HVAC experts

Chipg56

Member
HVAC experts: I have a fairly efficient gas furnace for my house. But it is somewhat like a fireplace. It draws its combustion air from within the house so when it is running it sucks warm air up the chimney. I am aware that the most efficient systems draw the combustion air from the outside. My first question is do the systems that draw air from the outside compensate for the temperature of that air by changing the air/fuel ratio as do modern fuel injection systems in cars? Can I gain in efficiently with my present system by simply sealing off the unit and drawing the combustion air from the outside? I am really concerned about heating my home next winter. Thanks for your help.
 
Last edited:
Good Qustion .
I wonder when you say chiminey are you talking about a regular metal type not the pvc on high efficency furnaces ?
Thats something ive never considered . they do have several boards . I am going with no . so most likely im wrong . hope a expert chimes in .
 
My house has a regular lined brick chimney which vents the furnace and gas water heater. House was built in 1955. I am thinking that I could seal the unit, have a PCV input and use the current exhaust. I just don't know if this furnace would operate properly with varied temps.
 
Carpenter not HVAC but have picked up a few things over the decades.

How old is your unit and was it installed according to factory specs by a professional?

If yes to the questions then you may need to accept that the company did not design your unit to be plumbed and operate as the more modern high efficiency units that do amazing with outside/cold air intakes.
 
Maybe looking at this wrong, but aren't you heating the air that goes up the chimney either way?

Now I realize heat continues to go out the chimney when burner is off, but you also must raise the temperature of the colder outside air with the heat of combustion.

Probably not going to make super noticeable difference in fuel bill.

Jmho

Edit
This may do what you are wanting.

https://www.energydepot.com/RPUcom/library/HVAC007.asp
 
Last edited:
Drawing the combustion air from the house creates a low pressure situation. The air that goes out the chimney is heated, the low pressure causes cold air from the outside to enter the house from any crevice. The current system is equipped with a damper, my concern is strictly the air used by the fan that supplies combustion air. The most efficient systems always draw the combustion air from outside thru a pvc pipe. I am certain that my system is not configured with an a/f or o2 sensor as it is not designed for wide swings in operating temp. My concern is that since I would be subjecting it to, easily, 50 or 60 degree temp swings if it could become so inefficient as to negate what is gained. Or worse inefficient combustion producing excessive carbon monoxide.
I guess what I am really trying to find out is if the ultra efficient units have, or need a system like modern fuel injection where the exhaust is monitored and the air fuel ratio is constantly adjusted. Maybe that level of sophistication is not even necessary.
 
I know nothing about HVAC but I have stayed at a holiday Inn express a few times, once you started pulling the outside air in wouldn't that create a condensation problem, you would need to compensate for.
 
I would guess that in colder weather the outside air is dryer then the warmer inside air, or do you mean on the exhaust side?
 
Drawing the combustion air from the house creates a low pressure situation. The air that goes out the chimney is heated, the low pressure causes cold air from the outside to enter the house from any crevice. The current system is equipped with a damper, my concern is strictly the air used by the fan that supplies combustion air. The most efficient systems always draw the combustion air from outside thru a pvc pipe. I am certain that my system is not configured with an a/f or o2 sensor as it is not designed for wide swings in operating temp. My concern is that since I would be subjecting it to, easily, 50 or 60 degree temp swings if it could become so inefficient as to negate what is gained. Or worse inefficient combustion producing excessive carbon monoxide.
I guess what I am really trying to find out is if the ultra efficient units have, or need a system like modern fuel injection where the exhaust is monitored and the air fuel ratio is constantly adjusted. Maybe that level of sophistication is not even necessary.
I can see what you are trying to accomplish.

Even though you eliminate the negative pressure from the inside. What about positive pressure from the outside pushing cold air thru the same crevices?

In my neighborhood, 40 mph winds from almost any direction occur. My house has a less than perfect aerodynamic shape, from any angle.

I think the incursion still will occur, even in the calm.
Stand by your triple pane window on a real cold day.

Just thinking you may be thinking small picture.
 
There is only one way you can stop heated air goin up chimney, is to power drive dampers on flew pipe. Then in must be interlocked with furnace and hot water tank. For less the 80% type furnace, it’s not worth it. Not heated air or cold out side will not change burner operation. Lack of air will. There is a lot more to get a tight house the the heating equipment. Single pipe vent heating, hot water, dryer vent, stove hood venter, bathroom vent fan, fireplace. If you can get the house to be very thought. Then what will happen is called a “sick house” that’s from no air change. If you want to get out side air just to furnace and hot water tank enclosed a room around them and put it outside air intake which is called make up air just for that room. Need 1 square inch for every 1000 btu’s total. All the work May not save on heat bills. I am getting new house (built 2014) that has a geothermal (6 ton unit) and solar. Solar panels put out 75amps @ 240vac. Electric bills avg $20-25 month. Winter $250 for 3 months less sun. If you want to replace to more efficient type furnace. 92-96%. The higher the efficiency the more it cost to repair it.
 
When I done construction we done work for The Habitat for Humanity houses. They done an infiltration test on them. If they didn't pass they had to be repaired before the house could pass. The HVAC guys told us that they were so well sealed up, that they HAD to put fresh air vents in the houses or they would get sick from no air movement as @JLSRacing said above. This is besides your cooking and bathroom vents.

It was a sight seeing them seal the front door up and put a fan unit to pull a vacuum in the house, then they used a meter to detect air leakage.

But, I get what you're trying to do, eliminate leaks and put them where they can be controlled to an extent.
 
You can seal up a new tight. If was mine. I would install ERV Energy recovery ventilation unit. It would change air with out much energy use. But is not want Chipg56 is trying to do. If you want to do it cheep. Put a 6” outside air intake with a barometric damper and run a flex line close to your furnace for outside air intake that will pull it close to the flex line and not the rest of the house. Most new houses has two out side air intakes. 1st for make air close to heating, for 80% type. 92%+ uses 2 vent PVC. 2nd is hooked to return air duct to pressure house to keep drafts out and make up air for vent fans. There is no best way. Most big building I worked in. Has a 20% air change. It’s like leaving your window open all winter. Energy cost will be going way up. Just like fuel costs.
 
I am sure I am far far away from being too tight. My house was well made back in 1955 but certainly does not approach well done modern construction.
 
Back
Top