Dynamic Real-Time Measurements and a Comparison of Gas and Wood Furnaces in a Dual-Fuel Heating System in Order to Evaluate the Occupants’ Safety and Indoor Air Quality

Abstract

Due to rising energy costs, there is a trend to return to conventional heating systems powered by solid fuel. A rise in the combination of new and old energy sources is creating unintended dual-fuel heating systems. These systems combine an old solid-fuel furnace and a new gas furnace. Usually, the old furnace was meant to be replaced by the new one and their cooperation was never intended when installing the new heating system. The occupants decided to leave the old system in fear of a rise in prices of gas or electricity or temporary problems with their supply. The study focuses on such a system and its influence on indoor air quality and thermal comfort. A series of dynamic measurements with an IoT remote sensor array in a chosen household was conducted to evaluate the behaviour of the system as well as effects on the indoor environment. Sensors measured the CO₂ concentration and thermal profile in a household when using a dual-fuel heating system consisting of an old wood furnace from the 1980s and a recently installed new gas furnace. The results showed that none of the heat sources posed a threat to the occupants. Contaminants were safely removed by the exhaust systems of the furnaces. The thermal comfort, however, was influenced more by the wood furnace where fluctuations in the temperature were noticed, especially during the night. The gas furnace maintained a stable temperature that was more suitable for the occupants.

1. Introduction

With the growing costs of energy and inflation, there has been a shift in the energy portfolio of households. This study focuses on the situation in central and eastern Europe, especially Poland, and investigates how the sources of energy for heating purposes are currently evolving and may change in the years to come.

Before the uncertainties of 2021 and 2022, there was an ongoing trend to receive government help to change old furnaces fueled by coal and wood into gas furnaces or heat pumps. This was due to the strong economic growth and increasing salary among citizens [1]. From 2010 to 2019, Poland’s gross domestic product (GDP) increased by 38% and its economic growth rate was 4.7% in 2019, significantly higher than the European Union (EU) average of 1.5% [1]. However, the COVID-19 outbreak had a major impact on Poland’s economy and energy system and from 2019 to 2020 GDP declined by 2.7%. This slowed investments by individual households and hindered the exchange of old solid fuel burners for new economical ones due to the lack of funds and energy uncertainty.

According to the “Clean Air” (Czyste Powietrze) program, run by the National Fund for Environmental Protection and Water Management (Poland), the number of applications for additional funding to change an old solid fuel furnace into a new one dropped from 132,302 in November 2020 to under 10,000 in November 2022 [2].

The goal of such programs was to improve the outdoor air quality that was among the worst in the world, especially in the south of Poland. However, since 2021, there has been a rise in gas and electricity prices along with an increase in inflation, and a return to solid fuels was observed [2]. In 2021 the share of coal in electricity production again exceeded 70%, while the amount of energy from renewable sources dropped [2,3]. Additionally, hard and brown coal accounted for approximately 45.4% of primary energy carriers [4].

Because of growing inflation and energy prices across Europe, energy poverty has increased and become more and more noticeable [5,6,7]. Energy poverty refers to the lack of access to affordable, reliable, and clean energy sources. It forces people to return to and to rely on unhealthy and dangerous forms of energy, such as kerosene lamps, wood fires, and diesel generators.

Due to the uncertainty of gas supply, increasing gas prices, and an unstable political situation, when investing in new heating systems, people often leave the old one as an alternative or cheaper solution. Such an approach creates an unintended dual-heating system. A dual-heating system typically includes a primary heating system, such as a furnace or boiler, and a secondary heating system, such as a heat pump or a ductless mini-split system [8,9]. Usually, for such installations, the primary system is used during extremely cold temperatures and satisfies the majority of the heating needs, while the secondary system operates during milder weather conditions and provides supplementary heat. The goal of a dual-heating system is to achieve improved energy efficiency and to provide backup heating in the event that one of the systems fails. Therefore, in a dual system, it is not typical for a new furnace to be paired with an old heating system due to the lack of knowledge of how the systems would influence each other. However, we are seeing more combinations like this, where a new gas furnace is added to a traditional solid fuel furnace. The goal of such a combination is to limit the impact of a temporary increase in prices and to limit energy poverty problems. As a consequence, this approach creates a system with one energy source of lower impact on outdoor air quality and one with a significant impact, with little knowledge of the systems’ influence on the indoor environment and of the systems impacts on one another.

In Poland, gas is generally more widely available, and the majority of households have a connection to existing systems. However, wood is widely available and easy to buy. The price of gas can fluctuate depending on the supply and demand in the market. The latest years have been especially uncertain as the price of gas has risen from PLN 0.11/kWh in 2020 to PLN 0.20/kWh in 2022. Wood is generally cheaper than gas, but the cost can vary depending on the availability of trees and the cost of processing the wood. Its price has also risen in the past years from PLN 190/m³ (around PLN 0.10/kWh) in 2020 to PLN 320/m³ (around PLN 0.16/kWh) at the end of 2022.

The increasing return to solid fuel will have an impact on the indoor and outdoor environment. It has been proven that burning such fuel increases contaminant concentration, including fine particles such as PM10 and PM2.5 [10,11,12,13]. Pyka and Wierzchowski proved that furnaces powered by solid fuel are also a source of mercury, which is extremely harmful to occupants [14].

Solid fuels, such as coal or wood, influence not only the outdoor air quality, but also the direct quality of the indoor air. Indoor air pollution can come from outside air that penetrates inside as well as from indoor sources [15,16]. Studies conducted by Załuska and Gładyszewska-Fiedoruk [17] as well as by Ścibor et al. [18] already showed increased levels of PM2.5 and PM10 in households. Various studies have emphasized the importance of evaluating indoor contaminant sources. For example, Jose et al. conducted simulations to show how contaminants, including CO₂, from different sources migrated through houses [19]. Hu et al. investigated different sources of particle matter within houses. Their findings showed that combustion sources were the main cause of contaminants [20]. Rahata et al. optimized the combustion parameters of a furnace to achieve lower NOx emission without resulting in combustion inefficiency. They identified different optimization methods to reduce NOx pollution [21]. CO₂ is an additional contaminant that, in high doses, can influence human decision-making performance [22]. It is often measured alongside thermal comfort indicators, such as the air temperature and relative humidity, to determine the quality of the indoor air as well as the effectiveness of ventilation systems, making it an important trace contaminant [23,24,25,26].

Long-term exposure to a high concentration of air pollutants causes illness and affects the comfort of work for occupants, and, to ensure human safety and health, the air exchange rate should be adjusted, providing that the outside air is less polluted than indoor air [27]. With additional factors, such as ventilation methods, occupant contribution, and faulty equipment, that have been proven to influence the air quality in previous studies [28,29,30], it is important to make sure that, when using solid fuel, the indoor air quality is not degraded even more by its operation.

This study looked at a hand-assembled dual-heating system and whether it posed a threat to occupants. In the area where the study was conducted, there are many systems like this. The system consisted of a wood furnace that has no brand name and was made locally with no attestation in the 1980s, with no information on the effect on the indoor environment. The second heating source was a new gas furnace with a brand name and warranties, with an assumed low impact on the indoor environment. In theory, an old furnace is supposed to be disposed of when installing a new gas furnace. However, most users keep the old furnace and connect it, without consultation, with the exhaust of the new system. What is more, because of the lack of availability of controlled equipment with an attestation in the 1980s, old wood furnaces have no certification, and we have no indication of how they work in practice. People keep these furnaces because they fear that they will not be able to afford heating when using gas. The question is not of the availability of good solutions in the market, but the effect of people not wishing to use them out of fear or of not being able to pay for fuel.

How each furnace affected the indoor air quality was examined and it was determined whether the dual-heating system was safe for the occupants. A series of dynamic measurements of indoor environmental conditions were conducted. As there is a trend to return to conventional energy sources, more mixed dual systems may be installed. Such systems may not be properly integrated and can pose a threat to the occupants’ health.

Finally, as such systems may become more prevalent in the future, it is important to provide adequate measurement methods and evaluation algorithms to be able to judge if they can be harmful to occupants. An algorithm for testing such systems is proposed based on the data acquired from this study.

2. Materials and Methods

Measurements of the influence of an old furnace used to burn wood in comparison to a new gas furnace were conducted. Both were located in a boiler room on the ground floor of a single-household building. A photo of the equipment is shown in Figure 1. The old wood furnace was installed in the year 1985; it had no attestation and no brand name as it was made by a local welder. This type of installation was quite common in most parts of central and eastern Europe due to the lack of funds, and often the difficulty of obtaining new types of equipment at that time. It has never undergone any testing or renovations since its instalment (except for the paint coating). The gas furnace is a condensing boiler that was installed one year prior to the experiment and had a power of 20 kW with an energy class of A+.

Both the furnaces were connected to the same exhaust system that was upgraded when the new furnace was installed due to the need to install a stainless-steel pipe for the discharge of the condensing boiler.

Measurement System

To measure the influence of the two different furnaces, a measurement system was set up in the area near the furnace room. A series of dynamic measurements was performed with the use of sensors that measured the carbon dioxide (CO₂) concentration, the relative humidity, and the temperature. The range and accuracy of the sensors are shown in

Table 1. Sensor range and accuracy.

Measured Parameter	Sensor Range	Measurement Accuracy
Temperature	−5 °C–+55 °C	±0.4 °C
Relative humidity	0–90%	±5%
CO2 concentration	0–5000 ppm	±20 ppm

.

The two furnaces are shown in Figure 1 as well as a schematic showing how the systems were connected. The extraction of the smoke was based on the natural airflow due to the difference in the buoyancy of the hot air coming out of the chimney. There was no fan or other device to help establish the airflow. The furnaces did not work simultaneously—only one was turned on at a time.

Carbon dioxide is a nontoxic gas at low concentrations, but high levels of CO₂ can be dangerous to human health. Previous ASHRAE standards (62-1989) [31] considered 1000 parts per million (ppm) as the highest acceptable concentration for indoor carbon dioxide and a minimum ventilation rate of 8 l/p/s. More recent standards (ASHRAE 62-1999, 62-2001 and 62-2004) [32,33] recommend that the indoor–outdoor differential concentration should not exceed 700 ppm. Concentrations of CO₂ above 1000 ppm can lead to headaches, dizziness, and other symptoms of CO₂ toxicity, and can be dangerous in enclosed spaces where ventilation is limited [34,35,36,37]. In extreme cases, high levels of the contaminant can lead to unconsciousness and death.

It is important to maintain good air quality and ventilation, especially in enclosed spaces where CO₂ levels can quickly build up [38,39,40,41,42].

Six sensors were located within the household, four on the same level as the furnace room, one on the staircase leading up to the living area, and one in the living area. The configuration of sensors is shown in Figure 2. The main goal of the sensors was to pick up the levels of CO₂. The measurement of the temperature and relative humidity were automatically registered along with the other measurements.

The relative humidity did not change drastically between the measurements with a value of around 40% during the night and 30% during the day.

The ground floor on which the furnace room was located was not occupied by the occupants on a day-today basis and was used as a storage space and additional kitchen area when needed. During the measurements occupants were asked not to enter the ground floor and to enter only when there was a need to add wood to the furnace. Each time this happened, it was noted down for future reference. The door between the levels of the household was closed permanently during the measurements.

The house in which the measurements were conducted was built in 1984 and had poor insulation due to the building regulations that existed back then. The external walls were built of internal plaster, 20 cm of concrete block, and 20 cm of slag block and external plaster. No additional thermal insulation had been added since the building was built.

The measurements were undertaken on four consecutive days to minimize the influence of the outdoor temperature on the results. The temperature during the night was between −17 °C and −15 °C, while during the day was between −7 °C and −4 °C. Due to the fragility of the sensors for low temperatures, there was no constant measurement of the outdoor air temperature.

Each measurement was performed for a minimum of 12 h; the measurement frequency was set to 30 s. For each furnace, two series of measurements were taken into consideration, one with the door to the furnace room open and the other with this door closed. The aim was to see if any contaminants would migrate into the adjoining rooms and if any of the configurations had a positive impact on the air quality.

3. Results

3.1. Carbon Dioxide

The main interest was in the concentration of carbon dioxide as it may influence the health of the occupants. The sensors measured carbon dioxide concentration every 30 s. The occupants did not enter the ground floor unless it was to add wood to the wood furnace. Sensors 1 to 5 were located in the no-go zone, while sensor 6 was in a living area on the first floor where the occupants moved freely.

The layout of the sensors was designed in such a way as to see if any of the furnaces influenced the air quality in the boiler room itself and if the gas would leak into the adjoining rooms.

3.2. The Wood Furnace

The results of the measurements of the wood furnace are shown in Figure 3. Two cases were taken into consideration: first, when the door to the boiler room was closed, and second, when it was open.

Because the furnace was fueled by wood, the occupants had to add wood every few hours. This was noted by the occupants and is marked on the graphs in Figure 3 by vertical lines.

The results from sensors 1–5, which were isolated from the occupants, show that, for the majority of the time, the carbon dioxide concentration was below the maximum threshold of 1000 ppm and, even if it rose above this level, it was only for a short period of time. There was also a strong correlation between an increase in CO₂ and the occupants entering the area. This was most likely due to the occupant exhaling carbon dioxide when entering the ground floor. The influence of human activity on carbon dioxide is well-documented; for this reason, the occupants were asked to note the time of entrance to the ground floor.

When the wood furnace stopped burning during the night (after 22:40 in Figure 3a, and after 23:10 in Figure 3b), the carbon dioxide concentration lowered to the same background level as when the furnace was used. This indicates that contaminants in the furnace did not leak into the household when it was being used. The system provided a proper chimney draught that efficiently expelled the smoke.

The highest concentration of CO₂ was noted by sensor 6. This sensor was the only one located on the ground floor where the occupants were present. The results were well above the limit of 1000 ppm during the day. However, this can be attributed to occupant activity as there were six people in the household at the time of the measurements and the sensor was in the main hallway. Additionally, the CO₂ concentration was elevated during the day and lowered during the night when the occupants did not use this area. There was no visible contaminant increase that could be correlated with the usage of the wood furnace.

It was also observed that the occupants entered the furnace room more often in the second case, as can be seen in Figure 3b. This was due to the fact that the second measurement was carried out on a Saturday and the occupants were present more in the house and wanted the temperature to be higher due to their presence.

3.3. The Gas Furnace

During the measurements conducted while using the gas furnace, the occupants did not enter the ground floor. The measurements were again carried out in two different configurations: with the door to the boiler room open and then when closed. The results are shown in Figure 4.

The results showed once again that the furnace did not influence the concentration of carbon dioxide in the boiler room or the adjoining rooms. The level of CO₂ was stable and did not rise when using the gas furnace.

Only the results from sensor 6 rose above the maximum hygienic level of 1000 ppm, mainly during the day due to occupant activity. This sensor was placed in a busy corridor through which the occupants frequently passed, directly interacting with the sensor. The reason for such high CO₂ levels can be attributed to the direct interaction with the sensor of the exhaled air and did not represent the mean concentration. The case of Figure 4b, shows that the concentration of CO₂ did not drop so significantly during the night. When the occupants were asked about this situation, they claimed that one of the occupants slept on the same floor as the sensor, whereas, on the previous nights, he normally slept on the upper level, which explained the elevated concentration.

For both the wood furnace and gas furnace, the difference between having the door to the boiler room closed or open did not influence the results. The contaminant levels remained stable.

3.4. Temperature

During the analysis of the results, it was noticed that the temperature on the ground floor was higher when using the wood furnace. The results are shown in Figure 5. The graphs show that, when using the wood furnace, it heated the adjoining room to around three degrees higher than when using the gas furnace. Fluctuations in temperature were also visible when using the wood furnace due to its temperature increasing when fuel was added.

To investigate this phenomenon, photos of the working furnaces were taken with a thermal camera, as shown in Figure 6; when one furnace was working, the other was turned off and cooled down. Figure 6 shows that the wood furnace heated up to a much higher temperature than the gas furnace. When the latter was operating, the highest temperature was noted on the exhaust pipe and not the furnace itself. When using the wood furnace, the temperature of the furnace walls reached up to 150 °C and heated the adjoining rooms.

The occupants later stated that they mainly used the wood furnace, and, because of its high temperature, there was no need for a heating system on the ground floor as it served as a heat source for them, especially in the adjacent kitchen. They stated that this was an additional advantage of the wood furnace along with the lower costs of the fuel needed to heat the house.

Additionally, the temperature on the ground floor fluctuated when using the wood furnace. This fluctuation occurred not only on the ground floor, but also on the higher level with the occupants resident (Sensor 6). This was due to increase in the temperature of the furnace itself and the water in the heating system when wood was added.

There was also a drop in the temperature in the household when the wood inside the furnace had burnt out during the night. The temperature in the living area dropped to around 17.5 °C, which could be a cause of thermal discomfort for the occupants.

4. Discussion

Furnaces powered by solid fuel have been shown to have a harmful effect on indoor air quality [26,43]. Constructing a self-made, dual-heating system without the proper tools and knowledge creates a risk that the extraction of contaminants may be worsened and that they may be released into the household instead of being expelled from the house. As studies have shown that a solid-fuel furnace is a potential source of hazards, such as contaminants, it is important to validate such systems [28]. Additionally, the exhaust of the furnace may be tampered with, and, as a result, not remove contaminants properly, so they may flow into the living areas and be potentially dangerous to the occupants [28].

The study showed that, in this case, the contaminants were removed by the exhaust system despite the system not being verified by a professional supplier. The results of the study showed that neither the wood nor the gas furnace had a negative impact on the air quality inside the house. However, if the connections to the exhaust system were to not provide a proper chimney draft for one of the furnaces, the exhaust could flow into the living area.

Carbon dioxide was used as a tracer gas to see if there was any leakage into the living spaces of the occupants. There were no large increases in the contaminant that would be harmful to human health in the room with the heat source or the adjoining rooms. There was, however, a large increase in CO₂ shown by the sensor in the corridor on the floor attended by the occupants. This was due to the placement of the sensor that interacted directly with the occupants in a busy corridor. Studies have shown that the presence of occupants has a large influence on carbon dioxide concentration [25,44]. Further measurements should be carried out to see if the ventilation system within the house is adequate, and with a different placement of sensors to observe a wider range of contaminant accumulation patterns

The presented wood furnace acted as a heat source in the boiler room and, when in use, the temperature in the house was much less stable. The furnace itself was not insulated and heat losses were noted. This could also be potentially dangerous for the gas furnace that was heated up in the process. The occupants were advised to insulate the furnace, at least on the side facing the gas furnace. Additionally, when using the wood furnace, the effectiveness of heating the water for the radiators was less efficient as the heat was dispersed into the room and not used in the central heating system. When using wood as a heating source, the temperature within the household was only optimal during the day and dropped rapidly during the night. This could leave the occupants with a sense of cold during the night and in the mornings before the furnace was restarted. When the gas furnace was used, the temperature was much more stable, which was better for thermal comfort.

As such dual-heating systems will continue to be created, it is important to evaluate them in a proper way to make sure that they are not harmful to the occupants. A proposed method for analyzing such hand-crafted, dual-heating systems is presented in the flowchart in Figure 7.

The flowchart in Figure 7 presents a methodology to assess the safety of a homemade dual-heating system and includes action steps in the event that there is a threat.

First, direct measurements of the trace contaminant should be conducted; if the concentration immediately increases, this means that there is most likely a problem with the extraction system as the chimney draft is not maintained. This problem could also be created by the furnace not being sufficiently airtight.

The second step involves measurements taken over time, ideally at least 12 h. This should be carried out to check if there is a small leakage of contaminants into the living space over a long period of time. Such leaks may not be observed using brief measurements as in the first step, and are the most risky as they are usually not noticed by occupants. If such accumulations occur, it may be due to a lack of proper ventilation, a lack of a chimney draft, or lack of airtightness of the furnace. This case is the most complex and would require the most detailed investigation.

Lastly, the third step would be to measure the thermal effects of the furnaces. If a new heat source (especially one fueled by gas) is in an area with a higher temperature than the manufacturer foresees, then the furnace should be insulated or moved to a different location to not damage the new heat source.

5. Conclusions

The difference in operation between an old wood furnace and a newly installed gas furnace in a dual-heating system has been presented. The former was made by a local welder, it had no attestation, no brand name, and had been in operation since 1985 with no conservation. The latter was a new gas furnace installed one year before the study was carried out and had all the necessary attestations. Both were connected to the same exhaust system, which was not recommended by the supplier of the gas furnace and was not performed by a professional.

It is well-known that using non-attested wood furnaces has a profound impact on the outdoor environment. However, the reason for this study was to see if there was any significant threat to the occupants inside the house. The use of a hand-crafted, dual-heating system with a wood furnace was tested and the potential impact on the inhabitants’ health due to contaminant leakage was evaluated.

The results showed that there was no threat to the occupants. This was thanks to the fact that the old furnace did not have any leakage into the surroundings and a good source of fresh air was provided that enabled a chimney draft to be maintained. However, with energy prices rising, there is a risk that energy poverty will continue to increase and people will return to conventional solid-fuel heating systems, or they will not cease to use their former heating source. While we know that this will have a negative impact on the environment (outdoor air quality), it is important to make sure that the well-being of occupants is maintained in case they have no other means to heat their household.

The study also showed risks of using such a system, as the wood furnace was not insulated and heated up to 150 °C when the gas furnace was placed next to it. It was advised to isolate the wood furnace to not damage the surrounding installations that could potentially damage the gas furnace.

Further studies should be carried out on how to improve the quality of such dual-heating systems and their influence on the indoor air environment. Methodologies for how to evaluate their safety should continue to be the subject of research as this may have a direct influence on human health. A proposed scheme of action was recommended in this paper, but further studies should be carried out to identify and exclude future risks.

If the price of energy continues to rise, such systems will continue to be installed even though they are not recommended for environmental reasons. While they are not the most efficient, the first priority should be to make them safe for occupants so that they do not pose a threat to human health.