A capacity margin looks like cheap insurance. At the quotation stage it costs almost nothing - one size up, a few thousand złoty of difference, and the reassurance that nobody will call complaining during a cold snap. The cost of that insurance shows up later, and it is the client who pays it, every month, for the life of the installation.
This article explains what actually happens inside an oversized heat pump, where the instinct to add capacity comes from, and how to size a unit so that the choice can be defended to the client.
What "oversized" means
It is not about a large number of kilowatts in itself. A pump delivering 16 kW at design conditions, in a building with a 15 kW load, is sized correctly; one delivering 8 kW there, in a building with a 4 kW load, is oversized by a factor of two.
Two relationships matter:
- the unit's output against the building's design heat load - that is, against the result of the heat load calculation for that specific building in that specific climate zone;
- the unit's lower modulation limit against the building's demand for most of the season - the relationship that gets discussed far less often, and the one that decides how the pump actually runs.
Why a pump that is too big cannot simply slow down
An inverter compressor modulates its output, but only within a range. Below a certain threshold it will not go, and the spread of that threshold between units is wide. The number on its own means little without two things: the operating point it was quoted at, and what it is expressed as a fraction of. Manufacturer data usually gives an output range at a specific point (A7/W35, for instance) - and it is that range, not a percentage read off the model name, that has to be set against the building's demand.
At the same time, a building needs its full design load only in the coldest hour of the year. With 20 °C indoors and a -20 °C design temperature, demand at zero degrees outside is roughly half the design load, and at plus eight about a third. Most of the heating season is spent in that part-load range, not in a cold snap.
Putting those two facts together gives you the heart of the problem:
The modulation floor against the building's demand
When the building's demand drops below the unit's minimum output, the pump has no way to deliver less. All that remains is to stop, wait and start again. That is cycling - and with an oversized unit it happens even when the hydraulics are faultless.
The bigger the machine, the higher its floor sits relative to the building's demand, and the larger the share of the heating season the hatched band covers.
What cycling actually costs
Seasonal efficiency. Catalogue COP is measured in steady-state operation to PN-EN 14511. SCOP is not measured at all - it is calculated to PN-EN 14825 for a reference building whose design load matches the unit's output. Cycling enters that calculation as a flat degradation coefficient, assumed for a correctly matched unit rather than for a machine that cycles through most of the season. The declared SCOP and the SCOP achieved in such a building are therefore two different numbers.
Compressor wear. Manufacturers cap the permitted number of starts per hour, because a start loads the compressor differently from continuous running: inrush current, oil migration and thermal cycling. A unit starting several times an hour instead of running steadily uses that allowance far faster.
Comfort. Intermittent operation means swings in flow temperature, and in a low-inertia system swings in room temperature too. This is the part the client notices without looking at a bill.
Conflict with hot water. Preparing domestic hot water interrupts space heating. In a pump that is already running in cycles, those interruptions simply multiply.
Where the instinct to add capacity comes from
Four causes, all of them understandable:
A habit carried over from boilers. A gas boiler modulates across a very wide range, and oversizing it costs little. The instinct of "I will take the bigger one and have peace of mind" is transplanted directly from a technology where it genuinely worked. With a heat pump it does not.
Adding hot water straight onto the heating output. Domestic hot water is an intermittent, time-shifted load, not a constant output added to the building's demand. Adding it linearly inflates the result, and with a properly sized cylinder and priority control it is usually not needed at all.
A rule of thumb instead of a calculation. The W/m² figure has a spread wider than the difference between one equipment class and the next. We measured this on our own module: for a single building whose full PN-EN 12831-1 calculation gave 8.40 kW, the rule-of-thumb estimate produces anything from 8.02 to 23.52 kW - depending purely on which insulation class the person estimating picks. We set this out in the heat load article.
Fear of the phone call during a cold snap. The most honest of the four causes and probably the strongest. It is worth naming plainly, because the answer to it is not a capacity margin but bivalent sizing - see below.
The bivalent point: a smaller pump does not mean a cold house
The fear of an undersized unit rests on a quiet assumption: that the pump alone must cover the entire demand in the coldest hour of the year. That is monovalent sizing, and it is not the only option.
The bivalent point
In bivalent sizing, below a certain outdoor temperature an immersion heater or a second source provides the top-up. How many hours a year that is depends on the bivalent point chosen and on the location - the higher you set it, the more hours. What matters more is how much energy those hours represent: the top-up covers only the difference between demand and the pump's output, not the whole demand, so in the annual balance it accounts for a few percent. Both can be counted from climate data for the location rather than merely guessed at.
The consequence runs against intuition: a smaller unit, sized bivalently, spends most of the season working inside its proper modulation range and achieves a higher seasonal efficiency than a larger unit that spends that same majority of the season cycling.
How to size a pump so the choice can be defended
- Calculate the design heat load room by room to PN-EN 12831-1, for the design temperature of the building's actual location. In Poland that ranges from -16 °C to -20 °C and below, and the same temperature has to hold across the whole chain of documents. Size the source on the BUILDING's design load, not on the sum of the room loads - ventilation is computed per room with a directional infiltration allowance that cancels at zone level, so the room sum comes out noticeably higher. The room-by-room results are what you size radiators and underfloor loops from.
- Verify the emitters room by room and establish the required flow temperature. A single room with an undersized radiator raises the required temperature for the entire system, and every degree higher means a worse COP all winter. Without a room-by-room breakdown you will not find that bottleneck - and without that figure you cannot carry out step 3.
- Read the unit's real output from its performance data at the design temperature and the flow temperature established in step 2 - not the nominal figure in the model name. A "12 kW" pump at -20 °C with 55 °C flow delivers considerably less than it does at the conditions its name was derived from.
- Check the lower modulation limit of the chosen unit - together with the conditions its data sheet quotes it at - and compare it against the building's demand at around +5 °C. That is near the median of the Polish heating season; at a -20 °C design temperature, demand there is about 40 percent of the design load. A floor above that value means cycling for most of the season.
- Decide deliberately between monovalent and bivalent operation, and record that decision together with the bivalent point assumed. With bivalent sizing, check the electrical supply and protection at the same time - the top-up heater is a load that has to fit within the connection agreement.
- Treat hot water separately - as an intermittent load with priority control, not as a constant output added to the building's demand.
- Check the system water volume. Too little water in the system causes cycling even with a correctly sized pump; manufacturers state a required minimum, and circuits closed off during use can take it below that.
What Czyste Powietrze changes after 20 July 2026
A new edition of the Czyste Powietrze subsidy programme has been in force since 20 July 2026, and the new terms apply to applications submitted from that date. The energy audit requirement is not new in this edition: an audit has been mandatory for every application, whatever the scope of works, since 31 March 2025, and its scope affects the qualifying scope of works. The announced audit voucher has not launched, so for now the applicant pays for the audit and settles it later as a qualifying cost.
For heat pump sizing the change is indirect but real: the sizing stops being a separate document. It now sits alongside a study prepared by somebody else, and any divergence in assumptions between them - a different design temperature, a different assumed envelope performance, a different demand - becomes visible to the client and to anyone assembling the file, and to the regional fund on inspection.
It is worth keeping a distinction in mind that is easily blurred: an energy audit is not a heat load calculation. An audit describes primarily the building's annual energy demand and the cost-effectiveness of modernisation options; it gives thermal power as a single whole-building figure, on the audit's own assumptions and with no room-by-room breakdown. You can size neither radiators nor underfloor loops from it. A heat load calculation gives instantaneous power under design conditions, room by room, and exists precisely to size equipment. Neither replaces the other, though both may end up in the same folder.
The detailed terms of the programme change during its lifetime, so amounts and deadlines are always worth confirming against the current programme document.
Common mistakes
- Sizing on the nominal figure in the model name rather than on output read from performance data at the design temperature and required flow temperature.
- Adding hot water capacity directly onto the building's demand.
- Ignoring the lower modulation limit when comparing two units of similar nominal output.
- Sizing from a W/m² rule of thumb and treating the result as the basis for ordering equipment.
- Sizing the source on the sum of the room loads instead of the building's design load. The room sum is noticeably higher, because ventilation is computed per room with an allowance that cancels at zone level - it is the simplest arithmetic route to oversizing.
- A "just in case" margin added after the calculation. The standard contains no safety factor - it contains assumptions that are themselves conservative: no internal or solar gains, every room at its design temperature simultaneously, and a statistically extreme outdoor temperature. A percentage added on top duplicates that margin. The only addition the standard provides for is reheat power for intermittently heated systems - and that is calculated, not estimated.
- No record of the bivalent point assumed, so that at handover nobody can reconstruct the basis on which the unit was chosen.
How we do it in HeatAlgo
The heat load module calculates the design heat load room by room to PN-EN 12831-1:2017, with PN-EN ISO 6946 for building elements and PN-EN ISO 13370 for ground contact.
The preliminary heat pump sizing module reads those calculations live and analyses existing radiators room by room, so the sizing happens for the same design temperature the building was calculated for. The solver picks one of 10 generalised power classes from characteristics averaged over catalogue data for 239 devices by 28 manufacturers. It is a starting point for choosing a specific unit, not a replacement for a full installation design.
The terms used in this article - bivalent point, monovalent sizing, SCOP - are explained in the installer's glossary. If you would rather commission the calculations than do them yourself, we also offer individual heat load calculations. More guides are in the help centre.
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