ᐅ KfW40 house offer with heat pump and mechanical ventilation with heat recovery— is it worth it?
Created on: 25 May 2019 21:19
Q
querys_
Hello everyone,
Today I received a house offer from a prefabricated house supplier.
They list the exhaust air heat pump Nibe F 750 with supply air module SAM 40 as the heating system. The maximum building heating load is said to be 7.5 kW.
Additionally, there is underfloor heating and a 180 L (48 gallon) domestic hot water storage tank.
The house is located near Cologne/Bonn Airport (for climate reference) and is about 120 m² (1,290 sq ft) in size (excluding the basement).
I have already read a lot of negative things about exhaust air heat pumps, so I want to ask again if this system is any good, and if not, what would be an alternative and how much more would that cost?
Thank you very much!
Today I received a house offer from a prefabricated house supplier.
They list the exhaust air heat pump Nibe F 750 with supply air module SAM 40 as the heating system. The maximum building heating load is said to be 7.5 kW.
Additionally, there is underfloor heating and a 180 L (48 gallon) domestic hot water storage tank.
The house is located near Cologne/Bonn Airport (for climate reference) and is about 120 m² (1,290 sq ft) in size (excluding the basement).
I have already read a lot of negative things about exhaust air heat pumps, so I want to ask again if this system is any good, and if not, what would be an alternative and how much more would that cost?
Thank you very much!
Regarding the building's heating load again. Are these values correct?
KFW-60 houses: approx. 50 W/m² (5 W/ft²)
KFW-40 houses: approx. 40 W/m² (4 W/ft²)
Passive houses: approx. 15 W/m² (1.5 W/ft²)
For my house from the offer, this would mean:
7500 W / 122 m² (1312 ft²) = approx. 60 W/m² (6 W/ft²)
Or calculated differently:
KfW40 house = 40 W/m² (4 W/ft²) × 122 m² (1312 ft²) = 4.8 kW
Although I already find 40 W/m² quite high and would rather expect around 30 W/m² (3 W/ft²) = 3600 W or less than half of the offered value.
I think I will ask again and inquire why the value is so high and, secondly, whether an air-to-air heat pump could also be installed.
KFW-60 houses: approx. 50 W/m² (5 W/ft²)
KFW-40 houses: approx. 40 W/m² (4 W/ft²)
Passive houses: approx. 15 W/m² (1.5 W/ft²)
For my house from the offer, this would mean:
7500 W / 122 m² (1312 ft²) = approx. 60 W/m² (6 W/ft²)
Or calculated differently:
KfW40 house = 40 W/m² (4 W/ft²) × 122 m² (1312 ft²) = 4.8 kW
Although I already find 40 W/m² quite high and would rather expect around 30 W/m² (3 W/ft²) = 3600 W or less than half of the offered value.
I think I will ask again and inquire why the value is so high and, secondly, whether an air-to-air heat pump could also be installed.
B
boxandroof27 May 2019 08:17Forget the general calculations and also ignore the figures provided by the construction companies for now. Do some more research and set clear requirements for the companies.
For reference: we have a calculated heating load of 3.9x kW for 150m² (1615 sq ft) without any allowance for domestic hot water. The house performs slightly worse than KfW 40, and the location is somewhat colder. The actual measured heating load in our house last winter was a maximum of 3 kW including hot water.
Search online for the “Crinks Heating Load Tool.” This tool is simple and sufficient for estimating the total heating load.
Unless your house is unusual in shape or glazing area, you’ll likely have a heating load between 3 and 4 kW. You don’t need to calculate much higher than that since it’s unlikely you’ll find a heat pump with lower or exactly matching capacity. More important is the calculation of the underfloor heating system.
Therefore, the advice is:
1. Make sure to use a fully modulating heat pump, not oversized (maximum 6 kW, preferably less)
2. Choose a standard air-to-water heat pump; definitely avoid exhaust-air or combined ventilation units
3. Have the underfloor heating designed for a maximum flow temperature of 30°C (86°F) at the standard outdoor temperature. For bathrooms, consider wall heating. (Perform room-by-room heating load calculations based on your desired temperatures per room)
4. And the usual: no buffer tank, no bypass valves, and omit individual room controls at the latest when you move in
If you follow this advice, you’re halfway there; otherwise, you risk unexpected consumption.
For reference: we have a calculated heating load of 3.9x kW for 150m² (1615 sq ft) without any allowance for domestic hot water. The house performs slightly worse than KfW 40, and the location is somewhat colder. The actual measured heating load in our house last winter was a maximum of 3 kW including hot water.
Search online for the “Crinks Heating Load Tool.” This tool is simple and sufficient for estimating the total heating load.
Unless your house is unusual in shape or glazing area, you’ll likely have a heating load between 3 and 4 kW. You don’t need to calculate much higher than that since it’s unlikely you’ll find a heat pump with lower or exactly matching capacity. More important is the calculation of the underfloor heating system.
Therefore, the advice is:
1. Make sure to use a fully modulating heat pump, not oversized (maximum 6 kW, preferably less)
2. Choose a standard air-to-water heat pump; definitely avoid exhaust-air or combined ventilation units
3. Have the underfloor heating designed for a maximum flow temperature of 30°C (86°F) at the standard outdoor temperature. For bathrooms, consider wall heating. (Perform room-by-room heating load calculations based on your desired temperatures per room)
4. And the usual: no buffer tank, no bypass valves, and omit individual room controls at the latest when you move in
If you follow this advice, you’re halfway there; otherwise, you risk unexpected consumption.
B
boxandroof27 May 2019 08:25querys_ schrieb:
why the value is so high and also whether it is possible to get an air-to-air heat pump. No, you want an air-to-water heat pump, not an air-to-air heat pump!
It’s better if you calculate the total heating load yourself for selecting the heat pump; anything else will be pointless. The energy-saving regulation calculation / energy performance certificate will differ anyway, so just file it away.
Insist on the smallest model and on receiving the room-by-room heating load calculation early, so you can check it yourself in time. If I were you, I would try to clarify the latter as soon as possible.
Thanks again, boxandroof! Of course, an air-to-water heat pump (those abbreviations can be confusing sometimes).
This is a standard “shoebox” house, nothing special.
I used the Crinks tool and entered the data from the offer, as well as the advertised U-value (0.148) from the manufacturer of the single-family house (the roof is a bit better at 0.133).
Key data:
Standard outdoor temperature -12°C (10°F)
H'T: (heat transfer coefficient) 0.148 W/(m²·K)
Desired average indoor temperature: 22 degrees
A: (building envelope surface area) 300 m² (3,229 ft²)
Ve: (heated gross volume) 440 m³ (15,535 ft³)
Maximum number of occupants: 4 persons
The values are quite interesting and significantly lower.
Central mechanical ventilation with heat recovery
Transmission load: 1510 W
+ Ventilation load: 193 W
+ Domestic hot water load: 400 W
= Total heating load: 2103 W
Decentralized mechanical ventilation with heat recovery
Transmission load: 1510 W
+ Ventilation load: 483 W
+ Domestic hot water load: 400 W
= Total heating load: 2393 W
Window ventilation or mechanical ventilation without heat recovery
Transmission load: 1510 W
+ Ventilation load: 966 W
+ Domestic hot water load: 400 W
= Total heating load: 2876 W
I think that with these values, a geothermal heat pump would be too expensive.
This is a standard “shoebox” house, nothing special.
I used the Crinks tool and entered the data from the offer, as well as the advertised U-value (0.148) from the manufacturer of the single-family house (the roof is a bit better at 0.133).
Key data:
Standard outdoor temperature -12°C (10°F)
H'T: (heat transfer coefficient) 0.148 W/(m²·K)
Desired average indoor temperature: 22 degrees
A: (building envelope surface area) 300 m² (3,229 ft²)
Ve: (heated gross volume) 440 m³ (15,535 ft³)
Maximum number of occupants: 4 persons
The values are quite interesting and significantly lower.
Central mechanical ventilation with heat recovery
Transmission load: 1510 W
+ Ventilation load: 193 W
+ Domestic hot water load: 400 W
= Total heating load: 2103 W
Decentralized mechanical ventilation with heat recovery
Transmission load: 1510 W
+ Ventilation load: 483 W
+ Domestic hot water load: 400 W
= Total heating load: 2393 W
Window ventilation or mechanical ventilation without heat recovery
Transmission load: 1510 W
+ Ventilation load: 966 W
+ Domestic hot water load: 400 W
= Total heating load: 2876 W
I think that with these values, a geothermal heat pump would be too expensive.
querys_ schrieb:
H'T: (overall heat transfer coefficient) 0.148 W/(m²·K) What kind of house is that supposed to be? I find that very unrealistic.
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