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?
H
Hausbau 552 Jan 2022 14:29Thank you for your effort. I will definitely read through some of this again once my heat pump is installed in the house and I start optimizing it. I will certainly get back to you then. It is very encouraging when initial expectations turn out successful in practice. Perhaps as a photovoltaic system owner, it is useful to not only focus on the heat pump and its parameters during optimization. Cycling can be reduced by using the energy integral of the heat pump or implemented alongside consistent application of lockout periods, among other things... Surely there are many options to adjust.
For our own house, I am interested in the possibilities and limitations of prioritizing the heat pump’s use with the photovoltaic system. From November to January there is less energy, but the forecast still predicts a total yield of 1,200 kWh (1,290 kWh) for these three months. How much of this can be consumed by the heat pump, and how much by the household electricity? The forecast for October is 880 kWh (940 kWh), for February 608 kWh (634 kWh), and for March already 1,110 kWh (1,175 kWh). Instead of feeding electricity into the grid at 6.5 cents/kWh (¢/kWh), the goal is to reduce purchases at about 35 cents/kWh (¢/kWh).
Overall, I expect an annual electricity consumption of 4,000 kWh (4,400 kWh), about half for the heat pump and half for household use. With the one-time investment in a photovoltaic system (net about 11,000 euros for a 13.68 kWp system), I want to have almost no electricity costs for the heat pump and household for the next 20 years by offsetting feed-in. Is that realistic? Of course, no one knows how electricity prices will develop.
For our own house, I am interested in the possibilities and limitations of prioritizing the heat pump’s use with the photovoltaic system. From November to January there is less energy, but the forecast still predicts a total yield of 1,200 kWh (1,290 kWh) for these three months. How much of this can be consumed by the heat pump, and how much by the household electricity? The forecast for October is 880 kWh (940 kWh), for February 608 kWh (634 kWh), and for March already 1,110 kWh (1,175 kWh). Instead of feeding electricity into the grid at 6.5 cents/kWh (¢/kWh), the goal is to reduce purchases at about 35 cents/kWh (¢/kWh).
Overall, I expect an annual electricity consumption of 4,000 kWh (4,400 kWh), about half for the heat pump and half for household use. With the one-time investment in a photovoltaic system (net about 11,000 euros for a 13.68 kWp system), I want to have almost no electricity costs for the heat pump and household for the next 20 years by offsetting feed-in. Is that realistic? Of course, no one knows how electricity prices will develop.
I wouldn’t rely on forecasts but rather wait to see what is actually possible in reality and then form an average or midpoint after 2-3 years.
You need to carefully consider or test lockout times to reduce cycles even further. However, I must say I haven’t tested this consistently enough yet.
It would be helpful if the Vaillant control system allowed defining more switching cycles. I believe you can only set three switch cycles per day, and one of those is lost if you want to use a night setback. You can still experiment with the reheating function at a certain temperature, but I don’t have a clear concept for that yet.
It’s pointless for me to run the system with a night setback or off at night and then compensate by running higher supply temperatures or counter-heating during the day.
So, from 6:00 to 13:00 I run at a higher flow temperature to make use of the photovoltaic system and higher outdoor temperature to compensate for the night setback. But here’s the problem: the household returns around 13:00 and it’s too warm inside, so the heat stored during the morning is then ventilated out again.
You need to carefully consider or test lockout times to reduce cycles even further. However, I must say I haven’t tested this consistently enough yet.
It would be helpful if the Vaillant control system allowed defining more switching cycles. I believe you can only set three switch cycles per day, and one of those is lost if you want to use a night setback. You can still experiment with the reheating function at a certain temperature, but I don’t have a clear concept for that yet.
It’s pointless for me to run the system with a night setback or off at night and then compensate by running higher supply temperatures or counter-heating during the day.
So, from 6:00 to 13:00 I run at a higher flow temperature to make use of the photovoltaic system and higher outdoor temperature to compensate for the night setback. But here’s the problem: the household returns around 13:00 and it’s too warm inside, so the heat stored during the morning is then ventilated out again.
H
Hausbau 552 Jan 2022 16:28lesmue79 schrieb:
I wouldn’t rely on forecasts but rather wait to see what is actually possible in reality and then form an average or middle ground after 2-3 years.
You need to carefully consider or experiment with blocking times to reduce the cycles even further. I must add that I haven’t tried this consistently enough yet.
It would be nice if Vaillant’s control system allowed defining more switching cycles. I think there are only three definable switching cycles per day, and one of those would be lost if you use the night setback. You can still play around a bit with the reheating function above a certain temperature, but I don’t have a clear concept for that yet.
It doesn’t help me if I run the system with a night setback or turn it off at night, only to then operate with increased supply temperatures during the day to compensate for that.
So, from 6:00 to 13:00, I run with a higher flow temperature to take advantage of the photovoltaic system and higher outdoor temperatures to compensate for the night setback. But here’s the catch: the household returns around 13:00, and it’s too warm inside, so the heat stored during the morning is ventilated out again. I think the Vaillant software is accurate enough so that the forecast won’t turn into a disaster. Of course, there are fluctuations year to year, but over several years they tend to balance out.
I can’t say anything specific yet about the possible switching cycles. I see potential when using photovoltaics. Producing hot water during the day definitely makes more sense. The efficiency of heat generation is also better during daytime. I have all the exterior and interior walls made from sand-lime brick, plus a 70mm (3 inch) cement screed, so there is a lot of thermal mass.
A moderate increase of the supply temperature during the day will bring two advantages: better efficiency because outdoor temperatures are higher, and better opportunities to use photovoltaic electricity. Then at night, fewer switching cycles occur. This will work better during transitional seasons compared to temperatures close to the natural setpoint.
Hausbau 55 schrieb:
With the one-time investment in a photovoltaic system (net approximately 11,000 euros for a 13.68 kWp systemMay I ask how this is possible? A lot of electricity?And 2000 kWh for a house—wait, when children are around, it will be double that.
H
Hausbau 552 Jan 2022 19:17tomtom79 schrieb:
May I ask how this is possible? A lot of electricity?
And 2000 kW for a house—wait until there are children, then it will be double.Waiting is no longer an option... occasionally our grandchildren, aged 10 and 8, visit. We purchased a full pallet with 36 modules, each 380 W, Trina TSM-380-DE09.05 Fullblack, price per module including tax 141 euros, bought inverter SMA STP 15000TL-30 and Sunny Home Manager 2.0, purchased mounting frame and small accessories, all via the Internet for transparent price comparison.
Order for mounting the frame placed with an independent solar installer, invoiced per number of modules. Installation used the existing scaffolding on site (precise scheduling was necessary, maximum time one week after final roof acceptance).
Connection notification and commissioning (initial operation) carried out by a specialized company (electrical installation).
Hausbau 55 schrieb:
Waiting is no longer an option... occasionally our grandchildren, aged 10 and 8, visit...
Purchase of a full pallet with 36 modules of 380 W each, Trina TSM-380-DE09.05 Fullblack, gross price per module 141 euros, purchase of inverter SMA STP 15000TL-30 and Sunny Home Manager 2.0, purchase of mounting frame and small accessories, all ordered online allowing transparent price comparison. Commissioned installation of the mounting frame to an independent solar technician, billed per number of modules. Installation using existing construction scaffolding (precise scheduling required, maximum time one week after roof handover). Registration and commissioning by a specialized company (electrical installation).Congratulations on being lucky enough to find someone like that. In our area, the saying goes more like: "If you buy the stuff online, then find someone online to install it for you," or
"I'll install the equipment for you but without any warranty; if you want it installed with a warranty, I have to charge double."
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