Does modern house w/high efficiency HVAC need fresh air intake or not?

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gdog

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Hi All,

Recently moved into this house that's about 20-some years old. This past summer replaced original furnace/Ac with 96% Carrier system; here's the furnace: https://www.carrier.com/residential/en/us/products/furnaces/59tn6/

We're in SE MI so just coming into cold season now. The humidity was too low so had the company that installed the system come out and ck the humidifier. It was working fine but based on specs, think they installed one that's too small. Convinced them to give me next step larger humidifier and we'll see in coming months if that fixes humidity issue.

But before he agreed to install larger humidifier he suggested we close off, or at least dampen the fresh air intake pipe that comes from outside the house into the air return ductwork in basement. Note I'm NOT referring to pvc piping for combustion intake and exhaust air; those are separate from this.

The fresh air intake is about 4" steel duct from outside and goes straight into air return plenum ducting, but looks like an inline valve or damper may constrain how much air comes in. This mixes outside "fresh" air with recycled air from the house before it's conditioned by the HVAC system. It's definitely pulling in outside air because pipe is cold to the touch.

The tech was telling me these fresh air intakes are not really necessary and suggested either constraining it, or blocking it completely. He said, it's code to have them installed, but they make the HVAC work harder as it has to condition more outside air as opposed to air already conditioned by the HVAC system. E.g. in FL, where he used to work, the AC can't keep up conditioning this outside air so they routinely block them to make the AC more efficient and effective in the hot humid summers. Of course in summer, the HVAC is trying to de-humidify inside air, while the opposite is true in winter (especially where/when it gets below freezing outside).

Anyone here have experience with this? I can see his point, but also see the benefit of pulling in, at least some outside fresh air for mixing/diluting the stale air inside the house. TIA for any thoughts on the subject!
 

Fitter30

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An HRV (Heat Recovery Ventilation) and an ERV (Energy Recovery Ventilation.) Are both types of unit to deliver fresh air. Blower door test will tell how tight the house is. Your electric company might have a energy audit with would include a blower door test.
Uncondition air coming into a the building envelope through that 4" pipe is like leave a window cracked.
 
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jadnashua

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TO be able to pull in outside air seems to mean there's leaks, otherwise, you'd end up pressurizing the house.

Yes, in a tight house, it's good to pull in some outside air, but wasting conditioned air to do it is not very efficient which is why the HRV and ERV devices came about. They preheat or cool the incoming air as stale air is exhausted. THe better ones also help with the humidity levels.

Pulling in outside air will generally lower the humidity level in the house. Relative humidity may be the same inside and out, but the amount of moisture the air can hold varies hugely based on the temperature...cold air can't hold anywhere near as much moisture as hot air does, so all of that you're pulling in will be drying out the house in the winter, and if you have central air, adding moisture to the house in the summer unless you live in the desert.
 

WorthFlorida

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If this is for just replacing air such as when the clothes dryer is running, bathroom exhaust fans or fireplaces, etc, is to reduce any drafts possible from doors and windows. A draft type gas WH or furnace air can be pulled down the chimney to make up lost air. For the winter I would close the damper almost shut to allow some air to flow in, for summer I would close it completely. Since the fresh air intake is on the return side of the air handler, you'll always pulling in air whenever the blower is running. As Jim stated above, you're pressuring the house by just pulling air in. This can push conditioned air out through the any exhaust vents or up a fire place chimney. From Spring to Fall you're more likely to open windows at times so the air inside the home gets exchanged anyway.
 

gdog

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Thanks for all the good input guys!

Yes, I can see if the house was tight, pulling in fresh air would be more important. But this house has a chimney w/damper; it's shut but that is no weather tight seal in there. I also just installed a ventilated range hood in the kitchen, so if house is pressurized at all, well it (conditioned air) will go right up that stack too.

And then there's the windows; looking to renew those as well soon: https://terrylove.com/forums/index.php?threads/replacement-window-recommendations.97722/

Maybe when I tighten this house up more, I'd look into an ERV, but doesn't seem like a necessary thing with the house in its current state. E.g. want to replace fireplace w/sealed unit eventually.

BTW the new larger humidifier is now capable of maintaining humidity into the 40% plus range, which makes the windows drool when it gets below freezing outside. The remodel journey continues..
 

jadnashua

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When running things like your range hood or dryer, you're exhausting air, so actually creating a lower pressure inside. This means that your house will be sucking air into any cracks it can. This means you're pulling in colder, often drier air in the winter, or more humid, nasty pollen filled air in the summer. This is where an 'engineered' leak is good, as they're designed to transfer heat and moisture in/out of the incoming makeup air.

Your furnace and water heater, if they're not closed combustion devices (i.e., has its own air intake) can consume a pretty large amount of already conditioned air during the combustion process. If you have a gas stove, that uses up a lot of air and can produce a fair amount of moisture as it burns the gas. A lot of range hoods just recirculate the air, but if yours exhausts it outside, that air needs to come from somewhere, and it comes through cracks and loose windows or doors.

A lot of combustion devices actually have a minimum volume requirement for them to work reliably, but that also assumes the house has a certain level of leakage. Long time ago, I looked into getting a tankless WH, but two things kept me from pursuing it: 1., the volume of space wasn't big enough per the installation instructions;, 2. my inlet water in the winter was too cold to provide the desired temperature rise.
 

gdog

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When running things like your range hood or dryer, you're exhausting air, so actually creating a lower pressure inside. This means that your house will be sucking air into any cracks it can. This means you're pulling in colder, often drier air in the winter, or more humid, nasty pollen filled air in the summer. This is where an 'engineered' leak is good, as they're designed to transfer heat and moisture in/out of the incoming makeup air.
When you talk about an "engineered leak" I assume you're referring to an ERV or equivalent device; yes?


FYI I ended up closing off the fresh air intake referenced above for the furnace, at least temporarily. Will likely re-hook it up to an ERV at some point. Yes, both my furnace and WH are closed combustion devices. We have a gas stove/oven but w/vented range hood.

Couple of updates since I last posted. Have six small (1'H x 3'W ?) windows in basement, which is poured concrete. Five of these are solid glass block construction, one is a single pane POS egress window. Noticed really bad drafts around all of them on the top edge and top of side corners. Looked like the drafts were coming from gaps between inner and outer structure (brick outside). Filled the gaps w/spray foam so no more drafts. Basement is noticeably more comfortable and a couple of degrees warmer than it was.

Another update is this is my latest toy: https://www.flir.com/Products/FLIR-ONE-Gen-3/
Very cool tool IMO! With this I've noticed the basement windows are leaking a lot of heat; esp the single pane one obviously. Plan to replace the single pane egress window with a better double or triple pane unit, but is there a good option for the glass block windows? These don't open; all the basement windows are only a few inches above exterior grade so have to be wary of water intrusion.
 

jadnashua

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Yes, an ERV ends up as an engineered leak. Closed combustion devices won't affect the home's air balance, but anything that actually exhausts air like a clothes drier, bathroom vent, range hood (unless it just runs it through a filter and back into the room). A gas stove will leave a lot of extra moisture and mostly carbon dioxide that you'd like to get outside, too.
 

Matthew Haberland

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What I have always found was putting in a SPRD damper in line with the fresh air in the system. That way it will pull fresh air from outside only when there is a negative pressure inside of your home. The furnace you gave us the link to is the top of the line modulating furnace and air conditioner. They are specifically designed to have longer runtimes, but with lower energy usage. So I do not recommend completely closing off your fresh air damper. The house needs it. Would you rather the cold air in the winter and warm humid air in the summer come through every nook and cranny of your house? Or would you rather it come through a designated duct that can be conditioned? The ERV units are very nice, in the correct application. In regards to the heating and cooling system you have, I believe it would be a waste of money on your end unless you have some serious bath fans and range vents with huge amounts of CFM. With the furnace fan constantly running on your infinity system, (which you should have running on MED) that will help tremendously with your humidity problems as well, in tandem with the humidifier. Do you have the Carrier Infinity control that goes with the system? Is the humidifier integrated into the circuit board? or is do you have a seperate designated control for you humidifier?
 
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