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

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)

Phew. Unfortunately, another huge combined storage tank designed to handle the oversized heat pump with an 8 kW rated capacity. A potential source of problems and efficiency loss.
Now it is here.
What do the heating curves look like in the heating circuit and the mixing circuit? For optimal efficiency, the mixing circuit should always be fully open.
I don’t have a good feeling about the bathroom setup. Heating on demand is usually more economical than raising the heating curve and throttling the underfloor heating, at least gentler on the heat pump.
I am not familiar with dimplex, so I can only offer general advice for support.
Do you have a clear and easy-to-understand diagram of the system hydraulics?

1. Obtain documentation for heating system design including hydraulic balancing.
2. Perform hydraulic balancing according to the provided documentation.
3. Run the heating system as designed (domestic hot water at 50°C (122°F) is fine; for example, I manage well with a 7°C (13°F) hysteresis, so the system operates once a day or twice a day if three people use the rain shower intensively. I consider lockout periods unnecessary—only possibly useful in combination with a photovoltaic system).
4. Return with the results and fine-tune the system accordingly.

First of all, forget about having temperature differences of more than three degrees within the thermal envelope!

So, having 18°C (64°F) in the bedroom and 23°C (73°F) in the bathroom is simply not feasible.
Trying to achieve that will only lead to frustration.
Interior doors and walls are simply not insulated.

Daniel-Sp schrieb:

Phew. Unfortunately, another huge combination storage tank designed to manage a heat pump that is oversized at 8 kW nominal output. A problem source and efficiency killer.
Now it’s installed.
What do the heating curves look like for the heating circuit and mixing circuit? For optimal efficiency, the mixing circuit should always be fully open.
I don’t have a good feeling about the bathroom. Demand-controlled reheating is usually more cost-effective than raising the heating curve and throttling the underfloor heating—at least it’s gentler on the heat pump.
I’m not familiar with Dimplex but can only offer general tips for support.
Do you have a clear diagram of the system hydraulics?

Although the Zeeh storage is also recommended by some efficiency enthusiasts. But yes, if I had had more time, I probably would have made a different choice. The idea was to eventually charge the storage using photovoltaic panels during sunny periods.
I don’t have a diagram. Unfortunately, the heating installer isn’t the type to explain things well. I would consider myself technically skilled, but whenever he was here, he was busy tinkering (and several frustrating things happened — for example, the auxiliary heating pump exploded during the screed program), and he’s very pressed for time, so I haven’t fully understood the system yet.

Benutzer200 schrieb:

1. Obtain the documentation for the heating design along with the hydraulic balancing plan
2. Perform the hydraulic balancing according to the documents
3. Run the heating system as calculated (50°C (122°F) for domestic hot water is fine; for example, I manage well with a 7°C (13°F) hysteresis so the system runs once a day or twice if the three girls take long rain showers. Lockout periods seem unnecessary—only useful if combined with photovoltaic systems)
4. Return with the results to fine-tune the system

The energy consultant required hydraulic balancing for the subsidy anyway. That’s already done. Some values seem very questionable to me (basement at 18°C (64°F) still with over 2 liters/min flow and bathroom aiming for 24°C (75°F) with 0.9 liters/min—can that be correct? Aren’t these values meant for systems with ERR?).
It will be carried out soon. Or rather, I’ve already adjusted the rooms with ERR myself (but there are only three). In all other rooms, I’ve turned down the flow significantly and run the bathrooms fully open?!
Regarding point 3: that practically sounds correct. However, as we only used the shower once early in the morning over the last six months and 40°C (104°F) is enough, and since we decided against circulation, we only draw warm water from the tap in the evening, mainly for showering and washing up. So I would still find it annoying if the water heats up at 11 pm instead of cooling down to 41°C (106°F) by 5 pm.

RotorMotor schrieb:

First of all, forget about trying to achieve more than a three-degree difference inside the thermal envelope!

So sleeping at 18°C (64°F) and bathroom at 23°C (73°F) just isn’t feasible.
Trying to reach that only leads to frustration.
Interior doors and walls are simply not insulated.

Well, three degrees would already be sufficient. And currently, things are looking quite good.
The bedroom is at 19.5°C (67°F) (with occasional evening ventilation it even goes down to 18.5/19°C (65/66°F) for a while), and the adjacent bathroom is at 23°C (73°F), sometimes even 23.5°C (74°F).
So even if the heating curve looks odd and the mixing seems suboptimal, I’m very satisfied at the moment. But it’s only been running like this for three days, and I was hoping the consumption would stabilize after the rooms are fully heated.

You should also heat the unused rooms. Otherwise, you will unnecessarily transfer heat from the heated rooms to the unheated ones. Which rooms are adjacent to the bathroom? Those must definitely be heated as well, even if they are not in use.
You asked about optimizing the settings. What are the current heating curves for the heating circuit and the mixing circuit?