How do I determine the capacity of my air handler?

Factual Questions
1

We’ve been contemplating finishing half of our basement for the last eon, and we’d like to start talking with contractors. But we’d like to get a bead, in advance, on whether we’re going to need to replace our air handler, which is less than 2 years old.

We had to replace the old system in a hurry the summer before last, when it died on us right before we were headed out of the country, and I believe I discussed this with the guy who replaced our old system, but things were crazy, I have no notes saying we talked about that, and I really can’t be sure.

The specs say nothing about how many cubic feet the system is meant to handle. And the manufacturer’s website has this useless bit in its FAQ:

Useful, huh?

I realize that, yes, demands on the system can vary with all that stuff, but I’d think square footage would be a basic starting point. (And all the other stuff shouldn’t matter that much for the air handler, since we leave the central fan running all the time, whether or not it’s heating or cooling.)

There’s also the question of the heat pump’s capacity, but I’m less concerned about that because (a) the basement is the coolest part of the house in the summer, so it shouldn’t add that much to the cooling load, and (b) we’re planning on installing a gas fireplace in the basement, which should keep it toasty in the winter without the heat pump’s help. So let’s skip this question for now.

I’d simply like to know how I’d figure out (or at least get a first approximation on) whether pushing air around roughly 25% more space than at present is likely to tax our air handler.

2

What you want to know is how much more cubic feet per minute (CFM) of air your air handler can provide.

This all depends on the fan and motor in the air handler. The relationship between airflow and power is governed by fan affinity laws, but the power requirements of the fan go up in a cubic relation ship to the volume. A 10% increase in volume requires 33% more power. If your fan motor is operating near it’s rated power, then you will have trouble, and may have to replace the motor with a higher Hp motor. The addition of additional duct is also going to increase the static pressure you fan is working at. 25% is a pretty big increase. On the other hand, if the system is oversized as it is, you may be ok.

Of course, even that may not fix the problem, as the rest of your system needs to be sized to operate at the higher flow rate. For instance, the cooling coil is designed to work at a certain volume of air. It may not be capable of cooling an additional 25% volume of air.

Finally, just because the load you are connecting to the system is cooler basement air don’t assume it is not a big load. The increase may be a substantial latent load if you have higher humidity in your basement.

What you need is an HVAC company to perform a load analysis and compare the capacity of your existing equipment with the new load requirements.

You may also want to consider a separate mini-split system if you simply want to add cooling to your basement. If your existing system doesn’t have the capacity or running ducts is a hassle, it may be a good solution.

3

Here is a quick check to determine if your existing system is oversized or not - does it run either constantly or for long period of time when it does run but keeps the house cool (which is ideal) or does it turn on, run a short time to cool the house quickly and then shut off?

The latter is called short cycling and an indication of an oversized system - in most cases it is a bad situation, but if you want to add areas to cool then you might have the capacity.

4

Why don’t you ask the guy who replaced the old system? They should have a record of what they installed and the estimated volume of the area of your house.

5

It’s hard to tell - we leave the fan running all the time, so we don’t really notice when the heating/cooling kicks in. And obviously it’ll be a few months before we’ll experience peak cooling load again. When July rolls around, we can switch the fan from ‘on’ to ‘auto’ and see what it does, but we’d really like to know sooner.

I guess I’ll first try contacting the outfit that installed my system back in 2010 (as RNATB suggested) and see what they can tell me; if they can’t, then I’ll get someone to do a load analysis. I know we did increase the capacity of both the air handler and the heat pump by a fair amount over the old one, but I’d be unable to say by how much.

6

It’s hard to make a determination without knowing the details of the equipment and the physical layout of your house. Getting a local HAVC guy or two to look at it would be best (including the installer).

But I’d also keep the option of a separate system open if adding ducts, etc, turns out to be difficult or expensive. Google “mini split sytems” to see what I’m talking about. It could be a good alternate solution.

7

Post the model number of the AHU.

Tell us your square feet, breaking out above grade vs below grade.

8

You’re going to want to get it looked at, but assuming your old system wasn’t tremendously undersized, the fact that you made the new one “a fair amount” larger will hopefully give you good news.

9

Heres some basic info that will help

A comprehensive “Manual J Load Calculation” is what will tell you *exactly * what your heating and cooling needs are. The manual J will address roof color, wall insulation, window U value, cubic feet and many other factors.

But there are many very reliable rules of thumb that will get you close, and in the vast majority of cases, get you more than close enough.

Some reliable rules of thumb: (for the Midwest/ Atlantic states type winters and summers)

For heating purposes you’ll need 5 Btu’s per cf, or 35-40 Btu’s per sf. Unless you have a lot of cathedral ceilings, the 35-40 Btu’s per sf will be just fine.

A/C needs will be 2-3 Btu’s per cf.

Another reliable rule is that you will get 500-700 sf per ton of cooling. Your needs are greater if your house is looser; an old house with no insulation and leaky windows may only get 500 sf per ton, while a super efficient house may get 1000 sf per ton. 700 sf is a good number for most houses built in the last 30+ years.

There are 12,000 Btu’s per ton of cooling, and 400 CFM per ton. From 1.5 tons through 4 tons they can be sold in half ton increments. The nomenclature on your air handler will reference the CFM, and that will dovetail with the unit outside.

So…an “18” in the model means 1.5 tons (18,000 Btu’s) “24”, 2 tons, “30”, 2.5 tons, “36”, 3 tons and so on…

By the same token, @ 400CFM/Ton an “18” is 600 CFM, “24” is 800 CFM and so on…

Is it confusing yet? :wink:

So let’s pull it together…

EXAMPLE:

You have a 2100 SF house, of average construction. At 700sf/Ton, you need a 3 ton A/C, and at 35 Btu’s/SF, 74,000 Btu’s heating output.

But there’s more! More I say!

The 700 sf per ton for cooling and 35 Btu’s for heating are great upstairs because the exterior walls are exposed to the weather. In the basement, however, it is 55° below the frost line, (about 30" where you live) so much of your below grade walls are exposed to 55°.

What that means is that both your heating and cooling needs are much less downstairs.