ᐅ Combine an air-to-water heat pump with a wood-burning stove connected to the central heating system
Created on: 29 Mar 2020 14:13
G
GSGaucho
Hello everyone,
We are currently planning a single-family home built with solid construction to KFW55 standard.
Two full stories, partially basement, without basement about 230m² (2,475 sq ft) of living space for 5 people.
The location is southern Germany at 550m (1,804 ft) above sea level. The shell construction planning is fixed, and the shell and gable roof have already been contracted. Construction start is week 22/2020.
I have also already contracted a 23kWp photovoltaic system on the south/west roof of the house and the south/east roof of the garage.
Due to economic reasons, a battery storage system is currently not an option.
The heating load according to calculation is about 5.5 kW at -15°C (5°F) ambient temperature.
Now it’s time to plan the heating system:
Current status is:
Now I have the first offer for a Stiebel Eltron LWZ 8 cs Premium.
Am I correct to assume that under the above parameters the LWZ 5 cs would also be sufficient?
How can I best integrate the Hoxter stove into the heating system?
As a layperson, I currently see two options:
Option 1
An 800-liter (210-gallon) buffer tank only for heating operation, without domestic hot water.
Domestic hot water is generated by the air-to-water heat pump during daylight; the heat pump runs mainly during the day and stores heat in the screed. From 4 p.m. onward, the Hoxter stove is fired.
The heating circuit would then have to switch to the buffer tank when a certain temperature X is reached in the stove circuit or the heat pump buffer.
Option 2
The air-to-water heat pump always charges the buffer tank with a maximum flow temperature of about 40°C (104°F). If this temperature is exceeded by the stove operation, the heat pump switches off. Also, the heat pump would be programmed to operate only during daytime.
I understand that the combination of air-to-water heat pump with a buffer tank is suboptimal. But a stove without hydronic integration also makes no sense, as it would quickly overheat.
My current bidder is almost unreachable for technical evaluation at the moment, fully booked, so I have no real information about integrating the stove yet.
What do you suggest?
Which other air-to-water heat pump manufacturers would you consider for this configuration?
Thank you very much,
GSGaucho
We are currently planning a single-family home built with solid construction to KFW55 standard.
Two full stories, partially basement, without basement about 230m² (2,475 sq ft) of living space for 5 people.
The location is southern Germany at 550m (1,804 ft) above sea level. The shell construction planning is fixed, and the shell and gable roof have already been contracted. Construction start is week 22/2020.
I have also already contracted a 23kWp photovoltaic system on the south/west roof of the house and the south/east roof of the garage.
Due to economic reasons, a battery storage system is currently not an option.
The heating load according to calculation is about 5.5 kW at -15°C (5°F) ambient temperature.
Now it’s time to plan the heating system:
Current status is:
- Underfloor heating in all rooms except for the pantry and storage/technical room in the basement.
- Central ventilation system combined with an air-to-water heat pump. Air-to-water heat pump installed indoors in the basement.
- A hydronic wood-burning stove from Hoxter with firing from a separate room, i.e., no wood/dirt in the living room. I still have 30rm (cords) of beech wood stored free of charge.
- Due to the high capacity of the photovoltaic system and the stove, I see no sense in a trench collector.
- An 800-liter (210-gallon) buffer tank can be placed almost directly under the Hoxter stove in the basement. The distance to the air-to-water heat pump is about 2.5m (8 feet).
Now I have the first offer for a Stiebel Eltron LWZ 8 cs Premium.
Am I correct to assume that under the above parameters the LWZ 5 cs would also be sufficient?
How can I best integrate the Hoxter stove into the heating system?
As a layperson, I currently see two options:
Option 1
An 800-liter (210-gallon) buffer tank only for heating operation, without domestic hot water.
Domestic hot water is generated by the air-to-water heat pump during daylight; the heat pump runs mainly during the day and stores heat in the screed. From 4 p.m. onward, the Hoxter stove is fired.
The heating circuit would then have to switch to the buffer tank when a certain temperature X is reached in the stove circuit or the heat pump buffer.
Option 2
The air-to-water heat pump always charges the buffer tank with a maximum flow temperature of about 40°C (104°F). If this temperature is exceeded by the stove operation, the heat pump switches off. Also, the heat pump would be programmed to operate only during daytime.
I understand that the combination of air-to-water heat pump with a buffer tank is suboptimal. But a stove without hydronic integration also makes no sense, as it would quickly overheat.
My current bidder is almost unreachable for technical evaluation at the moment, fully booked, so I have no real information about integrating the stove yet.
What do you suggest?
Which other air-to-water heat pump manufacturers would you consider for this configuration?
Thank you very much,
GSGaucho
nordanney schrieb:
So, this means you need an oversized photovoltaic system plus about €4 (plus constantly stoking the fireplace) worth of firewood every evening just to enjoy the low costs of the heat pump? ROFL The photovoltaic system was the best investment in the entire house. Anyone limiting themselves to 10kWp and being convinced to add a 10kWh battery obviously doesn’t understand the numbers.
The beech firewood costs me not a single cent, except for muscle power and a little fuel.
As I mentioned in one of my earlier posts, the alternative was to not install a wood stove at all.
But that’s not the topic of this discussion. The question was and still is how to technically integrate the two systems efficiently. And when the utilities soon reach €0.50 per kWh, we’ll have the conversation again at -10°C (14°F).
T
T_im_Norden16 Mar 2021 19:082 buffers, at least 3 pumps, and 1 mixing valve.
Let’s see what values you achieve in actual heating operation.
Let’s see what values you achieve in actual heating operation.
N
nordanney16 Mar 2021 19:26GSGaucho schrieb:
Beech firewood costs me not a single cent, except for muscle power and a bit of gasoline. Not for you. But for 99.99% of people, it does. So don’t praise a system that only makes sense in isolated cases (even if the land is not economically viable). Anyone who has to buy wood and cover the initial investment will call the system crazy.
T_im_Norden schrieb:
2 buffer tanks, at least 3 pumps, and 1 mixing valve.
Let’s see what values you actually achieve during real heating operation. Where does it say anything about 2 buffer tanks, 3 pumps, and a mixing valve??
Additionally, there are two pumps, one buffer tank, and no mixing valve. The heat pump works directly in the heating circuit. Is that clear?
nordanney schrieb:
Not you. But 99.99% of all people, yes. So don’t promote a system that only makes sense in rare cases (even if the plot is not economical). Anyone who has to buy wood and cover the initial investment will call the system crazy. I’m not promoting a system that only makes sense in rare cases, but one that represents the most effective solution for me. My actual additional costs are definitely lower than what a trench collector would have cost me doing it myself, especially considering the questionable results in purely gravelly soil.
Following your argument, 99.9% of homeowners should better not have a wood stove in the living room, just because it should only be used for economic reasons? Then we should also let 99.9% of firewood rot unused in the forest, releasing CO2 without benefit? Or should the affected forest owners and the state now all switch to wood chips with district heating? Or heat entirely with wood, even when they’re older?
But the main thing is that a deep geothermal drill costing $18,000, heavily subsidized by the public, can be highly praised.