Single stage vs. 2-stage HVAC system

kerfuffle6September 3, 2012

Building new house. 4800 sq ft. North and south sides of house will be controlled by separate HVAC systems. Contractor has recommended Coleman 5 ton 18 SEER split system with 110,000 BTU (Model AC8B048FC34T18S condenser; Model FC62D3XN15T coil; Model TM8X100C20MP11 furnace) for one side of house and Coleman 4 ton 18 SEER split system with 90,000 BTUs (Model AC8B036FC34T18S condenser; Model FC483XN15T coil; Model TX8X100C20MP11) for the other. I favor upgrading to a two stage system 9T, 9V or 9M-12. I also wonder about upgrading to CP9C at 97.5% efficiency. We live in southern New Mexico, so the winter can be cold but typically isn't very long, so the higher efficiency heating won't likely be recouped in energy savings in any reasonable period of time, but the better air conditioning should. Also the better systems have quieter fans and better components (or so I've read), and the quiet is very important. Any advice would be appreciated. Also, the estimate for the whole job is running in the 24K range -- does this seem like it is in the "reasonable" ballpark?

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Did you contractor do a heating and cooling calculation? Nine tons of cooling and 200,000 BTUs for heating sounds oversized for a new construction home for your climate.

Do you have to use this HVAC contractor? If not are you going to get additional quotes? Does the $24K quote include duct work?

    Bookmark   September 3, 2012 at 7:30PM
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Yes, he did calculations. The quote included the duct work. The house is well insulated with blown-in foam. Cooling is typically a bigger issue than heating in our region -- even in the winter the sun is intense so the windows are low e triple pane. Thoughts regarding single vs. two-stage?

    Bookmark   September 4, 2012 at 12:22AM
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Usually 2 stage has the advantage in humidity removal because you get longer run times by running in the lower stage for most of the time. I would think humidity is not much of an issue for you.

How far are you in construction? The only reason I ask is that your comment about triple pane shows you can learn a little more about things and perhaps make better choices regarding design and energy use. The design of overhangs and variable SHGC depending on orientation would make a huge difference in energy use for a lot lower cost than triple pane or spray foam.

The loads are very high and that is very typical in the industry. It is easy to always use worst case numbers in the calculator and double the loads.

It would be absolutely shocking to have a spray foam house with triple pane windows need 200k btus for 4500 sqft. My climate probably has similar winter (NC) and our similar size house was more like 50k btus and we don't have spray foam or triple pane windows.

The price seems good ballpark to me if it includes ductwork.

    Bookmark   September 4, 2012 at 6:07AM
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Something definitely wrong here with sizing.


    Bookmark   September 4, 2012 at 8:33AM
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Ask to see the calculations. For a house your size there should be a print out several pages long. Using rules of thumb of multiplying the square footage by a number of BTUs is not a calcualtion.

    Bookmark   September 4, 2012 at 9:45AM
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I agree that something is off with the load calcs.
inputs are very specific, for that amount of tons
some of the inputs must be off.

room by room calcs rather than whole house
should be done.
as calcs are from plans make sure that R-values
SHGC, Ufactors, and all information is included
on plans.

remember that the bigger the unit, the shorter
the run times. short run time, little dehumidification.

two stage, variable speed will allow larger units
to run in lower speeds. that may be hvac companies
stragegy. it isn't uncommon. that way they cya
for the few days of extreme temps and lots of
company so that you don't call them out on these
holidays. (fourth of july bbq for instance)

best of luck.

    Bookmark   September 4, 2012 at 9:35PM
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I insisted on a copy of the load calculations and it is many pages long but not easy to interpret. Are there specific items I should look for to determine whether the calculations are reasonable? Humidity here is very low as we are high desert (3300 ft above sea level). Temperature extremes are significant. It was still 96 today but in the 60s tonight. As an aside, overhangs are not part of a traditional southwestern adobe house, which is what we were seeking.

    Bookmark   September 5, 2012 at 12:59AM
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The overhang issue is a big one. That along with orientation can mean the difference between not using a/c or using a lot of a/c (particularly in your climate) - particularly with 96/60 and low humidity.

The unusual thing about that is that traditional construction/design keeps a house cool prior to a/c. I would presume they did it with mass in adobe construction.

High desert is so ideal for mass and solar design.

Manual Js are somewhat complicated but going line by line you can usually make sense of them. There is usually a summary page that includes the Heat gain and losses per item - somthing like 20,000 btuh from walls, 10,000 btuh from windows. There are measurements of walls, roof/floor, window sizes.

You can't ensure that the calcs are correct but you can check the inputs. The dead ringer is infiltration - find that number and see how your house is described. This is the easiest fudge factor in my mind. They often use average - go and read what average is supposed to be - it is not new house average, it is overall average.

The a/c load is possibly correct. Windows to the west can let in a lot of btus. Do you have a wall of windows to the west for a view?

    Bookmark   September 5, 2012 at 6:44AM
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4800 sf? 600k or so? Yay, hvac should cost 16k, not 24k. Now, if contractor wanted 55k than I might wonder. It is not really a big deal in your case.

    Bookmark   September 5, 2012 at 10:18AM
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Joe - I don't think anyone thinks 4% on HVAC is high. And the cost is extremely climate driven.

In his climate, a/c sizing drives the cost. I am guessing in MN, a/c size does not drive the cost.

    Bookmark   September 5, 2012 at 4:11PM
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You are correct about adobe mass maintaining heat in the winter and cool in the summer. The house is situated to allow enjoyment more in the late afternoon and evening Prime views face the mountains on the east and are in the back of the house. Front of house faces west with lots of sky. The windows are smaller and include a couple of bedrooms, laundry room, TV room in front but the western sun in the summer makes it too hot to sit out in the front even in the evening, as the house radiates the absorbed heat back out. Forgot to mention that all duct work is in conditioned space. The comments regarding the specifics of the Manual J are helpful, and I will tackle the reading this weekend. Many thanks.

    Bookmark   September 7, 2012 at 12:09AM
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