ᐅ Hydraulic balancing for air-to-water heat pump + high-efficiency circulation pump
Created on: 3 Jan 2021 23:07
L
lesmue79
Warning: wall of text and lots of theorycrafting:
I am currently trying to optimize or fundamentally adjust the hydraulic and thermal balancing of my air-to-water heat pump system, including underfloor heating, but I am running into the following issues:
First, about the house: KfW-55 bungalow with controlled mechanical ventilation
Nearly 105 m2 (1130 ft²) of heated floor area
Air-to-water heat pump with underfloor heating throughout, 10cm (5 inches) pipe spacing, max 30°C (86°F) flow temperature. At -12°C (10°F) outside temperature, the calculated heating load is 3276 watts.
According to the datasheet, the heat pump delivers 3200 watts at -10°C (14°F) outside temperature with flow 35°C (95°F) and return 30°C (86°F).
All rooms are designed for 20°C (68°F), including the bathroom (to avoid an oversized heat pump by the general contractor). Additionally, for the bathroom, an electric radiator is planned to achieve a room temperature of 2°C (4°F) higher than the rest. However, in reality, the toilet, utility room, bedroom, and guest room should only be heated to 18°C (64°F) (it won’t be much lower in a new building). The bathroom is intended to be warmer, at around 21–22°C (70–72°F).
Currently, I have the following questions (though perhaps I am too focused on the self-regulation effect and avoiding actuator valves):
1. Circulation pump: Various guides, manuals, and forum posts recommend setting the circulation pump of the underfloor heating to a constant flow rate.
My conclusion: my circulation pump is a high-efficiency variable-speed pump, so I can set the flow rate on the manifold in L/min (based on the calculations from the general contractor / heating engineer) to whatever I want, but the flow always settles around 600–630 L/h (10–11 L/min). The only significant flow changes I get are when I activate the actuators and room thermostats, which then open or close the valves. The only adjustment parameter on the circulation pump is the minimum flow rate; no other settings are available. But I don’t fully understand how this function works.
2. Operating times of the heat pump / self-regulation: I usually read that the heat pump should run as long as possible, though some sources say short cycling a few times is normal.
My conclusion: if I run the system without actuators and room thermostats, the energy integral control does not work; the system basically runs almost 24/7 at low temperatures, with interruptions only for defrosting. As a result, with a flow temperature of 27°C (81°F), I only get about 19–20°C (66–68°F) room temperature, but I’d prefer around 21–23°C (70–73°F), especially in the bathroom. If I do it the other way, with energy integral control (EIC) and actuators and slightly higher curve so that 30°C (86°F) flow is demanded, the actuators close in the first rooms, which causes the flow to increase to the other rooms because the pump still distributes the volume flow among the remaining open valves. At the same time, the flow temperature rises for rooms where the actuators are still open until the desired temperature is reached and the actuators close. Then the energy integral kicks in and goes negative because actual flow temperature exceeds setpoint flow temperature, until the heat pump shuts off once the energy integral has been reduced.
So right now, I’m struggling with what is better: should the system just run steadily at a flow temperature of 27°C (81°F) (which I might still optimize a bit), with heating only interrupted for defrosting or when the compressor’s hysteresis is exceeded, causing the compressor to be locked out for a certain time? Or should I define time windows during which the system is allowed to operate?
Maybe I could manage this better by refining the balancing, but I guess I’ll have to throttle down so much for the energy integral to work that the flow rate will fall below the minimum required, and the bypass valve will open.
Or should I run the system at 30°C (86°F) flow with room thermostats and actuators, allowing the energy integral control to function properly and reach the desired room temperatures?
Another strange issue is: according to the heating load and underfloor heating calculations, the system requires about 840 L/h (14 L/min) at 4.4 K (7.9°F) delta T in the design case. If I set the flow according to this calculation or slightly lower, the pump only delivers 600–630 L/h (10–11 L/min) at a delta T of about 3–4 K (5.4–7.2°F).
According to the datasheet, the optimal flow rate for the heat pump is 540 L/h (9 L/min) at 5 K (9°F) delta T.
540 L/h * 5 K * 1.163 = 3132 watts
620 L/h * 3.5 K * 1.163 = 2527 watts
840 L/h * 4.4 K * 1.163 = 4287 watts
Calculated heating load at -12°C (10°F) = 3176 watts (and this heating load is probably overestimated since controlled mechanical ventilation was not included in the calculation, and I want only 15–18°C (59–64°F) in four rooms instead of the calculated 20°C (68°F). Also, average outside temperatures for the heat pump in my area are closer to -10°C (14°F) rather than -12°C (10°F), so there is some margin).
Maybe I have now gotten too caught up in theoretical and calculated values and can’t see the forest for the trees?
I am currently trying to optimize or fundamentally adjust the hydraulic and thermal balancing of my air-to-water heat pump system, including underfloor heating, but I am running into the following issues:
First, about the house: KfW-55 bungalow with controlled mechanical ventilation
Nearly 105 m2 (1130 ft²) of heated floor area
Air-to-water heat pump with underfloor heating throughout, 10cm (5 inches) pipe spacing, max 30°C (86°F) flow temperature. At -12°C (10°F) outside temperature, the calculated heating load is 3276 watts.
According to the datasheet, the heat pump delivers 3200 watts at -10°C (14°F) outside temperature with flow 35°C (95°F) and return 30°C (86°F).
All rooms are designed for 20°C (68°F), including the bathroom (to avoid an oversized heat pump by the general contractor). Additionally, for the bathroom, an electric radiator is planned to achieve a room temperature of 2°C (4°F) higher than the rest. However, in reality, the toilet, utility room, bedroom, and guest room should only be heated to 18°C (64°F) (it won’t be much lower in a new building). The bathroom is intended to be warmer, at around 21–22°C (70–72°F).
Currently, I have the following questions (though perhaps I am too focused on the self-regulation effect and avoiding actuator valves):
1. Circulation pump: Various guides, manuals, and forum posts recommend setting the circulation pump of the underfloor heating to a constant flow rate.
My conclusion: my circulation pump is a high-efficiency variable-speed pump, so I can set the flow rate on the manifold in L/min (based on the calculations from the general contractor / heating engineer) to whatever I want, but the flow always settles around 600–630 L/h (10–11 L/min). The only significant flow changes I get are when I activate the actuators and room thermostats, which then open or close the valves. The only adjustment parameter on the circulation pump is the minimum flow rate; no other settings are available. But I don’t fully understand how this function works.
2. Operating times of the heat pump / self-regulation: I usually read that the heat pump should run as long as possible, though some sources say short cycling a few times is normal.
My conclusion: if I run the system without actuators and room thermostats, the energy integral control does not work; the system basically runs almost 24/7 at low temperatures, with interruptions only for defrosting. As a result, with a flow temperature of 27°C (81°F), I only get about 19–20°C (66–68°F) room temperature, but I’d prefer around 21–23°C (70–73°F), especially in the bathroom. If I do it the other way, with energy integral control (EIC) and actuators and slightly higher curve so that 30°C (86°F) flow is demanded, the actuators close in the first rooms, which causes the flow to increase to the other rooms because the pump still distributes the volume flow among the remaining open valves. At the same time, the flow temperature rises for rooms where the actuators are still open until the desired temperature is reached and the actuators close. Then the energy integral kicks in and goes negative because actual flow temperature exceeds setpoint flow temperature, until the heat pump shuts off once the energy integral has been reduced.
So right now, I’m struggling with what is better: should the system just run steadily at a flow temperature of 27°C (81°F) (which I might still optimize a bit), with heating only interrupted for defrosting or when the compressor’s hysteresis is exceeded, causing the compressor to be locked out for a certain time? Or should I define time windows during which the system is allowed to operate?
Maybe I could manage this better by refining the balancing, but I guess I’ll have to throttle down so much for the energy integral to work that the flow rate will fall below the minimum required, and the bypass valve will open.
Or should I run the system at 30°C (86°F) flow with room thermostats and actuators, allowing the energy integral control to function properly and reach the desired room temperatures?
Another strange issue is: according to the heating load and underfloor heating calculations, the system requires about 840 L/h (14 L/min) at 4.4 K (7.9°F) delta T in the design case. If I set the flow according to this calculation or slightly lower, the pump only delivers 600–630 L/h (10–11 L/min) at a delta T of about 3–4 K (5.4–7.2°F).
According to the datasheet, the optimal flow rate for the heat pump is 540 L/h (9 L/min) at 5 K (9°F) delta T.
540 L/h * 5 K * 1.163 = 3132 watts
620 L/h * 3.5 K * 1.163 = 2527 watts
840 L/h * 4.4 K * 1.163 = 4287 watts
Calculated heating load at -12°C (10°F) = 3176 watts (and this heating load is probably overestimated since controlled mechanical ventilation was not included in the calculation, and I want only 15–18°C (59–64°F) in four rooms instead of the calculated 20°C (68°F). Also, average outside temperatures for the heat pump in my area are closer to -10°C (14°F) rather than -12°C (10°F), so there is some margin).
Maybe I have now gotten too caught up in theoretical and calculated values and can’t see the forest for the trees?
T
T_im_Norden5 Jan 2021 18:51This cannot be properly assessed at the moment.
I assume the building’s thermal mass is still warming up.
It will become clear tomorrow whether the consumption remains this high.
However, heat meters for heat pumps are often inaccurate.
Do you have a hot water recirculation system?
What temperature is your domestic hot water?
I assume the building’s thermal mass is still warming up.
It will become clear tomorrow whether the consumption remains this high.
However, heat meters for heat pumps are often inaccurate.
Do you have a hot water recirculation system?
What temperature is your domestic hot water?
Yes, hot water circulation is available but is limited to once a day for a few minutes.
The set hot water temperature is 48°C (118°F).
It might also be possible to reduce the hot water cycle from three time slots to two time slots...
At the moment, I can’t really say much about the heating curve except that the house is getting warm. We are just starting to figure out how much I can lower the heating curve and thus the flow temperatures...
Tendency-wise, I usually feel cold here, while my partner currently would prefer to open all the windows at 21–22°C (70–72°F)...
Therefore, we will probably end up with a room temperature in the bathroom of 21–22°C (70–72°F) and in the rest of the house between 18–20°C (64–68°F)... If I want to push it to the extreme, I could say 20°C (68°F) in the bathroom via the heat pump and everything above that as needed via electric heaters.
The set hot water temperature is 48°C (118°F).
It might also be possible to reduce the hot water cycle from three time slots to two time slots...
At the moment, I can’t really say much about the heating curve except that the house is getting warm. We are just starting to figure out how much I can lower the heating curve and thus the flow temperatures...
Tendency-wise, I usually feel cold here, while my partner currently would prefer to open all the windows at 21–22°C (70–72°F)...
Therefore, we will probably end up with a room temperature in the bathroom of 21–22°C (70–72°F) and in the rest of the house between 18–20°C (64–68°F)... If I want to push it to the extreme, I could say 20°C (68°F) in the bathroom via the heat pump and everything above that as needed via electric heaters.
T
T_im_Norden5 Jan 2021 19:22Okay, then the issue is not with the hot water.
Have the temperatures increased further since then?
With KFW 55 standard, significantly lower consumption values should be achievable.
I believe you can definitely reach that.
Have the temperatures increased further since then?
With KFW 55 standard, significantly lower consumption values should be achievable.
I believe you can definitely reach that.
Yes, the temperatures are still rising. The lowest temperature is 20°C (68°F) in the bathroom and bedroom, while all other rooms have at least 21°C (70°F), with the living room just under 23°C (73°F).
The question is how much I can still lower the supply temperature once the flow rates are properly adjusted.
What’s the best approach tomorrow once the house has stabilized? Lower the heating curve first and then adjust the flow rates? Or the other way around—adjust the flow first and then see how far the curve can be reduced? I’d prefer the latter option (if it makes sense). Tomorrow I still have the whole day to make adjustments since it’s the last day of vacation.
The question is how much I can still lower the supply temperature once the flow rates are properly adjusted.
What’s the best approach tomorrow once the house has stabilized? Lower the heating curve first and then adjust the flow rates? Or the other way around—adjust the flow first and then see how far the curve can be reduced? I’d prefer the latter option (if it makes sense). Tomorrow I still have the whole day to make adjustments since it’s the last day of vacation.
T
T_im_Norden5 Jan 2021 20:26Once it has stabilized, the next step is to lower the heating curve.
After each adjustment, you need to wait again before making further changes. It can’t be done quickly.
You have to decide which room will serve as the reference.
Usually, this is the bathroom because it has a small amount of heating surface combined with higher temperatures.
You lower the heating curve until your reference room falls below the desired temperature, then raise the heating curve back to the last working value.
Only after that do you start adjusting the flow rates room by room.
Each throttled room affects the others, so again, wait and monitor the temperatures.
Feel free to share your values here so we can follow along.
After each adjustment, you need to wait again before making further changes. It can’t be done quickly.
You have to decide which room will serve as the reference.
Usually, this is the bathroom because it has a small amount of heating surface combined with higher temperatures.
You lower the heating curve until your reference room falls below the desired temperature, then raise the heating curve back to the last working value.
Only after that do you start adjusting the flow rates room by room.
Each throttled room affects the others, so again, wait and monitor the temperatures.
Feel free to share your values here so we can follow along.
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