Is it necessary to have passive ventilation, i.e., heat recovery in the house since 2021?
There is no single regulation that forces people to have heat recovery ventilation (HRV) in their homes. So why do many architects claim that HRV is indispensable?
The requirement comes from calculations. According to the energy class A0 for nearly zero-energy homes for 2021 and beyond, three main components of a building's energy performance are assessed:
- Thermal insulation properties of the building envelope
- Thermal loss and gains of the home
- Preparation of heat and production from renewable sources
Thermal insulation properties of the envelope
...have been gradually improving since 1960. Even the first prefabricated houses were insulated. They had so-called sandwich walls, where the middle layer formed thermal insulation concrete. The initiative for insulation in those years was largely driven by the desire to be independent of energy supplies from neighboring countries as well as ecology = the smell from numerous chimneys.
Later came the year 2010 when we began to take insulation more seriously. Since then, we have been regularly preparing the market for products, construction details, and craftsmanship to deliver the currently required A0 homes. The weakest link is windows, which need to be selected responsibly. Any architect or heating engineer will gladly advise on the final decision.
It should be said that a high-quality thermal envelope of a building is advantageous for its inhabitants because it reduces the number of critical points of the envelope (thermal bridge) and therefore mold. It reduces the need for heat and thus eliminates excessively dry air and the amount of dust particles clogging the lungs, carrying mites and viruses.
Like everything else, the thermal insulation envelope should not be exaggerated. From a certain thickness, adding insulation thickness has an impact on savings. For facade walls, we reached a thickness of 600mm of polystyrene, where theoretically it takes half a year for heat to escape from the structure (with total tightness of the house).
Thermal loss and gain, forced ventilation
...of the building is the heat that leaves the house through the above-mentioned building envelope. During the calculation, insulation thicknesses are modified, and possibilities for meaningful savings in details are sought. This is where one of the most important tasks of the heating designer comes into play, saving construction costs and house usage. Heat is also lost through ventilation. When adhering to the minimum air exchange n = 0.50, i.e., 50% of the air volume in the house per hour, we reach a thermal loss of 10-15 kWh/m2 * year. When we have to meet the standard and that maximum thermal loss of 20.40 kWh/m2 * year, we have very little left for transitions through the thermal envelope of the object. That is why architects will collectively reach for HRV with an efficiency of 85-90% (85-95% of the heat is recovered from the "dirty" air back when heating fresh air).
In earth-sheltered houses, we use the ground for this heat recovery and passive ventilation for ventilation. However, passive ventilation can only recover 10-15% of the heat, and the standard does not recognize heat storage in the ground in the green roof. Therefore, we choose to assess earth-sheltered houses according to the Passive House Planning Package (PHPP), which corresponds much more accurately to the measured values. In PHPP, it turns out that HRV helps only minimally. The PHPP calculation program captures reality with a high degree of accuracy. That is why we do not use HRV. Officially, however, STN and ČTN must be used for assessment, where HRV is indispensable. Gradually, we also get to research, i.e., verifying reality with universities.
In addition to heat losses, a house also has internal heat gains that need to be taken into account. This includes computers, stoves, hotplates, the expected number of people in the house, and refrigerators. Windows are factored into external heat gains, meaning the amount of heat gained from the sun through the window, like the cheapest solar panel. It should be noted that south-facing windows have greater heat gains than losses throughout the year. I consider windows with parameters for passive houses, i.e., Uw = 0.85 W/m2K (this applies from Uw = 1.10).
The big question concerns heat gains due to savings on insulation and heating elements, as well as preventing overheating in the summer. Overheating in summer will be the most common question for heating specialists as the climate changes. However, regulations do not require such an assessment for residential buildings. It is solely the responsibility of the designer and the willingness of investors to invest resources in a comprehensive solution for the house.
Heat generation and production from renewable sources, off-grid living
Here comes the alchemy of the heating designer. They must choose the fewest products that together provide a meaningful production of heat needed in the house. Products and devices must work together reliably, i.e., a foolproof solution. Naturally, energy production from the almost infinite energy of the sun is involved.
The smallest combination comes from three elements. System I:
- Radiant panels
- Stove with an exchanger for heating domestic hot water (DHW) and space heating
- Photovoltaics
An alternative is a compact unit that integrates all parts: System II:
- House heating - heat distribution
- Heat recovery
DHW heating and combined with photovoltaic panels or a ground-source heat pump. An air-source heat pump must be combined with preheating in cold months. - Supplementary stove - option
From the offered options, we undoubtedly choose simpler, smaller products. The reasons are:
- Simpler products are cheaper to install.
- Simpler products are more reliable and have a longer lifespan, even with partial dysfunction.
- Repairs are easier with a simpler system, separated parts. You only replace a small part or section with a new one. Repair is, of course, much simpler and distributed over time.
- The possibility of living off-grid, i.e., self-sufficiently
Off-grid and heat pump
Thanks to the very low heat demand for heating a short house, the client can choose the type of heating. Heat pumps and heat recovery are designed for very low consumption, but continuous. A switched-on heat pump consumes about 20 kWh/day in winter for an A1 house and about 10 kWh/day for an A0 house. Heat recovery consumes an additional 1 kWh/day. Household consumption (PC, cooking, laundry, Christmas lights) constitutes 8 kWh/day in winter. Heat from wood (wood as a battery) is easily produced and requires 1-1.5 cubic meters per year. However, there is a significant difference between consuming 8 kWh/day and consuming 19-29 kWh/day. Heat recovery combined with a heat pump would require more than twice the battery capacity and photovoltaics for an off-grid system. Therefore, we recommend a simpler heating system without heat recovery.
The installation of a 12-16 kWp system vs. a 29-38 kWp system is a significant difference in cost when living off-grid.
Example of Energy Consumption in a House
- Washing machine: 0.30 kWh/day
- Cooking: 1-2 kWh/day (cooked food - meat or cake)
- Lighting: 0.20 kWh/day
- Refrigerator: 0.40 kWh/day
- Electric kettle / to-go: 0.2 kWh/day (3 coffees / 1L tea per day)
Conclusion
Heat recovery systems are meaningful and necessary for most homes. However, it is essential to consider when to use them and when to bypass them. In the case of our short house, we choose to bypass the system because our clients' experiences suggest that the heat recovery system is not cost-effective. The ground accumulator around and under the house ensures the required thermal stability, and passive ventilation provides fresh air.