Plan for adding more rooms to existing A/C

[75gmck25]

Does anyone have a pointer to a web site or video that explains the best way to measure air flow and plan for adding more rooms to an A/C system? I would like to rent the air flow measurement equipment, if possible.

My A/C currently conditions about 1250 sq ft, and I am adding about 300 sq ft as part of a renovation. The A/C system is 2 1/2 ton, R410, and ducts are in the attic. Attic will be spray foamed and converted to conditioned space during the renovation, so I have flexibility to make adjustments in plenum path and duct size.

Location is in Northern VA, and it is an A/C only system (heating is hydronic). I would like to be a little more scientific in my planning, and not just start cutting holes in the plenum and adding ducts for the new space.

Thanks,

Bruce

 

[Jim greengo]

I usually figure around 450-500 sq ft per ton of ac,I'd say you'll be over according to my way of doing things.
It might cool it,but you'll be working it a lot harder/longer to get the job done.

 

[brewchief]

I usually figure around 450-500 sq ft per ton of ac,I'd say you'll be over according to my way of doing things.

It might cool it,but you'll be working it a lot harder/longer to get the job done.

As a rough rule of thumb here we would figure around 750 sq feet per ton, the problem with sizing anything like that is there are too many variables.



The proper way to do it is to do a manual J heat load calculation and then a manual D for duct sizing. Many of the heat loss calculation software programs can provide duct sizing as well.

An energy audit company may be able to do a proper heat loss at a reasonable cost and there are contractors that just do air balancing and measure airflow, we have used them to do airflow checks on commercial jobs.

Sent from my SM-G965U using Tapatalk

 

[LS6 Tommy]

If you were sized properly when the equipment was originally installed, you really "can't" add more load. That being said, just from a rule of thumb view, 2.5T for 1550 square feet is right at the edge of being undersized.


Tommy

 

[99LeCouch]

http://www.loadcalc.net/

Give it a run. It's as good as the information you give it.

Any other envelope tightening or duct reworking occurring during the reno? Could be that the existing system will do fine once the house gets much tighter with the spray foaming and other work, despite the increase in conditioned square footage.

My house is about 1600 sf, and a 2 ton A/C cools it fine. At 75*F inside on 92-93 degree days, it cycles. Having all the ductwork in conditioned space helps, as does having added more insulation and air sealed quite a bit.

 

[strutaeng]

loadcalc.net is the one I was going to recommend. Run the numbers and see if you are okay with 2.5 tons of cooling.


The 1 CFM per square foot is one that everyone seems to recommend. So, 300 sq ft = 300 CFM is something like a 10" round duct? Will also have to check if return is large enough for this increase. I think a 2.5 ton is something like 1000 CFM.

I've been told by some HVAC tech friends that you can go with a larger condenser by up to one ton. So, if you have the need for a 3.5 ton (for example) just get a new condenser and keep your air handler?

 

[rlitman]

If you were sized properly when the equipment was originally installed, you really "can't" add more load...

But he says he's spray foaming his attic and turning that into a conditioned space. He may or may not be adding more load. There's not really enough information to know.

 

[99LeCouch]

If you were sized properly when the equipment was originally installed, you really "can't" add more load. That being said, just from a rule of thumb view, 2.5T for 1550 square feet is right at the edge of being undersized.


Tommy

Odds are that it was oversized. Not many folks seem to want to take the time for proper sizing of HVAC equipment.

 

[SGKent]

1550 to 1600 is the limit on 2 1/2 ton in VA depending where you are. If you were in FL or AZ, parts of TX etc it would take 3 ton. The more energy efficient you can make your home the better it will work.

 

[yeldogt]

my 3k spray foam house us 3T ... have to do a load... better to be a tad under vs over.

in many cases the design temp only happens one day a year ....

 

[mrobins297aaa]

I think you'll have a hard time finding anything off the shelf to measure air flow, not sure it exists. We had a full time guy that did all our air balancing and air flow measurements. I remember him not really having much special equipment other than a manometer and a calculator.

I'm not sure about VA but in the Detroit area 2 1/2 for 1500 sq ft would be just about right. 1250 sq. ft would be a 2 ton.

here's one way to help you with your duct sizing.

The industry standard is 400 cfm per ton of AC.
so with 2 1/2 ton you have 1000 cfm.

now assuming your 1550 sq ft is pretty typical and you don't have a lot of west facing glass or tall ceilings and all the rooms are about the same you can use cfm per sq ft.
1000 cfm / 1550 sq ft = .65 cfm per sq ft.......

so 300 sq ft = 194 cfm

as far as duct sizes:
up to 100 cfm = 6" round
up to 250 cfm = 8" round
up to 450 cfm = 10" round
up to 750 cfm = 12" round

now the caveat here is there is a little bit of a art to this, ie the farther you are from the equipment the air cools/heats up and you have to move more air so if your at the end of the line don't be afraid to add a extra 6" run ,

 

[mrobins297aaa]

here's a way you can check the sizing on your existing trunk ductwork:
the formula A X V = CFM
A is the duct inside area in sq.ft
V is the velocity of the air moving thru the duct in feet per minute, standard residential is 900/1000 feet per minute FPM

so in your case A x 900 = 1000 cfm
1000/900 = 1.11 sq ft that is the inside area of the ductwork.
1.11 x 144 (that gives us sq inches) = 160 sq inches
160 sq inches would be = 20 x8 16x10 or any other combination

so you can check your trunk sizes with this and see if it's even capable of handling the air flow......it's not really that complicated and a lot easier than taking air flow readings.

 

[75gmck25]

Thanks for the all the input. It sounds like it may be marginal to use the 2 1/2 ton unit, but its only 6 years old and I would prefer to keep using it. I will have to measure the trunk size to get an idea of its maximum flow. I will also have to look at how to do a manual J calculation, since it will be useful for both heating and cooling estimates.

More specs for those who are interested
Existing 2 story house is 21x30 = 1260 sq ft total
New footprint is 21x37 = 1554 sq ft total
- A/C is located in attic
- Roof deck will be spray foamed and attic converted to conditioned space
- New walls are fire rated OSB and 5/8" drywall, and everything had to be fire caulked and sealed. New walls will be insulated with closed cell spray foam in 2x4 stud bays.
- New rooms are primarily bathroom and closet space (few windows) and all new windows are double pane Jeld-Wen.
- Existing house is 2x4 framed, with brick exterior and plaster interior. Most walls are not insulated in the stud bays, but they are thick and they are sealed well for air infiltration. Double-hung windows have add-on storm windows that appear to seal well.

Thanks,

Bruce

 

[strutaeng]

Thanks for the all the input. It sounds like it may be marginal to use the 2 1/2 ton unit, but its only 6 years old and I would prefer to keep using it. I will have to measure the trunk size to get an idea of its maximum flow. I will also have to look at how to do a manual J calculation, since it will be useful for both heating and cooling estimates.

More specs for those who are interested
Existing 2 story house is 21x30 = 1260 sq ft total
New footprint is 21x37 = 1554 sq ft total
- A/C is located in attic
- Roof deck will be spray foamed and attic converted to conditioned space
- New walls are fire rated OSB and 5/8" drywall, and everything had to be fire caulked and sealed. New walls will be insulated with closed cell spray foam in 2x4 stud bays.
- New rooms are primarily bathroom and closet space (few windows) and all new windows are double pane Jeld-Wen.
- Existing house is 2x4 framed, with brick exterior and plaster interior. Most walls are not insulated in the stud bays, but they are thick and they are sealed well for air infiltration. Double-hung windows have add-on storm windows that appear to seal well.

Thanks,

Bruce

Make sure you have enough insulation on the roof deck. R38 I'm guessing. Not familiar with foam insulation, but required thickness is probably more than rafter depth.

Use a "minimum thickness" acceptance criteria instead of an "average thickness." Talk to you installer about this.