HomeOwners : Dont play 'the register game' .

[hvacfella]

On Approx. 30% of my calls, the H.O. has all or nearly all of the first floor registers closed in order to move more air to the second floor ; it doesnt work , and more importantly, it will almost always return liquid freon back to the Compressor in the A/C Unit ... which is designed to pump vapor only. Damage can and often results. The reason liquid gets returned is due to the air not flowing fast enough over the Cooling Coil to evaporate all the liquid freon when you have a bunch of registers closed . Nearly all duct systems in residences are undersized/poorly designed, and closing down registers only makes the system more choked down. Just want to give you a heads up as this is a very popular thing H.O.'s do without realizing the ramifications.

Dave

[billygoat22]

ANd I thought this thread was going to be about moving new construction registers for the new sofa- that'll be on the curb in 5 yrs.

[jrbenny]

Then, it would be a good idea for contractors to install duct properly with balancing dampers.

[rtu]

Originally posted by jrbenny
Then, it would be a good idea for contractors to install duct properly with balancing dampers.

Ditto

[hvacfella]

Originally posted by jrbenny
Then, it would be a good idea for contractors to install ductwork properly with balancing dampers.

REPLY: I couldnt agree more. But, that is seldom the case even in custom built homes. Therefore, the H.O. needs to leave all the supply registers open and dont block returns with furniture, etc.

[scruit]

Some of the ducts in my basement were blocked by the previous owner of the house...

If the evap superheat is still good per manufacturer specs then there must still be enough airflow to allow evaporation, plus enough sensible heat loss to ensure there is no flooding to the compressor, right? (This would be accurate for the current load level at the time of testing, yeah? Would have to have the HVAC guy come back out check the superheat at low load also)

[slab]

I am just a homeowner who has also heard the tale concerning closing the registers to increase flow to the second story. I just want to make sure I understand the theory: By closing off the registers, it gives the air flowing through the 1st floor ducts no place to go, which increases the pressure. Zoning works differently by closing dampers at the beginning of the ducts, which allow the air to flow through the main trunk into the other open ducts, increasing flow to the open ducts (up to the limit of duct size). Am I getting it right?

My 2 story house doesn't have automatic zoning, but it does have manual dampers on each duct coming off the main trunk. Would partially closing these dampers on the 1st floor work to increase flow to the second, or is that just as bad as closing the registers?

[cem-bsee]

one must keep a certain amount of air flow across the coils -- one may use dampers to change the air flow thru given ducts, so long as the minimum air flow is maintained across the inside coil --
it does not matter which combination is chosen --

dampers may be switched between summer & winter operation --
with multiple stories, the dampers could be positioned in the trunks of each story, as opposed to necessitating attic access --

still, it is best to maintain some air flow into each conditioned space to control humidity, at least part of the time ea da | night.

[JoeSix]

Originally posted by hvacfella
On Approx. 30% of my calls, the H.O. has all or nearly all of the first floor registers closed in order to move more air to the second floor ; it doesnt work , and more importantly, it will almost always return liquid freon back to the Compressor in the A/C Unit ... which is designed to pump vapor only.

I'm curious, but doesn't a TXV help with this? (e.g. if the coil gets very cold, doesn't it meter less refrigerant into the coil?

(No, I'm not advocating blocking registers, just wondering about those dehumidifying stages of the air handler operation which also reduce air flow over the coil)

[dash]

We have found with proper Man D design,in our area,ducts in the attic ,curved blade adustable grilles,etc.,that grilles can be dampered ,at the grille and reduc air flow by 25% with no adverse effect.

Now,that would part of the grilles,< 30% of them, not all of the grilles.

[ct2]

ok so if blocking the regesters is a bad thing (and I understand why it is) what does a zoned system do differently. The zoned systems that we make will either close one side or the other or supply air to both outlets ( on a two outlet plenum)

but by closing off one zone you are diverting all of the airflow to the other, and by closing all of the regesters on one floor you would force the airflow to the other.....ooooo smaller ducts going to the regesters increases static and reduces airflow through the system--- right?

[hvac_czar]

Dave the hvac-hobbyist makes blanket statements like this, please forgive him as he doesn't have a complete grasp of how stuff works yet.

[JoeSix]

Originally posted by JoeSix
I'm curious, but doesn't a TXV help with this? (e.g. if the coil gets very cold, doesn't it meter less refrigerant into the coil?

I think I found my own answer...

...Now that more refrigerant is being introduced into the evaporator there is more availability to absorb heat, if there is insufficient heat to boil off all the refrigerant prior to it exiting the evaporator the temperature at the sensing bulb will decrease reducing the pressure at P1 and causing the valve to close...

http://www.hvacmechanic.com/txv.htm

Not that closing half the registers is a good idea, but I'd think a system with TXV might be less susceptible to floodback...

[scruit]

Because of the lack of a TXV to adjust of superheat, closing vents in a CapTube system would be much worse, especially when heat loads are low... Correct?

[billygoat22]

Difference between zoning and registers is that zoning duct (wehope) was designed to operate with dampers closed, such as each branch sized for total load, or a bypass damper and protective controls to prevent the floodback/freezeup issues.

Closing floor registers moves air up stairs because they were the path of less resistance, and by closing there is less resistance upstairs vs the closed register. but, as resistance in system increases, airflow across the coil decreases. Most systems are not designed to be adjusted in this manner (not that they thought about such things, leaving out balancing dampers and proper duct sizing)

Bottom line is, fix the duct so it won't damage the equipment.

[diegocentral]

Great Info.

So closing registers won't improve airflow in other registers? Or are you talking only about 2 story homes.

I live in a one story home and keep the washer/dryer and walk in closet registers closed. Since it's Wye Branched with the bedroom next to it, wouldn't it help a little?

On another note, when we zone system we do include a bypass damper with a counterweight and all the main runs are electronically dampened as well as manuals on the main runs. I thought this was in case one zone remained closed and the other opened it would relief into the R/A.
This sounds interesting

[jrbenny]

Bypass dampers are used to reduce noise in the supply duct. They do not protect by increasing air flow across coil. You are actually causing the equipment to reach operational limits faster when the bypass opens. In cooling, you are dumping cold air in the return. You are reducing the load on the evap by preconditioning the return air. Best zoned systems have supply ducts that can handle the airflow without using a bypass.

Shutting dampers will help non-zoned systems force the air to other registers. However, it only works to a certain point. It's best to get the duct sized properly with summer-to-winter balance dampers.

[Edited by jrbenny on 08-04-2005 at 06:58 PM]

[billygoat22]

Closing registers will increase airflow at other registers, but at the expense of reduced airflow at the coil or furnace heat exchanger.

[dash]

Depends on the duct design.we have tested ours and some can be partially losed without raising the static and reducing air flow.


When the Friction Rate is say .08 ,and you set your ductolator,for the cfms and fR,and it comes out to a 7.25" duct,so you go to an 8" duct.Because of this ,they can usually carry a little more air,so the system stays below the design static,when a few grilles are partially closed to balance things out.

[hvacfella]

'Not that closing half the registers is a good idea, but I'd think a system with TXV might be less susceptible to floodback...
'

Reply: Joe, your observation is correct ; With a TXV system, the valve will throttle down under low load to maintain its superheat setting as sensed at its bulb location (at the exit of the evaporator). Low load can occur via a low cooling load in the house , keeping the house cooler than normal , or, via restricted airflow whether it be a dirty air filter or blocked registers decreasing the airflow across the evaporator surface. Metering devices that employ a fixed orifice or cap tube are more prone to floodback by closing alot of registers.
Dave