2-stage to help with sizing innacuracies?

Reply

  #1  
Old 05-11-15, 02:08 PM
T
Member
Thread Starter
Join Date: Feb 2010
Location: USA
Posts: 23
Received 0 Votes on 0 Posts
2-stage to help with sizing innacuracies?

I'm interested in replacing my central AC and am looking for some opinions regarding sizing.

Some facts:

2180 sqft house 7.5 ft ceilings (except 330 sqft which is cathedral ceilings averaging 9ft). This is a Colonial style with the main foot print of 925 sqft, with an extra 330 sqft cathedral. ceiling family room on the 1st floor with garage underneath. Exchanger and ductwork are in a vented, unconditioned attic. The house was built in 1979. Current system was retrofitted, 4 supply ducts on 2nd floor, 4 on 1st floor (ducts through 2nd floor closets), and 1 return in 2nd floor hallway.

I'm assuming 88 degreess outside and 70 degrees inside. I'm in Massachusetts.

Current system is old and not very well marked, but AC contractor thinks it is 2.5 ton. Currently, this system struggles to keep house at 74 on a hot summer day (i.e. runs all day) . At night, the set point is 70, but it will run until well after midnight before it actually gets to this temperature, and only if the night is cooler. For example, it will likely only budge by 1 degree (down to 73 from 74) when I go to bed, after switching on at the hottest time of day and never turning off again before I'm asleep.

The AC contractor thinks that 2.5 is undersized and even 3 is right on the edge, so he recommends 3.5 tons (i.e. 42000 BTU).

I've done my own load calculation using the following site, but I'm not sure how accurate this is:
HVAC Load Calculation - Maunualj - Whole House Loadcalc
When I use 88 outside, 70 inside, I get right around 35000-36000 BTU which is 3 ton. The result that you get varies A LOT based on the outside/inside temperature difference that you put in.

There is an option to install a 2-stage unit (1 ton increments only) at 4 ton, which would be 2.8 ton at it's lower setting (70%). Also, a 2-stage, 3 ton would be 2.1 ton at the lower setting.

So the options are:

3 ton 1-stage
3.5 ton 1-stage
3/2.1 ton 2-stage
4/2.8 ton 2-stage
 
  #2  
Old 05-11-15, 02:13 PM
T
Member
Join Date: Jun 2012
Posts: 1,013
Received 19 Votes on 15 Posts
2.5 tons is to small for sure. I cool just under 1800 sq feet cape style with 3 tons. House built in 65, new windows, descent insulation, air handler in unconditioned attic space.

3 -3.5 should be what your looking at. I like it cool and would probably steer towards the 3.5 - 1 stage.
 
  #3  
Old 05-11-15, 02:18 PM
T
Member
Thread Starter
Join Date: Feb 2010
Location: USA
Posts: 23
Received 0 Votes on 0 Posts
I'm assuming you are in the same region? I'm in Andover, MA.

I like it cool too (70 is preferred vs. energy star recommended 78). Based on what you and my contractor are saying 3.5 should do it. A 2-stage 4-ton would give some "insurance" though, no?
 
  #4  
Old 05-11-15, 02:23 PM
T
Member
Join Date: Jun 2012
Posts: 1,013
Received 19 Votes on 15 Posts
I'm in NJ, summers can be HHH. Not sure you'll ever need 4 tons, your not buying insurance there and 2 stage units can be $$$$
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: