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!
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.