I'm sorta hijacking this from another thread, because it's of interest to me.

My original installer "dialed down" the CFM of my 4 ton unit due to excessive noise in the duct work.  I think it maxes at 1200 cfm (variable speed blower, usually starts up around 700 cfm, I think).  He showed me all the dip switches for controlling the speed, and they are clearly indicated on the panel.

I spotted in a thread about btu delivery at a given cfm, and there's certainly some discussion right now about increasing airflow and its effect on LAT(DAT).  I can understand that in *cooling* mode, you'd need a decent airflow otherwise the coil can ice up.

How does that work in heating?  I would assume generally that a lower CFM would increase LAT, but at the cost of delivering (more) heat at a slower rate to the home.  A faster overall CFM (regardless of the stage) would decrease LAT, but deliver the lower air temp at a faster rate.

I posted once about optimal gpm in the loop.  Now the question is about the opposite side of the system.

Is it best to maximize airflow in heating mode to the point where the ducts start sounding like jet engines, then back off slightly?  Probably if it sounds like a jet, the ducts are too small?  A 4 ton system should be able to do 1600 cfm, but I think my upstairs zone 2 ducts would explode with that amount of air going through the 10x10 zone 2 trunk.  Downstairs zone 1 would have no trouble, but upstairs...

Is it ok to dial it up a little and watch the EAT/LAT while being comfortable, or am I really shortchanging myself by running at a max of 1200 cfm?

higher CFM in geo compensates for the lower temperatures in the air coil. It needs to move more air across the coil to deliver the same BTUs as a comparable fossil fuel furnace.

Sounds like your system  is not optimally matched . Loop, heat pump, and duct work have to match within a small tolerance or else you have an imbalanced system that won't perform very well
Lowering fan speeds will certainly quiet the system and not deliver the btus that the heat pump is capable of delivering.
You can be quite far off on fossil fuel system design and still manage to get enough heat through just by oversizing and rule of thumb. It doesn't work well for geothermal.

Precise calculations and then some adjustments for customer preferences and you have a working system and satisfied customer.  Sounds like you are not satisfied yet.

The equation for sensible heat BTU is BTH delivered = CFM * temperature difference * 1.08. Why did the installer feel the need to dial down the airflow? Around my area, many do not understand the importance of proper duct sizing and airflow, comfort and economy suffer. In my personal house before I replaced the upstairs unit, my total static pressure was 1.2" and the fan was only designed to deliver the rated CFM against .5" Additionally, the supply was only sized for delivering <400CFM and even with the terrible static pressure, using TD to measure total CFM, the furnace was trying to push over 700 CFM. The result was a blast of air sounding like a jet taking off everytime the AC would kick in.

The Geo was a retrofit to existing ductwork from a previous a/c only unit. The previous system wasn't zoned, so the upstairs has a 10x10 trunk line that delivers all the air to the upstairs (about 900 sq feet of space upstairs). When he put the geo in, he zoned the system too. The downstairs has no problems (really), but when the unit would kick into maximum cfm (1600) for just the upstairs, the jet engine sound would be heard throughout the home. He therefore dialed the system down so that at maximum speed, it was quiet.
Ideally, I guess the ductwork to the upstairs should be replaced/enlarged?

Posted By decafdrinker on 08 Feb 2011 07:39 AM 

Ideally, I guess the ductwork to the upstairs should be replaced/enlarged?

Or you could install a bypass or dump.
j

Posted By decafdrinker on 08 Feb 2011 07:39 AM
Ideally, I guess the ductwork to the upstairs should be replaced/enlarged?

Are you comfortable with the new blower settings?
Pros: if so, what are the benefits to modifying the ductwork?  Should 1600 cfm have been going to the upstairs if only that zone called?

My ductulator shows that a 10 x 10 square duct will move ~ 500 CFM at .1 friction rate (best guess for steel ducting without seeing all design parameters) Now if your air handler is trying to move 1200 CFM thru this 10 x 10 duct, the velocity in the duct will be ~ 1700 FPM.  

A variable speed blower is not the catch all solution.  Zoned systems should be designed with a bypass damper.  Some good educational material can be found at this link. 

http://www.ewccontrols.com/hvac_news.htm

Would it be better to increase the size of the duct, install a bypass damper, or leave it as-is if we're comfortable?

IMO, I would increase the duct size and install a bypass. My reason is with the reduced airflow, you are not getting all the efficiency and performance designed with the system. As a result you may need 48,000 btus as per system design and the choked ductwork and dialed down fan speed may only provide 30,000 buts.

I'm not a huge fan of bypass dampers - kind of a last ditch fix. I prefer cracking dampers for zones not calling (adjust zero point so they don't completely close)

A dump is a viable option if there is a volume (foyer, great room, stairwell) that can take the air without casusing discomfort.

Too low airflow in cooling (whether via dip switches or bypass) can reduce efficiency and ultimately cause icing. BTW, I configure thermostats to go no lower than 68 in cooling mode to stave off icing.

Reducing airflow in heating mode seems attractive since it raises LAT. The trouble with that is the higher LAT comes at a price - lower efficiency and lower capacity. as LAT rises, compressor head pressure and amps rise significantly.

At bare minimum ask that technician verify compressor amps, parrticularly in stage 2, do not exceed full load for that compressor while operating with lower airflow settings. A bypass in heating has the same effect as lower airflow - EAT rises owing to the mixture of supply air with return air, and LAT rises as well.

Thanks Engineer,
I think I'm going to dial up the airflow a little and crack the zone 1 dampers and check comfort level. I agree about the thermos. I've locked mine to go no lower than 74 in cooling, and no higher than 75 in heating. I've also (for the winter) set the thermostat to Permanant 68 rather than messing with recovery/etc. The house is much more comfortable and the electric use is very predictable with no recovery. Once the house is better insulated, I might work on the settings and ductwork.

Posted By engineer on 09 Feb 2011 10:00 AM
I'm not a huge fan of bypass dampers - kind of a last ditch fix. I prefer cracking dampers for zones not calling (adjust zero point so they don't completely close)

A dump is a viable option if there is a volume (foyer, great room, stairwell) that can take the air without casusing discomfort.

Too low airflow in cooling (whether via dip switches or bypass) can reduce efficiency and ultimately cause icing. BTW, I configure thermostats to go no lower than 68 in cooling mode to stave off icing.

Good points.  One reason I like the bypass damper solution is the ability to mix pre-cooled/dehumidified air with the incoming return airstream.  Our standard procedure is to install a temp sensor at the discharge end of the supply plenum to prevent freezing and overheating.

Honeywell's DATS?

Yep, I have a honeywell DAT. It's not directly above the blower, but just round the corner.

I tried bumping the max speed up to 1400 CFM (it was set to 1200), but the LAT (according to the DAT) dropped from 93F to 87F, and the house definitely "felt" chillier.
I put it back to 1200 for now.

93 LAT sounds about right. 4T nominal makes around 3T actual in typical heating conditions, so 1200 CFM sounds OK.

LAT in upper 90s or 100s results in higher power lower efficiency.