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
Ok,
I’m happy to accept your data on the photovoltaic system since those are actual measured values.
My current electricity consumption comes from a house with a gas boiler and a hot water circuit running 24/7.
The chosen heating boiler has a huge advantage for me because of the backfiring feature. This means I have enough wood stored in the house for a week and no dust in the living room. I’m reluctant to give that up. While there are versions without the water circuit, that would basically feel like sitting in a sauna.
I will gladly take your advice regarding the Stiebel brand. My architect has installed the same system in a similar new build two years ago, so actual consumption values should be available by now.
As a newbie: what does BKA stand for? B...air conditioning?
I’m happy to accept your data on the photovoltaic system since those are actual measured values.
My current electricity consumption comes from a house with a gas boiler and a hot water circuit running 24/7.
The chosen heating boiler has a huge advantage for me because of the backfiring feature. This means I have enough wood stored in the house for a week and no dust in the living room. I’m reluctant to give that up. While there are versions without the water circuit, that would basically feel like sitting in a sauna.
I will gladly take your advice regarding the Stiebel brand. My architect has installed the same system in a similar new build two years ago, so actual consumption values should be available by now.
As a newbie: what does BKA stand for? B...air conditioning?
GSGaucho schrieb:
I will gladly follow your advice regarding the Stiebel brand. My architect has had the same system in a similar new build for two years now. Actual consumption values should be visible there.
As a newbie: what does BKA stand for? B... air conditioning? BKA stands for the mentioned thermally activated building system. You still have all the options to install this in the concrete floor slabs. With a timber beam ceiling, of course, this option is not available, and the alternative ceiling heating is comparatively not very cost-effective.
Regarding the architect’s experience, I would be cautious. Most people don’t want to admit to themselves or others that they might have made a wrong decision, so they tend to put a positive spin on it. I prefer to rely on the experience of users of such exhaust air heat pumps, which make an impact in forums. At first, sympathy is usually expressed to them. For example, have a look at this thread
Minuk1234567 schrieb:
Newly built timber house (prefabricated) with a comfort climate heating system.
After I developed dry skin, my husband bought a humidity meter.
These show an average humidity of around 25%. or just search for exhaust air heat pumps in forums.
Regarding hydronic heating... Do you want to save money or do you just want the feeling of radiant warmth?
Greetings, Nika
It was clear to me from the beginning that a wood heating system would never be economically viable. However, for me, it’s also about the hobby, fresh air, and teaching my sons some physical work. If I didn’t already have 30 cubic meters (1,060 cubic feet) of beech wood and regularly receive softwood for free, I would only heat with electricity.
On a day like today, for example, when everyone is at home, temperatures around 0°C (32°F), and the sky is overcast, I can safely turn off the air-to-water heat pump. The 5-10 kW output from the stove easily keeps the house warm.
I have now looked into Nibe and IDM a bit. IDM also promotes a combination of their heat pump with a wood boiler. I contacted them to ask how they recommend the hydraulic coupling. In Austria, a lot of heating with wood is common, so I trust their expertise the most in that regard.
I now also prefer to decouple the heat pump from the ventilation system. For example, a Nibe 2120 combined with a Nibe ERS 10-400 ventilation unit. Another advantage of Nibe is the separate control display that can be installed in a second room, like the kitchen.
On a day like today, for example, when everyone is at home, temperatures around 0°C (32°F), and the sky is overcast, I can safely turn off the air-to-water heat pump. The 5-10 kW output from the stove easily keeps the house warm.
I have now looked into Nibe and IDM a bit. IDM also promotes a combination of their heat pump with a wood boiler. I contacted them to ask how they recommend the hydraulic coupling. In Austria, a lot of heating with wood is common, so I trust their expertise the most in that regard.
I now also prefer to decouple the heat pump from the ventilation system. For example, a Nibe 2120 combined with a Nibe ERS 10-400 ventilation unit. Another advantage of Nibe is the separate control display that can be installed in a second room, like the kitchen.
Hi,
On Nibe’s website, you can find the hydraulic diagrams directly. Just check under the professional partners section. However, you should always question the manufacturer’s recommendations. For my Nibe system, I also opted for the matching hot water storage tank from them. It’s not a bad choice, but with a different tank without the small buffer, I would now have a better annual performance factor, and the system would have also cost less. Another downside is that later on, it would be more difficult to replace the heat pump with a unit from another manufacturer.
The same applies to the combination of controlled residential ventilation and heat pump. You end up being locked into one manufacturer, and if something breaks, you lose flexibility. Personally, I would choose the storage tank, ventilation system, and heat pump so that all three can be replaced independently.
Otherwise, you should prefer monobloc units for heat pumps. For IDM, a split unit could be an option. AI is also a good candidate for effective heat pumps. And as already mentioned, a ground source heat pump with a trench collector generally offers the best cost-benefit ratio in my opinion.
Regarding exhaust air heat pumps: well, physics can’t be fooled. After the cross-counterflow heat exchanger, the exhaust air is at outside temperature. However, the volume of air is so much smaller compared to a proper air-source heat pump that to extract the same amount of heat from the air, the exhaust air must be cooled about 15°C (27°F) below outside temperature. In comparison, a proper air-source heat pump might only need to provide about 2°C (4°F) of cooling capacity to extract the same amount of heat. This results in roughly a 30% difference in the seasonal performance factor.
Best regards,
Nika
GSGaucho schrieb:
I now also prefer separating the heat pump from the ventilation system.
For example, a Nibe 2120 with a Nibe ERS 10-400 unit.
An advantage of Nibe is the separate control display that can be installed in a second room, like the kitchen.
On Nibe’s website, you can find the hydraulic diagrams directly. Just check under the professional partners section. However, you should always question the manufacturer’s recommendations. For my Nibe system, I also opted for the matching hot water storage tank from them. It’s not a bad choice, but with a different tank without the small buffer, I would now have a better annual performance factor, and the system would have also cost less. Another downside is that later on, it would be more difficult to replace the heat pump with a unit from another manufacturer.
The same applies to the combination of controlled residential ventilation and heat pump. You end up being locked into one manufacturer, and if something breaks, you lose flexibility. Personally, I would choose the storage tank, ventilation system, and heat pump so that all three can be replaced independently.
Otherwise, you should prefer monobloc units for heat pumps. For IDM, a split unit could be an option. AI is also a good candidate for effective heat pumps. And as already mentioned, a ground source heat pump with a trench collector generally offers the best cost-benefit ratio in my opinion.
Regarding exhaust air heat pumps: well, physics can’t be fooled. After the cross-counterflow heat exchanger, the exhaust air is at outside temperature. However, the volume of air is so much smaller compared to a proper air-source heat pump that to extract the same amount of heat from the air, the exhaust air must be cooled about 15°C (27°F) below outside temperature. In comparison, a proper air-source heat pump might only need to provide about 2°C (4°F) of cooling capacity to extract the same amount of heat. This results in roughly a 30% difference in the seasonal performance factor.
Best regards,
Nika
In the meantime, I have looked into the subject in more detail.
The LWZ Premium from Stiebel Eltron, which is available as an option, has a separate supply and return connection for solar thermal systems. This means that warm water is circulated from the upper heat exchanger of the buffer tank—heated exclusively by the wood stove—to the heat pump via a pump circuit. The heat pump transfers this heat in a controlled manner through an internal heat exchanger into the supply line to the compressor.
So, with a target heating circuit supply temperature of, for example, 30°C (86°F), 30°C (86°F) is already fed into the circuit before the actual heat pump system.
The pump from the buffer tank is controlled by the heat pump. Optionally, a manual mixing valve can be integrated here to regulate the supply temperature at the solar thermal connection of the heat pump within the desired range. A temperature sensor in the upper part of the buffer tank signals the heat pump to switch to solar thermal mode and activate the circulation pump.
To heat the internal domestic hot water (DHW) buffer, energy is also taken from the controlled internal heat exchanger, just with higher internal supply temperatures.
So, where is the catch?
The LWZ Premium from Stiebel Eltron, which is available as an option, has a separate supply and return connection for solar thermal systems. This means that warm water is circulated from the upper heat exchanger of the buffer tank—heated exclusively by the wood stove—to the heat pump via a pump circuit. The heat pump transfers this heat in a controlled manner through an internal heat exchanger into the supply line to the compressor.
So, with a target heating circuit supply temperature of, for example, 30°C (86°F), 30°C (86°F) is already fed into the circuit before the actual heat pump system.
The pump from the buffer tank is controlled by the heat pump. Optionally, a manual mixing valve can be integrated here to regulate the supply temperature at the solar thermal connection of the heat pump within the desired range. A temperature sensor in the upper part of the buffer tank signals the heat pump to switch to solar thermal mode and activate the circulation pump.
To heat the internal domestic hot water (DHW) buffer, energy is also taken from the controlled internal heat exchanger, just with higher internal supply temperatures.
So, where is the catch?
N
neo-sciliar27 Aug 2020 13:07Hi,
We currently have a heat pump (specifically a ground-source water-to-water heat pump) and a hydronic stove, but that setup developed over time since we bought the house. I would never choose to build something like this from scratch. In fact, as already mentioned, low-temperature and high-temperature heating systems don’t really work well together. It took three winters to get the hydraulics configured so that everything works smoothly—and that only happened because a friend who is a heating engineer helped me, and I enjoy working on it. ………
I’ve read something here about supply temperatures… Are the NIBE units supply-controlled? I have an AiT, and it’s return-controlled… just curious.
What exactly is the difference between a monoblock and a split air-to-water heat pump? Does the monoblock not have an outdoor unit? How is that supposed to work?
Best regards, Andreas
We currently have a heat pump (specifically a ground-source water-to-water heat pump) and a hydronic stove, but that setup developed over time since we bought the house. I would never choose to build something like this from scratch. In fact, as already mentioned, low-temperature and high-temperature heating systems don’t really work well together. It took three winters to get the hydraulics configured so that everything works smoothly—and that only happened because a friend who is a heating engineer helped me, and I enjoy working on it. ………
I’ve read something here about supply temperatures… Are the NIBE units supply-controlled? I have an AiT, and it’s return-controlled… just curious.
What exactly is the difference between a monoblock and a split air-to-water heat pump? Does the monoblock not have an outdoor unit? How is that supposed to work?
Best regards, Andreas
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