Hello,

I have a new 95% efficient furnace installed. Instead of running the intake vent to the outside, they simply have it sucking air in from the furnace room. My old 80% efficient furnace was installed the same way and I did not have any problems that I was aware of.

Is there any potential problem with this? They claim no, but the owner's manual says otherwise. Manual states the furnace can be installed only as a direct vent in which the combustion air is taken from outdoors and the flue gases are discharged outdoors.

The exhaust vent is running to the outside.

If that was my house I would request it be installed to suck air from the outside because it reduces the need to provide venting of the utility room to adjacent spaces which is a sound containment strategy. Of course if you have a gas hot-water heater you still need venting but the size is diminished. Best case is a Hot water heater that has an outdoor air supply system and they do make them.

When you suck the air from the inside and send it up the flue where do you think it comes from? you are basically encouraging drafts in the rest of the house and reducing the overall efficiency of the system. You got short changed.

That's what I thought. Thanks. They are coming back out Monday to rectify.

Good point about the sound containment as well.

So twin furnaces and twin gas hot water heaters sucking air out of a basement furnace room is a bad idea?;)

My old furnaces sounded like a freight train. luckily, the new ones are very quiet.

Of course, per Murphy's law of HT, the furnace room is very close to the HT.

So twin furnaces and twin gas hot water heaters sucking air out of a basement furnace room is a bad idea

Of course, per Murphy's law of HT, the furnace room is very close to the HT.

Yea that really sucks....... I would investigate setting up an outside air system for that furnace room so that you can sound proof it and seal it up tight from the rest of the basement. Double drywall and GG on both sides of the furnace room walls, and insulated of course.

Highly unlikely that you have enough combustion air in the basement to supply all that equipment.

Definitely would have them follow the manufacturer's instructions.

Kudos to you for actually reading the instructions.

Tim

you could install a "fan-in-the-can".

i just had one installed (so could have a solid door from mechanical room out to finished basement instead of a louvered door----either meets code here). the fan in the can has an air intake pipe that connects to an exterior wall and pulls fresh air through. it cycles on/off with your furnace/boiler/etc.

parts cost me about $550 and install time was a couple of hours....hope this helps...i could prob post a pic if interested

I'd be very interested in a pic! Thanks

Sounds like an in-line duct fan. Plenty of options over at HVACquick.com. When I had a humidifier installed they mounted an outlet on the side of the furnace that cycles on and off with the furnace. Sounds like the same concept.

I imagine the issue is to determine the required cu ft of air per minute required for your equipment and that the fan needs to provide more.

Not sure fan-in-a-can would be compliant.

The code requires free air, not CFM.

Not to say it won't work, but I think you'd need an engineer to tell you that.

I've seen a lot of jobs, from homes to schools to gov't buildings. Never seen fan forced ventilation.

fwiw...

Tim

Fan-in-a-can with CFM guidelines

http://www.draftinducers.com/combustionair.htm

-

Actualy, I found the fan-in-a-can system here (http://www.fieldcontrols.com/cas.php).

I've never seen it before. Learn something new every day! I was picturing a duct booster fan/in-line fan like you mentioned BMDC.

This fan in a can is an engineered system. It also has a pressure switch that will interrupt combustion if the fan malfunctions.

Overall, I think it would be an approved alternative to traditional make-up air.

Tim

phew....your "code guy" did make me second guess for a second what i had heard from various general and HVAC contractors over the past year (and have seen some in other basements in the area). what you've found is exactly what i have (still need to get the camera out), though i do not have that additional "boot assembly" feature. rather the bottom of my system (called "Fan-In-A-Drum Assembly" in the link) sits right next to the air intake area of my boiler, and when activated it throws out a lot of fresh air (i should look into that additional boot assembly, though may not fit with my old boiler)

I'm a little confused.

The furnace burns gas - needs air and vents/releases air. The consensus is that the intake should be from outside.

This is completely separate from the large air handler intake, for the furnace?

My multiple basement rooms lack discrete intake vents, so all air in the basement passes through a single large vent return in 1 of the rooms. I believe this is the reason I have multiple grates in my walls, connecting all the rooms together.

***Can everyone/anyone confirm that I need multiple air return ducts, 1 for each room?

I bought my house ~ 1 year ago, and I'm pretty inexperienced (I wish I could say young), so I'm still learning.

Seems HVAC info is well-protected by installers, online. Anyone have a good textbook or online source? I've spent some time on HVAC Talk forum, but a reference would be ideal.

though I am not an HVAC expert by any stretch, no, i do not think you "need multiple air return ducts, 1 for each room." the conditioned air needs to circulate throughout the to be conditioned area. this starts with conditioned air being supplied to the various rooms (or even corners of rooms) via a trunk line coming off the air handler, branching off to individual vents (supplies). to work well, the same volume of air (plus or minus leakage, drafts, etc.) needs to return back through the other side of the system (return) to run back over the coil, etc. this is typically accomplished via one large return. to enable the air from throughout the space to find its way efficiently to that return, simple grates between rooms (or doors that are ripped 1/4 or 1/2 inch or so above the floor, etc.) might work. but the actual configuration, duct volumetric measurements, etc. are for the HVAC professionals to determine. you should have an HVAC contractor that you're comfortable with (pun intended), that can perform routine maintenance, etc. so i'd have a few come out and check over/critique/etc. your system and that's when you can also learn most of what you need to know...in terms of online source, hvac-talk.com (with the hyphen) is a great resource

Thanks, hmmm5.

I mentioned the HVC-talk forum, in my post. It's a decent resource, but much too 'disjointed' to get a good education.

These multiple grates in the walls are annoying. I guess I need a few returns added to those rooms I want to 'seal up'. I have 5-7 years to plan before the playroom is converted to a 'media room', with a wet bar and projector.

Edit - like I said, HVAC info is pretty well protected. Unless I can teach myself, I'll have to enlist an HVAC professional. If the info was widely available, they'd lose a lot of business.

Highly unlikely that you have enough combustion air in the basement to supply all that equipment.

Definitely would have them follow the manufacturer's instructions.

Kudos to you for actually reading the instructions.

Tim

I second Tims comments, and would follow the manufacture instructions.

I second Tims comments, and would follow the manufacture instructions.

I am a HVAC contractor, depending on the furnace model, you may or may not be able to use indoor air for combustion. Even if it was allowed, you might still have a choice to take outdoor air for combustion, and I would recomend it. For one ,it is more efficient to use unconditioned outside air instead of conditioned inside air for combustion. The house will have less of a negative air pressure meaning less cold outside air infiltration. And also the furnace will run more quietly, inportant in a home theater.

Bringing in outside air would mean running a PVC pipe equal in diameter to the existing exhaust pipe. It should be pitched at at least 1/4 inch per horizontal foot down toward the furnace, and should never angle down after leaving the furnace (except at termination). On the outside it should terminate next to the ehaust with a gooseneck 90 degri elbo with the intake opening facing down (at least 12 " above the highest anticipated snow fall level.
If your furnace is not rated to be used with indoor combustion air then I would force the contractor to fix it, no charge. If it is rated to be used with it I would either change it myself, or pay someone to do it for me. PVC pipe is very inexpensive, and easy to work with. The hardest part of the job would probably be making the hole to the outside.

Tony

That's what I thought. Thanks. They are coming back out Monday to rectify.

Good point about the sound containment as well.

So twin furnaces and twin gas hot water heaters sucking air out of a basement furnace room is a bad idea?;)


If you have 2 furnaces and 2 hot water heaters in the same room its possible the HVAC company has a single combustion air intake ducted from the outside rather than individual intakes for each piece of equipment. If it exists, it would be a duct (guessing 8" or larger in diameter not knowing the equipment sizes) coming from the outside and ending up in the mechanical room probably within a foot from the floor.

If you have invested in 95%+ efficient equipment I'd be shocked if they dont have some type of air to air heat exchanger in the exhaust/intake ducting too. It's possible the combustion air is ducted through a heat exchanger but I doubt it's acceptable in the code to do that. Either way, the comments about the furnaces/hot water heaters needing combution air is exactly correct and without a properly sized source for air you're sucking outside air from every crack and hole in the house to feed them. That would tend to negate a LOT of the effort spent on efficiency.

I'm a little confused.

The furnace burns gas - needs air and vents/releases air. The consensus is that the intake should be from outside.

This is completely separate from the large air handler intake, for the furnace?

My multiple basement rooms lack discrete intake vents, so all air in the basement passes through a single large vent return in 1 of the rooms. I believe this is the reason I have multiple grates in my walls, connecting all the rooms together.

***Can everyone/anyone confirm that I need multiple air return ducts, 1 for each room?

I bought my house ~ 1 year ago, and I'm pretty inexperienced (I wish I could say young), so I'm still learning.

Seems HVAC info is well-protected by installers, online. Anyone have a good textbook or online source? I've spent some time on HVAC Talk forum, but a reference would be ideal.


Yes, you are correct about the multiple grates around the basement. They would be called "transfer Grills", they work, but not very efficiently.

To answer your first question.
The air used for combustion is not the same as return air used for circulation.
To help explain, think of the supply ductwork as arteries and the return ductwork as veins and the furnace or air handler as the heart. Same Principal. The combustion air is the air needed to be mixed with the gas in order for the gas to combust. They are 2 separate things.
The ratio of combustion air to natural gas is anywhere from 9/1 to 13/1 meaning for every cubic foot of gas burned you will need 9 to 13 cubic feet of air for combustion. If your furnace has an input of 80,000 btus the it uses 80 cubic feet of natural gas in one hour. the translates to 720 to 1040 cubic feet of combustion air.
This air has to come from somewhere. If it is not piped directly into the 90 % furnace or you have an 80% or less furnace, then cold outside air that infiltrates through leaky doors, windows attics ect. It will mix with your conditioned inside air, however it makes its way into your house, to replace the air used in combustion. Now the air that has to be heated causing a little more run time on your furnace. One of the big advantages of a sealed combustion 90+% furnace is that the air used for combustion and the exhaust, NEVER has any contact with any of the air in the house!

To answer your question about having returns in every room, I would say that if I were designing a duct system for my own house, every room except the kitchen and the bathroom would have its own return. The kitchen and baths would not have it because I would not want to distribute any odors through the house. In the real world most builders do not want to pay the extra $ to do it this way . As long as there is adequate return air for the whole system the system will work. With separate returns though, you will have a more even distribution of you heating or cooling.

Tony
HVAC contractor

If you have 2 furnaces and 2 hot water heaters in the same room its possible the HVAC company has a single combustion air intake ducted from the outside rather than individual intakes for each piece of equipment. If it exists, it would be a duct (guessing 8" or larger in diameter not knowing the equipment sizes) coming from the outside and ending up in the mechanical room probably within a foot from the floor.

If you have invested in 95%+ efficient equipment I'd be shocked if they dont have some type of air to air heat exchanger in the exhaust/intake ducting too. It's possible the combustion air is ducted through a heat exchanger but I doubt it's acceptable in the code to do that. Either way, the comments about the furnaces/hot water heaters needing combution air is exactly correct and without a properly sized source for air you're sucking outside air from every crack and hole in the house to feed them. That would tend to negate a LOT of the effort spent on efficiency.

I agree with Klindy,

If the room is sealed and combustion air is ducted from the outside for 4 appliances, you do have the possibility of condensation problems or even freezing hot water pipes if there is no air/air heat exchanger, and you live in a cold climate where temps could get below zero for any length of time.
But I would still recomend piping the furnace intakes if poss.

Thanks, Tony. That clears it up.

I'll need multiple returns, if I want to get rid of the transfer grills.

Where to start...Tony, want to sell any of your old HVAC school books? ;)