Extremely high pellet consumption (200kg in 3 days) in a Kfw70 multi-family house!

In our latitudes, one can assume an annual heating output of about 150 kWh per m² per year. 10 kWh correspond to approximately 1 m³ natural gas, as well as 1 L heating oil and 2 kg pellets. In your case, 150/10*2=30 kg per m² per year. 30*550=16500 kg pellets per year Depending on the heating device, insulation, and heating habits, the value can vary somewhat. That means if you end up with 16-17 tons of pellets per year, your heating installer will likely not find anything unusual.

The reference building in my KFW-70 calculation has about 50 kWh/m^2 heating (excluding domestic hot water) energy demand, but that would still be 5.5 tons according to your calculation. If you now have somewhat more because of more glass or whatever else, that might still be realistic, since you also have to add domestic hot water, and there are only about 6 heating months, so it is indeed quite a lot in these months. Furthermore, November here seems somewhat cooler compared to previous years.

Hello Saruss,
you are of course right. If you take a look at the energy certificate, most modern houses will be found near energy classes A and B at about 50kWh/(m²a).
Looking at the comparison values, the average residential building stock in DE can be found at about 150kWh/(m²a).
As a rough assumption, I have been able to estimate consumption quite well with this value for existing buildings so far.
For the TE’s multi-family house, the values should even be a bit better than for a single-family house.
For a new building, however, I would still use the higher numbers for the first year because the three weeks of continuous heating of the building during screed drying with supply temperatures up to 55°C greatly distort the annual average consumption.

Hello Saruss, you are of course right. If you take a look at the energy certificate, most modern houses can be found near energy classes A and B at about 50kWh/(m²a). Looking at the comparison values, the average residential building stock in DE can be found at about 150kWh/(m²a). As a rough assumption, I have been able to estimate the consumption of existing buildings quite well with this value so far. For the tenant’s multifamily house, the values should even be a little better than for a single-family house. For a new building, however, I would still use the higher numbers for the first year because the three weeks of continuous heating of the building during screed drying with flow temperatures up to 55°C strongly distort the annual average consumption.

Hello. Thanks for the information.

So, of course I did not extrapolate the consumption from Nov. x10 months. I calculated 2 tons in Nov. only for the winter months Nov.-March with 2 tons (probably more if it gets really cold). Transition months Oct., April, May 1 ton, and summer 300kg (because there are collectors on the roof). That adds up to about 14-15 tons. According to Kfw70 and the building’s energy certificate, the kW value is 55kW per m2. That makes about 6 tons for 550m2 in the multifamily house. We would therefore be well above that with about 14-15 tons!

The screed is already dried! It was laid in spring 2016 and properly dried. The plaster in the apartments is also already dry (ventilated nicely over the summer, dehumidified etc.). The sun also warms a large part of the multifamily house through the windows. The collectors obviously only help during sunshine in the transition and summer periods!

The high flow temperature of 38 degrees (apartment) also confuses me.

Best regards

Annual consumption is distributed in Bavaria (e.g. Bamberg) as follows January 23% February 11% March 8% April 1% May 0% June 0% July 0% August 0% September 0% October 4% November 11% December 19% This results in a calculated annual demand of 17.5 tons with 2 tons in November - i.e. 4.8 kWh per kg * 17500 kg = 84000 kWh... per apartment therefore 7000 kWh - without ventilation system and including hot water this would still be somewhat acceptable even for one apartment.

Annual consumption in Bavaria (e.g. Bamberg) is distributed as follows January 23% February 11% March 8% April 1% May 0% June 0% July 0% August 0% September 0% October 4% November 11% December 19%

This results, with 2 tons in November, in a calculated annual demand of 17.5 tons – i.e. 4.8 kWh per kg * 17500 kg = 84000 kWh... per apartment that is 7000 kWh – without ventilation system and including hot water, this would still be somewhat okay even for one apartment.

Hello. Many thanks for your help. But isn’t 17.5 tons a lot for a new Kfw70 building with only 6 units and a total of 550m2? According to the energy certificate, the building has a heat demand of 55 kW per m2. That would mean about 6 tons per year. So 17.5 tons are almost triple that... I just think that the heating system (pellets with solar support) was not properly balanced. So rather on comfort (heating up quickly) than efficiency... Therefore, I am not surprised by the high flow temperature of the underfloor heating (currently 38 degrees).... Regards

55 kWh/(m²*a) primary energy or final energy?
As already mentioned, pellets always calculate the primary energy very quickly and nicely for you.
You are of course right - there was a calculation error; it would be 14,000 kWh per year and apartment or 150 kWh/m²a - which is rather high (not catastrophically high, but high).

I think I would bring this up at your next owners' meeting and see if you can get the heating engineer to come by again and check what else can be optimized.