ᐅ Fireplace and Underfloor Heating – Is Forced Air or Heat Storage More Efficient?
Created on: 21 Nov 2021 17:36
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erazorlll
Hello dear forum members,
We have planned a masonry fireplace as a divider between the living room and dining room in our new build.
=> Link to the planning thread
(the planned fireplace extends further into the room, unlike the current layout)
The primary goal is purely aesthetic (fire) and creating a cozy atmosphere. Heating is secondary since we have underfloor heating. Additionally, there is a controlled ventilation system. I am aware of the cost versus benefit issue and that this is purely a luxury feature.
The room where the stove will be installed is approximately 50m² (540 sq ft) with large windows.
I have repeatedly read about overheating problems in rooms when the stove is oversized or when there is simultaneous underfloor heating. Opening a window to compensate doesn’t really make sense.
I have now received two offers from stove builders, but they gave me different advice and would construct the fireplace differently.
Option 1: 12 kW as a convective fireplace (air intake, passing by the stove to be heated, then released back into the room) plus firebrick (refractory brick) storage.
Option 2: 11 kW as a storage fireplace with firebricks, without any convective air circulation.
Both fireplaces seem quite powerful at 11 and 12 kW. I raised the issue of overheating with both providers.
Option 1 says that with the air circulation, the room can heat up quickly when the outside temperature drops and the underfloor heating can’t keep up fast enough. The storage bricks would then release heat after the fire goes out. Without the convective air, the fireplace would be ineffective because it would take too long to warm up. Overheating is not a problem.
Option 2 says the underfloor heating is sufficient to heat the room, so a convective system would heat the room too quickly and cause overheating. The storage fireplace would deliver heat slowly and evenly, helping to prevent overheating. The glass door of the fireplace provides enough heat for short-term needs, so no additional convective air is required.
Now I wonder: who is correct, or which solution is more reasonable?
Thank you for your assessment and help.
We have planned a masonry fireplace as a divider between the living room and dining room in our new build.
=> Link to the planning thread
(the planned fireplace extends further into the room, unlike the current layout)
The primary goal is purely aesthetic (fire) and creating a cozy atmosphere. Heating is secondary since we have underfloor heating. Additionally, there is a controlled ventilation system. I am aware of the cost versus benefit issue and that this is purely a luxury feature.
The room where the stove will be installed is approximately 50m² (540 sq ft) with large windows.
I have repeatedly read about overheating problems in rooms when the stove is oversized or when there is simultaneous underfloor heating. Opening a window to compensate doesn’t really make sense.
I have now received two offers from stove builders, but they gave me different advice and would construct the fireplace differently.
Option 1: 12 kW as a convective fireplace (air intake, passing by the stove to be heated, then released back into the room) plus firebrick (refractory brick) storage.
Option 2: 11 kW as a storage fireplace with firebricks, without any convective air circulation.
Both fireplaces seem quite powerful at 11 and 12 kW. I raised the issue of overheating with both providers.
Option 1 says that with the air circulation, the room can heat up quickly when the outside temperature drops and the underfloor heating can’t keep up fast enough. The storage bricks would then release heat after the fire goes out. Without the convective air, the fireplace would be ineffective because it would take too long to warm up. Overheating is not a problem.
Option 2 says the underfloor heating is sufficient to heat the room, so a convective system would heat the room too quickly and cause overheating. The storage fireplace would deliver heat slowly and evenly, helping to prevent overheating. The glass door of the fireplace provides enough heat for short-term needs, so no additional convective air is required.
Now I wonder: who is correct, or which solution is more reasonable?
Thank you for your assessment and help.
erazorlll schrieb:
What kind of wood stove do you have? Or rather, which of the types I described? I said: I have no clue about this forced-air variant.
We now have a certified Olsberg Palena in our new build (required for controlled mechanical ventilation, room-air independent). I correct myself: not 7 kW, but 5 kW. Deliberately without storage stones, because we had storage stones in the old house and, considering our daily routine—leaving in the morning for work, etc.—we didn’t find any benefit or purpose in them. Everyone has to decide for themselves whether they want to sit by the stove in the morning. We don’t.
We use it as a comfort stove with the advantage that the underfloor heating doesn’t have to work as hard when the temperature rises. The room itself stays warmer for hours afterward than if only the underfloor heating was running.
My advice is to avoid choosing a stove with too high a kW rating for a closed room.
A recirculating chimney basically refers to anything that is not a masonry heater.
We have a rated output of 7.5 kW for a 70 m² (750 sq ft) open living space, using a recirculating chimney. When firing it with the amount of fuel recommended by the stove fitter to achieve a good burn (no blackened glass), we also get about a 3 to 4-degree Celsius (5 to 7°F) increase, which I find quite warm (22°C to 26°C / 72°F to 79°F). If I imagine 11 kW for 50 m² (540 sq ft), wow...
We have a rated output of 7.5 kW for a 70 m² (750 sq ft) open living space, using a recirculating chimney. When firing it with the amount of fuel recommended by the stove fitter to achieve a good burn (no blackened glass), we also get about a 3 to 4-degree Celsius (5 to 7°F) increase, which I find quite warm (22°C to 26°C / 72°F to 79°F). If I imagine 11 kW for 50 m² (540 sq ft), wow...
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hampshire22 Nov 2021 12:48RotorMotor schrieb:
11 kW is initially 11 kW, just to have a comparison—what heating capacity does your heating system have? That is correct but somewhat misleading. The heating output of a wood stove is standardized by measuring it over a set period (usually one or two hours, I don’t recall exactly). The way the stove distributes its heat affects the comfort in the room—a very rapid heat release causes a strong temperature spike. In well-insulated modern houses, this can lead to a feeling of overheating. The only solutions then are opening windows or turning on the air conditioning. The latter almost seems crazy.
The slower the heat release from the stove, the better. The better a house is insulated, the lower the required heat output to reach a comfortable temperature. In relation to wood stoves, this means either spreading the heat release over time (thermal storage) or directing the heat away from the room entirely (e.g., through the chimney).
Therefore, when sizing a stove, the stated heating capacity is less important than the heating curve and the minimum amount of fuel needed for a clean burn.
@erazorlll: Ask to see the heating curves for the offers (a flatter curve is better for you) and the minimum fuel amount for a clean burn. You don’t have to fill every stove to the maximum, and a smaller amount of wood still produces less heat but burns nicely.
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erazorlll22 Nov 2021 16:51Thank you for your replies. I will respond to them later this evening.
A quick and probably silly question:
When I see the specification "useful output 5-12 kW" from manufacturers (e.g., Hoxter UAK), does that refer to the minimum and maximum useful output depending on the wood load? So, less than 5 kW (or the corresponding wood amount of probably just over 1 kg (2.2 lbs)?) would not be possible in terms of combustion, and 12 kW would be at the maximum filling of the combustion chamber?
A quick and probably silly question:
When I see the specification "useful output 5-12 kW" from manufacturers (e.g., Hoxter UAK), does that refer to the minimum and maximum useful output depending on the wood load? So, less than 5 kW (or the corresponding wood amount of probably just over 1 kg (2.2 lbs)?) would not be possible in terms of combustion, and 12 kW would be at the maximum filling of the combustion chamber?
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hampshire22 Nov 2021 17:24erazorlll schrieb:
When I see the specification "useful output 5-12 kW" from manufacturers (e.g., Hoxter UAK), does this describe the minimum and maximum output depending on the amount of wood used? Yes, you can think of it like that, although converting kW to the actual amount of wood is not straightforward due to the heating curve-based standard.
A masonry heater with an efficiency of around 90% can be rated at 3.5 kW despite using 13 kg (29 lb) of wood. This is because most of the heat energy is stored in the thermal mass, and the heat output measurement is stopped after one hour, even though the combustion provides heat for up to 24 hours.
It is therefore better to know the minimum fuel load the stove can handle. For our masonry heater with an Ortner insert, this is specified (6.5–13 kg (14–29 lb)), but unfortunately, this information is not available for the Hase stoves owned by the guys (Sendai and Dheli, both without storage).
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erazorlll23 Nov 2021 11:12Thank you for your responses.
To summarize the (subjective) feedback:
@Snowy36 – masonry stove – comfortable warmth
@Benutzer200 – convection fireplace without storage – 16 kW for 75 m² (807 ft²) – about +3°C (5°F) (pleasant)
@ypg – fireplace without storage – 5 kW for 50 m² (538 ft²) – whole house heating / sometimes too warm (ventilation)
@HarvSpec – convection fireplace – 7.5 kW for 70 m² (753 ft²) – about +3-4°C (5-7°F) (felt too warm)
I am still unsure which option would be better for us.
From @hampshire, I understood that we should ask for the heating curves and minimum quantities for both offers. I will do that.
When sizing the stove, I have noticed that these fireplace inserts are quite similar. Variant 1 specifies a heat output range of 6.5 – 12.1 kW. Variant 2 states 5 – 12 kW.
I also did some additional research and found various heating curves for freestanding wood stoves with and without storage stones. Furthermore, I came across several articles about different types of stoves (unfortunately, I cannot link them here). At the moment, I tend to favor Variant 2 with a pure storage fireplace, since the heat release seems to be more even and “comfortable.”
However, I would appreciate further feedback from your side.
To summarize the (subjective) feedback:
@Snowy36 – masonry stove – comfortable warmth
@Benutzer200 – convection fireplace without storage – 16 kW for 75 m² (807 ft²) – about +3°C (5°F) (pleasant)
@ypg – fireplace without storage – 5 kW for 50 m² (538 ft²) – whole house heating / sometimes too warm (ventilation)
@HarvSpec – convection fireplace – 7.5 kW for 70 m² (753 ft²) – about +3-4°C (5-7°F) (felt too warm)
I am still unsure which option would be better for us.
From @hampshire, I understood that we should ask for the heating curves and minimum quantities for both offers. I will do that.
When sizing the stove, I have noticed that these fireplace inserts are quite similar. Variant 1 specifies a heat output range of 6.5 – 12.1 kW. Variant 2 states 5 – 12 kW.
I also did some additional research and found various heating curves for freestanding wood stoves with and without storage stones. Furthermore, I came across several articles about different types of stoves (unfortunately, I cannot link them here). At the moment, I tend to favor Variant 2 with a pure storage fireplace, since the heat release seems to be more even and “comfortable.”
However, I would appreciate further feedback from your side.
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