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Old 01-04-2018, 04:33 PM   #46
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Re: System Sizing


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Originally Posted by user_12345a View Post
Putting a 3 ton could cause issues with airflow if your ducts aren't sized for it. Not to mention low humidity removal.



Were you using 24k capacity or was it de-rated for 75 return?

At lets say more realistic 22k and that ratio, i get 18.26k sensible, his sensible is just over 20k.

Its total capacity was 22,300.

The black and white you read on a chart. Isn't the actual black and white. Using Manual S, helps to determine how it will really work.
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Old 01-04-2018, 04:41 PM   #47
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Re: System Sizing


22.3k x 0.83 = 18 509 btu/hr.

You're saying the chart is inaccurate and you have to use manual S only?

Unless you're assuming some airflow adjustments will be done to get the the shr closer to where you want it.
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Last edited by user_12345a; 01-04-2018 at 04:45 PM.
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Old 01-04-2018, 05:30 PM   #48
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Re: System Sizing


Which total is used for the calculation? Total Sensible Load, Total Btu's Heating/Cooling?
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Old 01-04-2018, 05:38 PM   #49
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Re: System Sizing


^total cooling but the machine selected has to be able to cover total sensible at the indoor conditions. so while your total load can be covered by 24k/2 ton it may not cover the sensible load though beenthere says it should.
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Old 01-04-2018, 07:27 PM   #50
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Re: System Sizing


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Which total is used for the calculation? Total Sensible Load, Total Btu's Heating/Cooling?
Manual S determines the actual capacity of the unit to choose. Then the total capacity, along with sensible, and latent are all three used to determine if the unit really meets the requirements.

As I said, a 2 ton will meet your 75 ID and 95 OD load requirements, but just barely. And the only time it will really have a problem, is during the once every 3 to 5 years we have those 2 to 3 day spells of 95 OD temps.

A 2.5 ton is definitely over sized. But with a VS blower and a proper thermostat, can be made to keep ID RH reasonably low. As lowering the blower speed/CFM will lower the 2.5 ton units actual capacity closer to around 2.15 to 2.2 tons total.
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Old 01-06-2018, 08:16 AM   #51
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Re: System Sizing


Thanks again for all the information. The house currently has a 3 ton heat pump and one of the contractors recommended a 3.5-4 ton inverter system based on a calc they completed. This seemed excessive since the current system heats and cools the house fine. That's why I was looking for a second opinion.
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Old 01-06-2018, 12:20 PM   #52
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Re: System Sizing


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Thanks again for all the information. The house currently has a 3 ton heat pump and one of the contractors recommended a 3.5-4 ton inverter system based on a calc they completed. This seemed excessive since the current system heats and cools the house fine. That's why I was looking for a second opinion.
When doing a heatpump, assuming your backup is electric, you have to look at heating performance or will be hit with very high bills.

If you put a 2-ton for cooling, you'll be using a lot more strip heat.

You may want to explore a 3 ton, 2 stage so you can get cooling capacity down to where it needs to be without sacrificing heating performance.

Inverter can be good, if you get one that can maintain capacity in cold weather much better than conventional heatpumps, you can almost completely eliminate your strip heat use.

Do not get a 4 ton because your air ducts would be too small, you can't just raise the size without enlarge air ducts. The high duct pressures can kill ecm motors.
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Old 01-06-2018, 12:33 PM   #53
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Re: System Sizing


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..

Do not get a 4 ton because your air ducts would be too small, you can't just raise the size without enlarge air ducts. The high duct pressures can kill ecm motors.
Inverter units operate differently. It will be at the lowest modulation most of the time for cooling mode. The airflow will be lower for heating mode, as it's trying to keep the discharge air temp up. You have to look at the specific model, as there are cheap ones out there that will cause problems.

Cheers!
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Old 01-06-2018, 12:39 PM   #54
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Re: System Sizing


They can still move up to 1600 cfm in certain modes (bringing the temp up in mild shoulder season weather or pulling it down in cooling mode), no?

I wouldn't risk it, personally.

ducts could be undersized for 3 ton.
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Old 01-06-2018, 01:30 PM   #55
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Quote:
Originally Posted by shocker View Post
Thanks again for all the information. The house currently has a 3 ton heat pump and one of the contractors recommended a 3.5-4 ton inverter system based on a calc they completed. This seemed excessive since the current system heats and cools the house fine. That's why I was looking for a second opinion.
This was close to my suggestion I said 4 to 4.5 roughly on the info you gave off the top of my head..a good contractor can mod your existing ductwork to match with your new unit..
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Old 01-06-2018, 01:50 PM   #56
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Re: System Sizing


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Originally Posted by user_12345a View Post
They can still move up to 1600 cfm in certain modes (bringing the temp up in mild shoulder season weather or pulling it down in cooling mode), no?

I wouldn't risk it, personally.

ducts could be undersized for 3 ton.
The only time that I'd expect it to move that much air is during hot pull down. The compressor will compensate for the reduced air flow due to the restricted ductwork. Yes, it's less then ideal, and I wouldn't normally suggest it. However, it will work.

During shoulder seasons, it would be a minimum modulation mostly, as you don't need most conditioning. During cold spells, the compressor will ramp up, but the indoor fan will modulate back to maintain air temp.

Cheers!

Last edited by supers05; 01-06-2018 at 01:54 PM.
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Old 01-06-2018, 03:41 PM   #57
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Re: System Sizing


I still don't think it's a good idea to put a 4 ton unless the duct system can take the airflow.

plus, even if it could ramp down to 40% or even lower to be right sized for a/c, the indoor coil would probably be warm (i don't know, above 45F) and do a lousy job of dehumidifying.

Could have this issue with a 3-ton 2-stage but not nearly to the same extent.
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Old 01-06-2018, 03:44 PM   #58
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Re: System Sizing


Quote:
Originally Posted by shocker View Post
Thanks again for all the information. The house currently has a 3 ton heat pump and one of the contractors recommended a 3.5-4 ton inverter system based on a calc they completed. This seemed excessive since the current system heats and cools the house fine. That's why I was looking for a second opinion.
Are your supplies or returns loud?

What temp do you set your thermostat to in the summer.
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Old 01-06-2018, 04:06 PM   #59
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Re: System Sizing


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Originally Posted by user_12345a View Post
I still don't think it's a good idea to put a 4 ton unless the duct system can take the airflow.

plus, even if it could ramp down to 40% or even lower to be right sized for a/c, the indoor coil would probably be warm (i don't know, above 45F) and do a lousy job of dehumidifying.

Could have this issue with a 3-ton 2-stage but not nearly to the same extent.
It's not a great idea, but it's not as bad as some people installing 4 ton conventional units on ductwork for 2 tons, lol.

At 75*f DB and 50% RH, the dew point is 55*. You'd still be dehumidifing. They slow air flow, to compensate for lower compressor speeds to control coil temp.

The higher the suction pressure, the better the efficiency, so they constantly attempt to allow it to rise as much as possible within usable limits. (Based on demand rate.)

Cheers!
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Old 01-06-2018, 04:16 PM   #60
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Re: System Sizing


I do have issues with noise out of the closest supply vent which makes me alter the damper. We actually don't use AC that much in the summer since we live in the woods and have substantial shade. On the hot/humid summer days when we run it we usually set it to 78. We use ceiling fans all the time instead. Thanks again for everyone's insight.
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