Combustion air requirement question.

[2MCHPSI]

I am finishing off my basement and currently my 80% gas furnace and water heater are open to the basement. I want to close it in due to the noise it makes ( Goodman GMS8 70000 btu makes loud buzzing noise when the furnace is running).

I have a total of 150000 BTUs with the water heater (71.5k btu) and furnace(70k btu). The HVAC closet will be around 10x10 and currently there is one 3" vertical pvc pipe open to my attic. I also have one horizontal run 6" insulated flexible pipe to outside air tied into my return air trunk (make up air).

I saw calculations of 2000 btu per sq inch horizontal which would be required around a 10" combustion air intake to the outside for a closed HVAC closet.. That seems like quite a lot.

Since I already have a 3" vertical pipe and a 6" horizontal pipe to my cold air return for make up air, can I factor in these 2 ducts in reducing the requirement I need of 10" combustion air for 150k btu?

10" cold air intake seems huge

Thanks

[beenthere]

Make up air is not considered combustion air. So you can't include it.

[2MCHPSI]

Thanks for the response.

I might need to change my idea of enclosing the HVAC in it's own air tight room and use the grills high and low to the basement. The basement is about 1200 sq ft finished space and the house is a 2 story and a basement. It has a separate zone upstairs, and uses the Goodman mentioned above for the 1st floor and basement. House is pretty tight. The basement came unfinished with 4 supply registers cut directly out of the main supply trunk, no return.

Air currently rushes up the stairs to the first floor returns...two of them 20x20 each placed high and low near the basement door

The basement door opens out to the first floor and has some resistance in closing due to the basement air coming up to the returns.

I would like to have a return installed in the basement about 20 ft from the HVAC unit Could there be any issues with that using the grills to the rest of the basement? I am thinking 20ft is far enough away but not sure....

If I were to hire a professional to come out and sort my options out, what is a good certification for them to have air quality checking wise? I do not want to just waste money on someone who does not thoroughly test what my system needs. I do everything myself construction wise (worked for a general contractor for years) but when carbon monoxide possibilities can arise from changing my setup, I take this very seriously as anyone should and will suck it up and hire a professional lol

[beenthere]

The return in the basement should be sized to take out slightly less air then supplies put in.

BPI

[skippedover]

Here's the deal.

First, with non-sealed combustion units, you need to have sufficient combustion air just to satisfy the needs of the burners. That's 50-cubic feet of air for each 1,000 Btu's of combustion (if the house isn't a 'tight' home). In the case of 150,000 Btu's, the need is 7,500 cu ft of space. Your 1200 sq. ft. basement should have approximately 8,400 cu ft of space in its entirety if the ceiling is 7-feet above the floor. In other words, you could reduce the size of the basement by 18-square feet and just meet the confined space calculation.

All of that said, if you've got air flooding to the main house level from the basement without assistance, then two things are happening in your home. The first is you've got a massive air leak somewhere above the basement, a leak that leads to the outdoors. The second is that you've also got a massive air leak in the basement that's compensating for the air losses above. You see, air doesn't just drift around willy-nilly, it needs a driving force. That force in a home is most often 'chimney effect' or warmer air rising up toward the ceilings and attic while colder air infiltrates from outdoors to make up for that which is rising to the attic or other leak location.

All that said, you should never introduce any return air in the combustion area zone (the room(s) where the furnace and water heater are located) or you run the very real risk of backdrafting the appliances and creating copious amounts of carbon monoxide.

[2MCHPSI]

Here's the deal.

First, with non-sealed combustion units, you need to have sufficient combustion air just to satisfy the needs of the burners. That's 50-cubic feet of air for each 1,000 Btu's of combustion (if the house isn't a 'tight' home). In the case of 150,000 Btu's, the need is 7,500 cu ft of space. Your 1200 sq. ft. basement should have approximately 8,400 cu ft of space in its entirety if the ceiling is 7-feet above the floor. In other words, you could reduce the size of the basement by 18-square feet and just meet the confined space calculation.

All of that said, if you've got air flooding to the main house level from the basement without assistance, then two things are happening in your home. The first is you've got a massive air leak somewhere above the basement, a leak that leads to the outdoors. The second is that you've also got a massive air leak in the basement that's compensating for the air losses above. You see, air doesn't just drift around willy-nilly, it needs a driving force. That force in a home is most often 'chimney effect' or warmer air rising up toward the ceilings and attic while colder air infiltrates from outdoors to make up for that which is rising to the attic or other leak location. Sorry as you can see I am clueless when it comes to testing and what can I expect from a professional test to the HVAC system.

All that said, you should never introduce any return air in the combustion area zone (the room(s) where the furnace and water heater are located) or you run the very real risk of backdrafting the appliances and creating copious amounts of carbon monoxide.

I did a very primitive test today with my kids bubble maker and the air coming up the basement stairs that is giving the basement door some resistance is going right into the 2 20x20 return vents right near the basement door on the first floor. That seems to be the cause of the air coming up the basement steps. The air movement up the stairs only happens when the blower is on, and there is also 4 4x10 registers supply going to the basement.
The 6" duct to the outside tied into my return can supply unconditioned air into the house I guess. The house is 5 years old and built Ok.. Tyvek wrapped, low-e windows and doors ect.

Before I go out and spend money on someone coming in and inspecting, I am trying to see opinions of a possible return in the basement. Like mentioned it is approx 20ft away from the HVAC.. My basement is a little over 1400 sq ft total with 8 1/2 ft ceilings.

I am not sure if I should test after routing a return in the basement to tell if it worked well, or have a test done before routing anything. not sure if a test without the return installed can determine if a return would work or not. Sorry as you can tell I have no idea what can be expected information wise having a professional test my HVAC system for air quality

[beenthere]

An easy test. Close off the basement supplies and see if you have the same air pressure pushing around the basement door. If not, you need a return in your basement as its pressurizing when the basement door is shut. And putting the rest of your house into a negative pressure.

[2MCHPSI]

thanks. If I have 4 4x10 supply's in the basement, and I have a 10" return duct to the main return air in the basement, what might be the grill size needed to pull enough air back that would equal the supply?

[beenthere]

Without knowing how much air is coming out of the supplies, no way to know.