# How to tap into existing ducts for basement?



## nateshirk

I did a search and the first page didn't seem to have anything relevant.

I want to condition the space in my basement. Can I just tap into the central air ducts already there? 

Any and ALL details would be appreciated. Thanks in advance.


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## beenthere

http://www.diychatroom.com/f17/duct-work-placement-99977/

http://www.diychatroom.com/f17/rearranging-ducts-99940/

Use the search feature, and it will search all threads.


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## nateshirk

Every time I use the search feature (including this time) it brings up LOTS of irrelevant topics, and I've wasted lots of time trying to find answers. It just seems easier to ask my questions, and someone like you posts (most likely) exactly what I'm looking for. Thanks and I'll take a look at what you posted.


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## nateshirk

OK. I read both posts, and it's not enough info for me (the first timer). Can I tap right off the big ducts already there? So the vent(s) will be right there in the soffit I build around them? If so, how? Do I have to tap into the return? Or will just a supply be ok? What size vents do I use? Other materials, and hardware that might be needed?


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## HVACDave

What type of existing ductwork do you have (dimensional size, metal ducting or flex, etc.) How many registers are you thinking of adding? (individual rooms or an open rec room etc.)


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## hvac122

You would want the supply airs to be near the outside walls so if your duct is in the middle of the basement you would tap off it with round pipe and extend out. 
You would also need to add return airs.


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## Rory Read

Not an HVAC contractor, but a few thoughts:

The easiest and most common place to see new registers in a basement is the ceiling where they are tapped into the first floor duct work. Unfortunately, you also get the least benefit from heat supplied in the ceiling.

The HVAC system may not be adequate to handle the added load. Adding registers may meet your inspection/sale/valuation needs, but it may not work all that well, and it may decrease the benefits you feel upstairs.

Very broadly, by what % are you increasing the conditioned space? Are you adding 33% more space, 50%, 10%? It might be fair to assume that a 10% increase is within the margin for most properly sized HVAC units but 50% will not be.

Another problem is that basements are often poorly insulated. Even if the walls are retro-fitted with proper insulation, the uninsulated slab works as a massive heat sink, which can increase the impact of basement square feet on any HVAC sizing calcs.

All that said, many (most?) clients say the heck with it, we want the space, we need registers (and sometimes returns) for permits, we don't want to buy a new HVAC unit, so do the work and get out of our home. I understand that, and say ok.

Don't rule out a few electric baseboard units. They are not exactly sexy, but they may meet the need, especially if the basement is going to see a little less use than the rest of the house.

Have fun with that duct work!

Rory


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## beenthere

Describe how your basement will be finished, and how it will be used, and how often.

Many times, electric baseboard will work better. using the same trunk line as the upstairs leads to a problem. The thermostat doesn't know that the basement is cold when its 65 degrees outside. Also, the coldest air from the first floor falls to the basement, and the warmest air in the basement rises to the first floor..


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## nateshirk

It will be a full basement (open, about 700 sq. ft.). Insulated, framed, and drywalled. I always hear people say it doesn't take much to warm the basement, so i figured it wouldn't take a huge toll on the system, and I could just add some vents that can be opened and closed. It will get used on a regular basis. Kids toy area, and an office area. I'll be closing off a portion to hide the HVAC system, water heater, and sump.

Here's a pic of what's there.


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## nateshirk

I was thinking I might be able to tap in at the opposite end of where the pump(?) is.


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## beenthere

It doesn't take much to heat a basement. but the furnace does need to be running to heat it.

When its 50 and above outside. During the day the radiant heat from the sun will decrease the run times and how often the furnace runs, which will tend to cause the basement to become cold. Since the first floor won't be calling for heat. Its during these times that the basement will become cold.

Often, a basement needs to be its own zone to have it at human comfort conditions year round.


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## nateshirk

One of my other concerns was being able to keep the basement drier using the central air.


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## beenthere

nateshirk said:


> One of my other concerns was being able to keep the basement drier using the central air.


Won't keep the basement drier per say. The RH will probably go up. You'll need a dehumidifier to keep the basement drier.


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## nateshirk

Thanks for all info.


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## nateshirk

So, I'll have a dehumidifier, or 2. 

I still can't find ANY "how-to's". What to cut with? What needs mounted? How big of a hole to cut? I just want to know "how-to" do this. Search isn't bringing up ANYTHING on "how-to" tap into existing ductwork.

Is it just a dumb idea? I guess I can do baseboard heat, but what about cooling in the summer? And I have gas heat, so wouldn't it be more econimical to use that?


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## kenmac

I don't know exactly what size you'll need or where your wanting to tap. I would use 1 of these . Mark your hole with marker & use razor knife to cut it out


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## beenthere

A duct knife is all you need to cut duct board.

Size hole is what ever size the take off/start collar needs.

What size duct, depends on how much air needs to be moved. Which is determined by how many BTUs you need to that area. Which in turn is dependent on how much insulation you are putting in. 

I would imagine a 6" supply or 2 is going to be needed. But I don't know how big of an area, or how much insulation your installing.

Make provisions now for adding electric baseboard later. The basement won't be warm when the outdoor temps are mild. The upstairs will be warm and not cal for heat.


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## nateshirk

Duct board? So it's not sheet metal under the insulated padding? 

It will be just under 700 sq. ft. And insulated pretty well.

Is a return necessary? Same size as the supply? Opposite sides of the room?

And I will take your advice for provisions for baseboard. Thanks


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## beenthere

From your pics is just ductboard, no sheet metal. Needs to have as much return as supply. if the air can't return to the unit, then the unit can't put the air in the room/area. it would also create a negative pressure issue with the first floor. And draw in fresh unconditioned outside air.


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## AllanJ

nateshirk said:


> Every time I use the search feature (including this time) it brings up LOTS of irrelevant topics, and I've wasted lots of time trying to find answers. It just seems easier to ask my questions.


(Netiquette in general) Feel free to post if a search does not reveal answers quickly.

You can put new louvers directly in the ducts if that is the location where you want the warm air to come out. But you should choose louvers that are adjustable so you don't overheat the basement and leave too little hot air for the upstairs. Ideally warm air should be introduced into the room near floor level but this requires running a small duct down from the overhead ducts along or inside a wall.

The relative humidity will go down as you heat up the basement. (The same air when warmed up will have its RH go down) But when/if you close the louvers when everybody goes upstairs then the relative humidity could go higher than it was before once the basement cools down again. This is because more upstairs air that usually has more moisture in it got commingled with basement air.

The return "vent" should not be (physically) close to the hot air vent(s).


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## nateshirk

Since I'm building a wall to enclose the Central Air unit*, I can just just run the duct down the inside of that wall. 

*I alays hear "heat pump". Is that the correct term for it, even though the A/C goes through it too?

Thanks for all the info guys.


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## kenmac

nateshirk said:


> *I alays hear "heat pump". Is that the correct term for it, even though the A/C goes through it too?
> 
> Thanks for all the info guys.


 

Yes, If you want to use a seprate unit ( heat & cool)for the den. FYI. I have a den down stairs that is mostly under ground. I use 3 supplies off my main unit for upstairs ( like in your pics). I installed a return in the den as well. The den stays nice & cozy year round


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## beenthere

If the outdoor unit provides both heating and cooling, its a heat pump.


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## nateshirk

Nope. The heat is definitely coming from the central air unit. With the A/C condenser outside.


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## beenthere

nateshirk said:


> Nope. The heat is definitely coming from the central air unit. With the A/C condenser outside.


Does the outside unit run when your thermostat calls for heat? Is your indoor unit using electric resistance heaters, gas or oil to provide the heat?


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