FAQ

It extracts heat from the surroundings (air, ground or water), the compressor concentrates it to a higher temperature and transfers it to the heating water. Think of it as a fridge in reverse – it cools outside and heats inside.

There are three main types:

• Air-to-water: extracts heat from outdoor air, simplest installation, slightly lower efficiency in freezing weather
• Ground-to-water: extracts heat from a ground collector or borehole, stable output year-round, higher initial investment
• Water-to-water: uses underground water from a well, highest efficiency of all types, requires a productive well and permits

Air-to-water – thanks to simpler installation and lower acquisition costs, it accounts for roughly 80% of the market.

Main advantages of a heat pump:

• Running costs are 3–4× lower than an electric boiler and 2× lower than gas
• Zero on-site emissions
• The pump can cool in summer
• No dependence on gas prices

Limitations of heat pumps:

• Higher acquisition cost than conventional boilers
• Air-to-water efficiency drops in severe frost
• A backup source (built-in electric boiler) is needed for extreme cold
• The outdoor unit produces noise, but this is manageable with correct placement

15–25 years with proper maintenance. The compressor is the most expensive component – quality brands last the full lifespan of the unit.

Heat pump maintenance:

• Regular inspection every 2 years
• Cleaning outdoor unit filters as needed (leaves, dust)
• Checking system pressure and sensor function
• Units under 20 kW do not require mandatory periodic inspections

COP is instantaneous efficiency – e.g. COP 4 means 1 kWh of electricity produces 4 kWh of heat. SCOP is the seasonal average that accounts for varying outdoor temperatures throughout the year. SCOP is the more realistic figure when comparing pumps.

The greater the difference between the source temperature and the required output temperature, the harder the compressor has to work. At −15 °C outdoors, the pump must bridge a larger temperature gap, which reduces efficiency.

Practically always when replacing a heating source or building a new house. Payback is typically 5–10 years, lifespan 15–25 years. With subsidies, payback shortens further.

Yes, but with care. For uninsulated buildings, we recommend at least partial insulation or a combination with an electric boiler for peak loads. The HOTJET ZETXp with R290 reaches output temperatures up to 75 °C, which suits older radiator systems.

Yes. HOTJET reaches output temperatures up to 65 °C (up to 75 °C with the ZETXp series). Low-temperature systems like underfloor heating or fan coils are ideal, but radiators work too.

Best from project documentation (building heat losses). As a rough guide: an insulated house is approximately 50–70 W per m², an older uninsulated house 100–150 W per m². For a 150 m² insulated house, that works out to approximately 10 kW.

An undersized pump relies on the backup electric boiler more often, raising costs. An oversized pump is unnecessarily expensive and cycles on/off more frequently (less of an issue with an inverter, which modulates output smoothly). Correct sizing is the foundation of efficient operation.

Simple formula: (pump cost minus subsidy) divided by annual savings. Typically 5–10 years. Example: investment CZK 300,000 minus subsidy CZK 90,000 = CZK 210,000. With annual savings of CZK 30,000, payback is 7 years.

A complete installation for a family house ranges from approximately CZK 250,000 to CZK 400,000 including assembly, before subsidies. The price difference between output ranges is not dramatic.

For a typical 150 m² house: heating approximately CZK 15,000 per year, hot water approximately CZK 5,000 per year. Total around CZK 20,000 per year. With photovoltaics, costs can drop close to zero.

Compared to gas: 40–60% savings. Compared to an electric boiler: 60–75% savings. Exact figures depend on insulation, temperature settings and electricity tariff.

Yes. The most common programme is Nová zelená úsporám (NZÚ). For air-to-water typically CZK 80,000–130,000, for ground-to-water CZK 100,000–180,000. In the Ostrava region, amounts are even higher: CZK 94,500 for air-to-water and CZK 130,000 for ground-to-water.

Applications are submitted via the NZÚ portal. You provide the project, invoices and a commissioning report. We recommend consulting the specific conditions with us or a subsidy advisor – we are happy to help with the entire process.

Usually not – a notification to the building authority is sufficient. Neighbours' consent may be required when placing the outdoor unit close to a property boundary due to noise limits.

A monoblock has all components in one outdoor unit; only water pipes enter the house – simpler installation. A split has outdoor and indoor units connected by refrigerant lines – a certified refrigeration technician is required. HOTJET primarily offers monoblocks.

Monoblock virtually always – trouble-free installation anywhere. Split only in specific space constraints. With a HOTJET monoblock, no intervention in the refrigerant circuit is needed, so a standard plumber can install it.

An inverter smoothly adjusts compressor output to the building's current demand. Yes, it is worth it – it delivers 20–30% savings, quieter operation and longer lifespan. All HOTJET models (except ONE2) are inverter-driven.

Modern inverter monoblocks range from 35–55 dB, comparable to a quiet fridge to an air conditioner. HOTJET is among the quieter on the market. For example, the ZETXp series has a sound power of 49 dB at A2/W55, while the ground-to-water WX series is just 38 dB.

Yes, with correct placement. The recommended distance from the property boundary is at least 2–3 metres, ideally directed away from neighbours' windows. HOTJET noise measurements follow the new, stricter European standard.

The indoor unit or hydromodule takes up approximately 0.5 m² of wall space. A buffer tank and hot water cylinder add another 1–2 m². A small utility room is sufficient.

Partially – behind a noise barrier wall or in a technical alcove. Free air intake and exhaust must be maintained for the outdoor unit to function properly. The alternative is the ZETXi indoor model, where nothing is visible outside at all.

Via room controllers (wired or wireless), a webserver for remote access over the internet, and a mobile app. The regulation automatically adjusts output based on outdoor temperature (weather-compensated control).

Yes, all HOTJET models provide active cooling. In summer they run in reverse and cool the house. Ground-to-water models also offer passive cooling (free cooling) with a power draw of just 50–100 W.

Yes, both. Hot water is handled by a cylinder (typically 200–300 litres) or flow-through heating. A pool connects via a 3-way valve – the pump automatically switches between heating, hot water and pool. All of this is included as standard with HOTJET.

An ideal combination. During the day, PV panels feed the heat pump, which heats water and stores thermal energy. Running costs can drop close to zero. PV support is included in the HOTJET base price – no surcharge.

Yes, this is called bivalent (hybrid) operation. The heat pump heats down to a certain outdoor temperature (e.g. −5 °C), below which the boiler takes over. HOTJET's regulation dynamically selects the most efficient source based on energy prices. Suitable for older uninsulated houses.

Check the error code on the display or webserver. Contact HOTJET service – technicians connect remotely and diagnose the issue. Most faults are resolved via remote diagnostics or phone guidance. A physical visit is only needed for hardware problems.

Most common faults and prevention:

• Frozen condensate – caused by a blocked drain; prevention: regular cleaning
• Low flow – caused by a clogged filter; regular filter checks
• Sensor error – due to age or moisture; addressed during a service inspection every 2 years

Yes, by an estimated 3–5%. Buyers appreciate low running costs and modern technology. The building's energy label improves significantly, which is an important factor when selling.

Very. 70–80% of the energy comes from a renewable source (air, ground, water). CO₂ emissions are 3–4× lower than a gas boiler. With photovoltaics, operation is practically emission-free. HOTJET pumps with R290 refrigerant (propane) have a GWP of just 3 – that is 225× less than the common R32 refrigerant.

The distributor assigns tariff D57d with 20–22 hours of low-rate electricity per day. The heat pump runs primarily during the cheap rate and the buffer tank stores heat for the more expensive hours. This significantly reduces running costs.

Yes, but with lower efficiency (COP drops to approximately 2). HOTJET operates down to −25 °C. In extreme frost, the built-in electric boiler kicks in automatically – comfort is maintained, only costs rise slightly. These extremes typically last just a few days per year.

Approximately 6–7%. Each extra degree means higher building heat losses. We recommend setting the temperature to 20–21 °C and reaching for a jumper rather than turning up the thermostat.

A fan draws outdoor air across the evaporator, where the refrigerant absorbs heat. The compressor concentrates the heat and transfers it via the condenser to the heating water. Even at −15 °C, there is enough usable energy in the air.

Family houses (new builds and renovations), smaller apartment buildings, commercial buildings up to approximately 50 kW, and anywhere a borehole or well is not feasible. It is the most universal heat pump type.

Advantages:

• Lowest acquisition cost
• Simple installation with no earthworks
• Wide output range

Disadvantages:

• Efficiency depends on outdoor temperature (drops in frost)
• Outdoor unit produces noise
• Slightly higher running costs than ground-to-water or water-to-water

Overview of HOTJET air-to-water series:

• ZETXe: proven series with R454B refrigerant for outdoor installation
• ZETXp: flagship with ecological propane R290, output temperature up to 75 °C and the new Copeland YHV compressor
• ZETXi: indoor variant for locations where an outdoor unit cannot be placed (historic buildings, apartment blocks)
• ONE2: budget variant without inverter

The ZETXp is the new generation with natural refrigerant R290 (propane) with a GWP of just 3. It reaches output temperatures up to 75 °C, ideal for older radiator systems. It uses the new Copeland YHV scroll inverter compressor. Models available in 7, 10 and 15 kW with real output up to 10, 15 and 25 kW. Energy class A+++ in both categories.

Size it according to the building's heat loss. As a rough guide: an insulated 150 m² house needs approximately 7–10 kW, an older uninsulated house of the same size 15–20 kW. An inverter pump (ZETXe, ZETXp) modulates output smoothly, so slight oversizing is not a problem.

With a HOTJET monoblock, two water pipes (supply and return) and one electrical cable are enough. No refrigerant lines through the facade, no refrigeration technician. A standard plumber can handle the installation.

Through automatic defrosting by reverse cycle. The pump briefly reverses the refrigerant flow direction, which defrosts the evaporator. The process takes just a few minutes and the house does not noticeably lose heat.

Yes. It is designed for year-round outdoor operation at temperatures from −25 to +45 °C. The casing is powder-coated, and the electronics are protected against moisture and dust. HOTJET also offers custom colour finishes.

The ZETXi is an indoor heat pump – the complete unit stands inside the house and air is ducted through the facade. Nothing is visible outside at all. Ideal for historic buildings, apartment blocks or locations with strict noise regulations. Technically identical to the outdoor series; only the installation method differs.

15–25 years with proper maintenance. The key is to regularly clean filters and the evaporator (leaves, dust) and maintain free space around the unit for airflow.

Ideally as close as possible – we recommend within 10 metres. Longer piping means higher heat losses and more complex installation. Correct placement regarding noise is also important – aim away from neighbours' windows.

Yes. The pool connects via a 3-way valve and the regulation automatically switches between house heating, hot water and pool. This feature is included as standard with HOTJET at no extra charge.

All features included in the base price:

• Photovoltaic support
• Hybrid operation (combination with a gas boiler)
• Pool control
• Active cooling
• Webserver with mobile app
• Remote diagnostics

No expensive feature packages like some competitors – you only pay for additional I/O modules if needed.

SCOP at W35 (underfloor heating): 4.87, equivalent to 192% efficiency. SCOP at W55 (radiators): 4.14, equivalent to 163% efficiency. These are among the highest values on the market in this output category.

Usually a notification to the building authority is sufficient. For R290 propane pumps, no refrigeration technician certification is required – R290 is not an F-gas, so the related legislative obligations do not apply.

Sound power of 49–52 dB under operating conditions A2/W55. At a distance of one metre, that corresponds to approximately 39 dB. Measurements follow the new, stricter European standard.

The ZETXe uses R454B refrigerant and a Copeland XHV compressor. The ZETXp has ecological R290 refrigerant (propane, GWP 3), the newer Copeland YHV compressor, a higher output temperature of up to 75 °C and better SCOP values. The ZETXp is technologically more advanced – the HOTJET flagship.

Yes. A cascade of multiple units covers even higher heat losses for large buildings. HOTJET's regulation handles cascade control as standard.

Often yes. New inverter pumps are 30–50% more efficient than models that are 10–15 years old. HOTJET offers a trade-in programme – we buy back old units at residual value. With a subsidy, the switch can be very cost-effective.

An antifreeze solution circulates in pipes buried in the ground (collector or borehole), where it absorbs stable heat from the earth (8–12 °C year-round). The compressor concentrates this heat and transfers it to the heating water. Thanks to the stable source temperature, ground-to-water achieves higher and more consistent efficiency than air-to-water.

A horizontal collector is laid 1.5–2 m below the surface, occupies 200–400 m² of garden and has lower investment costs. A borehole reaches 80–150 m deep, takes up minimal space, but requires a geological survey and is more expensive. A borehole offers more stable output; a collector fluctuates slightly with the weather.

Roughly 15–18 m of borehole per 1 kW of heating output. Example for 10 kW: one borehole of 150 m or two boreholes of 80 m each. The number and depth depend on geology – rocky subsoil needs fewer metres, sand needs more. HOTJET offers complete pipe sets for outputs from 4 to 21 kW.

The heat pump itself costs comparably (CZK 160,000–180,000). The main difference is the heat source cost: air-to-water is CZK 0 (air is free), ground-to-water is CZK 150,000–250,000 for boreholes. Total: air-to-water CZK 200,000–300,000, ground-to-water CZK 350,000–450,000. However, annual running costs are CZK 5,000–8,000 lower with ground-to-water.

Ground-to-water pays off when:

• You plan to live in the house for 15+ years
• You have higher heat losses (older house, radiators)
• You want maximum savings and stability
• Outdoor unit noise is a concern
• You want to use passive cooling in summer

Size it to cover 100% of heat losses, because bivalence with an electric boiler is not necessary – ground-to-water delivers stable output even in frost. The HOTJET WX series in models 7, 10 and 15 kW covers most family houses. For larger buildings, the 40WXi (6–40 kW inverter) or models 33W and 55W are available.

For boreholes you need:

• Geological survey – determines site suitability
• Hydrogeological assessment – evaluates impact on groundwater
• Water authority permit for boreholes deeper than 30 m
• Building permit or notification depending on scope
• Borehole approval

The entire process takes 2–4 months.

On a small plot, yes – a deep borehole takes up minimal space (approximately 2×2 m for the drilling rig). At higher altitudes, also no problem – underground temperature is stable regardless of elevation.

Trees cannot be planted above the collector, as roots would damage the pipes. Lawn and low plants grow without restriction. In winter, the grass above the collector may be slightly cooler. The area above the collector cannot be built on (garage, terrace) to maintain access for potential repairs.

Yes, and this is a major advantage of ground-to-water. Passive cooling (free cooling) means the compressor does not run – only the circulation pump operates. The ground in summer is at 10–12 °C and cools the floor to a comfortable 18–20 °C. Electricity consumption is just 50–100 W, a fraction of what air conditioning uses.

Yes. The pool connects via a 3-way valve and the pump switches automatically. For larger buildings, the HOTJET 40WXi with output up to 40 kW or a cascade of multiple units is available. The advantage is stable output even during all-day operation.

PE-RC piping lasts 50+ years (the material is resistant to pressure and ageing). The borehole itself has virtually unlimited lifespan. The antifreeze solution is replaced after 15–20 years. The heat pump itself lasts 15–25 years.

Water from a well (stable 8–12 °C year-round) is pumped directly into the heat pump's tube heat exchanger, where it transfers heat to the refrigerant. The cooled water (3–4 °C cooler) flows to an absorption well or watercourse. Thanks to the highest and most stable source temperature, it achieves the highest COP of all types.

Advantages:

• Highest COP (4.5–6)
• Stable output year-round
• Lowest running costs
• Quiet operation, no outdoor unit
• Tube heat exchanger withstands even freezing

Disadvantages:

• A productive well is required
• Legislation and permits
• An absorption well is needed
• Dependent on water quality
• Well pump maintenance

Water source requirements:

• Flow rate at least 1.5–2 m³ per hour per 5 kW of output
• Temperature ideally 8–12 °C year-round
• Quality – low iron, manganese and calcium content
• Stable water level that does not fluctuate significantly
• Two wells needed: extraction and absorption, at least 15–20 m apart

We recommend a hydrogeological assessment before investing.

HOTJET is one of the few manufacturers that allow direct water connection from the well without an intermediate heat exchanger. We use a special tube or coaxial heat exchanger that withstands freezing and handles impurities better than a standard plate heat exchanger. The well pump is external with a frequency converter – fully inverter-controlled.

For water-to-water you need:

• Hydrogeological assessment (source capacity and quality)
• Groundwater extraction permit from the water authority
• Discharge permit for the absorption well
• Building permit for wells and technology
• Final approval

The entire process takes 3–6 months. The legislation is stricter than for other types, but manageable.

A HOTJET WX heat pump costs CZK 166,000–182,000. Wells, pump and permits add another CZK 80,000–150,000. Total investment is CZK 250,000–350,000. Annual running costs for a 150 m² house are CZK 8,000–12,000. Compared to air-to-water, you save CZK 5,000–8,000 per year.

Yes. The HOTJET WX provides active cooling. Well water in summer (10–12 °C) is an ideal cold source. Cooling is very efficient and economical, and can be combined with underfloor cooling or fan coils.

Water-to-water makes sense when:

• You have an existing productive well or certainty of sufficient water
• Lowest running costs are the priority
• You have a radiator system with higher temperatures
• You want maximum efficiency and quiet operation
• You plan to live in the house long-term

On the other hand – if the water supply is uncertain, the local legislation is complex, or the budget is tight, choose a different type.

A buffer tank is a heating water reservoir that serves several functions:

• Hydraulic separation of the heat pump circuit from the heating circuits
• Reduced cycling (prevents frequent compressor starts)
• Heat storage for later use
• Balancing the difference between pump output and building demand

You may encounter various terms in practice:

• Buffer tank – general term, usually 500+ litres
• Buffer vessel – same thing, emphasis on temperature balancing
• Anti-cycling tank – emphasis on reducing compressor cycling, even smaller volumes 100–300 litres
• Hydraulic separator – minimal volume, hydraulic separation only

Tanks of 100–200 litres serve mainly for hydraulic separation and anti-cycling, not heat storage.

It depends on the pump type:

• For on/off pumps (ONE2) it is essential – without it the compressor cycles excessively
• For inverter pumps (ZETXe, ZETXp, WX) it is recommended, though not technically mandatory
• For systems with multiple circuits it is necessary for hydraulic separation
• When combined with photovoltaics it is strongly recommended for storing surplus energy

An on/off pump runs either at full power or not at all. Without a buffer, the compressor cycles every few minutes, causing high wear, lower efficiency (startups are energy-intensive) and shorter lifespan. With a buffer, the compressor runs longer cycles, ensuring stable operation.

Inverter pumps (ZETXe, ZETXp, WX) adjust output to building demand, so a buffer is not technically mandatory. We still recommend one for:

• Better use of photovoltaic surplus
• Bridging HDO lockout periods (low tariff)
• More stable operation during rapid demand changes
• Passive cooling capability with ground-to-water

Minimum volume against cycling: pump output in kW × 15–20 = minimum litres. Example: a 10 kW pump needs at least 150–200 litres. Optimal volume including storage: output in kW × 50–75 = optimal litres. For a 10 kW pump that is 500–750 litres.

One litre of water heated by 1 °C stores 1.16 Wh of energy. Example: a 500-litre tank heated from 40 to 55 °C (difference of 15 °C) stores 500 × 15 × 1.16 = 8,700 Wh, i.e. 8.7 kWh. That is approximately 2–3 hours of heating for a typical family house.

HOTJET buffer tank range:

• BF120 (120 litres) – anti-cycling tank, wall or floor mount
• BF200 (200 litres) – anti-cycling or small buffer
• B200 (200 litres, 167 litres usable volume) – cylinder with heat exchanger
• B300 (300 litres, 238 litres usable volume) – cylinder with heat exchanger
• B500 (500 litres, 426 litres usable volume) – for larger systems

The heat pump feeds the tank via the primary circuit. From the tank, secondary circuits are fed – radiators, underfloor heating, hot water cylinder. The tank acts as a hydraulic separator that decouples flows and pressures on both sides. Never connect multiple circuits with different flow rates directly to the pump without a buffer.

A hydromodule contains everything in one: regulation, circulation pump, electric boiler and valve. Price around CZK 45,500. A switchboard contains only the regulation for approximately CZK 21,000–22,000, and the other components are purchased separately. Use either a hydromodule or a switchboard – never both together. Hydromodules are not used with ground-to-water and water-to-water.

Surplus electricity from PV panels can be stored in the buffer tank as heat. When PV produces more electricity than the house consumes, the regulation turns on the heat pump and heats the water in the tank to a higher temperature (e.g. 55 °C instead of 45 °C). In the evening, the stored heat is gradually used. For PV systems, choose a larger tank (500+ litres).

Rule of thumb: PV output in kWp × 100–150 = recommended litres. Example: an 8 kWp PV system calls for 800–1,200 litres of buffer capacity for optimal surplus utilisation.

The distributor can switch off the low tariff for up to 6 hours per day in blocks of at most 2 hours. During lockout, the heat pump does not run. A buffer tank bridges the gap: a 500-litre tank with a 15 °C temperature difference holds approximately 8.7 kWh, which covers 2–3 hours of heating for a typical house.

Yes, two options:

• Separate tanks: buffer tank for heating (35–55 °C) and a separate hot water cylinder (55 °C+) – simpler regulation
• Combined tank (BOLLY): an inner hot water tank inside the outer buffer – saves space, more compact solution

Most common mistakes:

• Tank too small – go bigger, the price difference is modest
• Connecting multiple circuits directly to the pump without a buffer
• Missing tank insulation
• Incorrect sensor placement – must follow the manufacturer's documentation
• Underestimating HDO lockout – size for at least 2 hours of bridging

Yes, always. An uninsulated tank loses up to 10% of stored heat per day. An insulated tank loses under 2% per day. Standard insulation is 50–100 mm. The investment pays for itself within the first heating season. Insulate the connecting pipes too, not just the tank.

Buffer tanks are practically maintenance-free. Once a year, check:

• Tightness of joints and fittings
• Condition of insulation
• Function of the safety valve
• System air venting

Every 3–5 years, we recommend checking the anode protection on steel tanks and flushing if the water is hard.

Not in the heating circuit – legionella multiplies at 25–45 °C in stagnant potable water, but heating water circulates and is not intended for drinking. For combined tanks (BOLLY), heat the inner potable water reservoir to at least 55 °C. HOTJET's regulation includes a periodic anti-legionella function.

The tank itself does not – it is a passive reservoir. Electricity is needed for:

• The circulation pump (standalone or in the hydromodule)
• The regulation (controls charging and discharging)
• An optional electric boiler for backup heating

The price difference between BF120 (120 litres, CZK 10,140) and BF200 (200 litres, CZK 17,056) is under CZK 7,000, but capacity is 75% higher. A larger tank pays off especially when you plan photovoltaics, have frequent HDO lockouts, or combine multiple heating circuits.

Ideally in the utility room or boiler room, as close to the heat pump as possible, on a solid base (a full tank is heavy – BF120 weighs 143 kg full, BF200 235 kg, B500 over 500 kg). Ensure access for maintenance.

Yes. The BF120 is designed for both wall mounting and floor standing. Conditions for wall mounting: a load-bearing wall (not plasterboard), quality anchors into concrete or brick. Allow for a weight of approximately 143 kg when filled with water.

Recommended combinations:

• 7ZETXe/p: BF120 or BF200 (lower output, anti-cycling tank suffices)
• 10ZETXe/p: BF200 or B300 (mid-range output)
• 15ZETXe/p: BF200 to B500 (depending on system size)
• 20–30ZETXp: B500 and above (larger systems)
• ONE2 (on/off): at least BF200, mandatory against cycling
• WX (ground-to-water): BF200 or B300 per project requirements

Technically yes, but we recommend HOTJET tanks:

• Optimised for our systems
• Compatible connections and sensors
• Unified warranty and service
• Complete technical support from a single source

Commonly overlooked items:

• Expansion vessel for pressure compensation during heating
• Safety valve for overpressure protection
• Air venting – air in the system reduces efficiency
• Thermometer and pressure gauge for status monitoring
• Pipe insulation – not just the tank, but also the connecting pipes

The Siemens regulation monitors sensors in the tank: upper sensor (heating temperature), lower sensor (return temperature) and outdoor sensor (weather-compensated control). When the temperature drops below the set threshold, it starts the pump. When the target temperature is reached, it stops or reduces output. Inverter pumps modulate output smoothly.

Direct costs are practically zero – the tank itself consumes no energy. Heat losses from a well-insulated tank are 1–2% per day. Indirect savings, however, outweigh: fewer compressor starts = lower consumption, better use of low tariff = cheaper electricity, longer pump lifespan.

Common causes:

• Air in the system (needs venting)
• Expansion during heating (normal)
• Water circulation (normal operating sound)

Call service for persistent loud gurgling, water leaks or unusual knocking.

The BOLLY combines a hot water cylinder with a buffer tank in one unit:

• BOLLY250 (235 litres, inner cylinder 86 litres, price CZK 54,250)
• BOLLY300 (291 litres, inner cylinder 86 litres, price CZK 57,700)
• BOLLY500 (498 litres, inner cylinder 108 litres, price CZK 77,950)

Saves space, more compact solution.

Quick guide:

• On/off pump (ONE2): mandatory, at least 200 litres
• Inverter without PV: recommended, BF120 or BF200
• Inverter with PV: strongly recommended, 500+ litres
• Multiple heating circuits: mandatory for hydraulic separation
• Ground-to-water with cooling: recommended for passive cooling

Key points:

• Go bigger – the price difference is small, but the benefits are large
• Plan for PV – even if you do not have it yet, prepare with a larger tank
• Invest in quality insulation
• Place sensors exactly as per documentation
• Consult the design with a specialist – every house is different

Three basic methods:

• Cylinder heating – classic with a tank, water is heated directly in the heat exchanger
• Combined tank (BOLLY) – combines DHW cylinder with a buffer tank in one unit, more compact
• Flow-through heating (Fresh Water Heater) – only the heating water in the tank is heated, and domestic water is heated on demand in a plate heat exchanger

Flow-through heating offers:

• Hygiene: no deposits or sediment, minimal legionella risk, no cylinder cleaning needed
• Operational advantages: longer lifespan, easier serviceability, precise temperature control, more economical operation
• Flexibility: easy integration of multiple heat sources including solar, option to add more tanks

Recommended volumes:

• For 1–2 people: 300 litres is sufficient
• For 3–4 people (standard family house): 500 litres
• For 5+ people or multiple bathrooms: 800–1,000 litres

If you plan photovoltaics, choose a larger tank for better surplus use. Go bigger – the price difference is relatively small, but the operational benefits are noticeable.

HOTJET's regulation has an integrated anti-legionella function. It automatically monitors the cylinder temperature and when needed runs a thermal disinfection cycle that heats the water to a bacteria-killing temperature. With flow-through heating, legionella practically does not occur because water is not stored.

Recommended temperature settings:

• Nominal hot water temperature: 48 °C (regulation line 1610)
• Setback temperature: 40 °C (line 1612)
• Maximum temperature: 65 °C (line 1614)
• Switching differential: 5 °C (line 5024)

48 °C is the optimal compromise between comfort and energy savings – higher temperatures reduce pump efficiency.

One sensor (B3): simpler wiring, 5 °C switching differential, suitable for smaller cylinders. Two sensors (B3 and B31): more precise regulation, smaller switching differential (2 °C), better utilisation of cylinder capacity. Two sensors are suitable for larger cylinders and more demanding applications.

A special function for immediate hot water heating. When activated, it immediately starts charging the DHW cylinder regardless of the normal switching differential. Useful before an expected peak demand, for example in the morning before showers or in the evening.

Products for hot water heating:

• Classic cylinders B200, B300 and B500 for simpler installations
• Flow-through heaters FW300+, FW500+, FW800+ and FW1000+ for maximum hygiene
• Combined tanks BOLLY250, BOLLY300 and BOLLY500 for a compact solution

The choice depends on household size and priorities (hygiene, space, budget).