ᐅ Ground Source Heat Pump for a 200 m² Single-Family Home with Underfloor Heating, KfW55 Standard – Settings and Optimization

Created on: 4 Nov 2021 20:21

Hello. We moved into our new single-family house in March. I didn’t make any adjustments to the heating system at that time. Now that the temperatures are rising, I’ve started to take a closer look at it.

House details:

Single-family house with a (heated) basement + 2.5 floors (usable gable roof/also underfloor heating) -> 4 heating circuits
approximately 200 m² (2150 sq ft) of underfloor heating
2 bathrooms WITHOUT additional heating
kfw55 energy standard
ventilation system with heat recovery
Currently 2 rooms in the basement are unoccupied/unutilized + the technical room
There are also 2 children’s rooms in the attic that are unoccupied/unutilized

Heating system details (descriptions from the invoice):

High-efficiency brine/water heat pump Dimplex SI 8TU
High-efficiency brine system SZB 140E for brine/water heat pump with electronically controlled brine circulation pump Yonos Para 25/1-10
Multifunctional storage tank Geysir MTL-WP650 efficiency class B (150 mm (6 inches) insulation thickness) with connection options for multiple heat generators, with layering plate for large volume flows, capacity 850 liters (225 gallons), domestic hot water preparation using counterflow principle with stainless steel heat exchanger, including differential temperature controller and flow sensor for hot water tapping system
Hydraulic connection of the heat pump to the multifunctional storage tank with precision steel pipe 28x1.5 mm (1.1x0.06 inch) including insulation, 1 zone charging pump Dimplex UPH 75-25P with shut-off set, switchable between heating and domestic hot water charging
Integration of the heating system with heating circuit sets Easyflow DN 25 R1" with EPP insulation type 2 including 3-way mixing valve, mixing valve actuator and circulation pump Grundfos UPM3 Auto
(ERR in 3/4 of the rooms) - currently switched off
Cooling station Dimplex PKS 14 Econ for passive cooling via geothermal probes, consisting of heat exchanger, brine circulation pump, cooling modules for network operation with heat pump manager and temperature sensor
Room temperature controller Dimplex Smart RTC, for optimizing weather-compensated control via a reference room

My previous attempts:

Domestic Hot Water:

I first focused on the domestic hot water preparation. Initially, it was set to 50°C (122°F) with a 2° hysteresis. For Dimplex, this means that heating started again at 48°C (118°F). This setup was basically fine, but even without any hot water use, heating occurred 2-3 times a day. Since the pump ran only very briefly, the average summer consumption was about 0.7 kWh/day.
I then experimented with lowering the temperature, setting lockout periods, and increasing the hysteresis. Our "optimal consumption" turned out to be 50°C (122°F) and 7° hysteresis with lockout from 8 pm to 5 pm. This sometimes resulted in the pump not running for an entire day. However, energy use was only reduced to 0.5 kWh/day, meaning hardly any consumption reduction at the cost of noticeable comfort reductions. Currently, I am at 48°C (118°F) and 4° hysteresis with lockout from 10 pm to 5 pm. Since we mostly use hot water in the evening, this works well. For bathing or higher demand, the water is reheated if necessary. I am currently experimenting with 5 or 6 degrees hysteresis, as the heat loss during ongoing heating operation seems lower and we might be able to skip a day sometimes. We’ll see...

Now, regarding the heating, my attempts:

All ERRs switched off, heating circuits opened roughly by feel, and tried to adjust by regulation. Control was via fixed return flow temperature, which I tested between 23 and 26°C (73°F and 79°F). The consumption and COP results were very satisfactory. Unfortunately, I couldn’t get the bathroom above 22°C (72°F) without other rooms becoming too warm (rooms quickly reached 22°C, which I find too high).
Turned the ERR back on in the children’s rooms.
After a one-week vacation during which I completely switched off the heating, I started over. I tried the recommended approach of fully opening the warmest room (bathroom) to about 2.2 liters/min (0.58 gallons/min) and then increasing the temperature until satisfied. But this meant the heating was massively oversized?! The heat pump came on about 20 times for 10-15 minutes each, the supply temperature was nicely between 30 and 33°C (86°F and 91°F) but the temperature felt like it never really got away from the heat pump’s threshold. I am attaching a picture of the behavior.
Suspecting insufficient flow and hesitant to adjust the heating pump, I slowly opened other rooms slightly.
I also tried increasing the fixed temperature to 27 or 28°C (81°F or 82°F) and used hysteresis to make the pump run less often but longer. I am now quite satisfied with the temperatures in the house. However, the numbers still look a bit odd to me. I believe there are now many rooms/areas/storage volumes with such low flow that “cooled down” mass just circulates in the loop and eventually reaches the return line repeatedly. I don’t mind that but I also do not want to risk any damage. I will attach another picture.
Lastly, I reduced the temperature at night and in the morning so the heating starts at favorable times. Currently, two starts of about 2-3 hours each are sufficient.

Questions:

Am I completely off track here or are these approaches generally valid? Unfortunately, I can’t really rely on the heating engineer. He is surely competent but firstly hardly reachable and secondly probably overwhelmed by such optimization considerations.
What about rooms that are unused? Should I use the screed as a buffer and keep them slightly heated (<0.5 liters/min (0.13 gallons/min)) anyway?
I increasingly believe that managing the large temperature difference between the bathroom at 23°C (73°F) and the rooms at 20.5°C (69°F) is not well controlled – is there really no alternative to an additional heat source? We only use the bathroom for about 2 hours and in the evening for 4 hours at 23°C (73°F). Otherwise, 21-22°C (70-72°F) would definitely be sufficient there.
Any tips on settings?

Personal preferences:

The underfloor heating is off in the bedroom – yet it quickly reaches 19-20°C (66-68°F), which is almost too warm.
In the 3 children’s rooms, the ERR closes from 5 pm to 3 am (for sleeping – with time delay)
Other rooms 20-21°C (68-70°F)
Open-plan kitchen/living room 21-22°C (70-72°F)
Bathrooms 23°C (73°F)

Diagram showing temperature and pressure trends as well as setpoint curves of several sensors over time.

Multiple lines showing temperature and pressure trends of a heating/hot water system over time.

Time course of multiple sensor values: temperatures, humidity and pressure with legend.

Chart with several lines: temperature and setpoint trends from HVAC sensors over time.

Alessandro11 Nov 2021 11:06

#43

halmi schrieb:

sorry, I meant the values from @Alessandro

I ventilate every morning for about 10 minutes (I have a window and a balcony door in the bedroom). After that, depending on the outside temperature, it’s around 15°C (59°F). When I go to bed in the evening, the temperature is about 18.5°C (65°F). In the morning after getting up, it’s about 19.5°C (67°F).

RotorMotor11 Nov 2021 11:10

#44

Alessandro schrieb:

I ventilate every morning for 10 minutes across the room (I have a window and a balcony door in the bedroom). After that, depending on the outdoor temperature, it’s about 15°C (59°F). When I go to bed in the evening, the temperature is around 18.5°C (65°F). In the morning after getting up, it’s about 19.5°C (67°F).

Ah, so it really is 19.5°C (67°F). With 21.5°C (71°F) in the corridor, it’s just the expected 2°C (4°F) difference, not 5°C (9°F). 😉

Alessandro11 Nov 2021 11:12

#45

After 8 hours of sleep and 2.5 people in one room, it is inevitable that the temperature rises. The windows are closed during the night.
The bathroom has now reached 23.5°C (74.3°F). I need to adjust the final point of the heating curve slightly downward.

RotorMotor11 Nov 2021 11:23

#46

Alessandro schrieb:

After 8 hours of sleep and 2.5 people in one room, it’s natural that the temperature rises. The windows are closed at night.
The bathroom is now at 23.5°C (74°F). I need to adjust the endpoint of the heating curve a bit.

Sorry, but this all seems a bit uncoordinated.
Here a measurement with an open window, there one with the shower or hairdryer running, ...
And all of that with those rough ERR estimates.

But well, the main thing is that you feel comfortable and it’s pleasant for you!

Alessandro11 Nov 2021 11:27

#47

You need to read carefully.
I said that the bedroom does not get warmer than 18.5°C (65.3°F) with the adjoining bathroom at 23°C (73.4°F) and the adjoining hallway at 21.5°C (70.7°F).

It is obvious that temperatures change when people are present in the respective rooms.

This was in response to the statement that nowadays it is not possible to achieve significant temperature differences within the thermal envelope.

RotorMotor11 Nov 2021 11:45

#48

Alessandro schrieb:

You need to read carefully.
I said that the bedroom does not get warmer than 18.5°C (65°F) when the adjacent bathroom is 23°C (73°F) and the adjoining hallway is 21.5°C (71°F).

It is obviously logical that temperatures change when people are present in the respective rooms.

This was in reference to the statement that nowadays, it is not possible to achieve significant temperature differences within the thermal envelope.

I have read it and recognized that it is not particularly well structured. 😉

And I stand by my statement. Because when the window is open, the thermal envelope no longer applies.

It might be useful, for example, to calculate average temperatures. Otherwise, you just make others unhappy because they measure more systematically and do not reach these differences.

ᐅ Ground Source Heat Pump for a 200 m² Single-Family Home with Underfloor Heating, KfW55 Standard – Settings and Optimization

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