4 ton a/c with 5 ton coil?

[Brian]

Alright here's the question: I've got a

2800 sq ft house 3 bdr built in 2005 below ground basement open floor plan/high ceilings One stage furnace Denver, CO

I have heard that I could get a 4 ton 13 seer unit with a 5 ton coil (trane) or go with a 5 ton 13 seer(rheem) with a 5 ton coil. Will I get better efficiency with the 4 ton/5 ton coil trane?

Way confused! Thanks

[IBM5081]

If you have a 4 ton unit, then you only get 4 tons of cooling. If you have a 5 ton unit, then you get 5 tons of cooling (more cooling). Both units are rated 13 SEER, so the efficiency should be the same as far as energy efficiency is concerned. Why put a greater capacity coil on a smaller unit? Who recommended doing that?

As far as design is concerned - how much A/C is actually needed for this home? I assume that this is the only unit. What unit and capacity are you replacing, since it was built last year.

[Travis Jordan]

First you need a heat gain calculation to tell you how many tons (BTUs) of cooling you need. It sounds like your contractor(s) is/are guessing.

Once you know the BTU load then the contractor will assist you in selecting the right size condensor and coil. It is not uncommon to slightly oversize the coil to get somewhat better efficiency (SEER). However, you want to be sure that the sensible and latent heat requirements of the load are met.

[Robert Gammon]

Why in the world do you think they put these labels on them, 3 ton 4 ton, 5 ton etc?????

It is so the proper sized equipment outside gets matched up with the proper size equipment outside.

With a 2005 built house, why in the world are you considering a HVAC change after only one year????

5 ton compressor/condenser should always be matched with 5 ton coil/evaporator. Any money you save on purchase costs is likely to be eaten up by operating costs, and in fact, it may cost you more to operate as the 4 ton compressor will work harder/longer to cool the house in summertime.

The only way to know for certain, however, is to have a reputable HVAC contractor come out, inspect the system and make recommendations.

[Travis Jordan]

Virtually all manufacturers have certified upsized coils for use with their condensers.

Evaaporator coil size is a major factor in determining the balance between sensible and latent heat removal. Larger evaporator coils normally remove less humidity, because they operate at a higher coil temperature/pressure ratio --smaller evaporator coils tend to remove more humidity --because they normally operate at a lower operating temperature. The SEER is usually slightly higher with larger coils.

See ARI Primenet for specific ratings.

[Robert Gammon]

So in Denver with relatively lower humidity than Gulf Coast locations, a larger coil MAY be a recommended practice.

Here in Houston, or New Orleans, or Pensacola, or Mobile... humidity extraction is a big concern as the number of 90+/90+ days is very high. In these locations a 5 ton coil should be matched with a 5 ton compressor.

[Travis Jordan]

Or use a slightly oversized coil with a VS air handler / furnace, and achieve the same humidty with a higher SEER.

[HeatMan]

There are certain matches made with a 4 ton 13 SEER condenser and a 5 ton evap that will get you a higher SEER. You have to check the ARI ratings

[udarrell]

Denver, CO summer design conditions: 91 dry bulb 59 wet bulb! Approximately 12 to 13% Relative Humidity. You shouldn't have a latent load which frees up the sensible capacity handle the sensible temperature load.

In a ultra dry climate, go for an oversized evaporator coil, TXV, and

450-cfm of airflow per ton of cooling. I hope your supply and return air is at or near the ceiling for the cooling mode and near the floor for heating. This will optimize the temperature differentials of the air and the heat exchangers, which transfers more heat from a specific amount of a cubic foot of airflow. (More efficient operation)

Have a competent/trustworthy heat gain calc done, then do more to reduce the heat-gain and size the equipment accordingly. I would think you could get the sensible heatload and condenser sizing down to at least 3.5-ton with a matched 4-ton evaporator coil. Save needless expenses.

- udarrell

[Travis Jordan]

Excellent advice! Sometimes just the addition of some attic insulation can reduce heat gain by a half-ton or more.

[Brian]

Thanks for the info....just from the above responses, you can see how much of a "Science" HVAC systems are if done right!!! I now understand that an installer needs to do the calculations and take the right time doing it. Thanks again!