Looking to upgrade the AC system

[ChosenGSR]

I have a 97' built 3 story (all 3 above ground), 2100sqft townhouse with a single AC system. The top level of the house has cathedral ceilings (no attic). The air ducts and all but one return are all flex pipe. The unit inside (gas furnace) is a York Diamond 90 (P3URD12N07501A), the unit outside has a model number of H1RC030S06A.

The main problem that we're experiencing is that the unit is not able to cool off or even maintain temperature when it gets over 90 degrees (Maryland). In the summer I have the air distribution (adjustments at the ducts that sit on top of the unit, that split the flow between levels) set in such way that the first two levels are 20% open, while top level is 100% open. When temperatures are around 80-85 the AC unit is able to maintain 78 degrees, turning on and off as necessary. When the temperature is 95-100 (we had a week of 100 degree days), the AC unit will turn on in the morning and not turn off until 3-4AM at night. During the day it easily gets to 80 on the main level, and 84 on the top level.

If my temperature reading are correct (on a 95 degree day), the air coming out of the vents is anywhere from 68 degrees on the registers that are the closest to the main duct, to 72-73 in the top level room furthest from the main duct. Aside from temperature I would say the air flow is marginal at best, but I really don't know how to quantify this.

My understanding is that I have a 2.5 ton unit outside, as far as inside I have no idea, I am assuming it should match up as it's the original that was put in by the builder. In comparison my neighbor (townhouse next to mine) has a 3.5 ton unit, their townhouse is 500sqft larger. It seems to me that my unit is undersized based on what I've been reading online. Assuming 700sqft per ton I it should be at least a 3. Considering that it's a very tall townhouse (40ft to the roof, above ground) and cathedral ceilings (so much more heat is absorbed on the top level).

As far as the temperature readings I provided earlier, they seem to be 12-10 degrees below room temperature, which as far as I understand should be more in the 15 to 20 below range. I also don't know if there is simply enough air flow from the blower. I am assuming a larger unit also means a larger blower? The bottom line is I need to remedy the situation, and looking for some seasoned opinions from those in the know.

Thank you!

Edit: I would like to also note that the system has been serviced, the coil cleaned, the condenser cleaned and topped off with freon, all within the last year.

[luskys a/c]

Who ever "Topped" off your system needs to look at his readings again and see what his superheat and subcooling were . There is a reason why your Coil temp. td is so low.

[hcong]

Low airflow could be the cause of your house not cooling enough, with 3 floors and so many dampers, your blower might not be able to get enough airflow or there are leaky ducts.

Your supply air temperature coming out of your registers are too high. Likely a system problem, duct leakage, or ducts not insulated well enough.

Depending on your house insulation and windows... you might have a undersized unit... you should have someone do a load calculation.

[dan sw fl]

I have a 97' built 3 story (all 3 above ground), 2100sqft townhouse with a single AC system.

The main problem that we're experiencing is that the unit is not able to cool off or even maintain temperature when it gets over 90 degrees (Maryland).

In the summer I have the air distribution (adjustments at the ducts that sit on top of the unit, that split the flow between levels) set in such way that the first two levels are 20% open, while top level is 100% open. When temperatures are around 80-85 the AC unit is able to maintain 78 degrees, turning on and off as necessary.

When the temperature is 95-100 (we had a week of 100 degree days), the AC unit will turn on in the morning and not turn off until 3-4AM at night. During the day it easily gets to 80 on the main level, and 84 on the top level.

Aside from temperature I would say the air flow is marginal at best, but I really don't know how to quantify this.

I suspect that the heat gains are upto ~25,000 BTU/hr which should be handled by a 2.5 ton. The cooling issue is more about air distribution, TOTAL air flow and Performance than the unit size.

20% open on the first two levels is NOT appropriate - it significantly reduces the TOTAL air flow AND UNIT CAPACITY. Try 65% open. If that is not meaningful, the charge (superheat / subcool) needs to be re-checked.

At < 94'F outside, the inside air temperature should be < ~ 76'F / 78'F top.
__ > 99'F ___________________________ might be 80'F for upto 8 hours.

[ChosenGSR]

I suspect that the heat gains are about 25,000 BTU/hr which should be handled by a 2.5 ton. The cooling issue is more about air distribution, TOTAL air flow and Performance than the unit size.

20% open on the first two levels is NOT appropriate - it significantly reduces the TOTAL air flow AND UNIT CAPACITY. Try 65% open. If that is not meaningful, the charge (superheat / subcool) needs to be re-checked.

At < 94'F outside, the inside air temperature should be < ~ 76'F / 78'F top.
__ > 99'F ___________________________ might be 80'F for upto 8 hours.

If I do 50% flow to the first two levels, there is noticeably less flow going to the top floor, which ends up being even hotter. The top (3rd) floor of course is where the bedrooms are. Assuming that there are no leaks in the ducts (how does one check this?) could this be as simple as the blower being undersized for the needs of the house?

[ChosenGSR]

Low airflow could be the cause of your house not cooling enough, with 3 floors and so many dampers, your blower might not be able to get enough airflow or there are leaky ducts.

Your supply air temperature coming out of your registers are too high. Likely a system problem, duct leakage, or ducts not insulated well enough.

Depending on your house insulation and windows... you might have a undersized unit... you should have someone do a load calculation.

I don't know how to be certain that there are no leaky ducts, but I do know that depending on how I do the flow distribution between floors it most certainly has an effect on air flow from all registers on that floor. I suppose this means that there isn't a complete tear in the flex somewhere, but wouldn't eliminate a partial. The house is relatively new though. As far as insulation, I am not sure what can be done there without tearing up the house

If I remember correctly when the unit was serviced the temperature was checked at the evaporator coil, and it was reading 59 degrees. Then like I said most registers were registering 68, except for that one room on the top floor furthest from the supply. A 10 degree drop from the evaporator coil to the registers, is that out of line? Besides the discussed insulation of the ducts, could having less "flow velocity" as a result of a smaller blower be the reason for that kind of drop off in temperature?

[BaldLoonie]

Sometimes going bigger in a multistory just makes the lower levels cool faster and the upstairs hotter. A small unit for the 3rd floor or a mini-split up there maybe a better solution. Something to consider.

[beenthere]

If I do 50% flow to the first two levels, there is noticeably less flow going to the top floor, which ends up being even hotter. The top (3rd) floor of course is where the bedrooms are. Assuming that there are no leaks in the ducts (how does one check this?) could this be as simple as the blower being undersized for the needs of the house?

More like the duct work is under sized for the need of the house and system.

[ChosenGSR]

More like the duct work is under sized for the need of the house and system.

While it's very possible that the design is not optimal, it's a row of townhouses built at the same time by the same builder using the same materials and design. My neighbors say their AC systems are able to keep up.

[Doctorinky]

Sometimes going bigger in a multistory just makes the lower levels cool faster and the upstairs hotter. A small unit for the 3rd floor or a mini-split up there maybe a better solution. Something to consider.

Yeah, that would be the best solution. 2nd best solution would probably be a zoning system.

[dan sw fl]

I don't know how to be certain that there are no leaky ducts, but I do know that depending on how I do the flow distribution between floors it most certainly has an effect on air flow from all registers on that floor.

If I remember correctly when the unit was serviced the temperature was checked at the evaporator coil, and it was reading 59 degrees. Then like I said most registers were registering 68, except for that one room on the top floor furthest from the supply. A 10 degree drop from the evaporator coil to the registers, is that out of line?

9'F increase (59 TO 68) in Air Temperature in the duct seems to be 50+% OF THE UNIT CAPACITY.

Need a duct leakage test. http://www.energyconservatory.com/ap...lications1.htm

[ChosenGSR]

9'F increase (59 TO 68) in Air Temperature in the duct seems to be 50+% OF THE UNIT CAPACITY.

Need a duct leakage test. http://www.energyconservatory.com/ap...lications1.htm

I am now certain that the issue is duct related. Today I measured the temperature right above the evap. coil and it was a cool 57 degrees. Sadly even the nearest ducts in the basement were only registering 67, and the ones on other levels were closer to 70. Temperature is getting lost in the ducts. I have scheduled an appointment for an energy audit with what seems to be a reputable company in the area, I am terrified of what might come out of this

Everything I see online talks about insulting/repairing duct work that is clearly visible or reachable. Sadly in my townhouse the duct work is not accessible as it runs between the first two levels, and right under the roof. (there is no attic like I stated earlier, these are cathedral ceilings). I can't imagine having to tear up all the ceilings int he house to fix this, seems unrealistic.