Even the best ventilation unit, such as the heat recovery units in the PremAIR or SlimAIR series, will not reach its maximum efficiency (heat recovery of up to 95%) if the system is installed incorrectly. This article is a technical checklist for installers and property owners, highlighting the most common workmanship errors in HVAC systems.
Still at the planning stage?
If you are looking for the basics of how the system works, what affects its cost and how it is installed, start with our article: What is MVHR? A guide to mechanical ventilation with heat recovery »
An incorrectly installed ventilation system causes three main problems that drastically reduce comfort in the building:
Installing a system "by eye" is the quickest route to failure. The right duct diameters (e.g. for SPIRAL System circular ducts) and the air balance must come from accurate calculations, not from the installer's intuition. A professional design helps you avoid clashes with other services (plumbing, electrics) and optimises material costs.
The right approach: The design must include an air balance table for each zone (supply vs extract).
Unbalanced airflows (e.g. supply > extract) lead either to positive pressure, which forces moisture into the walls of the building, or to negative pressure, which can draw unpleasant odours in from the drains.
A common mistake is to fit the outdoor air intake too close to the exhaust outlet (causing a "short circuit" between the airflows) or in a place exposed to pollution.
Also remember to choose terminals one size larger than the main duct. This reduces flow resistance and, as a result, the noise on the building's facade.
An MVHR unit is a precision device. Although modern units such as the HRU-PremAIR have excellent internal insulation (e.g. an EPP casing), this is not enough for continuous operation at sub-zero temperatures without additional protection.
Rule: The unit should operate in a room where the temperature stays above 5°C. If you have to install it in an uninsulated loft, build a "warm enclosure" (an insulated cabinet) around it. Otherwise, moisture will condense on the casing and the electronics, which can lead to failure.
This is the most common cause of "dripping ceilings". Every section of duct that runs through a cold zone (an unheated loft) must be insulated with mineral wool at least 50 mm thick, or made from an innovative pre-insulated system such as EPP foam ducts (expanded polypropylene, 15 mm or 43 mm thick).
Critical points:
Flexible ducts ("concertina" ducting) have their uses, but they must not form the backbone of the entire system. Their corrugated structure creates very high flow resistance (up to three times higher than a smooth duct) and makes cleaning difficult.
Recommendation: Use traditional flexible ducts only to connect the air valves (max. 1–1.5 m), where they also act as silencers. Run the main ducts in rigid ducting (SPIRAL System) or in dedicated radial distribution systems with semi-rigid ducts, such as FLX System (PE-Flex), which are ideal for concealing in screeds and ceilings.
Boxing in plenum boxes, dampers or the unit itself with plasterboard without leaving access hatches is a mistake that will come back to haunt you at the first system clean (usually after 5–7 years). The system must be fully serviceable!
Starting up the MVHR unit "on factory settings" is not enough. Every ceiling air valve (supply and extract point) must be precisely adjusted in line with the design.
Procedure: The installer must use an anemometer to measure the airflow in each room and adjust the valve discs or airflow regulators (e.g. RAVAV). The measurement report (commissioning record) is the basis for the warranty and for the correct operation of the system.
Every leak in an extract duct risks drawing in dirty air from service shafts or ceiling voids. A leak on the supply side means a loss of pressure – fresh air will not reach the furthest bedrooms.
Use fittings with factory-fitted EPDM seals (providing airtightness class D) or dedicated sealing tapes (e.g. reinforced aluminium tape). Alnor spiral duct systems ensure the highest level of airtightness when installed correctly.
Strictly prohibited: Never connect the mechanical ventilation of the house to the ventilation of the garage or boiler room!
These rooms should keep their own independent natural ventilation in line with local regulations, and airtight doors should separate them from the living area served by the MVHR unit.
Without a drip profile, condensate from the exhaust outlet or driving rain on the outdoor air intake will run straight down the facade, leaving unsightly stains that are hard to remove and, over time, encouraging the growth of algae and fungi.
Electric duct heaters (e.g. HDE models) reach high temperatures. If the air is not filtered first, dust settling on the heating elements will start to burn (causing the typical "burnt dust smell") and, in extreme cases, can even cause a fire.
We recommend suitable filter boxes or advanced anti-smog filters (e.g. MOCarz), which protect both the heater and the heat exchanger in the MVHR unit.
Noise is the most common complaint from users of cheap and poorly designed systems. The fan in the unit always generates sound. The installer's job is to attenuate it BEFORE it reaches the ducts that distribute air around the house.
Solution: Dedicated duct silencers (e.g. rigid SIL silencers) must be installed directly after the MVHR unit on the main supply and extract ducts.
In a house with an MVHR system, the windows must be completely airtight. Leaving trickle vents in place (they are only needed with natural ventilation or extract-only systems) drastically upsets the pressure balance and causes uncontrolled cooling of the building. They should be permanently sealed, or windows without trickle vents should be ordered at the construction stage.
Avoid costly mistakes right from the planning stage of your mechanical ventilation system. Talk your project through with Alnor's experienced engineers. We will help you choose the right ventilation and heat recovery units, such as the advanced PremAIR models or the compact SlimAIR units, and select the necessary accessories, including HRQ-2ZONE zoning distributors and CO2 sensors.
We also encourage you to use our in-house design tools, such as AlnorSELECT, which makes it much easier to select the right components for an HVAC system. You can reach us by e-mail at export@alnor.com.pl or by phone on +48 22 737 40 00.
Find out more in our guide to mechanical ventilation with heat recovery. Take a look at our other articles too: