Pumps during heating renovation: replacing an old pump step by step

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Why replacing the pump during heating system renovation is so important

Renovating a heating system is always an opportunity to do it right – not only to replace the boiler or radiators, but to evaluate and set up the entire system so that it operates efficiently for the next twenty years. The pump is an element that is often underestimated during renovations. When a customer comes to us saying that their new condensing boiler "doesn't fit" in some way, in nine out of ten cases the problem is precisely the old, not replaced or improperly adjusted pump.

Old pumps – especially those from the late 70s and early 80s – are fixed-speed, without regulation, with high energy consumption and zero adaptability to current system conditions. Modern electronically controlled pumps (so-called ECM pumps) can reduce electricity consumption for circulation by 50 to 80 percent compared to older models. With continuous operation during the heating season (approximately 4,000 hours per year), this represents a significant difference in the electricity bill.

In this article, we will go through the entire process of replacing the pump during a heating system renovation – from diagnosing the old condition, through selecting the correct replacement part, to the physical installation, bleeding and setting. It is a job that an experienced DIY enthusiast can handle, but you can also easily include it in the overall renovation package with an installer, because the correct setting of the pump parameters has a direct impact on comfort and savings.

When is pump replacement necessary and when is it just recommended

The first question to ask during a renovation: is pump replacement necessary, or would it be enough to keep the original one? The answer depends on several factors:

Pump age: Pumps have a lifespan of 15 to 20 years with proper maintenance. If the original pump is older than 15 years, replacement is highly recommended – not because it doesn't work, but because its energy efficiency is much worse than that of today's models.
Type of regulation: Old pumps with a three-position switch (position 1, 2, 3) have no regulation. Modern electronic pumps adapt smoothly to the system load – they operate at minimum when it's warm outside and at maximum during cold spells.
Boiler replacement: If you are replacing the boiler from an atmospheric to a condensing one, or adding underfloor heating, the old pump may have an unsuitable performance for the new conditions – either too powerful (excessive noise, flow) or too weak (insufficient circulation).
Seals and bearings condition: Old pumps may have worn mechanical seals from which water leaks, or noisy bearings. Repair is sometimes possible, but spare parts for old models are hard to find and expensive.
Noise: If the pump buzzes, vibrates or makes irregular sounds, it is a sign of wear. More about this issue can be found in the article Pump noise: why it buzzes or vibrates and how to eliminate it.

In practice, I recommend replacing the pump whenever you change the boiler. The cost of the pump itself is relatively small in the context of a full renovation, but the savings from a modern electronic pump are noticeable every year throughout its lifetime.

Preparation before replacement: what to find out before disassembly

Before you reach for the wrenches, you need to gather some information. A chaotic replacement "blindly" leads to buying an unsuitable pump, or after installation you find out that the hoses don't fit, the threads are not the same, or the pump's performance does not match the system.

Measure the dimensions of the old pump

The most important dimension is the so-called construction length – the distance between the axes of the connecting nozzles (usually 130 mm or 180 mm for standard domestic pumps). Also note the diameter of the connection – most domestic circulation pumps have G 1½" or G 2" connections. Both of these values can be found on the pump's nameplate or in its documentation.

Also note how the pump is oriented – the motor axis can be horizontal or vertical. Some pumps allow the motor head to be rotated, others do not. This is important if you want to install the pump differently than the original.

Determine the system parameters

You need to know what flow rate and what head your heating circuit requires. If you don't have the design documentation, you can estimate or calculate these values – a detailed procedure can be found in the article Pump performance and flow: how to calculate what you really need. For a typical family house with an area of 150 m² and radiators, we usually get values of flow rate 1,000 to 1,800 l/h and head 2 to 5 m of water column.

Check the condition of the water in the system

Before disassembly, it is advisable to check what kind of water is in the system. If it is strongly turbid, rusty or contains visible sediments, replacing the pump alone is not enough – the entire system needs to be flushed. Contaminated water will shorten the life of the new pump to a fraction of its expected duration. More about system protection can be found in the article Maintenance and service of pumps: how to extend their lifespan and avoid failures.

You also need to check whether there is a non-freezing mixture (propylene glycol mixture) in the system. This is important when selecting a pump, as some types are not certified for operation with non-freezing mixtures, or alternatively, a different performance setting must be chosen due to the higher viscosity of the liquid.

Choosing the right replacement pump

When selecting a pump for a renovation, you basically have two options: buy the same type as the original (so-called drop-in replacement), or switch to a modern electronically controlled pump. The first option is simpler in terms of installation, but technically it is a missed opportunity. The second option brings real savings and better comfort, but it requires a bit more thought.

Circulation pumps for central heating

For most family homes with central heating, a wet-rotor circulation pump with an electronically commutated motor (ECM or EEI ≤ 0.23 according to European legislation) is ideal. These pumps have built-in automatic control – they can adjust performance according to the current needs of the system, which means that when thermostats on most radiators are open, the pump runs at full speed, and when they are closed (for example at night), the pump slows down to a minimum.

Specific parameters to compare when selecting:

Construction length: 130 mm (compact installations, common in apartment buildings) or 180 mm (family homes, older installations)
Connection threads: G 1½" or G 2" – most modern pumps are supplied with reductions for both sizes
Maximum flow rate: usually in the range of 2–8 m³/h for family homes
Maximum head: usually 4–8 m for family homes
Energy efficiency class: look for EEI ≤ 0.23 (since 2015 this is a mandatory standard in the EU)
Maximum water temperature: standard 110 °C, which is sufficient for condensing boilers

For more information on comparing pump types, see the article Circulation vs. pumping pumps: differences, advantages and when to use which, where the differences between wet-rotor and dry-rotor types are also discussed.

Step by step: removing the old pump

Once you have the new pump and all the necessary materials (gaskets, Teflon or hemp fiber with paste), you can proceed with the replacement. The procedure is the same for most common circulation pumps in family homes.

Step 1 – Shutting down the system and draining the water

First, turn off the boiler and let the system cool down to a safe temperature (below 40 °C). Never drain hot water – in addition to the risk of burns, you can damage system components with thermal shock.

If you have shut-off valves at the pump (which should always be the case – see the article Installing a pump in a heating system: procedure and most common mistakes), close them and limit the draining of the system to the section between the valves. If the valves are missing, you will have to drain the entire system via the drain cock at the boiler or at the lowest point of the piping.

I recommend having a bucket and a cloth handy – even after closing the valves, there will be a few deciliters of water left in the pump, which will flow out during disassembly.

Step 2 – Disconnecting the electricity

Before touching any electrical parts, disconnect the pump from the power supply. Either remove the fuses or switch off the circuit breaker for the corresponding circuit. Verify the absence of voltage using a voltage tester. Then unscrew the cover of the terminal box and disconnect the electrical wires. Note the color coding of the wires (L, N, PE) – this will make the connection easier with the new pump.

Step 3 – Mechanical disassembly

Loosen the flanged or threaded connections that attach the pump to the piping. In older installations, these connections may be "stuck" – the gaskets have hardened and the connections may resist. Use two wrenches at once (one holds the pipe, the other turns) so that you do not apply torsion to the piping.

After loosening both connections (or flanges), pull out the pump. If it was sealed with hemp fiber or Teflon, remove the residue from the threads using a brush – a clean thread is the basis for a reliable seal with the new pump.

Installation of the new pump: procedure and details to pay attention to

Pump orientation

Modern wet-rotor pumps can be installed with the motor oriented horizontally (horizontal motor axis) or vertically upwards. Vertical mounting with the axis pointing downward is in most cases not allowed – the bearings would not be sufficiently lubricated by water and the pump's lifespan would be drastically reduced. Always check the manufacturer's instructions to see which positions are permitted.

The pump should be mounted on the piping so that the rotor axis is horizontal – this is the standard and most reliable position for most types. At the same time, make sure the terminal box is oriented upwards or sideways – never downwards, to prevent condensation or water from dripping into it in case of a leak.

Sealing threaded connections

Sealing is a critical point. Do not use old, dried-out gaskets – always use new ones. For threaded connections, you have two options:

Hemp fiber with paste (e.g. Reinzaplast, Fermit): a classic, reliable option, suitable even for older threads that are not in perfect condition
Teflon (PTFE tape): quick and clean, but requires clean and undamaged threads; for larger threads (G 1½" and above), prefer hemp fiber

For flanged connections, a rubber gasket (O-ring or flat rubber gasket) is used. Check whether the new pump includes gaskets in its packaging – most modern pumps include them.

Tighten, but don't over-tighten

Threaded connections on pumps should be tightened by hand + about 1.5 turns with a wrench. Flanged connections should be tightened gradually in a cross pattern. Over-tightening a thread on a bronze or brass coupling is easy – it can even crack during the first start-up under pressure. If you have any doubts, use a torque wrench.

Electrical connection

New pumps are usually connected to a 230 V single-phase network. Follow the diagram in the pump manual. Most modern pumps also have an input for an external signal (e.g. ON/OFF from the boiler, bus communication for smart systems). If your boiler supports OpenTherm or PWM control, it is worth implementing this connection – the boiler and pump will communicate and optimize their joint operation.

Always use a cable gland (cable outlet) and secure the cable so that it does not come into contact with hot pipes and is not exposed to mechanical stress. Only close the terminal block once you are sure everything is correctly connected.

First start-up: purging and setting parameters

Purging the system

Air in the system is the number one enemy of the pump. Circulation pumps are lubricated by water itself – if the bearing runs "dry" for just a few minutes, it can be permanently damaged. Before starting, therefore:

Open the shut-off valves at the pump
Slowly fill the system with water until the boiler pressure gauge shows the desired pressure (usually 1.0–1.5 bar in a cold system)
Allow the system to release air through the radiators' bleed valves and the automatic air vent on the expansion tank
Many modern pumps have a bleed screw directly on the motor housing – turn it gently with a screwdriver by half a turn until water starts to flow without bubbles, then tighten it

Repeat the purging process 2–3 times during the first hours of operation – air is released gradually. Even automatic air vents need time to catch all the air pockets in the system.

Setting the operating mode

Modern electronic pumps offer several operating modes. The most common ones are:

Constant pressure (ΔP-c): the pump maintains a constant differential pressure regardless of flow. Suitable for systems with thermostatic valves, where flow varies.
Proportional pressure (ΔP-v): the pump reduces pressure in proportion to the drop in flow. More energy-efficient, recommended for most standard installations with radiators.
Constant speed (I, II, III): a classic mode, retained for compatibility. Not recommended as the primary setting for modern systems.

For a standard family house with radiators and thermostatic heads, I recommend starting with the ΔP-v (proportional pressure) mode and setting the maximum differential pressure value according to calculation or the boiler manufacturer's recommendation. The system will then find its optimal operating curve on its own. If after the first week you feel that some radiators are not heating sufficiently, try increasing the value slightly.

Typical mistakes when replacing a pump in practice

Over the years of working with customers and installers, I have seen several mistakes that repeat themselves. If you are aware of them in advance, you can easily avoid them:

Forgotten purging: The most common problem. The customer turns on the pump, the system is noisy and some radiators are cold. Solution: purge again, this time more thoroughly.
Incorrect pump orientation: Motor at the bottom – this happens when the installer copies the position of the old pump without checking if it was correct. The result is reduced lifespan and possible bearing failure.
Old gaskets not replaced: An old O-ring or hemp fiber is reused "because it looks fine". After the first start-up, the customer discovers a wet floor under the boiler room.
Old shut-off valves not replaced: Old ball valves at the pump may have worn gaskets. If you don't install new valves, you won't be able to isolate the pump during the next replacement (in 15 years).
Current collapse in ΔP-c mode: Setting too high a differential pressure in ΔP-c mode causes excessive noise in thermostatic valves (typical hissing). Solution: reduce the setting or switch to ΔP-v.
Not adapting pump performance when switching to floor heating: Floor heating has a higher hydraulic resistance than radiators, but a lower required flow. A pump that was optimal for radiators may operate at an unsuitable point on the curve. More on this in the article Pressure and head: what these parameters mean and why they are important.

Special cases: two-circuit systems and multi-zone systems

In modern renovations, we increasingly encounter systems with more than one heating circuit – for example, radiators + floor heating + hot water storage tank. In such cases, each circuit is equipped with its own pump, or a hydraulic distributor (collector/distributor) with a group of pumps is used.

In such configurations, the pump in the primary circuit (boiler → collector) must have sufficient capacity for the total flow of all secondary circuits, while the secondary pumps are sized only for their own circuit. This is a different approach from a simple single-purpose system and the calculation must take into account the hydraulic coupling between the circuits. In the case of more complex systems, I recommend consulting an experienced designer or read the article How to choose the right pump for heating or water: step by step.

For systems with heat pumps, even more specific rules apply – heat pumps require a higher flow rate at a smaller temperature difference (typically 5 K instead of 15–20 K with a boiler), which means significantly higher pump flow rates. A standard pump sized for a boiler will definitely be undersized in such a system.

Post-installation check: what to verify before handover

Once the pump is installed, the system is filled and bled, before the boiler room is finally sealed, check the following:

Tightness of all connections – visually inspect after the first heating to operating temperature (thermal expansion can reveal slight leaks)
Flow in the system – each radiator should become evenly warm after a short period of operation (of course, after balancing the system with thermostatic valves)
System pressure – 1.0–1.5 bar when cold, and should only increase by 0.2–0.4 bar when hot (75 °C); if the increase is greater, the expansion vessel pre-charge is likely incorrectly set
Noise – the pump should operate almost silently; any hissing, gurgling or cracking indicates air, incorrect pressure setting or a hydraulic issue
Water flow direction – modern pumps have an arrow on the body indicating the flow direction; check that it matches the actual direction in the piping (depending on the boiler room layout)

Documentation and warranty: what to keep

After replacing the pump, keep the following: the original documentation for the pump (manual, warranty card), a record of the installation date, set parameters (operating mode, set pressure value) and the name of the installer. These details are valuable for future service inspections or in case of a warranty claim. Modern pumps have warranties of 2 to 5 years, but the warranty usually requires proof of correct installation in accordance with the manual.

Also record the condition of the water in the system – if you added corrosion inhibitors or water softening agents, note the type and quantity. This information will be helpful for future maintenance. The topic of prevention and extending the lifespan of pumps is discussed in detail in the article Maintenance and service of pumps: how to extend lifespan and avoid failures.

Most frequently asked questions (FAQ)

Can I replace the pump myself, or do I need a professional?

Physically replacing the pump is technically feasible even for an experienced DIY enthusiast who has experience working on a heating system. The electrical connection is simple (230V single-phase supply), and the mechanical installation is also straightforward. I recommend calling a professional if: you are unsure about the correct pump performance selection, the system has multiple circuits and hydraulics, or if the pump regulation needs to be set in combination with the boiler via OpenTherm. Remember that when connecting to gas or replacing the boiler, an authorized technician must perform the work – the pump itself is not subject to this requirement.

How can I check if my old pump is still working?

You can easily check functionality: when the boiler is on, the pump should be audible (a soft hum) and the piping after the pump should be warmer than before it. If the pump is running, but some radiators are cold, the issue may be a clogged filter (a Y-filter should be installed before the pump), a blocked flow or hydraulic imbalance. If the pump does not start at all, check the circuit breaker and terminal block. More diagnostics can be found in the article Common pump failures for heating and water: causes and solutions.

Do I have to drain the entire system when replacing the pump?

Not necessarily, if you have shut-off valves (ball valves) at the pump. In this case, it is sufficient to close the valves, drain the water only between them (a few deciliters), replace the pump and open the valves. The system will refill itself and only needs to be bled. If you do not have shut-off valves, you must drain the entire system – and on this occasion, I recommend installing the valves.

What is the price difference between an old and a new pump type, and when is it worth it?

An old constant-speed pump without regulation can be bought cheaply – for tens of euros. A modern ECM pump with electronic regulation usually costs 80 to 200 euros. The difference in electricity consumption is significant: an old pump consumes 200–400 kWh per year, while a modern one consumes only 40–80 kWh. At an electricity price of 0.20 €/kWh, this results in a saving of 30 to 70 euros per year. The return on investment in a modern pump is therefore 1.5 to 4 years – after that period, you start making savings. With a pump lifespan of 15–20 years, this is a very good investment.

What to do if the new pump makes noise or is noisy after starting?

Noise after replacement is most commonly caused by three issues: air in the system (solution: bleeding, repeated), too high a differential pressure setting on the pump (solution: reduce the set value), or cavitation due to insufficient inlet pressure (solution: increase system pressure, check the Y-filter). A detailed diagnostic procedure can be found in the article Pump noise: why it buzzes or vibrates and how to eliminate it.

Do I need to adjust the expansion vessel after replacing the pump?

Not always, but it is a good opportunity. If you are carrying out a more extensive renovation (new boiler, new radiators, or possibly an expansion of the system), the system volume may have changed and the original expansion vessel may have an unsuitable pre-charge. The correct pre-charge of the expansion vessel is 0.1 bar lower than the cold operating pressure of the system. Incorrect pre-charge will be indicated by either a pressure drop when heated or excessive bleeding through the safety valve.

Conclusion: pump replacement as an investment with measurable return

Replacing the pump during a heating system renovation is not just a technical necessity – it is one of the few home investments you can calculate and whose return is measurable in euros on your electricity bill. A modern electronically regulated pump operates more quietly, runs reliably for longer, and consumes only a fraction of the energy compared to older models.

If you decide to replace the pump yourself, follow the procedure: first determine the system parameters and dimensions, select the correct pump, bleed the system thoroughly and set the operating mode. If you have doubts about the system hydraulics or the combination with the boiler, do not hesitate to consult – a well-adjusted pump in a properly hydraulically balanced system is the foundation of comfortable and energy-efficient heating for decades to come.

Do you have a question on this topic?

Struggling to decide or dealing with a specific situation in your home? Write to us – we are happy to help.