Bleeding and flow adjustment for bottom-connected radiators

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Air venting and flow adjustment for radiators with bottom connection

Air in the radiator system is one of the most common reasons why a radiator heats poorly, unevenly, or not at all. With radiators with bottom connection, this issue has several specific characteristics that do not occur with side connection – and that is precisely why it deserves its own detailed explanation. This article will explain to you why air enters the system, where it accumulates, how to reliably remove it, and how to correctly adjust the flow so that the radiator delivers exactly the thermal performance you expect from it.

If you are currently dealing with the installation of a new radiator, I recommend you also read the topics Installation of a radiator with bottom connection step by step and Connecting a radiator to existing pipes – what you need to know before installation, where you will find context for the entire installation process. Here we will focus exclusively on what follows the physical installation – that is, on starting up, air venting, and hydraulic balancing.

Why air in the radiator complicates things

Water and air will never become friends in a closed heating system. Air has several times lower thermal capacity and conductivity compared to water – where an air bubble takes up space, heat simply does not pass through. The result is very clear to you: the upper part of the radiator is warm or cold, while the lower third is hot to the touch. The humming, bubbling, and cracking sounds you sometimes hear from the radiator are only the acoustic manifestation of the fact that air and water are fighting for space inside.

With radiators with bottom connection – that is, those where both openings (supply and return) are located at the bottom, most often in the middle or at the bottom of the panel – air has a tendency to rise upwards and remain in the upper chambers of the panel. If the air vent is in the correct position (and at the same time correctly installed), this air can be relatively easily removed. However, if the radiator does not have its own air vent or the installation is set up inappropriately, the air holds on stubbornly and the flow of hot water is permanently limited.

Along with uneven heating, air in the system has another unpleasant side effect: it accelerates corrosion. Dissolved oxygen is aggressive towards the steel walls of panel radiators. Therefore, air venting is not only a matter of comfort, but also of the device's lifespan.

Where does the air come from and how does it get into the system

Air does not arise in the system on its own – it always gets there for a specific reason. If you know them, you can avoid them or at least know what happened when the radiator starts to make a dull noise again after years of operation.

Initial filling of the system: When a new system or a new radiator is filled with water, the air that was in the pipes and in the panel must be pushed somewhere. If the venting is not done thoroughly, the air remains trapped – and it is precisely this air that is the source of problems in the first weeks of operation.
Replacement or removal of the radiator: Whenever you open the system – whether you replace the radiator, add a new one, or simply unscrew the thermostatic head – air has the opportunity to enter. With bottom connection, this happens discreetly, without a significant water leak.
Corrosion and gaseous products: Long-term corrosion of steel panels produces hydrogen, which behaves similarly to air in the system – it rises and accumulates in the highest places.
Insufficient pressure in the expansion tank: If the pressure in the system is too low, water can locally turn into steam, which later condenses – but before condensation, it causes bubbling and noises identical to air.
Undersized air vent on the boiler or in the mechanical room: Some older pipe systems do not have an automatic air vent at the highest point of the circuit – air is removed at the radiators, but immediately returns from another part of the system.

Construction of radiators with bottom connection and air vent

Panel radiators with bottom connection – such as, for example, Radiator 21VK 300 x 600 output 447 W or Radiator 21VK 300 x 800 output 596 W – have a built-in air vent usually in one of the upper corners. Most of today's production has it in the upper right corner (view from the front), but manufacturers differ. Always check before installation which side the vent is on, because it will affect how you install the entire radiator and whether venting will even be possible without disassembly.

The air vent is usually a small screw with a square or slot, mounted in a brass body. Under the vent is a small rubber sealing ring. For venting, you need a venting key (so-called Entlüftungsschlüssel – a square 5–6 mm key or another shape according to the manufacturer), or a flat screwdriver, a small container to catch drops, and a hose or cloth.

Some radiators with bottom connection have an automatic air vent (so-called auto-vent) instead of a manual air bleed valve. It has a small float mechanism – when there is air in the upper chamber, the float drops and the air escapes on its own. As soon as water arrives, the float rises and closes the opening. Automatic air vents are more convenient, but in practice, you need to regularly check their sealing, because lime deposits from hard water can permanently open or close the valve.

Step-by-step procedure for bleeding

Bleeding a radiator with bottom connection is simple, but it has its rules. If you don't follow them, the result will be incomplete – air will be partially pushed out, but the problem will return in a few days.

Preparation before bleeding

Before you start the actual bleeding, check the pressure in the system. On the boiler manometer (or on the pressure gauge in the machine room), the cold pressure should be in the range of 1.0 – 1.5 bar for a standard residential system. If the pressure is lower (below 0.8 bar), add water to the expansion tank and then into the system – otherwise, during bleeding, the pressure will drop below the minimum and the pump may run dry.

Next, turn on the heating for at least 15–20 minutes before bleeding. Warm water has a lower ability to absorb air – air bubbles will be released and will collect in the upper part of the panel. If you bleed a cold system, the air may still be dissolved in the water and there will be nowhere to catch it.

The bleeding itself

Prepare a bleed key, a small container (glass or bowl), and a piece of cloth for any drips.
The bleed valve is located in the upper corner of the radiator – from the outside, a small screw with a slot or square opening is visible.
Hold the container directly under the valve – in bottom-connected radiators, the valve is high, so you need to stretch.
Use a key or screwdriver to open the valve slowly by 1/4 – 1/2 of a turn counterclockwise. There is no need to open it more.
Listen: at first, you will hear hissing – this is air escaping. When the hissing stops and a continuous stream of water starts flowing (not just drops), immediately close the valve.
Don't leave it – after the water stops flowing, wait 10–15 seconds and slightly open the valve again. Sometimes air comes in two waves.
After bleeding, check the pressure in the system – the pressure will slightly drop after bleeding. If the drop is more than 0.2 bar, add water through the boiler's filling valve.

Important: never bleed a radiator in a system where the circulation pump is running, unless it is absolutely necessary. The pump can create pressure surges in the system, which make bleeding difficult. Ideally, turn off the pump temporarily, bleed, and then restart it.

Order of bleeding with multiple radiators

If you are bleeding the entire system (for example, after a boiler replacement or after a complete renovation), always proceed from radiators closest to the boiler to those furthest away. And if you have a multi-storey house, start on the ground floor and continue upwards – you are "chasing" air ahead of you step by step to the highest points, where you finally push it out.

With a system with multiple branches (e.g., a living room loop, a bathroom loop), bleed one branch at a time – close the other branches with thermostatic heads or manual valves on the manifold. This ensures sufficient pressure and flow in the branch you are bleeding.

Flow setting – why it is as important as bleeding

Bleeding solves the problem with air. Flow setting solves an equally important problem – hydraulic balancing. In practice, this means that if you have multiple radiators in the system, water naturally flows along the path of least resistance. Radiators near the boiler get "too much" hot water, radiators at the end of the branch get very little. Result: you will overheat near the boiler and freeze near the window.

Radiators with bottom connection have a built-in regulating (flow) valve directly in the base block – so-called bottom valve block or integrated valve (depending on the manufacturer). This valve allows you to set the flow without the need for any additional fittings. It is one of the main advantages of bottom connection over side connection – you will learn more about it in the topic Bottom connection of a radiator vs side connection – advantages, disadvantages and when it is worth it.

What flow rates should be set?

The exact setting depends on the radiator's designed thermal output and the hydraulic calculation of the entire system. In practice, however, in most apartment and single-family homes without professional hydraulic calculations, the setting is resolved by trial and error and temperature measurements. Here is an approximate procedure:

Maximum opening (full flow): Set on radiators with the highest thermal output or those that are hydraulically furthest from the boiler (i.e., naturally have the highest pipe resistance).
Partial opening: Radiators close to the boiler or with lower output are throttled to ensure that water "must" also reach the more distant radiators.
Scale on valves: Modern integrated valves have a scale of 1 – 8 or 1 – 10. Step 5 corresponds to approximately 50% flow. On the more distant radiator, you set 7–8, and on the nearby radiator 2–4. These are only starting points, not exact values.

Let's take a concrete example from practice: a radiator system reconstruction in a brick apartment building, five radiators in a row. Radiator No. 1 (closest to the manifold) tends to overheat – supply temperature 75 °C and return 70 °C, a difference of only 5 K. Radiator No. 5 (at the end of the branch) has a supply of 75 °C, but the return is only 55 °C, a difference of 20 K – the radiator heats well, but only when the heat is actually needed. After balancing, the temperature difference (Δt) at radiators No. 1 – 5 should move within the same range, ideally 10 – 15 K. Radiator No. 1 is throttled to step 2–3, radiator No. 5 remains fully open or at step 7–8.

Setting the thermostatic head and its impact on flow

A thermostatic head (TRV – Thermostatic Radiator Valve) is not a tool for system balancing – it is a room temperature regulator. It works by sensing the ambient air temperature and opening or closing the hot water supply to the radiator accordingly. If the room is warm (e.g., the sun is shining through the window), the head automatically reduces the flow or stops it completely.

Hydraulic system balancing (using flow valves) and room temperature regulation (using thermostatic heads) should be understood as two different layers of control. Flow valves set the maximum that a radiator can receive – the thermostatic head then regulates the actual amount within this maximum according to need.

Thermostatic heads on radiators with bottom connections are usually mounted in one of the side openings on the bottom valve block. Note: some bottom valve blocks are designed for specific types of heads (e.g., M30×1.5 thread), others may have a different standard – always check compatibility before purchase.

Flow rate depending on radiator output

Each radiator has a calculated thermal output under standard conditions (usually at a temperature drop of 75/65/20 °C – supply temperature 75 °C, return 65 °C, room temperature 20 °C). This output corresponds to a certain water flow rate that must pass through the radiator per hour. From physics, we know that:

Q [W] = m [kg/h] × 1.163 × Δt [K]

where Q is the thermal output, m is the mass flow rate of water, and Δt is the temperature difference between the supply and return. For the Radiátor 21VK 300 x 500 output 373 W at standard Δt = 10 K, the flow rate should be:

m = 373 / (1.163 × 10) = 32.1 kg/h ≈ 32 l/h

For the Radiátor 21VK 300 x 400 output 298 W at the same Δt, the flow rate would be only about 25.6 l/h. These values are important when setting the flow meter or checking the correctness of the balancing. In everyday practice, you won't find flow meters on each radiator – balancing is usually done by measuring temperatures or using professional tools (ultrasonic flow meter, differential pressure gauge).

Common mistakes during air venting and flow setting

After years of working on customer sites, you still see the same mistakes. We list them so you can avoid them:

Venting without pressure check: People open the valve, let water flow out "just enough" and then wonder why the boiler reports low pressure. Rule: never vent more than 0.5 l of water during venting.
Venting only one radiator: Air moves through the entire system. If you have a problem with one radiator, check the venting of all others – otherwise, the air will return from another location.
Forgetting to vent the boiler and manifold: Modern condensing boilers have their own vent valve – this one should be vented first, before the radiators.
Setting the flow without measurements: Estimating "by eye" during balancing works only in simple systems with 2 – 3 radiators. In more complex networks, you risk one branch heating at full speed while another hardly at all.
Forgetting to open the thermostatic heads during venting: If the head is closed (room temperature reached), water does not flow through the radiator – venting does not work. Before venting, set all thermostatic heads to maximum or to 5 (or remove them).
Accidentally adjusting the flow valve during venting: Some bottom valve blocks have a flow valve and a vent valve in close proximity. By mistake, you might "touch" the flow valve and change the setting you carefully set before.

System balancing in practice – a real scenario

Imagine a new single-family house with nine radiators on two floors. The heating system is two-pipe, the boiler is condensing. After the first start-up, the heating technician set all flows to maximum 8 steps. The result? On the ground floor, where the radiators are close to the boiler, it was immediately several degrees warmer than on the upper floor. The thermostatic heads on the ground floor started to close – and thus let even more water go to the upper floor, which helped a bit, but not enough.

Solution: hydraulic balancing by the method of temperature differences. The heating technician, equipped with an infrared thermometer or clamp-on thermometers on the pipes, gradually throttled the flows on the ground floor radiators (reduced them to step 3 – 4), until the temperature differences between supply and return were within the range of 8 – 12 K on all radiators. The whole job took about 2 hours. Result: evenly heated house, lower gas consumption (the pump works more efficiently in a balanced system), and quiet operation without noise.

This type of problem is very common precisely during modernizations, when old cast iron radiators are replaced with new panel radiators with lower thermal capacity. New radiators react faster and hydraulic imbalances become immediately apparent – whereas with old cast iron radiators, uneven heating was often ignored for years.

Specifications of air venting in low-temperature and floor heating systems

If a radiator with bottom connection is part of a combined system (e.g., condensing boiler with floating temperature circuit + panel radiators + floor heating), air venting has additional specific aspects. Floor heating circuits have a manifold, which usually has automatic air vents. Panel radiators on the same boiler are vented separately.

In low-temperature systems (e.g., supply temperature 45 – 55 °C), Δt is smaller, but the flow must be higher to transfer the same power. This means that flows in the system must be adjusted differently than in a classic 75/65 °C system. Some installers underestimate this – they set the flow "as always" and then wonder why radiators heat insufficiently at low heating temperatures. The solution is either a larger radiator surface (longer or taller panel) or precise setting of maximum flows.

What to do if a radiator still heats unevenly after venting

It happens – you vent, set the flow, but one third of the panel is still cold or only warm. Here are several possible reasons:

Scale inside the panel: An older radiator may have lime or corrosive deposits inside that partially block the water flow. The solution is to chemically flush the system or replace the radiator.
Incorrect connection of supply and return: With bottom connection, it is crucial that the hot water supply is connected to the side leading into the "supply" channel inside the panel, and the return the other way around. If they are swapped, the water flows in the wrong direction and the radiator heats only partially. More about this issue in the topic Common faults in radiators with bottom connection and how to eliminate them.
Partially closed flow control valve: Check the setting of the flow control valve in the bottom block – if it was accidentally turned, the flow is limited.
Too low pressure in the system: At pressure below 0.7 bar, the pump cannot sufficiently push water into radiators at the end of the branch. Add pressure and repeat the venting.

Venting during renovation – special situations

During renovation, where only one or several radiators have been replaced (e.g., an old cast iron radiator with a new panel radiator with bottom connection), venting and flow issues appear in a different form. A new panel has a significantly smaller internal water volume than an old cast iron radiator. This can cause the expansion tank to be oversized – pressure fluctuations in the system are greater and air enters the system more easily. Check the setting of the expansion tank after replacing radiators and, if necessary, adjust the nitrogen pressure in the tank according to the system height.

Also: when replacing a radiator, always check the condition of the seals in the bottom valve block. If the old rubber seals are hard or cracked, air has an easy way in. Replace them preventively – they are cheap and available.

Most frequently asked questions (FAQ)

How often should radiators with bottom connection be vented?

In a well-functioning system with good sealing and no corrosion, venting once per season is sufficient – ideally at the beginning of the heating season (September – October), when you turn on the heating after summer. If you have to vent every 2 – 3 weeks, something is wrong in the system – look for the cause (leaks, corrosion, incorrect expansion tank).

Can I vent a radiator myself without a professional?

Yes, radiator venting is a routine home maintenance task that every owner can handle without special knowledge. You only need a venting key (a few cents in a hardware store), a container to catch the water, and a list of all radiators to go through. Hydraulic balancing of the entire system (setting flows) is more complex and for larger networks it is worth calling a professional with thermometers.

A radiator hisses even when the valve is closed – what does that mean?

If the hissing comes from the radiator with the venting valve closed, the problem is elsewhere – it is likely water flowing with air bubbles directly inside the panel or an incorrectly set flow (too high flow causes cavitation at the valve). Check the flow settings and if the hissing continues, have the system inspected by a heating technician.

Is one venting valve on a radiator enough or should there be more?

One venting valve on a radiator is standard and sufficient. A panel radiator has internally connected chambers so that air can escape toward one point – usually the top corner, where the valve is located. Exceptionally, very long radiators (e.g., over 2 000 mm in width) may have air pockets on both sides – in such a case, the manufacturer may install two valves.

Can flow be adjusted without professional tools?

Yes. For a standard apartment system with 3 – 8 radiators, it is sufficient to compare the temperatures of the supply and return using a contact or infrared thermometer. The goal is to achieve the same temperature difference (Δt) at all radiators in the same circulation loop. The ideal Δt for a standard 75/65 °C system is 8 – 12 K. You adjust the flows in the bottom valve block – restrict radiators with a small Δt (close to the boiler) and leave radiators with a large Δt (far from the boiler) fully open.

What to do if water leaks from the venting valve even after tightening?

If the valve leaks, the most common cause is a worn sealing ring under the screw or the valve seat is damaged by impurities from the water. The first step is to tighten the screw (but not too hard – brass threads can easily strip). If that doesn't help, replace the entire venting valve – it is a standardized valve (M10×1 or G1/8" thread depending on the manufacturer), available for a few euros. When replacing, do not depressurize the entire system – it is enough to locally close the radiator using the valve in the bottom block.

Conclusion

Venting and flow setting are not one-time tasks "during installation and done" – they are recurring service steps that determine whether the heating system operates economically, quietly, and evenly. Radiators with bottom connection – whether smaller types like Radiator 21VK 300 x 400 power 298 W for small rooms, or more powerful units like Radiator 21VK 300 x 700 power 522 W – have an integrated valve as a convenient tool for both tasks. It is important to know what each wheel does and why.

If you are wondering how to choose the right radiator power for a specific room, read the topic What radiator power do I need – calculation based on room area and height. And if you are interested in an overall overview of the correct panel dimensions, see Radiator 21VK dimensions – how to correctly choose panel height and width.

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.