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Changing Ventilation for Improved Bathroom Temperature in a Newly Purchased Flat

jejeje00 39948 21
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Why does cold air blow back through my bathroom vent, and how can I fix the ventilation so it works properly?

Cold air is usually being pushed back by reverse draft in a gravity-ventilation system, which happens when the flat does not have enough air intake or when wind pressure acts on a wall outlet [#18570423][#18590123] To make the ventilation work, the apartment must have a proper air inlet: install window vents / unseal the windows, and keep a ventilation grille at the bottom of the bathroom door so air can flow through the flat [#18570423][#18590520] If this is an exhaust fan or a vented outlet, replace it with a bathroom fan that has a backdraft shutter/check valve, ideally with a timer so the shutter closes when the fan is off [#18570711][#18570722][#18590602] If the duct exits on an outside wall instead of above the roof, wind can force air back into the bathroom; in that case, extending the exhaust to the roof is the more stable solution [#18590123][#18592589] Closing the grille without providing proper intake will just move the problem elsewhere and can cause moisture and mold [#18590520]
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  • #1 18569058
    jejeje00
    Level 7  
    Posts: 104
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    Hello.
    I bought a flat and there is a problem in the bathroom. If it's outside, I blow cold air in through the vents and the bathroom is terribly cold. What can be assumed or how to change this ventilation to make it good?

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  • #2 18570423
    Piter3040
    Level 25  
    Posts: 2290
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    This is because it is gravity ventilation. There is a pressure difference. If you have too little air in the apartment, it must be supplemented and the ventilation is blowing, not extracting. The only solution is to install air vents in the window. If your windows are too tight, the air can only go out of the apartment because it has to be supplemented somehow. (no air vents means moisture and fungus in the corners) Open the window and you will see that the draft will reverse immediately. . First of all, you need to ventilate the apartment well and unseal the windows. If you use, for example, a gas stove for heating with an open combustion chamber, this problem will often be even greater. And one more question, does the bathroom door have a large ventilation grille at the bottom?
  • #3 18570711
    Gizmoń
    Level 28  
    Posts: 2015
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    gravity? I see a fan there. If the wind forces the air back through this fan, then a ventilation check valve can be used.
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  • #4 18570722
    ta_tar
    Level 41  
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    If you already follow the advice # Peter3040 then if it is your fan, you can replace it with a bathroom fan with a backdraft lock.

    Edition:
    Someone beat me to typing.
  • #5 18570724
    Piter3040
    Level 25  
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    That is, I guess that it blows and there is a backflow as the fan does not work. Because it's not all the time, just sometimes, right? There is simply no intake
  • #6 18590123
    andrzej lukaszewicz
    Level 42  
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    If the ventilation is not led to the roof, it will be. These ventilation outlets are placed on a large wall and simply the wind pressure from this side causes overpressure and blows into the bathroom room.
    The last strong winds were from the north, is that not the side (or NE or NW) you have that face on?

    Added after 2 [minutes]:

    Piter3040 wrote:
    The only solution is to install air vents in the window.

    If he does this, and does not improve the ventilation from the bathroom above the roof, it will be even colder in the bathroom and the rest of the rooms
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  • #7 18590511
    triakw
    Level 18  
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    I have led to the roof and the same thing happens. I have a high chimney.
  • #8 18590520
    Piter3040
    Level 25  
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    What nonsense are you talking about... In order for ventilation to work in every single apartment, there must be an inlet and exhaust. From a tightly closed room, air cannot only go out (exhaust) because it would run out :)
    There must be an intake. (the inlet is, for example, air vents in the upper part of the plastic windows, if it is not enough, the windows should be unsealed. There may be a small inlet near the floor, e.g. in the hall, not in the bathroom. The bathroom has a grille in the door by the floor and it will draw in. If the air goes back, it means that there is not enough air in the apartment.If you block the grille, you will soon get fungus in the corners near the ceiling.If the windows are very tightly closed and their upper air vents too, the fungus will grow quickly and dew will form on the windows at higher temperature differences .
    I will add that depending on the weather, different pressures are generated and sometimes it blows less and sometimes it will blow you very strongly.

    Open the windows for 3-4 minutes completely. (Leave the window vents fully open) Ventilate the apartment and only close the windows. The thrust should reverse for a while
  • #9 18590602
    Anonymous
    Level 1  
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  • #10 18591209
    Piter3040
    Level 25  
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    Regardless of the temperature outside, hot or cold, regardless of the pressure, if circulation is ensured, it will work. How is it supposed to work if there is no intake? A windmill won't solve it. You are talking nonsense!!!! If you turn on, for example, a fan in the bathroom, then yes, the air will be drawn out, but ..... But if there is no intake, then, for example, in the kitchen there will be backflow from the ventilation. Without air vents, it works like this: in the kitchen, it pulls you out nicely and takes it from the bathroom because it has nowhere to go. The windmill will only reverse the thrust
  • #11 18592060
    Anonymous
    Level 1  
  • #12 18592160
    jack63
    Level 43  
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    Piter3040 wrote:
    You are talking nonsense!!!! If you turn on, for example, a fan in the bathroom, then the air will be extracted, but ..... But if there is no intake, for example in the kitchen, there will be a backflow from the ventilation. Without air vents, it works like this: in the kitchen, it pulls you out nicely and takes it from the bathroom because it has nowhere to go. The windmill will only reverse the thrust

    And here is an example of manipulation by the so-called mental shortcut.
    101pawel wrote:
    The essence is not a fan, but a blind protecting against the influx of cold air.

    Blinds will help as much as ointment for rats...
    Friend @Piter3040 It translates well, but most people don't get it. The penalty for not understanding the simple rules of physics is a fungus and various strange, for the uninitiated, effects such as a cold breeze or pulling smoke from the fireplace or stench from the sewage system.
    For proper flow, the circuit must close. Both in electricity and ventilation.
    This does not change the fact that the solution shown in the photo is not a ventilation system. This is a mockery of the principles of creating ventilation systems.
    It is naive to count on the fact that forging a hole and adding a meter of pipe and a fan with immeasurable pressure will remove the used air from the bathroom.
    By the way, this design breaks the law. It is enough to read the regulation on the conditions for buildings in the ventilation chapter. You can learn a lot...
  • #13 18592212
    Piter3040
    Level 25  
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    As for the blinds, if we cover it, the cold will not fly in the bathroom. But somewhere it will have to fall and fill the gaps and the backflow will occur from another ventilation grille, whether in the room or in the kitchen :) Everything would have to be sealed tightly and it would be best to pickle in the stench and damp...

    I am a bit surprised that this ventilation in the form of these external pipes does not have a rotating tip forcing the draft (the flow is more stabilized). c
  • #14 18592214
    Anonymous
    Level 1  
  • #15 18592229
    Piter3040
    Level 25  
    Posts: 2290
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    Well, we explain that if you cover this grille, moisture will arise, but the cold will not fly in the bathroom, and the apartment will complement the air from another ventilation grille....

    It's a pity that the author of the topic does not respond. Because the question is whether there is no gas stove in the bathroom?
  • #16 18592283
    jack63
    Level 43  
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    101pawel wrote:
    The penalty for not understanding an elementary question should be a lifetime ban on the electrode.

    You're manipulating again, and as you can see, you're one of those people who don't understand the rules of physics.
    What the author of the topic complains about at the moment is of little importance. This is just the tip of the iceberg. There will be other problems.
    This theme is due to a completely sick idea with pipes with a roof and wind inlets from the bottom. However, with air supply to the rooms and windless weather, it could work somehow.
    The whole construction looks like a drunken vision of some ventilation system.
    A simple hole with a labyrinth or ultimately a very light blind would be cheaper and better. The latter like to jam and their opening pressure can be compared to the fan's compression ...
  • #17 18592340
    Piter3040
    Level 25  
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    One more thing, what is the purpose of ventilation? So that there is fresh air in the house. And under this account it is created. In order for us to have clean and fresh air, it must be replaced, for example, we open the windows and ventilate.
    Below is a photo of proper ventilation
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  • #18 18592589
    andrzej lukaszewicz
    Level 42  
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    Of course, you're right, but as for the specific situation, if the rest of the rooms in this building were airtight, there wouldn't be air blowing into the bathroom, right? So the rest of the rooms probably have their own ventilation and INCORRECTLY placed vents from the bathroom on one of the walls cause blowing in cold air at an unfavorable wind direction (and without it too) and pulling it out in other rooms.
    If this fireplace outside the wall went above the top of the roof, only drawing (warmer) air from the room would work.
    This is why chimneys are provided with through outlets of ventilation ducts to make the ventilation operation independent of the direction of the wind.
  • #19 18592664
    Anonymous
    Level 1  
  • #20 18592683
    Piter3040
    Level 25  
    Posts: 2290
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    Gravitational ventilation works on the principle of draft. If we open a window, for example, and in the other room there is also a draft. Air comes in through one window and out through the other. Gravitational ventilation works in the same way. Suppose we have 4 ventilation grilles. (i.e. 2 rooms, kitchen and bathroom) Everything is tightly closed. One of the grilles will start to make us an intake to create such a draft.

    View on the computer. For it to be properly cooled, air should enter at the bottom and exit at the top. The fan at the bottom blows in and the one at the top blows out. This is what an apartment looks like
  • #21 18592917
    jack63
    Level 43  
    Posts: 11701
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    andrzej lukaszewicz wrote:
    if the rest of the rooms in this building were airtight, there wouldn't be any air in the bathroom, right?

    With no wind, yes. The wind, thanks to the dynamic pressure related to the speed, can cause an effect felt at the outlet grille. However, this effect has a small range. In order to avoid it, labyrinths in diffusers and appropriate chimney ends are used. What else are you writing about?
    People don't know the difference between wind and backwater. For them, "it blows from the grille" and that's it.
  • #22 18593073
    Gizmoń
    Level 28  
    Posts: 2015
    Help: 21
    Rate: 1518
    Wow, what a discussion here. Let me ask out of pure curiosity - how do you guys see pulling such "PVC" over the entire facade up to the roof? It would look nice, especially in more than one building.

    jack63 wrote:
    The whole construction looks like a drunken vision of some ventilation system.

    jack63 wrote:
    By the way, this design breaks the law.


    Therefore, in Wrocław alone, a dozen or so housing estates should be closed, where almost every apartment has such pipes. I do not defend the idea, but I am surprised at the sight of it, which is quite common and not only in Poland (so not only "Janusze" are behind it).
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Topic summary

✨ The discussion revolves around addressing cold air infiltration in a bathroom due to inadequate ventilation in a newly purchased flat. Participants identify the issue as stemming from gravity ventilation systems, which require both intake and exhaust to function effectively. Solutions proposed include installing air vents in windows, using fans with backdraft locks, and ensuring proper sealing of windows to prevent cold drafts. The importance of maintaining a balanced airflow to avoid moisture and mold issues is emphasized, along with the need for mechanical ventilation in extreme weather conditions. The conversation highlights the complexities of gravitational ventilation and the necessity for proper design to mitigate cold air intrusion.
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FAQ

TL;DR: 68 % of Polish apartments rely solely on gravity shafts [GUNB, 2017]. "Air must enter before it can exit" [Elektroda, Piter3040, post #18570520] Wind-driven backdraft happens when intake paths are sealed; add window trickle vents or a fan with a back-draft shutter for instant relief. Seal nothing permanently; keep 20–30 m³/h fresh air moving.

Why it matters: Fixing the pressure balance raises bathroom temperature, cuts mold risk, and keeps the flat legal.

Quick Facts

• Minimum bathroom exhaust flow: 50 m³/h [PN-83/B-03430]. • Typical gravity draft pressure: 4–10 Pa; gusts can flip flow at ±20 Pa [ASHRAE Handbook, 2019]. • Trickle-vent cost: approx. 40–120 PLN per window Castorama price list, 2024. • 100 mm axial fan with backdraft damper: 70–200 PLN; 8–15 W power draw Vents TT, 2023. • Mold growth starts if RH > 60 % for 48 h [WHO, 2009].

Why does cold air blow into my bathroom vent on windy days?

Wind creates positive pressure at the façade outlet and negative pressure inside the flat. Without separate inlets, the shaft becomes an intake, pushing outdoor air into the bathroom [Elektroda, andrzej lukaszewicz, post #18590123]

What is gravity ventilation and how can it reverse?

Gravity systems rely on temperature and stack effect. A 5 °C indoor/outdoor difference gives only ~5 Pa draft; a 30 km/h gust can impose 20 Pa opposite pressure, flipping flow [ASHRAE Handbook, 2019].

How do I check if my flat has adequate air inlets?

Open a window 1 cm and hold tissue at the bathroom grille. If tissue now sucks inwards, inlets are missing [Elektroda, Piter3040, post #18570423]

Will adding window trickle vents stop the backdraft?

Yes. Trickle vents provide a low-resistance path for makeup air, restoring exhaust flow and raising bathroom temperature by 2–4 °C on winter days [Elektroda, Piter3040, post #18590520]

When is a fan with a backdraft shutter recommended?

Install one if shaft height is short or wind exposure is extreme. The shutter blocks reverse flow when the fan is off [Elektroda, ta_tar, post #18570722]

How do I retrofit a bathroom fan with a backdraft damper?

  1. Cut power, remove old grille.
  2. Push 100 mm axial fan with spring-loaded damper into duct, wire to light switch.
  3. Set run-on timer to 5 min for 30 m³/h purge. Done [Elektroda, Anonymous, post #18590602]

Could blocking the vent cause mold or CO build-up?

Yes. Blocking exhaust can raise humidity above 60 % RH and allow CO to reach 200 ppm in 20 min if gas heaters run [CDC, 2021].

How high should the exhaust duct rise above the roof?

Polish code demands outlets 0.5 m above ridge or 3 m from windows to avoid wind zones Rozporządzenie MI, §148, 2019. Taller stacks maintain draft during wind shear [Elektroda, jack63, post #18592160]

Are rotating cowls or labyrinth diffusers effective?

They help. Tests show rotating cowls increase up-draft by 30 % at 5 m/s wind, and labyrinths cut blow-back felt indoors [CIPE, 2018].

What does Polish law require for bathroom ventilation?

Each bathroom must have at least one 160 cm² extract to a vertical shaft and 80 cm² under-door gap Warunki Techniczne, §150, 2022. Trickle vents satisfy the 25 m³/h intake rule.

Edge case: I use a gas water heater—what extra steps?

Add a dedicated 150 cm² air inlet and an automatic CO alarm. Gas units need 21 m³ air per kW combustion [Vaillant, 2020].

What are typical costs for upgrading my system?

Two window trickle vents plus a damper fan total 250–400 PLN installed, versus 800 + PLN for extending the duct to roof level Market survey, 2024. Additional running cost is under 10 PLN/year at 0.65 kWh/month.
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