Hydronic heating panels are not a new invention, and nobody serious claims they are. The steel panel radiator has been the standard heat emitter in European homes for decades, and the idea of heating a building with circulating hot water is older still. What has changed is the water. For most of the panel radiator’s history it was fed by a boiler at 70 or 80 °C, and every sizing table, every catalogue figure and every rule of thumb was built around that temperature. Heat pumps and condensing boilers work best at far lower temperatures, and a panel that was perfectly adequate in the old arrangement can turn out to be badly undersized in the new one.
This article explains how hydronic panels actually deliver heat, what the type numbers on a panel radiator mean, why output falls so steeply as water temperature drops, and how each panel is controlled and balanced. It is written for installers, specifiers and distributors choosing panels for a system, especially one that will run with a heat pump. For the basic definition and the general pros and cons of radiators, see our article on what a radiator is.
Radiation or Convection? How Panels Really Deliver Heat
The name suggests radiant heat, and it is one of the more misleading names in building services. A hot surface does emit radiant heat, which warms people and objects directly without heating the air in between. But a hot surface also warms the air touching it, that air rises, cooler air is drawn in at the bottom, and a convection current forms in the room. Which of the two dominates depends on how the panel is built, and for most modern panels the answer is convection.
A single flat steel panel with nothing behind it has the highest radiant share, because almost all of its output comes from its face. Manufacturers then raise output by welding corrugated convector fins to the back of the panel, and by adding second and third panels behind the first. Those fins and inner panels are hidden from the room, so they cannot radiate into it. They work almost entirely by heating air. A typical double panel with two rows of fins delivers the large majority of its output by convection, which is why a room heated by panel radiators has the familiar warm layer near the ceiling and cooler air near the floor.
None of this is a weakness, but it matters for honest comparisons. Underfloor heating has a much higher radiant share and a far more even vertical temperature profile, which is a large part of why it feels comfortable at a lower air temperature. Panel radiators respond faster, cost less to install in an existing building, and deliver more heat per square metre of wall. The trade-offs are set out in radiator vs underfloor heating.
A Short History, With the Dates Right
Heating a building through its structure goes back a long way. The Roman hypocaust, which passed hot gases from a furnace through a void beneath raised floors and sometimes through flues in the walls, dates from around the first century BC. Korean ondol, which channels heat under stone floors, is older still, and it shaped the modern Korean preference for floor heating, as we describe in underfloor heating in South Korea. Neither system used water, so neither is a direct ancestor of the hydronic panel.
Water-based heating took shape in the eighteenth and nineteenth centuries, and the cast iron column radiator appeared in the mid nineteenth century. It was heavy, slow to warm up and held a large volume of water. Pressed steel panel radiators replaced it across most of Europe during the twentieth century, being lighter, cheaper and much quicker to respond. The basic form has hardly changed since. What has changed are the fins, the coatings, the valve connections, and above all the water temperature the panels are now expected to work with.
Panel Radiator Types: What 11, 22 and 33 Mean
Panel radiators are described with a two-digit type number, and once you know the convention it tells you a great deal at a glance. The first digit is the number of water panels. The second digit is the number of convector fin rows. A deeper radiator with more panels and fins gives more output from the same wall area, at the cost of projection from the wall and a little more water content.
- Type 10: one panel, no fins. Slim, mainly radiant, lowest output. Used where projection must be minimal.
- Type 11: one panel, one row of fins. A common choice for bedrooms and small rooms.
- Type 21: two panels, one row of fins between them. A middle step in output.
- Type 22: two panels, two rows of fins. The most widely installed panel in Europe and the default for living spaces.
- Type 33: three panels, three rows of fins. Highest output per metre of wall, and the panel most often used when sizing for low water temperatures.
Heights typically run from about 300 to 900 mm and lengths from around 400 mm to well over 2 metres, so the same output can be achieved with very different shapes. A long, low Type 22 under a window and a short, tall Type 33 on a side wall can deliver similar heat. That flexibility is useful when upsizing for a heat pump, because the extra output often has to come from somewhere other than simply making the existing radiator longer.
Designer flat panels and vertical radiators sit outside this numbering. Many look excellent, but a flat face with no fins behind it often delivers considerably less heat than a standard Type 22 of the same size. When a designer panel replaces a standard one, check the output rather than assuming that a similar size means similar heat.
Why Output Falls So Sharply at Low Water Temperature
This is the section that matters most for anyone moving from a boiler to a heat pump, and it is the one most often skipped. Radiator output in European catalogues is rated under EN 442, usually at ΔT50. That figure is the difference between the mean water temperature in the radiator and the room air, and it corresponds to flow at 75 °C, return at 65 °C and a room at 20 °C. It is a standard test condition, not a promise of what the radiator will deliver in your building.
Output does not fall in a straight line with temperature. It follows roughly the temperature difference raised to the power of about 1.3, which means each step down costs a bit more than proportionally. Using that relationship, approximate output as a share of the ΔT50 rating looks like this:
| Mean water temp (room at 20 °C) | ΔT | Approximate share of rated output |
|---|---|---|
| 70 °C | 50 K | 100% |
| 60 °C | 40 K | about 75% |
| 50 °C | 30 K | about 51% |
| 45 °C | 25 K | about 41% |
| 40 °C | 20 K | about 30% |
Exact figures vary between manufacturers, because each publishes its own exponent, but the pattern is the same everywhere. Take a heat pump running at 45 °C flow and 40 °C return. The mean water temperature is 42.5 °C, so ΔT is 22.5 K, and a radiator rated at 1,500 W at ΔT50 will deliver only about 530 W. If that room needed 1,200 W, it will not get warm, however long the system runs.
There are only a few honest ways around this. You can raise the flow temperature, which works but costs heat pump efficiency, since a heat pump’s coefficient of performance falls as flow temperature rises, as explained in our article on coefficient of performance. You can enlarge the emitters, usually by moving from Type 11 to Type 22 or from Type 22 to Type 33, or by adding length. You can reduce the heat loss of the room with insulation and better glazing, so that the smaller output is enough. Or you can combine approaches, which is what most good retrofits end up doing. What does not work is fitting a heat pump to radiators sized for 75 °C and hoping for the best.
Sizing Panels for a Heat Pump
Good sizing starts with a room-by-room heat loss calculation, not with the existing radiators. The question is what each room needs at the design outdoor temperature, and then what panel will deliver that at the flow temperature the heat pump is meant to run at. Choosing that design flow temperature is the key decision, because it sets the size of every emitter and much of the system’s running cost. Many designers aim for around 45 °C or below on radiator systems, and lower where the building allows it.
Once the design temperatures are chosen, the catalogue rating has to be converted. Divide the room’s heat loss by the correction factor for the design ΔT, and the result is the ΔT50 rating the panel needs. A room needing 1,000 W at ΔT25 needs a panel rated at roughly 2,400 W at ΔT50. That sounds alarming, but it is often achievable by replacing a single-panel radiator with a Type 22 or Type 33 of similar length, and existing pipework can usually stay in place.
Temperature drop across the radiator matters too. Boiler systems were traditionally designed for a 10 to 20 K difference between flow and return. Heat pumps usually run with a smaller drop, often around 5 K, which keeps the mean water temperature higher for a given flow temperature but requires higher flow rates. Those higher flow rates affect pipe sizing, pump selection and balancing, so they have to be checked rather than assumed. Our article on air-to-water heat pumps covers the source side of the same design.
Controlling Each Panel: TRVs and Lockshields
Every panel radiator has two valves, and they do completely different jobs. On one side, usually the flow, sits the thermostatic radiator valve, or TRV. On the other sits the lockshield valve, which is set once during commissioning and then left alone. Mixing up their roles is one of the most common reasons a radiator system never quite heats evenly.
A TRV senses the air temperature around it and throttles flow to that panel as the room approaches its setting. Inside the head is a sensing element, most commonly a wax or liquid-filled capsule, which expands as it warms and pushes a pin that closes the valve. The quality of that element decides how accurately and how consistently the valve responds, which is why the element is the part that matters most. TRVs let rooms with sunshine, cooking or people in them stop drawing heat while colder rooms keep receiving it. Our dedicated article on thermostatic radiator valves goes into selection and setting.
A few practical rules apply. A TRV should not be fitted in the same room as the main room thermostat, since the two will fight over control of that room. The TRV head needs free air around it, so it should not be buried behind curtains or furniture. And on heat pump systems, it is worth keeping enough radiators open to guarantee a minimum flow through the heat pump, either by leaving TRVs in the main living spaces fully open or by providing a bypass or buffer volume. Otherwise, when most TRVs close on a mild day, the heat pump can short cycle. The room thermostat itself and how it fits into the wider system are explained in the role of the central heating room thermostat.
Balancing With the Lockshield
Water takes the easiest route, so radiators close to the pump get more flow than those at the far end of the circuit. Balancing corrects this by partly closing the lockshield valves on the near radiators, pushing more water out to the distant ones. The usual target is the design temperature drop across each radiator, measured with clip-on thermometers on the flow and return pipes. A system that is well balanced heats up evenly, and an unbalanced one leaves the far rooms cold while the near rooms overheat.
Balancing becomes more important, not less, when moving to a heat pump. The smaller temperature drop and higher flow rates make an unbalanced system more obvious, and a heat pump does not have the spare temperature that once masked poor balancing on a boiler. It is not glamorous work, but it often makes the difference between a retrofit that works and one that gets blamed on the heat pump.
Panels and Underfloor Heating on One System
Many buildings use both: panel radiators upstairs and underfloor heating on the ground floor, or radiators in an older part of a house and underfloor heating in an extension. The two emitters want different water temperatures. Panels on a boiler system may run at 60 to 70 °C, while a screed floor needs roughly 35 to 45 °C. Feeding the floor directly at radiator temperature would overheat the surface and can damage the screed and the floor covering.
The usual answer is a separate underfloor circuit with its own pump and a mixing valve that blends return water into the flow to bring it down to floor temperature. The manifold then distributes that water to the individual loops. Our article on underfloor heating mixing valves explains the mixing stage, and UFH manifolds with a pump covers the pumped manifold arrangement.
On a heat pump system the picture is simpler in one way. If the radiators have been sized for low temperature, both emitters can sometimes run from the same flow temperature, and the mixing stage becomes a safety limit rather than the main temperature control. That is one more reason to size the panels properly rather than raising the heat pump’s flow temperature to suit undersized radiators.
Water Quality and Maintenance
Steel panels are durable, but they corrode from the inside if the water allows it. Oxygen entering the system, through a leak, frequent top-ups or non-barrier plastic pipe, reacts with the steel and forms black magnetite sludge. That sludge settles in the bottom of the panels, causing cold spots near the bottom of the radiator, and it collects in pumps and valves. A radiator that is cold at the bottom and warm at the top is often showing sludge, while one that is cold at the top usually has trapped air.
Prevention costs very little compared with the cure. A system should be cleaned and flushed when new panels are fitted, dosed with a corrosion inhibitor, and fitted with a magnetic filter on the return. Air should be bled from each panel after filling, and system pressure checked against the expansion vessel charge. Hard water is also a consideration in heat exchangers and in the heat source itself. A small amount of attention here protects the TRVs, lockshields and pump that the rest of this article depends on.
On installation, panels on an existing wet system are about as easy as heating upgrades get. Swapping a Type 11 for a Type 22 on the same pipe centres can be a morning’s work. Installing hydronic heating in a building that has no wet system at all is a different project, since it needs a heat source, pipework to every room and space for both. It is still often worth doing, but it is not the light job sometimes described.
Frequently Asked Questions
Do hydronic heating panels heat by radiation?
Partly. A flat single panel with no fins delivers a relatively high share of radiant heat, but most modern panels have convector fins and multiple panels, and for those convection delivers the larger share of the output.
What does Type 22 mean on a radiator?
It means two water panels and two rows of convector fins. The first digit is the number of panels and the second is the number of fin rows. Type 11, 21, 22 and 33 follow the same convention.
Can I keep my existing radiators with a heat pump?
Sometimes. Radiators are rated at ΔT50, which matches a 75 °C flow. At typical heat pump temperatures they may deliver only a third to a half of that rating. A room-by-room heat loss calculation will show which panels are big enough and which need replacing.
How much output does a radiator lose at 45 °C flow?
With a 45/40 °C flow and return and a room at 20 °C, a typical panel delivers roughly 35 percent of its ΔT50 rating. A radiator rated at 1,500 W would deliver around 530 W.
Where should a thermostatic radiator valve be fitted?
Usually on the flow side, with the head in free air rather than behind curtains or furniture. It should not be fitted in the same room as the main room thermostat.
What is the lockshield valve for?
Balancing. It is set during commissioning so that radiators near the pump do not take more than their share of flow, and then left alone.
Why is my radiator cold at the bottom?
Most often because magnetite sludge has settled in the lower part of the panel. A radiator cold at the top usually has air in it and needs bleeding.
Hydronic Components and Heat Pumps from Legom
Jiaxing Legom Technology Co., Ltd. manufactures hydronic heating components and heat pumps in Jiaxing, Zhejiang, supplying distributors and brands in more than 90 countries. Our air-to-water heat pumps are designed for low flow temperatures, which is exactly the condition this article is about, and work with properly sized panel radiators, underfloor heating or a combination of both. For floor heating on the same system, see heat pumps for underfloor heating.
We make our own wax thermostatic elements, the sensing component behind thermostatic valves, rather than buying them in. That is what lets us keep response temperature consistent from one production batch to the next. Alongside them we supply HVAC valves including mixing and control valves, manifolds for mixed radiator and underfloor systems, and room thermostats, including programmable models for scheduled control.
For distributors and brands, our OEM and ODM service covers branding, connection standards, temperature ranges and market documentation.
Reviewed by Maggie Shen, Director at Legom, on November 3, 2026. This article was reviewed for technical accuracy, including the split between convective and radiant output in panel radiators, panel type nomenclature, EN 442 output rating at ΔT50, output correction at lower mean water temperatures, sizing panel radiators for heat pump flow temperatures, the roles of thermostatic radiator valves and lockshield valves, hydronic balancing, mixed radiator and underfloor heating systems, and magnetite sludge prevention.