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_Norden4 Jan 2021 13:15Defrosting every half hour?
How long does the defrosting take?
What are the outside temperature and humidity levels?
How often was it defrosted before?
The system keeps heating constantly, right, or does the compressor shut off?
Basically, I would currently assume that the system is adding much more heat into the house and therefore has to work harder.
This was to be expected since all the valves are open.
The defrosting should not take long and should not have a significant impact on the indoor temperature.
Still, keep an eye on the electricity consumption and see how much heating energy it produces from that.
How long does the defrosting take?
What are the outside temperature and humidity levels?
How often was it defrosted before?
The system keeps heating constantly, right, or does the compressor shut off?
Basically, I would currently assume that the system is adding much more heat into the house and therefore has to work harder.
This was to be expected since all the valves are open.
The defrosting should not take long and should not have a significant impact on the indoor temperature.
Still, keep an eye on the electricity consumption and see how much heating energy it produces from that.
Currently, according to the outdoor sensor, it is 2.5°C (36.5°F), humidity unknown...
According to my phone and weather app, temperatures are around 0 to -1°C (32 to 30°F) with nearly 90% humidity, and since early December there has often been fog, which has caused the system to freeze more frequently.
I can’t really assess the long-term performance yet since this is the first heating season for the house and heat pump.
The defrosting process takes a maximum of 5 minutes.
However, the defrost cycles do not affect compressor starts because the compressor basically continues running during defrost due to the reversing valve. Otherwise, I would see many more compressor starts in the statistics.
I still need to figure out power consumption and related data within the complex control system, which is a bit tricky and nested with Vaillant.
According to the control system, I have an overall seasonal performance factor (SPF) of around 5.5.
I just wanted to mention the defrosting so it doesn’t cause problems later if there is no noticeable change in performance. According to the display, the system also only modulates around 50% most of the time.
According to my phone and weather app, temperatures are around 0 to -1°C (32 to 30°F) with nearly 90% humidity, and since early December there has often been fog, which has caused the system to freeze more frequently.
I can’t really assess the long-term performance yet since this is the first heating season for the house and heat pump.
The defrosting process takes a maximum of 5 minutes.
However, the defrost cycles do not affect compressor starts because the compressor basically continues running during defrost due to the reversing valve. Otherwise, I would see many more compressor starts in the statistics.
I still need to figure out power consumption and related data within the complex control system, which is a bit tricky and nested with Vaillant.
According to the control system, I have an overall seasonal performance factor (SPF) of around 5.5.
I just wanted to mention the defrosting so it doesn’t cause problems later if there is no noticeable change in performance. According to the display, the system also only modulates around 50% most of the time.
T
T_im_Norden4 Jan 2021 13:45The 5.5 is probably a rather optimistic estimate from Vaillant 🙂.
With the temperatures and humidity, frequent icing is to be expected.
At a duration of 5 minutes, not much will be lost. The heat pump basically only uses the water available in the circuit for defrosting and does not draw anything from the house.
The problem would be if it additionally uses the electric heater— is that disabled in the control system?
Do you have a buffer tank in the heating circuit, or does the supply line go directly into the underfloor heating?
With the temperatures and humidity, frequent icing is to be expected.
At a duration of 5 minutes, not much will be lost. The heat pump basically only uses the water available in the circuit for defrosting and does not draw anything from the house.
The problem would be if it additionally uses the electric heater— is that disabled in the control system?
Do you have a buffer tank in the heating circuit, or does the supply line go directly into the underfloor heating?
I have a small buffer tank of about 18 liters (5 gallons) acting as a series storage in the return line, and a bypass valve which I have completely deactivated. The bypass valve only became noticeable when nearly all actuators were closed due to an error. The buffer and bypass valve were non-negotiable requirements from the general contractor / prefab house manufacturer. Once the system is out of warranty, the bypass valve will be removed.
Yes, this will also be the approach of my currently preferred replacement heating engineer. He explained to me that it is designed to ensure a minimum run time of the compressor when the ERR (expansion valve) is closed, because otherwise it would keep turning on and off immediately. This is bad since the compressor would not be sufficiently lubricated. The refrigerant also acts as a lubricant for the compressor. It is, however, a flow buffer and only slightly reduces efficiency. If, as recommended, the ERR is inactive (actuators constantly open), nothing happens except the buffer being continuously flowed through.
Mine is only larger (40 liters (10.6 gallons)), but I currently have the Arotherm plus 75/6 planned due to BAFA subsidy...
Mine is only larger (40 liters (10.6 gallons)), but I currently have the Arotherm plus 75/6 planned due to BAFA subsidy...
The first 24 hours without ERR have passed. Attached are the resulting temperature readings.
Change compared to yesterday:
Outdoor temperature dropped from 2.5°C to 1°C (36.5°F to 33.8°F)
Basically, I noticed that, regarding my defrosting theory from yesterday and the associated heat extraction, the system is now coming out of the defrost cycle, and the supply temperature is stabilizing at 28.3°C (83°F) compared to the target supply temperature of 29.5°C (85°F). Yesterday, after defrosting, it was still at 26–27°C (79–81°F) with the same target supply temperature.
This would mean to me that the screed temperature is slowly increasing until there is no temperature difference between the underfloor surface and the room air temperature. As a result, the actual supply temperature will keep rising until the target supply temperature is reached or exceeded. At that point, no more heat transfer to the rooms will take place. Then the energy integral should start to count down (as long as the actual supply temperature is above the target supply temperature), and when the integral reaches zero, the system will switch off until the integral reaches the pump's turn-on point (120 minutes). Then the compressor should run again.
@ T in the north, I have a few questions regarding this:
Can anything be concluded from the last 24 hours yet? Should the system continue running as is, until the supply temperature rises further? Or can the flow rates already be adjusted? According to today’s measured temperatures, all rooms except the bathroom are at the target temperature, so is it possible to reduce flow so the bathroom can warm up? Or should the system be left alone for now? How much does window ventilation interfere with the balancing? The controlled ventilation system is running, but the household still insists on keeping windows ajar.
Change compared to yesterday:
Outdoor temperature dropped from 2.5°C to 1°C (36.5°F to 33.8°F)
Basically, I noticed that, regarding my defrosting theory from yesterday and the associated heat extraction, the system is now coming out of the defrost cycle, and the supply temperature is stabilizing at 28.3°C (83°F) compared to the target supply temperature of 29.5°C (85°F). Yesterday, after defrosting, it was still at 26–27°C (79–81°F) with the same target supply temperature.
This would mean to me that the screed temperature is slowly increasing until there is no temperature difference between the underfloor surface and the room air temperature. As a result, the actual supply temperature will keep rising until the target supply temperature is reached or exceeded. At that point, no more heat transfer to the rooms will take place. Then the energy integral should start to count down (as long as the actual supply temperature is above the target supply temperature), and when the integral reaches zero, the system will switch off until the integral reaches the pump's turn-on point (120 minutes). Then the compressor should run again.
@ T in the north, I have a few questions regarding this:
Can anything be concluded from the last 24 hours yet? Should the system continue running as is, until the supply temperature rises further? Or can the flow rates already be adjusted? According to today’s measured temperatures, all rooms except the bathroom are at the target temperature, so is it possible to reduce flow so the bathroom can warm up? Or should the system be left alone for now? How much does window ventilation interfere with the balancing? The controlled ventilation system is running, but the household still insists on keeping windows ajar.
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