What pump capacity do I need for my home
What pump power do I need for my house – a complete guide to selection
This is a question we encounter literally every day. A customer comes in, says that the old pump on the boiler is failing and needs to be replaced, and wants to know what to buy. Or they are building a new house, the plumber has left, and they want to double-check for themselves whether the recommended model is suitable. Or they are undergoing a major renovation and are afraid that the existing pump is not sufficient for the new underfloor heating being added to the radiators.
The answer "it depends" may sound like an excuse, but in the case of circulation pumps, it is the actual truth. The pump power does not depend only on the house area. It depends on the type of heating system, the length of the piping, the number of circuits, the building height, the water temperature, and a few other factors as well. In this article, we will go through all of them in detail, with specific numbers, real-life examples, and finally with recommendations for specific models for typical situations.
What a circulation pump actually does – basics you cannot move without
A circulation pump is the heart of every hot water heating system. Its task is simple: to overcome the hydraulic resistance of the piping network and ensure that hot water circulates from the boiler to the radiators (or underfloor heating circuits) and back. Without a pump, gravity circulation would work only under very specific conditions – in modern systems with plastic pipes and thermostatic valves, it is practically unusable.
The pump is therefore described by two basic parameters:
- Flow rate (Q) – how much water the pump transports per unit of time, given in m³/h or l/h
- Head (H) – the pressure the pump can generate to overcome the network resistance, given in meters of water column (m w.c.) or in Pa/kPa
These two values together form the pump characteristic – the Q/H curve. When you look at any circulation pump, for example, the IBO OHI 25/60-130, the "25/60" in the name indicates precisely these values. But more on that in another topic of our Knowledge Center – in the article "What do the numbers in the IBO pump name mean – how to read technical model designations."
Two key calculations: required flow rate and required head
Every selection of a circulation pump is based on two independent calculations. Both must be done – and then you look for a pump that satisfies both at the same time.
1. Calculation of the required flow rate
The flow rate depends on the thermal power that must be delivered to the heating system. It is based on a basic physical formula:
Q = P / (ρ × c × ΔT)
where:
- P = boiler power / house heat loss (W or kW)
- ρ = water density (≈ 1 000 kg/m³, at 70 °C ≈ 978 kg/m³)
- c = specific heat capacity of water (4 186 J/(kg·K), rounded to 1 163 Wh/(m³·K))
- ΔT = temperature difference between supply and return (typically 10–20 °C)
In practice, this formula is simplified to:
Q [m³/h] = P [kW] / (1,163 × ΔT [°C])
Example 1: A family house with a 15 kW boiler, temperature difference 80/60 °C (ΔT = 20 °C):
Q = 15 / (1,163 × 20) = 15 / 23,26 = 0,645 m³/h ≈ 645 l/h
Example 2: A larger house, condensing boiler 24 kW, temperature difference 55/45 °C (ΔT = 10 °C):
Q = 24 / (1,163 × 10) = 24 / 11,63 = 2,06 m³/h ≈ 2 060 l/h
Note that a lower temperature difference (modern condensing boilers, underfloor heating) significantly increases the required flow rate. This is one of the most common mistakes in dimensioning – a customer replaces an old radiator system 80/60 with a condensing boiler and underfloor heating 40/30 and asks why the original small pump is no longer sufficient. It is no longer sufficient because at ΔT = 10 °C, it requires double the flow rate compared to ΔT = 20 °C.
2. Calculation of the required head
The head (pump pressure) must overcome the total hydraulic resistance of the heating system. This resistance depends on:
- length of the pipes and their diameter
- number and type of fittings (valves, elbows, T-pieces, distributors)
- thermostatic valves and mixers
- heat exchangers, safety groups, etc.
An accurate hydraulic calculation is a job for a designer with appropriate software. In practice, however, for family houses, a verified approximation can be used:
H [m w.c.] = R × L × (1 + Z)
where R is the specific resistance of the longest circuit (typically 100–200 Pa/m for a properly dimensioned radiator system), L is the length of the longest circuit in meters, and Z is the coefficient of local resistances (0,3–0,5 for simple systems, 0,5–1,0 for systems with multiple thermostatic valves).
For practical use: most standard family houses with a properly designed heating system require a pump with a head of 2 to 6 meters. Houses with underfloor heating with long circuits or multiple filters may require up to 8–10 m.
Heat loss of the house as the basis of everything
Many people confuse the boiler's power with the house's heat loss. A boiler may have a nominal power of 24 kW, but if the house requires only 8 kW at -15 °C outside, the pump is dimensioned according to the actual heat loss, not the boiler's power. The boiler runs at a much lower power most of the year due to modulation.
The heat loss of a single-family house in practice is roughly as follows (approximate values for standard construction):
- Old house without thermal insulation (before 1990): 80–120 W/m² of usable area
- House built in the 90s, partially insulated: 50–80 W/m²
- House insulated according to current standards (minimum requirements): 30–50 W/m²
- Low-energy house (A1, A2): 15–30 W/m²
- Passive house: under 15 W/m²
Example for a 150 m² house with standard insulation: heat loss ≈ 150 × 55 W/m² = 8 250 W ≈ 8.25 kW. This is the value from which the boiler and pump selection is based.
Influence of the type of heating system on the choice of pump
The type of heating system has a huge impact on the choice of pump. It is not only about power – it is also about hydraulic resistance and the head that the pump must overcome.
Radiator system with thermostatic valves
This is the most common situation in existing houses today. Radiators with thermostatic valves have a dynamically changing hydraulic resistance – when the thermostat closes the valve, the resistance increases. Therefore, it is highly recommended to use pumps with electronic control – so-called EC motors with automatic speed adjustment. The pump reduces the speed when the valves close, saves energy and prevents noise (characteristic "hissing" in radiators).
For a standard radiator system in a single-family house (100–200 m², boiler power 12–20 kW, temperature drop 75/65 or 80/60 °C), a pump with a flow rate of 0.5–1.5 m³/h and a head of 3–5 m is usually sufficient.
Floor heating
Floor heating operates at low temperatures (supply 35–45 °C, return 25–35 °C) and with long pipe loops. This means a higher flow rate (due to a small ΔT) and higher hydraulic resistance in the pipes. Floor heating loops are usually supplied via a manifold (distribution unit), where each loop has a regulating valve.
For floor heating, we therefore need a pump with a higher flow rate and often also a higher head. In addition, a manifold with connected loops creates higher resistance than simple radiator circuits. Therefore, a head of 4–8 m is usually required here.
Combined system (radiators + floor heating)
This is the most complex situation. Both types are usually solved via a separating unit (hydraulic balancer, group regulator), and each circuit has its own pump. Do not try to cover the entire system with one pump – hydraulic problems will arise.
Practical dimensioning according to the area of the house – approximate tables
Although we have said that the calculation is more complex, for the average house owner it is useful to have a quick overview. The following table is based on many years of experience and applies to typical single-family houses in the climatic zone of Slovakia (design outdoor temperature -12 to -15 °C):
| Area of the house | Estimated heat loss | Required flow rate (ΔT=20°C) | Required flow rate (ΔT=10°C) | Recommended head |
|---|---|---|---|---|
| Up to 80 m² | 4–6 kW | 0.17–0.26 m³/h | 0.34–0.52 m³/h | 2–4 m |
| 80–120 m² | 6–9 kW | 0.26–0.39 m³/h | 0.52–0.77 m³/h | 3–5 m |
| 120–180 m² | 9–14 kW | 0.39–0.60 m³/h | 0.77–1.20 m³/h | 4–6 m |
| 180–250 m² | 14–20 kW | 0.60–0.86 m³/h | 1.20–1.72 m³/h | 4–7 m |
| over 250 m² | over 20 kW | over 0.86 m³/h | over 1.72 m³/h | 5–9 m |
Note: The values apply to standard insulated houses (50–60 W/m²). For older buildings without insulation, multiply the flow rate by 1.5×, for low-energy houses you can divide by 1.3–1.5×.
How to read the parameters of specific IBO OHI models and choose the right one
Once you know the required flow rate and head, selecting a specific model becomes relatively simple. The IBO OHI series offers several models, each suitable for slightly different application ranges.
The model IBO OHI 15-60/130 is a compact pump with DN15 connection, maximum flow rate of 0.86 m³/h and maximum head of 6 m. It is an excellent choice for small to medium-sized family homes up to about 100 m² with a standard radiator system, where you do not require high-flow performance. The three-speed setting allows you to find the right operating point.
Models IBO OHI 25/40-130 and IBO OHI 25/40-180 are suitable for houses where a higher flow rate is needed (DN25 connection), but the hydraulic resistance of the system is not extremely high – maximum head of 4 m. Typical use: radiator systems in well-designed houses up to 150 m², where the piping has a larger diameter and shorter circuits.
If your system requires both a higher flow rate and higher system resistance – for example, houses with more complex piping, multiple floors or partial floor heating – choose the models IBO OHI 25/60-130 or IBO OHI 25/60-180. These models offer both flow rate and head of 6 m, making them versatile for most family homes in Slovakia. The difference between 130 and 180 is only in the flange center distance (construction length) – 130 mm and 180 mm respectively. This is important when replacing an existing pump, as you need to maintain the original dimensions.
Concrete examples from practice – what customers most often deal with
Case 1: Replacement of an old Grundfos UP 15-60 pump in a Protherm Panther boiler
We deal with this scenario almost every week. The boiler has an integrated pump that has failed after 12–15 years of operation. The owner wants a replacement, preferably cheaper than the original from Grundfos. Parameters of the original pump: DN15, center distance 130 mm, performance at 3rd speed corresponding to approx. 0.8 m³/h at 3 m.
For a house with an area of 110 m², radiator system 80/60 °C, boiler 14 kW: required flow rate = 14/(1.163×20) = 0.60 m³/h, required head ≈ 3–4 m. Solution: IBO OHI 15-60/130 – fits dimensionally (130 mm), has the required performance, works at 3 speeds.
Case 2: New construction, 190 m², combined system of a condensing boiler with floor heating in a ground floor layout
Boiler 20 kW, floor heating in the ground floor (8 loops, each approx. 80 m long), radiators on the upper floor. System temperature difference 45/35 °C for floor heating, 55/45 °C for radiators. Two separate pumps are being considered here:
- Primary loop (boiler → hydraulic balancer): power 20 kW, ΔT = 10 °C → Q = 1.72 m³/h, H ≈ 2–3 m
- Floor heating loop (behind a mixing unit): 8 loops × 80 m pipe = high resistance, Q ≈ 1.3 m³/h, H ≈ 6–8 m → IBO OHI 25/60-130 or 25/60-180 depending on construction length
- Radiator loop: Q ≈ 0.6 m³/h, H ≈ 4 m → IBO OHI 25/40-130
Case 3: Renovation – adding hot water heating (boiler) to an existing boiler
The owner is adding a storage water heater to an existing gas boiler, which had only radiators before. A new loop with a make-up pump is created. This pump is used only to pump water from the boiler to the storage tank – the resistance of the coil in the boiler is relatively low, Q = 0.5–0.8 m³/h is sufficient, H = 2–3 m. Here, the IBO OHI 15-60/130 is exactly right.
Case 4: Three-story family house, old radiator system, steel pipes DN15–DN20
Old steel pipes have high hydraulic resistance (corrosion and deposits reduce the internal diameter). The system has three floors – the flow must overcome vertical differences (although height does not directly affect the required pressure if the system is properly filled and vented, the resistances from each floor add up). For a house of 200 m²: Q ≈ 0.9 m³/h, but H ≈ 6–8 m due to the old pipes. In this case, we recommend the IBO OHI 25/60-130 with the correct speed scale selected.
Why bigger is not always better – the problem of an oversized pump
This is important – and many installers make this mistake. When they are unsure, they install a larger pump "just to be safe." However, an oversized pump causes a number of problems:
- Hydraulic noise – the flow is too fast, water "hums" in radiators and fittings (cavitation at thermostatic valves)
- Increased energy consumption – the pump runs at a higher power than necessary
- Impaired regulation – fast circulation prevents proper heat transfer in radiators
- Shortened lifespan – operating far from the optimal point damages the pump bearings more quickly
On the other hand, an undersized pump cannot deliver enough heat to distant radiators – and it is warm only near the boiler, while the radiators at the end of the system remain cold. The goal is therefore the correct operating point, not maximum performance.
Energy class of the pump and operating costs
Modern EU directives (ErP – Energy Related Products) since 2013 have significantly changed the requirements for circulation pumps. Older three-speed pumps with a consumption of 60–100 W have gradually been replaced by EC (electronically commutated) pumps with automatic regulation, which consume only 3–15 W during normal operation.
For comparison: an old pump with a consumption of 80 W operating 4,000 hours per year consumes 320 kWh ≈ 80 €/year at a price of 0.25 €/kWh. A modern EC pump with a consumption of 8 W consumes 32 kWh ≈ 8 €/year. The difference is therefore about 70 € per year – over 5 years this is 350 €, which already represents the full price of a new pump.
Models in the IBO OHI series are standard three-speed pumps – they are cost-effective and reliable, but their energy efficiency is lower than that of EC pumps. For most applications in family homes, this is an excellent price-to-utility ratio, but if you are looking for maximum energy savings and long-term operation, it is worth considering EC variants. More on this in the article "IBO OHI vs other circulation pump brands – a comparison of price-to-quality ratio".
Construction length 130 vs. 180 mm – when does it matter
In each model of the IBO OHI series (except for the 15-60, which is only available in 130 mm), both construction lengths of 130 mm and 180 mm are available. This value indicates the distance between the centers of the flanges attached to the pump – i.e., the physical length the pump occupies in the piping.
When is this important? Only when replacing an existing pump, where you want to maintain the original dimensions and do not want to extend or shorten the piping. Always measure the distance between the centers of the existing flanges before purchasing. If you have 130 mm – choose the 130 mm model, if 180 mm – choose the 180 mm model. In a new installation, it does not matter, the difference is only in the mounting comfort.
Practical tips before placing an order
From our experience with customers, we know that many problems arise from simple oversights. Before placing an order, double-check the following:
- Connection size: DN15 (G 1") or DN25 (G 1½")? Be careful, most modern boilers have G 1" inputs, so DN25 pumps need to be reduced – this is not a problem, but you need to think about it.
- Shaft distance: 130 mm or 180 mm? Measure it before removing the old pump.
- Mounting position: A circulation pump can be mounted horizontally or vertically, but the shaft must always be horizontal – not vertical. More details in the article "Installation of IBO circulation pump – procedure, position and wiring".
- Power supply: 230 V / 50 Hz – this is standard, all IBO OHI models meet this requirement.
- Maximum medium temperature: For IBO OHI, this is 110 °C – this always suits heating systems.
- Closed vs. open system: Circulation pumps are intended for closed pressurized systems with an expansion tank. Do not install them in open gravity systems without adaptation.
Most frequently asked questions (FAQ)
What if I don't know the heat loss of my house – can I estimate it somehow?
Yes, roughly. If you have a normally insulated family house (10–12 cm of wall insulation, plastic windows, roof with insulation), calculate with 50–60 W/m² of usable area. For an old, non-insulated house, double it to 100–120 W/m². A low-energy house or one that has undergone a major renovation: 25–40 W/m². Multiply the area of the house by the coefficient – you get an approximate heat loss in kW, from which you can calculate the flow.
Do I have to buy exactly the same model that was originally in the boiler?
No. It is important that the new pump meets the required flow and head (or slightly exceeds it – i.e., the operating point is in the upper third of its curve), has the same connection size and the same construction length. The specific brand or model can be different. IBO OHI models are compatible replacements for Grundfos UP, Wilo Star-RS and similar models of the same class.
What if my pump is humming or vibrating, but otherwise works?
Pump humming can have several causes: air in the system (vent the pump and radiators), deposits in the rotor (disassemble and clean), an unsuitable operating point (the pump is running too close to the edge of the curve), or worn bearings (end of life is approaching). First, try to vent the system – it is free and solves 40% of the cases. More advice is available in the article "IBO circulation pump is not pumping water or is humming – solutions for common problems".
Is it worth buying a pump with electronic regulation (EC motor) instead of a three-speed one?
For standard radiator systems in smaller homes (up to 130 m²) with simple piping, the price difference between a three-speed and an EC pump is rarely justified from a purely financial perspective. An EC pump saves about 50–70 € per year, but costs 100–200 € more – the payback period is 2–4 years. In larger homes, systems with underfloor heating, and systems with thermostatic valves on each radiator, an EC pump is definitely worth it.
Can I connect two IBO OHI pumps in series or in parallel if one is not enough?
Connecting in series doubles the head – used for very long circuits with high resistance. Connecting in parallel doubles the flow – for systems with high thermal output and low network resistance. In practice, however, this is technically more complex and requires proper hydraulic sizing. For most single-family homes, it is simpler and cheaper to choose one larger pump of the correct model.
What pressure (bar) must the water have in the system for the pump to function properly?
The circulation pump itself does not create pressure in the system – this is ensured by the expansion tank and pressure relief valve. The pump only overcomes the hydraulic resistance of the network. The minimum operating pressure in the system with cold water should be 1 bar (the pump does not suck it – the water must be under pressure in the pipes to prevent cavitation). The maximum pressure for IBO OHI is 10 bar – standard heating systems operate at 1.5–2.5 bar, so this is not a problem.
Conclusion – summary of selecting the correct power
Do you have a question about this topic?
Not sure or dealing with a specific situation in your home? Write to us – we are happy to help.