Author: Mycond Technical Department
The recovery of rejected heat from air dehumidifiers is an increasingly relevant area of energy saving in modern indoor climate systems. Combining dehumidification with space heating or water heating can significantly improve a building’s energy efficiency. In this article, we take a detailed look at the engineering foundations of heat recovery from refrigeration-type dehumidifiers, integration schemes with heating, criteria for feasibility, and common design mistakes.
Thermal balance of a refrigeration-type dehumidifier — the source of rejected heat
The thermodynamic cycle of a refrigeration-type dehumidifier is based on cooling air below its dew point at the evaporator, which causes moisture to condense, followed by heating of the dehumidified air at the condenser. As air passes through the evaporator it is cooled and dried, and then it is heated as it passes through the condenser.
The energy balance at the condenser includes three main components:
1. Latent heat of moisture condensation — the energy released when water vapour condenses on the evaporator. It is calculated as the dehumidification capacity (kg/h) multiplied by the latent heat of vaporisation (kJ/kg), which depends on the condensation temperature. The latent heat of vaporisation is not a constant and can vary within 2300–2500 kJ/kg depending on the exact temperature.
2. Compressor work — the electrical power consumed by the compressor. This value is taken from the dehumidifier’s technical data or calculated as part of the refrigeration cycle analysis.
3. Sensible heat of air — additional heating of the air passing through the dehumidifier. It depends on the unit’s design and operating regime.
On an h–d diagram, the psychrometric process in the dehumidifier looks as follows: the air is first cooled to a temperature below the dew point (a straight line with decreasing enthalpy), then moisture condenses (a curve with a simultaneous decrease in humidity ratio), and finally the air is heated at the condenser (a straight line with increasing enthalpy but no change in humidity ratio).
For a numerical example: if a dehumidifier removes 10 kg/h of moisture at 25°C, the latent heat will be approximately 10 kg/h × 2450 kJ/kg ≈ 24,500 kJ/h or about 6.8 kW. Taking into account compressor work, for example, 3 kW, the total heat at the condenser will be 6.8 kW + 3 kW = 9.8 kW.
The temperature rise of the air at the dehumidifier outlet is determined by the ratio of the condenser heat output to the air mass flow rate and is calculated via the thermal balance for the specific system.
Theoretical foundations of heat recovery — condenser potential and temperature levels
The refrigerant condensation temperature and the heat transfer fluid temperature are different quantities. Refrigerant condenses at a temperature that depends on the temperature of the cooling medium (air or water) at the condenser plus the heat exchanger temperature approach. For example, for an air-cooled condenser in a room at 25°C, the condensation temperature may be 35–45°C. For a water-cooled condenser with water at 30°C, the condensation temperature may be 40–50°C. These values are not universal constants; they are the result of calculations for specific conditions.
The coefficient of performance (COP) of a dehumidifier has a clear definition. The heating COP equals the ratio of heat at the condenser to compressor work. It is an indicator of heat output relative to electricity consumption. The cooling COP is the ratio of heat at the evaporator to compressor work, an indicator of cooling capacity relative to electricity consumption.
It is important to note that dehumidifier catalogues often state SMER (Specific Moisture Extraction Rate) in litres per kilowatt-hour or kilograms per kilowatt-hour, which is a different metric from COP.
To calculate heat at the condenser, we use the energy balance: latent heat plus compressor work, with the work taken from the dehumidifier’s technical data.
Compared with a traditional air-to-water heat pump, a dehumidifier has the advantage of taking heat from indoor air at 20–25°C, while a heat pump draws heat from outdoor air, the temperature of which can vary from −10°C to +10°C in winter. This provides more stable operating conditions for the dehumidifier’s evaporator.
The potential for heat recovery depends on temperature differences, heat exchanger type, and operating regime. With proper heat exchanger selection and alignment of temperature levels, it is possible to transfer most of the condenser heat into a useful load. However, increasing the condensation temperature as the cooling water temperature rises can reduce the efficiency of the refrigeration cycle.
Integration schemes — three basic approaches
There are three main schemes for integrating a dehumidifier with a heating system:
Scheme one: separate water heat exchanger — a plate or shell-and-tube heat exchanger is installed on the condenser side. On the hot side there may be refrigerant or air after the condenser, depending on the dehumidifier design. On the cold side — water from the heating system or domestic hot water. The connection hydraulics include tying into the heating return or the domestic hot water circuit, a circulation pump, an expansion vessel, and balancing valves. Advantages are simplicity and the possibility of retrofitting existing systems. Disadvantages are additional hydraulic resistance and the need for a dedicated circulation pump.
Scheme two: cascade connection with a heat pump — the dehumidifier heats water from temperature T1 to T2 (e.g., from 20°C to 40°C), and the heat pump boosts it from T2 to T3 (e.g., from 40°C to 60°C) for domestic hot water. A buffer tank is installed between them to smooth operating modes. Advantages include reduced load on the heat pump and increased overall system COP, as the heat pump operates with a pre-heated source. Disadvantages include control complexity and the need to coordinate the operating regimes of two devices.
Scheme three: direct low-temperature consumers — condenser heat is directed to underfloor heating (supply temperature 30–40°C), ventilation supply air preheating (20–30°C), or pool heating (26–30°C). Advantages are well-matched temperature levels and maximum recovery without additional equipment. The disadvantage is the need to have such low-temperature consumers on site.
The choice of scheme depends on available consumers, their temperature level, and operating regime throughout the year.
Compatibility characteristics of different heat consumers:
Underfloor heating (30–40°C): good compatibility, direct connection is possible.
Domestic hot water (55–60°C): limited compatibility, cascade or post-heating required.
Radiators (50–70°C): limited compatibility, only in cascade with a heat pump.
Swimming pool (26–30°C): excellent compatibility, an ideal year-round consumer.
Calculation of recovered heat — one detailed example
Let’s consider a specific calculation example for a swimming pool.
Input data:
- Dehumidification capacity G = 20 kg/h (from the pool moisture emission calculation)
- Indoor air temperature = 28°C
- Indoor relative humidity = 60%
- Dehumidifier electrical power N = 6 kW (from technical data)
Step one: calculation of latent heat of moisture condensation.
Latent heat of vaporisation at 28°C r ≈ 2435 kJ/kg (from water vapour tables).
Latent heat Q(latent) = G × r = 20 kg/h × 2435 kJ/kg = 48,700 kJ/h = 13.5 kW.
Step two: condenser heat balance.
Heat at the condenser Q(condenser) = Q(latent) + N(compressor) = 13.5 kW + 6 kW = 19.5 kW.
This is the total heat output released at the condenser.
Step three: recovered capacity via the water heat exchanger.
Assume heat exchanger effectiveness of 80% (a realistic value for a properly selected plate heat exchanger; depends on type, area, and temperature approach).
Recovered heat Q(recovered) = Q(condenser) × 0.80 = 19.5 kW × 0.80 = 15.6 kW.
Step four: pool water heating.
Water flow through the heat exchanger m = 0.5 kg/s (selected by temperature approach and circuit hydraulics).
Specific heat capacity of water c = 4.19 kJ/(kg·K).
Temperature rise ΔT = Q(recovered) ÷ (c × m) = 15.6 kW ÷ (4.19 kJ/(kg·K) × 0.5 kg/s) = 7.4 K.
If the inlet water is 26°C, the outlet will be 26°C + 7.4°C = 33.4°C, which is suitable for pool heating.
Step five: what this provides to the pool heating system.
Without heat recovery, all pool heating is provided by a gas boiler or electric heater at the gas or electricity tariff.
With 15.6 kW of recovered “free” heat, the load on the primary heater is reduced.
Annual savings depend on the dehumidifier’s operating hours over the year, gas/electricity tariffs, and the availability of alternative heat sources. A specific calculation requires project input data.
Seasonal operation — winter, shoulder seasons, summer
A dehumidifier heat recovery system has different operating modes throughout the year.
Winter mode: condenser heat is directed to space heating or pool heating. The dehumidifier operates on humidity control, and the heat is fully recovered without dumping into the room. If the consumer is low-temperature heating (underfloor heating), the system can operate autonomously without an additional source. If a higher temperature is needed (domestic hot water at 60°C), the dehumidifier provides base heating to 45–50°C, and additional post-heating is provided by a boiler or heat pump.
Shoulder seasons (spring–autumn): part of the heat is recovered when heating is still required, but some may be surplus when heating is already off while dehumidification still runs. A switching system is required — an automatic three-way valve that directs heat either to heating, or to rejection if heating is no longer needed while the dehumidifier still operates, or to a buffer tank.
Summer mode: if there is a year-round consumer (pool, process heating), the heat is directed there. If there is no consumer, a heat rejection system is needed: a dry cooler, a cooling tower, or simply disabling the water circuit. In the latter case, the dehumidifier rejects heat into the room, increasing the load on the air conditioner.
A specific control scheme may include a three-way valve and a dry cooler with logic: IF the outdoor air temperature is greater than 20°C OR the indoor temperature is greater than 26°C OR there is no heating demand from the thermostat, THEN the heat is directed to the dry cooler or into the room, OTHERWISE the heat goes to the heating circuit.
Automation includes temperature sensors on supply and return of each circuit, valve control by algorithm via a programmable controller or DDC system.
Impact of integration on dehumidification efficiency, condensation temperature, and performance
The physics of the process forms a clear chain of cause and effect: increasing the cooling water temperature at the condenser increases the refrigerant condensation temperature, which raises the condensing pressure, which in turn reduces the refrigerant mass flow rate through the compressor, which lowers the evaporator cooling capacity and, as a result, reduces dehumidification capacity.
A quantitative assessment of this effect depends on compressor type, refrigerant, and initial conditions. For typical R410A scroll compressors, a 10 K increase in condensation temperature can lead to a reduction in compressor mass flow rate by an amount that depends on the compressor design (specific values are taken from the compressor manufacturer’s performance charts for the exact model), proportionally reducing dehumidification capacity.
A compromise solution is to limit the maximum outlet temperature of the heat transfer fluid. For example, if 55°C water is needed for domestic hot water, and the dehumidifier can only provide 45°C without a critical drop in performance, then a cascade scheme is used: the dehumidifier heats water from 20°C to 45°C, and a heat pump boosts it from 45°C to 60°C.
Systems with inverter compressor control can partially compensate for capacity drop by increasing rotational speed, but this increases electricity consumption. A balance between capacity and energy use must be found.
When integration makes engineering sense — application criteria
Integrating a dehumidifier with a heating system is advisable if all of the following conditions are met simultaneously:
1. Stable moisture loads — the dehumidifier operates not sporadically, but at least 10–15 hours per day for 6+ months per year. Typical facilities: swimming pools, laundries, drying areas, vegetable stores, pharmaceutical production.
2. Presence of a continuous low-temperature heat consumer up to 50°C: underfloor heating, pool heating, supply air preheating, low-temperature radiators, process heating.
3. A solution for the summer period: a year-round consumer (pool), a heat rejection system (dry cooler, cooling tower), or a coordinated operating regime (the dehumidifier runs at night when the heat does not interfere with daytime air conditioning).
4. Rational ratio of capacities — the dehumidifier’s heat output is at least 20–30% of the site’s base heating load; otherwise, the complexity of integration does not pay back the capital costs.
Integration does not make engineering sense if:
- The dehumidifier operates sporadically (1–2 hours per day, summer only).
- There are no low-temperature consumers (only high-temperature heating >70°C or domestic hot water without the possibility of a cascade scheme).
- The project economics are irrational: the cost of integration (heat exchanger, piping, controls, installation) exceeds 8–10 years of savings on energy carriers at current tariffs.
Boundary conditions under which approaches do not work or require correction:
- Indoor temperature <15°C — dehumidification efficiency drops sharply; condensation on the evaporator is impeded due to low evaporation temperature.
- Condensation temperature >60°C — most residential and commercial compressors are not designed for such high pressure, which can lead to high-pressure trips or failure.
- Regions with a very short heating season (<3 months) — the payback of integration decreases due to the small number of hours of recovered heat use.
Common design mistakes
Mistake one: ignoring the dehumidifier’s heat rejection when calculating the cooling load. Consequence: in summer the air conditioner cannot cope, indoor temperatures exceed the norm, discomfort ensues. For example, in a pool a dehumidifier has 25 kW of heat output, but the cooling design accounts only for moisture emissions from occupants and solar gains, ignoring the dehumidifier’s heat rejection. Result — a 3–5 kW shortfall in cooling capacity, overheating of the room.
Mistake two: no provision for summer heat rejection. Consequence: in summer the dehumidifier either cannot operate (high-pressure trip due to excessive condensing pressure) or overheats the room (additional load on the air conditioner). Solution: plan a dry cooler or summer consumer (pool, process heating) at the design stage, allocate space, piping, and power supply.
Mistake three: incorrect selection of heat transfer fluid temperature without analysing the impact on dehumidification. For example, the client wants 60°C for domestic hot water, and the designer connects the dehumidifier directly without a cascade. Result: condensation temperature rises to a critical level (55–60°C), dehumidification capacity drops, and indoor humidity is not maintained at the design level. Solution: cascade scheme (dehumidifier heats to 45°C, boiler or heat pump boosts to 60°C) or limiting the maximum heat transfer fluid temperature.
Mistake four: no buffer tank in a system with variable heat demand. Consequence: the dehumidifier is controlled by humidity (switching on and off via a humidistat), the heating consumer is controlled by temperature (thermostat), leading to mismatched operating regimes; frequent compressor starts/stops cause equipment wear. Solution: a 300–500 litre buffer tank for commercial systems to smooth short-term mismatches in regimes.
Mistake five: long distances between the dehumidifier and the consumer without calculating heat losses. Example: dehumidifier in the basement, consumer on the roof, 50 m distance, uninsulated pipes or with thin insulation. Result: heat losses in the piping can constitute a significant part of the useful capacity. Solution: place the dehumidifier closer to the consumer or provide quality pipe insulation 50–100 mm thick.
Mistake six: overestimating the dehumidifier as a full replacement for a heat pump or boiler. Reality: a dehumidifier provides as much heat as the moisture it removes. If moisture loads are small or seasonal, the heat is also limited. In winter, with low indoor humidity, the dehumidifier hardly runs, hence there is little or no heat when it is most needed. Solution: a realistic calculation of heat potential considering the annual moisture generation profile and the building’s heat losses.
Mistake seven: ignoring the need for water circuit maintenance. If water is hard and not treated, scale forms on the heat exchanger surfaces, reducing heat transfer efficiency. Solution: water treatment (softening or demineralisation) or periodic chemical cleaning of the heat exchanger every 1–2 years.
Frequently asked questions (FAQ)
Question one: What are the temperature limits for the heat transfer fluid when recovering heat from a dehumidifier condenser?
Answer: The minimum temperature is limited by the need for sufficient temperature difference for heat exchange (typically 5–7 K), i.e., not lower than 15–20°C, which in practice is not a limitation for heating systems, as heating return water is usually higher. The maximum temperature depends on the compressor’s permissible condensing pressure. For most R410A dehumidifiers, the outlet temperature of the heat transfer fluid should not exceed 50–55°C, depending on compressor model. Industrial models with high-pressure compressors can provide up to 60–65°C. Exceeding these values can lead to high-pressure trips or compressor failure.
Question two: Can a dehumidifier fully replace a heating system?
Answer: For facilities with stable moisture loads (swimming pools, laundries, drying areas, vegetable stores) and low-temperature heat consumers (underfloor heating 30–40°C, pool heating 28°C) — possibly as a primary heat source in the shoulder seasons (spring–autumn) and partially in winter, provided there is a backup source (electric boiler, gas boiler) for peak frosts. For typical residential, office, or retail premises without significant and continuous moisture loads — no, because the amount of available heat is limited by dehumidification capacity. If moisture loads are small, in winter (when indoor air is dry) the dehumidifier hardly runs, hence there is little or no heat when it is most needed.
Question three: What should be done with heat in summer if heating is not needed?
Answer: There are three options. First — a year-round heat consumer (a pool requires heating even in summer; process heating in industry) so heat is continuously recovered. Second — a dry cooler or cooling tower to reject heat to atmosphere (additional capital costs for equipment plus operating costs for fan electricity). Third — disable the water circuit in summer; the heat goes into the room as in standard dehumidifier operation, but this requires greater air conditioning capacity.
Question four: How does integration affect dehumidification efficiency?
Answer: Increasing the cooling water temperature at the condenser increases the refrigerant condensation temperature, which raises condensing pressure, reduces compressor mass flow, and as a result reduces dehumidification capacity. The magnitude of the effect depends on compressor type, refrigerant, and operating regime. A compromise is to limit the maximum outlet temperature of the heat transfer fluid at the heat exchanger (usually not above 45–50°C) to preserve acceptable dehumidification performance, or to use a cascade scheme in which the dehumidifier operates with a lower condensation temperature and supplementary heating is provided by another source.
Question five: How to evaluate the economic effect of integration?
Answer: The calculation consists of several steps: determining recovered heat over the heating season, determining the substituted energy, calculating annual savings, and determining the payback period. This requires data on average dehumidification capacity, total operating hours per season, gas and electricity tariffs, and the cost of equipment and installation. Specific figures depend on many factors, and without specific project input data an accurate calculation is not possible.
Conclusions
Integrating a dehumidifier with a heating system or heat pump via condenser heat recovery is an effective engineering solution for facilities with stable moisture loads and low-temperature heat consumers. However, it is not a universal solution but a tool for specific conditions.
Key success conditions: correct thermal balance, a clear energy balance at the condenser, alignment of temperature levels (maximum heat transfer fluid temperature matched to compressor capabilities), the presence of a solution for the summer period (dry cooler, year-round consumer, or coordinated operating regime), and realistic expectations (understanding that the amount of heat is limited by moisture generation, not by the building’s heat losses).
Recommendations for design engineers: always analyse the possibility of heat recovery at the design stage, even if implementation is deferred; provide provisions and reserve space for equipment; perform a detailed calculation with specific input data; allow for future upgrades.
Feasibility criteria for integration: stable moisture loads for 6+ months, the presence of a low-temperature consumer up to 50°C, and a solution for the summer period. If at least one condition is not met, integration requires additional techno-economic justification.
Common mistakes to avoid: ignoring the thermal balance when designing cooling, overestimating the ability to replace primary heating, no summer heat rejection system, and incorrect equipment placement leading to large heat losses in piping.
Heat recovery from a dehumidifier is not a universal solution but an engineering tool for specific conditions. Success depends on design quality, a detailed thermal balance, and a realistic calculation of economic feasibility for a specific facility with specific input data.
