Mismatched A/C unit sizes

[Bruce Ramsey]

Todays inspection had mismatched size air conditioning units. Interior units were 4 ton and exterior units were 3 ton.

HVAC is not my strongest suite but am I correct in assuming the exterior unit can only cool so much and so the extra capacity of the interior units is wasted? Or is there some other awful consequences of which I am not aware?

Exterior Unit: Lennox, Model #: 13ACD-036-230-02, Serial number: 5807C06557
Exterior Unit: Lennox, Model #: 13ACD-036-230-02, Serial number: 5807C06560
Interior Unit: Lennox, Model #: CH22-44/48B-2F-2, Serial number: 6006K66815
Interior Unit: Lennox, Model #: CH33-48C-2F-2, Serial number: 6007G27739

3900 sq ft two story with a unit for each of the floors, one in the attic and one in the crawl. Not a tree in site to provide any shade.

Similar Threads:

• Missing A/C Condensing Unit - Take 2
• alternating joist sizes
• A/C condensing unit location?
• Mismatched Lennox heat pump and high temp diffs
• Fireplace Had It's Own Air Handler Unit

[Jon Randolph]

It is a common practice for some HVAC thech to recommend installing a slightly larger evaporator coil than condensing unit. This increases the surface area of the evaporator coil and allows it to take more heat out of the air passing trough it. A larger evaporator coil can be used but a smaller coil can not.

Only issue that I can think of is that a larger coil may reqiure a slightly larger coolant charge than the condensing unit came from the factory with. They are usually sized for a matched coil and up to 25 feet of lines. This, of course, should have been checked by the HVAC tech at installation.

[Jerry Peck]

It is very common to up-size the evaporator coil 1/2 ton to 1 ton for best efficiency and full use of the Btu produced by the condenser unit.

[Jim Luttrall]

The SEER rating is not going to be accurate except with the factory specified units. Before worrying too much though, check the factory specs to see if they allow or even recommend the installation. Different needs can call for different solutions.

[Jerry Peck]

The SEER rating is not going to be accurate except with the factory specified units.

Correct,

When we lived in South Florida and replaced our a/c system in 1996, the condenser unit was a 4 ton and the evaporator unit was a 5 ton, with a SEER rating of 14.7 based on that combination. While 14.7 SEER was not the highest available at that time (16 SEER combination was available), the price to make the next step up was dramatic, so we only went with the combination we did.

[Ted Menelly]

Is there anything else that has to be done when upsizing an evaporator coil for the system to function properly/efficiently

[Billy Stephens]

Is there anything else that has to be done when upsizing an evaporator coil for the system to function properly/efficiently

.
.....
.

[Scott Patterson]

Todays inspection had mismatched size air conditioning units. Interior units were 4 ton and exterior units were 3 ton.

3900 sq ft two story with a unit for each of the floors, one in the attic and one in the crawl. Not a tree in site to provide any shade.

I'm trying to understand why they have systems in the attic and the crawl? Is this a SIP's home? Does the home have a sealed crawl space?

[Jerry Peck]

I'm trying to understand why they have systems in the attic and the crawl?

Because square footage costs so much and no one wants to spend the money to pay for it to be used for 'my a/c unit'.

An example is the house I was consulting on day before yesterday, 9,000 sf (give or take) and 4 AHU, all in the attic.

Figure that each one really needs about a 4' x 3' area minimum, or 12 sf, at about $600 sf that means each AHU is taking up $7,200 worth of floor space, 4 AHU means you are spending $30,000 for floor space for the AHU.

Which costs more, the AHU or the space to put the AHU? The space to put them! So, they go up in the attic.

As though, out of 9,000 sf, you cannot find a place to put those AHU? Give me a break. It's all about the money it costs to give up that square footage.

[Scott Patterson]

Because square footage costs so much and no one wants to spend the money to pay for it to be used for 'my a/c unit'.

An example is the house I was consulting on day before yesterday, 9,000 sf (give or take) and 4 AHU, all in the attic.

Figure that each one really needs about a 4' x 3' area minimum, or 12 sf, at about $600 sf that means each AHU is taking up $7,200 worth of floor space, 4 AHU means you are spending $30,000 for floor space for the AHU.

Which costs more, the AHU or the space to put the AHU? The space to put them! So, they go up in the attic.

As though, out of 9,000 sf, you cannot find a place to put those AHU? Give me a break. It's all about the money it costs to give up that square footage.

No, I understand the placement of the units in the attic, etc. The majority of the homes I see have the AHU in the attic. What I was questioning is that it sounds like the Attic and the Crawl are temperature controlled.

[Bob Spermo]

Some builders are now conditioning the attic and putting the air handlers and the ducts in this conditioned space. The rafters are insulated with open cell foam and the ceiling joists are not insulated. The walls are usually a loose fill (cellulose or fiberglass). This would obviously be an unvented attic. This technique is big with the "Green" community. They also install a whole house ventilation system. Many jurisdictions have approved theunvented attic.

[Bruce Ramsey]

The HVAC systems are located in the attic and crawlspace. The duct work is spidered through the attic and crawlspace. Ceiling registers for 2nd story and floor registers for 1st story. Neither the attic or crawlspace are conditioned.

The other common configuration is HVAC in attic and exterior package system for the first story. Same configuration for ducts and registers.

This is land of crawlspaces, virtually no basements, and slabs for the tract homes under 1800 sq ft.

[Ted Menelly]

Some builders are now conditioning the attic and putting the air handlers and the ducts in this conditioned space. The rafters are insulated with open cell foam and the ceiling joists are not insulated. The walls are usually a loose fill (cellulose or fiberglass). This would obviously be an unvented attic. This technique is big with the "Green" community. They also install a whole house ventilation system. Many jurisdictions have approved theunvented attic.

Where is the cost savings of conditioning additional space. The cost to condition it or get it ready for conditioning plus conditioning that additional space kinda counters saving energy cost all together. Makes no sence.

[Bob Spermo]

Ted,

What really makes no sense is putting the air handlers and the ducts in the hottest area of the house - the vented attic. The efficiency loss is approximately 30-40%. The unvented attic is conditioned marginally with 4" ducts. It is not kept at the same temperature as the living space. The insulation (foam and loose fill) is more expensive than the old batt insulation. However, a house properly insulated, properly ventilated with properly sized A/C units will more than make up the extra cost in a very few years. In addition, the indoor air quality will much better. We need to spend more effort on improving the whole house building shell.

[Ted Menelly]

I am all for the units in the home. Space requirement is minimal and the duct work (hopefully bet than the old days) in enclosed soffits or ceilings. No build out of any sort in the attic and the duct work is below the attic insulation. The only thing I ever found in doing it in the past was they never made the soffits or dropped ceilings to look like they were part of and incorporated into the design of the home.

Just to me to partially build out an attic space and then condition that space (you might as well just make more living space) was a waste of material and energy when they had it right to begin with but just did not do it right. Some might argue that a small closet takes up square footage of the home. For the end savings over all it is certainly not a waste of space and I believe much more cost efficient and economical.

JMO

[Jerry Peck]

For the end savings over all it is certainly not a waste of space and I believe much more cost efficient and economical.

Ted,

How about if we just did away with the duct work and all of its losses, putting a unit in each room, that way each room has individual control.

Wait ... we used to have that ... it was called 'room a/c units' or 'window a/c units' ... maybe they will make a comeback?

[Ted Menelly]

Ted,

How about if we just did away with the duct work and all of its losses, putting a unit in each room, that way each room has individual control.

Wait ... we used to have that ... it was called 'room a/c units' or 'window a/c units' ... maybe they will make a comeback?

Less expensive and more efficient. The best of both worlds.

Attic space of a 3000 sq ft 2 story home is 1500 square feet. That is 1500 square feet of partial build out and conditioned space. Like I say an extra 32 square ft (floor space for unit on each floor (if that) at a minimal cost for build out (no flooring, interior finish and trim) comes to a whole lot less that partially building out and attic and then conditioning it for years to comes. first floor duct work goes through the truss joists, second floor has minimal soffits, problem solved, little expense, great savings over time.

[Wayne Carlisle]

By installing the foam between the rafters it keeps the heat from ever getting inside the attic. All of the ducts are within the thermal envelope of the house therefor no loss of efficiency.

Since the structure is more energy efficient (tighter) you have to downsize the AC unit so that it will run long enough to take out the humidity from the structure thus saving costs on utilities.

[Ted Menelly]

By installing the foam between the rafters it keeps the heat from ever getting inside the attic. All of the ducts are within the thermal envelope of the house therefor no loss of efficiency.

Since the structure is more energy efficient (tighter) you have to downsize the AC unit so that it will run long enough to take out the humidity from the structure thus saving costs on utilities.

Yes, right to a *certain degree* and when I finish my inspection report I will add to it.

[Ted Menelly]

Anyway

All the trouble and expense to slightly reduce long term savings, and it is very slightly because you are adding a huge amount of cubic feet in total (even though you a minimally conditioning the space) you are conditioning the space, It is not free. I would think one would be hard pressed to recover the output seeing how the come back would be so slow. Most folks on average are in their homes for 5 to 7 years. Now adays it will more than likely to become longer. Just put good ventilation and solar board for sheathing. Install heat pumps in the home with internal ducts (not in the attic). The sheathing cuts the attic down (with good ventilation) by about 20 degrees. That cost would be more recoverable.
My way once the additional cost is saved the rest of time is free and you are not putting out that extra cubic foot of conditioning. You pocket the money for the rest of your breathing life. Or someone else does. Either way, much greener.

Just my wonderful opinion.