Duct fans (or empty air handlers?), to extend system?

Keith W.

Hey Prof:

I look forward to further thinking and info here. I'd just posted a related question on another thread, after someone mentioned lowering air speed across the coil to increase latent heat removal. I know that some products, like the spacepak system, use a deeper coil to remove additional humidity, which they argue allows for a higher sensible setpoint and ultimately, energy savings. I'm not convinced, but would be interested in hearing thoughts (that is, can one remove latent heat and move the sensible comfort point up, and save energy, or is the sensible energy savings lost by the cost of latent heat removal)?

And the question I asked there was, essentially, can you "choke" the system in order to reduce coil airspeed and increase humidity control, or are you asking for trouble, as you note above in your catastrophe scenario.

Thanks for any insights.

Keith

Keith W.

extending an existing system

I am a contractor, working on a house with a major addition that has an existing (relatively new) 3-ton AC system. I did a heat loss calc on the house, which is getting doubled in size (it's still small) and much, much better insulated, and it calls for just about a 2-ton unit TOTAL. The HVAC sub wants to put in a separate unit on the new (back) of the house, mostly because he's concerned there's no way to get air moved around suffiently to use the existing unit for the back half of the house: that is, the duct runs would be too long.

I'm wondering if we couldn't add a duct booster fan, or even use a air handler without a coil in it, to create the pressure differential that would move air into that section of the house. Is this done often. We're talking about the second floor of the back addition, around 900 sf and 4 rooms.

Thanks for any thoughts you wish to share!

Keith

don_156

Most

airhandler for home use will support 400 hundred feet of duct.

Putting in two system will give you nothing but poor comfort and humidity issue.

Maybe a zone damper system is in order with a blower motor
that can over come the pressure drop of the distribution system and devices.

The only way to know is to do the math.

Ps..no them fan booster are junk..Forget it.

Eugene Silberstein 3

No Booster Fan

If everything you posted is accurate, there is no need for a booster fan. In addition, the booster fan will actaully DECREASE the amount of air that will flow to the areas located upstream of the booster fan or downstream of the original system blower.

Consider a system blower that was originally delivering 1200 cfm of air to the home. Now add a "booster" fan that delivers 800 cfm. This booster fan will get its 800 cfm from the original duct system and reduce the amount of air availiable to the original areas to only 400 cfm. Be careful when attempting to do this, as an oversized booster fan can, in the extreme case, turn all supply grills into return grills that supply air to the booster!

The one way to ensure that you have ample airflow to all areas in the home is to perform a Manual D calculation which will ensure that the duct system is properly sized.

If the home origianlly had a 3 ton system and the new home (with the addition) calls for only 2 tons, the existing duct system should actually be oversized. An oversized duct system will result in low air velocities which can lead to improper cooling. The CFM that is delivered by a duct system is equal to the cross-sectional area of the duct mulitplied by the velocity of the air. For example, air moving at a speed of 700 feet per minute through a 24" x 12" will result in the delivery of 1400 cfm. If the duct size was doubled to 24" x 24", the velocity of the air would drop to 350 feet per minute. This reduction is velocity will affect the air patterns in the occupied space and can result in improper conditioning of the space.

When doing a Manual D calculation, the total equivalent length of the longest duct run (supply and return) is used to determine the available static pressure for the system. If designed to be with in the desired range, the system blower will be able to deliver the proper volumes of air to the occupied space, no matter how far the duct runs are. The purpose of using Manual D is to DESIGN the air distribution system so that it works in the space being conditioned.

If the HVAC contractor is unsure about whether or not the air will get to the remote locations in the home, suggest that he use Manual D to give him the answer. Rememer, there's nothing like getting it right the first time!

Hope this helps.

will smith_4

One problem Prof.-

> If everything you posted is accurate, there is no

> need for a booster fan. In addition, the booster

> fan will actaully DECREASE the amount of air that

> will flow to the areas located upstream of the

> booster fan or downstream of the original system

> blower.

>

> Consider a system blower that was

> originally delivering 1200 cfm of air to the

> home. Now add a "booster" fan that delivers 800

> cfm. This booster fan will get its 800 cfm from

> the original duct system and reduce the amount of

> air availiable to the original areas to only 400

> cfm. Be careful when attempting to do this, as an

> oversized booster fan can, in the extreme case,

> turn all supply grills into return grills that

> supply air to the booster!

>

> The one way to

> ensure that you have ample airflow to all areas

> in the home is to perform a Manual D calculation

> which will ensure that the duct system is

> properly sized.

>

> If the home origianlly had a 3

> ton system and the new home (with the addition)

> calls for only 2 tons, the existing duct system

> should actually be oversized. An oversized duct

> system will result in low air velocities which

> can lead to improper cooling. The CFM that is

> delivered by a duct system is equal to the

> cross-sectional area of the duct mulitplied by

> the velocity of the air. For example, air moving

> at a speed of 700 feet per minute through a 24" x

> 12" will result in the delivery of 1400 cfm. If

> the duct size was doubled to 24" x 24", the

> velocity of the air would drop to 350 feet per

> minute. This reduction is velocity will affect

> the air patterns in the occupied space and can

> result in improper conditioning of the space.

> When doing a Manual D calculation, the total

> equivalent length of the longest duct run (supply

> and return) is used to determine the available

> static pressure for the system. If designed to be

> with in the desired range, the system blower will

> be able to deliver the proper volumes of air to

> the occupied space, no matter how far the duct

> runs are. The purpose of using Manual D is to

> DESIGN the air distribution system so that it

> works in the space being conditioned.

>

> If the

> HVAC contractor is unsure about whether or not

> the air will get to the remote locations in the

> home, suggest that he use Manual D to give him

> the answer. Rememer, there's nothing like getting

> it right the first time!

>

> Hope this helps.

will smith_4

One problem Prof.-

The biggest problem I've seen through the years with load calcs, be it rules of thumb or various programs, is that many people don't seem to get into the duct sizing aspect deep enough. Knowing what the heat loss/gain is key, but meaningless if it isn't attached to the right duct. If 50% of the systems I've seen (commercial and residential) had proper supplies, and 30% had proper returns, I'd eat my hat. Even bring the hot sauce. Just replaced a system, duct and all (moonlighted too-) which had been installed by a well known national chain. Just at 1000 square feet, 8 supplies-all off of 10X10. 150,000 btu's. Had been in the house for 19 years. Two heat exchangers, 12 (count 'em) limits. We need more commonsense info regarding duct sizing. Duct work is just as important as near boiler piping on a steam system, but I think it gets largely ignored. Then again, so does the near boiler piping. And refrigerant piping. Hmmmmm.....How bout it Professor? Build another Wall for the House of HVAC?

Keith W.

Thanks a bunch!

Thanks to everyone for your thoughts here. Here's the trick...

I've got HVAC-calc residential, with which I did the heat loss. I can do a Manual D with it too, I guess (I assume that's what it's doing when I size ducts). The guy the owner hired is moonlighting this project, I think: he's doing beautiful duct work, but I pretty sure he's rule-of-thumbing all the sizing. Since getting ducts up to the second floor is hard and will result in long runs, he's intending on putting in a separate system. But the present system is already a bit oversized, and this is in the Washington DC area (very high humidity) so I think that will lead to less comfort, not more). The client wants me to help get this part right, since I've shared some of my misgivings, but I don't completely want to step on this other guys toes, since he's an HVAC professional and I'm closer to a general contractor sort of (I do integrated energy consulting, mostly for the purpose of integrating solar, etc, into residential energy systems: which is why I own HVAC-calc).

Thanks for the specifics on booster fans, they always looked like junk to me. I guess my hesitancy to do the manual D has been that so much of the duct is already in place, but I suppose that doesn't change anything: I should do the calc, after figuring out the branching structure of existing duct, and then see where it succeeds, where it fails, and how difficult it would be to retrofit something up to the second floor, and then see if I can diplomatically get all this across to the HVAC guy.

I don't know if that generates any further thoughts from anyone out there, I'm always open. Thanks again.

Keith

Eugene Silberstein 3

You are correct, Will

In the early years, when all of this HVAC stuff was more art than science, as long as air came out of the registers, all was good. Don't get me wrong, the individuals who did this work were craftsmen in their own right, just as the dead men were with their works of art that we call steam systems.

But just because the air distribution systems looked great and did deliver air to the occupied space does not mean that the system was designed propely. It's also important to mention that the cooling "system" is not only the air handler in the basement or attic and the condensing unit in the back yard, but also the air distribution system and everything else that goes along with it.

An improperly sized duct system can choke the blower, resulting in improper airflow through the evaporator coil which, in turn will result in lower system operating pressures. Lower operating pressures result in the freezing of the evaporator coil, which further reduces the airflow through the coil.

Ice acts as an insulator, which further reduces the refrigerant's ability to absorb heat, causing the operating pressures to drop even further and the evaporator superheat to drop.

Low evaporator supeheat, if allowed to reach zeo degrees, will allow liquid refrigerant to flow back to the compressor, resulting in major component damage.

Whew!

Does it all seem worth it?

So how can we ensure that, once the system is put into operation, it is actually delivering the amount of cooling capacity that the nameplate says?

Stay tuned.

Eugene Silberstein 3

Deeper coils

Deeper evaporator coils offer greater resistance to airflow and must be accounted for at the time of system design.

I can understand completely that the contractor has been doing AC work for years and probably, up to this point, has not had any customers call back with comfort related complaints. The main issue is this... There is no logical connection between "customers have not complained" and "the system is installed correctly". In the past, designing and installing ductwork was just as much art as science. Nowadays, there is a lot more science involved.

The system that this contractor may be installing may work just fine in his eyes and in the eyes of the home owner, but the overall efficiency of the system, relies on the efficient operation of all of the system components, including the air distibution system.

Using the right tool for the job being performed is something that we all take for granted. Heat gain and heat loss software, manuals, charts and tables are tools that enable us to to our jobs right the first time.

Just as we would not use an adjustable wrench as a hammer, why would we size up an air distribution system by saying, I installed 18 x 12 inch duct in a house that looked just like this one and that homeowner never complained, so I'll use that size again here.

Sorry to over simplify the issue, but nobody likes change and the mentality seems to be that since the old school method of duct sizing seemed to work well in the past, why fix it, especially if it ain't broke?

The bottom line is this... Why install a 13 SEER unit on a completely inefficient duct system. If I change the engine on an old clunker, the car is still an old clunker with a new engine.