“I have a big fire in the stove, the living room is warm, but the radiators are barely lukewarm. How can that be? After all, I have a stove with a heat exchanger, which is precisely designed to heat the whole house!”
This can really annoy the owner of a stove with a hot water heat exchanger.
Wood burns in the firebox, the stove gives off heat to the room, but the radiators in the other rooms remain cold.
This does not automatically mean that the stove is defective. The problem could be due to the water circulation, the settings of the heating system, or simply that the water is not yet heated sufficiently.
Let us explain the most common causes.
1. I’m currently preheating the oven. Shouldn’t the radiators already be warm?
Not always.
After flooding, the hot water heat exchanger needs a certain amount of time to heat the water to the required temperature.
In some heating systems, the circulation pump only starts once the set temperature is reached. Until then, the water only circulates in a short circuit near the furnace, or the pump is not yet running.
One reason for this is to protect the heat exchanger from excessively cold return water.
So if you turn on the water and the radiators are cold for the first few minutes, it may not mean that there is a problem.
It is important that the heat transfer to the heating system starts properly after reaching the prescribed operating temperature.
2. What happens if the circulation pump doesn’t work?
This is one of the possible causes.
In forced circulation systems, the circulation pump ensures the flow of heating water between the furnace and the individual heating elements.
If the pump is not working properly, it can happen that the water in the heat exchanger heats up, but the heat does not reach where it belongs.
The cause could be, for example, a pump failure, a problem with the power supply or control system, or a blockage of the operation.
Caution: If the water in the heat exchanger quickly reaches a high temperature while the radiators remain cold, this can pose a dangerous situation.
In such a situation, do not add any additional fuel. Observe the manufacturer’s safety instructions.
Do not interfere with the electrical wiring or heating system yourself.
3. Can radiators not be filled with air?
Yes.
If air gets into the heating system, this can impede or interrupt the flow of water.
A typical symptom is a radiator that remains warm in some places and cold in others. Sometimes you can also hear a gurgling or bubbling sound.
Air pockets can occur, for example, after filling the system, after repairs, or as a result of another operational problem.
Radiators are usually equipped with vents, but venting must be done safely and in accordance with the system instructions.
If the device is hot, has unusually high pressure, or shows signs of overheating, do not attempt to bleed it.
Furthermore, repeated ventilation is not a normal condition and its cause should be determined by a specialist.
4. Could there be a problem with the radiator’s thermostatic valve?
Yes, and sometimes the solution is surprisingly simple.
The radiator can remain cold because its thermostatic valve limits the flow of heating water.
Once the desired room temperature is reached, the valve can be closed or almost closed.
Sometimes the valve can get stuck after a longer period of disuse.
However, if only one radiator is cold and the others are working properly, it makes sense to check its valve.
If none of the radiators get warm, the cause is probably somewhere else.
Important: With solid fuel stoves, safe heat dissipation must always be ensured. Therefore, it is not advisable to arbitrarily close all radiators or other heat outlets.
5. What is a mixing or thermostatic valve and why can it affect the heating system?
“I have a valve on my stove that the heating engineer installed. Could that be the problem?”
It can.
In systems with a hot water heat exchanger, valves are often used to maintain the correct temperature of the water flowing back to the stove.
If water that is too cold is returned to the heat exchanger for an extended period of time, the operating conditions can deteriorate and condensation and deposits can form.
Therefore, in some connections, the water is first heated in a shorter circuit and only then is the heat gradually released to the radiators.
For example, the manufacturer requires a minimum return water temperature of 60 °C for some Haas+Sohn ovens.
However, this does not mean that all ovens meet this prescribed value.
If the valve is incorrectly selected, connected, or defective, this can restrict heat transfer to the heating system.
Have it checked by a professional and replaced if necessary.
6. Could the pipes or filter be clogged?
Yes.
Dirt, sludge and corrosion products can accumulate in heating systems, obstructing the flow of water.
Some circuits therefore contain filters or dirt separators.
If the filter is heavily clogged, this can lead to insufficient flow.
Similar symptoms can also occur due to a faulty hydraulic connection or incorrect pump settings.
However, do not clean the filters if the pressure circuit needs to be accessible when the system is hot.
7. What if the stove is too weak to heat the whole house?
That’s also possible.
Imagine you have a stove with a total power output of 12 kW.
However, this does not mean that all 12 kW will be thrown into the water.
For example, if the manufacturer specifies the power output of the 4 kW for the room and the 8 kW for the water, the water component of the power output is primarily decisive for radiator heating.
If the house needs more heat than the heat exchanger can supply, the radiators may not be warm enough, even with a properly functioning system.
The result also depends on the heat loss of the house, the size of the radiators, the temperature gradient and the current outside temperature.
Therefore, when choosing an oven with a heat exchanger, it is important to assess the performance for the water and the performance for the room separately.
8. What if I have a storage tank?
In a system with an expansion tank, the behavior of the radiators can change.
The heat from the oven can first be transferred to the storage tank and then, depending on the connection and control system, to the individual heating circuits.
If the control is not set correctly or part of the system is not working, there may be enough heat in the oven, but the radiators may remain cold.
The problem could be related to the pump, the mixing valve, the temperature sensors, or the control settings, for example.
However, the exact cause cannot be determined without knowledge of the specific wiring.
9. How do I know that it’s no longer just cold radiators, but a dangerous situation?
This is the most important part of the whole problem.
If the radiators do not get warm, but the water temperature in the heat exchanger continues to rise, this may mean that the heat is not being dissipated properly.
The following symptoms are particularly alarming:
- The water temperature quickly rises to the limits set by the manufacturer.
- The pressure in the system is rising unusually sharply.
- Loud, unusual noises are coming from the pipes or the heat exchanger.
- Hot water or steam escapes from the fuse or other safety device.
- The stove or control unit is signaling overheating.
In this case, it is no longer a common problem in the area of heating comfort.
Do not add any more fuel, do not open the hot water circuit, and never attempt to quickly cool an overheated heat exchanger with cold water.
In case of danger from steam leaks, scalding or other accidents, leave the danger area and call the fire department at 150 or 112.
10. What can I safely check myself before calling a heating engineer?
If the oven shows no signs of overheating and the heating system is working under normal conditions, you can check some basic things without taking the appliance apart.
Check if all radiators are not heating or only some.
Check the normally accessible settings of the room thermostat and thermostat heads.
If you have a control display, you can read the displayed temperatures, pressures, and any error messages.
Also, pay attention to whether the problem only occurs when heating or persists even after prolonged operation.
This information can be very helpful to the heating installer in finding the cause of the problem.
However, do not attempt to change the operating parameters of the controller, disassemble the pump, or tamper with the safety elements without knowing the wiring.
What can you take away from this?
If a wood-burning stove with a hot water heat exchanger is burning, but the radiators are not getting warm, the fault may not lie with the stove itself.
The cause could be too low a water temperature during heating, a defective circulation pump, air inclusions, a faulty control system, or insufficient performance of the heat exchanger.
However, one thing is crucial: whether the heat from the heat exchanger is safely dissipated.
Cold radiators may represent a minor malfunction, but if the temperature or water pressure rises rapidly at the same time, this can lead to a dangerous situation.
Therefore, a simple rule applies:
Don’t judge a heat exchanger furnace solely by its combustion performance. What happens to the heat transferred to the water is also important.