The question “heat pump or gas” usually reaches me too late: the house is drawn, the heating layout set, the client holding two quotations. By then the decision has largely been made, it just was not said out loud. The economics of a heat pump depend not on the brand but on the water temperature the heating system was designed around. The wider context of a house energy concept is covered in Passive House from A to Z.
Why does the answer change depending on what you weight?
The strengths of the two options sit at opposite ends of the timeline. A condensing boiler wins at the entrance: the appliance is small, the installation routine. An air-to-water heat pump wins on running cost, delivering several times as much heat as the electricity it consumes, but its capital cost including hydraulics, tanks and electrical work is typically a multiple of a boiler's.
Rather than guessing future energy prices, the comparison can rest on a ratio taken from your own bills. If the electricity price per kilowatt hour divided by the gas price is lower than your system's seasonal coefficient of performance (SCOP), the pump is cheaper to run; if higher, the boiler is. Two variables remain: the price ratio, which you do not control, and the SCOP, which the designer sets. Hence the opposite answers in two different houses.
One item gets overlooked here: the fixed gas distribution charge is independent of consumption, so the better the building envelope, the larger the share of the bill made up of standing charges.
| Criterion | Condensing boiler | Air-to-water heat pump |
|---|---|---|
| Capital cost of source and hydraulics | Lowest of the common options | Typically a multiple of a boiler |
| What sets the running cost | Gas price and whether the boiler condenses | Electricity price divided by SCOP |
| Sensitivity to design temperature | Moderate: above 55 degrees C on the return it stops condensing | High: every extra degree cuts SCOP |
| Regulatory risk over 20 years | Higher, regulation runs against fossil fuels | Lower, the risk sits in the electricity price |
Why does design flow temperature govern heat pump economics?
The coefficient of performance (COP) is a property not of a machine but of a pair of temperatures: the wider the gap between outdoor air and heating water, the more electricity the pump needs. Catalogue figures therefore come with a test point, for example A7/W35 (air at 7, water at 35 degrees C). It says nothing about a January morning; the only meaningful comparison is SCOP at the design temperature the system will run at.
Here is the core of it. Design flow temperature is not a controller setting, it is geometry: how much heat the house loses in a freeze and how much surface area is available to give it back. Radiant floor heating has the area of the whole floor and works at low temperature; radiators on a classic drop need hot water. Emitter area cannot be changed after completion without demolition, so the decision about the heat source is really taken when a low-temperature heating system is designed, or quietly abandoned. The other side of the equation is the envelope: a lower heat loss lets the same emitter area work at a lower water temperature.
| Design temperature (flow/return) | What it means in practice | Effect on the pump | Effect on the boiler |
|---|---|---|---|
| 30/25 to 35/28 degrees C | Floor heating throughout, good envelope | Best achievable SCOP | Condenses continuously, efficiency 90 to 95 percent |
| 40/33 to 45/38 degrees C | Wider loop spacing or oversized radiators | Good SCOP | Condenses for most of the season |
| 50/45 to 55/45 degrees C | Tightly sized radiators | SCOP noticeably lower | Condenses only at part load |
| 70/55 degrees C and above | Classic radiator drop | The pump loses its advantage | Almost never condenses |
What does a condensing boiler need in order to condense?
A condensing boiler gets its efficiency not from the burner but from cooling flue gas below its dew point and recovering the latent heat of the water vapour. That happens only when the return water is below roughly 55 degrees C, which on a 70/55 radiator drop almost never occurs. The second reason is oversizing: a boiler that turns down only to twenty or thirty percent of output reheats quickly on a mild autumn day, shuts off and fires again, losing efficiency at every start and holding the return high.
A low-temperature circuit is not an investment purely in a heat pump, it benefits the boiler equally. Design floor heating and then choose gas after all, and you have lost nothing: the source can be swapped later without touching a pipe. The reverse order means an expensive renovation.
What is the bivalent point and what back-up does a pump need?
The output of an air source pump falls in a freeze, exactly when you need it most. The outdoor temperature at which its output equals the heat loss of the house is the bivalent point; below it an electric element in the tank or a second fuel-burning source has to help. The bivalent point is a design choice, not a defect: a machine sized for the coldest hour of the year will cycle below its minimum output all winter. It is smarter to size for the majority of the season's hours, taking the design outdoor temperature from the standard for the locality.
- Electric back-up raises the electrical demand of the house; breaker rating and reserved capacity belong in the discussion with the distribution operator while the connection is designed.
- Defrosting consumes part of the heat produced and repeats more often in a humid freeze. The seasonal figure accounts for it; the brochure COP does not.
- If the back-up is to be a gas boiler, you need the connection, the flue and the inspections, which is nearly the whole boiler branch of the investment.
Where should the outdoor unit go so it does not become a neighbour problem?
The outdoor unit is loudest in a freeze at full load and during defrost, on a winter morning, and this is the most common reason a finished heat pump gets reworked. What decides the outcome is not the label decibel figure but the level arriving at the neighbour's side: night-time limits are set by health legislation and assessed at the protected location, not on your terrace.
- Setback from the boundary does more than any add-on enclosure.
- Never put the unit in a corner between two masonry walls or in a narrow recess: reflections raise the level and the restricted air supply cuts output.
- The fan discharge must not face a bedroom window, on your side or the neighbour's, and defrost condensate needs a drain, or the path turns to ice.
Siting belongs in the site plan, not the installation day. The new building act 25/2025 Z. z., in force since 1 April 2025, and the spatial planning act 200/2022 Z. z. both work with documentation that parties to the proceedings can comment on. A neighbour who sees the unit on a drawing comments on a project; one who first sees it finished under their window comments with a complaint.
What changes for hot water and the buffer tank?
Space heating and hot water are two different jobs for a heat pump. Tap water needs roughly 55 to 60 degrees C, and that lift is where a pump is least efficient, while a boiler handles it with no penalty. In a house with a low heat loss, hot water can therefore be a comparable share of annual consumption to the heating itself.
Two vessels get confused on the hydraulic side. A buffer tank holds system water and sits on the heating circuit; a hot water cylinder holds potable water for the taps. The tank on the circuit provides enough volume that a modulating pump with a minimum output around thirty percent does not cycle when most floor zones close; oversizing it only adds standing loss and takes up space. For the cylinder the critical figure is the heat exchanger surface: a pump delivers heat at a lower temperature than a boiler and so needs a larger coil. A cylinder bought by volume alone is the mistake behind a house that never has enough hot water.
How do regulation and subsidies treat the two options?
A new build is designed as a nearly zero-energy building and demonstrates its class through an energy performance certificate. The certificate counts primary rather than delivered energy and accounts for the renewable share. A heat pump counts part of the heat it delivers as energy taken from the environment; a boiler contributes no renewable share, so the choice of source feeds directly into the assessment.
On subsidies, stay sober. Zelená domácnostiam, administered by SIEA through vouchers, and Obnov Dom both target existing houses and heat source replacement, not new builds, so a subsidy is usually not an argument on either side. The argument is the future replacement: a boiler installed today will be replaced in a different regulatory environment, and European regulation is moving towards pricing emissions from heating buildings and ending support for fossil sources. Nobody can put a figure on that risk, but it is one-sided.
When should you choose the boiler and when the heat pump?
Choose a condensing boiler if several of these hold at once:
- The gas connection to the plot is already built and paid for.
- The budget is tight and the capital difference would come out of the envelope or the windows; the envelope is always the better first investment.
- There is no defensible place on the plot for an outdoor unit: a narrow lot, a courtyard surrounded by neighbouring windows.
- The house will be used intermittently and you need fast, high output on arrival.
- The system will stay at a higher design temperature and you do not intend to change it.
Choose a heat pump if several of these hold at once:
- The system is low-temperature from the outset, ideally floor heating throughout.
- The envelope is above standard and the calculation confirms a low heat loss.
- You have a location with a real setback from the boundary and no reflective corner.
- You are planning photovoltaics, so there is a synergy with self-consumption.
- You want to cool through the same system in summer.
- You do not want combustion, a flue, inspections or standing charges for a second connection.
- You are deciding on a twenty-year cost basis, not a completion-day one.
When is a hybrid the right answer?
A hybrid heat pump is not indecision but an answer to a house where the design temperature cannot be pushed down everywhere. The pump covers the majority of the season's hours at low temperature, the boiler takes the freezing peaks and possibly hot water. In a new build it earns its place where the gas connection already exists and part of the house has radiators, or where the house is built in phases. With floor heating throughout it is pointless: you pay for two appliances and more complex controls to cover a few percent of the annual heat.
Conclusion
The clients who came out best committed to a low-temperature system first and deferred the source decision until they had a heat loss calculation in front of them. That bought them a basis for an honest comparison and a system in which the heat source is a replaceable component. Where prices and rules change faster than a house wears out, that is the most valuable thing a heating design can buy.