HVAC Units in “open” home-how size themm
edwardh1
| Posted in Energy, Heating & Insulation on
How do you size HVAC units when there are two units in an “open” type house?
Say with 2900 ft2 total and 1900 of it downstairs and 1100 upstairs?
house has a cathedral ceiling in the downstairs living room and then 10 steps up to an open TV room then 5 more steps up to an open 12 long hall with 2 bedrooms and a bath off it.
That is my house.
It has an old 3 1/2 ton heat pump downstairs and an old 2 ton upstairs.
Due to either oversizing at new construction 18 years ago, and/or my weatherizing , neither unit runs much in hot or cold weather — 20 min on 20min off on 95 degree day
If I downsize when I replace them, which unit size gets cut first?
I have never seen this subject discussed anywhere.
Replies
You size the units the same as you would in any other house or building. You calculate the heat losses and gains based on the climate, construction and exposure. Based on that and how you use the house (like do you run the AC from April to September, do you keep is set to 70 in the summer and 80 in the winter?) you pick the units to meet the loads. In air conditioning, it is better to be a little undersized than oversized. The key to making the systems serving a space with two-story ceilings and the like is the location of adequate returns and proper placement of the thermostat.
BTW, 1100 and 1900 add up to 3000. 5.5 tons in a 3000 sf house is probably way too much cooling. I have 4 tons doing a good job of cooling almost twice that. In 95 or hotter, it runs 12 hours a day, dries the air very nicely and keeps the house less than 80.
You didn't say where you live or anything about the wall construction, windows or insulation present.
If I had to upgrade them one at a time, I would replace/resize the unit serving your sleeping area first.
Thanks
esp the sleeping area first comment - its is downstairs
sorry about the math -
actual numbers are like 2000 ft2 first floor with full 2 story kitchen and 2 story living room ceilings.
250 on an intermediate level TV room
and about 650 on the 2 nd floor.
located coastal south carolina - good actic insulation, medium to good tightness, contemporary design so it has lots of windows, double pane.
thermostat and return in hall on first floor
thermostat and return in hall on second floor
Have you ever seen anything on the web about how the unit sizes are selected - like initially by the designer of the house/ the total heat loss is an easy one - just use a manual j or other calc program to figure it up, but the house is almost an open box. --ever seen anything published about what size goes where??
I wrote someone involved with heat calculation and he said: "your contractor will know what to do" which seemed a little imprecise to me.
Edited 12/16/2002 8:14:35 AM ET by wain
No, I haven't seen anything like this on the web, but I haven't looked. I have designed forced air systems for large houses with big open spaces, two story great rooms and high cathedral ceilings.
You're not likely to find anything useful on this issue "on the web".
Some contractors will know what to do, some will not. I saw a Parade home last summer that had a three story "atrium"- basement, first and second floor all open with a wall of windows. The HVAC contractor that did that job did not "know what to do".
Besides being way oversized, does the system you now have work ok? You have a single return in the hall for each system?
Air will go where you make it go or where you let it go. A single return in the hallway is great, for a one story, 1200 sf ranch. Multiple floors, open floor plans and high ceilings require a better system of returns to prevent stratification. I bet the second story is always hotter than the first floor. Not being able to see a floor plan, I can't really tell you where to put the returns. Something to consider, a 12 x 12 return is good for 400 cfm or 1 ton of cooling. A 12 x 6 is good for half of that. When you redo this system, I would add returns where necessary. If you can find a contractor that tells you the same thing, good deal. If not you may want to consider finding a good engineer to review or to design the system. It may cost too much, though. We seldom do residential design because people are seldom willing to pay our fee and because "the contrcator will know what to do".
I run calculations to size equipment. There is no other way to do it correctly. If I have to guess for the purposes of estimating a job, for a house I would expect to use 800 to 1000 sf per ton (in my own home I have 1250 sf/ton) for cooling and 30 btuh/sf for heating. For a 3000 sf home 3-1/2 tons of AC would be my best GUESS. Depends on the climate, shade, windows/treatmens, roof and wall colors, etc. However the house is laid out, the calculated heat gain/heat loss determine the size(s) of the unit(s). The layout determines the details of the duct work and register locations and thermostat location(s).
Thanks again.
dark roof, zero shade. More windows than normal. House has 3 6 ft high 10 ft wide patio doors. Sunroom has 21 windows 1 1/2 ft wide 5 ft high each.
One central return for each system seems to be the norm here.
Sometimes on real large home you see a second one. My son lives in an older home in mid state NY and each room there has a return.
Units cool very well now. Scrubs out water pretty well, rel hum in summer usually 55% as shown on a radio shack electronic gage.
I am in process of removing interior window trim and foaming around the windows.
units do not run much even on a 100 degree day.
My instinct is to downsize the 3 1/2 ton to a 3 ton when it dies.
that would only be 10% 5.5 tons total going to 5.