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
Daniel-Sp schrieb:
Is the buffer tank completely disconnected from the heating circuit at temperatures below 30°C (86°F)? The buffer tank circuit for the heat pump does not need to be disconnected because it is not hydraulically connected. The connection is only through the internal heat exchanger within the heat pump.
If the buffer temperature drops to the flow temperature of the heating circuits, the pump in the buffer tank circuit to the heat pump stops operating, and the heat pump continues running with the compressor.
When the temperature in the buffer rises to a set value, for example 45°C (113°F), the heat pump cycles the above-mentioned pump back on and the compressor shuts off.
D
Daniel-Sp6 Sep 2020 20:03I don't quite understand that. Do you have a picture of the hydraulics?
GSGaucho schrieb:
Option 1
A buffer tank with 800 liters (210 gallons) exclusively for heating without domestic hot water.
Domestic hot water is generated during daylight hours by the air-to-water heat pump, which normally operates only during the day and stores heat in the screed. From 4 p.m. onward, the Hoxter stove is running.
The heating circuit would then have to switch to the buffer tank when a specific temperature is reached in either the stove or air-to-water heat pump circuit. Hello,
very interesting solution ... I am planning something similar.
I would like to install the same Stiebel Eltron air heat pump in our planned new build, but combined with the water-bearing wood stove from Walltherm (wood gasification technology).
Have you been able to implement your plan as described? If so, what experience have you had with your system in recent months?
I would appreciate your reply.
Best regards,
A Anton
Hello Anton,
Yes, I implemented the solution with the Stiebel LWZ8.
It has a solar connection and controls a circulation pump for the buffer tank based on the temperature sensor at the top of the buffer tank.
I ran the screed heating program in mid-February and operated the stove heavily during that time. About 700 kWh have already been transferred from the buffer to the Stiebel. That means roughly 240 euros less in electricity costs for running the heating element.
For example, with the Stiebel, you can also set the domestic hot water temperature somewhat higher when operating with solar power. Or better yet, set the hot water generation to occur during the times of day when the buffer temperature is sufficient.
I recommend downloading and studying the user manual and commissioning instructions for the Stiebel as PDFs from their website. That’s more informative than any opinion you find online.
There is a common strict opinion online that the low-temperature system of an air-to-water heat pump cannot be technically or economically sensibly combined with the high-temperature system of a hydronic wood stove.
That may be true for heat pumps without a solar connection.
I am convinced, however, that with my system here in Austria, I can get through the night with no more than 5 kWh electricity consumption at temperatures down to -5°C (23°F), if I load the stove with 10–15 kg (22–33 lbs) of beech wood in the evening.
My 27 kWp photovoltaic system delivers between 1.5 and 2.5 kW during a cloudy winter day. That’s enough for heating.
If you have any further questions, just get in touch.
Regards,
GSGaucho
Yes, I implemented the solution with the Stiebel LWZ8.
It has a solar connection and controls a circulation pump for the buffer tank based on the temperature sensor at the top of the buffer tank.
I ran the screed heating program in mid-February and operated the stove heavily during that time. About 700 kWh have already been transferred from the buffer to the Stiebel. That means roughly 240 euros less in electricity costs for running the heating element.
For example, with the Stiebel, you can also set the domestic hot water temperature somewhat higher when operating with solar power. Or better yet, set the hot water generation to occur during the times of day when the buffer temperature is sufficient.
I recommend downloading and studying the user manual and commissioning instructions for the Stiebel as PDFs from their website. That’s more informative than any opinion you find online.
There is a common strict opinion online that the low-temperature system of an air-to-water heat pump cannot be technically or economically sensibly combined with the high-temperature system of a hydronic wood stove.
That may be true for heat pumps without a solar connection.
I am convinced, however, that with my system here in Austria, I can get through the night with no more than 5 kWh electricity consumption at temperatures down to -5°C (23°F), if I load the stove with 10–15 kg (22–33 lbs) of beech wood in the evening.
My 27 kWp photovoltaic system delivers between 1.5 and 2.5 kW during a cloudy winter day. That’s enough for heating.
If you have any further questions, just get in touch.
Regards,
GSGaucho
N
nordanney16 Mar 2021 12:42GSGaucho schrieb:
In any case, I am confident that my system in Austria can manage temperatures down to -5°C (23°F) with no more than 5 kWh of electricity consumption overnight, if I stoke the stove in the evening with 10–15 kg (22–33 lbs) of beech wood.
My 27 kW peak solar photovoltaic system produces between 1.5 and 2.5 kW during a cloudy winter day. That is enough for heating. So, this means you need an oversized solar photovoltaic system plus around €4 worth of firewood every evening (plus constantly tending the wood stove) just to enjoy low operating costs of the heat pump? ROFL
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