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Please help me pick the best option for HVAC replacement

hifiaudio2 | Posted in Mechanicals on

We moved into this 20 year old house here in Nashville, TN in June and have been doing various projects to bring the home up to and above current building best practices. We have put open cell foam in the attic (average ~8 inches with rafters covered) and have encapsulated and foamed the crawl with 1.5 in of closed cell and 12 mil vapor barrier. We replaced the upstairs HVAC with a new 5 ton Trane xr17 and S9v2 furnace.

With all of those items complete… I am running out of funds for improvements! BUT.. we “have” to replace one of the two existing downstairs HVAC package units because one of them is 20 years old and apparently has a cracked heat exchanger, so I am told I will have no heat at all from that unit this winter. So I have come up with a few options and bids to fix this issue, and want to get input from the community here please.

1) Simply fix / replace the heat exchanger only on this 20 year old package unit. I am advised against this by the local HVAC contractors as being a waste of money long term, and this option is around $1900 I am told.

2) Replace ONLY that broken package unit with a new, cheap as I can get unit. Would be probably a 2 ton unit. I am told that would run around $6k. This sounds really high to me, though.

3) Remove the current old unit, and rework and replace most of the existing ductwork in the crawlspace (which I want to replace anyway since I believe the old ductwork is nasty inside and seems to be a place where musty smells come from. I smell the smells sometimes in the house, but NOT in the crawlspace, leading me to believe they come from the ductwork). I would then hook up ONLY the other existing ~4 year old 3.5 ton Everest model unit to the downstairs and let it be the only HVAC unit for the downstairs. This may be a bit undersized, but I think together with all the insulating work I did plus the hefty upstairs unit possibly being a bit oversized in my opinion, this is an option that intrigues me. The current two downstairs units were installed in an odd way where the old unit we are discussing replacing was serving half of the downstairs as well as about 800 sq feet of the upstairs. That upstairs load has already been removed from that downstairs unit’s duties. So the result of this would be the current 3.5 ton unit serving the roughly 3200 sq feet of downstairs. I would ask that they do this new ductwork in such a way as to allow the #4 option below to basically be a drop in installation should this scenario prove to be asking too much of the 3.5 ton unit. This reworking / replacement of the ductwork would be about $2k I think.

4) Completely replace the current downstairs units and all ductwork with the exact same 5 ton Trane xr17 and S9V2 furnace in a pancake unit in the crawlspace. This is what I intended to do in the first place, but since funds are limited I now wanted to go through these other options before biting this bullet. $13,500 for this including the whole house air filter / UV light / two zone system.

Thanks for any help on this and my other questions in the last month!!

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  1. GBA Editor
    Martin Holladay | | #1

    I doubt whether anyone will be able to provide advice over the internet -- a lot depends on details that would require a site visit to ascertain.

    My first reaction is that your equipment is oversized. You have a 5-ton unit downstairs and a 5-ton unit upstairs -- in other words, a total of 10 tons? That's a lot.

    So, as usual, the first question is: Have you hired anyone to perform a careful room-by-room Manual J calculation to determine your design heating loads and design cooling loads?

  2. hifiaudio2 | | #2

    I have not paid for that manual J yet, no. The one BPI / Resnet company that came out (he spent about a total of 3 hours here) was actually concerned that 10 total tons was not ENOUGH... but I thought that seemed crazy. I think he wanted around $400 to perform the manual J. Is that in line with normal charges for something like that?

    I guess as far as the other questions... are any of those options just "ridiculous" on the surface? In other words is it clearly a waste to just repair a heat exchanger on a 20 year old inefficient unit? Or is it absurd to think a 3.5 ton unit could handle the load of a 3200 sq ft main floor with volume ceilings, etc?

  3. Expert Member
    Dana Dorsett | | #3

    This has to be one gia-normous house for the upper floor to have loads that would call for 5 tons of AC and a big gas-burner! Ten tons of AC for a 2- story house would usually be about right for a 12,000-14,000 square footer (6000-7000 square feet downstairs, 6000-7000 square feet up.) Sounds like your house is more like half that size(?).

    Does this house have huge "sunset view" west facing windows or something?

    ".. the result of this would be the current 3.5 ton unit serving the roughly 3200 sq feet of downstairs."

    That would be a more realistic sizing ratio, maybe even a bit oversized for the true load, unless there is some factor like big high-gain windows catching the afternoon sun. Take a look at the tonnage to square feet ratio's in the info-graphics in this article, and read the rationale:

    You'll have to sign up for the 10 day trial to see it, but you'll see just what an outlier your house might be. Most houses built with glass in the windows, doors that shut, and insulation in the walls would have first floor cooling loads of less than a ton per 1000 feet, though there are some that come in a bit heavier than that. The infographic can be seen in a direct link (without the discussion) here:

    The average in Allison Bailes' graphic is about a ton per 1400', and includes both upper and lower floors. Most of those homes are (like yours) in the southeastern US. Lower floors with conditioned space above typically have a smaller tons per conditioned square feet ratio, since lower floors generally have better shading factors, and don't have a hot attic above them. A ton for every 1500' or more would be typical for lower levels. That makes 3.5 tons for 3200' is about 1.1 tons per 1000' (or a 915' per ton) on the high side for a lower level of a 2-story, but it's probably not 2x oversized (though it could be.) An aggressive and careful Manual-J would be able to determine that.

    Charging $400 for a Manual-J would be on the cheap side in my neighborhood, but if the person doing the calculation is nervous that 10 tons might not be enough they are prone to skewing it (intentionally or otherwise) toward the high side. Most use software packages with pull-down menus for selecting contstruction type for making these calculations, and it's very easy to end up with a "garbage in = garbage out" type numbers when they are selectively guessing (consciously or otherwise) to the conservative side. That is more common error to find with HVAC contractors than with RESNET raters, but none of us mere humans are immune.

    Some crusty old schoolers would look at a 6400' house, squint at it with an upheld thumb and say "A ton per 500', in to 6400 square feet comes to about 13 tons, let's make it 14 just to be sure, or maybe 15.". Their now middle-aged children seem stuck on a ton per 750' for newer house which would put a 6400' house in the 8.5-9 ton range. But real loads are nowhere near that high.

  4. hifiaudio2 | | #4

    Thanks Dana! That is exactly the info I needed. (and by the way, while I have very tall 20 ft volume ceiling and and a catwalk above in the middle of my house, I have no west or east facing windows).

    And you are right about the HVAC guys not "buying" this line of thinking at all. Here is the response I got when I tried to talk to one of the companies that I have done business with about just changing the duct work to allow the single existing 3.5 ton unit to serve the main floor:

    "It's not accurate. Call around and you will see. That would mean a 1400 sq ft house would need a 1 ton unit. They don't even make a one ton unit. You are wasting your time."


    "If the energy guys you are reading from online are correct then the TVA (tennessee valley authority that this contractor does Energy Right work for) is wrong. I think I would put my money on them."

    I think I will just get one of these guys to change the duct work for me and go from there. I would certainly rather spend $2k on new ductwork than $13.5k on a new unit right now.

  5. Expert Member
    Dana Dorsett | | #5

    Changing ductwork around isn't always simple or practical and for high volume/low load spaces you may need higher cfm air handlers than come in a package unit to achieve optimal distribution. But in terms of raw compressor output you can probably get there with what you have.

    As a general rule the smaller the house, the greater the ratio of exterior surface area to floor space, which results in a higher BTU/square foot ratio for both heating & cooling. If you look at Bailes' infographic, of the 8 sample houses between 4000 and 8000 square feet only two anywhere near a ton per 1000'. The rest are in the ton per 1500' range or higher, some much higher. The cluster of seven between ~750-1000 square feet per ton (that would then be covered using the ton per 750' rule of thumb) are all under 3200 square feet, and only one ( ~1500') house would need more than a ton per 750'.

    Even so, regarding the contractor's comments, there are plenty of 1400' houses that can be heated & cooled with a 1-ton ducted mini-split, (about half the houses in that range in Bailes' sample set), though many or even most houses that small would be looking at 1.5 tons (about a ton per 1000'.) They probably don't make package units as small as1 ton, but so what? There are plenty of full sized air-handler heat pump options in the 1.5 ton range.

  6. hifiaudio2 | | #6

    Well the install of the new 5 ton unit starts today. You guys arent kidding about contractors not buying into the tons per sq ft ratios above. The same contractor that put in my upstairs Trane x17 and s9v2 is putting the exact same unit in my crawlspace, and he was basically freaking out saying even the 5 ton is not enough... he wants 7-8 ton. He says it will run "all the time" in the summer and may not be enough, but I told him to just go with it. Its a two zone unit, and he wants to size one of the zones so it can take all 5 tons if needed (so that a 2nd two ton unit can be added later to the ductwork they install if needed), and that leaves the bedroom zone on a 2 ton max I guess, if I understand how this works. But in any case, the bedroom suite / office was only on a 3 ton standalone unit anyway, so this should be fine. Should I NOT let him zone it that way, or make him simply zone it evenly to split the sq footage? They started work but it will be a couple of days before the actual unit install I think. They are removing the old unit and then ductwork today.

    I am sure I wont be happy with how dirty my nice encapsulated crawl gets after all this work in there! :(

    EDIT: nevermind on the "5 ton to one side" question above... I asked the installers and there is no difference in whatever that meant and simply zoning both systems as normal. The tonnage to each side would be dictated by the number of supplies anyway. The owner / head guy was just making it sound different I guess...

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