You have finally made your mind up not to settle for gravity ventilation, but install Mechanical Ventilation with Heat Recovery (MVHR) instead. Question: What MVHR or HRS (Heat Recovery System) do I choose? Being an investment for years, this is not an easy decision to make, as a “mistake” can cost you thousands of euros.
The differences between commercially available MVHR units, rather off-puttingly, depend on numerous parameters and variables. We then resolved to select the 5 most important things that you should, in our opinion, consider when choosing a heat recovery unit for Your ventilation system.
First of all, you need to decide on how efficient your HRV should be, in order to limit the range of products you will choose from. Setting the heat recovery rate or the volume of air per hour (m3/h) you need requires the number of residents and rooms in the house. According to EU guidelines, the volume of 20-30 m3/h of filtered airflow per inhabitant per each habitable room must be supplied. And the same volume of stale air must be extracted from wet rooms (bathrooms, toilets and kitchens), bearing in mind the minimum exhaust rates from these rooms specified in relevant regulations.
For instance, a family of four living in a house with an area of 150-200 m2 will look for an MVHR unit with a heat recovery rate of 350 m3/h. Calculating the correct efficiency value is key because if you buy too small MVHR unit for your ventilation system, it will not have the performance to do the job. In other words, no proper air exchange in your house will happen, as it fails to perform the main task of the ventilation system. It is also good practice to pick a sufficient heat recovery rate for your MVHR unit to work with about 70% capacity (or even less) of its maximum rating. This will mean less noise, lower power consumption and, most vitally, reserve ample enough for the purposes of airing or increased demand.
Once you have chosen a unit with the right efficiency rate, you, and what I mean by ‘you’ is the homeowner together with the design agency or installer, need to agree on:
WHERE to install the MVHR unit, e.g. in the attic or in a utility room
HOW MUCH space is available, which will determine the type of installation required: ceiling, floor or wall mounted
TYPE of connectors layout: from the top or from the side
If you are in short supply of space, you will opt for a suspended HRS kit, which is a unit that features a flat structure and is hung directly from the ceiling. Compact-build HRS units are another type of HRS units that you will be able to fit handily into, say, a closet in your laundry room and save space.
What we recommend as a solution for older buildings are wall-mounted HRS kits, especially during energy retrofit in the building, where the installation of a classic mechanical ventilation system with heat recovery is sometimes difficult, if not impossible. The wall-mounted version uses a ductless unit, so you don't have to figure out where to accommodate the heat recovery ductwork. It is to be routed inside the wall in every room of the building. As a disadvantage in comparison with conventional devices, these MVHR units come with lower air filtration levels and heat recovery rates.
This section will have a look at the performance rating of the heat recovery unit. Checking the energy label, which you will find either on the product data sheet or in your MVHR manual, you will see 3 values: heat recovery rate, which we have already discussed, energy efficiency class and noise or, more precisely, sound power level.
As the efficiency rating involves heat recovery and power consumption, this should come as an important fact for us. Similarly to a washing machine or refrigerator, A+ is the best class, then A, B and so on. Most appliances on the market are A rated, but there are a few models that are A+ rated too. Energy efficiency rating is a clear indicator to customers of whether an appliance features efficient performance.
The third rating, acoustic power or the MVHR unit noise level informs you how many decibels the unit “emits” at a distance from its housing (Important: this is not the noise level of the whole system). Needless to say, the lower the value of acoustic power, the better as it works more quietly.
It is worth noting, however, that even few decibels’ difference between one unit and another could mean that one of them is, roughly speaking, half as quiet as the other one. You are not often impressed by 1-2 decibels less or more, but in fact it can help you choose a quiet heat recovery unit.
MVHR units can also be classified according to a type of heat exchanger used in them. An exchanger is an extremely important component in the heat recovery unit, as it impacts the heat recovery rate of the device. Main types of heat exchangers:
Rotary heat exchangers are the earliest types of exchangers used in the ventilation industry. The MVHR unit with a rotary exchanger incorporates, apart from heat recovery, the recovery of part of moisture from the exhaust air. Moisture recovery can be of particular importance in very cold climates where frost or even ice formation on the exchanger could be an issue. Rotary exchanger MVHR systems are common in dry and cold climates where moisture recovery is highly desirable. This is one of the reasons why a heat recovery unit with a rotary exchanger is a very popular choice in Scandinavian countries.
Compared to plate-type exchangers, a “disadvantage” in MVHR units with rotary exchangers lies in their lower recovery rate of 65-85%, as well as a greater rate of mixing of air flows. However, the so-called EATR or exhaust air transfer ratio is gradually reduced with some new developments in the field.
Until a few years ago, rotary (regenerative) heat exchangers were in common use in Poland, but this trend has been changing in favour of the units with cross-flow heat exchangers and counter-flow heat exchangers (i.e. so-called heat recovery/plate heat exchangers). A cross-flow exchanger is a structure in which air streams flow crosswise. They are separated from each other by thin plates that allow the heat to flow and prevent the two streams from mixing.
These exchangers have a heat recovery efficiency in the range of 50-75%. Due to such relatively low recovery efficiency, their use in devices intended for the domestic market and in large MVHR units has continued to decrease. While their simple design and low production cost are an advantage, unfavourably, they lack moisture recovery and there is a risk of exchanger frosting at temperatures below 0˚C.
Counter-flow heat exchangers have a similar design (plates), however the streams additionally flow in opposite directions, which significantly increases heat recovery (over 90%) and makes them the most efficient exchangers available on the ventilation system market.
Enthalpy counter-flow heat exchangers (with moisture recovery) seem to look the same as a typical counter-flow heat exchanger on the surface. They differ from each other by the material which the membranes, which separate the two air streams, are made of. They are made of materials that let water vapour molecules pass through when they stop other gases or contaminants.
Two solutions are available on the market: cellulose and polymer membranes. The first product is relatively cheap but is subject to wear over time. After about 10 years, you may need to replace such an exchanger. The other solution is more expensive, while it is estimated to last for many years.
The benefit of these exchangers is that they maintain relative humidity in rooms at comfortable level during the heating season and their resistance to frosting is higher (defrosting algorithms can be run with lower temperatures as compared to conventional exchangers), which results in better average annual heat recovery. In addition, the enthalpy exchanger recovers the so-called latent heat, which increases its actual efficiency.
Is a climate zone a factor when I choose an MVHR system? Definitely yes. However, because Poland is located in a moderate climatic zone, we have no restrictions as to which type of MVHR unit and which type of exchanger to choose.
As we have already mentioned (see Rotary Heat Exchangers), rotary exchangers are installed more frequently in cold climates due to the risk of exchanger icing at lower temperatures. In the case of a heat recovery unit with cross-flow heat exchangers or counter-flow heat exchangers, the downside is that they can get iced and consequently damaged. This is why the cross-flow heat exchanger unit is fitted with a special system preventing such icing, e.g. it takes the form of an anti-freeze algorithm or an additional heater is installed.
To sum up, we've listed 5 aspects that we consider to be crucial when choosing a heat recovery unit. Is that all? Absolutely not. However, once You'll define the above parameters, we can go into details, such as:
Would you like to know more? Read other articles on heat recovery ventilation: