Oversized air ducts a problem?

[rumn8r]

This is related to a different post I had regarding converting from a commercial air handler to residential unit(s):

http://hvac-talk.com/vbb/showthread.php?threadid=77203

The conversion will likely involve 2 or more multi-zoned units. It looks like the diffusers can be adjusted or partially blocked to increase air velocity but it would be too costly to replace the ducts. I would like to know what problems I might encounter if my ducts and diffusers are much larger (2X to 4X) than required.

[rumn8r]

Can anyone help on this?

[darthwade]

There are really no problems with oversized ductwork, as long as you have balancing dampers to balance the system.

[rumn8r]

I am considering Trane and Carrier zone controls, both with modulating dampers.

[darthwade]

Definately not a problem. I am an HVAC engineer and we love oversized ducts, since it reduces the fan static required and raises the efficiency of the system.

[rumn8r]

That's good to hear but I've been told that there would be insufficient air flow at the diffusers for adequate mixing with room air. All rooms have return air returns so maybe mixing won't be a problem?

A couple of contractors thought that excessive duct size is a problem and that is why I am asking. It has to do with how many different systems I have to put in for airflow when only one system is required for heating and ac.

[dash]

If the supply grilles are oversized too,the "throw" may be thought to be insuffiecent,to mix the air in the room.


I say "thought" to be ,others may say it will be .


I was trained to size grilles to "Throw" at least 75% of the distance to the otherside of the room.You likely won't get that.

That's what I was taught and thought it was critical.However we have been installing Carrier two speeds for years,and the air flow is cut in half on low,there have been no issues at all.

In rereading Man. D,I found that even though Throw is sized as I was taught,there's a section that states"the cfms delivered to the room,will cause 10 to 20 times that amount of cfm movement in the room,thru entrainment and the delta T of the air"Not an exact quote ,but something like that.


The Carrier Infinity zoning is likely your best bet as it "adapts" to the actual duct system.
Should the static be too low for proper operation,it's simple to9 add a dampers to the main plenum/trunk to bring it up.

[rumn8r]

Dash, good point on the two speed units. I hadn't thought of that. One contractor that sells Bryant told me that each zone must be able to handle 70% of the total airflow. He said that is why I need 2 separate units, each zoned. I don't quite understand that if the units are variable speed. However, if that is true and if mixing is adequate then with oversized ducts and diffusers should be desirable to reduce air noise, right?

While I would prefer to pay for just one unit (with 8 zones) it is probably better to have two separate units because there are two air returns: one for the bedroom wing and one for the rest of the house.

What do you mean "should the static be too low for proper operation"?

Regarding Carrier Infinity Zoning: this is one of the two I am considering. However, I thought that the Trane system is comparable (modulating dampers)and "adapts" also (maybe I misunderstood).

Darthwade, why is the efficiency of the system raised with large ducts/diffusers?

[dash]

The Infinity Zoning would not require each zone to handle 70% of airflow.

Each zone should be sized at 125%,of the air flow,per Man. J,although it can work ,even on existing ducts.There will be a compromise ,if the ducts are on the small side,in that at times it may overcool or overheat a zone by a degree or two,plus it will be noisier at times.

I think you'll find that Infinity,is the better zoning system,a lot is going on other then just modulating damers.It will stay in low speed if that's all that is needed,assuming you go with two speed ,which is part of why the ducts don't need to be so large.

[Edited by dash on 06-09-2005 at 09:51 AM]

[htrguy]

Originally posted by darthwade
There are really no problems with oversized ductwork, as long as you have balancing dampers to balance the system.
I hate to be a wet blanket but... oversized ductwork using a PSC blower will increase airflow, increase blower watts and can cause water blowoff on the coil. If a balance damper is used to maintain system system static, maybe.
Someone else says that it will increase effy? I don't think so. Nothing is free the blower watts will probably decrease effy.
I would look towards reducing the register size and add upstream branch dampers to maintain static.

[rumn8r]

If there is less resistance to airrflow why would blower watts increase? What is a PSC blower? What about variable speed blowers? I am considering Carrier or Trane units, both with variable speed blowers and their high end zoning systems (Carrier Infinity and the Trane equivalent).

What about Dash's comment that two speed units (and I assume variable speed units as well) work fine despite the fact that ducts and diffusers are "oversized" when the blower slows down?

[htrguy]

The reason watts go up is the less restriction causes the blower wheel to grab more air (load up) the additional loading of the wheel applies requires more torque to turn which causes the permanent split capacitor (PSC) motor to slow down; CFM is increased. High statics acually unload the blower wheel and unload the motor and the blower motor speeds up; CFM drops. In an ECM motor (in constant CFM mode) the blower senses the drop in CFM at high statics and increases the blower speed to keep the CFM constant. In low static situations the motor slows down to keep the CFM constant and a watt reduction occurs. Constant CFM ECM motors watt consumption can meet or exceed a PSC blowers at high statics.

[htrguy]

Originally posted by rumn8r


What about Dash's comment that two speed units (and I assume variable speed units as well) work fine despite the fact that ducts and diffusers are "oversized" when the blower slows down? [/B]

An ECM in constant CFM is the best for oversized duct work. There is a watt savings that can be captured. Two speed PSC blower systems are not known to save watts on the blower in low speed. Your situation is unique. All this is going to come down on the expertise of the installer and his/her familiarity of the system capabilities in the install. No one will be able to give you all the specifics unless a system layout is provided. Even then it will be opinions.

[dash]

When I mentioned Two speed,it's the compressor,not the Fan motor(although the fan would be on low cfms),and it would be with a variable speed fan motor.

[rumn8r]

Dash, OK, sorry I misunderstood and thought you were talking about a two speed motor. I will likely get varaible speed gas furnaces and two speed AC units.

htrguy, Do the variable speed furnaces from Carrier and Trane use ECM motors?

If so, what about the zone control changing the speed of the motor?




"No one will be able to give you all the specifics unless a system layout is provided."

Is this by a mechanical engineer?

[tuccillo]

Originally posted by rumn8r
Dash, OK, sorry I misunderstood and thought you were talking about a two speed motor. I will likely get varaible speed gas furnaces and two speed AC units.

htrguy, Do the variable speed furnaces from Carrier and Trane use ECM motors?

If so, what about the zone control changing the speed of the motor?




"No one will be able to give you all the specifics unless a system layout is provided."

Is this by a mechanical engineer?

Dash can give you a better description of the Infinity zoning capabilities but I will also toss in my 2 cents. I have the 2-speed compressor and 4 zones with the Infinity (Evolution) control. It is pretty interesting to watch. The postion of the modulating dampers (and therefore the CFM delivered to each zone ) changes fairly often in response to the temperature changes in the zones. The CFM also vary based on the actual RH and the RH you have set the control to. It runs a lot at low-speed - often in response to the humidity level. It does pop into high-speed occasionally but based on the heatpump statistics reported by the user interface, it has run in low-speed about 80% of the time. It doesnt draw much power in low-speed - about 1.2 KWs Temperature control in the zones is remarkable - it is *always* at the requested temperature in all 4 zones. The RH control is also remarkable - it never varies by more that 3% RH. My only complaint is that the RH sensor is way off - I will have my HVAC guy replace it. It is off by about 17% RH - reports 58% but it is really 41% based on wetbulb and drybulb measurements.

[htrguy]

Originally posted by rumn8r
Dash, OK, sorry I misunderstood and thought you were talking about a two speed motor. I will likely get varaible speed gas furnaces and two speed AC units.

htrguy, Do the variable speed furnaces from Carrier and Trane use ECM motors?

If so, what about the zone control changing the speed of the motor?




"No one will be able to give you all the specifics unless a system layout is provided."

Is this by a mechanical engineer?


Carrier and Trane and just about all manufacturers use the GE ECM.
True observation about the changing speeds. As has been suggested, there are variable speed (MANUFACTURER RECOMENDED!) zone systems out there.
System layout. A ME is not necessary unless you plan on gaining several quotes and educted solutions quickly. Contractors need to know what the system is as there are several options to solve any problem.
On this site, the information gleaned is based on general statements I.E oversized ducts. You are not going to get absolutes from here nor from Contractors unless it's all spelled out up front.
A system layout with specific modifications will need to be originated. Up to you how you'd like to address. I would pay for the work of the quote and the load estimates from the contractors who quote. You sound as though you want a higher level of involvement than the run of the mill residential customer.

[rumn8r]

htrguy,

"You sound as though you want a higher level of involvement than the run of the mill residential customer."

True, but I wish it wasn't. I knew when I bought this house (9 years ago) that I would replace the system. I knew it wasn't going to be simple but I have received quite varied opinions from contactors as to what I need. I don't mind paying a fair price to a contactor and I don't mind hiring an engineer to design it. I don't want to be taken advantage of but I don't want to pay for it twice either (if the first install is a problem). Once this beast is removed from the house, there is no going back. That's why I felt I needed to get more involved.

You all have been very helpful. I talked to a highly recommended mechanical engineer (does mostly commercial systems) today but he can't get to it for a month or so. I am still receptive to opinions/comments/things to consider.

[dash]

There's EWCM motors and there's Carrier Infinity,with the version 2.5 GE ecm,like no other for control of zoning.Any more questions??

[rumn8r]

I hired a ME to do a heat loss/heat gain and design a replacement system so I can get a couple of bids (yes, it has taken a very long time). He didn't think the oversized ducts were a problem and specified that the registers be adjusted or adapted to match the reduced airflow of the conventional furnaces (rather than the large Trane air handler). He specified 3 high efficiency condensing variable speed furnaces. I just had a HVAC contractor (very knowledgable) inspect prior to bidding and he was concerned about using the variable speed furnaces with the low static air pressure in the oversized ducts. I am looking at high-end Trane or Carrier systems with zoning (Infinity level). I mentioned that he could use dampers to increase the static air pressure and he said that would reduce airflow (if I undersood him correctly). So, is this a problem or not?

Also, htrguy, as I understand it, these are ECM motors so does your comment mean that I won't see any energy savings?



Originally posted by htrguy
The reason watts go up is the less restriction causes the blower wheel to grab more air (load up) the additional loading of the wheel applies requires more torque to turn which causes the permanent split capacitor (PSC) motor to slow down; CFM is increased. High statics acually unload the blower wheel and unload the motor and the blower motor speeds up; CFM drops. In an ECM motor (in constant CFM mode) the blower senses the drop in CFM at high statics and increases the blower speed to keep the CFM constant. In low static situations the motor slows down to keep the CFM constant and a watt reduction occurs. Constant CFM ECM motors watt consumption can meet or exceed a PSC blowers at high statics.

[htrguy]

I'm flying blind, not seening anything on your system and you're mediating (not by choice). Lower statics do save power with ECM over PSC blowers. I can't fathom why the ME has said dampers will reduce airflow; ECM is constant CFM. I would think that they would be able to place dampers to maintain balance and minimum static. Increasing statics will cause an increase in power consumption with ECM not reduce airflow.
Also, I do not understand why they opine that there is a concern on low static operation using ECM blowers. I have heard that there were issues when manufacturers first started using ECM blowers because the ECM had a glitch concerning low static sensitivity. I believe that has all been resolved. There must be somthing I'm missing or something is lost in translation

[dash]

The ECM may need a minimum level of static (resistance to air flow).

This can easily be achieved by a manual damper at the main supply and return ducts.

Other branch ducts may need dampers to get the correct cfms(aif flow) to each room ,to maintain similar temperatures for room to room.

The increased static of these dampers will not be enough to cause concern for ,high static or low air flow,when using a variable speed motor,IMHO.

[rumn8r]

Htrguy,

The ME didn't say that dampers will reduce airflow, it was the contractor (if I understood him correctly). However, even if I misunderstood he was still hesitant to install variable speed furnaces but said that he need to check with Carrier.

I also thought you said that ECM VS motors don't necessarily save energy if used with oversized ducts. Is that correct?



Originally posted by htrguy
I'm flying blind, not seening anything on your system and you're mediating (not by choice). Lower statics do save power with ECM over PSC blowers. I can't fathom why the ME has said dampers will reduce airflow; ECM is constant CFM. I would think that they would be able to place dampers to maintain balance and minimum static. Increasing statics will cause an increase in power consumption with ECM not reduce airflow.
Also, I do not understand why they opine that there is a concern on low static operation using ECM blowers. I have heard that there were issues when manufacturers first started using ECM blowers because the ECM had a glitch concerning low static sensitivity. I believe that has all been resolved. There must be somthing I'm missing or something is lost in translation

[pstu]

>>The ECM may need a minimum level of static (resistance to air flow).

Are we talking about less than 0.1 inch water column? My variable speed furnace documents near-constant airflow down to that level. If we are talking about a duct system which results in less than 0.1 inch w.c. then I am amazed. But as a homeowner perhaps it doesn't take much.

I'm just curious, this is the first case I've heard of where the problem was too *little* static pressure.

Best wishes -- Pstu

[mcjo tech]

I agree. ECM variable speed blower is the best way to go to compensate for the oversized ductwork but it can only compensate so much. Make sure you hire a competent professional to evaluate duct work including perhaps the use of dampers prior to the install of any system.

[htrguy]

Originally posted by htrguy
In low static situations the ECM motor slows down to keep the CFM constant and a watt reduction occurs. Constant CFM ECM motors watt consumption can meet or exceed a PSC blowers at high statics.

I'm quoting myself.

[mcjo tech]

I just completed a course from Trane on their ECM variable speed motors. It was my understanding that they consume only about 60 watts of power. By having a pre-magnetized rotor along with Pulse Width Modulation and the ability to turn the windings on and off electronically this 3 phase motor has very low wattage consumption even at full ramp up. I would like to know how it would be possible for the ECM motor to use more power then a conventional PSC motor?

[dash]

mcjo,

Carrier MVB-100-20 specs;

.4 ESP @1525 cfms=486 watts

.4 ESP @ 2000 cfms=947 watts


.9 ESP @ 1525 cfms = 696 watts

.9 ESP @ 2000 cfms = 1129 watts


Only the the 060 and 040 use less than 60 watts ,and that's at .1 or .2 ESP,would need to be heating only ,with no cooling coil,to get the ESP that low.

[htrguy]

I am not repeating what I've heard at a training class. It's based on motor design and measured results. Look at at airhandler airflow tables youself. Think about it. An ECM blower motor speeds up to maintain CFM at higher statics using more power. A psc speeds up because the blower wheel unloads (less CFM) and the motor uses less power.

Quick example, some are worse than others:

.1" static
PSC 1547 CFM @ 504 Watts
ECM 1171 CFM @ 181 Watts

1.0"
PSC 1143 CFM @ 377 Watts
ECM 1185 CFM @ 390 Watts

[rumn8r]

So oversized ducts = low static = lower watts with VS (ECM) blowers. Right?

[htrguy]

Yes

[rumn8r]

Thanks! ...and the consensus is that the HVAC contractor can add dampers to increase static pressure if needed so I can use the Infinity system??

[htrguy]

Yes

[rumn8r]

A HVAC contractor emailed me to say that he had a conference call with a couple of Carrier engineers (state and regional) and they were very concerned that the Infinity variable speed blowers would NOT be compatible with my oversized ducts. That doesn't fit with info in this thread (and other threads) and I'm not sure what to do. I definitely want to use VS blowers to save energy but it might be tough to get a warranty on the equipment. Any ideas on how I should proceed?



[Edited by rumn8r on 12-16-2006 at 12:04 PM]

[pstu]

Bear with me here. Do you mean to say, they suspect the Carrier VS air handler will NOT be compatible? I'm just a homeowner but that sounds absurd, you could always adjust for higher restriction by having dampers. So probably what I am thinking, is not what he really thinks.

What I think, find out who are these engineers and try to identify exactly what they did and did not say. Your engineer should have the mojo to get a direct contact with these guys. Then get a clear statement of what is their professional opinion, in writing! I can imagine a Carrier dealer talking a line of manure in trying to sell, but not one of their engineers.

Again, my VS air handler is documented to be within spec for as little as 0.1 inch water column (and up to 0.9 inch w.c.). I would not know how to build either a supply or a return duct that would have that little resistance. Far more typical is my system which has about 0.4 inch w.c. on the return alone, and about 0.55-0.60 total.

The very concept of having too much duct for the air handler sounds so very unlikely, it is like saying a person has too much money saved for his retirement. You hear all the time about the opposite, practically never about having too much.

Best wishes -- Pstu

[Edited by pstu on 12-16-2006 at 09:35 AM]

[sskzekeman]

What they are probably thinking is that when you run the ECM motors to satisfy low CFM demands on a wide open duct system, you are talking very low motor RPMs and at those levels, the system may run into control instablities, precluding its use without sufficient dampers.
But as others have pointed out, properly placed dampers used in moderation should make the system work just fine for either PSC or ECM motors, although the PSCs may require more dampening to match the CFM requirements to the limited RPMs available.

[AIR PRO]

Let me explain how I learned about the backward falacies of the PSC motor many years ago. Most inteligent people would think that a motor with NO LOAD will use less amperage because it is simply free wheeling... This couldn't be further from the truth. Everyone in our industry knows(I hope!) that if you take an amp draw reading on a blower motor with the door off.....you will NOT get an accurate reading, it will read high.....because of no load.

Simple test for you rookies, find an old 110 volt blower assembly with a psc motor, hook it up and put your amp probe around one of the motor leads, then take a piece of cardboard and cover 3/4 of one side of the blower housing and watch the current draw go down. You will also hear the motor speed up.

The PSC motor draws LESS amperage when loaded than it does when unloaded or freewheeling. The ECM variable speed motor overcomes this by sensing resistance and overcoming static pressure deficiencies in the duct system. Hence why a ECM /variable speed motor is used in 90% of the air handlers/furnaces made today.

[jacob perkins]

Originally posted by Hawk327
Let me explain how I learned about the backward falacies of the PSC motor many years ago. Most inteligent people would think that a motor with NO LOAD will use less amperage because it is simply free wheeling... This couldn't be further from the truth. Everyone in our industry knows(I hope!) that if you take an amp draw reading on a blower motor with the door off.....you will NOT get an accurate reading, it will read high.....because of no load.

Simple test for you rookies, find an old 110 volt blower assembly with a psc motor, hook it up and put your amp probe around one of the motor leads, then take a piece of cardboard and cover 3/4 of one side of the blower housing and watch the current draw go down. You will also hear the motor speed up.

The PSC motor draws LESS amperage when loaded than it does when unloaded or freewheeling. The ECM variable speed motor overcomes this by sensing resistance and overcoming static pressure deficiencies in the duct system. Hence why a ECM /variable speed motor is used in 90% of the air handlers/furnaces made today.

htrguy

The reason watts go up is the less restriction causes the blower wheel to grab more air (load up) the additional loading of the wheel applies requires more torque to turn which causes the permanent split capacitor (PSC) motor to slow down; CFM is increased. High statics acually unload the blower wheel and unload the motor and the blower motor speeds up; CFM drops. In an ECM motor (in constant CFM mode) the blower senses the drop in CFM at high statics and increases the blower speed to keep the CFM constant. In low static situations the motor slows down to keep the CFM constant and a watt reduction occurs. Constant CFM ECM motors watt consumption can meet or exceed a PSC blowers at high statics.


-------------------------

thunderdome!

[mark beiser]

Originally posted by Hawk327
Let me explain how I learned about the backward falacies of the PSC motor many years ago. Most inteligent people would think that a motor with NO LOAD will use less amperage because it is simply free wheeling... This couldn't be further from the truth. Everyone in our industry knows(I hope!) that if you take an amp draw reading on a blower motor with the door off.....you will NOT get an accurate reading, it will read high.....because of no load.

Simple test for you rookies, find an old 110 volt blower assembly with a psc motor, hook it up and put your amp probe around one of the motor leads, then take a piece of cardboard and cover 3/4 of one side of the blower housing and watch the current draw go down. You will also hear the motor speed up.

The PSC motor draws LESS amperage when loaded than it does when unloaded or freewheeling. The ECM variable speed motor overcomes this by sensing resistance and overcoming static pressure deficiencies in the duct system. Hence why a ECM /variable speed motor is used in 90% of the air handlers/furnaces made today.

You have it backwards, when you pull the blower panel off, the motor slows down and amps go up because it is loading up.
When you put the panel back on, increasing the ESP on the blower, it unloads some, so the RPM and amps go down.
If you totally block the supply or return, the blower will totally unload, since it can't move any air. It will speed up to near its synchronous speed, and the amps will go way down.

[mark beiser]

My main concern with oversized ductwork would be if it was installed in unconditioned spaces. If it is very badly oversized, the velocity will be very low, wich can cause undesireable levels of heat gain/loss in the duct system.

If the ductwork is mostly all in conditioned spaces it isn't an issue.

Other than that, as others have mentioned, you will need to do something with all the registers so that the exit velocity of the air is a minimum of about 400 fpm, or you will have problems with the mixing of the room and supply air.

[cem-bsee]

let me repeat --the dampers need to be placed near the start of the duct being controlled --
that is, do not use a supply register as a damper

[rumn8r]

Originally posted by rumn8r
they were very concerned that the Infinity variable speed blowers would be compatible with my oversized ducts.

Sorry, I meant to write "NOT compatible" and have since edited my post.

[oceana/c]

Originally posted by htrguy
The reason watts go up is the less restriction causes the blower wheel to grab more air (load up) the additional loading of the wheel applies requires more torque to turn which causes the permanent split capacitor (PSC) motor to slow down; CFM is increased. High statics acually unload the blower wheel and unload the motor and the blower motor speeds up; CFM drops. In an ECM motor (in constant CFM mode) the blower senses the drop in CFM at high statics and increases the blower speed to keep the CFM constant. In low static situations the motor slows down to keep the CFM constant and a watt reduction occurs. Constant CFM ECM motors watt consumption can meet or exceed a PSC blowers at high statics.
That's it in a nutshell, Thanks htrguy.
As far as ductwork you need properly sized (takeoffs & can's) to throw or distribute the air. If you have a 50'deep room & side wall registers you would never make it with...say 4-12x18's...........6-8x20's would throw the air across the room. (Ok it's just an example.) :)

[dash]

How oversized are the ducts,has anyone varifed that they are oversized??

What is the tonnage reduction from the old to the proposed new system??

[rumn8r]

Originally posted by dash
How oversized are the ducts,has anyone varifed that they are oversized??

What is the tonnage reduction from the old to the proposed new system??

Here are the existing registers and ducts and how the ME proposed to rebalance the registers but using the existing ducts. Currently the system is hydro air and the blower is a Trane Climate Changer for all 6 zones. Plan is to replace with 3 air handlers:
1) a 5T for the living room, entryway and study, kitchen and dining room (79 k btu/h heat loss, 35 k btu/h heat gain by my calculations using HVAC Calc)
2) a 3T for the bedroom wing (30 k btu/h heat loss, 13 k btu/h heat gain)
3) a 2T (16 k btu/h heat loss, < 1 k btu/h heat gain but no AC) for the rec room (lower level).

The air handlers for 1 and 3 will share a common return.

Living room: existing registers total 2500 cfm but will rebalance to 1,000 cfm; supply duct is 21" round. The dramatic drop in cfm is because the long wall of the LR is all glass which was originally an outside west facing wall. This was enclosed by a heated solarium/pool room and heat loss decreased greatly.

Entryway and study: existing registers total 510 cfm but will rebalance to 250 cfm; supply duct is 12" x 8"

Kitchen: existing registers total 700 cfm but will rebalance to 500 cfm; supply duct is 15" x 9"

Diningroom: existing registers total 380 cfm but will rebalance to 300 cfm; supply duct is 16" x 6"

Rec. room (lower level): existing registers total 605 cfm but will rebalance to 700 cfm; supply duct is 14" x 8"

The above zones are open to the living room and share one common return; return duct is 30" x 20"

Bedroom wing: existing registers total 1100 cfm but will rebalance to 1250 cfm; supply duct is 18" round
Another common return for the three bedrooms: return duct is 18" round

[Edited by rumn8r on 12-17-2006 at 07:04 PM]

[cem-bsee]

why the wide variation between the calc load & the output rating of the units?

[tinknocker service tech]

I dont want to bust any bubbles here but you need to relize one thing in this experiment.

you ducts are designed for a certain static pressure and a certain amount of cfms at each register in the house. You have three systems doing this at this time.

By installing one unit with a damper system what most likely will happen when two or more damper open the static will drop off and the air flo or cfm will stall.
You will find the air will fill the ducts and just bearly push out of any of the registers

Also with a v=drive blower you will find with one zone it will ramp up and be strong but with all zones calling and no static pressure it will ramp down to slow just barely pushing the air
Varitable motors are designed to read static and ajust accordenly to deliver the cfm you want and with out any static they wont ramp and think the cfm is correct

My ex boss tried this same thing and i did worn him before hand. WE had to replaced the ducts for it to ramp and work properly and all was fine.

size the zone acordinly to your loads with the proper unit or replace three systems same
imo only

You are takeing 10 ton 5000cfm and feeding with a max of 2500cfm 5 ton think about it before you regret it

[Edited by tinknocker service tech on 12-17-2006 at 07:49 PM]

[rumn8r]

cem-bsee,
I think to get enough airflow but I'm not sure. Both heat and air are oversized for the Main house and the BR wing but the ME specified Trane VS furnaces with high/low fire and 19i AC units with hi/low. Should I question this? I don't know what his heat gain/ heat loss figures are because he didn't share them with me. I am completely inexperienced but took great care with measuring everything using HVAC-calc. Now, it looks like I will go with hydro air and I will be able to match the heat supplied to the air handlers but maybe I should downsize the AC units.

[rumn8r]

Originally posted by mark beiser
My main concern with oversized ductwork would be if it was installed in unconditioned spaces. If it is very badly oversized, the velocity will be very low, wich can cause undesireable levels of heat gain/loss in the duct system.

If the ductwork is mostly all in conditioned spaces it isn't an issue.

Other than that, as others have mentioned, you will need to do something with all the registers so that the exit velocity of the air is a minimum of about 400 fpm, or you will have problems with the mixing of the room and supply air.

Mark,
The bedroom ducts run under the living room concrete floor but are insulated, then run up through a closet to the attic and run to each of 3 bedrooms in the attic space but they are insulated.

The livingroom duct runs under the slab floor but is encased in concrete and insulated with 1" rigid insulation.

All other ducts are in conditioned spaces.

The size of the ducts are in my previous post.

[rumn8r]

Originally posted by tinknocker service tech
You have three systems doing this at this time.

I think you might have misunderstood; I currently have one air handler for the house and the proposal is to switch to three.

[dash]

You stated,"The air handlers for 1 and 3 will share a common return."

This should be changed.VS blowers check the resistance to air flow and adjust to deliver the vcfms they are set up for.Sharing a common return they will"hunt" as one ramps up or down ,the other will adjust and it justgoes on and on.It's even worse with two stage equipment.

Run this by your ME and contractor before installing as it needs to be changed.

The rest all sounds okay other then sizing,IMHO,but we can't see the entire duct system ,so can't say for sure.ME's usually oversize ,at least that's what we see around here.Oversizing can be negated ,to some degree,by using two stage heating and cooling,with great dehumidification controls on the cooling. Oversized cooling,to some degree,is a great way to handle the "party" load on the 4th of July.

[Edited by dash on 12-18-2006 at 12:24 PM]

[rumn8r]

Dash,

Thanks for the tip on the shared return with VS air handlers; I will discuss this with the ME.

Mark,

Using the duct sizes and the rebalanced register sizes it looks like the registers in the Main (air handler #1) part of the house have an air velocity of 375 to 533 fpm. The bedroom registers will be ~700 fpm and the basement rec room will be ~900 fpm.


If I go with hydro heat (heating coils) and 2 stage AC with a VS air handler, is there any downside to oversizing the air handler relative to the AC unit (i.e. use a 5 T air handler with a 4 T AC condenser and coil). The speed can be changed to match the heating/cooling needs right?

[tinknocker service tech]

Originally posted by rumn8r

Originally posted by tinknocker service tech
You have three systems doing this at this time.

I think you might have misunderstood; I currently have one air handler for the house and the proposal is to switch to three.



sorry your right i did read it wrong and thought you were going the other way arround