Don't know if this helps, but the following formula is used to determine cfm.

CFM = (BTUhs) / (1.08 x Delta T)

It may need to be adjusted for altitude, as with my area. We have to add 19% to the number.

Clark, thanks for the assistance.

I have 3 zones on my 3-ton Envision NDV038 unit.  My duct sizes are such that when only 1 zone is calling for cooling, I've got air flow with much greater than 900 fps velocity.  I therefore want to set my CFM, for when just 1 zone is on, to as low as the NDV038 is designed to run at.  This would be the 2nd of 5 fan speed settings on my IntelliZone panel if Dehumid is set to Normal, or the 1st of 4 settings if Dehuhid is set to On.

My 1st 3 choices of speeds are 650, 750 or 850 CFM.  I'd like to run at 650, when just 1 zone is on, if indeed the NDV038's 1st stage is designed to run this slow.  At 650, along with slightly opening 1 of my zone dampers, I can get the velocity of the most restrictive zone down to about 900.

A confusing subject for me is that without WaterFurnace's zoning panel, the NDV038's minimum fan speed, when the compressor is in 1st stage, is 1100 CFM (or 85% of 1100 if Dehumid is set to On).  Since the zoning panel simply makes sure the tstats and dampers are properly coordinated, and makes sure the right compressor stage is enabled as a function of number of zones turned on, and doesn't make an refrigeration related changes, I don't yet understand what it is about the panel that supposedly enables the NDV038's 1st stage minimum CFM to drop so significantly.

I'm new here, and am looking forward to contributing my knowledge as best as I can based on what I've learned after having my 2 Envision units for 1 year - I've learned a lot, particularly in this Dallas heat at the moment.  And, I have a pretty good system for monitoring the performance of the units, such that I can comment with facts.

What I'm hoping is that there may be some WaterFurnace experts here who can answer the questions on minimum CFM for the Envision model.

Thanks and best regards,

Bill

A:

I have an Envision 038 with intellizone with 4 active zones, including a couple small ones - approximately four 6" runouts each, so I need the slowest possible blower speed when only one zone is calling. My fan speed dip switches are set at 1,2,3,5,8 and dehu mode is on. Unit runs in low stage pretty much always and hold relative humidity around 50% most of the time.

Without Intellizone my understanding is you have only 3 fan speed settings to choose from, as well as dehu option. I don't see why you couldn't select the lowest speed as speed 1. The only caveat I'm aware of is the need to avoid icing the evaporator. Do that and the compressor is at risk. Avoid that by keeping air filter clean and being reasonably judicious with thermostat settings (stay above 70 or a bit higher)

Might be simpler if we talked. Feel free to email me: waterleo at comcast dot net

engineer, this is great!  Someone not only with an Envision model and an IntelliZone panel, but also with the exact same Envision model.  Mine is a vertical unit, with a DSH option, BTW.

I appreciate the email reference - I think I'd like to keep the discussion here as it allows everyone, myself included, to all learn together.

BTW, a large amount of real-time performance monitoring for my units is available here: http://welserver.com/WEL0043/ .

Since you have the Dehu mode On, your IntelliZone fan speed 8 switch setting is not used - you only get 4 speed settings with Dehu mode On (i.e., in 2nd stage mode, all zones open, max CFM, you're running at 1100, which corresponds to speed 5).

Like you I too have my first 3 speed settings at 1 (650), 2 (750), and 3 (850), but I didn't have Dehu mode On.  Since my smallest zone is only good for 525 CFM @ 900 fps, and since my slowest fan speed for 1st stage cooling was thus 2 (750), my air velocity coming out of my supply registers was far greater than 900 fps.

I've now set Dehu mode to On, lowering my 1st stage cooling / 1 zone open fan speed to 650.  I've also set the damper motor on my largest zone to be always slightly open to 'spill off' the remaining 125 CFM (650 minus 525).

My concern is running the 3 ton unit, in 1st stage (which is about 2 tons), forever at 650 CFM when only one zone is On, and whether or not the 3 ton unit is designed for this.

Certainly without the IntelliZone panel, WF setup instructions, for a 2-stage model, state the minum CFM is 1100 (or 85% of 1100).  This sure is a lot more than 650.  So why such a discrepancy, and what is really the minimum CFM that the 3 ton 2-stage unit can run at?  That's what I'm trying to figure out.

Best regards,

Bill

I did surf the WEL info - neat stuff.

I have a much lower tech monitoring system - redington hourmeters on the powered water heater, Y1 and Y2 outputs. I log those meter by hand as well as the utility's kWh meter to keep a handle on what's going on.

I'm not sure why the discrepancy between CFM settings. They probably are walking a fine line between keeping the zoning system flexible and ensuring minimum airflow. If an ignorant user sets a thermostat at 60 degrees thinking that'll somehow make the system cool faster, and then forgets about that setting there's a good chance the evaporator will ice up overnight, possibly subjecting the compressor to liquid in the suction, not a good thing.

There's some history to this as well - the product line before Envision ran at 50% capacity on low stage, allowing lower airflow. Envision runs at 67% at low stage owing to its use of Copeland's Ultratech 2 stage compressor. WFs intellizone doc took a while to reflect this change, and finally got around to calling for larger minimum zone sizes.

Again, as far as I know, the only constraint against lower airflow settings is potential for icing the evaporator. Note also that efficiency drops as CFM / ton is reduced, but most of us in the muggy south cheerfully accept that in exchange for the better dehumidification that results

engineer, thanks.

I have what I think is the most current IntelliZone Installation Manual, dated 10/06.

The Manual references both the Premier and the Envision models.  Min 3-ton CFM for the Premier is 600, min for the Envision is 650.

There are a couple of statements in the Manual I don't understand:
 
(1) "Minimum Airflow Setting - Envision dual capacity units have 70% low capacity, compared to ... Premier two speed units with 50% low capacity.  This will change the minimum zone CFM from 25% of nominal (400 CFM/ton) to 40% of nominal.  Therefore, when using the IntelliZone with an Envision dual capacity unit, the zone branch ducts must be sized to handle more airflow."

(2) "Dual Capacity Envision has a 70% low capacity output, therefore min CFM required per zone is 40% of nominal CFM, significantly higher than the (Premier) Series with a 50% low capacity output."

My interpretation of the two statements is:  if the NDV038 is 3.2 tons (38/12), that puts nominal CFM for the NDV038 at 1267.  40% of this number is 507 CFM.  This is as compared to the minimum of 650 from the IntelliZone document, and 935 (85% of 1100) from the Envision document.

But I don't know if the above interpretation is correct.

Best regards,

Bill

I have a 10/2007 intellizone manual, but I remember reading about the same language in it.

Beware rules of thumb such as translating model numbers into thousands of Btuh and 400 CFM per ton - much depends on your actual operating conditions. I have 70 deg entering water so my 038 can make 42 kBtuh on high and about 30 KBtuh on low. Your conditions likely vary considerably from that...or not.

In the spec catalog for Envision (downloadable by anyone from WF site) they provide capacity correction factors for airflows as low as 240 CFM / ton, from which I infer that such flows are permissible provided one insures against evaporator icing. 30 KBtuh at 240 CFM / ton works out to 600 CFM, so 650, lowest dipswitch setting should be OK.

Reducing airflow reduces unit capacity as described in correction tables.

Site specific analysis is what is called for by ACCA Manual S. All aspects (waterside temps and flows, airside temps, humidity and flows) of each specific installation must be accounted for in the course of the design process.

Let me reiterate that you should be able to safely reduce airflow to well below 400 CFM per ton so long as you avoid evaporator icing via judicious airflow selection and thermostat settings. My thermostats, Honeywell 8321s, allow the installer to clamp allowed settings to prevent too low a cooling setpoint and too high heating setpoints. If you select lower airflows, consider such clamps. I set mine so that minimum cooling SP is 65 and Max heating is 80.

BTW, I meant to respond earlier to your observation that my dipswitch 8 (5th and highest speed) will be ignored since I selected dehu mode. True, but only while cooling. The dehu mode switch airflow reduction of 15% does not apply in heating mode, so the system will flow air at the switch 8 setting in heating mode if sufficient zones call for heat.

Hope this helps

Bill and Engineer. Thanks for touching on this topic. Interesting discussion which drives a question. I've had a Manual J done for our upcoming single level 3100sf ICF home. Result: Cooling load 42,270 BTuh, Heating loss 42,289 BTuh. Design CFM 1309. We didn't specify mult-zones. Now awaiting Manual D report. In general if the fan runs on low speed to cool/heat the entire house most of the time, do we need to consider a zoned system? We're empty nesters and also considering cost of zone dampers, controls, etc. Is a zoned system more for room comfort, operating cost savings or both? Much to learn. Thanks Dave in Kansas

Zoned systems will mainly be for comfort. Mostly to do with different needs of different levels of the home.

A basement has a low cooling requirement. An upstairs/loft has a low heating requirement (with most forced air systems). If just a main with basement, the main usually has a completely different load than the basement. With this being the case, zoning can help condition that particular area to its individual needs--to a point.

What do you mean that the fan runs on low most of the time? You mean with a 2 stage system?

Clark, Yes I would prefer a 2 stage compressor with a variable speed fan. Thanks for the explanation regarding zoning. Dave

You might get away without zoning a single level home, and I understand the reluctance to deal with cost and complexity. You may be able to balance the system once and be fine. However - consider the following:

1) You state that you are empty nesters - will two of you daily use all the rooms in the 3100 SF home

2) Is there significant west facing glass with some or no shading? If so, you'll need to take specific steps to deal with it.

BTW, since you are already using ICF I hope you are also planning to sprayfoam the underside of the roof, thus sealing the attic and bringing all that ductwork into the conditioned envelope - this alone can cut loads but 15% or more.

Response to Engineer
1) Won't use all rooms daily.
2) Yes to west facing glass, about 220sf. Half is under a covered porch. All windows are double pane, low E, with mini blinds and a third pane covering the blinds (Pella). What steps would you suggest to deal with this scenario.

yes, will spray Icynene under roof deck and keep ducts in conditioned space.

If we were to consider perhaps 2 or 3 zones, should the duct design address it or does a manual S or the installer work it out? Dave

Minblinds behind an additional pane should do the trick but you have to remember to use them. Another option may be to specify a shading or tint for the none-porch covered west glass. Andersen calls theirs "low-E Sun" and it cuts the SHGC (Solar Heat Gain Coefficient) to 0.20. Pella likely has something similar.

If rooms with heavy west glass share a zone with rooms without such glass then it may be hard to get both spaces comfortable - either you overcool the non-west rooms or the west rooms get warm. That's a good place to zone. Rooms with East glass get hit during morning.

Rooms not daily used might be better off in their own zone, allowing them to be cut back a bit. Zoning typically doesn't allow big differences in temps between zones, but good savings can result from a 3-5 deg differential.

The duct design, Manual D, encompasses zoning. (Manual S concerns matching equipment to site conditions to ensure the whole house load is properly met).

Many contractors will tell you that zoning isn't needed, won't result in savings or is otherwise problematic. That is typically because they don't understand it or don't want to deal with it, or both. If you do go with zones, it'll probably go better if it isn't the contractor's first attempt at it.

Thanks for your sensible explanation. I'll talk to the fellow doing the Manual D for his thoughts regarding zoning for our floorplan. I can see 2 logical zones (1) our main living area including a Great Room with west facing windows and an office with the large west facing windows... and (2) the bedroom wing with minimal windows facing south. One zone occupied primarily in the daytime and the second primarily at night. Think I'll reread the previous discussion of WF zone controls and CFMs. Dave

I got another response from WaterFurnace yesterday.

The bottom line is, for the Envision 038 model, minimum airflow required for 1st stage cooling is 650 CFM.  (This can be achieved by setting Dehum to On).

I tried to get an understanding of why the Envision's Specification Catalog (page 18) and Installation Manual (page 15) say otherwise (minimum is 1100, or 85% of 1100 CFM in cooling mode when Dehum is On), but I wasn't successful.  And thus I don't understand why the minimum CFM required in these documents doesn't match the IntelliZone's Installation Manual.

But clearly, the minimum is 650 for 1st stage cooling (and 750 for 1st stage heating).

Best regards,

Bill

Dave, I too believe a zoned system is primarily for comfort, although I'm sure there's some operating cost savings (able to cool rooms to different levels) and there's potential capital savings (purchase smaller HVAC equipment and move the cooling capability to rooms that need it as a function of time of day).

The biggest need for zoning residences, like mine, are for those that have a lot of room temp variations - solar gains due to a lot of glass (my case), differing solar gains in Summer vs. Winter, multi-level homes where hotter air accumulates in the upper levels, etc.

Running the fan constantly may solve the problem, but it will depend on where you live.  If you don't live in an arid climate, in the cooling season, I do not recommend you run the fan when the compressor's not running.  There's too much water sitting in your evaporator coil, and you'll have a higher indoor RH.  Here's a chart from a Lennox study to show what's happening: http://www.pbase.com/neukranz/image/88853062 .

Best regards,

Bill

engineer, many thanks.

Your IntelliZone Manual is a later revision than mine.  If you have the manual in electronic form, I'd appreciate a copy of it.  I'll send to you an email on this.

I assume the capacities you're quoting above are Heat of Rejection (HR) numbers.  My 038 unit is running similarly right now, at about 30 kBTU/h (4.2° deltaT @ 14 gpm), for 1st stage.  (I don't know how it performs for 2nd stage because it never runs in 2nd stage).

I have my WR 90 Series Blue Universal Tstats set to low temp SP limit of 74°.  Thanks for affirming this strategy.  Sounds like this is plenty of protection against evap coil icing up.

Yes, your postings are exceptionally helpful!

Best regards,

Bill

engineer, this comment helped tremendously!

It caused me to realize that my heat mode fan speeds would be very different than my cool mode speeds if i have Dehum turned On, since I have significant gaps between fan speed settings.  I didn't realize this until reading your comment.

My zoned ducts are undersized.  This prevents me from running different air flows for heat versus cool and still staying below 900 fpm air velocities.

Thus, what I need to do is set fan speeds and damper motor settings to always accommodate at least 750, not 650 CFM going somewhere, at less than 900 fpm, regardless of heat/cool mode.

So first I've set Dehum Off.  This sets my minimum CFM to 750 in first stage heat/cool.  Plus, I don't need Dehum On to reduce RH because I don't have difficulty keeping my inside RH in the low 40s with Dehum Off.

Next I've set my fan speeds at 1 (650), 2 (750), 4 (1000), 5 (1100) and 9 (1500).

750 CFM is too much for 2 of the 3 zones on a single zone basis.  So lastly I've additionally opened up damper motors on 2 of the 3 zones to have some place for all 750 CFM to go without exceeding 900 fpm at the supply vents.  (Since I have a variable-speed fan I don't have a bypass duct.)

For 2 of 3 zones On situations, I can only accommodate about 1100 CFM, again due to duct sizes.  And since 2nd stage cooling will engage if at least 2 zones are calling for 2nd stage, that forces the 4th fan speed (lowest of 2nd stage speed choices) to be no higher than 5 (1100).  And thus, dropping down 1 notch, the higher 1st stage speed can be no higher than 4 (1000).

In summary, the comment helped me appreciate that I didn't have my speeds set to correctly take into account my air distribution limitations.  And I've now completed the adjustments (above).

Thanks and best regards,

Bill

Just spoke with our duct designer about whether to zone or not. Based on our layout which is fairly open on one level and that we seek the same temp throughout, he didn't think zoning was mandated. By ensuring duct, eqpt and controls are sized and installed properly and that we minimze heat gain thru windows by closing blinds we should achieve a comfortable temp throughout. He can design it so that we can add dampers/controls if we choose to zone later.

We have 2 ft overhangs all around and windows not under covered porches will be recessed about 5-6". I should note we're focused mainly on A/C as we will heating via radiant in a finished concrete slab.

Bill, that's an interesting chart, along with some of the others.

Bill, I