I had 4 HVAC contractors out to quote me a new AC system today. All of them quoted me for a 2.5 ton, considering that my house is 1000 sqft in florida, concrete slab, with a flat roof. They all said it was the "right system for this size of house."

However, according to this image attached I should only need a 2 ton AC for florida. Also, according to a few other sites I've visited, a 2 ton is appropriate.

I also did a BTU calculation on www.loadcalc.net, using the most extreme temperatures I can expect (65F indoors, 110F outdoors) and found that I need around 20K BTUs of cooling, which would be just about where I could find myself with a 2 ton system (cause, yknow, efficiency, it's not actually 24k btu the system will output closer to 20k btu...)

So I can only assume that all 4 of these contractors are either: A) looking out for my best interest and telling me that I should slightly oversize my system, or B) trying to squeeze an extra few dollars out of me

And I'd just like to know, is it wise to oversize the system when according to Manual J's BTU calcs I know precisely what I need?

enter image description here

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    I note that "manual J" while terribly easy to use is obviously making assumptions about insulation levels, which could be wildly wrong in either direction, and likewise does not care about windows, trees, or other things that affect your heating and cooling needs. Which doesn't mean your contractors are doing that, simply that this is a VERY simplified/dumbed-down approach to a very complex subject. Precise, I doubt rather highly, from my very well insulated building. If your house is bult the way they assume, it might be closer to right.
    – Ecnerwal
    Jun 18 '20 at 1:50
  • What size is your old system? Any neighbors with similar houses that you can ask about what size/type systems they have? Jun 18 '20 at 2:24
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    Personally I would take the consensus of 4 HVAC professionals over a free online calculator. But that's just me...
    – jwh20
    Jun 18 '20 at 11:01
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    @jwh20 I think the OP raised a valid question about the HVAC contractors as they are dealing with other people's money. I also read that many boilers have traditionally been oversized for the same reason. So it's probably good to understand what factors drive the decision to oversize by 25%.
    – Roc W.
    Jun 18 '20 at 18:58
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    Another thing that can throw the estimates off is windows. Depending on how many and how big (think of a big glass wall), the heat gain through windows (and doors) and can be significant, particularly if your wall are well insulated.
    – SteveSh
    Jun 19 '20 at 0:08

Oversized A/C systems come up short, dry, and sticky

Most people treat air conditioning in the same fashion as they handle heating: namely, as a simple mover of BTUs. However, that isn't quite true, otherwise we wouldn't have condensate coming out of our indoor units. The truth of the matter is that air conditioners both cool air and dehumidify it, at the same time, and have two different capacity measurements as a result. However, manufacturers lumped both functions into a single BTU sizing number for a given set of conditions. The proper sizing procedures using Manuals J and S take this into account (note that even an aggressive Manual J calculation with no padding at all will still yield a somewhat oversized system, just not a grossly oversized one); however, "rule of thumb" sizing does not.

Worse yet, the ratio of latent (moisture removal) to sensible (direct temperature dropping) capacity an air conditioner supplies is not constant. Instead, it depends on the speed of the indoor blower and the ability of the air conditioner to run long enough to get condensing conditions on the indoor coil (i.e. the indoor coil needs to be colder than the dewpoint of the air). With a grossly oversized system, the indoor blower speed will be high, leading to very little "residence time" for a given parcel of air to hang out near the coil and give up humidity. Worse yet, the air conditioner will only need to run briefly to reach the temperature setpoint, meaning that the indoor coil never gets below the dewpoint of the air and you get none of that wonderful condensate coming out of the drain pan. Instead, you get a house that is cold, clammy, and downright icky, as all that humidity is now sitting in air that's much colder than the outside air.

Avoiding this is simple: do not go above proper Manual J, Eighth Edition numbers for a house, ever. In fact, it is indeed more acceptable to slightly undersize a system and not have perfect temperature control on one or two days of the year than to grossly oversize it and find yourself wondering why the air conditioning is making your life miserable.

There is a caveat to all this, though

There is one caveat to what I just said, and that's that Manual J/S sizing assumes that your house has properly designed ductwork, as per Manual D. Improper ductwork can cut your HVAC system's net effective capacity by 25% or more; sadly, this is not an uncommon situation in the American South, where "lob a bunch of flex into the scorching hot vented attic and call it a day" is all too often the way it's done. As a result, many systems wind up oversized to compensate for the sloppy, crappy, money-wasting, energy-hogging nightmare known as a 'ductopus' that invaded someone's attic.

As a result, you'll want to have your HVAC installer banish any metallic many-tentacled monstrosities currently present from your attic when they put the new system in. If you can't bear to have the ducts dropped into conditioned space, then you'll want to go to an unvented/conditioned attic instead, or at a minimum use a steaming heap of spray foam to encapsulate them thoroughly, although the latter's only practical if the air handler's in a closet, not sleeping in the attic next to the ductopus that's trying to strangle it.

  • Pretty funny response, very descriptive - i'll avoid the ductopus next time i'm up there!
    – Tyler M
    Jun 19 '20 at 1:40
  • OT Question: the "someone's attic" link talks about cooling, can we presume the same applies to heating for those of us in cooler climes?
    – FreeMan
    Jun 19 '20 at 12:34
  • @FreeMan -- houses in heating climates often have basements, or at least crawlspaces, where the heating plant (furnace and ducts) can live. There is some merit to encapsulating/insulating crawlspaces, or building basements as "inside" assemblies, but even a vented crawl is not going to experience quite the wild temperature excursions that a vented attic will. Jun 19 '20 at 22:51
  • The root cellar and crawl space are where my furnace and 90% of my duct work live, so that makes sense.
    – FreeMan
    Jun 20 '20 at 13:28

I am from Pennsylvania and have been retired for many years so let me give you my thoughts on how I would size your system. If you choose the smaller 2 ton system the only time you would notice that it may be undersized would be on extremely hot days when the inside temperature would rise above the set point. I have always thought that a smaller system would be better overall since it would better control the humidity during the milder days of a normal cooling season. On an extremely hot day, the inside temperature may rise slightly but that is a trade-off I could live with. I would ask my neighbors the size of their units and compare the construction of their homes to mine to get a good idea of the needed A/C size. This is an addition to my above statement, Years ago, we would up-size the "A" coil to the next size, say a 3.5 ton coil with a 3 ton A/C unit. This would yield slightly more A/C cooling capacity and would give the coil more surface area to remove more humidity. Now to be honest, I have not sized or installed an A/C unit in many years so I would have to consult an HVAC company to make sure that I am correct in my statements. my 2 cents

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    "have been retired for many years" - retired from what? Did you spend 30 years doing HVAC? If so, this is very relevant and useful background info. If you were a school teacher (kudos to you for that!) it's meaningless. Also, standards, requirements, and even conventional wisdom that applied "many years" ago when you were still working could be totally different today.
    – FreeMan
    Jun 18 '20 at 12:08
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    Check my profile ; started in HVAC and migrated to high and low pressure steam and hot water boilers for commercial and industries almost any fuel. that burned.
    – d.george
    Jun 18 '20 at 22:52
  • Very good info! People don't always think to check profiles (guilty!), so throwing a very brief summary "I retired after 40 years in HVAC" (or similar) lets everyone know right up front what value to put on the info. I could make the same assertion you did and it would carry zero weight - I've been a computer programmer for 20+ years and don't touch my own HVAC system... ;)
    – FreeMan
    Jun 19 '20 at 12:10

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