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Discussion Starter · #1 ·
Didn't want to hijack another thread.

Y'all know my stance on it. Curious if some has any proof either which way. I had read a few papers on it, but I'll have to find them again. Essentially what I read was, the higher the surface area, and /or the lower the face velocity, the better the dehumidification, when the coil temp is kept constant.
 

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Didn't want to hijack another thread.

Y'all know my stance on it. Curious if some has any proof either which way. I had read a few papers on it, but I'll have to find them again. Essentially what I read was, the higher the surface area, and /or the lower the face velocity, the better the dehumidification, when the coil temp is kept constant.
Had a thread like this a few years ago.
As evidence larger coils dehumidify worse, I provided performance data for a 2-stage showing a higher sensible heat ratio on low, if i remember correctly.
 

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Discussion Starter · #3 ·
Had a thread like this a few years ago.
As evidence larger coils dehumidify worse, I provided performance data for a 2-stage showing a higher sensible heat ratio on low, if i remember correctly.
But they all have very high airflow rates on low stage. >450 cfm/ton. I've seen a few north of 600cfm/ton.
 

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Discussion Starter · #6 ·
Your google-Fu is strong. I forgot that I made those points.
 

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I forget about this point I made in that thread:
"
Goodman has their specs available to general public.

GSX13.

1.5 ton - at 80f indoor, 67f wb, 95 outdoor has a sensible heat ratio of 0.75.

2 ton at same conditions, same indoor coil, shr of 0.73.

Both use the same indoor coil, 2 ton. "

The specs I can get now actually show the 2 ton 13 seer using a 2.5 ton nominal coil, so can't make that comparison any more. Probably they do that now to cut back on outdoor coil area and reduce costs -> get the lost efficiency back with a larger evap coil.
 

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Discussion Starter · #8 ·
I forget about this point I made in that thread:
"
Goodman has their specs available to general public.

GSX13.

1.5 ton - at 80f indoor, 67f wb, 95 outdoor has a sensible heat ratio of 0.75.

2 ton at same conditions, same indoor coil, shr of 0.73.

Both use the same indoor coil, 2 ton. "

The specs I can get now actually show the 2 ton 13 seer using a 2.5 ton nominal coil, so can't make that comparison any more. Probably they do that now to cut back on outdoor coil area and reduce costs -> get the lost efficiency back with a larger evap coil.
Here's something similar. 1½ ton coil used for 1½ and 2 ton condensing unit. They rated it with an orifice instead of a TXV. However, it shows that it does a slightly better job at dehumidification at birth higher and lower airflows when the coil to compressor ratio is higher.

 

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Here's something similar. 1½ ton coil used for 1½ and 2 ton condensing unit. They rated it with an orifice instead of a TXV. However, it shows that it does a slightly better job at dehumidification at birth higher and lower airflows when the coil to compressor ratio is higher.

I believe CA*F1824 was/is a 1.5 ton to 2 ton nominal coil. (18 to 24)
But yes, it shows the 1.5 ton has a lower shr, granted cfm per ton of 2 ton very slightly higher.
 

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Discussion Starter · #10 ·
I believe CA*F1824 was/is a 1.5 ton to 2 ton nominal coil. (18 to 24)
But yes, it shows the 1.5 ton has a lower shr, granted cfm per ton of 2 ton very slightly higher.
Yea I saw that weird cfm number. Almost looks like a typo. Maybe just playing that numbers game.
 

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For the same coil temperature, and same CFM, a large coil will have a lower air speed, air will be in contact more with the coil, so there is more surface and contact-time for water in the air to condensate. Better de-humidification.
On the other hand that means that the heat transfer is also better. So to reach the same setpoint, that air handler will run less time.
So that means less time to do de-humidification of the air in the house.
Those are pulling in different directions, but not necessarily with the same "intensity".

I guess there is no exact answer, the efficiency of the coil is depending on several factors, like set point, RH in the house, actual sizing of the compressor for the house size.
My gut feel is that the manufacturers, by experiments and calculations, have taken all those factors in consideration, averaged them to get the proper (compromise?) size of the coils.
When we deviate, we favor one or other of the factors and in the end we end up theoretically with a worse optimized system.
 

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Discussion Starter · #13 ·
Have you thoroughly tested the theory?
Not lab level testing. I know that when I commission, my SHR can be lower without much change to airflow. But it's not like I have 2 identical units side by side to test with. Thinking about putting together a trainer for it though.
 

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Discussion Starter · #14 ·
For the same coil temperature, and same CFM, a large coil will have a lower air speed, air will be in contact more with the coil, so there is more surface and contact-time for water in the air to condensate. Better de-humidification.
On the other hand that means that the heat transfer is also better. So to reach the same setpoint, that air handler will run less time.
So that means less time to do de-humidification of the air in the house.
Those are pulling in different directions, but not necessarily with the same "intensity".

I guess there is no exact answer, the efficiency of the coil is depending on several factors, like set point, RH in the house, actual sizing of the compressor for the house size.
My gut feel is that the manufacturers, by experiments and calculations, have taken all those factors in consideration, averaged them to get the proper (compromise?) size of the coils.
When we deviate, we favor one or other of the factors and in the end we end up theoretically with a worse optimized system.
One of those factors is price. If physical size and price weren't an issue, coils would be massive. They would use low outdoor fan speeds, or even none at all, if they could get away with it. It's just not cost effective or practical.

I think of it in terms of energy transfer. While sensible heat transfer is linear, latent heat is not. But essentially at the same air conditions, and flow rate, the bigger coil will transfer a bit more energy. The coil will be a tad warmer as you have a fixed size compressor, and we haven't increased it. It will move slightly more refrigerant, increasing it's efficiency, so the coil temp won't increase as much as you'd think.

If the coil temp was already right on the dew point, as larger coil may be warmer then it and dehumidify a bit less. But lowering airflow even a little bit, brings it back under dew point. Lowering airflow seems to effect SHR much more on a larger coil and actual SST less when invert the dew point.
 

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So it's a compromise. In my case I have high humidity usually, so I put the speed at the lowest position.
If the humidity was not a consideration, I would use faster speeds for better homogenization.
 

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So it's a compromise. In my case I have high humidity usually, so I put the speed at the lowest position.
If the humidity was not a consideration, I would use faster speeds for better homogenization.
Can have the best of both, slowing down only as necessary with a humidistat or thermostat equipped with cooling humidity control. Can take wiring mods to do it - ie, adding relay, unless air handler supports the feature as is.
 

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Discussion Starter · #20 ·
Can have the best of both, slowing down only as necessary with a humidistat or thermostat equipped with cooling humidity control. Can take wiring mods to do it - ie, adding relay, unless air handler supports the feature as is.
I use second stage for that, but dehum terminals should work too. A humidistat mounted to the return from Y² to Y¹ or dehum works great, and cheap.
 
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