“I bought a wood-burning stove with a hot water heat exchanger, but my central heating system isn’t connected yet. Can I use it normally and connect the heat exchanger later?”
Or another situation:
“It’s not so cold outside anymore. I only want to heat the living room in the evenings, but not the whole house. Can I turn off the pump and turn off the radiators?”
Both questions seem similar, but they address two different problems. In both cases, one needs to understand exactly how a stove with a hot water heat exchanger works.
With conventional wood-burning stoves that have a hot water heat exchanger, it is not safe to heat with an empty heat exchanger or without ensuring that heat is being dissipated from the heating system.
Why must there be water in the heat exchanger?
Wood-burning stoves with hot water heat exchangers release some of the heat directly into the room and some into the water in the heating system.
The heat exchanger is located in a room where it is exposed to the high temperatures caused by wood combustion.
The water flowing through the heat exchanger transfers the absorbed heat to the radiators, the storage tank, or other parts of the heating system.
If there is no water in the heat exchanger, it lacks the normal cooling method.
Its metal parts can overheat, deform, or be permanently damaged.
Therefore, manufacturers such as ABX and Haas+Sohn explicitly state in their instructions that ovens with hot water heat exchangers cannot be operated without being connected to the heating system and filled with the prescribed heat transfer medium.
1. What happens if the heat exchanger is empty, but the stove otherwise works?
“After all, the fire is burning in the combustion chamber. What should happen if there is no water in the heat exchanger?”
That’s the problem.
The heat exchanger is designed to transfer a significant amount of heat to the water during operation. When empty, its temperature can considerably exceed the values intended for operation.
This can lead to deformations, weld damage, leaks, or other permanent damage.
Such a defect may not be apparent at first glance.
After subsequent filling and connection to the heating system, a water leak or other problems may occur.
Therefore, an empty heat exchanger is not a safe substitute for a conventional stove without a heat exchanger.
2. What happens if there is water in the heat exchanger, but I switch off the pump?
That’s not the right solution either.
If the heat exchanger is filled with water, but the water cannot circulate sufficiently, the heat may not be dissipated safely.
If burning continues, the water temperature can rise rapidly.
In extreme cases, this can lead to overheating, steam formation and a dangerous pressure increase in the heating system.
Some systems are designed for gravity circulation and therefore do not require an electric pump. However, this must be verified by a qualified professional for the specific system.
With a conventional convection system, the pump should not be arbitrarily switched off when a fire is burning in the oven.
3. Can I heat the living room only in summer or only in spring?
That’s a very common question.
Imagine you have a furnace with a total output of 12 kW, of which 8 kW is delivered to the water and 4 kW to the room.
You simply want to create a pleasant atmosphere in your living room.
The heat exchanger also transfers heat to the water during combustion. It’s not simply a matter of deciding that all of the stove’s heating output will be directed solely into the room this time.
Closing the radiators or switching off the circulation pump reduces or completely stops the necessary heat consumption.
One solution could be, for example, a properly constructed storage tank or another suitable method of heat transfer.
However, such a connection must be designed by a professional and must correspond to the performance of the oven as well as the manufacturer’s requirements.
4. Does a storage tank help?
Yes, in some cases that is possible.
The storage tank serves to store the heat that the oven is currently generating, but which the household does not need immediately.
For example, if you do not want to heat the radiators in the bedrooms, a suitably designed system can divert some of the heat to the storage tank.
However, several conditions apply.
The tank must have sufficient capacity, be able to absorb additional heat, and safe heat transfer from the heat exchanger must be ensured.
If the tank is already heated up or the necessary pump fails, it may not be able to dissipate any further heat.
The storage tank alone does not guarantee that you can use the oven at any time, regardless of whether you need to heat the house.
5. Are there any ovens where the heat exchanger can be removed?
Yes. And that is an important exception.
In some Haas+Sohn wood-burning stoves, the design allows the hot water heat exchanger to be removed and replaced by a blanking plate specified by the manufacturer.
Such ovens can also be used in hot air mode after appropriate conversion, provided that the manufacturer of the respective model allows this.
However, this is not about operating with an empty heat exchanger.
The heat exchanger must actually be replaced with a suitable replacement part, and the oven must be operated in the configuration approved by the manufacturer.
Furthermore, it cannot be assumed that all Haas+Sohn ovens or products from other brands have this option.
When you talk about such things