New AC Unit Questions

[shawn]

Usually on pretty tight construction you can do 400sqft/ton and be great....given all the variables are NORMAL. Older or potentially less than stellar conditions 325.

That's a normal "out of your ass rule of thumb" for commercial ballpark estimates, where you can assume leakage, minimal insulation and typical heat transfer

 

[CasterTroy]

How are those installed in a wall by the way; do you need a full length (length of the slot diffuser) header for a standard wall to accommodate the duct boot unless it's a non load bearing interior wall? They're usually a couple of feet long, so you're spanning at least a few stud bays.

Sidewall....sort of. It has to be framed like a window. The plenum on a slot diffuser is the length of the grille..

The above is a ceiling mounted slot

 

[CasterTroy]

Then how else do you account for the heat loss/gain via the stair?

There is an assumed leakage. Ideally if both units are functioning at setpoint on design day, filters are fresh, and the system is properly ballanced, there is minimal heat migration. (again, you design based on COOLING, not heating. Heating MORE than takes care of itself if cooling is satisfied)

But of course the low static solid ductwork has been VE'd with yards of flex duct, the system most certainly wasn't balanced, and diffusers were placed where they LOOKED best and not where they functioned correctly. Then, add furniture, drapes.....a slew of other factors and now you have massive heat migration to the upper floor. (this is where the experience of a good HVAC contractor trumps our design efforts)

But, no, the upper unit does't assume the "other half of the work" in reverse conditions.

 

[jeepinmatt]

but usually I pull a number out of my ass, then spend 2 days modelling a space and come within a 1/4 ton of what I ballparked it at

Sounds like my job. Throw out a rough guess, spend 2 weeks modeling part setup, tooling, machining processes, labor, etc in deep detail, and end up within 5% of my initial guess when we only needed plus or minus 20% accuracy anyway...but managers love to see a big stack of calculations that they don't understand.

 

[CasterTroy]

but owners love to see a big stack of calculations that they don't understand.

FTFY

Now you can come work for me, since you really seem to understand 99% of my job. The other 1% is arguing with architects over ridiculous fees where they EXPECT 10,000 hrs of your time, but want to PAY for 122hrs of fee

 

[Fabrik8]

Sidewall....sort of. It has to be framed like a window. The plenum on a slot diffuser is the length of the grille..
View attachment 250908
The above is a ceiling mounted slot

Yep, that's exactly what I was thinking then, header the length of the plenum box. No free lunch when it comes to adding something like that. I'm not sure if I'd want to stick that in a sidewall anyway, because it's a huge hole the the envelope insulation at that depth.
I'm going to research those ductless units though, I'm not afraid of tearing up some drywall to do cool stuff.

 

[Cherokeekid88]

A lot of what you guys are talking about is going through one ear and out the other, but what I did gather is I just need to have someone come out and do an estimate. I have been using the same AC guys for the past couple years and they have always done me right, but none of them seem to hve any personality and never really explain things to me.
Do most AC companies charge a service call fee to come out and do an estimate?

 

[obullfish]

Most around here will do free estimates

 

[rockcity]

Don't hire someone that charges for an estimate.

 

[awheelterd]

Yeah if you see their ad on tv, don't call them. Talk to your neighbors and find the local guys. The big companies will rake you over the coals.

 

[rodney eppes]

I had gotten 3 estimates to start with. Two local companies, which 1 came out & did a "walk-thru''. The other based his estimate on what he knew of the house & the current unit. Third estimate I got just for comparison, & it was a TV add Company [$10,000]. Then a NC member recommended a company. That Company did everything Proper, & I got a middle of the line unit for $7k. So Glad I called them!

 

[336wheeler]

That's not at all correct. Loads are calculated on cooling. The heating phase (heat pump) is grossly oversized if the cooling load is designed correctly

Envelope loads are based on delta T from desired space temp. Equipment sizing, is based on the bigger load - which may not always be cooling. Sure, latent load adds a substantial amount of coil capacity to a sensible cooling load. But heating load calcs generally discard dehumidification. In climate zones 5+, heating is generally the majority load. Unless the space has large internal loads, like a data center or other heat generating equipment.

500-600 SF/ton is a decent ROT for residential... 400 SF/ton is a bit on the oversized side of things. Most contractors use a dumbed down version of Manual J or TRACE to expand their repertoire beyond rule of thumb.

 

[CasterTroy]

Typically, yes. But I'm pretty sure no one on here is running a hot deck cold deck data center, or a surgical suite on the 3rd floor of their home on top of mount Mitchell and worrying about satisfying ASHRAE 90.1

 

[shawn]

So, anyway. It sounds like we're back to the basic assumption that there's probably nothing wrong with the unit that's installed on the house, but there probably is a problem with the design. And fixing the design is going to mean ripping big holes in the walls to replace/resize/damper ductwork.

Are there balance dampers anywhere in the system? Have you checked everywhere for filters? If you have regular 1" pleated filters at the returns, make sure there isn't also a filter at the return side of the unit.

 

[336wheeler]

To the OP... consider something like a 1-1.5 ton ductless mini split for your upstairs and a 2 ton split system with AHU for your existing downstairs ductwork. Assuming your ductwork is reasonable.

Check for holes or breaks in the ducts along the runs, sharp turns that would restrict airflow, and duct leakage where the ducts connect to your AHU, registers, or where the branch ducts connect to the trunk duct. And mastic/duct tape them sumbitches up. If you have sharp bends that restrict airflow, consider replacing that section. Help it help you.