Resizing old air vent opening to house smaller vent cover

The air vent size was worked out on original house so diminishing the size could cause airflow issues in the future.If you proceed with smaller openings you will have to form more openings to have same airflow rate

It's great that you are considering the ventilation aspect of your property, as proper airflow is essential for maintaining a healthy indoor environment. I'm glad you received advice against filling in the air vents. Keeping the vents open ensures adequate ventilation, which helps prevent issues like condensation, mold, and stagnant air. Regarding your query about fitting new vents that are smaller than the current openings, it is indeed possible to adjust the openings to accommodate the new vents. However, this process requires careful consideration and proper execution to ensure a seamless integration. Here are some steps to help you with the process: Measure the new vents: Begin by carefully measuring the dimensions of the new vents you intend to install. Take note of the exact sizes and shapes of the vents, which will aid in determining the modifications needed for your existing openings. Assess the existing openings: Examine the current air vent openings to understand their construction and any potential obstacles or structural elements around them. This evaluation will help you determine how much adjustment is necessary. Seek professional advice: I recommend consulting a skilled carpenter or a professional experienced in working with ventilation systems. They can assess your specific situation, provide tailored recommendations, and ensure that the modifications are carried out safely and efficiently. Make adjustments before plastering: If possible, it is generally better to make the necessary adjustments to the openings before the plastering work begins. This way, you can ensure a clean and precise installation of the new vents without damaging the fresh plaster. Ensure proper sealing: Once the new vents are in place, make sure they are adequately sealed to prevent any air leaks. Proper sealing will help maintain the effectiveness of the ventilation system. By addressing the vent modifications before plastering, you can achieve a seamless integration of the new vents into the wall's surface and maintain the functionality of your ventilation system. As with any modifications to your property, it is essential to take a cautious and informed approach. Seeking professional advice and assistance will ensure that the adjustments are done correctly, preserving the integrity of your home's structure.

Stick with the same size get some nice new air vents

as above..... Especially if you have a gas fire or backboiler in the same room