DN 25 vs DN 32 - how to choose the correct size of the thermostatic unit
Why even address the size of the thermostatic mixing unit
The thermostatic mixing unit with a pump and check valve is in solid fuel boiler circuits a component through which practically the entire volume flow of return water passes during boiler startup. Its task is to keep the temperature of the water entering the boiler above a minimum threshold (typically 55 °C, 61 °C, 72 °C or 80 °C depending on the type of boiler - this choice is discussed in a separate article on selecting the appropriate temperature of the thermostatic mixing unit), thus protecting the boiler's heat exchanger from low-temperature corrosion and soot buildup. Simplified: the unit works like a three-way thermostatic valve that, at low return water temperature, bypasses hot water from the boiler circuit directly back to the boiler (bypass), and only after the system has warmed up does it gradually mix cooler return water from the heating circuit.
When selecting this unit, technicians commonly focus on the correct opening temperature, as this is the parameter that directly affects boiler protection. The size (DN) is often underestimated or selected "according to the pipe already installed," which is, however, a mistake that manifests itself either immediately during system startup or later at full boiler capacity in freezing weather. DN 25 and DN 32 are the most commonly used sizes in the segment of small and medium solid fuel boilers, and the difference between them is not cosmetic - it is a matter of whether the unit can transfer the required flow without unnecessary pressure losses and without limiting the pump's performance.
What exactly does DN mean and why not confuse it with the thread
DN (Diametre Nominal, nominal diameter) is a dimensionless number that approximately corresponds to the internal diameter of the pipe in millimeters, but it is not an exact physical dimension - it is a normalized value according to EN 1333. DN 25 corresponds approximately to a pipe with a 1" thread, DN 32 corresponds to a 5/4" (1 1/4") thread. A common mistake in practice is that the installer looks at what thread the boiler connection has and automatically assumes that the thermostatic mixing unit must have an identical thread - without calculating the actual required flow.
It is important to realize that the thermostatic mixing unit does not necessarily have to have the same size as the boiler's connections. The boiler may have a 5/4" port, but at a lower output (e.g., 15-20 kW), a DN 25 unit may easily be sufficient, because the limiting factor is not the diameter of the boiler port, but the actual mass flow of water required to transfer the thermal power.
Visual comparison of cross-sections DN 25 and DN 32
The cross-sectional area increases with the square of the diameter, so the difference between DN 25 and DN 32 is not just an additional 7 mm - the actual flow area of DN 32 is approximately 64% larger than that of DN 25. This directly affects the speed of water flow and the pressure loss of the fitting at a given flow rate.
How flow and boiler output determine the required size
The basic formula we use in dimensioning is:
Q = m × c × ΔT
where Q is the thermal power (kW), m is the mass flow (kg/s), c is the specific heat capacity of water (approximately 4.18 kJ/kg·K), and ΔT is the temperature difference between the inlet and outlet of the boiler (usually 15-20 °C for solid fuel boilers).
Converted into a practical form for a temperature difference of 15 K, the approximate relationship is: the flow in liters per hour is equal to the power in kW multiplied by approximately 57 (at a 15 K difference) or 43 (at a 20 K difference). For a quick orientation in the field, we use simplified tabular values:
| Boiler output | Flow at ΔT 15 K | Flow at ΔT 20 K | Recommended size of TRJ |
|---|---|---|---|
| up to 20 kW | ~1150 l/h | ~860 l/h | DN 25 |
| 20-25 kW | ~1150-1430 l/h | ~860-1075 l/h | DN 25 (borderline) |
| 25-40 kW | ~1430-2290 l/h | ~1075-1720 l/h | DN 32 |
| 40-60 kW | ~2290-3440 l/h | ~1720-2580 l/h | DN 32 (borderline, assess the pump) |
These values are approximate and are based on experience with typical wood, coal, and pellet boilers in the segment of family homes and small businesses. At outputs above 40-50 kW, it is always necessary to assess the specific characteristics of the pump integrated in the unit and the actual pressure loss of the entire boiler circuit, or consider a parallel solution or a larger size outside the standard DN 25/32 range.
Why "bigger is not always better"
It might seem that the simplest solution is to always use DN 32 to avoid any flow limitations. In practice, it is not that simple, and for several reasons:
- At low outputs (e.g., 15 kW boiler) with a DN 32 unit, the flow speed in the fitting may be so low that the sensitivity of the thermostatic element to temperature changes deteriorates - the valve reacts more slowly and less accurately.
- The built-in circulation pump in the thermostatic mixing unit has its own characteristic (dependence of head on flow) and at an excessively large size in a small circuit, it may operate in an unsuitable part of the characteristic where efficiency is low and electricity consumption is disproportionately high compared to the actual need.
- A larger fitting = higher costs and higher space requirements in the boiler room, without a real benefit.
On the other hand, an overly small size at high output causes the opposite extreme, which is more serious in terms of boiler safety - we will address it in more detail in the next section.
Consequences of underestimating the size (DN 25 where DN 32 is needed)
This is the most common mistake we encounter in complaints and additional modifications in boiler rooms. A real-life scenario: the customer has a solid fuel boiler with an output of 30 kW, the original piping in the boiler room is 1" (so it seems logical to use DN 25), the installer installs a thermostatic mixing unit DN 25; 55 °C and the system works seemingly without problems at partial load. The problem appears only at full boiler load in freezing weather, when maximum flow is needed:
- The pressure loss on the DN 25 fitting increases quadratically at a flow approaching the upper limit of its capacity - doubling the flow increases the pressure loss approximately fourfold.
- The integrated pump must overcome higher resistance, operates in a part of the characteristic with lower flow than needed, and the actual flow through the boiler is lower than the designer calculated.
- Lower flow at the same thermal output means a higher temperature difference at the boiler - the temperature at the boiler outlet rises, sometimes approaching the limits of the boiler's safety thermostat, leading to cycling of the burner/fan, reduced output, or even boiler shutdown by the safety thermostat.
- In extreme cases, local overheating in the boiler heat exchanger occurs, which accelerates limescale deposition and thermal stress on the heat exchanger material.
Simplified: an underestimated size does not cause an immediate failure, but systematically limits the boiler's performance reserve exactly when you need it most - during the coldest weather.
Graph: pressure loss vs flow for DN 25 and DN 32
The solid line represents DN 25, the dashed line DN 32. At low flows, the difference is minimal, but with increasing flow, the pressure loss on DN 25 rises significantly faster - this is precisely the phenomenon that causes problems at full boiler capacity.
Consequences of overdimensioning the nominal diameter (DN 32 where DN 25 would be sufficient)
The opposite case is less dramatic, but equally uneconomical. In the case of a small boiler (e.g., 12-18 kW) with an oversized DN 32 unit, we encounter the following phenomena:
- Low flow velocity in the valve (below approx. 0.2 m/s) causes slower and less accurate response of the thermostatic element to changes in return water temperature - the unit "lazily" switches between bypass and mixing, and the temperature at the boiler inlet fluctuates in a wider range than would be ideal.
- At very low flows, air or impurities may settle locally in the valve, as the flow is not sufficiently turbulent to carry them away.
- The investment in a larger and more expensive valve is not utilized - the pump with higher power consumption operates unnecessarily outside the optimal point.
In practice, the goal is to choose the nominal diameter so that at the boiler's rated power, the valve operates within a reasonable flow velocity range, approximately 0.3 to 1.0 m/s, ideally around 0.5-0.7 m/s.
Practical procedure for selecting the nominal diameter on site
When solving a specific order, we proceed according to the following algorithm:
Step 1 - Determine the boiler's rated power
The data can be found in the boiler's technical documentation or on the production label. In the case of older boilers without documentation, we estimate based on the heated area of the building and the type of fuel, or possibly based on the size of the heating surface of the heat exchanger.
Step 2 - Determine the expected temperature difference
In the case of solid fuel boilers, a temperature difference of 15-20 °C between the outlet and inlet of the boiler is commonly designed. A lower difference (e.g., 10 °C) requires a higher flow rate at the same power, which may shift the choice towards DN 32 even at a lower boiler power.
Step 3 - Calculate or estimate the required flow rate
We use the formula Q = m × c × ΔT, converted to volumetric flow. For a 25 kW boiler and a difference of 15 K, the flow rate is approximately 1430 l/h, which is already at the upper limit of the recommended range for DN 25 and in many cases it makes sense to choose DN 32, especially if there is a power reserve or if future connection to a storage tank is planned.
Step 4 - Compare with the pump characteristics in the unit
Thermostatic units have an integrated circulation pump with a certain maximum head and maximum flow rate. It is necessary to verify that at the required flow rate, the pump still provides sufficient head to overcome the resistance of the boiler circuit (including the boiler itself, piping and valves).
Step 5 - Take into account future reserves and planned system modifications
If connecting a storage tank in the future or replacing the boiler with a more powerful one is planned, it is reasonable to choose a larger nominal diameter now, as replacing the thermostatic unit later means another intervention into the piping, draining the system and additional labor costs.
Case studies from practice
Case 1 - family house, wood-burning condensing boiler 25 kW
A house with a floor area of 150 m², a condensing boiler with a rated power of 25 kW, connected to a 1000 l storage tank. In the design, we calculated with a temperature difference of 15 K, which at full power means a flow rate of around 1430 l/h. The original installer's design calculated with a DN 25 unit with an opening temperature of 61 °C, but after recalculation, we recommended DN 32, because at full boiler power and planned later addition of a solar circuit, DN 25 would be at the edge of its capacity and with simultaneous use of the storage tank, it is desirable to have a flow reserve for rapid heating of the tank during heating up.
Case 2 - cottage, coal boiler 15 kW
A smaller cottage with a direct heating circuit without accumulation, a coal boiler 15 kW. Here the choice is clear - thermostatic unit DN 25; 55 °C is fully sufficient, the flow rate at full power is around 860 l/h, which is comfortably in the middle of the recommended range for DN 25 and the flow velocity remains in the optimal range for sensitive response of the thermostat.
Case 3 - incorrect complaint that turned out to be a sizing error
The customer complained that a 32 kW pellet boiler was constantly tripping on the safety thermostat in the cold. During the inspection, it turned out that a DN 25 unit with an opening temperature of 72 °C had been installed (both the temperature and the nominal diameter were incorrectly chosen - a pellet boiler with an automatic burner is more sensitive to exceeding the maximum outlet temperature than a wood boiler). After replacing it with DN 32 and re-evaluating the opening temperature to 61 °C (according to the boiler manufacturer's recommendation), the problem was completely resolved - the boiler maintained a stable temperature at full power without cycling.
Connection between the nominal diameter and other components of the circuit
Choosing the correct DN of the thermostatic unit is not an isolated decision - it is connected to a whole range of other components in the boiler circuit:
- System air venting: At higher flow rates in a DN 32 circuit, it is important to have functional air venting at the highest points of the system, as higher flow can carry air bubbles further into the system, where they then accumulate. We recommend a combination of automatic air vent valve 1/2" at the highest points and possibly manual air vent valve at locations where controlled manual operation is needed. A detailed guide on selection can be found in the article How to choose an air vent valve for a heating system.
- Pipe cross-section in the entire boiler circuit: It makes no sense to install a DN 32 unit on piping that is DN 25 or smaller in other sections of the circuit - the valve would be limited by the surrounding piping and its benefit would not be utilized.
- Pump power and characteristics: As we have mentioned, at DN 32, the pump integrated in the unit must be able to provide sufficient head at the higher flow rate.
- Setting of the opening temperature: DN and temperature are chosen together, not independently - high flow at an incorrectly low opening temperature can cause the boiler to take a long time to reach a safe temperature, while low flow at a high opening temperature can cause overheating.
Installation and practical recommendations during installation
During the installation of the thermostatic unit (detailed procedure can be found in the article Installation of a thermostatic unit with a pump and check valve), several principles apply that are directly related to its proper function in relation to the selected nominal diameter:
- The unit is always mounted in the direction of flow according to the arrow on the body - with DN 32 units the emphasis on the correct direction is even greater, because higher flow in the reverse direction can cause the mixing mechanism to become completely non-functional.
- It is recommended to install a dirt filter before the unit (on the cold/return water inlet), especially in older systems where deposits from the original piping are a risk - with DN 32 units the space between the valve seat and the body is relatively larger, but protection against coarse dirt is still recommended.
- The electrical connection of the pump must correspond to the power of the impeller - with DN 32 units the pumps are usually slightly higher in power, so the capacity of the fuse in the boiler room distribution board should be verified.
- The check valve integrated in the unit prevents free circulation when the pump is turned off - with the larger DN 32 size it is important to verify that the check valve actually seats and that reverse flow does not occur, which could in certain circumstances cool down the boiler even when the pump is off.
Maintenance and long-term functionality
Regardless of the selected size, it is true that the thermostatic unit requires regular inspection, especially at the beginning and end of the heating season. We recommend checking the free operation of the pump (unblocking after the summer shutdown), the functionality of the check valve and the tightness of the connections. With DN 32 units with higher flow, the seals can wear out a bit faster due to frequent temperature cycling, so we recommend checking the tightness at least once a year. General maintenance guidelines for control and safety valves can be found in the separate article "Maintenance and inspection of control and safety valves in heating."
Summary - quick decision table
| Parameter | DN 25 | DN 32 |
|---|---|---|
| Typical boiler power | up to approx. 25 kW | 25-50 kW |
| Flow at ΔT 15 K | up to approx. 1400 l/h | 1400-2900 l/h |
| Thread | 1" | 5/4" |
| Risk of underestimation | - | flow limitation, boiler overheating at full power |
| Risk of overestimation | slower thermostat response, low flow speed | - |
| Suitable for planned accumulation | only at low power | yes, recommended |
Flow speed in the valve - why it is important for thermostat sensitivity
This diagram shows why it is not correct to assess the suitability of the size only according to the tabular power limit, but also according to the speed range in which the valve will actually operate at a specific boiler. The same DN 25 unit at a 15 kW boiler moves in the middle of the optimal range, while at a 25 kW boiler it is already approaching the upper limit, where noise, pressure loss and mechanical stress on the valve seat increase with every switching between bypass and mixing. This very reserve (or its absence) is the reason why at borderline power levels we recommend leaning towards a larger size, even if the tabular value would still allow DN 25.
Common questions about choosing DN 25 vs DN 32
Can I use a DN 32 unit on a boiler with a 1" connection without reducing?
Yes, many thermostatic units DN 32 have inlet/outlet nozzles with a reduction or are supplied with transition fittings. It is important that the reduction is not too long and abrupt, as this would cause local turbulence and increased pressure loss. In practice, a reduction on a short section usually does not cause a problem.
How do I know that I have a unit with the wrong size if it is already installed?
A typical sign of an underestimated size is that the boiler often shuts off on the safety thermostat at full power (especially in cold weather), the temperature at the boiler outlet fluctuates well above the set value, or the pump "rumbles" at high speeds without corresponding effect. A sign of an overestimated size is, on the contrary, slow and unstable temperature rise when heating up and larger temperature fluctuations at low power.
Is it reasonable to always choose a larger size "just to be safe"?
Not entirely - with a significantly oversized valve, the sensitivity of regulation at low flows deteriorates and the investment is not used. It is better to do a calculation according to the boiler power and temperature drop, or choose DN 32 only if the boiler power is at the limit or if you plan to expand the system in the future (for example, a storage tank).
Does the size of the unit also affect the choice of opening temperature?
Not directly, the opening temperature (55, 61, 72 or 80 °C) is chosen according to the type of boiler and the manufacturer's recommendation, independently of the size. However, both parameters must be chosen together so that the resulting combination corresponds to the actual operating mode of the boiler - we go into detail on this in the article on choosing the right temperature for a thermostatic unit.
What happens if I don't replace the thermostatic unit when replacing an old boiler with a more powerful one?
If the new boiler is more powerful and the original unit was DN 25 designed just on the edge of the previous boiler, the flow limitation will be exactly as described in the section on underestimating the size - therefore, when replacing the boiler, it is always necessary to recalculate the capacity of the thermostatic unit, not just its opening temperature.
Is the choice of DN related to problems with system air venting?
Indirectly yes - the higher flow with DN 32 can carry micro air bubbles further into the system, where they then accumulate at higher points. If you also have a problem with a leaky or non-functional air vent, we recommend looking at the article "Why the air vent leaks or doesn't work," where we discuss the most common causes.
Conclusion
The choice between a DN 25 and DN 32 thermostatic unit is not a formal question of thread size, but a direct technical calculation based on the boiler's nominal output, the expected temperature drop and the characteristics of the integrated pump. An underestimated nominal size limits the flow exactly at full boiler output, when the protection of the heat exchanger is most important, and can lead to overheating and boiler cycling. An overestimated nominal size unnecessarily reduces the sensitivity of the regulation at low flows and represents an uneconomical investment. We recommend performing a simple calculation according to the table in this article for each project, taking into account planned future system modifications (especially the buffer tank), and in case of doubts, choosing a nominal size with a small technical reserve, not unnecessarily oversized. When selecting a specific model, we also recommend reviewing the parameters of the thermostatic unit DN 25 with an opening temperature of 61 °C and comparing it with the needs of your specific boiler, ideally in combination with information from the control and safety valves category, where you will also find related components for complete protection of the boiler circuit.
Do you have a question on this topic?
Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.