Oversized Coil Drawbacks?

[teeoff]

I am considering installing a Carrier Infinity dual fuel system in my house. A three ton heat pump should be sufficient for cooling capacity, but I see that the coil size changes the output significantly. With the 25HNA6 heat pump and the CNPH*3617A** coil I would get 33400 BTU/Hr, with the CNPH*4821A** coil I would get 34600 BTU/Hr. The EER, SEER,COP, & HSPF all increase with the larger coil. Other than size, is there any drawback to using the larger coil? This would be matched with a 58MUV-60-14 furnace. Also, would anyone recommend going to the 25HNA9 heat pump for its higher efficiency and scroll compressor?

[lolson]

this looks like a DIYer. call a pro
an over sized coil is better in some areas but has some drawbacks in others

[teeoff]

I don't plan to install it myself. I do plan to select the system from several choices. My house is not the the same as the average house in the St Louis area, requiring me to do my homework to pass on to the contractors.

[catmanacman]

oversized coils due increase the seer rating but they due not dehumidify as well oversized coil will require a lower fan setting to keep the coil cold enough for proper dehumidification

[BigJon3475]

Humidity control is the main problem with going with "to large" of a coil.....However you do gain more sensible heat removal which is why you see 1200 btu difference......with more btu's being removed/added if your still using the same power you get a higher efficiency. It would all depend on your latent load and your sensible load to determine if a larger coil is a better for you or not which is where the load calculation comes in to play.

[teeoff]

Thanks for the information. Using HVAC-Calc i get 24,938 BTU/Hr sensible gain and 6,911 BTU/Hr Latent gain. 72% of the 3 ton coil output of 33,400 BTU/Hr is 24,048 BTU/Hr. Does that mean that the full 1200 BTU/Hr increase can be applied to the sensible gain? As you can see, I am very close to the edge on this and I would rather not jump to the 4 Ton unless needed. We currently have a 3.5 ton lennox AC unit (actual 38,000 BTU/Hr according to the Carrier rep.), it has had no trouble cooling the house at the hottest outside temps.

[BigJon3475]

The amount a system is rated for is not what it will do on a regular basis. You need to match the required gain/loss load to the manual D procedures so that you purchase the correct equipment for your needs. The specifications that are presented with the equipment are @ 95º outside temp and 80º dry bulb with a 67º wet bulb. If the outside temp goes up you loose capacity. If the inside temps drop you lose capacity. If I remember correctly for every 1º drop in inside temps you loose about 835 btu's of sensible capacity per/1000 cfm of air flow. So if you have a 3 ton system moving 1200 cfm of air and you want it to actually cool to 75º instead of the 80º that it's tested for efficiency at the system will be putting out 5000 btu's less than the 36,000 it may be rated at.



It would depend on your design conditions and the equipment that your wanting to get.

[teddy bear]

The cooling btus are the results of the size of the coil, the capacity of the compressor, and the amount of air flowing over the coil. The main changes to improve the efficiency of the current a/c is to increase the size of the coil and increase the flow of air over the indoor/outdoor coils. Smart contractors slow the air flow over the indoor coil to get a very cold coil to remove enough moisture. Sensible capacity declines and latent remains the same. This improves the latent ratio to a point. An extra half ton of capacity to handle the reduction in sensible capacity makes sense. In addition, we have many days that exceed design load. You have visitors on a hot or you wish to set the t-stat up during routine time you are not in your home, makes extra capacity desirable.
Something not addressed in your calcs is the variable sensible/latent ratios for cooler damp days. What is the s/l ratio when it is raining and 70^F for a week. You still have the 7,000 btus/hour of latent load with low/no sensible cooling load. Even the undersized a/c is unable to deal with the latent load. For real comfort and control, size your a/c an extra .5 ton and add a 90 pint/day whole house dehumidifier to maintain <50%RH without any a/c operation. This allows turning the a/c off when unoccupied and maintaining <50%RH. This can cut your a/c cost in half while maintaining ideal %RH. Regards TB

[thorton]

The cooling btus are the results of the size of the coil, the capacity of the compressor, and the amount of air flowing over the coil. The main changes to improve the efficiency of the current a/c is to increase the size of the coil and increase the flow of air over the indoor/outdoor coils. Smart contractors slow the air flow over the indoor coil to get a very cold coil to remove enough moisture. Sensible capacity declines and latent remains the same. This improves the latent ratio to a point. An extra half ton of capacity to handle the reduction in sensible capacity makes sense. In addition, we have many days that exceed design load. You have visitors on a hot or you wish to set the t-stat up during routine time you are not in your home, makes extra capacity desirable.
Something not addressed in your calcs is the variable sensible/latent ratios for cooler damp days. What is the s/l ratio when it is raining and 70^F for a week. You still have the 7,000 btus/hour of latent load with low/no sensible cooling load. Even the undersized a/c is unable to deal with the latent load. For real comfort and control, size your a/c an extra .5 ton and add a 90 pint/day whole house dehumidifier to maintain <50&#37;RH without any a/c operation. This allows turning the a/c off when unoccupied and maintaining <50%RH. This can cut your a/c cost in half while maintaining ideal %RH. Regards TB

Teady Bear, your a RH guru. Your answers are second to none!!! I, too think that sensible heat removal is # 1, in my mind, anyways. When I'm too hot, I want sensible load, not latent load. I don't want to wait for hours for a system that is sized marginally, according to manual J equation. Don't get me wrong, I know that dehumidification is important, too. Putting in equipment that is .5 over manual J, makes sense to me. I sure wish I had of put in a whole house deumidifier. Where I live the last 6 weeks or so I didn't need heat or cooling, yet the inside of my home was 60RH most of those six weeks. Bears Rule!!!

thorton
_______________
Birthdays are good for you
The more you have, the longer you live

[pstu]

Thanks for the information. Using HVAC-Calc i get 24,938 BTU/Hr sensible gain and 6,911 BTU/Hr Latent gain. 72&#37; of the 3 ton coil output of 33,400 BTU/Hr is 24,048 BTU/Hr. Does that mean that the full 1200 BTU/Hr increase can be applied to the sensible gain? As you can see, I am very close to the edge on this and I would rather not jump to the 4 Ton unless needed. We currently have a 3.5 ton lennox AC unit (actual 38,000 BTU/Hr according to the Carrier rep.), it has had no trouble cooling the house at the hottest outside temps.

Teeoff, since you are analyzing this in more detail than the usual homeowner, have you considered buying ACCA Manual S and sizing equipment the right way? It will cost you only about as much as you have already spent for the Hvac-Calc license. This is no more or less than I have done. I myself am a homeowner in S.Texas who used to live in St. Louis, just curious where are you?

I know that humidity is distinctly a concern in the St. Louis area, as it is where I live just west of Houston TX. Are you figuring sensible capacity is a flat 72% of total capacity? The sensible heat ratio is sometimes 72%, often not. It also depends very much on operating conditions, I could email you some Trane technical manuals which prove it is a function of outdoor temperature, indoor temperature, and indoor humidity. Email me if you want that, using the address in my profile. If your chosen brand is not Trane, you can either seek the appropriate manual or use Manual S for some close estimates.

Some of the rules important to Manual S are to:
1) Adjust for the equipment output, it will be de-rated compared to the ARI ratings because your design day will be a hotter temperature.
2) Adjust for inside design temperature, ARI is for 80F and you probably want 75F or so.
3) If your latent capacity is in excess of what is needed, approximately half can be considered as additional sensible capacity, I think.

Hope this helps -- Pstu

[teeoff]

Teeoff, since you are analyzing this in more detail than the usual homeowner, have you considered buying ACCA Manual S and sizing equipment the right way? It will cost you only about as much as you have already spent for the Hvac-Calc license. This is no more or less than I have done. I myself am a homeowner in S.Texas who used to live in St. Louis, just curious where are you?

I know that humidity is distinctly a concern in the St. Louis area, as it is where I live just west of Houston TX. Are you figuring sensible capacity is a flat 72% of total capacity? The sensible heat ratio is sometimes 72%, often not. It also depends very much on operating conditions, I could email you some Trane technical manuals which prove it is a function of outdoor temperature, indoor temperature, and indoor humidity. Email me if you want that, using the address in my profile. If your chosen brand is not Trane, you can either seek the appropriate manual or use Manual S for some close estimates.

Some of the rules important to Manual S are to:
1) Adjust for the equipment output, it will be de-rated compared to the ARI ratings because your design day will be a hotter temperature.
2) Adjust for inside design temperature, ARI is for 80F and you probably want 75F or so.
3) If your latent capacity is in excess of what is needed, approximately half can be considered as additional sensible capacity, I think.

Hope this helps -- Pstu

I am actually in Illinois, just across the river from Arnold. I did not plan to get this involved in sizing the equipment, but the only contractor who offered to do an actual load calculation has not called back for 2 weeks. I was just using 72&#37;, just trying to get in the ballpark. As you can see, we are right on the edge with a 3 ton heat pump & with the Carrrier Infinity we were looking at, the next step up is 4 tons. Since we were looking for a dual fuel system, that also required a jump to an 80,000 BTU furnace instead of the 60,000 to get enough airflow capacity. Our house is built with insulated concrete forms which also complicates the calculations, the extra mass in the walls has a smoothing effect on the thermal load. Our AC may not run as much during the hottest part of the day, but the heat does not disappear when the sun goes down, so our AC runs more at night than our neighbors'. We also don't do the setbacks because my wife teaches piono in the house, so there is always someone home.

[dcmcm5]

I guess I'll have to disagree with the suggestion to add 1/2 ton above the calculated value. While a professional that knows what he is doing may choose to do that given the specific conditions, house, requirements, etc., a blanket suggestion can be misinterpreted and create problems elsewhere.

If the load calculations are right and the equipment selection is done correctly, why add additional capacity? If you do have periods of several weeks where no cooling is required, why not just use the dehumidifier and install the correct size unit?

And yes, according to Manual S, up to 1/2 of the excess latent capacity of the equipment can be used for sensible heat removal.

[John Markl]

Aside from the minutae of load calculations, here in Texas, I like the extra half ton on a coil. Too often, calculations that look great on paper, don't work so well when it's 95+ for days on end........with the extra size coil, I can flip a larger condensing unit and have a happy customer.

I've done this dozens of times over the years, mostly on systems where another company did the installation, but I came in years later and turned a disgruntled homeowner into a happy camper, just by installing a new condensing unit that matched the original coil size.

[dcmcm5]

I have had great success with the correct size without having to add the extra capacity. The answer to "undersized" units may not be a larger one, because it may be lacking the capacity because of duct problems or other reasons. I don't ever remember seeing a calculation done right and an installation done right to match the calculation being a problem. One of the two have to be wrong, or a combination of minor problems with both.

I was not really addressing a professional that knows what they are doing such as you and TB installing what is right for a specific situation. My point was for the casual person surfing this board that sees a pro recommend extra capacity, then takes that to someone that does not do things right and asks for the extra capacity on top of a system that is probably already too large.

I think we have all seen disgusting jobs (too small, too large, poor design, poor installation, etc.) done by other "professionals" and it makes it hard when we say "call a pro" yet the person asking has been already burned by a "pro". But I hope anyone that reads a few of these posts will be able to learn enough to sort out the good from the bad.

[John Markl]

The particular contractor that I followed so many times, was notorious for using larger indoor coils to gain higher SEER "rating" as opposed to providing the actual desired SEER equipment.

For example, they would install a 9 SEER 3 ton condenser, with a 9 SEER 4 ton coil, and call it 3 ton 11 SEER.

A half ton oversized coil is no biggie, in my book, but I see some really radical stuff. I have an elderly customer who has a five ton cap tube slab coil with a 3 1/2 ton condensing unit. She's convinced that it works fine, and since she's on a "fixed income" I can't convince her to replace either component.

[dcmcm5]

Those are a couple of good examples to store away in my "really dumb" file. I would agree that the 1/2 ton is not a big deal. I wish we could educate homeowners to understand that those types of situations are to be avoided and to find someone that knows what they are doing, but there will always be someone that will give in, mostly because of the price.

[teddy bear]

I was hoping for more discussion about the lack of latent capacity at the lower/no sensible cooling loads. Maintaining <50%RH during cool wet weather with dehumidification, eliminates the critical a/c sizing issue. An extra .5 ton of cooling capacity is great as equipment ages or the equipment is not available in .5 ton sizes. The most negative issue with large cooling coils is the 2 lbs. per ton of moisture retained on the coil between cycles. A 4 ton has 6-8 lbs. of retained water that slowly evaporates into the ducts keeping the ducts wet for days. One lb. of water evaporating raises the %RH in 1,000 sqft. of home 8%RH. Supplemental dehumidification allows for daily drying of the ducts to prevent mold growth over the long term. IAQ requires maintaining <50%RH throughout the home and the air handling systems to avoid mold growth. Low humidity is also a comfort issue. Regards TB

[teeoff]

That is exactly why I was asking about the negatives that come with an oversized coil. But, when the alternative is to go to a 4 ton heat pump with the same 4 ton coil, wouldn't you end up with the same amount of moisture in the coil along with longer periods without air movement?

[drsmith012]

The most negative issue with large cooling coils is the 2 lbs. per ton of moisture retained on the coil between cycles. A 4 ton has 6-8 lbs. of retained water that slowly evaporates into the ducts keeping the ducts wet for days. One lb. of water evaporating raises the &#37;RH in 1,000 sqft. of home 8%RH. Supplemental dehumidification allows for daily drying of the ducts to prevent mold growth over the long term. IAQ requires maintaining <50%RH throughout the home and the air handling systems to avoid mold growth. Low humidity is also a comfort issue. Regards TB

A four ton unit holds almost a gallon in its coil! I had no Idea they held that much water. I knew my coil had a lot of water when I ran the fan continously during low demand cooling days. I swear I could feel the humidity actually rise.

Also I am supprised no one has suggested a dual stage unit. My heat load calc came out on the edge of three ton or 2.5 ton so I went with a dual stage unit. two ton low, three high. Unfortunantly/fortunantly my ground loop is so efficient due to the saturated soil I am actually running in the four ton range on high.

[dcmcm5]

I was hoping for more discussion about the lack of latent capacity at the lower/no sensible cooling loads. Maintaining <50%RH during cool wet weather with dehumidification, eliminates the critical a/c sizing issue. An extra .5 ton of cooling capacity is great as equipment ages or the equipment is not available in .5 ton sizes. The most negative issue with large cooling coils is the 2 lbs. per ton of moisture retained on the coil between cycles. A 4 ton has 6-8 lbs. of retained water that slowly evaporates into the ducts keeping the ducts wet for days. One lb. of water evaporating raises the %RH in 1,000 sqft. of home 8%RH. Supplemental dehumidification allows for daily drying of the ducts to prevent mold growth over the long term. IAQ requires maintaining <50%RH throughout the home and the air handling systems to avoid mold growth. Low humidity is also a comfort issue. Regards TB

TB - Why would you need the extra .5 ton if you used a dehumidifier? If you don't need to remove the sensible heat, wouldn't the dehumidifier maintain the humidity?