How do I figure out the right way of adding more supply and return registers?

Hello everyone,

We're renovating our finished basement. It has a bedroom that has no HVAC registers so, needless to say, it's not the most comfortable room in the house in the summer or winter. (It has a small window but it's not enough). We want to add a register. The duct that's right above the bedroom supplies air to the kitchen and a living room upstairs on the main level, so it already has several registers. Can we simply cut another one there? Does it depend on the size of the duct and the number of existing registers? Does it depend on the specs of the HVAC unit? How do we figure out the right way to do it? And is it possible that we will need to drag a brand new duct to the downstairs bedroom from the HVAC unit (from the plenum chamber, right?)?

Also, the basement has no return registers so we want to add one for better air circulation. Do we need to do similar calculations for it?

Thank you!

Air conditioning is probably not your biggest issue in the basement, heating will be. I'd want a return air duct, if for nothing else to get the benefit of your AC system dehumidifying the air in the summer. Do you have your system serviced on a regular basis? If so, I'd ask your HVAC guy for some on-the-ground advice and to determine if the system can handle the additional volume of your basement. I suspect it won't be a problem for AC it may not have enough to support it in the winter. You could always add a couple of units of baseboard electric to supplement the heat when you're using the basement and on especially cold days. If the basement is fully in ground the walls will be a fairly consistent termperature.

Thanks. We live in MD near DC, so humidity is a serious issue year round. Windows in the room are at the ground level so I guess the basement is fully in ground. By the way, we're renovating after a flood caused by a failed sump pump. When the contractor removed drywall at the bottom, they discovered old mold that's being treated now. So my hope is that the extra returns/supplies will help circulate the air, keep it drier (as you said) and prevent new mold from growing.

That said, our house seems to be fairly cold in the winter. I keep the thermostat at 72-73 degrees and still it's cold. Part of that is the house was build around 1960 so wall insulation is poor. Plus we have a couple of skylights. But, other than the HVAC system not running properly, I am also concerned about too many supply registers, and surely don't want to make the problem worse. The downstairs bedroom is only used by guests while the rest of the house is used by us daily, so having more comfort there is more important.

I would cut in ductwork and add the baseboard electric for those times when you have guests. Adding a dehumidifier in the basement might be a good idea, especially if the humidity is high year-round. Are you tearing out all the drywall? If so, I'd do a moisture test on the walls. Tape a piece of plastic to the wall sealing all four sides, do this in several places. If after a few days water has collected between the plastic and the foundation you have water vapor coming in from outside. If you have water condensing on the room side you have high humidity and the water is coming from elsewhere, possibly the floor. If you have water vapor, but not liquid water coming in from outside sealing the walls from inside with Drylok or Thoroseal may help.

When finishing basement walls you do not want a vapor barrier on the insulation or against the concrete block or poured concrete walls. You also don't want to paint the drywall with an oil based paint, you want the walls to be able to breathe. Go here: https://buildingscience.com/portfolio and read up on designs that work for "mixed humid" climates. There should be some good information about basements there as well.

ilyaz said:

Hello everyone,

We're renovating our finished basement. It has a bedroom that has no HVAC registers so, needless to say, it's not the most comfortable room in the house in the summer or winter. (It has a small window but it's not enough). We want to add a register. The duct that's right above the bedroom supplies air to the kitchen and a living room upstairs on the main level, so it already has several registers. Can we simply cut another one there? Does it depend on the size of the duct and the number of existing registers? Does it depend on the specs of the HVAC unit? How do we figure out the right way to do it? And is it possible that we will need to drag a brand new duct to the downstairs bedroom from the HVAC unit (from the plenum chamber, right?)?

Also, the basement has no return registers so we want to add one for better air circulation. Do we need to do similar calculations for it?

Thank you!

Click to expand...

Completely off subject but, it sounds like this bedroom may not have a legal egress window size.

If you add a supply air then you will also need to add return. Without the return air the supply is basically useless.

The new ducts will need to be added to a main trunk line that has enough additional CFM capability to handle the vent that you want to add.

Your unit probably wasn’t sized assuming it needed to heat and coil the basement also but with luck it will be over sized like most older units are and work for your situation. A proper heat load calculation will need to be preformed to determine how many CFM will need to be added to heat and coil this room. Or the entire basement?

You should really call your local HVAC tech to help you out with this one. Once you board it up it’s hard to change it if you guessed it wrong. They will be able to run the calculations and determine if your existing duct can handle the additional cfm, help you plan your return air solution, and determine if your unit is sized correctly.

Another thought here, in another post you mentioned that you were having a hard time keeping the temperatures even throughout the house. Adding heat to your basement will keep your floors heated and in result it will help keep the rooms above more comfortable. Because they will have lower heat loss. It can make a big difference because you usually don’t have any insulation in your floor... Just a thought.

That's an interesting idea although the devil is probably in details: heating the basement might mean heating the rooms less which might offset any benefits from warmer floors. I suspect I am losing a lot more heat through walls that through the floors.

ilyaz said:

That's an interesting idea although the devil is probably in details: heating the basement might mean heating the rooms less which might offset any benefits from warmer floors. I suspect I am losing a lot more heat through walls that through the floors.

Click to expand...

How long are your heat cycles? It’s not a bad thing for the furnace to run a little longer, in fact too short of a run cycle can become problematic in some situations.

By keeping your feet warm you can offset some of the cold “feeling” you get as you walk around. This more applies to those who walk around bare footed rather than someone who always wears slippers. I don’t doubt for a moment that you feel colder on the outside rooms, this is common because of the heat loss through the walls and windows. The warm floor thing would help to slightly reduce that (if your floors were noticeably cold all the time).

Have you used a thermometer to measure the air temp difference between the rooms that feel cold and the room with the thermostat? What is the difference between the rooms?

I’m going by the information you mentioned earlier, since your unit is able to satisfy the heat demand I’m going to assume that your furnace is not undersized. If it ran constantly and just couldn’t keep up then you would have an issue. It sounds like what you have is a balancing issue, or an insulation issue. You said you upgraded your insulation already so that leaves us with balancing. The room that has your thermostat is the room that controls the temperature of the whole house. When it comes up to temperature then the thermostat thinks the whole house is up to temperature. If the rest of the house is still cold then the room with the thermostat has too much air going into it. It’s heating up too quickly. Reduce the air into that room so that it doesn’t heat up as quickly and therefore giving the other rooms a chance to warm up too.

If all the rooms are comfortable, and at the same temperature at the end of the heat cycle but cool down exceptionally quickly then you still have a heat loss issue or drafts somewhere that is causing issues.

The heat naturally wants to rise, so you need to protect the heat that stays low. You have an un heated basement and that is a heat drain. It’s allowing the heat that is low to transfer through the floor. This is taken into consideration when heat load calculations are preformed. Additional heat is required to be supplied to a room if the floor is not insulated or over an un heated area. If you can not add additional CFM to each of those rooms then heating the space below would reduce the required CFM and achieve similar results (within limitations). Some of that heat below is going to travel through your floor and into the space above so it will reduce the speed at which those upper rooms cool down. If lack of insulation is still an issue then this won’t be a substitute for it but it sounds like you’ve fixed that previously so I’m just pointing at the next smoking gun that I see from where I’m sitting. I don’t recall exactly where you said you added insulation though...

Of course, you are my eyes on this so if I’m misunderstanding something or not seeing the full picture then you would be the best one to make that call. All I can do is give you my opinion from the information that I’ve been given. I’m still going to recommend a heating contractor go there to preform tests and calculations because that is ultimately going to give you the best answers to the problems that you are having, providing that you get a technician capable of performing those calculations and tests correctly... that may be your biggest challenge. Be sure to let the service company know exactly what you want done so they send the right tech for the job, and not the next available apprentice.

Im really curious how long the furnace runs on each heat cycle and how long it’s off between cycles. If you could time those and report back it could help us know if you have something vastly abnormal. Record the outside temperature when you time it so I have a rough idea on what it’s working against. These measurements should be taken on its natural run cycles without increasing the temperature setting to make it turn on. Also, you should not be standing beside the thermostat while you time this, so body heat doesn’t affect the results.

Thanks for all the details. Right now there is too much additional variation: outside temps keep jumping up and down plus our basement restoration is full swing so the contractor is cutting drywall, opening and closing windows and so on. So I want to wait until it's over and then do accurate measurements.

Insulation is definitely not a solved problem. We added some to the attic and to a couple of walls when we're doing major work in those rooms, but I think most of the walls in the house are still without adequate insulation, and we won't be cutting those open any time soon.

The house is a 2-story split level and the furnace and the thermostat are on the bottom level which is half-story above the basement. Which probably helps a bit in the winter but not enough.