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Adding new HVAC vents to basement

I originally posted this in the Living Space forum, but no responses. I thought this might be a better place for it.

I'm remodeling my basement, knocking out a couple partition walls (non-bearing) and removing a drop ceiling and will be drywalling it. There are currently no HVAC vents in the basement. I have access to the ductwork in the area where the drop ceiling was, a supply and a return trunk. These two trunks hang belong the joists. And I can probably get access to some of the other ducts in the ceiling joist space that is already drywalled.

My question is how should I go about determining where to cut in the new vents? The area is L-shaped. I'm assuming it would be best to have both a supply and return. The basement doesn't really need to be cooled in the summer and the humidity is not bad. It does get a bit cool in the winter time during the WI winter.

What kind of vents would be best? I'd like something that would close tightly . So I could close the supply during the summer, but keep the return open. I'm imagining that that would work the best to drop the humidity out of the air but not make it crazy cold.

Re: Adding new HVAC vents to basement

How big is the room your trying to heat (sq. footage) this matters so you get the right amount of cfm's to the room so it will heat right. From what you say about knocking down walls it sounds like your going to have a fairly big room and will be taking air from other parts of the house. Not very efficient and will make your furnace run longer and won't heat right and cost you $$$. Worse case, you will have upsize your furnace. Also how is your cellar layed out, that matters as for where the supply and returns are installed, you definitely want a return where the air is the coldest. Since you won't be cooling that room both the supply and returns should have dampers. Make sure your pulling back at least as much air as you are supplying. Good Luck!!! :)

Re: Adding new HVAC vents to basement

Install zone dampers after the main duct. You would also need to install a thermostat this way you make the room a different zone from the rest of the house and can easily control the temperature

Re: Adding new HVAC vents to basement

You can zone the cellar but you will also need a zone control as well as zone dampers and a stat, you could still have a problem if you try to heat both floors at the same time with an undersized furnace, again this depends on the size of the new space. First find out the size of your furnace and what size your house needs, what your using now may be oversized. Are you using oil or gas? If your using oil you could change the nozzle to give you more BTU's, when I bought my house I had to go to a smaller nozzle. Good Luck!!! :)

Re: Adding new HVAC vents to basement

I have about 500 sq feet of finished space in the basement. A little over 1000 for the first floor. My furnace is natural gas, Carrier brand, on a info plate it says

Input 75,000 BTU/hr

I don't know about zoning since the basement doesn't have it's own runs of ductwork. So as it is run I don't think there would be a way to shut off the first floor and only run heat to the basement. I'm a novice at this.

Re: Adding new HVAC vents to basement

That's not as easy a question as you may think. Cutting into a main line will unbalance the system and can cause heating and air conditioning problems for the rest of the house.

Basically you want heat entering at one end and the return at the other. To prevent over heating the basement the vent should be thermostatically controlled. It will not control the furnace just the vent.

Re: Adding new HVAC vents to basement

First I know nothing of your house, Ins, doors, windows, etc. so telling me it will be 1500 sq. ft. does me no good. 75,000 BTUs sounds pretty close but there is no way of knowing without doing a Load Calc. You also want the output, not input of the BTUs. If your looking for the cheap way out you should do three eight inch runs, full open will give you 600 cfms and you can balance them out with the rest of the house with the dampers. You also want to pull back as much air as your supplying, I would run the supplies about 18" off the outside wall and have the return closer to the middle. Use three eight inch returns it will pull back around 600 cfms. Try to space out the supplies evenly. Like I said I know nothing of the layout of you cellar so I could be off a little bit. When you put your take offs on the ductwork make sure your supply takeoffs have dampers for balancing. Make sure both the supply and return grills have dampers so you can shut them when the cellar is not being used. Good Luck!!! :)

Re: Adding new HVAC vents to basement

MadisonDan ....
Part of this equation has to do with how well insulated the basement space is.
The better the insulation the easier to condition the space.

Around these parts our basements are always supplied with HVAC supply ducts.
It depends on the square footage but usually 2 or 3 supply drops of 5 inch .......8 inch seems over kill to me. It's common that furnaces are somewhat oversized when installed that will allow for this conditioning of the basement .
However , as was stated earlier no one over the internet can say what size you need for your home.

Installing 2 or 3 supplies in strategic locations may be enough for your situation.
A return should be installed in an unobstructed location near the floor.

**Caution ........ when installing air returns in a basement there is a need to be carefull not to create too much negative pressure when there is combustion equipment within close proximity ... such as fuel fired water heaters , non-direct vent furnaces etc. Otherwise you could create backdrafting situations which is not a good thing.

A rule of thumb for air returns in a basement is to have half the amount of return as supply.
For example ...... 2 supplies of 5 inch there should 1 return of 5 inch
3 supplies of 6 inch would have 1 return of 6 inch ( or 2 three inch )

The air return will do the dehumidification and not the supplies this why it's benifical to have these in a basement.

The supply and return branches should run directly from the trunks ..... DO NOT tap into existing lines to other rooms. When these branches are cut into the trunks ensure these connections are sealed well with either metal foil tape ( true duct tape ) or appropriate mastic.
Manual dampers can be added to the ducting as well the grills that are closeable can be used to shutoff air flow from these.

As Jack mentioned earlier ... the HVAC system should be balanced .... adding more takeoffs will alter the existing balance to the rest of the home somewhat.

If you are unfamiliar with doing this type of work you might consider having an HVAC pro evaluate your heating system , do the takeoffs and balance the system for you.

Re: Adding new HVAC vents to basement

Like I said you can damper down the 8 inch to balance at least that way you will know you have enough.I thought I said that. Good Luck!!!

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