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

[545GAlady]

Hello. It's me again. I have a question about the blower cfm of our upstairs system.

Our upstairs system is a 2012 Amana Distinctions (Goodman, of course) SSX14 2 ton A/C unit matched up with a 2012 Lennox SL280v gas furnace. The A/C was installed back in early April and the furnace was installed last July. The upstairs is 1400 sq. ft. I am wondering what the cfm is suppose to be in cool mode. I know it's kinda too late to be talking about the A/C unit when A/C season for us is over, well whatever. The system is running in about 883 cfms and sometimes the unit short cycles. If the A/C has been on for 7 1/2 minutes, the blower speed will increase to about 1005 cfm. This is what the tech told us during our yearly tune-up. The evaporator coil is a CAPF3636A6 which is a 3 ton.

The blower ramping option is set to the option A (factory setting) where it would run at a very low speed (50%) when the system starts up and after 30 seconds on running on this speed the blower will ramp up to 82% cfm. If the cooling demand is not met within 7 minutes then the blower will ramp up to 100% of the cfm setting. When the cooling demand is met the blower will ramp back down to 50% for about 30 seconds then ramp down to off. I got this information from the specifications/installer's manual on Lennox's website. This is also what the tech told me.

So how much cfms should a 2 ton system have? Is this indoor coil and A/C unit the proper match up? Sorry about the long post. Thanks in advance.

[wahoo]

Hard to tell from here. Regardless of what the blower is set at, we have no idea of what your actual air flow is. Even if the cfm is set at 1050, the duct system may not be designed to deliver that amount of air flow. A two ton system should be putting out about 800 cfm. maximum.

[545GAlady]

Hard to tell from here. Regardless of what the blower is set at, we have no idea of what your actual air flow is. Even if the cfm is set at 1050, the duct system may not be designed to deliver that amount of air flow. A two ton system should be putting out about 800 cfm. maximum.

The ductwork is all 8 inch. I should have said that in my first post. Hope this helps.

[George2]

The ramping of the blower is good for de-humidification. I'm lost on the 883 cfm-1,005 cfm.

Normally, it should start around (I would guess) 150-200 cfm, then go to (maybe) 300-350 and finish around (no more than) 400 cfm. Note: I'm not a tech and have not stayed at a Holiday Inn (lately).

The ramping is in the furnace is timed. A better thermostat would control the airflow differently.

[545GAlady]

The ramping of the blower is good for de-humidification. I'm lost on the 883 cfm-1,005 cfm.

Normally, it should start around (I would guess) 150-200 cfm, then go to (maybe) 300-350 and finish around (no more than) 400 cfm. Note: I'm not a tech and have not stayed at a Holiday Inn (lately).

The ramping is in the furnace is timed. A better thermostat would control the airflow differently.

I don't exactly if the cfm is at that setting, though I believe it's around that. Did you check Lennox.com to see the airflow settings in the specifications sheet? I could be wrong you never know. We plan to upgrade our thermostats on our first floor and upstairs systems to Honeywell Prestige 2.0 IAQs so that we can control the temperature over the internet.

[RyanHughes]

There's no one "proper" cfm for a 2 ton system. Nominally, 2 tons is 800 CFM @ 400 cfm/ton. But some building loads dictate slightly higher or lower airflow depending on the latent and sensible capacity the conditioned space requires. For instance a home in Florida would be better suited at 350 cfm/ton for improved humidity removal, whereas a system cooling a server room may be configured for 450 cfm/ton to obtain greater sensible capacity.

The 070 size SL280V has a 3 ton blower. The factory cool speed setting is HIGH which corresponds to a 3 ton outdoor unit. The cfm range on that particular furnace is 645 cfm - 1372 cfm for cooling. What cfm your furnace is actually delivering depends on whether they wired the compressor to "stage 1" or "stage 2" cooling on the furnace board as well as whether they adjusted the dipswitches from the factory default (there are 4 speed settings as well as an adjustment). By what the technician said, it sounds like they may have hooked up the compressor to Y1 (stage 1) and left the blower speed on default. 883 cfm is HIGH speed for "stage 1" cooling (even though you don't have a 2-stage outdoor unit). If the enhanced ramp-up profile is enabled, the blower will ramp up as you describe to this speed (100%), but it won't go higher.

I hope this is clear to you. See attached literature (pg. 15) for more info.

ehb_sl280uh_icomfort_1207_210600_011.pdf

[545GAlady]

There's no one "proper" cfm for a 2 ton system. Nominally, 2 tons is 800 CFM @ 400 cfm/ton. But some building loads dictate slightly higher or lower airflow depending on the latent and sensible capacity the conditioned space requires. For instance a home in Florida would be better suited at 350 cfm/ton for improved humidity removal, whereas a system cooling a server room may be configured for 450 cfm/ton to obtain greater sensible capacity.

The 070 size SL280V has a 3 ton blower. The factory cool speed setting is HIGH which corresponds to a 3 ton outdoor unit. The cfm range on that particular furnace is 645 cfm - 1372 cfm for cooling. What cfm your furnace is actually delivering depends on whether they wired the compressor to "stage 1" or "stage 2" cooling on the furnace board as well as whether they adjusted the dipswitches from the factory default (there are 4 speed settings as well as an adjustment). By what the technician said, it sounds like they may have hooked up the compressor to Y1 (stage 1) and left the blower speed on default. 883 cfm is HIGH speed for "stage 1" cooling (even though you don't have a 2-stage outdoor unit). If the enhanced ramp-up profile is enabled, the blower will ramp up as you describe to this speed (100%), but it won't go higher.

I hope this is clear to you. See attached literature (pg. 15) for more info.

ehb_sl280uh_icomfort_1207_210600_011.pdf

Thanks. I'm also wondering why the system is short cycling (only on days that are not too hot).

[RyanHughes]

Thanks. I'm also wondering why the system is short cycling (only on days that are not too hot).

What kind of thermostat do you have now? Some have a cycles per hour setting that can be adjusted (lowered) to increase average runtimes. 3 is a good setting. Decreasing the airflow a bit can also help increase runtimes and possibly improve comfort, but there are lots of things to consider when setting airflow. How short are the cycles? Runtimes are going to be shorter on milder days.

[545GAlady]

What kind of thermostat do you have now? Some have a cycles per hour setting that can be adjusted (lowered) to increase average runtimes. 3 is a good setting. Decreasing the airflow a bit can also help increase runtimes and possibly improve comfort, but there are lots of things to consider when setting airflow. How short are the cycles?

Thanks. The thermostat is a Honeywell RTH6450D. The cycles usually range from 5 minutes to 9 minutes. Except on hot days the cycles are like 30 mins to 1 hour. Sometimes 2 hours depending how hot it is.

[RyanHughes]

Thanks. The thermostat is a Honeywell RTH6450D. The cycles usually range from 5 minutes to 9 minutes. Except on hot days the cycles are like 30 mins to 1 hour.

Glancing through the installation guide, looks like there's a heating cycles per hour setting but no cooling cycles per hour setting. Go figure. You might find that the Prestige thermostat with CPH set appropriately allows for better (longer) runtimes. 5-9 minutes is short. It takes at least that time for the system to reach its peak efficiency and capacity. Surprised the runtimes are that short even with the ramp-up profile providing low airflow. 10-15 minutes is more ideal on mild days and likely achievable with a better thermostat controlling to a particular CPH during partial-load conditions.

[jtrammel]

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

[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.

[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.

[545GAlady]

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.

It's been a few days since I've been on this thread, but I was able to lower the cph setting down to 3 for the furnace and so far this increased the run time by at least 2 minutes. This also helped balance the temperatures in all the rooms upstairs. When we do get the IAQ thermostat installed, I'll make sure the cph setting is lowered in both low and high stage.

[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.

[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]

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.

[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.