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?
D
Deliverer6 Mar 2022 09:57kati1337 schrieb:
Because with such a high flow temperature, your heating costs will increase significantly. The plumber will probably be very sorry about that! ;-)
So – I think it’s clear that almost everyone here, including myself, actually considers a 40°C (104°F) flow temperature to be outrageous. BUT, for the sake of completeness, I wanted to mention that just last year a study was conducted. It was about the suitability of heat pumps for older buildings and up to what flow temperature heat pumps can be used effectively.
And to my surprise, it showed that for modern air-to-water heat pumps, the efficiency difference between 35°C (95°F) and 55°C (131°F) flow temperature is only 13%. So it’s very likely that a 40°C (104°F) flow temperature won’t bankrupt you. I’m not allowed to share the link, but the full study can be found online.
Maybe this helps.
R
RotorMotor6 Mar 2022 10:05lesmue79 schrieb:
Calculated for a maximum of 20° does not mean that you cannot actually achieve more later on.It just means the calculation has been made optimistically.RotorMotor schrieb:
It's just conveniently calculated then.Viewed that way, yes, but I can still raise the supply temperature later if I want or need to.
If I set the system to 35°C (95°F) with a 15cm/20cm (6 inches/8 inches) pipe spacing and later want to lower it to 30°C (86°F), I can only go as low as the pipe spacing and other factors allow. Once the screed is in, I can't change the pipe spacing anymore, but I can still adjust the supply temperature.
Raising it is always possible, lowering it is not...
Especially when working with a general contractor (GC) or a prefab house manufacturer, you often encounter little flexibility with such special requests. The usual argument is, "We've always done it this way, it worked fine for 20 years, so running at 35°C (95°F) or, as in this example, 40°C (104°F) supply temperature will be fine for the next twenty years as well."
But as you said, you can calculate it however it suits you.
S
Stefan0016 Mar 2022 10:49HoisleBauer22 schrieb:
The investment would only have paid off after about 24 years.Unlike solar systems, I would expect the service life of underfloor heating to be significantly longer, over 30 years, so I would still consider payback after 24 years a good investment.Especially if you can then switch to a smaller heating system, it might become even more cost-effective. Try discussing the whole topic again with your general contractor.
lesmue79 schrieb:
If I design the system for 35°C (95°F) and a pipe spacing of 15cm/20cm (6 inches/8 inches), and later want to reduce it to 30°C (86°F), it will only work down to the point where the pipe spacing and other factors allow.Why would anyone want to reduce the target temperature (supply temperature) later? That doesn’t make any sense! The design is done now.Similar topics