The heat recovery unit and how it works to save energy
A heat recovery It is an equipment that allows to recover part of the energy of the air conditioned air inside a room or premises, through the mechanical ventilation system of said air, by means of a exchanger that puts the interior air that is extracted in contact with that of the exterior that is introduced, without mixing the air from the two circuits.
In winter it works by heating the cold air that comes in from outside, while in summer it allows cooling the hot air from outside, also having filters that reduce the level of pollutants and considerably improve the quality of said air.
As required by current regulations, any home or commercial premises requires a ventilation system to renew the indoor air to the outside and simultaneously take air from the outside, allowing to maintain the quality of the indoor air, so that incorporating this equipment we will reduce energy consumption considerably.
It is normally presented as a box with openings and filters that allow this exchange to be carried out and are placed in the mechanical ventilation units that incorporate the supply and return fans of the home or premises.
These equipments allow to recover up to 60% of the heat that would be lost in a mechanical ventilation system in which the intake and extraction air flows are independent, allowing energy savings that can reach over 40% of the consumption in air conditioning equipment.
There are three types of exchangers, the cross-flow exchangers that reach an efficiency of between 50% and 85%, the parallel flow exchangers with an efficiency of approximately 90% and the rotary flow exchangers, which have a rotor or mass that accumulates heat. , a motor and a housing and have an efficiency of approximately 70%:
Cross Flow Exchanger:
In heat exchangers the fluids can circulate in directions orthogonal to each other, so that the hot flow and the cold flow intersect (cross flow). The cross flow is classified into mixed (between the fluids it flows freely in a direction orthogonal to the other without limitations) and unmixed (plates are arranged to guide the flow between the fluids).
Parallel Flow Exchanger:
Some advantages of heat recovery units:
- Improved energy efficiency.
- Heat recovery from mechanical ventilation systems and energy saving.
- Energy and economic saving.
- They can work by cooling the air that enters the interior in summer or heating it in winter.
- It has an economic price that allows to amortize the cost of the investment in a short time, requiring an economic study to analyze its viability.
- They allow its implementation with highly energy efficient equipment and together with renewable energies, such as geothermal with Canadian wells.
Efficiency of heat recovery equipment:
It has fans that allow the flow and circulation of air both to take it from outside and to expel that from inside (both in the impulsion flow and in the return flow), which implies energy consumption and its efficiency should be evaluated based on the air flow rate and the difference in temperature between the outside and inside air.
- The higher the flow, the lower the efficiency of the recuperator.
- The greater the thermal gradient or temperature difference between the outdoor and indoor air, the greater its efficiency.
Applicable regulations for heat recovery units:
Residential home use: Technical Code and DB HS 3.
The technical code within the Basic health document DB HS, indoor air quality, in section 3 which specifies the conditions of the ventilation systems for homes, the provision of a hybrid or mechanical ventilation system is established as mandatory to renew the indoor air.
If we consider a hybrid type ventilation system in which the intake openings will be in direct contact with the outside (Section 3.1.1 point d, of the DB HS-3), we could not consider this heat recovery system since the intake is not channeled by grille and ducts.
The technical code does not require the installation of this equipment in a home mechanical ventilation system, but we can reduce energy consumption by inserting a recuperator between the intake and extraction systems, being able to achieve energy savings of up to 40% , according to the following image.
Tertiary use: RITE Commercial Premises.
In this case, the indoor air quality is regulated in the RITE, which establishes that for the different categories of indoor air (IDA 1, IDA 2 OR IDA 3), and is regulated when it is necessary to place this equipment.
It is required to have a heat recovery unit, according to Technical Instruction IT 1.2.4.5.2: "Heat recovery from exhaust air", in the air conditioning systems of buildings in which the flow of air expelled to the outside, by mechanical means, is greater than 0.5 m3/ s.
Application case of heat recovery systems with renewable energies
As it is evident to incorporate heat recovery units together with renewable energy systems They will significantly improve the reduction of energy consumption and energy efficiency, forming part of the design strategies of passive or high energy efficiency buildings, we put as cases of their application:
Canadian Well Ventilation and Heat Recovery:
In this case, the air that enters the house is already preheated in winter or pre-cooled in summer, coming from the buried duct circuit of the Canadian well, in this way the air that enters in winter is less cold and that that enters in summer is less hot, making it easier to achieve comfort conditions, as can be seen in the following image: (Right image, summer situation and left, winter)
Another application case would be in systems of energy-efficient air conditioning with heat pumps and air pretreatment through conduits in Canadian wells with heat recovery units, in which these heat exchangers allow for improvement and greater use of energy.
Links of interest:
- New energy efficiency measures in products from the EU
- Economic feasibility of an energy efficient building
- Saving energy and improving energy efficiency in buildings
- Sustainable buildings for non-residential use
- Sustainable development example
- Resources for the Energy Certificate-
Article prepared by José Luis Morote Salmeron (Technical Architect - Energy Manager - Google plus profile) Access to his website HERE, in collaboration with OVACEN.
