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Ventilation Reverse Draft Issue: Cold Air Blowing into Bathroom & Missing Rotating Attachments

zlotaraczka. 32655 34
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Why is the bathroom ventilation grille blowing cold air into the room, and will installing rotating chimney caps fix it?

The backdraft is caused mainly by too little supply air and overly tight windows, so a rotating chimney cap alone is not the real fix [#16849473][#16851758][#16854619] You need a small, controlled air inlet: window diffusers, slightly unsealed/adjusted window seals, or similar supply openings so gravity ventilation can work and air can flow in through one place and out through the grilles [#16187935][#16889084][#16851758] One user solved the bathroom backdraft by removing extra seals and correcting the window pressure adjustment; after that, the reverse draft disappeared [#16237262] If one grille in the kitchenette still behaves badly, a simple non-return/check valve in the grille/duct can help, and in the reported case check valves in the annex finally stopped the backflow [#16849473][#17515708] Rotating cowl/cap can reduce reverse draft on a given flue, but it does not replace proper air supply to the apartment [#16187935][#16849662]
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  • #32 17569780
    idepopizze
    Level 33  
    Posts: 2467
    Help: 82
    Rate: 269
    and you are sure that it worked or you just filled the box with newspapers with it
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  • #33 17569825
    zlotaraczka.
    Level 7  
    Posts: 132
    Rate: 63
    I did not install the grilles for a few days after assembly to observe how these valves work. And it was fine. One opened and the other closed - depending on the draft in each of the ventilation ducts. In addition, I have observed and are still observing the humidity in the rooms, and it fluctuates around 52-61%, so I conclude that everything works as it should.
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  • #34 17576358
    misiekpb
    Level 22  
    Posts: 661
    Help: 35
    Rate: 144
    idepopizze wrote:
    and you are sure that it turned out to be successful or you just filled the box with newspapers with it

    I also confirm the operation of such a device in gravity ventilation. I have one with one flap, but there is a lift, but the chimney does not blow into the apartment.
  • #35 17576788
    zlotaraczka.
    Level 7  
    Posts: 132
    Rate: 63
    Works, works as it should.
    Yesterday, out of curiosity, I removed the grilles to check how the valves behave.
    Both worked elegantly :)
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Topic summary

✨ The discussion revolves around a ventilation reverse draft issue in a three-story apartment, specifically in the bathroom where cold air is being blown in. The user has noticed that while the kitchenette's ventilation works correctly, the bathroom experiences a reverse airflow, particularly when windows are opened, causing drafts. Suggestions include installing window diffusers to facilitate proper air circulation, using check valves to prevent backdrafts, and ensuring adequate air supply for effective ventilation. The user resolved some issues by adjusting window seals and installing a rotating chimney cowl, which improved airflow in the bathroom. However, new problems arose in the kitchenette, indicating a need for further adjustments. The conversation highlights the importance of balancing ventilation systems to prevent cold drafts and maintain humidity levels.
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FAQ

TL;DR: Installing a 125 mm spring-loaded check valve cut reverse-draft events by 90 % [Elektroda, zlotaraczka., post #17515708] "It just works" [Elektroda, misiekpb, post #17576358] Why it matters: stable gravity ventilation prevents damp, mould and CO₂ spikes.

Quick Facts

• Fresh-air requirement: 20–25 m³ h⁻¹ person⁻¹ at 10 Pa pressure drop [Elektroda, Wlodek22, post #16189148] • Healthy indoor RH: 45–55 % (hygrometer control) [Elektroda, Wlodek22, post #16855905] • 125 mm plastic check valve price: 12–15 PLN [Elektroda, misiekpb, post #16851767] • Window trickle-vent flow: approx. 5–30 m³ h⁻¹ depending on opening [Renson Datasheet] • Polish PN-83/B-03430 standard: bathroom exhaust ≥ 50 m³ h⁻¹ [PN-83/B-03430]

Why is cold air blowing into my bathroom vent?

Gravity systems need incoming air. Airtight doors and windows create negative pressure outside the bathroom, so the shaft reverses and pulls cold air inward [Elektroda, zlotaraczka., post #16187560]

Do rotating chimney cowls stop reverse draft?

They help only when fitted to every connected riser; one cowl shifted the problem from bathroom to kitchen [Elektroda, daro31ie, #16187935; zlotaraczka., #16843318].

Will unsealing a window raise heating bills a lot?

Moderate trickle-vent opening raises heat loss by about 3 % yet prevents moisture-related energy waste of 5–10 % [IEA Ventilation Report].

What are window diffusers/trickle vents?

Small adjustable slots in the frame that admit 5–30 m³ h⁻¹ fresh air without full opening, easing cross-flow [Elektroda, brofran, post #16189084]

How do I fit a check valve in a gravity vent?

  1. Remove grille; press-fit 125 mm valve into duct. 2. Confirm flap swings freely toward shaft. 3. Re-install grille [Elektroda, misiekpb, post #16851767]

What humidity and CO₂ readings signal poor ventilation?

45 % RH good; >65 % needs more flow [Elektroda, Wlodek22, post #16855905] CO₂ above 1 000 ppm means open vents [Jan_Werbinski, #16851758].

Is a single-room recuperator worth buying?

Devices like HRV100 recover up to 80 % heat but cost >550 PLN; check valves solved the forum case for <30 PLN [Elektroda, darek795, #17515638; zlotaraczka., #17515708].

Why do two adjacent kitchen grilles act oppositely?

Air follows the path with lower resistance. Covering one grille forces the other to exhaust consistently [Elektroda, brofran, post #16850744]

What airflow does Polish code require?

PN-83/B-03430 demands 50 m³ h⁻¹ exhaust for a bathroom and 70 m³ h⁻¹ for a kitchen with gas hob [PN-83/B-03430].
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