May 31, 2015 #13
    Besides the obvious leakage and old age / corrosion problem, correct sizing is a complex calculation. It seems the most rigorous analytical method used is ACCA's Manual J sizing methodology. It's based on many factors like house orientation, region, insulation levels, type & quantity of windows, etc. You can buy the Manual J for $100+ if you want to do your own. I found this website which claims to do the same for no cost.

    http://www.loadcalc.net/

    However, my Engineering Professor buddy who is a HVAC Guru says it's not all that necessary to do it the rigorous way. So I'm confused.

    After much research for my own renovation project, there seems to be a consensus that the all-to-common rule of thumb "X-tons per 1,000 square feet" sizing method is very likely to oversize a system. But is done because it requires effort to do a rigorous calculation. I've discovered the modern thinking is that oversizing is as bad as undersizing. Oversizing will blast conditioned air into the home at the cost of rapid-cycling the equipment. And after much research, I will probably go for the 2-speed systems but not the Variable Frequency Drive systems. I've installed VFD air compressor units in factories, and those are fabulous devices. My research found operations analysis charts showing the VFD systems are definitely more efficient than 1- or 2-speed systems, but not by a percentage increase that justifies the (the assumed) premium cost. We'll see when I start getting firm prices.
     
  15. May 31, 2015 #14

    jim hardy

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    you dont want to be leaking Freon 22 if you're greenhouse-gas sensitive
    and it's becoming hard to get , except in China.

    New refrigerants operate at higher pressure hence dont lend themselves so readily to home-handyman maintenance....

    Were your old system leak-free i'd say keep it. But that is not the case.
    My neighbor's 30 year old R22 system has come back to life several times now with one new indoor and one outdoor fan, one control relay , & most recently new capacitors for outdoor fan and compressor. Had to free up the reverse cycle freon valve some years back by exercising it a couple hundred times. He's postponed the replacement cost .


    old jim
     
  16. Jun 1, 2015 #15

    Borg

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    Thanks for all of the advice. It has helped us to make decisions and put our mind at ease about keeping the existing lines. New system is getting installed on Thursday.
     
  17. Jun 5, 2015 #16

    Borg

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    The HVAC has been installed and I think that it's perfect. It's a 3 ton system instead of the previous 3.5 ton plus we were able to keep the existing A/C lines which also saved a lot. It's really quiet both inside and outside and you can hardly tell that it's running. It's been unusually cold the last two days but we will be back in the 90's Monday to see how it handles the heat. I'm tempted to purposely let the house get hot and humid just so that I can see how long it takes to cool it down and remove the humidity. Thanks again for all of the advice everyone.
     
  18. Jun 5, 2015 #17

    jim hardy

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    While it's all new and clean study the inside coil arrangement, in particular the drip pan and its drain.

    When i lived in Florida where you run airconditioning ten months a year in incredible humidity, that was the most common trouble spot.
    Dust that made it through(or around) the air filter accumulated on the fins and washed down into the pan where mildew sprouted on it, eventually plugging the drain. Then the pan rusted and i was fighting water leaks.... Admittedly S Florida really tests air conditioning equipment.
    While you're in there be sure there are no air leaks that bypass your filter.

    A good cleaning of the inside unit's fins every cooling season helped mine. I used "Tide" laundry soap, a scrub brush and garden hose.

    It'd be interesting to connect a power monitor to your system . Then you could trend..... i'm told that things like planting a tree to shade the outdoor unit help , as does of course shading windows. If the drain is where you could collect and measure the condensate, the cost of humidity might show up.
    In peak of summertime i spray my roof with the garden hose a couple times per day .


    glad you're up and running !

    old jim
     
  19. Jun 5, 2015 #18

    Borg

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    The old system had mold and mildew issues in the drain line. I had to replace the drain at one point because it was clogged to the point of backing up into the furnace so I'll definitely watch for that. The new system has PVC pipe that I can't see through. I may replace it eventually with some clear tubing to make it easier to monitor.

    It's under the patio deck for the second floor where it doesn't get any sun. On the longest days, the sun hits it for no more than an hour before sunset.
     
  20. Jun 5, 2015 #19

    jim hardy

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    Sorry, i didnt intend to belabor the obvious.... no offense meant.

    You are obviously one who looks into things....

    Regards ! old jim
     
  21. Jun 5, 2015 #20

    Borg

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    None taken. You and Russ have been very helpful.
     
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