Can I tap into existing ductwork?
The answer is “maybe,” meaning yes, if you have a system that’s bigger than you currently require, and no, if the equipment is already at maximum output. There are a few other considerations, but that’s the main one.
Both answers are addressed in detail below, but here’s the bottom line:
A smart homeowner is going to have HVAC professionals make the necessary inspection and calculations to determine if your HVAC equipment and ductwork system can handle more ducts, vents and additional space.
This Pick HVAC ductwork guide provides insight into:
- Whether it is OK to add ductwork to an existing system – When it will work and when it won’t.
- How to make it work– There is one option, a rather inconvenient and possibly expensive one, that might make it work.
- Alternatives– If you choose not to tap into existing ductwork, there are other ways to get the new space heated and air conditioned.
Can My HVAC System Handle More Ducts?
Let’s explore when it’s OK and when it won’t work to tap into existing ductwork to serve a finished basement, converted attic or garage, newly constructed addition or an existing room that doesn’t get enough heating or AC.
Yes, it can!
If your current HVAC and ductwork system is a little (or a lot) too big, then yes, you likely can tap into existing ductwork and adequately heat all the spaces in your home.
Some HVAC designers still go with a “bigger is better” methodology even though this is proven to be false. Equipment that is too large for the home creates energy and cost waste, temperature swings, hot/cold spots and cool, clammy conditions in summer. Does any of that sound like a good idea?
It probably can’t!
If your current heating and air conditioning system was correctly sized for your home at the time of installation, then it probably won’t adequately serve additional space unless it is small space.
For example, if an upstairs bathroom is chilly in winter, and you want to add a duct run and vent to get more heat into the room, you’ll probably be OK.
But if you’re converting an attic or basement that isn’t currently heated and cooled, then your current system isn’t designed to handle that much space. And there’s no way it has enough heating and AC capacity for a large addition.
Why? A Pro Look at Ductwork and Airflow
To summarize – if the new space is small, a bath or small home office, then you might be fine adding ductwork or a “drop” running off the main ductwork trunk. For larger space, you’ll need an alternative way to heat/cool the area.
Here’s why this is the case.
Airflow: The blower motor/fan in your air handler or furnace can only push so much air, measured in cubic feet per minute, or CFM.
When ductwork is added, there is more air to be pushed and pulled through the network of ducts. If it exceeds the CFM rating of the blower, then you’ll experience low airflow through vents, and rooms won’t really get the heat or AC you want. This is one of the top reasons for low airflow through vents as described in our Guide on the topic.
Example: 1600 CFM is a common blower rating. Let’s say it serves 8 vents. That’s 200 CFM per vent. If you add two more vents for a converted attic, the math shows 160 CFM per vent. Will that be strong enough airflow to get warmed and cooled air properly circulated? Probably not, especially when you also consider the extra air in the added ductwork, if ductwork is added.
What if you add four vents and ductwork for a finished basement or bedroom suite addition? Now you’ve reduced CFM to 133 per vent. Your current HVAC equipment won’t do the job. Even if you don’t add ductwork in this project, there’s not enough air flowing around to make all the rooms in your home comfortable.
How to Make Your Current Equipment Work
There is one decent option for using your current HVAC system and ductwork to serve more space.
That is to add a zoning system with dampers to your ductwork. In the example above, we said the system’s CFM airflow could handle 8 vents but won’t if you add two or four more.
With a zoned system, you could close two or four dampers, so that the system was only sending air to 8 vents.
A cheap “zoning system” is to add dampers, each with a mechanical handle that allows you to open or close it. To work, the ducts must be exposed in order to open or close the handle. They cost about $150 each installed.
Frankly, having to mechanically open and close dampers based on what rooms you want heated or cooled at the moment will prove a regrettable hassle.
A better way – but way more expensive: The preferred way to zone a home’s ductwork is to use motorized zone dampers with a zone controller that you can program to open or close – or partially open/close – various dampers. The dampers cost about $800 each installed when wiring is included. The controller costs $300-$1,000 depending on the model. Our guide called Do I Need a Zoned System covers basics, equipment and costs.
Some zoning systems use a separate thermostat for each zone – and the cost adds up!
An Alternative to More Ductwork and Vents – Go Ductless!
While not cheap either, a ductless heat pump system that delivers efficient and effective air conditioning and heat when needed is an ideal option for additions, converted space, finished basements and anywhere that it.
A single-zone ductless system costs less than most zoning control equipment and installation, so it might be a cost-effective option. And if you enjoy doing the work yourself, there are a range of easy-to-install DIY ductless heat pumps from Pioneer, MrCool and others. They’re pre-charged with the right amount of refrigerant, and that’s why you don’t need to hire an HVAC technician to install and charge them.
Our recommended brands for ductless mini split systems include Mitsubishi, Fujitsu, Gree, Daikin and LG plus the DIY ductless brands.
DIY Ductwork Installation
OK, maybe you’ve come to this point and are determined to install new vents, and possibly ducts, to get more heat and/or AC to the hot/cold rooms or the newly converted space in your home.
Rather than give a ductwork installation step-by-step guide, we have a few recommendations.
First, flexible ductwork is the easiest to install. You’ll cut the right-sized hole into the main line or trunk of the ductwork, fix one end of the flex duct to the trunk and the other to the opening where the vent will be.
Be sure to stretch the duct tight, because compressed loops in ductwork create friction, low airflow and higher static pressure. You won’t be happy with the results, and your HVAC system will work unnecessarily hard – meaning you will shorten its life.
Next, it will improve airflow if you also add a cold air return duct and tap it into the main cold air return line.
Finally, seal and insulate ALL your ductwork, not just the new ducts.
We’ve created a range of pages to help homeowners with their ductwork dilemmas:
- How to Seal Your Ductwork: 3 Simple Methods
- How to Insulate Ductwork in Basement or Attic (With Steps)
- When to Choose Flexible Ductwork (Pros and Cons)
Wrapping it Up – Pro Conclusions
There’s more than one way to get extra space heated and cooled off. While it is tempting to “go for it” when considering adding ductwork and vents, it is wise to have an HVAC pro inspect the system to determine whether it can serve the additional area you want it to.
Why waste money on a “cheap fix” that isn’t a fix?
In the end, you’ll enjoy your new room or zone much more if you determine which method outlined above will deliver the most effective solution.
Other Questions? Pick HVAC has the best library of FAQs and Answers to your heating, venting and air conditioning questions plus a wealth of information on related topics.