Gas furnace Combustion air... (HVAC)

[Member 5605]

Vent to attic right side, crawl access to right under ½ chair.


Door closed,



I have the register off right now but there is a vent grate.


@bbbass not wanting to kill anyone which is clear I hope... I also don't want inefficient systems in our house when a remedy can be had for less than $100-.

 

[308]

Combustion air and exhaust air. Need to be per manufactures recommendations...obviously.

As far as professionals go, I've yet to encounter a residential 'professional' with a Shortridge Instruments multimeter, an airfoil and/or a pitot tube in their tool box.

 

[bolus]

consider having someone out to check it. We just got a new furnace and it is amazing how much better it is and how much more efficient. I'm saving a $100 a month heating in the winter just on the gas bill while keeping the house 4 degrees warmer on average.

Ours was in the basement and drawing cold air from the basement as well. The new one is much better

 

[tarster]

Combustion air and exhaust air. Need to be per manufactures recommendations...obviously.

As far as professionals go, I've yet to encounter a residential 'professional' with a Shortridge Instruments multimeter, an airfoil and/or a pitot tube in their tool box.

Its probably back at the shop.. in the original box sitting on a dusty shelf.. gets a glance every now and then when someone is looking for another item..

 

[Jamie6.5]

According to this, One Sq In of cross section in the fresh/combustion air supply per 4k BTU rating of the furnace should be adequate to prevent "spillage" while the furnace is operating:

Combustion Air for Furnaces | The ASHI Reporter | Inspection News & Views from the American Society of Home Inspectors

Opening to Outdoors
One of the other ways to get adequate combustion air to the burner is with an opening to the outdoors from the furnace room or enclosure. Where the furnace is in a basement, crude ductwork typically runs from the screened hole in the outside wall down to near the floor level, close to the burner. In some cases, a trap is created to minimize the cold draft. In this case, one square inch of opening is required for every 4,000 BTU.

The fact that they put louvered doors on the closet it's housed in leads me to believe they knew this would be an issue when it was installed.
I would put in a fresh air supply sourced from the outside, and seal the louvers on the closet doors, or replace them with solid doors that seal better.
Just make sure that the fresh air supply is sized adequately, and constructed of low restriction materials (NOT corrugated).

ETA: Where does the rectangular hole in the ceiling lead to, and what is its purpose?

 

[308]

ETA: Where does the rectangular hole in the ceiling lead to, and what is its purpose?

It sounds like it's the OSA/Makeup air opening.

 

[Jamie6.5]

It sounds like it's the OSA/Makeup air opening.

That's what I wondered.
I also wonder if it's blocked on the other end.

 

[Member 5605]

According to this, One Sq In of cross section in the fresh/combustion air supply per 4k BTU rating of the furnace should be adequate to prevent "spillage" while the furnace is operating:

Combustion Air for Furnaces | The ASHI Reporter | Inspection News & Views from the American Society of Home Inspectors



The fact that they put louvered doors on the closet it's housed in leads me to believe they knew this would be an issue when it was installed.
I would put in a fresh air supply sourced from the outside, and seal the louvers on the closet doors, or replace them with solid doors that seal better.
Just make sure that the fresh air supply is sized adequately, and constructed of low restriction materials (NOT corrugated).

ETA: Where does the rectangular hole in the ceiling lead to, and what is its purpose?

Goes to attic. Cold in winter and blazing hot in summer.

Open, is about 10" above insulation.

 

[Jamie6.5]

Goes to attic. Cold in winter and blazing hot in summer.

Open, is about 10" above insulation.

Okay, then that's the fresh air source.
But it's apparently causing thermal stratification of the interior house air, because the furnace is drawing warmer house air, and the cold air from the attic is allowed to settle to the floor.
I would just extend it down to the level of the combustion air intake on the furnace housing, and put a small plenum on the end of it, with an opening towards the furnace. Preferably with a flapper door in it along the way, to prevent airflow when the furnace isn't operating.

And seal the closet doors.

 

[bbbass]

Combustion air and exhaust air. Need to be per manufactures recommendations...obviously.

As far as professionals go, I've yet to encounter a residential 'professional' with a Shortridge Instruments multimeter, an airfoil and/or a pitot tube in their tool box.

Yep, there are some bozos out there...obviously. But are you saying that we shouldn't trust anybody in the biz to do a professional job?

 

[bbbass]

Attic is a poor source for combustion air unless it has gable vents. Otherwise, snow may cause blockage. As mentioned by others, the air intake ducting should terminate outside. I've forgotten, was that your plan?

William Barns
Oregon Certified Home Inspector #005 - retired
Past member of the Oregon State Home Inspection Advisory Committee

I'm no expert on HVAC, I haven't kept up with it, there's soooo many regs, and my brain don't work anymore (which is why I retired). Sorry to be tedious.

Regardless of your plans for modification, the pics show what seems to be a low quality unit, poorly installed. I'm sure you will be glad when you get a mini-split unit.

 

[Member 5605]

So @Jamie6.5 if I need 1in^2 of vent per 4,000 BTU and I have a 60,000 BTU furnace... when the furnace runs on high which it does not EVER... so it's really a 42,000 BTU furnace. S
"Amana ADVC80603BX"

I need 15in/sq of venting if we base it on the maximum output that this furnace is capable of doing:

• if my vent above is 5.5"x13" = 71in^2 of vent
• If I allowed both attic and crawl space vents to feed the furnace with fresh air then I would have 140in^2 of fresh air venting... more than sufficient.
• IF I left vent alone and sealed doors from inside to prevent flow.
• IF I were to use a 5" pipe then I'm looking at 3.14x5 =15.7 which is the minimum venting for that furnace.

according to link: Combustion Air for Furnaces | The ASHI Reporter | Inspection News & Views from the American Society of Home Inspectors
that you referenced.

@bbbass I do have gable vents and every so many feet along the eves I have vents as well as many vents along the peak (well close to it). Attic is far from sealed for airflow but it is sealed from critters, water and that's about it. if I run it up and through it's not a problem to run it out to one end of the attic, I would only have about 20' to go to hit the north gable.

Originally running out to exterior was not the plan but could be done.

 

[308]

Yep, there are some bozos out there...obviously. But are you saying that we shouldn't trust anybody in the biz to do a professional job?

No, just taking a bit of an exception that there are no professionals commenting in this thread.

 

[bbbass]

Didn't mean to imply that, however nobody identified themselves as being in the business of HVAC professionally. And I'll stick by my comment about making recommendations without seeing the installation in-situ.

 

[Member 5605]

No, just taking a bit of an exception that there are no professionals commenting in this thread.

and if there were I'd rather them not start out saying "according to xyz where I work, you could do... or get by with... "

Not because of me but whoever else down the road reads thread and says "well this person working at this facility said I could do this.... and it's their fault that this happened.."

I do my own stunts...

 

[bbbass]

 

[bbbass]

View attachment 330468

I see the space is shared by a gas fired water heater. While not likely with your opening to the attic, IMO once you close the system the louver doors should be blocked and an adequate outside air supply provided to prevent a powerful kitchen air vent fan from backdrafting the water heater.

 

[Jamie6.5]

So @Jamie6.5 if I need 1in^2 of vent per 4,000 BTU and I have a 60,000 BTU furnace... when the furnace runs on high which it does not EVER... so it's really a 42,000 BTU furnace. S
"Amana ADVC80603BX"

I need 15in/sq of venting if we base it on the maximum output that this furnace is capable of doing:

• if my vent above is 5.5"x13" = 71in^2 of vent
• If I allowed both attic and crawl space vents to feed the furnace with fresh air then I would have 140in^2 of fresh air venting... more than sufficient.
• IF I left vent alone and sealed doors from inside to prevent flow.
• IF I were to use a 5" pipe then I'm looking at 3.14x5 =15.7 which is the minimum venting for that furnace.

according to link: Combustion Air for Furnaces | The ASHI Reporter | Inspection News & Views from the American Society of Home Inspectors
that you referenced.

The cross section SqIn of 5" pipe is 19.635 Sq In, as the formula is [Pi*R Sq]. The radius of 5" being 2.5. So (2.5*2.5)*3.1416=19.635
So if the overhead vent is 71 SqIn, it's 300+% as big as it needs to be, which should be more than adequate.

Why not, as a test, seal up the doors temporarily with newspaper or plastic and close them and see what happens, before you do anything else?
I think the biggest problem you have at the moment is the air is taking the path of least resistance, which is NOT down through the attic vent.
The warm air escaping out through it, is mucking up its intent.

 

[Member 5605]

The cross section SqIn of 5" pipe is 19.635 Sq In, as the formula is [Pi*R Sq]. The radius of 5" being 2.5. So 2.5*2.5*3.1416=19.635
So if the overhead vent is 71 SqIn, it's 300+% as big as it needs to be, which should be more than adequate.

Why not, as a test, seal up the doors temporarily with newspaper or plastic and close them and see what happens, before you do anything else?
I think the biggest problem you have at the moment is the air is taking the path of least resistance, which is NOT down through the attic vent.
The warm air escaping out through it, is mucking up its intent.

oops, I took diameter not radius... uugh..

I will be doing that shortly. I was out running errands this am and now I'm in "get it done" mode at the hacienda.

 

[bbbass]

• If I allowed both attic and crawl space vents to feed the furnace with fresh air then I would have 140in^2 of fresh air venting... more than sufficient.

Originally running out to exterior was not the plan but could be done.

I have seen furnaces and homes ruined by drawing moisture laden combustion air from the crawlspace. To reiterate, go completely to the exterior is best practice. And regardless if you have an opening to the crawlspace, you need to close it off!!!