Home Pressure Effects of Unbalanced ERV, Ventilating Dehumidifier, or Supply-Only Ventilation
What this covers
Modern ventilation devices have finely tunable airflows- but what does tuning them actually do to your home? Let's dig into the math on unbalanced ventilation with ERVs, ventilating dehumidifiers, and supply-only ventilation.
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The speaker demonstrates that creating positive home pressurization with ERV systems is physically challenging and often economically unfeasible in real-world residential applications due to fundamental building science constraints, requiring careful system design and trade-offs rather than simple unbalancing.
- Pressurization requires specific CFM outputs that exceed most residential ERV capacities and building physics allows depressurization more readily than pressurization
- Mathematical modeling shows leaky homes need 230-350 CFM of unbalanced supply air to achieve modest 2-3 Pascal pressurization, requiring system upsizing or multiple equipment
- Supply-only ventilation with dehumidifiers offers lower-cost alternatives but still face practical CFM limitations relative to actual building air leakage
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Depressurizing a home (creating negative pressure) is generally easier than pressurizing it in American residential construction because kitchen exhaust dampers open more readily to outflow than to inflow, and building paper/house wrap often have untaped bottom seams that prevent inflow but allow outflow.
“quick nerd note to depressurize a home is generally easier with the way we construct homes in America than to pressurize a home and it has to do with the way that we um the dampers that are in place on your kitchen exhaust hood for example close when you pull on it but it opens when you push on it also house wrap if you use building paper and house wrap things like that often they won't tape the bottom seam of that because it avoids the warranty if you do so that'll blow right open if you push on it but not if you pull on it”
Even in very airtight homes with minimal intentional air leakage, some uncontrolled air movement still occurs due to wind pressure differentials across building surfaces and HVAC system pressure imbalances, requiring either perfect design or active pressurization strategies.
“even with a very airtight home like the one that I'm talking to you from you'll have a little bit of air leakage when the wind blows a certain way or when the HVAC system unbalances the house in a certain way it's pretty engineered but it's real life and there's like a little bit of floppiness”
ERVs are chosen for balanced ventilation primarily because they equalize airflow between incoming and outgoing air, not primarily for energy efficiency, and the balanced characteristic is the key functional requirement in airtight homes.
“as you've heard me say on this channel ervs are the ventilation strategy that we need to use in very airtight homes and more and more people are choosing to build airtight because they're starting to see the benefit and really want that now when you're using an Erv and you have this balanced ventilation and by the way that is the reason that you get an Erv is because it's balanced it's an equalizing ventilator not because of the Energy Efficiency um we hardly ever talk about Energy Efficiency on this channel”
A typical residential home with a blower door test of 1500 CFM50 would require 230 CFM of continuous incoming air to achieve a 3 Pascal positive pressurization, which exceeds the capacity of most residential ERVs on the market and is essentially unfeasible in residential applications.
“if we use a more typical home let's say somebody that's got a blower door test of 1500 in order to produce a three Pascal pressurization on the home we're going to need 230 CFM of incoming air to be able to do that at that point I mean there are very few ervs in residential applications that blow 230 CFM at all much less 230 CFM more on the incoming stream than whatever we have on the outgoing stream”
Adding 350 CFM of supply ventilation to a 2-ton air conditioner system (which normally moves 800 CFM) would require upsizing all ductwork by approximately 50 percent, representing a massive infrastructure investment that makes the ventilation strategy economically prohibitive even before considering operational costs.
“and we're going to bring that in through a mirv 16 filter and we're going to blah blah we're going to dehumidify it I would say you you want to plan to build a home for yourself like that is pretty hard to do 350 CFM on top of your 800 CFM of two-ton airflow is going to be upsizing your ductwork by 50 percent in order to get the air to move the right way and not build up a lot of pressure”
When a home requires 120 CFM of bathroom exhaust and needs 44 CFM of unbalanced pressurization, the total required supply is 164 CFM, which exceeds the 150 CFM total capacity of the reference ERV, making it impossible to achieve the desired pressurization without purchasing a larger unit like a 180 or 200 CFM ERV.
“we're gonna need 44 more CFM coming in than going out if I am gonna have my exhaust be 120 then that means that I'd have to have my Supply be 164 which I can't do with that Erv that we were just looking at I'm now going to have to shop for an even bigger one a 180 or 200”
Modern ERV systems with EC (electronically commutated) motors allow independent variable control of incoming and outgoing fan speeds, enabling unbalanced ventilation strategies where supply and exhaust CFM can be set to different values within a single unit.
“now we have access to what's called EC motors which are just means that they're infinitely variable you can actually turn a knob and it'll slow the fan down or speed the fan up so we can now on several Brands including this brown right here uh churn the incoming Airstream to a different number than the outgoing Airstream”
Inexpensive supply-only ventilation systems (like products from Aprilaire and AC Infinity) that push air into the home through a duct between the return plenum and outdoors offer a more economical alternative to unbalanced ERVs for attempting modest home pressurization.
“there is a way to uh get around this air cycler has very inexpensive components um there this is like anything that would be considered a supply ventilation at least you can find things that like from Aprilaire and from AC Infinity that just have Supply fans that can just push into the home”
For a very leaky home with a blower door test of 3000 CFM50, even achieving a modest 2 Pascal positive pressurization would require 350 CFM of continuous unbalanced ventilation, making active pressurization strategy completely infeasible without massive equipment and infrastructure investment.
“in a very leaky home what we're talking about like maybe a 3000 on the blower door if we wanted to even induce let's just say a two Pascal pressurization which by the way remember that when the wind blows it's going to change you know the the pressurizations on the rooms in the windward side of the house versus the leeward side of the house it's going to change when the doors are closed but even just with a two Pascal pressurization we're looking at 350 CFM”
To maintain continuous positive home pressurization (1-2 Pascal above ambient) using supply-only ventilation, the air handler must run continuously and the supply duct must remain open at all times, which creates ongoing energy consumption and dehumidification requirements.
“if you want to constantly have control over the pressure in your home and constantly have it be a high pressure with respect to outside and high being just a pascal or two you would need to have your air handler running all the time and you need to have that thing open all the time”
Successful home system design requires understanding that 'everything is a system' and recognizing that incompatible design goals (like high pressurization without infrastructure upsizing) cannot be solved through equipment alone when fundamental physics constraints apply.
“because I remember everything is a system so just remind yourself of that”
In a balanced ERV system with separate supply and exhaust fans, the location of supply and exhaust ductwork placement in different rooms creates exploitable pressure differentials that can be used to influence home air flow physics even though the system is nominally balanced overall.
“you've got supplies and exhaust and you put those in different rooms then you can start to influence the physics of how your home is working and I'm going to have another video about why unbalanced across rooms is very important I.E you wouldn't put the Erv in the same room so that it's like giving to and taking from in the same room”
Bathroom exhaust requirements in modern airtight homes should differentiate between shower exhaust (requiring more ventilation due to moisture generation) and toilet exhaust (requiring less), rather than applying the same CFM requirement to both as some industry standards suggest.
“and that's 25 CFM from each hour 15 CFM from each toilet and if you hear Zender talk about it they're talking about 20 CFM from either a toilet or a shower and I think that it's strange to treat both of those things the same way I think that they're very different the amount of pollution that they would give to the home”
For a specific airtight home with a blower door test of 290 CFM50, 3000 square feet, and 47000 cubic feet of air volume, achieving a 3 Pascal positive pressurization requires 44 CFM of unbalanced ventilation (more supply than exhaust).
“we can do let's do my house if we had a blower door test of 290 which is my home's blower test for 45 000 47 000 cubic feet of air within my 3 000 square foot home um and to depressurize or pressurize the home to three pascals let's just say we're going to need 44 CFM of unbalanced”
The fundamental problem of residential pressurization strategies is that the mathematics consistently show unfeasible requirements across all home types and all equipment approaches, making perfect home pressurization economically and practically impossible for most applications despite being theoretically desirable.
“so you can see that the it the math doesn't really add up and I have this happen a lot of times where people want a certain thing out of their home now that we're learning more and more about this from our channel from Matt risinger's Channel and many others um as you know the information gets out there more and more and I'm so excited about that but there are some scenarios where you just can't do certain things in a home affordably because of other factors in the home because I remember everything is a system”
A ventilating dehumidifier unit rated at 350 CFM at zero water column pressure but only 280 CFM at 2.2 inches water column (realistic ductwork pressure) with 70 CFM from outside and 210 CFM from inside would only be able to pressurize a home with blower door test of 600 CFM50 to 2 Pascal, making it insufficient for most homes seeking pressurization.
“at 2.2 inches in water column which is probably pretty reasonable we have less than 300 CFM a flow we'll call it 280. now 280 is nice because we can divide that by four and the reason that we want to divide it by four is that this is a 10 inch round duct that is 78 square inches roughly this is a six inch round duct that is approximately 25 square inches it's 28 but what we're going to use a three to one so we've got 75 square inches set and 25 square inches that's a three parts air in the big color one part error in the small collar so if this whole thing only blows 280 CFM we divide that by four which is 70 a piece so what's coming from outside in this case is only 70 CFM and then the remaining 210 CFM comes from this big collar that's coming from inside the house so you can see that if we need to be able like let's just see what uh at what point 70 CFM is going to induce a two Pascal we would need to have a lowered or test number of 600 cfm50 in order to get that to work with that 70 CFM pressurizing my home to two pascals so you can see that the it the math doesn't really add up”
Hospital operating suites use continuous positive pressurization so that air always flows outward and no contaminated air from surrounding rooms can enter, establishing a model that could theoretically be applied to residential homes seeking perfect air quality control.
“if you want it to be perfect perfect you can do things like they do in hospitals so like an operating Suite is always pressurized so that no air from any rooms around it in the hospital gets in there air always goes out from an operating theater so likewise here we can try and do that with a home”
Using free online calculators and tools like the depressurization analysis tool on RedCalc and ASHRAE 62.2 calculations can help homeowners and builders understand the real-world requirements and constraints of ventilation system design before making equipment or design decisions.
“learn to use these tools these calculators that are online that are free really amazing um please do make sure the comment ask questions below if you have them like And subscribe tune in next time”
ASHRAE 62.2 standard provides the calculation methodology for determining required ventilation rates based on building characteristics and occupancy, with newer versions (2013-2016) providing more current calculations than older standards.
“let's imagine that if you use the Erv calculation which is ashrae 62.2 which you can do on this red calc website that I'm looking at right here you'd use any of these ashrae 62.2s depending on what time frame you want to be if you want a more modern calculation you use the 2013-2016”
Ventilating dehumidifiers (like the unit discussed) that draw air from both outside and inside the home through separate ducts of different sizes can provide a lower-cost alternative for supply ventilation by mixing outdoor air with indoor air in controlled proportions.
“you can also use something like this this is a vote it's called a ventilating dehumidifier that outdoor uh a hole right here this collar is meant to go Outdoors this one is meant to come inside”
A 10-inch round duct (approximately 78 square inches) connected to a 6-inch round duct (approximately 25 square inches) creates a 3:1 mixing ratio, allowing a ventilating dehumidifier to proportion outdoor supply air versus indoor recirculation based on duct diameter.
“this is a 10 inch round duct that is 78 square inches roughly this is a six inch round duct that is approximately 25 square inches it's 28 but what we're going to use a three to one so we've got 75 square inches set and 25 square inches that's a three parts air in the big color one part error in the small collar”
A specific ERV model with 150 CFM total capacity operating with EC motors can be used as a reference point for calculating whether unbalanced ventilation is feasible given typical home exhaust requirements and depressurization analysis.
“this particular Erv that we're looking at right here blows 150 CFM total”