Although the case buildings differ in construction year, ventilation systems and ownership, they remain comparable for the purpose of this study because each renovation measure targets the same underlying issue: ventilation related heat losses in multi family buildings with district heating. The analysis focuses on relative improvements before and after each measure rather than on absolute performance levels. This enables meaningful comparison of the energy and economic impacts of C HRV, DE HRV and EAHP solutions across different building contexts.

The energy declarations were carried out by a certified independent energy expert in the Swedish National Board of Housing, Building and Planning's programme Gripen according to BBR29. For normalized energy consumption for domestic hot water, 22.5 kWh/A_temp/year has been used, which is a 10% reduction from 25 kWh/A_temp/year. The reduction has been made because the properties have efficient mixer taps. [15] The primary energy use for each case building was calculated according to the BBR29 methodology, following the formulation in:

where:

E_el (kWh): delivered electricity to the building,

f_el (–): primary energy factor for electricity,

E_dh (kWh): delivered district heating for space heating and domestic hot water,

f_dh (–): primary energy factor for district heating,

A_temp (m²): heated floor area.

The equation expresses the total primary energy use as the sum of electricity and district heating, each weighted by their respective primary energy factors, divided by the building’s heated area. This allows comparison of energy performance across buildings and renovation measures on a common, normalised basis.

Input data has been for 2022 consumption, except before measures for Case Building 2_DE-HRV. Regarding Case Building 2_DE-HRV, district heating consumption includes another building, consumption is distributed with 59% to Case Building 2_DE-HRV. For Case Building 2_DE-HRV consumption after the measure, measured values were used from March to December 2022 and January and February 2023. Consumption before the measure were values from 2021.

The district heating consumption at Case Building 1_C-HRV after the measure was 67,262 kWh, which then included all renovation measures at the block. According to the design calculations, the HRV measure was estimated to reduce the primary energy figure by 20 kWh/Atemp/year. Before the measure was implemented, the primary energy use was estimated at 139,504 kWh. The total reduction in the primary energy number for all measures combined, according to the design, was 37.7 kWh/A_temp/year.

The property electricity at Case Building 1_C-HRV before the measure was based on projected values and included electricity for fans before the measure and electricity for pumps. To ensure that the new, more efficient pumps did not influence the results, their electricity use was included in the property electricity both before and after the measure. Interior and exterior lighting were unchanged before and after the measure and are included in the property electricity.

Total consumption for the three apartment buildings at the premises of Case Building 3_EAHP in 2022 was consumption after the measure. Combined with information on the heat pump production and electricity to the compressor, the district heating requirement before the measure could be determined. It would also have been possible to estimate the district heating requirement before the measure based on the 2021 consumption.

TMF Energi is an Excel program developed by Research Institutes of Sweden (RISE) on behalf of the Swedish Wood and Furniture Companies (TMF) [16]. For Case Building 3_EAHP, TMF Energi has been used to investigate the difference between heat recovery from exhaust air from one of the buildings compared to all three. It has also been used to see how the primary energy figures change when installing solar cells.

Energivision is a program used for energy declarations and energy analyses [17]. In this Excel-based program, proposed measures can be added to see how they affect the energy declaration, energy needs and the profitability of the investment. Climate data and energy prices are available in Energivision. In the present study, Energivision has been used to determine energy needs/energy losses before measures Case Building 1_C-HRV, Case Building 2_DE-HRV and Case Building 3_EAHP.

No consideration has been given to whether the measure resulted in increased electricity supply security for the properties. Consideration of increased electricity demand due to charging was included only for Case Building 3_EAHP with the exhaust air heat pump and was only relevant for prices regarding Växjö. The power price for the highest average electricity output was 102.5 SEK/kW/month (incl. VAT) [18]. District heating tariffs and electricity network fees for Ljungby were obtained from Ljungby Energi [23], [24] and were used in the economic calculations wherever Ljungby price conditions were applied, including for Case Building 2_{DE HRV} and in the comparative scenarios for Case Building 1_{C HRV} and Case Building 3_EAHP.

The increased electricity output was based on an average value between two methods for estimating the output. One was based on the exhaust air heat pump's COP which was 4.89 and that the district heating output decreased by 20 kW according to invoices, which gave an electricity output of 4.09 kW.

The second method was to take the compressor output of 39,970 kWh and divide by the year's 8,760 hours, which gave 4.56 kW. The average value was thus 4.33 kW as a result of increased power output from the exhaust air heat pump operation at Case Building 3_EAHP. Reduced charged district heating output for Case Building 1_C-HRV and Case Building 2_DE-HRV was estimated at 10 kW, half of what Case Building 3_EAHP reduced according to costs from Växjö Energi AB.

The reduced district heating consumption for the properties has been distributed in percentage terms based on the previous distribution over the year. As a result of this assumption, the HRV units at Case Building 1_C-HRV and Case Building 2_DE-HRV reduces the district heating demand during the summer months of June, July and August, when the only remaining load is domestic hot water.

The power cost for Växjö Municipality was based on the so-called signature power of Växjö Energi AB. Daily average power measurements are taken for weekdays between November and March, if the outdoor temperature is below 10°C. These data points build up characteristic power consumption at different outdoor temperatures. Billing is done for characteristic consumption at -10°C, which is based on the expected power drawn at -10°C. [19]

The economic analysis was based on simple payback period (SPB)(Eq. (2)), detailed payback period (DPB) (Eq.(5)), annual energy cost savings, and annual electricity cost increases. Annual net savings consist of reduced district heating energy cost, reduced district heating power charges (where applicable), increased electricity cost due to operation of the HRV or EAHP systems, and annual service costs. A rent increase was also included for Case Building 1_{C HRV}.

The simple payback period (SPB) is defined as:

where the annual net savings are:

The detailed payback method accounts for monthly variations in electricity and district-heating prices, as well as monthly distribution of heat savings. Monthly savings are expressed as:

The detailed payback period (DPB) is then defined as:

In Eqs. (3–5), B-terms denote annual or monthly benefits, including savings from reduced district heating demand for domestic hot water and space heating (B_dhw+sh), reductions in district heating power charges (B_eff), and additional income from rent adjustments (B_rent). C-terms represent annual or monthly costs, comprising electricity use associated with the HRV or EAHP system (C_el) and service and maintenance costs (C_serv).

The economic analysis was based on collected investment costs and electricity and district heating prices. The electricity price consists of two parts, electricity grid and electricity trading costs. The electricity grid and district heating prices were supplemented with spot prices from Vattenfall [20]. Spot prices for 2021 were used to avoid including the record high prices for 2022. However, the price for electricity grid and district heating has been based on the 2023 prices.

Payback periods for the renovation measures were evaluated using both simple and detailed type, in accordance with Eq. (2) and Eq. (5). The two simple payback periods were based on a representative electricity and district heating price derived from data from [21] and [22]. The two detailed payback periods, for Växjö and Ljungby municipalities, were implemented with a higher level of methodological detail. The method accounts for monthly electricity and district heating prices, the monthly distribution of annual district heating reductions, increased electricity consumption, and the reduced delivered district heating output, where the monthly savings are computed according to Eq. (4). Increases in rent or fees have also been considered in both the simple and detailed payback period calculations, where the annual net savings follow the formulation in Eq. (3).

This section presents the results of the current study with respect to energy performance and economic outcomes.

Energy Calculations and Energy Declarations

The energy performance of the properties before and after the measures is presented in Figure 1. All values are normalized to a standard year and weighed using primary energy factors. Case Building 1_C-HRV had the largest percentage reduction with 22.7%, as the improvement was 22 kWh/m²/year. Case Building 2_DE-HRV increased its primary energy number by 2 kWh/m²/year, corresponding to an increase of 2.1%, while Case Building 3_EAHP decreased by 12 kWh/m²/year, corresponding to a reduction of 15%.

Figure 1.

Summary of the properties' primary energy numbers before and after the measures based on the energy declarations

In Figure 2, the consumption before and after the measures is indicated in terms of specific energy use, meaning that the consumption is unweighted and uncorrected. This means that 1 kWh of electricity is equated with 1 kWh of district heating. The energy for domestic hot water is normalized. Case Building 1_C-HRV reduces energy use from 125.5 to 93.5 kWh/m²/year, which corresponds to a 25.5% reduction and 32 kWh/m²/year. Case Building 2_DE-HRV reduces energy use by 24.3% and 25.6 kWh/m²/year, from 106.5 to 80.6 kWh/m²/year. Case Building 3_EAHP reduces its specific energy use by 10 kWh/m²/year, from 134.5 to 124.5 kWh/m²/year, compared to the increase in primary energy number shown in Figure 1. The increase is mainly due to the weighing of electricity and district heating consumption. Normal year correction can also be a contributing factor depending on the year.

Figure 2.

Summary of specific energy use for each of the Case Buildings, separated into heat, DHW and Electricity, before and after the measures

To perform a detailed payback period that follows monthly price variations, the district heating savings of the measures need to be distributed per month, the distribution is shown in Figure 3. The data are based on measurements from 2020–2022 for Case Building 2_DE-HRV and Case Building 3_EAHP for the years 2020 to 2022, and the Figure 3 displays the averaged percentage distribution.

Figure 3.

Monthly percentage distribution of annual district heating consumption

Figure 4 shows the amount of kWh per m³ of district heating. The values were used for detailed payback period to estimate the reduced amount of district heating flow after the renovation measures. Flow savings are relevant for payback period prices for Växjö Municipality as Växjö Energi has a flow rate during the winter months. The figure compares the calculated average energy content in kWh/m³, derived from district heating cost data for Case Building 3_EAHP, with the values reported in [21]. For detailed payback period, the calculated values from Case Building 3_EAHP have been used, while the values from [21] have been used as a reference.

Figure 4.

Energy content per m³ of district heating

The profitability of investments is affected by electricity prices. Växjö and Ljungby belong to electricity price area SE4, which is Sweden's southernmost area. The prices exclude VAT and include Vattenfall's electricity certificate fee and surcharge [20].

Investment costs and other costs for the renovation measures are reported in Table 2. The most expensive investment was Case Building 1_C-HRV which cost SEK 2,667,750 incl. VAT, distributed across 12 apartments, the cost was approximately SEK 220,000 per apartment. The investment for Case Building 2_DE-HRV with 16 apartments was SEK 2,042,667 incl. VAT and approximately SEK 128,000 per apartment. The least expensive renovation measure in terms of total cost per apartment was the exhaust air heat pump in Case Building 3_EAHP which cost SEK 1,418,750. The property has 76 apartments, corresponding to approximately SEK 19,000 per apartment.

Table 2.

Economic summary for the Case Buildings

Category	Case Building 1 C-HRV	Case Building 2 DE-HRV	Case Building 3 EAHP
Investment costs excl. VAT	920 000 SEK	1 634 134 SEK	1 100 000 SEK
Other investment costs excl. VAT	200 000 SEK	N/A	35 000 SEK
Cost for fan room excl. VAT	1 014 200 SEK	N/A	N/A
Total investment excl. VAT	2 134 200 SEK	1 634 134 SEK	1 135 000 SEK
Total investment incl. VAT	2 667 750 SEK	2 042 667 SEK	1 418 750 SEK
Total investment incl. VAT per apartment	222 313 SEK	127 667 SEK	18 668 SEK
Service costs excl. VAT	2 600 SEK	14 000 SEK	3 500 SEK
System lifespan	20 years	15 years	20 years
Discount rate	5%	5%	5%
Annuity factor	0.0802	0.0963	0.0802
Conversion factor to present value	12.46	10.38	12.46
Service costs over the entire usage period incl. VAT	40 502 SEK	181 644 SEK	54 522 SEK

The external financial support that Vidingehem AB applied for and was granted for all measures at Case Building 1_C-HRV was SEK 784,670. All investment costs including VAT are calculated using a VAT rate of 25%.

Table 3 shows monthly savings as a result of reduced district heating demand. Växjö Energi AB has different tariffs for winter months (November - March) and summer months (April – October). The flow savings of district heating are based on specific energy content (kWh/m³) per month and are derived from district heating costs for Case Building 3_EAHP that were used for the calculations. Ljungby Energi AB has no flow tariff in 2023. Based on Figure 3, the total annual district heating savings have been distributed per month. These values are used for the calculation of the detailed payback period.

Table 3.

Monthly distribution of district heating savings due to renovation measures

	Case Building 1 C-HRV				Case Building 2 DE-HRV				Case Building 3 EAHP
	Savings [SEK]				Savings [SEK]				Savings [SEK]
	? E [MWh]	? V [m3]	Växjö	Ljungby	? E [MWh]	? V [m3]	Växjö	Ljungby	? E [MWh]	? V [m3]	Växjö	Ljungby
Jan	5.7	112	4 948	3 700	3.5	69	3 431	2 278	29.0	573	22 129	18 199
Feb	5.2	99	4 575	3 413	3.2	61	3 202	2 102	26.4	505	20 230	16 735
Mar	4.8	99	4 348	3 196	3.0	61	3 062	1 968	24.4	506	19 072	15 632
Apr	3.6	79	2 648	2 503	2.2	49	2 015	1 541	18.1	402	10 402	12 095
May	2.4	59	2 090	1 830	1.4	36	1 671	1 127	12.0	300	7 558	8 661
Jun	0.9	24	1 399	995	0.5	15	1 245	612	4.4	124	4 032	4 404
Jul	1.0	30	1 473	1 085	0.6	19	1 291	668	5.2	155	4 413	4 864
Aug	1.0	31	1 482	1 095	0.6	19	1 297	674	5.3	156	4 456	4 916
Sep	1.7	47	1 779	1 454	1.0	29	1 480	896	8.6	238	5 974	6 749
Oct	2.7	66	2 267	2 043	1.7	40	1 780	1 258	13.9	335	8 461	9 752
Nov	3.8	87	3 714	2 653	2.4	54	2 671	1 634	19.5	446	15 837	12 863
Dec	5.6	117	4 923	3 654	3.5	72	3 416	2 250	28.6	596	22 002	17 964
Sum	38.3	851	35,645	27,621	23.6	524	26,560	17,009	195.4	4,338	144,567	132,833

Note: ?E = district heating energy reduction; ?V = flow reduction. All monetary values in SEK.

The last row in Table 3 shows the annual cost reduction of district heating after the measure. Case Building 1_{C HRV} saves SEK 35 645 with prices from Växjö Energi AB and SEK 27 621 with prices from Ljungby Energi AB. Case Building 2_{DE HRV} saves SEK 26 560 (Växjö) and SEK 17 009 (Ljungby). The largest savings are achieved by Case Building 3_EAHP, with SEK 144 567 (Växjö) and SEK 132 833 (Ljungby). Only Case Building 3_EAHP shows higher district heating cost savings under Ljungby tariffs. This is due to the assumption that Case Building 1_{C HRV} and Case Building 2_{DE HRV} reduce the district heating power by 10 kW in Växjö. Växjö Energi AB has changes its energy price for district heating from 1 September 2023. The price increase was +29% for the winter energy price (November–March), rising from 458 SEK/MWh to 590 SEK/MWh including VAT. For April–October, the energy price increased from 266 SEK/MWh to 464 SEK/MWh including VAT, corresponding to a 74% increase.

Case Building 1_{C HRV} experienced the most significant improvement. Its primary energy consumption decreased by 23%, corresponding to 22 kWh/m²/year, while the specific energy use dropped by 25.5% (32 kWh/m²/year). The pre renovation electricity and district heating consumption numbers were based on aggregated design values from several properties included in Vidingehem’s renovation project. The allocation of these aggregated values to Case Building 1_{C HRV} was done proportionally based on floor area, introducing a certain margin of error. This type of discrepancy between design assumptions and operational data is well documented in the performance gap literature, highlighting the need for calibration of real life performance against calculated values, as recommended under EPBD provisions [5] and supported by empirical studies showing that calculations tend to overestimate savings [11]. The strong performance of the C HRV system can be explained by both its technical configuration and the local tariff structure in Växjö. The system is fully centralised, ensuring uniform airflow, high heat recovery efficiency and low electricity demand. Since the original system was exhaust only, introducing balanced ventilation provided a large net reduction in heating needs. In addition, Växjö’s district heating tariff includes a relatively high power based fee, making reductions in peak heat demand economically valuable. This combination of centralised heat recovery and tariff driven benefits explains why Case Building 1_{C HRV} achieves the most favourable improvement despite the high investment cost. Economically, the project involved the highest total cost (SEK 2.7 million), partly due to the construction of a fan room. The shortest simple payback period was 61 years based on 2021 price levels [22]. These calculations do not consider future price developments, and investment decisions for deep renovations that occur within policy frameworks such as the EPBD recast and EU Taxonomy [3], [2].

Case Building 2_{DE HRV} was the only building where the primary energy number increased. The building originally had natural ventilation, resulting in lower airflow compared to mechanical systems. Installing decentralised HRV units increases ventilation rates to meet indoor air quality standards, which raised heat losses even if comfort improves [6], [8]. Its specific energy use increased by 7.4% (10 kWh/m²/year). The apartments also lack thermostat limitations, unlike in Case Building 1_{C HRV} and Case Building 3_EAHP, further contributing to higher energy use. In buildings without thermostat limitations, occupants often raise indoor temperatures, which increases space heating demand. Combined with the higher airflow introduced by the DE HRV units, this leads to more heated air being exhausted and replaced, helping explain the increased energy use.

It should be noted that no measured ventilation flow or IAQ data were available for Case Building 2_{DE HRV}, which limits the precision of the energy balance interpretation. The results should therefore be viewed as indicative rather than exact. The increase in energy use aligns with the behaviour of decentralised systems in older buildings. When passive ventilation is replaced, ventilation volumes typically rise, increasing thermal losses. Each apartment unit also uses its own fans, raising electricity consumption. Since Ljungby has no district heating power charges, the measure gains little from reduced peak loads. From an economic perspective, the DE HRV renovation produced a negative annual net outcome, since the additional fan electricity consumption exceeded the corresponding district heating savings. Combined, these factors make the DE HRV measure the least cost effective of the three.

Case Building 3_EAHP achieved a 15% reduction in primary energy use (12 kWh/m²/year), and its specific energy use decreased by 24.3% (25.9 kWh/m²/year). Only one of the three on-site buildings provided exhaust air to the heat pump, but the recovered heat served all three buildings, achieving two thirds of the theoretical potential savings at one third of the cost. The system also benefits from low supply temperature requirements, resulting in a high COP of 4.89. These results align with studies showing that EAHPs can be effective deep renovation measures when distribution temperatures are low [7], [10], and that pairing EAHPs with PV can further improve economic viability [12]. The performance of Case Building 3_EAHP reflects the characteristics of exhaust air heat pumps: stable exhaust air temperatures support a high COP, but the system simultaneously increases electricity use and raises the building’s subscribed electricity power level. This makes the economic outcome highly dependent on electricity prices and local tariff structures. Under 2021 prices, the payback period was 19 years [22], but under 2022 prices profitability decreases sharply. Field evidence from Swedish multi family buildings indicates that hybrid control and smart energy management can improve cost performance [12]. This sensitivity to electricity price levels explains why EAHP systems show strong energy performance but mixed economic results.

This study is based on real world data from three renovated apartment buildings, which strengthens the practical relevance but also introduces some limitations. The buildings differ in age, construction characteristics and initial ventilation conditions, which influence their baseline energy performance. Although efforts were made to isolate the effects of each renovation measure, the presence of other parallel renovations and the use of normalized hot water values introduce some uncertainty. Economic results are directly dependent on local energy tariffs and may vary under different price scenarios. Furthermore, the detailed payback analysis assumes stable monthly consumption patterns, which may not fully reflect long term behavioural or climatic variations. These limitations should be considered when interpreting the results and when generalizing the findings to other building stocks or regions.