chscl:

I had a high velocity system added to my 1925 house this spring. It has been wonderful. Based on your room sizes, the number of ducts appear fine. We have 2 or 3 ducts per room (15x15 to 10x10). Overall, I think the HVAC guy said 6 ducts per tonnage. We have a three ton unit, so we have 18 ducts on the 2nd and 3rd floors with one return on each floor.

Other than that, I think you may need to look into insulation and air sealing. We had that done two years ago and our HVAC bills have been the better for it. Finally, your system could be old or oversized, but overising is more for the HVAC board. It too complicated for me to explain. I do remember that bigger is not better for AC.

Schroads

The inspector had mentioned the system is approx 10 years old ...is that considered "old"?
we have brand new windows so think the insulation is better than with the original windows.
I guess we just feel as if we need more ducts in each room so it can cool better ..these ducts are only 2" in diamter!
Can you recommend a site where we may find a tech in our area that is familiar with these systems? We are located in NYC

chscl, do you close the upstairs doors to keep the cool air in? We used to have trouble keeping our bedrooms cool during the day until we realized all the cold air was rushing downstairs. Closing the doors helped a lot.

For comparison, here is a link to some unico (not affiliated) high velocity ac house plans.

Sample 1, for example, is a 3500 sq ft house with a cooling load of about 60000 btuh, and they've used 18 outlets and 3 returns. It seems a bit of a sprawling floor plan, so the number of returns might be more than needed in a more traditional stacked house (we're counting on one return on the first floor servicing the whole thing, but it just depends on the situation).

So, you could look at these as a starting point. There are so many variables in hvac--kind of makes it fun. Our third floor AC was performing terribly the month after we bought our house--until we figured out that the previous owners had set the thermostat on our attic vent fan to 175F!!! We turned it down to 90, attic is now vented regularly, ducts sit in a cooler space--voila! AC works like a champ. I hope your problem has such an easy fix.

You've probably already done this, but you should visually inspect every connection you can see in the system. We're housesitting for our neighbors while they're out of town. I happened to notice that I could see into their basement through a grate in the dining room. Odd, I thought, so I went down to investigate. In the basement, I find 8' of 12" flex return duct coiled in a heap on the floor under the return grate; the whole thing's come loose. I shudder to think what their heat was like if this had gone unnoticed all winter, feeding cold basement air into the system instead of simply boosting warmish return air--yikes!.

These installers are human with deadlines, and I can easily see a rushed connection failing and throwing off the system. I'm not defending it--heck, I work in quality assurance on projects that require high reliability. I just shake my head at what goes on in the "real world" where there is little inspection, oversight, or feedback on most people's daily work. In such a setting, I've learned that (no surprise) mistakes are the norm.

Here is a link that might be useful: High velocity ac plans

Forgot to say, plant some trees in strategic locations. Fast growers (other forums will have good suggestions but tulip poplar and birch come to mind) will start to affect your load calculations in less than 5 years, enhance the beauty and value of your properly (if properly sited), and increase your enjoyment too.

Thanks, we will reinspect every visible connection to make sure there are no leaks. But I guess there isn't anything we could do able ducts in the walls that we can't see.