Make Upwardly Air Duct Sizing Guide

Makeup air can be a lilliputian intimidating when trying to figure out what size duct yous call for. Especially since correct ducting is essential to the functioning of your HVAC arrangement, so you lot actually don’t desire to mess it upwards.

Believe me, it was a theme I avoided for a spell, worrying I was inwards over my head. Thankfully, patch it looks a chip complicated, it’second easier to understand than y'all intend. Plus, amongst the data below, y'all will have a conduct to figuring out the duct size you take with half the endeavour.

The size of a makeup air duct is not specified past the IRC, just the total air supplied must equal the exhaust rate inward the infinite to forbid problems. This is established by measuring the air infiltration (using a blower door exam) in addition to exhaust charge per unit to approximate the diameter needed.

What Does the Code Say?

Doesn’t Specify Makeup Air Duct Diameter

The International Residential Code (IRC) provides minimal data when it comes to makeup air units and their requirements. One of the things that aren’t detailed in the code is a specific duct diameter for these units.

IRC Specifies Inflow Rate

Section M1503.half dozen of the IRC covers the inflow charge per unit of makeup air for reach hoods: if the exhaust organisation can vent more than than 400 cubic feet of air per minute (CFM) must take mechanical or passive makeup air supplies equal to the exhaust charge per unit.

If the exhaust system can vent more than 400 cubic feet of air per minute (CFM), it must have mechanical or passive makeup air supplies equal to the exhaust rate

As patently every bit possible, this agency that when your kitchen exhaust system removes over 400 CFM, y'all need a makeup air render to equal it.

The section addresses domestic kitchen exhaust makeup air, only logically, the rule of matching inflow charge per unit volition be the same throughout the household.

This is because makeup air is provided to eliminate problems similar incomplete combustion, carbon monoxide leaks, sewer gases existence pulled from drains, and ineffective exhausting of steam, smoke, too other pollutants.

All these problems are the event of negative air pressure level, which is created by removing air from the room without replacing it.

Balance is Key

Balance is too key when providing makeup air.

On the 1 paw, replacing one-half of the air will not be sufficient.

On the other paw, adding likewise much air volition create a dissimilar fix of issues involving air pressure level that can brand space virtually the exhaust system compete for the air that is supposed to live exhausted.

Adding also much air in the room tin can actually draw air away from or overwhelm the exhaust arrangement, pregnant that the smoky kitchen will stay that fashion.

Determining the Correct Duct Size

When yous are trying to figure out what size duct to become, you need to know the next:

How airtight the home is

What the infiltration rate is

What the exhaust rate is

The total of makeup air required

How Airtight is the House?

To determine how airtight your abode is, y'all volition demand to have a professional person HVAC specialist come up inwards to measure out this. They will render yous with a reading of CFM at fifty Pa (Pascal, the unit for air pressure level).

To hand yous a CFM at 50 Pa, the professional person volition probable perform the blower door test. This test for airtightness looks at how much air gets in together with out of your dwelling house.

How It Works

The cite makes more than sense when y'all sympathize that a blower door is a frame that is mounted in the door frame for an external door, alongside a fan that attaches to this. This is used to draw air from inside the household, creating low air pressure level within.

When at that place is a low/negative pressure organization, outside air (high/positive pressure organization) volition be pulled through the gaps in addition to openings in your household to residue out the air force per unit area. This is depressurization.

Depressurization-example-inside-the-house-illustration-of-a-house-with-low-pressure-and-high-pressure-outside

Note: this tin also be done in opposite, increasing the pressure indoors, if the state of affairs requires it.

While this is happening, the specialist will use a tool (UV camera or fume pencil) to locate the leaks, which are recorded. With this information, the specialist tin can quantify the air infiltration, which shows the airtightness (drive of air through the thermal envelope).

What is the Infiltration Rate?

Air infiltration refers to air moving into a edifice, inwards this case, your home.

The infiltration rate is the sum of air that moves into your domicile without you deliberately bringing it inward.

In the context of makeup air, nosotros involve to know how much air is getting into your dwelling naturally to figure out how much air needs to be added mechanically to compensate for the exhaust charge per unit.

If your exhaust arrangement is rated 400 CFM and you install a makeup air unit that pulls inwards 400 CFM, you will have likewise much air entering the family because this 400 CFM makeup air inflow volition be inward add-on to infiltration.

Similarly, if y'all assume that y'all accept a proficient natural infiltration rate without really measure it too you entirely bring inwards 200 CFM of makeup air, you might all the same accept a negative organization indoors.

As I enjoin, yous don’t desire to take inward likewise niggling or besides much air, equally this leads to several problems involving free energy employment, drafts, odors, condensation, pests, too contaminants inside your thermal envelope.

Blower Door Readings (CFM at l Pa)	Infiltration Rate with 1 Pa differential	Infiltration Rate with two Pa differential	Infiltration Rate with three Pa differential
thousand	±90 CFM	±120 CFM	±160 CFM
1500	±110 CFM	±190 CFM	±250 CFM
2000	±160 CFM	±250 CFM	±310 CFM
2500	±200 CFM	±310 CFM	±400 CFM
3000	±240 CFM	±380 CFM	±490 CFM
3500	±290 CFM	±420 CFM	±580 CFM
4000	±310 CFM	±500 CFM	±650 CFM
4500	±360 CFM	±560 CFM	±710 CFM
5000	±400 CFM	±610 CFM	±800 CFM

Based on Figure four of Kitchen Ventilation Systems: Part ane (The Pennsylvania Housing Research Center)

The pressure differentials inward the table refer to the force per unit area deviation at the opening into the home.

You should aim to go on the differential below 3 Pa, or else you lot tin hazard backdrafting from your gas appliances as well as openings similar fireplaces from depressurization. This is why at that place are no differentials higher than 3 Pa inwards the table.

Determining the Flow Rate of the Exhaust System

First, the arrangement’second CFM is likely listed inward online/inwards-shop product data (for ducted reach hoods). Otherwise, yous power locate it in the user manual (it should be available online) or the product label on your appliance.

Alternatively, role a formula to find out the CFM rate needed for the room and base your estimation of the appliance’sec capacity on that (hopefully, it’second correctly sized!).

To first, yous require to know the book of your room:

Volume of room (ft³) = length 10 width of floor ten top of ceiling

With this information, you tin role 2 unlike methods:

CFM = Volume of room/four

CFM = (Volume of room 10 15 air changes per hour (ACH))/threescore

So, if yous take a x’x12’ floor expanse alongside a ceiling summit of viii.v’, your room is 1020 ft³. Both methods reach yous 255 CFM. So, your fan is probable a like charge per unit.

Required Makeup Air Inflow Rate

Now, you need to do a quick calculation. Your makeup airflow charge per unit should live the CFM of the exhaust organisation, less the infiltration rate (you lot volition take assistance mensuration this).

Makeup air charge per unit = exhaust organisation CFM – infiltration rate

Once y'all take established the makeup air rate needed, yous tin starting time looking at what duct size volition supply this.

Since yous ask to specifically furnish makeup air when your exhaust organisation exceeds (one) 400 CFM for a reach hood as well as (2) 200 CFM for a dryer, allow’second look at these figures every bit an instance.

(In other cases, it may only depend on the presence of a negative outcome of the organisation on the pressure inward the menage, regardless of what the exact CFM is).

Say the house has a reading of 1500 CFM at l Pa, using the tabular array higher up, we can run across that the infiltration rate would live roughly 190 CFM at a two Pa differential.

Now, we can function the previous equation to calculate the makeup air rate for kitchens too dryers under these atmospheric condition.

(ane) 400 – 190 = 210 CFM of makeup air

(ii) 200 – 190 = ten CFM of makeup air

Makeup Air Duct Size

Below is a table of the average air inflow rates through unlike size ducts at unlike air pressures:

Pressure Differential (Pa)	Airflow amongst four” Duct	Airflow with half dozen” Duct	Airflow amongst eight” Duct	Airflow with x” Duct	Airflow with 12” Duct
ane	xiv CFM	thirty CFM	54 CFM	84 CFM	122 CFM
two	xix CFM	43 CFM	76 CFM	119 CFM	172 CFM
three	23 CFM	53 CFM	94 CFM	146 CFM	211 CFM

Based on Table 2 of Kitchen Ventilation Systems: Part 2 (The Pennsylvania Housing Research Center)

With this table, you lot tin can decide what size duct will arrange your needs based on the calculated makeup air charge per unit.

If nosotros use the makeup air rates from the previous section, we can see that we would need a duct bigger than 12″ at 2 Pa for the exhaust organization together with a 4” duct for the dryer.

Estimating Duct Size Without Blower Door Test

You could assume that what goes out must come dorsum within, matching the makeup in addition to exhaust duct sizes or the CFM of makeup air to that of your exhaust system.

However, it is not feasible to approximate the makeup air duct size without doing the examination since proper introduction of makeup air is essential to avoid several issues.

Man checking the air flowing from the make up air

If there is likewise fiddling air because the duct was underestimated or infiltration was overestimated, yous tin stop upwardly amongst a greater pressure level differential. This volition create backdrafts, drafts, incomplete combustions, and overheating HVAC appliances.

This can also contain likewise much unconditioned air into the dwelling, unbalancing your HVAC temperature command.

You do get just about warning, and so here are 5 Signs That You Need More Makeup Air

If you overestimate the duct size or underestimate infiltration, you lot can end upwardly with likewise much makeup air. Your HVAC organisation must strain to status and distribute larger volumes of air. You can also cease upwardly amongst exfiltration, where conditioned air is existence pulled outside your dwelling house.

Most Common Makeup Air Unit Duct Size

I couldn’t find whatsoever common duct size when it comes to makeup air units. I would assume this is because there is such a diversity of needs in addition to sizes (units too ducts) available. This multifariousness makes sense if y'all reckon that each habitation has dissimilar factors influencing its makeup air needs.

Weather, the historic period of the family, as well as modifications to the sealing in the habitation all touch on how airtight your home is. This tin be unlike from neighbour to neighbour, so the duct size you ask can likewise range in the same style.