How to Set Up and Control a Low-Temperature Boiler
How to Set Up and Control a Low-Temperature Boiler
Low-temperature (classic) gas boilers are among the most widespread heat sources in older Slovak households. Unlike modern condensing boilers, they don't use the condensing heat of the flue gases, but their operation can still be relatively easily and cheaply regulated so that they heat exactly as much as is actually needed. A correctly set heating water temperature combined with a suitable type of control can reduce gas consumption without any intervention in the boiler itself – you just need to add or reconfigure the controls. In this article we explain the difference between a simple room thermostat and weather-compensated control, what temperatures are actually recommended, and what accessories are worth buying even for an older boiler.
Classic Room Thermostat vs. Weather-Compensated Control
In practice, most low-temperature boilers are regulated in the simplest possible way – with a room thermostat. It measures the air temperature in the room where it's installed (typically the living room or hallway), and when the temperature drops below the set value, it signals the boiler to switch on the burner. When the desired temperature is reached, the boiler switches off. This is the ON/OFF principle, which is cheap, reliable, and most older boilers have worked with it without problems since they were manufactured.
Some models of low-temperature boilers (though not all) also support what is known as weather-compensated control. With this, an outdoor temperature sensor measures the current outdoor temperature, and the boiler's control electronics continuously adjust the heating water temperature according to a pre-set curve – the colder it is outside, the more the boiler "heats up" the water going to the radiators, and vice versa. The result is more even heating without unnecessary temperature fluctuations in the room and less burner cycling (switching on and off).
Important note: while for a condensing boiler, lowering the heating water temperature brings a significant increase in efficiency (condensation of the flue gases occurs at a return water temperature of approximately below 55°C, and the lower it is, the more extra heat is "extracted" from the flue gases), this effect is absent or much weaker for a low-temperature boiler. So a low-temperature boiler won't gain the same efficiency bonus just by setting the water as low as possible – on the contrary, too low a heating water temperature can mean the radiators can't adequately heat the room during heavy frost. The benefit of control for a low-temperature boiler therefore lies more in not heating more than necessary, rather than chasing the lowest possible water temperature at any cost.
Before deciding on weather-compensated control, you need to check whether the specific boiler model supports it at all (this usually means control electronics with what's known as an eBus, OpenTherm, or other digital communication bus). If the boiler doesn't have such a bus, a simple room thermostat remains the option – which isn't a problem, you just need to expect the room temperature to fluctuate slightly around the set value.
Recommended Temperatures
Exact values vary depending on the specific house, radiator sizing, and the intensity of frost, but in practice the following approximate ranges have proven effective:
| Parameter | Recommended value |
|---|---|
| Heating water temperature (radiators) | approx. 55–70°C depending on frost and radiator sizing |
| Room temperature in the living room (daytime) | approx. 20–22°C |
| Room temperature in the bedroom / at night | lower, setback value (typically 2–4°C less) |
How to approach this in practice: during heavier frost, you need to set a higher heating water temperature (closer to 70°C) so the radiators can keep up with covering the house's heat losses. During milder weather, a lower water temperature (closer to 55°C) is sufficient – this saves gas even without touching the room thermostat. If the boiler has the option to set a so-called weather compensation curve, this adjustment is taken over automatically. Without it, you need to change the water temperature manually on the boiler according to the current weather, or set it to a medium value that suits most ordinary days of the season.
We recommend not raising the room temperature unnecessarily above 22°C – every extra degree means noticeably higher gas consumption. In rooms that aren't normally used (for example a bedroom during the day), it's worth maintaining a lower temperature using setback or a thermostatic radiator valve.
Control Accessories
Even an older low-temperature boiler can be fitted with accessories that improve the accuracy and convenience of control. The choice depends on whether the boiler supports weather compensation (needs an outdoor sensor) or can manage with a quality room/weekly thermostat.
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Protherm – Outdoor Temperature Sensor (wired) for boilers with an eBus bus – allows you to additionally activate weather-compensated control on boilers with an eBus communication bus. Thanks to it, the boiler automatically adjusts the heating water temperature according to the current outdoor temperature, reducing the number of unnecessary burner on/off cycles. Suitable only for models that support weather compensation – compatibility needs to be verified for the specific boiler type. Price from €33.54. |
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Vaillant VRT 50 – a room, or optionally weather-compensating, thermostat with a weekly programme. It allows you to set different temperatures for individual days and hours (for example setback overnight or during the working week when nobody is home), so you don't have to manually switch the temperature on the boiler. Price from €71.64. |
Before buying any accessories, we recommend checking compatibility with the specific boiler model and manufacturer – especially for an outdoor sensor, support for the relevant communication bus is required (e.g. eBus for Protherm/Vaillant boilers). For a standard room thermostat with a weekly programme, compatibility is usually broader, since most boilers can be controlled by it via a simple switching contact.
A simple saving even with an older boiler.
Setback and Time Programme – A Simple Saving Even With an Older Boiler
One of the simplest and yet most effective savings with an older low-temperature boiler doesn't relate directly to the boiler itself, but to when and to what temperature you actually heat. If a flat or house is normally heated to the same temperature 24 hours a day regardless of whether anyone is home, or whether it's night or day, this is almost always unnecessary waste.
This is exactly what a thermostat's weekly programme is for – it lets you set in advance when the house should be warmer (for example in the morning before leaving for work and in the evening after returning) and when a lower, setback temperature is sufficient (overnight, during the day when the flat is empty, over the weekend if the family is away). This means nobody has to worry about manually switching the temperature on the boiler – the thermostat does it automatically according to the programmed schedule.
A typical setup example in an ordinary household:
- 6:00 – 8:00 – comfort temperature (e.g. 21 – 22°C), the family is getting ready for work/school
- 8:00 – 16:00 – setback (e.g. 18 – 19°C), the flat is empty
- 16:00 – 22:00 – comfort temperature, the family is home
- 22:00 – 6:00 – night setback (e.g. 17 – 18°C), better sleep and lower consumption
Setback shouldn't be understood as completely turning off the heating – it's more of a mild reduction in temperature by a few degrees. Thanks to the thermal inertia of the masonry and radiators, the house doesn't cool down all at once, and after the setback period ends, the temperature quickly returns to the comfort value, since the gap that needs reheating is only small.
This form of control is one of the cheapest investments you can make even with a boiler that's many years old – it requires no intervention in the boiler itself or the gas installation, just replacing or supplementing the room thermostat with a programmable model.
Approximate values from the table earlier in the article.
Two approaches to control discussed in the article.
Accessories that can be added even to an older boiler.
Frequently Asked Questions
Can a low-temperature boiler be controlled with weather compensation?
Only some models – it depends on whether the boiler has control electronics supporting a communication bus (e.g. eBus) and whether the manufacturer offers an outdoor temperature sensor for that type. Compatibility needs to be checked for the specific boiler model before buying a sensor.
What temperature should I set on an old gas boiler?
As a guide, a heating water temperature of 55 – 70°C depending on frost and radiator sizing, a room temperature of around 20 – 22°C during the day, and a lower (setback) temperature at night or when nobody is home.
Is it worth buying a programmable thermostat for an old boiler?
Yes, it's one of the cheapest and quickest-to-pay-back upgrades – it allows automatic setback overnight and when the flat is empty, without having to manually switch the temperature, and it also works with an ordinary older boiler without a weather-compensation bus.
Does a low heating water temperature bring the same savings as with a condensing boiler?
No. At a low return water temperature (below approximately 55°C), a condensing boiler also uses the condensing heat of the flue gases, which significantly increases its efficiency. A low-temperature boiler doesn't have this effect, or it's only minimal, so the benefit of lowering the water temperature is much smaller – it's more important not to heat unnecessarily beyond actual need.
Is a simple room thermostat enough to control an old boiler?
Yes, a room thermostat working on the on/off principle is a common and reliable solution for most low-temperature boilers. For greater comfort and savings, it's worth choosing a model with a weekly programme.
How much does frost affect the heating water temperature setting?
Significantly – during heavy frost, a higher water temperature is needed (closer to 70°C) so the radiators can cover the house's heat losses, while in milder weather a lower temperature (closer to 55°C) is sufficient, saving gas.