MVHR, Range Hoods & Airtight Homes: How to Keep Your Kitchen (and Fire) Happy
Designing an airtight home with mechanical ventilation and a functioning kitchen shouldn’t feel like trying to negotiate a peace treaty between your range hood, your MVHR and your wood burner.
Yet here we are.
This article is for architects, builders and homeowners who are trying to make sense of MVHR (mechanical ventilation with heat recovery), range hoods (kitchen extract) and make-up air in modern airtight homes – especially in New Zealand, where we also have to play nicely with the Healthy Homes ventilation standard and the local love affair with fireplaces. (Tenancy Services)
No magic boxes. No “just crack a window, bro” hand-waving. Just clear trade-offs, real numbers and a few polite reality checks.
If you’re ready to check how your project tracks along? go straight to the calculator here:
Why MVHR Doesn’t Magically Fix Your Cooking Smells
What MVHR is actually designed to do (and what it isn’t)
An MVHR system is absolutely brilliant at:
Providing continuous background ventilation
Recovering heat from exhaust air
Keeping indoor air quality and humidity in a sensible range
Making airtight homes habitable instead of stuffy boxes
What it is not designed to do:
Sit directly above your hob
Slurp up frying oil, wok steam and curry vapour in one go
Replace a proper kitchen extract system
MVHR works with moderate airflows spread out over time and space. Your cooking does not.
Background ventilation vs “I just fried fish” events
Think of MVHR as the quiet, reliable staff member who keeps the office running.
Your range hood is the crisis-management team.
MVHR: 0.3–0.5 air changes per hour, all day, every day
Range hood: 200–600 m³/h, short bursts, right where the steam or smell is
So yes, your MVHR helps with moisture and smells overall, but it’s simply not built to catch everything exploding off a hot frying pan in real time.
Why your shiny new MVHR won’t replace a proper range hood
Every so often someone says:
“We’re putting in MVHR, so we don’t really need a range hood, right?”
No. That’s not how any of this works.
Even New Zealand’s Healthy Homes Standards are very clear: a kitchen must have an extractor fan or range hood that vents to the outside. Recirculating fans that dump air back into the room do not meet the ventilation standard on their own. (Tenancy Services)
MVHR is there to keep the whole house healthy. The range hood is there to stop tonight’s dinner from becoming tomorrow’s house smell.
Do You Still Need a Range Hood in an Airtight, MVHR-Ventilated Home?
The honest answer: yes, here’s why
Short version:
If you cook anything more ambitious than toast, yes, you still need a range hood.
Reasons:
Local extraction right at the source is still the most effective way to deal with grease, steam and odours
MVHR extract points are usually in the ceiling or nearby – not over the pan
Regulations expect proper kitchen extract in most real-world scenarios
Clients expect to be able to fry fish without smelling it in the bedrooms for two days
Range hood types: ducted vs recirculating (and what actually works)
Two main options:
Ducted range hood (vented to outside)
Best for odour and moisture control
Complies with NZ Healthy Homes ventilation requirements when sized and ducted correctly (myRent.co.nz)
Needs a decent duct run, backdraft damper and sensible termination
Recirculating hood (charcoal filter, no external duct)
Popular in very airtight Passive House-style homes in Europe
Reduces some smells and grease, but doesn’t actually remove moisture (which can help keeping humidity inside in check during colder seasons - which make the interior air drier)
Doesn’t count as an extractor that “vents to outside” under Healthy Homes, unless modified
Recirculating hoods can work in some owner-occupied, all-electric, super-airtight homes – but if you’re designing for New Zealand rentals or mainstream clients, a proper ducted range hood is the safer, more robust choice.
Why “smell control” is different from “ventilation rates on a datasheet”
Designers love numbers. (I say this lovingly – I am one.)
But:
Meeting a whole-house ventilation rate on paper
≠Stopping a strong stir-fry smell travelling down the hallway
Good kitchen design is about:
Capture efficiency (actually catching the plume)
Enough airflow at the hood
Not dumping that airflow straight into the roof space
Having somewhere sensible for the makeup air to come from
Which neatly brings us to…
Makeup Air: Where Does All That Extracted Air Come From?
What happens when the fan sucks and the house doesn’t leak
In the old days, houses were leaky enough that no one worried about this. The range hood just pulled air through all the gaps and called it a day.
In a modern airtight home:
The hood wants to remove, say, 300–600 m³/h of air
The building envelope is not keen on giving that up
Something has to give: pressure, leaks, or your fireplace flue
If you don’t design for makeup air, the system will find its own path – which may be:
Down the chimney
Through trickle vents and door undercuts
Or through every tiny gap you swore you sealed
How much air your range hood is really moving
Let’s say you’ve got:
120 m² house
2.4 m ceilings
Volume = 288 m³
Range hood at 300 m³/h on boost
That’s roughly 1 air change per hour just from the hood while it’s running.
At 600 m³/h, you’re at about 2 air changes per hour – in a house that might be tested down at 1.0 ACH@50 Pa or better. In other words, you’re asking a very tight envelope to leak like an old villa for the duration of cooking.
Why short, sharp bursts of extraction are usually fine
The good news: most people don’t cook on “boost” for hours.
Typical pattern:
Turn on hood
Smash out dinner in 10–15 minutes
Turn it down or off
If you extract 300 m³/h for 10 minutes, that’s:
300 × (10/60) = 50 m³
Which is about 17% of the house volume
In a house without a fire and with halfway decent operable windows, this is totally manageable as long as you give the air a civilised way back in.
When “just open a window” is good enough – and when it isn’t
Practical rule of thumb:
No fireplace / wood burner / open-flued gas
modest hood (≤ 300 m³/h)
→ “Open a nearby window when you hit boost” is generally acceptable.
Big hood (400–600+ m³/h)
very airtight home
combustion appliance in the same space
→ You’re in “design proper makeup air” territory, not “hope and pray” territory.
Fireplaces, Wood Burners and Backdrafting: The Bit No One Likes Talking About
Why big range hoods and open fireplaces don’t play nicely together
Imagine you’re a fireplace flue.
You’re used to warm air floating up you happily. Life is good.
Now someone turns on a 600 m³/h range hood in the same open-plan space and seals all the windows. The room pressure drops. Suddenly it’s easier for air to be pulled down your flue than in through the rest of the envelope.
Congratulations: you now have backdrafting.
Negative pressure, backdrafting and the not-so-fun side of “cosy”
Backdrafting can drag:
Smoke
Fine ash
Carbon monoxide
General nastiness
back into the room instead of venting it outside. Guidance around combustion safety tends to get twitchy once room depressurisation goes beyond a few Pascals with a flame appliance running, especially in airtight homes. (JSTOR)
If your ventilation design never even looks at that risk, they’re not doing their job.
Simple safety rules if you have both a hood and a fire
If there’s a wood burner or open-flued gas appliance in the same pressure zone as your kitchen:
Don’t rely on “she’ll be right”
Limit hood flow if you’re not providing proper makeup air
Prefer room-sealed appliances for new builds
At the very least, educate the client that big hood + closed windows + roaring fire is not a fun science experiment
When you should be thinking about pressure sensors and interlocks
If you want to be thorough (and you should), consider:
Pressure switches that cut or throttle the range hood if room pressure goes below a safe threshold
Interlocks that stop the hood running on full chat when the fire is in use
Controls that force a makeup air damper open whenever the hood is on a high setting
It sounds fancy. It’s less fancy than dealing with a smoky living room and an insurance argument.
Separate Kitchen Extracts and Airtight Dampers: When They Actually Make Sense
What a dedicated kitchen extract with airtight damper looks like in practice
A separate kitchen extract with an airtight damper usually means:
You still have a ducted range hood to outside
You add a dedicated makeup air duct from outside into the same space
That makeup air duct has a motorised or pressure-controlled airtight damper
The damper opens when the hood runs, closes when it stops
So instead of dragging air through random cracks (or down the flue), you:
Give the air a clear, low-resistance path in
Control where it appears (not as a draught across someone’s ankles)
Keep the house pressure closer to neutral
Pros, cons and costs compared to “just use the MVHR”
Pros
Much safer with fires and high-flow hoods
Reduces the risk of backdrafting
Allows you to actually use the hood at the flow rate it was tested at
Cons
More ductwork, more hardware, more commissioning
Needs decent detailing to keep the damper genuinely airtight when closed
Slightly more to design, more for the electrician to wire and more for the installer to get wrong if not supervised
When a separate extract is overkill (and when it’s a very good idea)
Overkill for:
Small to medium all-electric homes
Moderate range hoods
Reasonable airtightness and clients who are happy to open a window
Very good idea for:
High-end, ultra-airtight homes
Big statement kitchens with serious extract rates
Any project with solid-fuel fires or open-flued appliances in the same zone
Three Common Design Approaches (and Where They Break)
Option 1: Classic ducted hood + MVHR extract nearby
This is the default that works for most projects:
Ducted range hood over the hob
MVHR extract valve in or near the kitchen (not above the hob)
Possibly a boost mode on MVHR for general moisture removal
Where it breaks:
When someone specifies a monster hood and forgets makeup air
When the duct is bodged, full of kinks or vented into the roof space
When no one tells the client they should open a window on boost if there’s no dedicated makeup air
Option 2: Recirculating range hood + MVHR extract only (the Passive House favourite)
This is common in certified Passive Houses and ultra-airtight homes overseas.
Recirculating hood captures grease and some odours
MVHR extract does all the actual air removal
Great for pressure balance and energy efficiency
Where it breaks:
Clients who cook a lot of aromatic food can feel underwhelmed
In New Zealand rental stock, it won’t meet Healthy Homes ventilation rules on its own (Tenancy Services)
It relies entirely on filter maintenance, which… let’s be honest, is not the average household’s strong point
Option 3: Ducted hood + dedicated makeup air with airtight damper
The grown-up solution for big hoods in very airtight homes.
Where it breaks:
When it exists only on the drawing and no one checks it was actually installed
When the damper is uninsulated and drips condensation
When controls are not commissioned and the client never learns how it works
Real-world trade-offs: noise, smells, safety and client expectations
In the real world you’re balancing:
Noise (clients hate loud hoods)
Smell control (clients hate fish-smell bedrooms more)
Safety (don’t cause backdrafting)
Energy efficiency (don’t undo the whole point of airtightness)
Cost and complexity (no one wants a spaceship control panel to boil pasta)
Your job is to pick the simplest approach that actually works for the specific house, not the flashiest diagram.
Worked Examples: How to Sanity-Check Your Own Design
Example 1: Normal cook, modest hood, reasonably airtight house
Take our earlier example:
120 m², 2.4 m ceilings → 288 m³ volume
Airtightness around 1.0 ACH@50 Pa
Ducted hood at 300 m³/h
MVHR ticking along at ~0.4–0.5 ACH
For a typical household cooking 10–20 minutes on boost with a nearby openable window, this is a no-drama situation:
House pressure doesn’t go crazy
Smells go out
MVHR keeps background air quality even
Example 2: Big hood + very airtight home + wood burner
Now change just a few things:
Same house volume
Airtightness closer to Passive House levels
Hood upgraded to 600 m³/h
Wood burner installed in the same open-plan space
Now:
The hood can easily out-pull what the envelope will cough up
The flue can become a makeup air path
You’re at realistic risk of backdrafting, especially in calm weather
Here, “open a window and hope for the best” is not good enough as a design strategy. You’re firmly in dedicated makeup air + controls territory.
How to tell when you’re in the “no drama” zone vs the “test this properly” zone
Red flags that you should stop guessing and actually test pressure and flows on site:
Very low airtightness numbers (the good kind)
High advertised hood flow rates
Combustion appliances in the same pressure zone
Clients who love serious cooking and will actually use the high settings
This is where commissioning and a blower door test become your friends, not just a compliance tick-box.
Designer Checklist: MVHR, Range Hoods and Makeup Air in One Page
Step 1: Get your numbers – volume, airtightness, flows
Do not design this stuff on vibes.
You need:
House volume
Target or tested airtightness
Range hood flow rates
MVHR supply/extract flows
Presence and type of fires / gas appliances
Step 2: Pick a kitchen extract concept on purpose (not by accident)
Actively choose one:
Ducted hood + MVHR extract nearby
Recirculating hood + MVHR extract only (special cases)
Ducted hood + dedicated makeup air
If you can’t explain why you chose it in one sentence, you probably haven’t chosen.
Step 3: Quick risk filter for fireplaces and open-flued appliances
If there’s a wood burner or open-flued gas appliance in the same zone:
Assume you need proper makeup air
Prefer room-sealed appliances for new builds
Consider pressure-based interlocks
Step 4: Choosing the right makeup air strategy for your project
Low risk: window nearby + client education
Medium risk: trickle vent / louvre plus good envelope detailing
High risk: dedicated makeup air duct with airtight damper and proper controls
Step 5: Setting up the MVHR so it helps (and doesn’t fight the hood)
Keep MVHR balanced in normal operation
Use boost modes for bathrooms and kitchen area
Don’t unbalance the MVHR wildly to “feed” a giant hood – it’s inefficient and doesn’t really solve the problem
Step 6: Controls, interlocks and what to tell the client at handover
At handover, the client should know:
What the hood does
When to use boost
When to open a window (if that’s your strategy)
What the makeup air damper is and when it operates
How all this interacts with their fire, if they have one
If the only instruction is “press that button, it’ll be fine”, expect trouble.
What This Looks Like on a Real Job
Where we actually put terminals, ducts and dampers
In practice, good layouts:
Keep the hood duct as short and straight as possible
Avoid venting into soffits that blow straight back into open windows
Put makeup air where it doesn’t create cold draughts across the floor
Keep MVHR terminals away from the immediate cooking plume
How we explain “boost mode” and makeup air to clients
Clients don’t need a lecture on Pascals.
They do need:
“If you’re really going for it in the kitchen, hit boost and crack this window.”
“If you see this light, the hood is throttling because the fire is on.”
“No, it’s not broken; yes, it’s doing that on purpose.”
The point where “nice theory” meets “installer with a holesaw”
Most good designs die in the last 10%.
That’s where the:
Duct gets squeezed through the wrong joist bay
Damper ends up in the wrong place
Termination is swapped on site for whatever was in the van
So the real game is detail + supervision. Draw it clearly, then check it actually happened.
When NOT to Over-Engineer the Kitchen Extract
Projects where a decent ducted hood and a window is all you need
Not every house needs a PhD-level ventilation strategy.
If you have:
All-electric cooking
No fire in the same space
Reasonable airtightness but not Passive House
A modest ducted hood and a decent operable window
…then a simple “hood + window + MVHR background” setup is absolutely fine.
How to avoid gold-plating the ventilation on a tight budget
On tighter budgets, the priorities are:
Continuous background ventilation (MVHR or equivalent)
A compliant, ducted kitchen extract
Good envelope detailing to avoid hidden moisture problems
Gold-plated controls, pressure sensors and fancy dampers are nice, but not mandatory in every three-bed in Southland.
Red flags that you’re designing for the brochure, not the building
If your design decisions are mainly driven by:
“But it looks great on the brochure”
“The supplier says this is what Europe does now”
“The client really liked that render”
…pause and ask what the building will actually do on a wet Tuesday when someone lights the fire and cooks dinner.
Why BEO Is Fussy About This Stuff
After 15+ years testing buildings, we’ve seen too many “perfect” homes misbehave as soon as the fan goes on
After 15+ years doing this and testing hundreds of homes that look flawless on paper, I can tell you: most ventilation problems don’t show up in the drawings – they show up the first evening someone turns a fan on and starts cooking.
Why we care more about pressure, safety and comfort than brand names
We’re not here to push a particular brand of MVHR or range hood.
We are here to make sure:
The building behaves
The fire doesn’t backdraft
The kitchen doesn’t smell like last week’s dinner
The mechanical systems actually support the envelope you’ve just carefully detailed
What you’ll get from us: straight answers, not magic boxes
If you want help on a project, you’ll get:
Plain English explanations
Numbers where they actually matter
Clear recommendations on when you need something fancy and when you really don’t
