What's the proper cfm for a 2 ton system?

[RyanHughes]

I forgot to mention that we have a solar attic fan and it keeps the temperatures down usually in the 115 deg range in the attic.

Okay. Still likely picking up heat in the attic since very few duct systems are fully sealed and well insulated. Next cooling season, you might want to take supply and return temperatures both at the furnace/coil and at the supply registers/return grilles to compare them. Ideally your supply air is ~ 55-60 deg in cooling. My own system generally has colder supply air, but most of my supply ducts are within conditioned space and have minimal leakage, so that makes a difference. My return temperature is also ~72 deg which makes a difference. Your return air is likely picking up heat before it reaches the blower and could be closer to 80 deg. Similar situation for supply air in an attic system.

[545GAlady]

Okay. Still likely picking up heat in the attic since very few duct systems are fully sealed and well insulated. Next cooling season, you might want to take supply and return temperatures both at the furnace/coil and at the supply registers/return grilles to compare them. Ideally your supply air is ~ 55-60 deg in cooling. My own system generally has colder supply air, but most of my ducts are within conditioned space and have minimal leakage, so that makes a difference. My return temperature is also ~72 deg which makes a difference. Your return air is likely picking up heat before it reaches the blower and could be closer to 80 deg.

I checked the temperatures back in August and for the supply air it read about 67 deg. The returns were closer to 80. When the new furnace was installed, the ductwork was sealed. The ductwork has R6 insulation.

[RyanHughes]

I checked the temperatures back in August and for the supply air it read about 67 deg. The returns were closer to 80. When the new furnace was installed, the ductwork was all sealed up. The ductwork has R6 insulation.

Were these temps at the unit or at the registers? Hopefully at the registers. 67 deg supply air is not very good. In fact I'm surprised it's cooling the space within 5-9 minutes even on milder days. If the ductwork is all sealed up, I would want to make sure the system is charged properly. Even at 883 cfm you should be getting much cooler supply air. The delta T at the coil is more telling though. Could have around 60 deg supply air off the coil, which would be reasonable with 80 deg return air, and the supply ductwork is still just picking up heat.

[jtrammel]

R8 would be better. A 20-22F temp drop between supply and return is about as good as it gets

[545GAlady]

These were at the registers. Thanks for asking.

[545GAlady]

R8 would be better. A 20-22F temp drop between supply and return is about as good as it gets

Thanks. I was thinking the same thing about R8 insulation ductwork. It seems to be better to use in unconditioned spaces.

[545GAlady]

I also have a question about the cfm in heating mode. For the btu's the furnace runs on 52,000 btu output on high stage and 35,000 btu output on low stage. The cfm is about 850 on first stage and 925 on high stage. The problem is the system also short cycles in HEAT mode. Sometimes the cycles are only about 3 minutes. I thought that having a two-stage furnace will run longer and provide more even temps. With the short cycling, the unit provides uneven temps upstairs. I am wondering if the cfm could be lowered but I think it won't be a bright idea due to the fact that furnaces can not have too little airflow because the limit switch will keep shutting down the burners due to the low airflow. Can the cfm be lowered? Thanks in advance.

[545GAlady]

The system has 5 returns (one 8 inch in each bedroom, one 10 inch in one end of hallway, and one 8 inch in other end of hallway). I don't think that the returns are causing the problem. Well I really don't know. Hope this helps.

[54regcab]

Normally MORE airflow will help even out temperatures since it stirs the air up more.

[RyanHughes]

I also have a question about the cfm in heating mode. For the btu's the furnace runs on 52,000 btu output on high stage and 35,000 btu output on low stage. The cfm is about 850 on first stage and 925 on high stage. The problem is the system also short cycles in HEAT mode. Sometimes the cycles are only about 3 minutes. I thought that having a two-stage furnace will run longer and provide more even temps. With the short cycling, the unit provides uneven temps upstairs. I am wondering if the cfm could be lowered but I think it won't be a bright idea due to the fact that furnaces can not have too little airflow because the limit switch will keep shutting down the burners due to the low airflow. Can the cfm be lowered? Thanks in advance.

Let us know how the system performs when you get the Prestige IAQ installed, configured to control both stages properly. I think there will be a difference. CPH for high and low stage heat can be set.

[George2]

Normally MORE airflow will help even out temperatures since it stirs the air up more.

I agree with ur statement to a point but, through balancing, you will "increase" the airflow to the areas that need it without the need to install a oversized blower.

Just having more airflow, i.e., larger blower, WILL help when the homeowner or contractor doesn't know how to do balance a system.

That is why I like zoning so much. A homeowner can easily change the temperature to an area of the home depending on the weather or load differences as they may arise.

[motoguy128]

But even zoning requires that you have a minimum airflow to run the equipemnt on low stage with min&max supply temps AND that you still ahve adequate flow to each zone to match the heat loss or gain.

A good zone systme starts as a properly sized, distributed and balanced duct system for equipment sized maybe 20-25% larger that what a load calc calls for. IF you're using zoning ot correct for ductwork imbalances, it won't work as well and may even fall on it's face and be worse than non zoning at all. It shold only adjust for changes in load only. I'm not saying that not what good pros do, just clarifying.

[motoguy128]

Normally MORE airflow will help even out temperatures since it stirs the air up more.

Only when the equipment is off (fan only). When running it might even make the problem worse. Althouhg is restricted duct systems, the register grills at some point can act as a flow limiter at some point, and force proportionally more air to the the underconditoned spaces.

Plus more airflow on AC will increase system capacity and further shorten run times.

If they are the typcial cheap registers, the throw is so poor, increased velocty probably won't make too much difference anyway.