Radiators and a Condensing Boiler
Many people associate condensing boilers exclusively with underfloor heating – and that's no coincidence, since underfloor heating is the textbook example where a boiler condenses almost non-stop and saves as much as possible. But what if you have classic radiators at home, or an older house with old, undersized units below the windows? Is it even worth buying a condensing boiler in that case, or is it just "for underfloor heating people"? The answer is clear – yes, it's almost always worth it – but it's important to understand why the savings won't be quite the same as with underfloor heating, and what you can do to maximize them.
Summary of the conclusion reached by the article.
Why radiators have different temperature requirements than underfloor heating
A condensing boiler achieves its high efficiency by also using the heat contained in the water vapor from the flue gases – this vapor condenses back into water and releases its heat into the heating system only when the temperature of the water returning to the boiler (the so-called return) is low enough, generally below approximately 55 °C. The lower this temperature, the more vapor condenses, and the higher the efficiency the boiler actually achieves – in an ideal case, it can exceed 100% relative to the calorific value of the gas.
What water temperature goes to the radiators
And this is exactly where the key difference from underfloor heating lies. Underfloor heating works with a flow water temperature typically around 30–40 °C, because the large surface area of the floor can deliver the required heat even with such "lukewarm" water. Radiators have a significantly smaller surface area than a floor, so to deliver the same amount of heat to a room they need noticeably warmer water – typically in the range of 55–70 °C, and with older or undersized radiators on freezing days, easily even more.
What this means for condensation
When the boiler has to heat water to 65–70 °C, the return water comes back warmer than the condensation threshold, and at that moment the boiler works practically like a classic (non-condensing) boiler – the condensing effect doesn't kick in. This typically happens during the coldest days of the heating season, when the system needs to work at full output and with the highest water temperature.
Conversely, during transitional periods – in autumn, in spring, during milder winters, and on most days when it's not extremely cold outside – the house needs significantly less heat, and modern weather-compensated control (which automatically adjusts the system's water temperature to the outdoor temperature) can keep the water in the radiators at a much lower level, easily below 50 °C. This is exactly when condensation kicks in fully again and the boiler runs at its highest efficiency. Since such milder days actually make up the majority of the heating season in Slovakia (the heating season lasts roughly from September to May, but genuine frost is only part of it), a condensing boiler with radiators, averaged over the whole season, condenses for a significant share of its operating hours – not constantly as with underfloor heating, but far from negligible either.
Put simply: with underfloor heating the boiler condenses practically all year round, with radiators only part of the year – but even this "part" is enough to make the savings compared to an old atmospheric boiler noticeable, usually tens of percent in gas consumption. You can find more on the real savings figures in a separate article, How much a condensing boiler saves compared to an old boiler.
Does it also work in older houses with old radiators?
This is the question we deal with most often with clients. The answer is: yes, it works, but with a caveat – in an older house with old (often undersized) radiators, the savings and overall system efficiency will be somewhat lower than in a house with newer or larger units, or with insulation.
Why old radiators "can't keep up"
Radiators installed 20–30 or more years ago were generally designed to work with classic (non-condensing) gas or even solid-fuel boilers, which commonly supplied water at 70, 80, even 90 °C. At the time, the designer assumed that a radiator of that size, at such a high water temperature, would cover the room's heat loss even during the hardest frost.
When you connect such a radiator to a condensing boiler that "tries" to run as much as possible in economical (condensing) mode, i.e. with lower water temperature, it can happen that on extremely cold days the radiator simply doesn't release enough heat into the room – the apartment or house feels noticeably colder than expected. The boiler either automatically compensates by raising the water temperature (thereby partly or fully losing the condensing bonus exactly when consumption is highest), or – if it's set too conservatively – the room doesn't get properly heated.
Solution 1: a boiler with a reasonable output reserve
The first and simplest solution is not to size the boiler "right on the edge," but with an adequate output reserve, and to have it work with a weather-compensation curve set so that on the coldest days it can still deliver the higher water temperature that old radiators need. Condensing savings will be reduced somewhat during frosty days, but the rest of the season continues to run efficiently. Correct output sizing is such an important topic that we've dedicated a separate article to it – see What condensing boiler output do I need, where you'll also find a specific calculation method.
Solution 2: enlarging or replacing the radiators
The second, more thorough solution is worthwhile especially during a larger renovation – increasing the surface area of the radiators (for example by replacing them with a taller or wider panel unit, or with a radiator that has more panels/fins), so they can deliver the same amount of heat even at a lower water temperature around 50–55 °C. This is exactly why you'll find radiators of the same type in different sizes and outputs in the catalog – for example, a common type 21K panel radiator sized 300 × 400 mm delivers an output of approximately 298 W, while a somewhat larger 300 × 500 mm size already delivers around 373 W. Even such an apparently small difference in size can, at a lower water temperature, make up for what the original undersized radiator was lacking.
As an example of a common panel radiator unit you might encounter when replacing or extending a system, take for instance the Radiator 21K 300 x 400, output 298W (price approximately €40–60), or the somewhat more powerful Radiator 21K 300 x 500, output 373W (price approximately €45–65). The choice of a specific size and type always depends on the heat loss of the particular room – ideally have it assessed by a professional, or at least get an approximate calculation using our calculators, which we describe in the article What condensing boiler output do I need.
The good news is that these two solutions can also be combined and don't need to be done at the same time – you can install a boiler with the right output reserve right away, and gradually replace or supplement the radiators as part of further renovation steps (for example together with facade insulation or window replacement, which will also reduce the required heating water temperature).
Thermostatic heads and valves – room-by-room control
Regardless of whether your radiators are new, old, large, or small, one element can significantly increase the efficiency of the whole system in practically every case – a thermostatic valve with a head on every radiator. Thanks to it, each room "takes" exactly as much heat as it needs, and no more. In rooms that get plenty of sunlight, that have appliances generating heat, or where you simply don't want it as warm as elsewhere (a bedroom is a typical example), the head will restrict or fully shut off the water supply.
From the condensing boiler's point of view, this matters too, because it reduces the average amount of heat the system needs to deliver to the house at any given moment, which lets the control run with a lower heating water temperature – and thus condense for longer and more intensively again. In other words, good room-by-room control isn't just a matter of comfort, it directly helps the boiler save gas.
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Two-way thermostatic valve – 1/2"xEK; angled – allows precise, two-stage adjustment of the water flow into the radiator and, combined with a thermostatic head, ensures automatic temperature control in the given room independently of other rooms. The angled version is suitable for a typical radiator connection from below/the side. Price from €14.27. |
When replacing or modernizing a radiator system, it's therefore worth investing in good-quality thermostatic valves on all units at once – it's a relatively inexpensive element (on the order of tens of euros for an entire apartment or small house), which thanks to gas savings typically pays for itself very quickly. If you want to go a step further, you can replace the thermostatic heads with electronic ones that can be programmed to a time schedule (for example a lower temperature at night or during the day when you're at work) – the principle of the benefit for the condensing boiler is the same as with a classic mechanical head, just with more control comfort. You can read about setting up the whole system, including the weather-compensation curve, in the article Thermostat and weather-compensated boiler control.
Low-temperature radiators as a compromise between a classic radiator and underfloor heating
If you're attracted by the efficiency of underfloor heating, but a complete renovation involving milling out floors or raising the floor level isn't realistic for your house (for example because of door heights, stairs, or simply budget), there's also a middle path – so-called low-temperature radiators. These are radiators specially designed and sized to deliver the required output even at a lower heating water temperature, typically around 45–55 °C, i.e. much closer to the mode in which the boiler condenses the most.
This is usually achieved with a larger heat-exchange surface (more panels/fins, or a built-in fan that increases heat transfer through air movement even at a lower unit temperature) or simply a larger radiator size than the usual standard. As a result, you get a combination of benefits – a faster response to a temperature change than with underfloor heating (a radiator heats up and cools down faster than a massive floor structure), while at the same time working better with a condensing boiler than classic high-temperature radiators.
A low-temperature solution is worth considering especially during a larger heating renovation, when you're replacing the radiators anyway, or for an extension or new build, where nothing is fixed yet. In apartments and houses where it's enough just to supplement or slightly oversize the existing system, ordinary panel radiators in a somewhat larger size, as mentioned above, are usually sufficient too.
Summary – is a condensing boiler worth it with radiators?
Yes, definitely yes – even in an older house with older radiators, a condensing boiler saves a noticeable amount of gas compared to a classic atmospheric boiler, because for a large part of the heating season (outside the hardest frosts) it runs in condensing, i.e. highly economical, mode. The difference compared to underfloor heating is that with radiators, condensation doesn't happen continuously, but only during milder days and transitional periods – which is nonetheless most of the heating season.
If you want to increase efficiency even further, three steps make the most sense: (1) have the boiler sized with an adequate output reserve so it easily covers even frosty days, (2) supplement or gradually replace undersized radiators with larger or low-temperature ones, and (3) fit good-quality thermostatic valves on every radiator, so room-by-room control lets the system run at the lowest possible water temperature. Combining these steps can bring the efficiency of a radiator system close to what we know from underfloor heating.
Approximate water temperatures by type of heating unit.
The two approaches discussed in the text above.
How comfort in individual rooms can be fine-tuned.
Frequently Asked Questions
- Is a condensing boiler worth it if I only have old radiators at home and don't plan to replace them? Yes. Even without replacing the radiators, you'll save compared to an old atmospheric boiler, because the boiler will condense at least during milder days of the heating season. The savings will be somewhat lower than if you had new or low-temperature radiators, but still significant.
- How do I know if my radiators are undersized for a condensing boiler? A typical sign is that even with a visibly warm radiator, the room isn't warm enough on frosty days, or that the control has to permanently keep a high water temperature to reach the desired room temperature. In that case it helps to either raise the boiler's temperature curve (at the expense of some of the savings during frost), or over time enlarge/replace the radiators.
- Do I have to replace all my radiators at once when switching to a condensing boiler? No. You can install the boiler right away and gradually supplement or replace the radiators as needed and budget allows, for example as part of further renovation steps. The system will work fine with the original radiators too, just with a somewhat lower degree of condensation during the coldest days.
- Are low-temperature radiators a necessity with a condensing boiler? No, they're more of an optional upgrade. Ordinary panel radiators in a reasonable, or slightly oversized, size work well with a condensing boiler. Low-temperature radiators are an advantage especially during a larger renovation or new build, where you want efficiency as close as possible to underfloor heating.
- Do thermostatic heads actually help save money, or is it just about comfort? They actually help save money, in two ways – they prevent unnecessary overheating of individual rooms and at the same time reduce the average water temperature needed in the system, which lets the boiler condense for longer and more intensively. Investing in good-quality heads on all radiators therefore usually pays for itself fairly quickly.
- What's the difference in savings between a house with underfloor heating and a house with radiators? The exact figure depends on the specific house, but in general a house with underfloor heating achieves a somewhat higher average annual saving, because the boiler condenses almost all year round. A house with radiators still saves significantly compared to an old boiler, it's just that these savings are concentrated mainly in the milder periods of the year.