I am setting up return air for a new goodman furnace. There will be two return air ducts that travel through the house. I don't know the cubic inches these two ducts need to be. The two return air ducts will come together at a 10 inch by 20 inch main return air duct at the furnace. The duct work has to travel through the house and I am not looking for the length, but the cubic inches of the end of the two ducts combined. the furnace I have is a Goodman model number GMH80904BN. The product number below has the letter "A" at the end of it and mine does not.
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Re: need the cubic inches for the return air duct
For proper minimum opening you need to do duct calculation. This is not a typical math equation for the typical homeowner. And unfortunately alot of contractors do not take the time to calculate this.
If you insist on doing this yourself...... I would recommend the largest opening that a common filter will fit. probably 20" by 25". Going to big on a return air will never hurt. But going to small will.
20" x 25" will work safely 95% of the time. But... If you are running airconditioning that is 5 tons in size, you will need the design a return air system that will feed from BOTH sides of the furnace.
If you are just sizing the trunk line you should use a ductulator to calculate the minimum size. A good rule of thumb would be to size the "return air" trunk 1 - 1 1/2 times the size of the supply trunk line.
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<span style="font-weight: bold;">Installing a new furnace</span> can be done by a good handy type homeowner. There are a few things that you will need to know. I will not be able to convey everything that I know in an article such as this, but I will attempt to give the important things that will help the<span style="font-weight: bold;"> handy person</span> to be able to overcome some of the major hurdles that may be encountered when <span style="font-weight: bold;">changing a furnace.</span><br /><br />The first thing you want to do when <span style="font-weight: bold;">changing out your furnace</span> is to research different new models. Do not assume that you need to same BTU furnace as you have. You may be much better off with a <span style="font-weight: bold;">smaller furnace</span>, especially if you are changing out an older <span style="font-weight: bold;">inefficient furnace</span> with a <span style="font-weight: bold;">higher efficiency</span> model. Also check that the dimensions of the new furnace will allow it to fit into the same space, and adapt to the existing <span style="font-weight: bold;">supply and return duct work.</span><br /><br />Next you will have to remove the <span style="font-weight: bold;">old furnace</span>. When you do this make sure to carefully plan for the installation of the <span style="font-weight: bold;">new furnace</span>. By doing some careful planning you can make the hookup of the new furnace much easier. Do not take apart more then is necessary to remove to old furnace. Then position the new furnace to take advantage of as much of the existing parts from the old furnace as possible.<br /><br />Remove the <span style="font-weight: bold;">electrical supply</span> and the <span style="font-weight: bold;">gas or oil lines.</span> Then carefully take apart the supply and return duct work. Many times the return duct work can simply be reattached to the new furnace after cutting the appropriate hole into the side of the furnace. As long as you do not have <span style="font-weight: bold;">air conditioning</span> on your furnace you can often just strap up the supply duct-work temporarily to the ceiling and hold it there till you can get the new furnace under it again. Many new furnaces are not as tall as the old ones, so you will either need to block up the furnace and shorten the return duct, or you will have to attach it to the existing duct work and then support the supply duct and build new duct to go up to the old supply. Duct-board material is easy to work with and will work well for doing this.<br /><br />If you have air conditioning on you system you can often support the indoor coil along with the duct-work and just make the swap underneath it. If you cannot do that then you will need to get a professional to help you so that you can pump down and recharge the system. That process takes a <span style="font-weight: bold;">special license</span> and special equipment to get the job done.<br /><br />Now that you have everything removed from the furnace and marked so you know how it goes back together, you can slide the new furnace in place. I usually start by hooking up the supply duct, then the return duct. Once these two major things are in place then the <span style="font-weight: bold;">gas line</span> and wiring can be installed to the new furnace. I usually make sure to use an approve <span style="font-weight: bold;">flexible gas line</span> so that the piping part is easier. Often the <span style="font-weight: bold;">electrical lines</span> will fit to the new furnace, however if not then changing the wires is really not that hard.<br /><br />Also <span style="font-weight: bold;">MAKE SURE</span> to keep the manufacturer directions!!! Inside the <span style="font-weight: bold;">installation manual</span> there are very clear<span style="font-weight: bold;"> installation instructions.</span> Make sure to follow all of the directions exactly. There also are instructions for the <span style="font-weight: bold;">start up</span> of the furnace in there, those instructions will help you through the <span style="font-weight: bold;">start up and check procedures</span>. <span>Make sure to keep the installation manual for future reference</span>. There are also<span style="font-weight: bold;"> troubleshooting</span> procedures and <span style="font-weight: bold;">flow charts</span> in there that will make <span style="font-weight: bold;">troubleshooting a problem</span> in the future much easier.<br /><br />After over twenty years of <span style="font-weight: bold;">installing furnaces</span> I still get out the installation manual and read it, as they are always changing things that are needed for <span style="font-weight: bold;">proper operation.</span> However after these many years I also have been able to trim the time down to less then a day for some installations and usually always less then two days for even a difficult one. Your time will be more then that, but with some planning ahead and creating lists of the<span style="font-weight: bold;"> materials and tools</span> needed you can still get the job done well and in a respectable amount of time. Or you may choose to have some else do it for you....
Usually the return air will enter the side of the furnace near the bottum, in order to hook up the return air duct to it you have to cut a opening into the side of the furnace to accomodate the return air duct.
The blower motor is only going to pull a certain amount of CFM for circulation air. There is some restriction of air flow that is created when the return air system is ducted causing the air flow to "slow down" a bit. The air going into the furnace firebox is probably moving at a faster rate than if the return air was ducted. It will work and you will get the same amount of heat (energy) released but the faster moving air will make it feel cooler. Also the unit might be undersized(BTU output) for the size of the cabin and amount of airflow the blower is creating. Adding a extra filter(double) might slow down the air flow a bit to create more warmth(feel) coming from the registers. Also if the furnace and air intake is pulling air from outside(garage,closet, or under house) then the colder air from outside going into the system will decrease the heat from the furnace. It is better to have a closed system (return air inside cabin). You might want to try adding a extra filter to slow the air flow down some or use some sheetmetal and square it from the intake to try and concentrate the incoming air a little better and see if that improves the warmth coming from the furnace.
The Cold Air Return is really the Return Air. The Purpose of the Return
air duct is to pull the cooled or heated air back to the furnace. It then
filters the air before getting cooled or heated again.
This is what controls the thermostat, which starts and stops the unit. In
the end this is how your air stays conditioned. In some states it is
required by law that a 4-6inch fresh air vent be tied into the return air
duct. Everytime the furnace fan comes on, it pulls in fresh air from
Filters are usually added to the sheetmetal seperate from the furnace. All air must be filtered before entering the furnace. Turn on heat and feel ductwork to determine which direction air flow goes, warm will be supply and room temp. will be return. Filter should be on return side of furnace. Some of them may be filter grilles located in a central reurn on older homes with only one return in a hallway or sometimes the correct ductwork is not installed and they are inside the bottom panel where return ducts connect
Could be that the fan on you new furnace is more powerful than your old one and is forcefully blowing the cold duct air out causing a temporary chill or it could be that you need to adjust the fan delay.
I'm not familar with this unit so I wouldn't have aclue on how to do this or even if you can.
Your main limit switch is activated. Maybe for good reason. This switch keeps the furnace from overheating. I have the same problem right now and I think my a/c coil may be blocked, limiting air flow through the heat exchanger and the furnace. Does your gas and main circulating fan come on for a few minutes before the switch activates and the light begins blinking?
Check your furnace filter and any return air ducts in your home - see if they are blocked or restricted.