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HVAC sizing

Andrew Bennett | Posted in Mechanicals on

I purchased a home in 2015 and in 2016 got a price on a 2.5 tonne split HVAC system.  Because of a lack of money I wasn’t able to move ahead with replacement.  Just recently I had the same firm come out and re-quote the system. 

We found the email from 2016 where they quoted a 2.5 tonne Amana system and they suggested that I go with the Goodman brand since it’s essentially the same, but would be less expensive bring the quote in very near the 2016 level.

Later in the day the guy calls back and says, “Hey we took a photo of the plate on the outside unit and it’s a 3.5 tonne not a 2.5 tonne.  If you want to downsize to a 3.0 that would probably be okay, but we don’t think a 2.5 tonne is sufficient.  Especially on the heating unit they are recommending that I  stay with the current sized unit as the ductwork is sized for the bigger blower motor of a 3.5.”  The ductwork in the garage and at the returns is ductboard.  I am not sure what it is under the house.  

Now…more detail.  The home is a two story with only a single thermostat downstairs.  We also want to install a second thermostat upstairs and put in some dampers to give us somewhat of a zoned system.  

The house is about 1800 sq ft with more sq footage on the 2nd floor as there is a room over the garage.  

Currently during the summer we shut quite a few of the downstairs vents to get better cooling upstairs.  If we have a dampered system (BTW redoing all the ductwork just isn’nt the budget) then in the summer when we need more cooling upstairs it seems logical to me that a 3.5 or 3 tonne unit would just be way the heck too much for a an upstairs that is maybe 1000 sq ft.

Finally, the current system cycles way too often.  I’ve gotten used to it, and it’s not summer right now, but from memory I think it cycles on and off multiple times within a single hour during the summer.  

My thought is that the advice is going to be have someone do a manual J, but I’m wondering if I can find someone who will do a manual J in Knoxville, TN.  The firm I am using is the firm that did the HVAC for  some experimental homes for the lab in Oak Ridge (course the engineers in Oak Ridge probably specified what they wanted to see in the system).  

This firm in the past (at a house I built back in 2009) did do a manual J, but the owner (who I am pretty sure is the one who did it) is now deceased.  

I know I spoke with the owner back in 2016 about the sizing, but I don’t remember details.  Anyway…I don’t even know what I’m asking.  Does it sound like a 2.5 tonne unit is enough?  Answer…manual J

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Replies

  1. Doug McEvers | | #1

    As I remember, a zoned forced air system has to be in the original design and duct layout. Trunk lines are separate for each zone and a motorized damper directs the heating and cooling depending on the zone calling for it.

  2. Expert Member
    Dana Dorsett | | #2

    Measure the duty cycle on the existing system on days when it's close to the 99% &/or 1% outside design temperature, or do an analysis of it's duty cycle across a range out outdoor temperatures. That will MEASURE the heating/cooling loads empirically rather than estimating it based on construction & shading factors (as with Manual-J). For tips on how to go about it:

    https://www.greenbuildingadvisor.com/article/how-to-tell-if-your-air-conditioner-is-oversized

    https://www.greenbuildingadvisor.com/question/ways-to-graph-air-conditioning-run-times

    From the description of the summertime cycling behavior it's likely to be QUITE oversized, and even a 2.5 tonner might be oversized for the cooling load. A typical decently built reasonbly insulated 1800-2500' home would have a design cooling load less than 2 tons. Check out this graphic plotting house size against square feet per ton on a few dozen Manual-J's performed by Energy Vanguard in Decatur GA:

    https://www.energyvanguard.com/sites/default/files/styles/panopoly_image_original/public/square-feet-per-ton-air-conditioner-sizing.png?itok=vsJxOobH

    Only a the handful of worst performing homes either side of 2000 square feet would call for a 2.5 tonner.

    >"Especially on the heating unit they are recommending that I stay with the current sized unit as the ductwork is sized for the bigger blower motor of a 3.5.” "

    Tell us more about the heating unit. (I suspect it's ludicrously oversized for the load- most are.)

    Oversized ducts are fine for lower cfm air handlers, but if you have a single-stage gas-burner or something that needs higher CFM than could be delivered with the ducts as-is there may be solutions other than a completely new duct system to get you there. Mapping the static pressures on the system with a dual-port manometer with the current air handler would be useful for figuring this out too.

    Oversizing is the enemy of comfort, and to some degree efficiency too. Take the time to read/watch Nate Adams' take on the subject. Even though his discussions are primarily addressing heating comfort issue, the principles are the same for AC. When ridiculously oversized the room over the garage is likely to see really big temperature swings, and not track well with the rest of the zone. When perfectly right-sized it's usually fine. These are Nate's primer-pages:

    http://www.natethehousewhisperer.com/home-comfort-101.html

    http://www.natethehousewhisperer.com/hvac-101.html

    http://www.natethehousewhisperer.com/hvac-102.html

  3. Andrew Bennett | | #3

    Attached are some pics of the heat part of the unit. The labels I could find were so old they were unreadable.

  4. Andrew Bennett | | #4

    I feel much more comfortable with the 2.5 tonne unit after seeing the Allison Bailes data. I also spent some time on Nate's site and sent him a message to see if he knows anyone legit in the Knoxville, TN area.

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