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I noticed today that there is a massive air return in my unfinished basement. The opening is about 2 meters from the blower, and as large in diameter as the return duct coming from the upstairs. This makes me think that the furnace is pulling most of its air from that intake rather than from the upstairs rooms. Also, the supply vent in the basement blows very hard compared to the upstairs rooms.

Will capping off this return or limiting the basement's supply vent cause any problems for the equipment, or lower efficiency?

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    This is impossible to answer definitively, without a comprehensive inspection of the system. – Tester101 Nov 12 '14 at 13:12
  • The basement should be conditioned space, so there should be no problem drawing air from it. You'll want to make sure you change your filters often though, since basements tend to be dusty places. – Tester101 Nov 12 '14 at 13:15
  • Do you feel that this configuration is causing any problems? Is the rest of the home not warm/cool enough? – Tester101 Nov 12 '14 at 13:20
  • Yes it does seem to be a problem. There is more air coming out of the single vent in the basement than all of the upstairs vents combined. Again, i suspect its because of the air preferring the lower resistance path of the intake in the basement over the intakes coming from the upstairs. Often the basement is 10 or more degrees warmer than the upstairs. – John Moffitt Nov 12 '14 at 13:34
  • You might try installing a damper, and restricting the air rather that completely closing it off. This also leaves you the option to reopen the duct if you notice any problems. You'll want to monitor the furnace for overheating, and not ignore any high limit trips. – Tester101 Nov 12 '14 at 14:05
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Your basement return air should have a damper installed which should allow you to restrict the amount of air drawn in by it. If your house is older it may not have this however in all newer homes this damper is installed by the ductwork installers. If it is not it is not very hard to add one. The damper is slightly smaller than the opening of the return which will still allow it to draw air even when completely closed.

When i am normally balancing a new homes system i will normally close the basement return so that the system will draw more air from the finished parts of the house, and specifically the top floor or bedrooms where people complain about poor air circulation the most.

Now even if you were to completely seal the basement return, i don think it would cause any issues or excessive static pressure on the system. The furnace will pull more air from the other returns to overcome it. Also if you have a furnace with an ecm blower motor it will ramp up to try and overcome additional restriction caused by added static pressure (to some degree) which is normally caused by a dirty filter.

Also in summer time you will need to close all your basement supply registers for cooling. Your air conditioner is sized for your top floors, and the cold air will migrate to the basement anyways. I find a lot of people decide to just leave them closed year round when there basement is unfinished as they are rarely down there, and it saves them from having to remember to close them. This is always homeowner preference, but for the cooling season they must be closed.

So i would close the damper if you have one installed. If not i would either install one or have one installed by a hvac contractor. Just remember if you decide to finish the basement that you have proper return airs installed in it. And not to just close it up and forget about it.

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The previous person answering your question does not have a sufficient understanding of HVAC systems. For the past fifteen years, I have been working on such systems five days a week. I have learned a few things about such systems.

As long as you have more than one return in your home, capping one return will not damage your system in any way. I set up some homes to have a single return on the main floor. And in other homes, I install as many as five returns. If you have only one or if you have five returns, the volume of air being returned is the same. It is difficult to imagine than any duct work is so bad that only having a single return would cause any issues. If that is the case, the systems in many of the homes in which we have only installed one return should have had numerous issues. But that is not the case.

During the summer, leave your basement returns open. That allows for the cool air from the basement to be pumped into the upper stories. When it is chilly outside, still leave your basement return closed. If it is open, it means that your heat pump is having to also warm the cold air from your basement. But when things are reversed and the temperature drops into the tens or the single digits, keep the basement return open. At that point, you basement is warmer than the rest of the house because the basement atmospheric ground temperature is warmer than the cold air seeping into your home. Limiting your system to only one or two air returns will not overload system and will not damage your duct work in any way. That is simply a myth.

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Capping the return and/or the supply in the unfinished basement could both potentially damage the system by causing it to overheat or putting too much stress on the blower motor; without knowing more about the system, here's what I suggest:
1. You're probably okay to close/block the supply register as long as the rest of the registers in your house are open. This is assuming that there are plenty of other registers. For example, if there are 8 registers that are open and 1-2 that are blocked, then that's fine. There's a reason they have louvers on them. Never close more than 1/4 of them at once, and you'll be alright.
2. In regards to blocking up the return, that's a little more risky. Usually systems with 2 returns are designed that way so that heat/cool is distributed through the house more evenly, not because they need 2 returns worth of air coming in. That being said, you shouldn't block the return until you've confirmed that you'll get enough airflow. You'll need pictures/dimensions of the ductwork and furnace as well the information off the unit's data plate (BTU/hr, fan capacity, etc.) for anybody to tell you if its okay to block. That's my professional opinion as an HVAC technician.

That being said, if you feel like experimenting, here's another option:
1. With both returns open turn the fan on at the furnace (either via thermostat or by connecting your R terminal to the G terminal on your circuit board)
2. Measure the current on the neutral (white wire) to the blower motor with an amp clamp.
3. Block the return downstairs temporarily, make sure its sealed up well.
4. Measure the amps on your blower motor again.
5. If the current increased by more 10%, then you shouldn't block the return.

If you don't have a multimeter with an amp clamp you can do the same test by running the furnace normally and comparing the temperature at a supply register with and without the return blocked. When you're comparing the temperatures, remember to subtract the room temp. first.
For example: 135 F - 65 F = 70 F temperature rise
If the temperature rise increases by more than 10% or 5 F (whichever is lower) then you shouldn't block the return.

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