Gas and Oil Home Heating Furnaces - Mismatch size of condendor and evaporator units?

I am in the process of replacing both of my evaporators (separate units for unstairs/downstairs). I have had a number of AC companies give me their opinion on what I should do. Most of the companies want to match the size of the evaporator units with the condensors (i.e. 3 ton to 3 ton).

One company wants to install a smaller evaporator coil (i.e. 3 ton condensor to 2.5 ton coil) Their claim is that this system will remove more humidity since the coil will get much colder (I live in Houston - a very humid climate). I can understand the logic of this. However, this company installs custom systems not off-the-shelf units so their systems are 2 - 3 times the cost of an off-the-shelf unit. Plus their warranty 3 - 5 years parts and labor isn't as good as some of the off-the-sheld units.

My questions are:

1) Should I consider getting an off-shelf-model with an evaporator coil that is smaller than my condensor outside? My only concern doing it this way is the fan will be rated too small and I won't get enough airflow.

2) If I do go with an off-the-shelf unit, can I (or should I) go with a unit other than Carrier? Carrier comes with only a 3 -5 year parts and labor warranty. The American Standard that I have been considering comes with a 10 year parts and labor. In addition, the Carrier uses a metal primary pan whereas the American Standard has a plastic one. Since the main problem with the old units is that the primary pans have rusted out, I would think that going with a plastic primary pan would be better.

Here are some facts about my home. As you can see, the evaporators are pretty old.

Location: Houston, TX
House: 2000 sq feet (1000 upstairs, 1000 downstairs)
Condensors: 2.5 ton Carrier Heat Pump upstairs (1998)
3.0 ton Carrier Heat Pump downstairs (1995)
Evaporators: 3.0 ton Carrier Upstairs (1988)
3.0 ton Carrier Downstairs (1988)

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I presume that the new units considered will be heat pumps (like the old ones are). Unless the factory lists the smaller coil and the chosen condensing unit as an approved match, don't go there...especially with a heat pump.

A smaller coil does NOT mean reduced airflow. I believe that the custom design is based on full airflow for an outdoor unit larger than the manual J design recommendations and a smaller coil so it will run colder. I don't believe that they ever do this with heat pumps. The custom design is likely to be an approved off the shelf design AC with electric strip heat.

I would recommend the American Standard heat pump system with a variable speed air handler. Chose the fan speed at 350 cfm per ton. Then size per manual J for the smallest unit that will do the job. This will get the maximum run time for optimum humidity control. This would be very effective. You can even put a humidistat on the system to optimize it further.

I'm a little confused. You're replacing ONLY the indoor coils or you're replacing both the indoor and outdoor sections? If you're replacing only the indoor coils, you'd better grill the contractor about warranty. A lot of manufacturers won't warranty a mismatch, either by size or brand. [especially on heat pump systems] Am I reading correctly that you currently have a mismatch on the upstairs system?

Hi MZW....after reading your post I've got to jump in here with a couple comments. Like BigJohn, I too am a litle confused with exactly what you're going to replace, but let me say first, that if you now have a 2.5T rated indoor coil and a 3T outdoor unit upstairs, you've had a mismatched heat pump system from the gitgo. Regardless of whether someone told you that was the way to go, or whether or not you've been "satisfied" with its preformance, to get the most out of a heat pump both from its rated efficiency or from the refrigeration cycle, by all means match both indoor and outdoor units. That 2.5 T replacement coil you spoke about will be a mistake, even with a straight cool system. Lousy humidity removal. In straight cool systems, its common to oversize the evap. by a half ton to give better humidity removal while keeping the fan speed/volumne the same. The larger coil has more surface area for airflow and the compressor has the extra capacity needed to keep the coil saturated. This also is accomplished with high efficiency equiptment and the manufacturer dials in the proper fan speed to match coth coils. If you're going to replace indoor coils, my suggestion would be to go with Carrier because of their expansion device. I'm not sure what Amer. Std. uses, but a coil is a coil. Plastic versus steel drain pans? I'm old fashioned. I'd go with steel. If good humidity is a great concern, then you might rethink about replacing one or both units with high efficiency ht. pumps...... matched. This way you will get good warranties on both equiptment and labor along with a trouble free system, so long as you keep clean filters. In any event, keep it simple and go with "off-the-shelf" equiptment and a good installer.

I plan to only replace both of the evaporators. As a few of you pointed out, I currently already have one set of mismatched units. Unfortunately, it appears to be mismatched in the opposite of what I would want.

I think if money were no object, I would seriously consider their custom designed system. But of course, money is always a driver. So my biggest question is, should I ask an off-the-shelf installer to put in a unit with a smaller coil and get him to up the fan speed? My gut says this is probably not a good thing to do.

Below is the info on using smaller coil sizes. This link only gets you to the home page. One the left side of the page, about half way down you will see a link called "Downsized coils". In it is a 30 page engineering document on why smaller coils are good in a high humidity environment. They also include some information on how the fan should be set up, and that the overall efficiency is lower but not significantly lower. After browsing the engineering document (most of it is over my head), it appears that there was some debate 5 - 10 years ago that "oversizing" the evaporator would increase humidity removal. Lennox has done some research to show that this really isn't the case and that a smaller coil size removes more humidity.

I will keep you posted on what I end up doing. Unfortunately, for cost reasons I will probably go with a matched system. If I do go the other route, I will let you know what the results are.


Central_City_Air (http://www.centralcityair.com/Index2.htm)

Sometimes its is hard to keep track of all of the great info that everyone provides while posting a new reply.

From the info everyone has given me, I will not try and mismatch off-the-shelf systems to incorporate a smaller coil. Sounds like if I did I would be making a serious mistake.

If I break down and go custom (and like I said in the prior post, I probably won't due to cost), I will follow their recommendation and go with the smaller coil design. If I do this, I will let you know the results.

Thanks again for the good advise and the quick responses. You can't imagine how valuable the service you provide is!:)

If you had straight cooling, you could experiment, BUT because you have heat pumps, you can't undersize the indoor coil relative to the capacity of the outdoor section. If you do, you wind up with an undersized heat rejection coil during the heating cycle which is a sure death warrant for the compressor.

That must be why the custom install company said they could design a custom evaporator to go with the heat pump condensor, but in doing so they would be required to disconnect the heat pump cycle. As a result, I would only be able to use the electric heat strips for heating.

Makes sense. Thanks again for the info.

Look at:
http://hvac-talk.com/vbb/showthread.php?threadid=17197

Airman1 is the contractor who is promoting the custom solution. One of the best points he makes is

"Again downsizing cooling coils is not new and the only reason. I repeat the ONLY reason it is not popular is because it affects the manufacturers SEER...where was the mold before SEER ratings...how many people moved out of their house before 1992 because of mold? Why are there 1000 times more mold cases today then 6 years ago? What has changed to cause this...it is more than just A/C but the A/C system is the one factor WE control and therefore can correct the mold problem in high humid areas...with better designs and performance...no humidity no mold..."

I happen to live in the hottest and driest climate in the US. I have seen runaway mold even here. Also, the dirty socks syndrome was unknown in Yuma, AZ several years ago even though the problem has been in the Southeast for over 20 years. We see it now. The question is WHY? If we see this in YUMA, I can scarcely imagine the issues in Houston.

The discussion shows that the problem is complex and that there are a lot of low bidders out there doing what they do best. Knowledge and competency is not what they bring to the jobsite.

Sizing the equipment, airflow, air filtration and maintenance are all critical to humidity control and mold prevention. A standard system can give good results if installed and maintained by a knowledgeable and competent contractor. Don't get to enamored with a low price or a pleasant personality.

Quote from the same thread cited before:

" have to say that we have a 3 ton Carrier 38TKB condensing unit for 10 years now. When we got the unit, we did not replace our old indoor coil and furnace. The indoor coil was a 2 ton coil. The installer knew this so he put a TXV valve on the indoor coil, and for 8 years that system worked just fine. Never had any, and I mean ANY problems.It felt so comfortable because the indoor humidity was from 43-45%RH and we had the thermostat at 72-73° and felt great. Then in 2000, the furnace developed a crack in the heat exchanger and we got a new furnace and coil. The company increased the indoor coil from a 2 ton to a 4 ton matched to the 3 ton condensing unit. The furnace size was was the same size as the old with the exact same blower size because they did a load calculation. To make a long story short, the installing company said they would increase our SEER, but mind you, this is only a 10 SEER air conditioner. Ever since they put in that new coil, our humidity has gone up to 60% and we now have the thermostat at 71° because it feels clammy with the extra humidity. We all are allergic to mold and have since had to get a dehumidifier just to lower it to 55%. Our eletric bills have gone up with this setup because of the dehumidifier and the lower temperature we have to set it at to feel comfortable. Can anyone tell me how you can convince a heating and cooling contractor to downsize your indoor coil, since we would be paying for installation and everything, just to get the humidity down?We live in out in the county in southeast Missouri and it gets very humid around here. Sorry to vent.

Chris"

MZW.....just a short two cents, for what its worth to you. I pulled up CCAir who I gather you've talked to. Their home page is put together very well by someone. I was surprised to note they DO NOT do Commercial work where himidity control sometimes is an absolute must. Having said that, manufacturer's with the higher SEER's, give better humidity control because of a combination of larger coils, correct fan speed to match the coil temperature, etc.,etc.. Think about it.....with flooded evap's., the smaller the coil surface, the less moisture removal. Larger coil (to a degree), better moisture removal because of the larger surface for airflow. With an undersized evap., you invite trouble because the system is out of balance with the principals of refrigeration period. As I previously stated, keep matched units with heat pumps. Good humidity control is a must with allergies, so you may want to consider a humidistat which will keep the unit running longer. Gets too cold, then its time for reheat. Again , keep it simple with stock equiptment. Good luck.

I’m gonna add my opinion without reading the other replies so I may duplicate something or confuse the issue even more.

Assuming your systems are sized properly in the first place, down sizing an evaporator coil is totally wrong. Up sizing the indoor coil will gain better humidity control and not the other way around, provided you don’t go overboard with the sizing. As for this company doing custom work yada, yada, yada. That’s sales and nothing more than justification for the higher price. If their systems are 2-3 times the norm, you’re getting ripped. You’d be wise to walk away.

Off the shelf brands –VS- the more widely known names. Consider this, the laws of physics don’t change. Three tons of air is three tons of air no matter how you add it up. Each manufacturer makes the same thing for a given model group with slight differences in physical design. What you need to consider more than name is what you’ll get for your money. Take the better warranty and do your best to check out the contractor your considering.

If your existing coils are rotted out it’s due to improper leveling of your equipment causing water to stand in the drain pan and/or the drain itself is dirty. You’ll need to be sure the new one’s are level, front to back and side-to-side, also the drain is clear.

Phil

What does alarm me is 5.5 tons of air on a 2K sq. ft. house. That seems a bit much and may be a reason to downsize the coil but is detrimental to the operation of your system and not the proper solution to an oversizing problem.

So, Phil and Ty, both of you think that raising the SST will result in removing more latent heat from the air flowing thru the evaporator?

MZW;
The price you got for downsized coils was actually from AirMan1's company, Central City Air in Houston, right?

His downsized coil method will prevent you from going back with heat pumps. In fact, he says heat pumps do not work in Houston, due in most part to his downsized coil design.

Airman1 is held in high esteem by ACCA, and is invited to speak at all sorts of IAQ symposiums, but if you read the thread Lynn referenced above, you'll see that it is almost impossible to get facts from him.

The two brands of equipment he sells are Amana and Lennox, both of whom have posted their warranty requirements on the Internet. They both require matched or rated components. Airman saya his components are rated and meet the manufacturer's warranty, but Robin Boyd (the Goodman/Amana tech rep) says they don't. Being involved in manufacturing myself, I can tell you the systems he sells are not ARI rated, nor UL/ETL approved. That does not mean you'll have a problem, it just means any problem you have will be between you and Central City Air, as the manufacturer may or may not warranty your system.

You have to make your own decision, but my first question would be, "Have you had complaints about high humidity?" My second question would be, "Do you have issues with mold?" If you answered no to these questions, I'd recommend passing on the downsized coil design.

As far as the coil downsizing, look deeply into the subject, with other contractors.

The fact is, there are no facts, just lots of evasive double talk.

There are areas that suffer from high humidity, no doubt Houston is one such area.

My particular part of the country, (PA) however, rarely has extreme humidity.

I've been installing HVAC systems here to the ACCA "Manual J" for quite a few years, and have NEVER ONCE had a house suffer from high humidity. This is a direct contradiction to what the gentleman from Houston says about manual J. He (Airman1) is telling everyone that will listen, that manual J is wrong.

Please use extreme caution, and your own good judgement before going with a downsized coil.

It seems strange that coil sizing has become an issue in the past couple years.

Lots of systems out there installed htirty years ago, that are still operating, and are not full of mold. With standard coils...

Originally posted by condenseddave
As far as the coil downsizing, look deeply into the subject, with other contractors.

The fact is, there are no facts, just lots of evasive double talk.

There are areas that suffer from high humidity, no doubt Houston is one such area.

My particular part of the country, (PA) however, rarely has extreme humidity.

I've been installing HVAC systems here to the ACCA "Manual J" for quite a few years, and have NEVER ONCE had a house suffer from high humidity. This is a direct contradiction to what the gentleman from Houston says about manual J. He (Airman1) is telling everyone that will listen, that manual J is wrong.

Please use extreme caution, and your own good judgement before going with a downsized coil.


It seems strange that coil sizing has become an issue in the past couple years.

Lots of systems out there installed htirty years ago, that are still operating, and are not full of mold. With standard coils...

All this coil sizeing for what?? Many many years ago i went to service a unit in a home about 35k with lots of glass to the west.
Looked at the coil first and it had two set's of lines going into it
now what I found out it had a split coil with 2 two ton A/C in it with one blower and duct set up for the 4 ton,on a 2 stage t-stat. This set up held the home just fine.Later i had some eng. and sales rep.run J loads on the home they all came up with 6 to 7 ton.
But the 4 ton worked just fine.After that i used this set up a lot in homes and commercial.At one time a company made a 5 ton unit with a 2 and a 3 ton compresser's in it that worked fine. I have put in restaurant's say with a 10 ton load two 5 ton on a 2 stage stat with the duct work layout for the 10 ton and the blowers locked together.We never had any mold in any of our duct work.
If you have mold in the duct work ill bet the ac unit is over sized.
Also a good way to keep the mold out is put a germicidal light in the duct work and have it turn when the blower is running.Have done that also. ;) ED

This is a reply to bigjohn's post concerning raising the SST for more latent heat removal. I made no mention of raising the sataurated coil temperature and would be hesatant to do so. Perhaps I should have used the term "full evaporator" rather than the term "flooded". My intent was to indicate that with a given volumne of air flow, the larger proper sized coil will remove more moisture (latent heat) than the smaller.

If you read much of the thread that I referred you to, you see that there is no agreement to recommend the downsized coil solution. That doesn't mean that the downsized coil is not effective when done by Central City Air. Most of the industry feels that the standard available equipment, properly sized and installed, will provide excellent results. The contractor is always the key. No manufacturer has any control over the end results.

So I will restate my recommendations to stop your headache:

"I would recommend the American Standard heat pump system with a variable speed air handler. Chose the fan speed at 350 cfm per ton. Then size per manual J for the smallest unit that will do the job. This will get the maximum run time for optimum humidity control. This would be very effective. You can even put a humidistat on the system to optimize it further."

Make sure you get a good, established, knowledgeable and honorable contractor. Brand, price and efficiency are secondary.

excellent advice, lynn.

Originally posted by condenseddave
excellent advice, lynn.

Ditto.

I agree with lynn. I believe that both Trane(American Standard) and Carrier have a great variable speed air handler system. With Trane you can add a seperate humidity control. With Carrier you can get a Thermidistat. It is a thermostat and humidistat combo. I have looked in both Carrier and Trane's catalog and Carrier does not list any approved systems with smaller air handlers. Trane does. But remember this. On a lot of systems, say 2 1/2 ton and 3 ton air handler, The same coil is used. You just change the piston or accurator to match what the compressor pumps. Here where I live, you are not supposed to be able to install a system that does not have an approved ARI rating. Ask this contractor to show you in the ARI book the approved match he is wanting to install.

Again, Great advice Lynn.