HVAC Experts - Payback on Combustion Air Intake

[TWG1572]

I've done some searching and haven't come with an answer so I'm hoping I might get some opinions here.

My house was built in 2001 - it's pretty tight with decent windows and spray foam insulation. However, the furnace is set up to burn inside air for combustion. Most of the new installs I see have an outside air intake for the furnace. It seems to me like burning inside air would be inefficient, but I've not been able to quantify exactly how bad it is. Maybe it's minor in the scheme of things - I just don't know.

I could have an outside air intake installed on my furnace. I'd hire someone, simply because I'm too chicken to saw a hole in the side of my house. I'd assume that's going to cost me $100-$200. I'm curious if I would see a payoff to doing this, or if I should just ignore it and move on in life to larger energy wasters.

Edit: We do have an outside air intake plumbed into the return HVAC air. So we are getting outside air into the home, we just need to heat it before we burn it.

Based on an old document I found online, it takes 873 cuft of air to burn 1 gallon of propane. Given that and our 1,100 gallon usage - we are burning 960,000 cuft of conditioned air. I'm just not sure where to go from there.

 

[HerrStig]

IF your burner is in an unheated basement and there is little or no air infltration from your heated spaces it won't save you enough to bother with. If you use air you have already paid to heat to burn your fuel you are sending a little money up the stack. I think it will be a long payback in stopping that, though. In VERY cold climates an oil burner which is fed with outside air can be hard to start if it has been cold soaking and the vent damper on the inlet doesn't seal well. Gas burner systems don't have that problem.

 

[leeharvey418]

Six of one, half dozen of the other...

You're either using inside air, thus causing additional inward leakage with the attendant chilling effect, or you're using outside air that doesn't get your combustor as hot for a given fuel input.

At the end of the day, the system can be designed either way, but either way it's a compromise.

 

[A_Harman]

Why would cold outside combustion air be more efficient in heating than warm inside combustion air? Seems like you would have to burn more fuel to heat the combustion air enough to transfer heat at the same temperature in the heat exchanger.

 

[bepperb]

Originally Posted By: A_Harman
Why would cold outside combustion air be more efficient in heating than warm inside combustion air? Seems like you would have to burn more fuel to heat the combustion air enough to transfer heat at the same temperature in the heat exchanger.


I'm not willing to go into that argument and I've heard both sides. Efficiency isn't the only (or most important) concern. The furnace will use quite a bit of combustion air in operation, and while all of this should be coming in the fresh air intake that isn't always the case. When the furnace is running that will create a vacuum in the enclosure and draw air in elsewhere. That air will be cold. Even if the furnace is operating in the same efficiency that cold air will reduce comfort if it's coming in a window, open door... etc.

Also, the negative pressure creates a problem for dryers, fireplaces, water heaters. You have all these things competing for a fairly limited supply of fresh air. A 3" pvc pipe from the furnace and a 4" dryer vent both force fed by blowers don't share a naturally aspirated 5" supply line very well.

At the risk of this going further, why would you preheat the combustion air at 96% (or less efficiency) before you burn it? What does that gain, besides not needing a second pipe, which you really do need for safety and comfort concerns outlined above.

 

[Quest]

before you proceed to hiring someone to cut a hole in your house, one question remains:

if your house is tightly built (based on 2001 local/regional construction code), then there must be the following happening:

(1) a fan running around 80~120cfm continuously on the highest point of the house. This is to evacuate any stale air, contamination (incuding but not limited to VOC of all sorts), possible radon gas, etc.

(2) because it's constructed that way (with point 1 within), your house shall exhibit negative pressure and cold air will try to creep in from nooks and crannies.

typically, for these types of house, there are 1 of the 2 possible implementation of heater furnace. For mid-efficiency type (single stage burner, hot surface ignitor type of ignition, the possibility of burner requiring fresh outside air (needs an air intake from the outside )is high; for 2 stage furnace, typically air is taken in from the outside via ABS pipe (with a return exhaust type ABS pipe). For the latter type: no additional inside air is consumed.

You'll need to investigate closely as to how your furnace gets it's fresh air from, for the type (1) burner, most likely it's not forced exhaust type (relying on hot air to create enough rise on the metal chimney to exhaust). Too much of negative air pressure within your house will cause inverse effect (backdraft), where the exhaust gas is not hot enough to rise up the chimney but due to negative air pressure inside the house, the exhaust gas from your furnace will come down the chimney causing inside air contamination, with CO, CO2, etc.

Please take this matter seriously, for your family and your life depends on it.

Also: if there's really a cold fresh air inlet that leads into your furnace/boiler room, so be it. While you may lose a bit of heat due to cold air intake that feeds your boiler, furnace, etc. but the reduction of negative air pressure will keep your house inside air pressure fairly balanced, reducing the possibility of backdrafting (which will lead to CO poisoning)

 

[SrDriver]

It takes 29 cubic feet of air to burn 1 cubic foot of gas there therefore if you can use 100% outside air for combustion you are lightyears ahead in virtually any application.

There are several methods to do this with older applications a HVAC shop can come up with the best method for your application there.

 

[TWG1572]

Originally Posted By: Quest
Please take this matter seriously, for your family and your life depends on it.


Absolutely. My intent would be to use a local HVAC contractor we've used in the past. I'm just trying to understand the situation before calling them, and having a service call for something that isn't worthwhile or even possible. I like to be informed, before I have the discussion.

Our house has a radon fan, which exhausts through the roof. It pulls it's air from under the slab, so I'd assume the makeup air would come from the basement. I believe it's a 140cfm model. That said - the house was not built with the radon fan. It was added when we purchased the house.

We have a single stage furnace. It is not forced exhaust (I'm assuming the forced exhaust on a furnace looks like that on a hot water heater). It is a solid ABS pipe with no fan on it.

The only existing fresh/cold air intake is teed into the cold air return on the furnace. It's probably a 4 or 5" round duct (I'm at work so can't measure) There is a damper, and it does move to the open position while the furnance is running. So I am drawing some outside air, which is being fed through the furnace and conditioned. I'd assume that if the air pressure got negative enough within the house, the damper would open even if the furnace wasn't running.

 

[bepperb]

If your furnace exhausts into a white pvc pipe through the side of your house it's forced exhaust. You can't see the blower because it's inside the metal furnace chassis. When the thermostat calls for heat you can probably hear the blower come on 30 seconds before the big blower for the heated air.

A dual stage furnace is a unrelated, that would burn at two different levels and run the blower at multiple speeds. That would also have forced combustion air, that doesn't mean your "not dual stage" furnace doesn't have it.

It would be uncommon for a home built in 2001 in WI to have a natural draft 80% efficiency furnace, though not completely out of the question. That would vent vertically through the roof in a metal b vent. You can't do that horizontally or in pvc, so I doubt it.

Yes, that's normal for the fresh air intake to run into the cold air intake. I think that's good, not great. Great would be a combustion air supply line.

Maybe post a pic of your furnace, or post the model.

But as stated, combustion air intake would be more efficient, but probably not a huge payback. If you have a power vent water heater and no fireplace you might not even benefit from it from a safety standpoint. That might be why they didn't do it in the first place. Then maybe the only reason to run one would be if you walled in the furnace limiting the available air.

 

[TWG1572]

Originally Posted By: bepperb
If your furnace exhausts into a white pvc pipe through the side of your house it's forced exhaust.


Ahh, then you are correct. It is a forced exhaust. It's a higher efficiency model - I'll grab a picture tonight.


Originally Posted By: bepperb
But as stated, combustion air intake would be more efficient, but probably not a huge payback. If you have a power vent water heater and no fireplace you might not even benefit from it from a safety standpoint. That might be why they didn't do it in the first place. Then maybe the only reason to run one would be if you walled in the furnace limiting the available air.


We do have both a fireplace (origional) and they did a basement finish job approximately 5 years after the house was built. At that point, the furnace was walled off into it's own room. The room itself is large - 500 to 600 sqft - but there are only two intakes from the rest of the house into that room. They basically put two cold air return grills into the wall between the furnance room and the rest of the basement. That's the source of combustion air.

On a semi-related note, I really wish they had thought to plumb a fresh air intake into the fireplace when built. The insert has that feature, but they didn't use it. Unfortunately, it's on an inside wall of the house and with the basement finished I don't have much ability to retrofit one in.

I think the furnace intake is one of those things where if I can make a good case for paybeck or safety, I'll do it. Otherwise, I'll leave well enough alone.

 

[TWG1572]

The furnace is a Heil DC90

 

[1 FMF]

in general about 1 cubic foot of air releases 100 btu's of heat.
if your burner is 50k btu-hour then 500 ft3/hr or 8 cfm of air is required. if your burner is 100k btuh then you need ~16 cfm of air.
regardless of how the furnace is installed and how it gets fresh air to burn, if the burner is adjusted properly and burning cleanly at stoichiometric then there is no need for an outside source of air to be pumped in to the burner. it's getting plenty of oxygen and the combustion process is as good as it's gonna get.

when you decide to pump cold air in with a cold air intake, you run the risk of pushing too much cold air into the burn box and creating a heat loss up through the flue, but that shouldn't happen provided the cold air intake is adjusted properly.

the biggest factor is where the furnace is located and how/where it is pulling it's air from. if it's not in a living space and that furnace area is sealed off well from the living space then the vast majority of cold air coming in from outside is not influencing the living space... so no need for cold air intake. like previously said, when the furnace is located within a finished living space then it's drawing air in it just heated and you're pushing that out the flue at 8/12/16 cubic feet per minute depending on your burner size... while 8/12/16 cfm of cold outside air infiltrates the living space from wherever cooling those parts of the house as that cold air makes it's way to the suction point of the burner.
the cold air intake is rarely used to increase combustion efficiency, almost always there's enough air/oxygen making it to the burner unless things are over sized and the area the furnace is in is sealed tight- generally apartments and condo's where the furnace is in your coat closet as you walk in for these you could really use a cold air intake but ironically there's no easy way to plumb one.

https://www.myodesie.com/index.php/wiki/index/returnEntry/id/3054