AC system died, time to replace

[jimmyswimmy]

Hope I'm not bothering anyone here. I've looked around and haven't found the answer to my question yet so I'll pose it here and hope it interests one of you.

Bought my house 3 years ago, the seller had "installed" a 4 ton (way too big) Fridgidaire AC system (some guys on the side) about 5-6 years ago (unit mfr date 2001). It's never been able to cool the house more than 10 degrees below ambient. Insulation is not up to par but it's not bad enough to account for that. The compressor died and I'm told the price to replace the whole AC system is similar to the price to replace the compressor.

So I'm starting to get quotes. I can put in a 13 SEER Carrier Comfort series AC system for 15% over the cost of the compressor repair, so far. I also have a quote for doign that plus replacing the furnace with a VS motor blower and a two stage burner. I asked for a quote for the 15 SEER Carrier Performance series system. It seems like a no-brainer to do the Performance series unit if I'm thinking about replacing the whole unit, considering rebates and tax incentives (Virginia offers something as well).

I'm basically wondering - how much of a difference in comfort can I expect by spending xx for a new furnace? The VS motor should help even temperatures, right? Which would be nice. Would having a properly sized system make more of a difference than a VS blower? I'm also hoping to reduce humidity. I am not willing to spend the cash to put in the Infinity system, since I'm still toying with the idea of either not having AC for awhile or replacing the compressor in this cruddy system myself.

Any comments or thoughts or anything would be useful. I'm a controls engineer and I am getting frustrated with this business, manufacturers don't provide specs to us regular guys and pricing is hard to get. I'm getting tired of trying to pry information out of these sales guys. It's just hard to make comparisons to come to an informed decision. I appreciate your time in advance.

[RoBoTeq]

The variable speed blower in a new furnace will save you money in electrical usage all year round. The variable speed blower can also be set up to greatly reduce humidity.

With todays advanced technology, it makes no sense to not get all of the benefits from a total system rather then just parts of a decent system connected to technology that cannot take advantage of the benefits of the new.

Otherwise; just go with a standard entry level 13 SEER system and save the bucks.

[BaldLoonie]

The compressor should be in warranty so I'm not sure how you can get a new system for 15% more???

Has anybody explained why the oversized A/C won't cool the house?

[jimmyswimmy]

Okay, I haven't seen anybody doing heat load calcs yet, so I don't know if these number are accurate. I was told the system is oversized by multiple vendors, including one who said they did the calc. I have a 4 ton unit and ought to have a 2 or 2.5 ton, depending on who's talking. Similar houses (orientation, design, shading, etc.) have 2 and 2.5 ton units in my neighborhood.

Nobody's explained to me exactly why an oversized unit doesn't cool the house. I would love to know why that would be the case (I'm an electronics guy, I understand the controls stuff but the fluids/air side of HVAC is a mystery to me). I've run four or five vendors through the house and haven't gotten an answer yet. It's a 60's era house with most outlet vents low on the walls and two intakes, one in the basement about 8 feet from the unit and one on the ceiling on the main floor (one story house with a basement) which is not that far away but has a terrible long high-resistance run to the unit.

The compressor is supposed to be in warranty but the only guy who's given me a quote on replacing it said it would be about 3k, mostly in labor. The really fun part for me is that the unit is supposedly not covered by warranty since it was not professionally installed, it was installed by a bunch of hacks worse than me.

Anyway thanks for the good words about the variable speed blowers, RoBoTeq, that helps. Can someone point me towards a good explanation of what that Honeywell IAQ thing really does? I've seen it referenced on here a couple times and it sounds pretty impressive; is it a drop-in replacement for a thermostat/control system?

Thanks much!

[smokin68]

Get the HvacCalc off this site and run your own load calc. The only reason a 4 ton doesn't cool a space that requires 2.5 tons is it's malfuntioning. If you know your system was installed by hacks, start from scratch.

[deq1269]

Get the HvacCalc off this site and run your own load calc. The only reason a 4 ton doesn't cool a space that requires 2.5 tons is it's malfuntioning. If you know your system was installed by hacks, start from scratch.

An oversized unit may not cool the house because its not removing humidity. It may lower the temperature but it would still feel humid in the house thus not providing comfort. The purpose of an air conditioner is to lower the temperature and remove humidity. If its too big it wont remove humidity.

[pstu]

An oversized unit may not cool the house because its not removing humidity. It may lower the temperature but it would still feel humid in the house thus not providing comfort. The purpose of an air conditioner is to lower the temperature and remove humidity. If its too big it wont remove humidity.
True but that would not explain the OP statement that the old system would never cool more than 10 degrees below ambient.

I totally agree with the recommendation to get HVAC-CALC licensed for $XX, and get a heat load model done. I'm a homeowner in S.Texas and have done so, it will only require a couple hours work measuring things and keying in numbers.

Best of luck -- Pstu

[tpa-fl]

I definitely want to echo the comment that you can't judge your cooling need based upon the performance of your existing system. I didn't realize how poor my old 2 ton heat pump's performance was until the 2 temporary window ACs I used could easily freeze the place out (total capacity, a hair less than 1 ton). Now my new 2-stage system is installed (1 ton/2 ton staging) and sure enough, 1 ton of cooling is all this place needs most of the time.

[ckr74]

An oversized unit will cool your house. You could put a 5 ton unit in it and it will cool just fine. Problem is that it won't remove much humidity because of short cycles. Unit needs to be sized for your home.