ᐅ Increase the supply temperature from 40°C to 35°C or not?
Created on: 5 Mar 2022 00:47
H
HoisleBauer22
Hello everyone,
My home construction company specified a flow temperature of 40 degrees Celsius (104°F) in the contract "to save money," meaning a larger pipe spacing, probably increasing from 10-15 cm (4-6 inches) to 20 cm (8 inches).
The key data for the house (planned): KfW 55 standard, Daikin Altherma 3 R ECH2O 308/508, annual performance factor around 4, living area 145 m² (1560 ft²) with the basement also heated (this was required for KfW), the basement itself has 80 m² (860 ft²) of usable/floor space, but is not considered living space according to standards due to insufficient natural light.
We have a controlled ventilation system with heat recovery. We also plan to install a photovoltaic system of about 10 kW peak on the roof at some point.
Now I’m wondering if it would be worthwhile to reduce the flow temperature from 40 degrees Celsius (104°F) to 35 degrees Celsius (95°F) given an assumed electricity price of around 45 cents per kWh. The additional costs are about €15 per m² for the upgrade. The crucial point is how much energy savings percentage is achieved per degree of reduction. It is usually assumed to be 2.5 percent per degree, which would amount to 10–12.5 percent savings at 5 degrees lower flow temperature.
Does anyone have any ideas on how to calculate this or if there is a website available for such calculations?
My home construction company specified a flow temperature of 40 degrees Celsius (104°F) in the contract "to save money," meaning a larger pipe spacing, probably increasing from 10-15 cm (4-6 inches) to 20 cm (8 inches).
The key data for the house (planned): KfW 55 standard, Daikin Altherma 3 R ECH2O 308/508, annual performance factor around 4, living area 145 m² (1560 ft²) with the basement also heated (this was required for KfW), the basement itself has 80 m² (860 ft²) of usable/floor space, but is not considered living space according to standards due to insufficient natural light.
We have a controlled ventilation system with heat recovery. We also plan to install a photovoltaic system of about 10 kW peak on the roof at some point.
Now I’m wondering if it would be worthwhile to reduce the flow temperature from 40 degrees Celsius (104°F) to 35 degrees Celsius (95°F) given an assumed electricity price of around 45 cents per kWh. The additional costs are about €15 per m² for the upgrade. The crucial point is how much energy savings percentage is achieved per degree of reduction. It is usually assumed to be 2.5 percent per degree, which would amount to 10–12.5 percent savings at 5 degrees lower flow temperature.
Does anyone have any ideas on how to calculate this or if there is a website available for such calculations?
R
RotorMotor6 Mar 2022 07:54hanse987 schrieb:
I think 35 degrees is still too high. For a new build, I would never go above 30 degrees.With KfW 55 standard, this is rarely achievable. Almost always only possible with wall heating and/or additional electric heaters.I set the following requirements:
Maximum supply temperature of 30°C (86°F) in NAT mode
Maximum pipe spacing of 10cm (5 inches) specified
Maximum room temperature of 20°C (68°F) in all rooms
And the smallest possible heat pump matching the heating load
(otherwise I would have a 5 kW instead of a 3.5 kW air-to-water heat pump)
Conclusion: even the 3.5 kW unit still cycles in a KFW-55 house at temperatures above 0°C to 5°C (32°F to 41°F).
The general contractor (GC) also wanted an additional charge for the smaller pipe spacing (compared to 15 cm (6 inches) spacing) of 1500€, which I negotiated down to 500€ by telling him that the smaller heat pump saves him money on the purchase cost. They offset the costs, and I ended up paying less, although reluctantly.
Just try to find out (by Googling) what the next smaller Daikin unit is in terms of capacity. Then compare the prices you find online, maybe that will give you a good basis.
But generally, 40 cm (16 inches) pipe spacing is outrageous and unreasonable, and definitely not state of the art for heat pumps.
With inverter air-to-water heat pumps, it’s important to check on big capacity steps whether both units actually modulate down equally, because often they are almost identical and just limited by software.
For example, a 4 kW unit that delivers a minimum of 2 kW at 0°C (32°F), and an 8 kW unit delivers 5.6 kW.
Always keep an eye on the minimum and maximum flow rates of the heat pump compared to what the underfloor heating design calls for. It’s pointless if the heat pump can deliver 800 l/h at full load but your underfloor heating requires 1500 l/h.
The connection line to the heating manifold should be at least 28 mm copper pipe and not 25 mm plastic pipe "straw" with an inner diameter of 20 mm—you won’t be happy with that. Based on my last experience, if the GC is already trying to cut costs, he will definitely not use copper.
I won’t go into details about the bypass valve and buffer tank since that has already been discussed thoroughly here and in other forums. But hey, maybe he’ll skip those too to save money—then suggest a smaller heat pump plus no bypass valve or buffer tank (and preferably no individual room controls), and instead more underfloor heating pipe.
Maximum supply temperature of 30°C (86°F) in NAT mode
Maximum pipe spacing of 10cm (5 inches) specified
Maximum room temperature of 20°C (68°F) in all rooms
And the smallest possible heat pump matching the heating load
(otherwise I would have a 5 kW instead of a 3.5 kW air-to-water heat pump)
Conclusion: even the 3.5 kW unit still cycles in a KFW-55 house at temperatures above 0°C to 5°C (32°F to 41°F).
The general contractor (GC) also wanted an additional charge for the smaller pipe spacing (compared to 15 cm (6 inches) spacing) of 1500€, which I negotiated down to 500€ by telling him that the smaller heat pump saves him money on the purchase cost. They offset the costs, and I ended up paying less, although reluctantly.
Just try to find out (by Googling) what the next smaller Daikin unit is in terms of capacity. Then compare the prices you find online, maybe that will give you a good basis.
But generally, 40 cm (16 inches) pipe spacing is outrageous and unreasonable, and definitely not state of the art for heat pumps.
With inverter air-to-water heat pumps, it’s important to check on big capacity steps whether both units actually modulate down equally, because often they are almost identical and just limited by software.
For example, a 4 kW unit that delivers a minimum of 2 kW at 0°C (32°F), and an 8 kW unit delivers 5.6 kW.
Always keep an eye on the minimum and maximum flow rates of the heat pump compared to what the underfloor heating design calls for. It’s pointless if the heat pump can deliver 800 l/h at full load but your underfloor heating requires 1500 l/h.
The connection line to the heating manifold should be at least 28 mm copper pipe and not 25 mm plastic pipe "straw" with an inner diameter of 20 mm—you won’t be happy with that. Based on my last experience, if the GC is already trying to cut costs, he will definitely not use copper.
I won’t go into details about the bypass valve and buffer tank since that has already been discussed thoroughly here and in other forums. But hey, maybe he’ll skip those too to save money—then suggest a smaller heat pump plus no bypass valve or buffer tank (and preferably no individual room controls), and instead more underfloor heating pipe.
lesmue79 schrieb:
max. 20°C (68°F) room temperature in all rooms.
Conclusion: even the 3.5 kW still cycles in a KfW-55 house at temperatures above 0°C-5°C (32°F-41°F)Not everyone lives in a “refrigerator.”What exactly do you mean by cycling?
At 0 degrees, in the usual sense, it certainly doesn’t cycle.
And is your system actually running “properly” now…
lesmue79 schrieb:
max. 20°C (68°F) room temperature in all rooms.You mean at least 20 degrees, right?Yes, it’s working properly, with the lowest heating curve set to 21-22°C (70-72°F) everywhere. The bypass valve is removed.
Calculating for a maximum of 20°C (68°F) does not mean you can’t achieve higher temperatures later.
This could be due to a higher supply temperature, electric radiators, or because the heating load is still oversized despite calculated room temperatures of 20°C (68°F).
Calculating for a maximum of 20°C (68°F) does not mean you can’t achieve higher temperatures later.
This could be due to a higher supply temperature, electric radiators, or because the heating load is still oversized despite calculated room temperatures of 20°C (68°F).
Strange approach by your home construction company.
Ours insisted on a floor heating system with closer pipe spacing when choosing the heat pump package. It was practically impossible to purchase them separately.
Because with such a high supply temperature, your heating costs increase significantly. A heat pump (at least an air-to-water heat pump, I’m not familiar with others) operates most efficiently at a low supply temperature. Ours even stays below 30°C (86°F) almost all the time during winter.
Ours insisted on a floor heating system with closer pipe spacing when choosing the heat pump package. It was practically impossible to purchase them separately.
Because with such a high supply temperature, your heating costs increase significantly. A heat pump (at least an air-to-water heat pump, I’m not familiar with others) operates most efficiently at a low supply temperature. Ours even stays below 30°C (86°F) almost all the time during winter.
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