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_Norden6 Jan 2021 17:40The base temperature, also called the setpoint, is the desired room temperature at floor level, which you have already set to 20°C (68°F).
This depends on the heating curve, which is a calculated value based on the outside temperature.
If it becomes too warm inside despite the lowest heating curve setting and the heat pump cannot compensate by modulation, you could try forcing the heating system below the heating curve by limiting the supply temperature.
For example:
The heating curve would calculate a supply temperature of 27°C (81°F) at an outside temperature of 0°C (32°F) and a desired room temperature of 20°C (68°F).
If that is already too warm for you, limiting the supply temperature to 26°C (79°F) will usually cause modern heating systems to recalculate and adjust the curve accordingly.
This depends on the heating curve, which is a calculated value based on the outside temperature.
If it becomes too warm inside despite the lowest heating curve setting and the heat pump cannot compensate by modulation, you could try forcing the heating system below the heating curve by limiting the supply temperature.
For example:
The heating curve would calculate a supply temperature of 27°C (81°F) at an outside temperature of 0°C (32°F) and a desired room temperature of 20°C (68°F).
If that is already too warm for you, limiting the supply temperature to 26°C (79°F) will usually cause modern heating systems to recalculate and adjust the curve accordingly.
T
T_im_Norden6 Jan 2021 18:04I have limited the supply temperature to 25°C (77°F) in my system, which uses a gas boiler, and set it to the lowest heating curve available.
The calculated supply temperature is then 22°C (72°F). The boiler runs until it reaches 27.5°C (81.5°F) and then shuts off for several hours until the supply temperature drops below the setpoint.
My boiler now always operates at its lowest modulation level (20% of capacity), except when heating domestic hot water.
With this setup, I achieve run times between 9 and 19 hours, with 1 to 5 burner starts per day.
Consumption (150m² (1,615 ft²), built to standard energy-saving regulations with improved wall insulation, controlled ventilation with heat recovery) ranges between 28 and 60 kWh depending on hot water use (bathtub or frequent showers).
Since we only moved in on December 12, these figures are likely to decrease further, as a general increase in consumption of up to 25% is expected during the first 1-2 years.
The calculated supply temperature is then 22°C (72°F). The boiler runs until it reaches 27.5°C (81.5°F) and then shuts off for several hours until the supply temperature drops below the setpoint.
My boiler now always operates at its lowest modulation level (20% of capacity), except when heating domestic hot water.
With this setup, I achieve run times between 9 and 19 hours, with 1 to 5 burner starts per day.
Consumption (150m² (1,615 ft²), built to standard energy-saving regulations with improved wall insulation, controlled ventilation with heat recovery) ranges between 28 and 60 kWh depending on hot water use (bathtub or frequent showers).
Since we only moved in on December 12, these figures are likely to decrease further, as a general increase in consumption of up to 25% is expected during the first 1-2 years.
This is basically the compressor hysteresis in my system, which I think can be set from 3,000 up to 15,000, but then the compressor is locked, and I have no control over this lockout period.
Normally, the system is controlled by the energy integral, which is fixed at -180 degree-minutes.
For every minute that the supply temperature (due to compressor hysteresis) is above the supply setpoint, the -180 moves toward 0. When it reaches 0, the compressor would turn off until the activation threshold (compressor start currently at -120) is reached again. Unless the supply temperature reaches the maximum hysteresis before the energy integral is depleted, in which case the compressor would be locked accordingly. Sounds a bit confusing, but I can’t explain it any better.
In short:
When the actual supply temperature is above the supply setpoint, the 180-minute timer counts down to 0 (unless compressor hysteresis intervenes earlier). When the actual supply temperature falls below the supply setpoint, the timer counts back up towards 180, and when it reaches 120, the compressor switches on and starts heating until the actual supply temperature exceeds the supply setpoint again, and the cycle repeats.
Normally, the system is controlled by the energy integral, which is fixed at -180 degree-minutes.
For every minute that the supply temperature (due to compressor hysteresis) is above the supply setpoint, the -180 moves toward 0. When it reaches 0, the compressor would turn off until the activation threshold (compressor start currently at -120) is reached again. Unless the supply temperature reaches the maximum hysteresis before the energy integral is depleted, in which case the compressor would be locked accordingly. Sounds a bit confusing, but I can’t explain it any better.
In short:
When the actual supply temperature is above the supply setpoint, the 180-minute timer counts down to 0 (unless compressor hysteresis intervenes earlier). When the actual supply temperature falls below the supply setpoint, the timer counts back up towards 180, and when it reaches 120, the compressor switches on and starts heating until the actual supply temperature exceeds the supply setpoint again, and the cycle repeats.
I'm still struggling with my consumption data and need to create a proper table for it.
If I’m not mistaken, compared to yesterday, I generated about 16.5 kWh less energy to maintain the temperature. However, the COP is worse (but this could be due to the hot water setting, as I tried something different today).
If I’m not mistaken, compared to yesterday, I generated about 16.5 kWh less energy to maintain the temperature. However, the COP is worse (but this could be due to the hot water setting, as I tried something different today).
Similar topics