under performing AC

Discussion in 'HVAC Heating & Cooling' started by slpcarpentry, Jul 8, 2012.

  1. slpcarpentry

    slpcarpentry New Member

    Messages:
    1
    Location:
    NJ
    Have a 15 year old Goodman 14 seer 3 ton that has never been able to keep up with the load. Is it possible to increase my capacity by installing a larger/more efficient compressor without changing the A frame?
  2. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    Definitely NOT. If is sometime possible to have an outdoor unit slightly UNDER the ton rating of the indoor evapoarator. It is NEVER ok to have the outdoor unit oversized. Not only mismatch is the evaporator unable to keep up with the liquid being sent to it, but now your condensor coil is wrong also!

    Was the evaporator installed at the same time as the condenser?

    At 15 years, it might just be time for a new system.

    Has a good HVAC tech evaluated this system to see if it is truly undersized? A good inspection would include all the duct work in the attic to make sure you are well insulated and have no leaks.
  3. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    Since you say it has never been able to keep up with the load, perhaps it would be best to ask why? What size home? What sort of set point are you trying to hold (and how hot does it get inside with AC running full out on a given outside temp?) New Jersey is pretty mild, so it makes me wonder why the load is so high. Is the AC in the attic? Are the ducts sealed, insulated, etc.? Does part of the home run cold and the rest hot? The devil is in the details.

    I have a 4 ton unit in a climate that is about 5 degree higher design temp than yours, and mine is oversized, running at about 3 tons effective in the hottest part of repeated 105 F afternoons like yesterday (design guides say 100 F for here.) I run the thermostat at 79 F in summer, which might be on the high side for some people's tastes. I went with a 4 ton because it is a two stage, so there is little efficiency penalty for oversizing...plus the older 4 ton had trouble keeping up in a hot summer when I moved here. However, I have done some insulation improvement, as well as duct sealing, window sealing, and rebalancing, so I've shed some of the heat load.

    If you change the compressor, you will have to match it with coil and air handler. Most likely the duct work is already on the small size for a 3 ton AC, making a larger installation dubious without some heavy retrofit. A better bet is to improve the insulation and sealing of the envelope and ductwork.
  4. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

    Messages:
    303
    Location:
    Minneapolis
    Here in Mpls we have been beyond design conditions for a few weeks. This means the unit was not designed for the weather (10 year event) but the average or mean.

    I find many of my customers - especially in the cooling season, leave the units off until they get home...sorry, no residential HVAC is properly designed that way. If the AC is in fact designed properly and operated 24/7, one has to take advantage of the lower night time temperature and cool the house as much as possible. Limiting cooking, showers and proper ventilation can help.

    Solar gain is also a killer; cover those windows folks and leave the fan on full time.

    Finally, have a look in the attic. If you are short of R-50 or have fiberglass without the added benefit of cellulose on top, call the insulating guy.

    Stay cool.
  5. Dana

    Dana In the trades

    Messages:
    2,812
    Location:
    01609
    Attacking the load is job-1.0 as BadgerBoilerMN suggests, but job 1.1 is attacking the parasitic losses. You can attack these separately if you like, but the results are more assured if done as part of a master plan.

    At "typical" duct leakage levels about 20% of the air is directed to/coming from places other than intended, but worst-case scenarios of 40% aren't rare. Any duct leakage creates pressure differences between rooms, driving outdoor air infiltration through air-leaky walls/basements/attics well beyond that of natural infiltration forces, which increases the very significant latent loads of a NJ climate.

    Sealing the ducts and air handler with the appropriate duct mastic &/or FSK tapes is the first step. Any ducts that are routed in attics above the insulation need to be insulated to at least R8 (R12 better), with a non-air-permeable material (closed cell foam works), after which you can safely bury the ducts in attic insulation.

    Back at job 1.0, air sealing the house, particularly the attic floor/upper floor ceiling is as-important as going high-R in the attic, and more easily done prior to blowing in more attic insulation. The foundation sill & band joist is the other single largest unseen untreated air leak in most homes. If you seal the top of the house and the bottom, stack effect driven air leakage is reduced immensely. Treating the rest of the air leaks still counts, since air-handler driven pressure differentials between rooms don't go away completely, even with better-sealed ducts and a well designed & balanced duct system.

    When killing solar gain, exterior shading on the east and west facing windows are more important than on the south side due to the much lower angle of the sun on those sides adding up to much higher solar gain. The southwest or west facing windows have the highest priority for reducing peak loads, since it's taking up that gain at a time of day when the air temps are higher and the roof & walls are already sun-baked and hot and the load already pretty high. Awnings & overhangs can usually take care of extreme gains on the south facing windows.

    If you don't have low-E windows, low-E storm windows can be added to help both heat rejection in summer, and heat retention in winter at a fraction of the cost of a replacement window. (And a low-E storm over any double-pane will outperform many decent-performance replacement windows.)
  6. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    3,932
    Location:
    NW Ontario, Canada
    I agree whole heartedly that duct sealing is important. So too is sealing the building envelope but you kinda lost me on the "stack effect". I've always seen stack effect as warm air is less dense and rises, like a hot air balloon. If you have a hole in the top of your balloon, the hot air leaks out and you lose heat (and loft).

    When we have cooler (conditioned) air, would it not be heavier than the outside air and so not prone to stack effect? In that case it would be more like a bathtub and the water represents the heavier cold air, so the lower portion of the house needs to keep the cold air in.
  7. Dana

    Dana In the trades

    Messages:
    2,812
    Location:
    01609
    If ANY conditioned space air is escaping, it's sucking in humid air somewhere else. The stack works both ways- yes, when denser cool air is allowed to escape out the bottom, it's sucking air into the house at some other point. It's still follows the math & physics of the stack effect even though there's a change of sign (and therefore direction) compared to a combustion-flue context. If you prefer to call it something else, fine, but the principle is the same.

    If hot and humid air is allowed to enter conditioned space from from an overheated attic, it's a double-whammy but all air-infiltration is a significant cooling load in a NJ climate due to relatively high summertime dew points. It takes ~2x the compressor energy to lower the dew point of air 10F than it takes to lower it's temperature 10F. A typical steamy July day in NJ will have a dew points in the high 60s to low 70s F, which translates into uncomfortable, humid, and mold-inducing conditions in a 78F conditioned space. To be comfortable and healthy at 75-78F you need the dew point to be lower than 63F (60% RH @ 78F), but under 58F (50% RH @ 78F) is noticeably better.

    Bottom line, for the AC to be able to keep up, lowering the latent-load issue by fixing infiltration is as important as the lowering sensible load with insulation & shading. The higher the summertime dew point, the more important air-sealing becomes. If you only fix the sensible load aspect of the building you end up being cool, but also dank, clammy.

    In a tight house with reasonable insulation & windows and no unusual solar gain factors a 3 ton compressor should be capable of handling 3000-4000' of conditioned space. The old-school rule of thumb was 500'/ton, but it takes a pretty crummy building envelope for the cooling loads to actually hit that. Some propose 1000'/ton as a better rule of thumb, but even that is overkill for most homes if you keep the AC at reasonable setpoints during unoccupied hours rather than let the full thermal mass of the house soak at some high temp for hours, then try to cool it down to 75F right after you get home from work/school/etc. Rules of thumb aren't a substitute for a real heat gain/loss calc, but even if you undersize it by quite a bit it should be able keep up with the load most days.
  8. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    In a conditioned space at 78 F the AC knocks the relative humidity down to around 40%. 50% at 78 F becomes uncomfortable and sticky. For this house 79 F setpoint is about the transition point where the AC keeps the indoor humidity at a tolerable level. 78 F set point is even more pleasant, but I can see the impact in electrical use as well.

    Once tightening and insulating have been completed, there is still reason to be conservative in design temp selection (meaning don't be afraid of picking annual maximums.) One of these is that we keep breaking max temp records year after year. And going forward I expect more of the same. So if one uses current guidelines there is a good chance of regularly getting several weeks each summer that the unit can't keep up...and that is if we don't get even hotter over the next decade (a bet I'm not willing to take.)

    Another reason is that as units age one can expect some loss of capacity even with efforts to keep the condenser coils clean. And of course, other users (potential buyers) may prefer 75 F or lower setpoints.

    If one is considering dual stage designs, then there is less of a concern with oversizing as the unit will run at half capacity most of its life anyway. With mine the second stage begins cycling somewhere at or above 95 F, never before. Now, if I was one to let the home heat up during the day, and program it to drop the set point around 5 pm, the unit would use most if not all of its surplus capacity and take considerable time to catch up. I've seen this when returning from vacation and dropping the set point back down to normal.
  9. Dana

    Dana In the trades

    Messages:
    2,812
    Location:
    01609
    The actual conditioned space RH that it'll settle to at 78F will vary by many factors, including your actual infiltration rate & outdoor dew point, the sensible load, the interior moisture sources, the sizing of the compressor relative to the loads (run-time). The assumption that it will be somewhere around 40% isn't a good one. High-R/low-gain very tight houses with low sensible loads end up with much higher interior RH from interior moisture sources (but these are usually manageable with dehumidifiers.)

    My house is neither high-R nor super-tight, but it has low sensible cooling loads due to site shading factors and good south-side roof overhangs. With the AC set to a 78F, limiting the interior RH to 50% is achievable only with other mechanical dehumidification except perhaps on the afternoon of very hottest day of the summer. The rest of the summer the RH will creep up if not controlled by other means. The 70-pint dehumidifier in the basement uses more kilowatt-hours during the May-September cooling season than the central AC. Tightening up the house further would help, but we'd still need to dehumidify, since about half that latent load is from internal moisture sources (cooking/bathing/breathing activities.)

    A 14 SEER 3 ton is still pretty efficient, and replacing it makes no sense if you can bring the load to within it's output range by fixing the parasitic losses and tightening up the house. Depending on the layout of the house and exactly where the big-gain rooms are located, it may even make more sense to supplement with a very-high SEER fully-modulating mini-split than to rip out/redesign the old system to drop in a bigger one, multi-stage or not.
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