Variable air handlers

[BilltheCat]

I'm a homeowner looking to replace my old AC system, and I'm getting conflicting information from different contractors on the benefit of installing a variable speed air handler. One contractor says it saves on energy because of the fact that it can draw more moisture out of the air. The other contractor says you shouldn't install one, because without installing more supply and possibly another return line (according to the contractor something you can't know until a month or two after its installed) it won't pull enough air across the coil to function properly? I appreciate any and all help I can get.

[HVAC2B]

I'm a homeowner looking to replace my old AC system, and I'm getting conflicting information from different contractors on the benefit of installing a variable speed air handler. One contractor says it saves on energy because of the fact that it can draw more moisture out of the air. The other contractor says you shouldn't install one, because without installing more supply and possibly another return line (according to the contractor something you can't know until a month or two after its installed) it won't pull enough air across the coil to function properly? I appreciate any and all help I can get.

IMHO,they are very efficient
sounds to me,that dipstick can't set a dipswitch

[hangfirew8]

I'm a homeowner looking to replace my old AC system, and I'm getting conflicting information from different contractors on the benefit of installing a variable speed air handler. One contractor says it saves on energy because of the fact that it can draw more moisture out of the air.

If properly set up, this is true. Either using an algorithm, or a (de)humidistat, a variable speed system can run at lower fan speed part of the time in cooling mode, which will pull humidity out faster.

The other contractor says you shouldn't install one, because without installing more supply and possibly another return line (according to the contractor something you can't know until a month or two after its installed) it won't pull enough air across the coil to function properly? I appreciate any and all help I can get.

That's funny, because some have come here after being told that a VS system was the solution to their undersized ducts problem. It's not.

A contractor that knows what he is doing can tell you whether your duct system is good enough, either by doing sizing according to "Manual D" or actual airflow measurements on your old system.

-HF

[Morrison]

Variable speed air handles usualy provide better comfort and normly helps with air flow,down side is if they need to be replaced they are very expensive

[BilltheCat]

Okay so it looks like they could be good, but the contractor should do a manual D calc first to see if my duct work can handle it as it stands now, is that correct?

BilltheCat

[key1cc]

Okay so it looks like they could be good, but the contractor should do a manual D calc first to see if my duct work can handle it as it stands now, is that correct?

BilltheCat

From someone who has one...and wishes he had gotten the manual D...that is correct.
Key1

[aintitfun]

Variables are definately not a cure all, for poor duct design, but there should be no reason, that it cannot be installed, The manual d is to evaluate as to how much this is going to help your airflow.

[dash]

Okay so it looks like they could be good, but the contractor should do a manual D calc first to see if my duct work can handle it as it stands now, is that correct?

BilltheCat

Easier to just test the static of the existing system first,then Man. to redesign if needed.

[BilltheCat]

Thanks for your help so far, I feel like at least I have somewhere to turn to. Understand, I am not trying to DYI, but its not like I just have $6000 lying around to plunk down on the first whiz bang system someone tries to sell me. I've got three different contractors that I've contacted for quotes. None has so far done the Manual J calc, though when I questioned the first contractor about it he said that since nothing had changed in the house since it was built (16 years ago).

I want a good reliable system that will not only cool my house, but will be energy efficient and reliable as well. Since I can't rely on Consumer Reports or any independent source like that, I'm having to try and figure this out myself. Thanks for your time.

[DanW13]

Whose to say that when your equipment was installed 16 yrs ago that it was done right ???

[firecontrol]

No matter what kind of air handler you put in the ductwork needs to be verified to be adequate.

[beenthere]

As Dash said. Your duct work can be checked now before a new system is installed.
Don't have to install one and see how it does.

[everythingair]

Easier to just test the static of the existing system first,then Man. to redesign if needed.

This is sound advice. Even if the ducting system is not perfect, which most are not, an ECM motor can be beneficial as long as the current static pressure parameters are met.

For example; if your system is a 2.5 ton system, the nominal amount of air needed, based on the average 400 cfm per ton, would be 1000 cfm. Let's say your duct system calculates to be between .7 and .8 SP, which is not far from what I've seen nationwide. This would put the system at the far end of the blower charts for a standard psc motor but still within the capabilities of a VS motor.

If the blower chart for a psc motor blower rate the required 1000 cfm of air at the standard SP of .5, then at .7-.8 SP that psc blower will only be providing about 700-800 cfm of air. With a psc motor, this can be a problem during humid days when a wet coil from low airflow will create even more resistance which will lower the cfm ability even more until the coil eventually freezes up.

With the same system using a VS blower, the VS motor set at 1000 cfm is going to continually ramp up to provide that 1000 cfm of air. There is no SP diferential chart for a VS blower because the motor continues to attempt to achieve the amount of air it is programmed despite varying SP conditions...within reason.

Also, a VS blower can be safely set to a lower cfm rating without the fear of the additional resistance from a wet coil causing a coil freeze up. By setting a VS blower to a lower cfm rating, you are decreasing the SP of the system, creating an all around quieter and better dehumidifying system.

If a duct system is just too restrictive, a VS motor will do what is called "huffing". This is where the motor ramps up to achieve the desired cfm of air but reaches a critical point where the motor cannot safely operate, so the motor decreases the rpm in order to operate within a safe speed, and will continue going up and down until if eventually dies. In this case, a redesign of the ducting to reduce resistance is needed.

If the ducting can not be practically altered to decrease the SP, then the next best thing to do is to have a larger HP psc motor installed at the same rpm in order to overcome the SP. This will void any motor warranty of a unit and will cost more to operate, so it is only suggested is all else is just not practical.

[beenthere]

If the blower chart for a psc motor blower rate the required 1000 cfm of air at the standard SP of .5, then at .7-.8 SP that psc blower will only be providing about 700-800 cfm of air. With a psc motor, this can be a problem during humid days when a wet coil from low airflow will create even more resistance which will lower the cfm ability even more until the coil eventually freezes up.

With the same system using a VS blower, the VS motor set at 1000 cfm is going to continually ramp up to provide that 1000 cfm of air. There is no SP differential chart for a VS blower because the motor continues to attempt to achieve the amount of air it is programmed despite varying SP conditions...within reason.

Some have a derate percentage to use above X amount of static.

If a current system with a PSC blower is moving 800 CFM at a .75" static. Then increasing CFM to 1000 would raise the static to roughly 1.17"
They end up using about the same amount of electric in that type of an install, and making the system loud.
VS blowers don't have CFM ratings at that high of a static.

[everythingair]

Some have a derate percentage to use above X amount of static.

If a current system with a PSC blower is moving 800 CFM at a .75" static. Then increasing CFM to 1000 would raise the static to roughly 1.17"
They end up using about the same amount of electric in that type of an install, and making the system loud.
VS blowers don't have CFM ratings at that high of a static.

This is what I was referring to when I mentioned the psc motor blower performance chart. If the psc motor that is installed in a particular blower is not rated to perform above a certain SP, then it is better to use a larger HP motor. This is essentially what equipment manufacturers do with high static blower furnaces. I believe the most that I have seen for SP for VS motors is .8 SP. A system that is that close to the edge can have issues with just a dirty filter.

[key1cc]

My static for my VS is at 1.1....so i am having duct work done. I've never noticed any huffing though nor any "oil-canning" (ducts making loud noice from bowing in and out due to the pressure). My duct guy said the VS motors can run at 1.5 SP (of course I don't believe him). He is really good with sheet metal....but does not appear to possess the knowledge of the Pro's on this site.
Key1

[BilltheCat]

Sorry if I'm asking dumb questions, but is the static pressure test something that can be done fairly easily by the contractor while he's here the first time, or is that something that needs special equipment and time to set up? Also I've got the flexible duct work installed, is that going to make a difference in terms of the SP?

[dash]

The test equip. needed is very resonable,problem is many aren't trained to do so.

Flex designed correctly wouldn't be an issue.Flex would be larger ducts ,for the same static ,then meatl.

[RoBoTeq]

The test equip. needed is very resonable,problem is many aren't trained to do so.

Flex designed correctly wouldn't be an issue.Flex would be larger ducts ,for the same static ,then meatl.

I sure wish more people understood this and stopped making flex out to be some sort of air killer.