Retun plenum much smaller than supply


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Old 12-03-13, 09:22 PM
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Retun plenum much smaller than supply

Hi folks. We have a 70s Ranch w/ central air and, fortunately, individual room returns, but a furnace that is > 2/3 to one end of the basement..
I've been battling poor air flow in a far bedroom, it's cold in winter, warm in summer, but only with the door closed. Sadly it must stay closed most of the time due to keep a cat and 3 year old out...

Also if we close the doors to the living room, which is right overt he furnace, it gets warm quickly.

After sealing all seams well, I'm about to install a booster fan w/ ductstat etc to that far room.
However something really bothers me. The primary horizontal trunk for the returns in nice and beefy, about the same as the supply, yet main plenum that connects this down to the fan/filter & furnace seems quite small, and is only ~ <1/2 the size of the primary vertical plenum on the supply side coming off the furnace/coil.
Is this normal? It seems to me like this would be really restrictive to the fan and limit the vacuum at the vents, and potentially a contributor to our flow. Shouldn't these two plenums be about the same size?
 
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Old 12-05-13, 06:51 AM
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What size is it? Also your booster fan will do you no good. You have a duct issue to that room.
 
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Old 12-05-13, 09:07 AM
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From the picture it looks like your supply duct--the horizontal run is about the same size as the return duct. It's only the vertical plenum on top of the furnace that looks too big--but it needs to be to house the AC evaporator coil assembly.
 
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Old 12-07-13, 07:11 AM
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Here are some numbers.
The giant vertical plenum (housing the condensor) is 27x20.
The narrower vertical plenum on the return side is only 8x20.
The trunk connecting them on bottom, which houses the filter, is 16x20.

The main horizontal trunks go both ways, one direction is quite long, ~37 ft, other side ~18.5 ft.
On the long side, supply trunk is 20x8, then after ~10 ft reduces to 16x8, then to 12x8 after about 20 ft. Other short side is 16x8 then reduces to 12x8.
The return trunk is almost all 12x8, except for the last ~6' going into the plenum which is 20x8 (there is a room return above that section, all of my returns use joist spaces to connect to rooms).

So, yes, the internal volume of that small vertical trunk is less than the sum of what is feeding it from both directions, but much smaller than the supplied side. I would think it should be 16x20 to match the sum of what is above + the space below housing the fan + filter.

The "problem room" is of course a single line at the end of that 37' run. Pressure is low, and a lot of un-heated/cooled air gets pushed out of the vent before it warms up. Hence the plan for a booster fan, tied to a thermostat to kick in only when vent temp has been reached.

If the return side is restricted, I can see how that would be of limited help... but I've heard from a lot of people about booster fans helping in this scenario. Band-aid? Absolutely. But much cheaper than redoing ductwork.
 
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Old 12-07-13, 04:28 PM
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I'm thinking your instinct may be correct. More so in that the entire return truck seems to be smaller than the supply system. In simple terms --- with a forced air system the air flow for both the supply and return should be roughly equal (balanced).
For example -- If the system is set to deliver 500 CFM then that should be for both supply and return to be able to recirculate the air.

If there is insufficent return air flow in a room, the supply air will fill the room to a point that it becomes positvely pressurized. This causes resistance to the incoming supply air ( increased static pressure ) which reduces the amount of conditioned air supplied to the room --- cold in winter and warm in summer.

Obviously the other situation is insuffient supply air which simply doesn't provide enough volume of conditioned air.


Personally , I think the booster fans are a waste of time & money and they usually are noisey --- but that's me.

Something you might consider is adjusting the balancing dampers in the duct work.

ROOM REQUIREMENTS VARY

Also every room in the house has its own heating or air conditioning requirements. This means that you do not want the same flow of air into each room, but the quantity of air that each room requires. That sunny south facing room needs less heating air than a colder north facing room -- but during air conditioning it requires more cooling than that cooler north facing room. That also means that the same furnace fan and same ductwork needs to work differently in the winter than the summer! Most houses are simply set up to work with a compromise in both seasons.



LOCATE YOUR BALANCING DAMPERS

If you have no dampers, or they are all wide open, you are probably getting more air coming out of the floor grills in the rooms close to the furnace than in those upstairs rooms or the back bedroom. In fact, the thermostat is often close to one of these ducts with maximum air flow -- turning off the furnace long before the far away rooms have warmed up.



STEP BY STEP BALANCING

A homeowner is better suited to balance both their heating and air conditioning air flow than a professional simply because they live in the house and can fine tune the balancing over a period of time. A professional will get it basically right, but constant tweaking will make it really comfortable. Remember that any time you touch one single part of the air flow system, everything changes because there is only one air source and any small change redistributes it all -- so make a change and live with it for a few days before making another change.

This whole procedure should be done once in the heart of the heating season and once in the heart of the air conditioning season , if you have air conditioning running through these same ducts.

Here are the instructions for the heating season -- they are the same for the air conditioning season, all that changes is my mention of getting a room warm enough or cold enough.

1- Open all the dampers on all the floor or wall grills wide open. You will not be doing any adjustment with these because people can play with them or accidentally change their settings by such actions as vacuuming over them. They can be useful for temporally turning off a grill when a room is providing too much air flow and you don't want to turn the whole system off, but generally should be left wide open.

2- Open all the BALANCING DAMPERS in the ductwork above the furnace.

3- Totally close off the one or two ducts that feed air to the rooms in the house that have no problem at all. Actually totally closing these dampers does not actually close off all the air flow (like when you close a water valve), as they are built so loose that some air goes by anyway. You will most likely open these a bit in a later step.

4- Live in the house a few days to see if the once cold rooms are now warmer and if perhaps the once warm rooms are now a bit cold. Actually you will hear that the furnace is working more than before. That is simply because it is not turning off prematurely as it takes longer for the thermostat in the now restricted room to warm up. Yes this costs more to run the furnace more, but we are approaching actually heating the whole house, not just a couple of rooms.

5- Now you start the fine tuning. Any room that has restricted air flow and is not warm enough, open the damper a little bit, then live in the house a couple of days. Any room that is warmer than the other ones, close down the damper. Don't make a lot of changes, or large changes all at once as you will find it impossible to figure out what did what. Change one thing, test, change again.


7- Once you have the house heating evenly, take a permanent marker and draw a line on the wall of the duct extending out from the damper handle, like an extension of the handle. Now mark this line as the WINTER setting. When you do this same balancing task in the middle of the air conditioning season, you will find that the dampers have moved. Make another line and mark it as the SUMMER setting.


With Summer and Winter settings for the air flow dampers clearly identified, at the beginning of each season you can quickly set your entire system to its optimum performance by simply shifting the dampers to the appropriate positions. When you sell the house, don't forget to explain this to the new owners -- they will appreciate it.
 
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Old 12-09-13, 07:35 AM
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8 x 20 only good for 750 - 900cfm what size AC do you have?
 
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Old 12-10-13, 07:01 PM
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I'm sorry, not sure what exactly you are asking for.
The compressor unit is a Bryant 597c, 12 seer, unsure on tonnage rating for the whole system (don;t have a lot of docs) I'm thinking somewhere I'd seen 2 ton (house is 1780 sq ft) but cannot confirm.
 
 

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