Should I get rid of unused central AC equip when replacing furnace?

Discussion in 'HVAC Heating & Cooling' started by tracyballard, Dec 16, 2019.

  1. tracyballard

    tracyballard Member

    Feb 3, 2015
    It looks like I'm about to have to replace our forced air updraft furnace pretty soon (installed in central utility closet, slab floor single level home). This house had a central AC system when we moved in, and it is part of the furnace, the AC coil sits right over the furnace, but the AC system has been non-functional for 6 or 7 years and we use window AC's. To replace this furnace unit should I just get another similar unit and leave everything else in place, or should I get rid of all this unused central AC stuff? still have a unit out in the backyard and all lines in addition to the coil over the furnace, no freon in them though. My furnace is an 80% unit and I plan to replace with the same. I would try to fix up the current furnace, but it looks like it needs a new inducer fan, and the big air circulation fan at the bottom is so corroded that it's not really balanced that well anymore. Any info will be highly appreciated!
  2. Dana

    Dana In the trades

    Jan 14, 2009
    Furnace replacement time is an opporunity moment for right-sizing the systems. If your house is like 9 out of 10 in the US the furnace is more than 3x oversized for the design condition load, and the AC is more than 2x, all of which has a strongly negative effect on comfort. Bigger is NOT better- oversizing is the enemy of comfort & efficiency.

    If by some freak coincidence yours is one of the 10% of houses with reasonably sized AC, and if the non-functional system was running on R410A refrigerant (rather than R22) you may be better off from a comfort point of view buying a right-sized 2 stage or modulating heat pump system compatible with the existing coil, and skipping the gas furnace entirely, despite probably having a somewhat higher operating cost than the gas burner (TBD- gas and electricity rates vary widely.) In much of Texas the cooling loads are higher than the heating load, but not all locations. A heat pump right-sized for a larger cooling load can still deliver good efficiency on a smaller heat load if it's a 2 stager modulating.

    If the heat load is small enough it's sometimes more comfortable and efficient to run a right-sized hydro-air coil off the water heater. FirstCo has a wide range of hydro-air units, many come with a cooling coil that could be optimized.

    So, how many BTU is the furnace, and how many tons of AC, and how big is this house? Are the ducts above the insulation in the attic, or below he insulation (either buried, or in soffits below ceiling level)?

    If you're in a cooler climate zone 4 part of TX (the panhandle) or north or west of the middle of zone 3 (in brown below) it's reasonable to estimate the heat load using wintertime gas bills against local heating degree-days. The method for how to go about that is detailed here.


    In the cool half of zone 4 heating loads are usually higher than cooling loads, and there is usually a consistent enough heating load in the January/February for reasonable accuracy using those methods.

    Otherwise, run a Manual-J type heating and cooling load calculation using freebie online tools, and aggressive (rather than conservative) assumptions about R-values, air tightness, etc, taking into account any factor that might lower the load numbers (as per the instructions in ACCA Manual -J.) One reasonable Manual-J-ish online tool that's pretty easy to use is LoadCalc. Even when using aggressive inputs that tool tends to oversize by 20-25% or more, so be extra-aggressive on air-leakage (select for the tightest possible construction & ducts), and reduce the ventilation rate assumption to zero and it's usually reasonably close- close enough for sizing a furnace anyway. Another true Manual-J registered freebie online tool is CoolCalc. While not as flexible as professional tools, if used with properly aggressive assumptions it's pretty good with most standard construction types.

    As much as I detest seat of the pants rules of thumb for sizing equipment in most of the cool-half of Texas a heating load of 10BTU/hr per square foot is usually about right ( for houses that have glass in the windows and insulation in the walls and attic, and a ton of cooling per 1000' is usually more than adequate capacity, and often oversized, despite commonly used rules of thumb that many HVAC contractors use. But there is a wide range of actual load ratios. One HVAC consulting company in Decatur GA compiled this graphic of Manual-J cooling loads plotted against house size of a few dozen load calculations they did for clients (most of them in the Gulf Coast states):


    Note that only the worst performing houses came in under 1000 square feet per ton, with a handful of "normal" sized house north of 1500' per ton, yet there are still HVAC contractors in the Gulf Coast states (or even New England) insisting that 500' or 750' per ton is the "right" rule of thumb to use. In the more humid parts of Texas (about everything east of San Antonio, drawing a line north to south) a 2x or larger oversize factor usually results in poor latent load (humidity) control during most of the cooling season.
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  4. WorthFlorida

    WorthFlorida The wife is still training me.

    Oct 28, 2009
    Orlando, Florida
    As far as the A/C for your home, if the duct work was designed for heating only, most likely it is undersized for today's high efficient heat pump systems. Honest techs will tell you if the duct work is sufficient.

    Most HVAC techs don't want to use unused old lines without at least doing a flush with nitrogen and a long vacuum test but it takes time and money. Copper lines do not last forever and some techs will use it to save money but if there is ever a leak, you'd be paying twice.

    When I worked two bids for a new AC/heat pump unit for my sons house, the first tech said we'll use the old lines (it was an old R22 system) and stated he does it all the time. The second company didn't think twice and included new lines and the bid was $800 less. Warranty work cost a lot of money if the old lines blew on the original installation.
    Last edited: Dec 18, 2019
  5. Dana

    Dana In the trades

    Jan 14, 2009
    If the heating-only system was 3-4x or more oversized for the actual loads (possible, even likely), it may be just fine with a right sized high efficiency heat pump or ducted mini-split running lower cfm.

    Finding honest techs who actually perform proper load calculations seems like a harder problem.

    That sounds right. I'd be suspicious of reusing much of the system if it had a lot of years on it before crapping out, even if it was an R410A coil, and somehow magically right-sized, but it would all have to be checked out and properly flushed & tested first. Re-using the lines & air handler coils of an R22 system with a shiny new R410A condenser/compressor seems pretty sketchy. I know some people still do it, but I don't know why.
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