Air Handlers

Brad White · October 2007

It is all about match-up. A larger coil will get you longer retention time (time air is resident on the coil during which heat is transferred).

Change airflow with a given coil and things can change rapidly. Too much condensing unit matched to too little coil and too little air means colder air. Maybe this is not a problem but any deficiencies in your duct insulation and vapor barrier may find you chasing down a drip.

Too much evaporator matched to too little condensing unit means tepid air that may not dehumidify the space. Just as John Mills said, know your loads, figure your conditions and work from there.

Slower air velocity increases latent heat (moisture) removal. The faster the air velocity lessens that process. Too fast and it does less of each.

Deeper coils and those with more dense fin spacing favor latent heat even better. Shallower coils and those with wider fin spacing favor sensible heat removal.

As you might vary the volume across a given cooling coil, the ratio of sensible to latent heat changes.

Here is an oblique example to illustrate how this might be applied:

Take a conventional 3-ton (say a full 36.0 MBH) standard type A-type cooling coil in a fan coil unit.

Say it moves 1,200 CFM and takes that air from 75 degrees DB down to 54 degrees saturated, a 21 degree drop. That would be about 27.3 MBH or about 76% sensible heat ratio (SHR). The remainder, 8.7 MBH, is the latent or moisture removal portion, about one gallon per hour.

Take the same 3-ton system and apply it to a Unico/Space Pak High velocity system. Half the air, say 600 CFM, but the same tonnage. Such a system might, for discussion purposes, have a 0.60 SHR. That means 21.6 MBH sensible; in other words the air temperature can drop from 75 degrees to 41.8 degrees saturated. That is quite a bit more latent removal to be sure, 14.4 MBH . Same tonnage, half the airflow.

Does that help? Hard to explain without illustrating.

Paul Fredricks_3 · October 2007

I haven't done all that much in the way of sizing and installing air handlers, but I do recall that a company I used to work for like to size the air handlers 1/2 ton larger than the condensing unit. I don't recall why.

We just came upon a house with 4 hydro air units. All 3 ton air handlers with 2.5 ton condensing units. The units are pretty old so we are giving them a price to upgrade. With efficiencies the way they are, would this practice still make sense? I know we'll have to review the specs from the manufacturer to make sure things match up.

Empire_2 · October 2007

Hi Paul

I believe they are utilizing the larger coil for more dehumidification. Chances are that the unit will run longer and Delta T's will be say 16 or 17*. The Super heat will also be greater due to the coil being over sized.

Mike T.

John Mills_5 · October 2007

Larger coil

usually means warmer coil, higher sensible, lower latent and lower dehumidfication.

First step: ACCURATE LOAD CALC! Then size your equipment based on that. Air handler should be ARI matched to outdoor unit. It is common for 1/2 ton larger especially with higher SEER. If in a humid area, look for a brand/model with a lower sensible heat ratio (.7 for example) meaning higher humidity removal.

Techman · October 2007

Coil size

Hi Paul ,Mike T ,& John .I think both Mike T & John are correct!It depend which side of the coin you are looking at! If the load is 2.5t with a 2.5t cond and a 3t evap then one is right ,then ,if the load is 3t w/ a 2.5t cond and a 3t evap then the other is right. So is it oversizing the evap or undersizing the cond unit?Let's hear from an engineer or the Professor!!!!

Paul Fredricks_3 · October 2007

I must be too early in the AM. My mind is spinning.

Let's assume that 2.5 ton is required. Am I to assume a 3 ton evap will give me....what? Air that's not so cold and less humidification? Is that based on 3 tons of airflow? Or if the blower runs slower, say 1000 CFM for 2.5 tons, that more humidity can be wrung out of the air since is spends more time on the coil? Or cooler air since it spends more time on the coil?

Is it more efficient to run a larger coil?

:-{

Brad White · October 2007

Had coffee yet?

Hi Paul-

No easy answer without specific equipment performance tables. Absent that, I would stick to known match-ups.

I can see a modest increase such as your 2.5/3.0 being in the range of reasonable provided you have enough refrigerant charge and can make the apparatus dewpoint. If that falls short, could you reduce your airflow to a point where moisture drops off? I will let the professor speak to the details of refrigerant control, a weak spot of mine for now.

If you were in dry Arizona, I would not worry so much but in humid areas, it is a concern. In the northeast, it could work well enough until it gets muggy for prolong periods.

Also, coil ratings are usually given as "nominal" meaning at ARI conditions of 80/67 entering air which is outside of my comfort zone.

Not a definitive answer but it is the best I can do in the moment.

Paul Fredricks_3 · October 2007

Not a coffee guy

When I need the caffeine I head for a Pepsi.

Thanks Brad. I guess sticking with the match ups makes the most sense. I'll pass the info on to those who need it.

Air Handlers

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