Evaporator sizing - is bigger better?

[cobra2411]

While I can't answer about multiple evaporators, on the conventional split systems I install I usually go a half ton larger on the evap coil. The added surface area slows airflow and you get better heat and dehumidification.

[doug30293]

Originally Posted by cobra2411

While I can't answer about multiple evaporators, on the conventional split systems I install I usually go a half ton larger on the evap coil. The added surface area slows airflow and you get better heat and dehumidification.

Based on ARI ratings for several heat pumps I looked at it appears that 1/2 ton up on the evap coil is good for one point on the SEER scale. This seems to be the standard approach between 14 and 15 SEER.

I could not find any evidence that one ton (two sizes) up on the evap coil results in further improvement. Perhaps 1/2 ton is the edge of diminishing returns.

[AC_Hacker]

Originally Posted by doug30293

Based on ARI ratings for several heat pumps I looked at it appears that 1/2 ton up on the evap coil is good for one point on the SEER scale. This seems to be the standard approach between 14 and 15 SEER.

I could not find any evidence that one ton (two sizes) up on the evap coil results in further improvement. Perhaps 1/2 ton is the edge of diminishing returns.

> " it appears that 1/2 ton up on the evap coil is good for
> one point on the SEER scale."

It this relative to a 1/2 Ton heat pump, or a 4.5 Ton Heat pump, or larger, or smaller?

Size matters.

-AC

[jeff5may]

Originally Posted by doug30293

Based on ARI ratings for several heat pumps I looked at it appears that 1/2 ton up on the evap coil is good for one point on the SEER scale. This seems to be the standard approach between 14 and 15 SEER.

I could not find any evidence that one ton (two sizes) up on the evap coil results in further improvement. Perhaps 1/2 ton is the edge of diminishing returns.

This is in reference to the indoor coil in an air handler. With a heat pump, oversizing the indoor coil too much will kill your supply air temperature. Instead of 120 degF air coming out of registers, you will have 110 degF air, which feels "not so hot". Efficiency may rise a smidgen, but the overall "warmth" provided may be seen as diminished by the average user. Not a good thing to most.

What we are talking about here is the outdoor unit heat exchanger being oversized. With the correct refrigerant metering scheme, heat transfer will be increased. This will lead to increased mass flow through the compressor, and higher discharge temperature and pressure. Indoors, this leads to increased condenser temperature and pressure in the air handler, and increased supply air temperature at the registers. Perceived "warmth" effect will be increased regardless of whether you change the indoor coil or not.

[MN Renovator]

If the indoor coil has more surface area, you might get more EER/SEER out of it but when cooling that extra surface area can diminish some of its dehumidification capability because the coil needs to be saturated before the water starts to run off of it and once you've met the temperature demand of the cycle and it shuts off, all of the water remaining in the coil will evaporate back into the air after the cycle and be return to the building. Taking a look at the AHRI ratings for 1.5 ton and 2 ton units of the Bryant variety in their 16 SEER models, I'm usually only seeing a boost of about 1/2 SEER by jumping from a 2 ton to a 3 ton coil. I'd rather sacrifice a partial SEER point and have better dehumidification and get a condenser and blower that are as efficient as they can be if I'm going with a central air unit because reducing latent heat(removing moisture) is very important, it isn't just about removing sensible heat(reducing temperature).

I currently have a 2 ton and if I went with a whole house heat pump I'd be willing to keep the slight oversize because I can manage to run a single cycle and still be plenty comfortable but a 1.5 ton would cause the temperature to be more constant while its running rather than have it get to 75 to 78 before powering on and have it shut off at 70-72 when it is done for the day. I've found that this approach removes enough moisture in the air to make the next day's high 70's feel very comfortable. I'd only go to a 2 ton for if I wanted a heat pump because I'd get closer to the capacity I'd need for heating without too much cooling oversize for my house, also the HSPF and EER/SEER usually is higher in the 2 ton units for the brand I'm looking at because people who have low loads to use a 1.5 ton aren't usually going to use too much energy in comparison to someone who needs a 3 ton, for example, and it's harder to sell premium efficiency units in that size.

[NiHaoMike]

Originally Posted by MN Renovator

If the indoor coil has more surface area, you might get more EER/SEER out of it but when cooling that extra surface area can diminish some of its dehumidification capability because the coil needs to be saturated before the water starts to run off of it and once you've met the temperature demand of the cycle and it shuts off, all of the water remaining in the coil will evaporate back into the air after the cycle and be return to the building. Taking a look at the AHRI ratings for 1.5 ton and 2 ton units of the Bryant variety in their 16 SEER models, I'm usually only seeing a boost of about 1/2 SEER by jumping from a 2 ton to a 3 ton coil. I'd rather sacrifice a partial SEER point and have better dehumidification and get a condenser and blower that are as efficient as they can be if I'm going with a central air unit because reducing latent heat(removing moisture) is very important, it isn't just about removing sensible heat(reducing temperature).

You decrease the airflow to regain the dehumidification lost by upsizing the evaporator. The net efficiency still improves, especially if you're using an ECM fan motor and a humidity sensor to vary its speed.

[MN Renovator]

Originally Posted by NiHaoMike

You decrease the airflow to regain the dehumidification lost by upsizing the evaporator. The net efficiency still improves, especially if you're using an ECM fan motor and a humidity sensor to vary its speed.

Aren't you still dumping the extra moisture that is in the larger coil back into the air after the cycle is complete? You may be removing more at the beginning of the cycle with the reduced airflow but you aren't really removing the humidity from the building until the moisture has saturated the coil and it starts to drip off. These are things that I'm not sure really get measured in the lab tests and I think the latent heat that gets added back after the cycle isn't insignificant. ..and I think that for the average oversized system in most homes this is a detriment because short cycles make this an issue. In my case, I probably would never notice because I make sure that the shortest cycle that ever runs is at least 2 hours. Most systems are oversized and probably run 30 minutes and 20 minutes of that is saturating the coil and only 10 minutes of true moisture removal and so 20 minutes of cooling essentially was left as sensible only but since the moisture removal reduces sensible BTU removal the overall sensible efficiency is less and the latent removal not what it could have been.

Granted system design is extremely important and I think a slightly undersized system(rounding down from manual J instead of rounding to the next .5 ton size) is the key to the best efficiency but contractors aren't willing to do it and homeowners generally don't understand the benefit of better moisture control and efficiency of longer cycles. Looking at the expanded performance charts with systems that use TXVs and ECM motors, it seems to me that you can get decent latent removal with the most efficient sensible removal at 375-400 CFM per ton. 350 CFM per ton is usually used for the beginning 30 minutes of a cycle by many manufacturers to saturate the coil and they go to 400 CFM for most efficient sensible removal after that but if you run a long enough cycle, it seems to me that the math works better for the 375-400 CFM per ton range. ECM motors can move the 400CFM per ton without much increase in blower power, the increased load to the condenser might increase power to it but it would be a good use of that power.

[NiHaoMike]

Originally Posted by MN Renovator

Aren't you still dumping the extra moisture that is in the larger coil back into the air after the cycle is complete? You may be removing more at the beginning of the cycle with the reduced airflow but you aren't really removing the humidity from the building until the moisture has saturated the coil and it starts to drip off. These are things that I'm not sure really get measured in the lab tests and I think the latent heat that gets added back after the cycle isn't insignificant. ..and I think that for the average oversized system in most homes this is a detriment because short cycles make this an issue. In my case, I probably would never notice because I make sure that the shortest cycle that ever runs is at least 2 hours. Most systems are oversized and probably run 30 minutes and 20 minutes of that is saturating the coil and only 10 minutes of true moisture removal and so 20 minutes of cooling essentially was left as sensible only but since the moisture removal reduces sensible BTU removal the overall sensible efficiency is less and the latent removal not what it could have been.

Granted system design is extremely important and I think a slightly undersized system(rounding down from manual J instead of rounding to the next .5 ton size) is the key to the best efficiency but contractors aren't willing to do it and homeowners generally don't understand the benefit of better moisture control and efficiency of longer cycles. Looking at the expanded performance charts with systems that use TXVs and ECM motors, it seems to me that you can get decent latent removal with the most efficient sensible removal at 375-400 CFM per ton. 350 CFM per ton is usually used for the beginning 30 minutes of a cycle by many manufacturers to saturate the coil and they go to 400 CFM for most efficient sensible removal after that but if you run a long enough cycle, it seems to me that the math works better for the 375-400 CFM per ton range. ECM motors can move the 400CFM per ton without much increase in blower power, the increased load to the condenser might increase power to it but it would be a good use of that power.

You can play around with the fan delays to minimize that problem. And, of course, tricks to increase the cycle time like slowing down the thermostat and installing a VFD.

[MN Renovator]

This might be helpful, some AHRI data that I pulled when doing this research. Bryant equipment.

For the 180BNA024 condenser EER is .4 higher, SEER .6 higher with the same furnace and a 3 ton coil versus 2 ton.

For the 116BNA024 condenser EER is .5 higher SEER .7 higher.

With the 986 furnace(with a variable speed ECM versus the constant torque ECM of the 925) the gain is .2 EER and .3 SEER for the 180BNA024 condensor.
The 116BNA024 condensor the gain is .2 EER and .5 SEER.

This is why the efficiency gains seem insignificant to me versus the extra moisture load that the larger evaporator holds over and releases back into the air and the lag before it actually removes it from the building. If you need to use energy again in the next cycle to remove the extra moisture added back with the larger coil you aren't saving energy while its doing that.

EER, SEER, BTU, furnace model(40k BTU size), condenser model name, condensor model, coil size.

13.5 19.3 26400 925 Evolution 20 180BNA024****A 37k coil
12.5 15 24000 925 Preferred 16 126BNA024****A** 31k coil
12.5 15.2 24000 925 Legacy RNC 16 116BNA024****A* 36k coil
13.2 18.5 25000 986 Evolution 20 180BNA024****A 30k coil
12.7 15.5 24000 986 116BNA024****A* 36k coil

13.1 18.7 25400 925 Evolution 20 180BNA024****A 24k coil
12 15.3 24800 925 Preferred 17 127ANA024****A* 24k coil
12 14.5 24000 925 Legacy RNC 16 116BNA024****A 24k coil
13 18.2 24600 986 Evolution 20 180BNA024****A 24k coil
12.5 15 23800 986 116BNA024****A* 24k coil
12.5 15 23800 986 Preferred 16 CA16NA024****A 24k coil
11.2 13.5 23200 925 13 SEER ENTRY CA13NA024**** 24k coil
11.2 13.5 23200 925 Legacy RNC 13 113AN(A,W)024-D 24k coil

[Servicetech]

Sensible to latent heat ratio (humidity removal) closely follows the Saturated Suction Tempature of the evaporator coil. Larger coils and more airflow increase SST, trading better efficiency for less humidity removal. Smaller coils/lower airflow lowers SST, give up efficiency for better humidity removal. Go above 50 degrees and humidity removal will be minimal, go below 35 and you will freeze up the coil. Most systems are balanced well between 40f-45f SST. A TXV will keep superheat constant so most of the coil surface remains active. With a fixed orfice the amount of of the coil that is active wil vary depending on the load.