2 ton coil with a 1.5 condensor

[jud aley]

I’m planning to install AC into my own house. The Manual J calculation calls for 1.5 to 2 tons of cooling. My HVAC sub is recommending a 1.5 ton condenser with a 2 ton coil. He claims this will provide increased dehumidification without the cost of a standalone dehumidifier. Brand and SEER of the coil and the condenser would be matched.
Anyone ever heard of this strategy before and if yes does it really work?

-Jud

[johnny watt]

Re: 2 ton coil with a 1.5 condensor

I have heard of it but it didn't make sense to me.

I wanted a larger coil in a system I did last year because the 1.5 ton put too much restriction on the furnace airflow, (which is the part that runs most months of the year around here). My HVAC guy said it was common to use the next size coil up and that I would benefit with more humidity removal to boot. He said all he had to do is change the TXV or orifice.

But in my mind, more humidity removal comes from a colder coil. That means slowing the air speed down or using a smaller coil for the same BTU, not larger.

I have also heard people claim you get more humidity removal from longer run times. This again does not make sense in my mind. The longest run times would be from a coil running at 69.9 degrees. But that would not get any condensation on it to speak of, thus very little humidity removal.

Even if I am wrong, I am certain you should get all the humidity control you need by just sizing the coil and condenser to each other. No need to try to get extra humidity control.

But if for some reason you have extra humidity being introduced into the home that needs to be removed, it is far more efficient to do so with a dehumidifier than with an AC system.

[dgbldr]

Re: 2 ton coil with a 1.5 condensor

Anyone ever heard of this strategy before... Jud

Yes, absolutely, but that's not exactly how it works. The installer doesn't get to make the call, i.e. he can't just pull a combo out of his arse.

Each manufacturer publishes a list of SEER "matches", where a SEER rating is given for a match of indoor unit A and an outdoor unit B. If the combo your guy wants to use is listed, great. If it's not, it means the manufacturer doesn't approve it as a match.

Around here, the HVAC contractor is required to provide a certification showing the manufacturer's listing of the combo as a match with a SEER rating.

Edit: If humidity is an issue, have him calculate the latent and sensible loads and size accordingly. Manufacturers have specific ratings for latent and sensible for their combos, if yours doesn't, consult the manufacturer.

[dgbldr]

Re: 2 ton coil with a 1.5 condensor

But in my mind, more humidity removal comes from a colder coil. That means slowing the air speed down or using a smaller coil for the same BTU, not larger.

Wrong. You can't make the coil colder, it's operating at very low temp already and making it colder would make it ice up ("freeze").

I have also heard people claim you get more humidity removal from longer run times. This again does not make sense in my mind. The longest run times would be from a coil running at 69.9 degrees. But that would not get any condensation on it to speak of, thus very little humidity removal.

No. Longer run time lets more air pass by the coil and condense. Sure, a coil at 69F won't condense, but it also won't cool the house much. So coils are never run at 69. Longer run times are achieved by choosing a slightly lower system capacity (undersizing the system, or at least not oversizing it). The most common cause of insufficient humidity removal is oversized systems.

[Allan Edwards]

Re: 2 ton coil with a 1.5 condensor

I’m planning to install AC into my own house. The Manual J calculation calls for 1.5 to 2 tons of cooling. My HVAC sub is recommending a 1.5 ton condenser with a 2 ton coil. He claims this will provide increased dehumidification without the cost of a standalone dehumidifier. Brand and SEER of the coil and the condenser would be matched.
Anyone ever heard of this strategy before and if yes does it really work?

-Jud

Jud

From the land of air conditioning and high humidity, yes sometimes we do mis-match equipment sizes, for various reasons. However, in your case a smaller coil is better for removing humidity, not larger, because it does run longer and it will be colder. That is my experience and what we often do. The larger coil is better to remove heat and but smaller coil is better to remove humidity. And of course lower humidity makes air "feel" cooler.

[jud aley]

Re: 2 ton coil with a 1.5 condensor

Allen-
On a web site called HVAC talk I found a thread on this subject and a contractor from Maine said that in his area they are more interested in humidity removal than cooling as it rarely goes over 85 degrees. His strategy and belief is that an oversized coil and a smaller condenser will pull more humidity out of the air stream, the smaller condenser will run for a longer period while the larger coil traps more moisture. He said he had been using this strategy with good success.
Today my HVAC sub told me that the coil they are spec’ing will work with either a 1-1/2 ton condenser or a 2 ton.
Seems to me that the best strategy is to match the tonnage of the coil to that of the compressor.

[Ted S.]

Re: 2 ton coil with a 1.5 condensor

Curious if you can get an AHRI Certificate with this mix matching.

[Allan Edwards]

Re: 2 ton coil with a 1.5 condensor

At least in our hot humid climate over-sized, very efficient units do not run enough to dehumidify. Temperature is satisfied and fan coil unit does not run enough not enough to remove humidity.

[johnny watt]

Re: 2 ton coil with a 1.5 condensor

Wrong. You can't make the coil colder, it's operating at very low temp already and making it colder would make it ice up ("freeze").

I can make a coil colder by running the air through it slower. If I run it slow enough, the coil will freeze up. Or if I want to keep the air speed the same then I could use a smaller coil to run it colder for a given BTU.

The concept of running the coil colder to remove humidity is something I read years ago in an HVAC design book, which talked mostly about chillers and 34-50 degree coils, but also covered packaged units. It also mentioned the unlikelihood that a packaged unit would be a well designed system.

No. Longer run time lets more air pass by the coil and condense. Sure, a coil at 69F won't condense, but it also won't cool the house much. So coils are never run at 69. Longer run times are achieved by choosing a slightly lower system capacity (undersizing the system, or at least not oversizing it). The most common cause of insufficient humidity removal is oversized systems.

I brought up a 69 degree coil to illustrate a concept, not as a design option.

From googling this morning, it looks like the most common cause of insufficient dehumidification, is too large a coil, which holds too much water, which is reintroduced into the conditioned space while the fan continues to run, in order to harvest the residual cooling of a cold coil, and increase the bogus rating known as SEER

[jud aley]

Re: 2 ton coil with a 1.5 condensor

Johnny-
I also saw a thread about a larger coil holding too much water, which is reintroduced into the conditioned space while the fan continues to run, but I also found a thread where a guy states just the opposite.

I have the cut sheet for the Trane condenser and it states “Cooling capacities are matched with a wide selection of air handlers and furnace coils that are AHRI certified.” So does that mean that the condenser is designed to work with different size coils with out problems?

[Ted S.]

Re: 2 ton coil with a 1.5 condensor

The matched system should have it's own certificate.