Combine air-water heat pump with water-carrying wood stove

Hello everyone,

we are currently planning a single-family house in solid construction KFW55. Two full floors, partially basemented, without a cellar there are around 230m² of living space, 5 people. The location is southern Germany at 550m above sea level. The shell planning is fixed, the shell and the gable roof have already been contracted. Construction start is KW22/2020. I have also already contracted a 23kWp photovoltaic system on the S/W house roof and S/O roof of the garage. A power storage system is currently not an option due to lack of economic viability. The heating load according to calculation is around 5.5 kW at -15°C outside temperature.

Now it is about the heating system planning: Current status is:

[*]Underfloor heating in all rooms except pantry and storage/technical room in the basement.
[*]central ventilation system in combination with the air-water heat pump. Air-water heat pump installed indoors in the basement.
[*]a water-bearing Hoxter wood stove with firing from a separate room, i.e. no wood/dirt in the living room. I still have 30rm of beech wood free in stock.
[*]due to the high capacity of the photovoltaic system and the stove, I see no sense in a trench collector.
[*]a buffer tank 800L can be placed almost directly under the Hoxter in the basement. Distance to the air-water heat pump is also only about 2.5m.

Now I have the first offer for a Stiebel Eltron LWZ 8 cs Premium. Am I correct in assuming that under the above parameters the lwz 5 cs would also be sufficient?

How do I best integrate the Hoxter into the heating system?

As a layperson I currently see two options:
Option 1
An 800L buffer tank only for heating operation without domestic hot water. DHW is generated by the air-water heat pump in daylight, the air-water heat pump only operates in normal mode during the day and buffers in the screed, from 4 p.m. the Hoxter burns. The heating circuit would then have to switch to the buffer tank when reaching a temperature X in the circuit of the stove or buffer from the air-water heat pump.

Option 2
The air-water heat pump always operates into the buffer tank with a maximum flow temperature of perhaps 40°C. If this is exceeded by the operation of the stove, the air-water heat pump switches off. Additionally, programming the air-water heat pump only during the day. I am aware that the combination of air-water heat pump with buffer tank is suboptimal. But a stove without water circulation is also nonsense, otherwise it overheats in no time.

My current bidder is almost unreachable for technical evaluation as he is fully booked, so he has no real information about the integration of the stove yet. What do you suggest? Which other manufacturer of air-water heat pumps would you still consider for this configuration?

Thank you very much
Gaucho

Incorporating a heat pump into an 800l buffer tank is inefficient. Therefore, solution 2 is out. Solution 1 would mean too much control technology for me... My opinion is either or. Combining a low-temperature system with a high-temperature system is demanding in terms of control technology and usually reduces the efficiency of the heat pump.

Hi,

oh dear. Where to even begin....

because of the high capacity of the photovoltaic system and the stove, I see no point in a trench collector

1. A trench collector, with proper planning and approach, is not significantly more expensive than an air-source heat pump. In return, you get higher efficiency in the end, no noise issues (and no trouble with neighbors), the option for very cheap passive cooling, and a longer lifespan of the heat generator due to indoor installation and gentler operation. Just the higher operating comfort alone would be worth spending a bit more money to not have to worry about when the fan freezes up in unfavorable weather. Oh yes, the photovoltaic system barely produces anything in winter because the majority of the electricity is already consumed by Hamburg demand.
Although I have a very good air-source heat pump (not Stiebel) and a large photovoltaic system (22kWp), I would always prefer a geothermal heat pump with trench collector. Oh yes, there is also a stove.

Single-family house in solid construction KFW55. Two full floors, partly basement, without basement around 230m² living space, 5 people.

That practically screams for concrete core activation in the ceilings. At this stage, no problem at all. You get a lot of benefits for little money:
- higher comfort due to radiant heat from the ceiling
- cooling with heat pump works much better. Cooling via underfloor heating is less pleasant and does not work as well.
- higher efficiency of the heat pump due to lowered supply temperatures

Two full floors, partly basement, without basement around 230m² living space, 5 people. [...]The heating load according to calculation is about 5.5 kW at -15°C outdoor temperature[...]Am I correct in assuming that under the above-mentioned parameters the lwz 5 cs is sufficient?

That seems unrealistic to me. In my KfW40 with 240m² living space, occupants and outdoor temperature = -14°C, I have a heating load of just under 7 kW. Therefore, I would recommend a modulating heat pump with somewhat higher capacity in your place. This way, you can better utilize the photovoltaic surpluses during the day, if any are present at all, and run the heat pump less at night. In the case of an air heat pump, you can also better exploit the higher daytime temperatures and partially avoid inefficient night operation.

central ventilation system combined with the air-water heat pump. Air-water heat pump installed indoors in the basement.

If you do a bit of googling, you’ll find that such a heating system is basically an electric direct heating and one of the most inefficient options you can install.

Option 1
An 800L buffer storage tank solely for heating operation without domestic hot water.
DHW is generated during daylight by the air-water heat pump, which operates only during the day in normal mode and stores heat in the screed; from 4 p.m. the Hoxter is running.
The heating circuit would then have to switch to the buffer storage when a temperature X is reached in the circuit of the stove or buffer of the air-water heat pump.

Option 2
The air-water heat pump always works into the buffer storage with a maximum supply temperature of maybe 40°C. If this is exceeded by the operation of the stove, the air-water heat pump switches off. In addition, the air-water heat pump is programmed to operate only during daytime.
I am aware that the combo of air-water heat pump with buffer storage is suboptimal. But a stove without water circuit also makes no sense, otherwise it overheats in no time.

The described options are pretty much the worst case for the efficiency of a heat pump and especially an air-source heat pump.
1. Instead of a water-bearing stove, you can use a grund or small storage stove. This avoids almost all problems you may have with a water-bearing stove (in combination with heat pump) without giving up the advantages. Furthermore, a grund stove runs without electricity, unlike the water jacket, saving you from the notorious efficiency killer heating buffer tank.
2. If you absolutely want to install a water-bearing stove, you can still hydraulically integrate the heat pump in such a way that it feeds the heating circuits directly without a heating buffer and thus runs efficiently. Operation with supply temperature = 40°C into the heating buffer will almost certainly wipe out any savings, if any, from wood heating.
3. Wood heating is not cheap. Calculate what a kWh of heat costs at your wood price. The efficiency of the stove is maybe around 70%. Wood heating is okay as a comfort factor. However, you won’t save money with it. Rather, you will pay extra.

Which other manufacturer of air-water heat pumps would you consider under this configuration?

If at all an air heat pump, then one that achieves a calculated seasonal performance factor >4.5. The ones that come to mind are Nibe F2120 and IDM. Google “seasonal performance factor calculator”. This is only possible with supply temperature <35°C. In new buildings with concrete core activation, you can easily manage supply temperatures below 30°C. You will also get BAFA subsidies, which should cover the extra costs. Additionally, a geothermal heat pump is cheaper than a comparable air heat pump.
However, I will never understand people who install an air heat pump when a geothermal heat pump would have been possible at almost the same cost. And I am even a happy owner of an air heat pump.

Regards Nika

Thank you for the prompt answers.

Regarding the ring trench collector vs. air-to-water heat pump: The offered Stiebel does not have an external heat exchanger. In my opinion, nothing can freeze there.
The photovoltaic system has a predicted yield at our location in January of 24 kWh per day. According to our own records of actual values, we need around 14-16 kWh of household electricity per day in January, thus 10-12 kWh during the day. Accordingly, at least 12 kWh should be available for operating heating/DHW and ventilation.

Regarding ceiling heating: The ceiling on the upper floor towards the cold attic space is a wooden beam ceiling with some exposed beams. We do not want to install two systems there, i.e. underfloor heating is fixed.
I have wood now for free, so it is not a factor.
A good storage tank or basic stove costs >20k. And I know people who have one and rather advise against it, as it still overheats, especially when the sun comes during the day. Our house also has large window areas.

Stiebel advertises this air-to-water heat pump with "can be combined with solar thermal..." I should probably write to them once to see how the hydraulic integration is supposed to work. Or maybe it is only meant for summer operation. ops:

What I will take a look at is the ring trench collector and the option of the heat pump directly into the heating circuit and buffer tank in parallel.
One big advantage of my solution is, among other things, that when it is really cold, I can fully heat with wood.

The photovoltaic system at our location has a forecasted yield of 24 kWh per day in January. According to our own records of the actual values, we need around 16 kWh household electricity per day in January. So around 8 kWh should be free for heating and ventilation.

You should not confuse the average value with actual values. What use is the yield if the majority of it is generated on 5-10 days in January and on the remaining days it is not even enough for the household? My yield this January was 470 kWh. Most of it on 7 days. January is still quite good. December is more interesting. If you size your heat pump too narrowly, it will have to run at night without photovoltaic power.

Furthermore, the actual values probably refer to your current consumption (in the apartment?). In the house, you will also have controlled residential ventilation, heating pumps, etc. additionally. That means consumption will increase even without the heat pump.

So that you can roughly estimate how far the photovoltaic system will cover your heat pump demand... With my 22 kWp photovoltaic system I was able to cover the heating demand of the KfW40+ house with 240 m² at an actual annual performance factor of 4.5 as follows:
Nov: 33%
Dec: 28%
Jan: 28%
Feb: 46%
Mar: 66%
Attribution of photovoltaic yields to the heat pump or Hamburg was done proportional to demand. That means, if you correctly attribute only the remainder to the heat pump, coverage is significantly worse. It will not be much different for you. Maybe even somewhat worse due to the higher heating demand at KfW55. If you implement your hydraulic concept as proposed, it will get even worse. Therefore, the heat pump should definitely be operated without a buffer, otherwise you would push your annual performance factor in practice well below 3.7.

Regarding ceiling heating: The ceiling on the upper floor towards the unheated attic is a wooden beam ceiling with partly exposed beams. We do not want to install two systems there, i.e. underfloor heating is fixed.

That was not the idea. Ceiling heating should be an addition, not a replacement for underfloor heating. Ceiling heating is also not possible in a wooden beam ceiling. Additional ceiling heating is more a question of how much you want to improve the heat pump efficiency and whether you also want efficient cooling. Regardless of ceiling heating, I would prefer a solid ceiling, if possible.

A good storage stove or masonry stove costs over 20k. And I know people who have one and rather advise against it because it still overheats, especially when the sun comes during the day. Our house also has large window areas.

Compare the air-side output of a storage stove (around 2 kW) with the selected water-carrying stove. But I don't want to talk you out of your water-carrying stove. Everyone may indulge their hobby.

Stiebel advertises this air-to-water heat pump as "can be combined with solar thermal..." I should probably contact them about how the hydraulic integration is supposed to work. Or maybe this is only meant for summer operation. ops:

I better say nothing about Stiebel. Only this much: I don't know anyone who operates an efficient heating system with it. But at least the lwz 5 cs premium achieves a good annual performance factor on paper. It would be interesting whether Stiebel is more open to other manufacturers there. I would be glad about reports. Since the normal version is not listed in the annual performance factor calculator, you should take the premium for BAFA purposes, if there is even a normal version.

Regards Nika

Stiebel advertises this air-to-water heat pump with "can be combined with solar thermal..." I should probably write to them to ask how the hydraulic integration is supposed to work. Or maybe that's only meant for summer operation.

Oh dear... I just looked at the documents for the lwz 5/8cs premium... I have to warn you right away. Since the heat is extracted from the exhaust air, at low temperatures the air volume flow has to be increased, which causes the humidity of the air to drop to unbearably low levels, due to ventilation losses you have a higher heating load, which then also has to be covered additionally by a higher flow temperature, which further kills the efficiency of the heat pump. There is a parallel thread where the residents are desperate because the air is too dry. And if you Google a bit, you will quickly understand why such systems are only criticized as direct electric heating by experienced heat pump users. The most desperate inquiries from heat pump owners come exactly about such systems. I first thought it was a normal indoor air heat pump. But that is not the case. So please, please, please don't do it to yourself and rather look for an alternative. The calculated annual performance factor for such heat pumps is even further from reality due to calculation assumptions than fuel consumption figures are for cars. Regards Nika