# Thread: Proper size of unit

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## Proper size of unit

Upstairs unit is old and broken. I'm going to replace the unit soon but I have a question. #1 How do you figure out the proper size unit to install? Square footage? (I have about 1,400 sq ft) Cubic feet?

2. A manual J load calc is what you want. The calculation takes the building structure and calculates your heat loss or heat gain. At the top of the page it says hvac calc in the blue banner. I believe you can purchase a program that will allow you do it.

3. Cubic ft of air: 1400-sf X's 8-ft(?) ceilings = 11,200-cu.ft. home's air volume.

Say you have a 2.5-Ton A/C, the blower delivers 1000-cu.ft. per minute (CFM) X's 60 minutes is 60,000 cu.ft. of air exchange per hour.

If it ran steady for a full hour: 60,000-blower's cu.ft. of air / 11,200-cu.ft of home air volume is 5.357 (ACH) air changes per hour; Blower operating 30-minutes, half that or, 2.67-ACH.

4. For a furnace you cna use your old utility bills. But that's a little tricky for a AC system because hte delievered effciency takes a little more work to determine. But you cna use your heating bills to give you a ballpark so yo at least know if your close.

But mainly you need a proper load calculation done.

Using suqre footage is almost useless. I have a 3200sqft 2 story 86 y/o home. some "rules of thumb" calculation would say I need as much at 6-7 tons. I calculated that I only need combined 4-1/2 tons.

Another way ot get a rought estimate is to look at equipment run times during design conditions. So where I am in Iowa, if it's 95F outside and sunny & fairly humid, I'm at design conditions. My system should be running constantly to keep up if properly sized. If it cycles and run for only 50% of the time over a 1 hour period, then it's roughly 2X the size I need.

5. If I understand correctly you have 2 systems now. If that is the case you need to figure the load on the whole house, then seperate out the rooms onto the differnt systems. If you try and figure just on the rooms involved with the system to be replaced there is a good chance you will oversize it becasue the other system isn't taken into acount.

6. Originally Posted by mastermike
Upstairs unit is old and broken. I'm going to replace the unit soon but I have a question. #1 How do you figure out the proper size unit to install? Square footage? (I have about 1,400 sq ft) Cubic feet?
Can you read the model numbers off the old A/C?

It takes a knowledgeable Tech to test an A/C for delivered performance to the rooms; without that you're guessing whether the old A/C was delivering near its Rated Btuh.

Take the Supply Air & Return Air "wet bulb temperatures" & interpose them on the enthalpy chart linked below.

TESTO 605-H2 - for fast wet bulb temps

Use an anemometer to get CFM X* 4.5 @sea-level, or use X* 4.35 if 1000' above sea-level, X* change (difference) in enthalpy = BTUH (Ballpark) Operating Performance.

"U Must Right Click Link & open in New Tab," look-up wet bulb enthalpy figures on enthalpy chart," & figure enthalpy change times the result of the CFM tests X* the factor difference off the chart.
Wet Bulb Enthalpy Chart Print chart

Usually, most existing equipment will be over-sized.
I prefer doing an Energy Efficiency Audit, doing the good ROI retrofit & then the heat-gain/heat-loss load calcs; & equipment sizing from those numbers.
Last edited by udarrell; 03-30-2012 at 01:20 PM. Reason: Clarity: change (difference) in enthalpy between the two readings...

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