Hello
We are building a KfW 70 house. We have a lot of window area, so the calculations showed that we need to take additional measures to meet the KfW 70 standard.
We decided against a ventilation system, but instead opted for extra panels on the roof, not only to supply hot water via collectors but also to support heating.
However, the 300-liter (79 gallons) tank has now been replaced by a 750-liter (198 gallons) water storage tank. That seems huuuuge! Does the tank’s capacity affect the calculations? I haven’t been able to reach the engineering firm yet to ask, so I wanted to get some input from the forum first.
We are building a KfW 70 house. We have a lot of window area, so the calculations showed that we need to take additional measures to meet the KfW 70 standard.
We decided against a ventilation system, but instead opted for extra panels on the roof, not only to supply hot water via collectors but also to support heating.
However, the 300-liter (79 gallons) tank has now been replaced by a 750-liter (198 gallons) water storage tank. That seems huuuuge! Does the tank’s capacity affect the calculations? I haven’t been able to reach the engineering firm yet to ask, so I wanted to get some input from the forum first.
That may be true... but for various reasons, we decided against installing the system. For one, we wouldn’t be able to use it as intended. My wife and I are fresh air enthusiasts and we constantly ventilate. The ventilation system wouldn’t stand a chance against us :-) In other words, we wouldn’t stop ventilating just because we had a ventilation system.
Secondly, what happens if the ductwork was poorly done during construction? You can never access it again. What if something settles or nests in there? These were all considerations that don’t necessarily apply to everyone... And in the past, houses were built without ventilation systems anyway ;-)
Secondly, what happens if the ductwork was poorly done during construction? You can never access it again. What if something settles or nests in there? These were all considerations that don’t necessarily apply to everyone... And in the past, houses were built without ventilation systems anyway ;-)
Hi,
it is normal for the hot water storage tank to become significantly larger when additional heating support is included. Common sizes are about 600 liters (160 gallons) for heating and 300 liters (80 gallons) for domestic hot water. In this context, 750 liters (200 gallons) is rather small, but may be appropriate with low heating demand thanks to good insulation.
The reason is that when heating support is included, the storage tank also needs to hold a heat reserve for the heating circuit. This reserve usually has to be sized larger than the buffer for domestic hot water.
it is normal for the hot water storage tank to become significantly larger when additional heating support is included. Common sizes are about 600 liters (160 gallons) for heating and 300 liters (80 gallons) for domestic hot water. In this context, 750 liters (200 gallons) is rather small, but may be appropriate with low heating demand thanks to good insulation.
The reason is that when heating support is included, the storage tank also needs to hold a heat reserve for the heating circuit. This reserve usually has to be sized larger than the buffer for domestic hot water.
Hello,
In general, it is well known that for energy storage, both the state of charge of the storage and the supply or demand power are important factors.
Initially, it does not matter whether the storage is thermal, batteries, or for example, pressure vessels. The basic physical principles are initially completely identical (exponential function)!
If the storage is completely empty, the theoretically available power can be easily used to raise the storage to a desired energy potential.
While batteries and pressure vessels are supplied by an electric motor with constant power, the situation is completely different for solar thermal power input.
The irradiance in a solar thermal system follows a seasonal and daily pattern.
Especially the daily pattern, depending on the storage’s state of charge, prevents the actual use of the solar thermal input, as well as comfort behavior. Who wants to shower in the morning with water at 10°C (50°F) temperature (storage = empty)?
Assuming a comfortable domestic hot water temperature of 48…55°C (118…131°F) in the tank, depending on size (charging temperature is higher!), all solar thermal power inputs below the charging temperature are not usable at all!
Unfortunately, theoretical idealized calculations often do not consider this ;-)
During the day, irradiance usually peaks around midday. Until then, irradiance can be used depending on the storage’s state of charge. After passing the peak, there are hardly any possibilities to use the solar power input economically.
Unfortunately, theoretical idealized calculations also do not consider this either ;-)
This results in a significant deficit in the annual energy balance!
More than 80% of single-family household users leave in the morning and return in the evening. This means solar thermal input is barely usable during these times!
In reality, this leads to only about an 8…12% reduction in domestic hot water demand.
For commercial buildings or single-family homes with warm water usage distributed throughout the day, the situation looks much more favorable!
A “back-of-the-envelope calculation,” although I usually dislike these, should at least provide a rough orientation:
Single-family house, domestic hot water demand 2400 kWh/year (without solar thermal system):
Gas condensing boiler, ($0.07/kWh) => consumption 2900 kWh/year => $203/year
Air-source heat pump, ($0.20/kWh) => consumption 686 kWh/year => $137/year
Ground-source heat pump, ($0.20/kWh) => consumption 533 kWh/year => $107/year
With a moderate, realistic coverage of necessity (10%) by the solar thermal system for domestic hot water, the following “savings effects” result:
Gas condensing boiler: $20/year
Air-source heat pump: $14/year
Ground-source heat pump: $11/year
With an investment of $5,000…7,000 for the solar thermal domestic hot water system, the following payback periods result:
Gas condensing boiler: 300 years
Air-source heat pump: 429 years
Ground-source heat pump: 545 years
Who would consider installing a solar thermal system for domestic hot water with these numbers, especially when financed externally, and without considering depreciation, electricity consumption for control systems and pumps until then ;-)
Naive or optimistic people may hardly understand this, especially when bought as a general contractor (GC) or main contractor (MC) package. Intelligent builders definitely see this differently and strive for independent, objective advice, planning, and dimensioning, even if it costs a few dollars!
Best regards.
Explosiv schrieb:
....The background is that when supporting heating, a heat reserve is also stored in the tank for the heating circuit. And this usually needs to be larger than the buffer for domestic hot water.
In general, it is well known that for energy storage, both the state of charge of the storage and the supply or demand power are important factors.
Initially, it does not matter whether the storage is thermal, batteries, or for example, pressure vessels. The basic physical principles are initially completely identical (exponential function)!
If the storage is completely empty, the theoretically available power can be easily used to raise the storage to a desired energy potential.
While batteries and pressure vessels are supplied by an electric motor with constant power, the situation is completely different for solar thermal power input.
The irradiance in a solar thermal system follows a seasonal and daily pattern.
Especially the daily pattern, depending on the storage’s state of charge, prevents the actual use of the solar thermal input, as well as comfort behavior. Who wants to shower in the morning with water at 10°C (50°F) temperature (storage = empty)?
Assuming a comfortable domestic hot water temperature of 48…55°C (118…131°F) in the tank, depending on size (charging temperature is higher!), all solar thermal power inputs below the charging temperature are not usable at all!
Unfortunately, theoretical idealized calculations often do not consider this ;-)
During the day, irradiance usually peaks around midday. Until then, irradiance can be used depending on the storage’s state of charge. After passing the peak, there are hardly any possibilities to use the solar power input economically.
Unfortunately, theoretical idealized calculations also do not consider this either ;-)
This results in a significant deficit in the annual energy balance!
More than 80% of single-family household users leave in the morning and return in the evening. This means solar thermal input is barely usable during these times!
In reality, this leads to only about an 8…12% reduction in domestic hot water demand.
For commercial buildings or single-family homes with warm water usage distributed throughout the day, the situation looks much more favorable!
A “back-of-the-envelope calculation,” although I usually dislike these, should at least provide a rough orientation:
Single-family house, domestic hot water demand 2400 kWh/year (without solar thermal system):
Gas condensing boiler, ($0.07/kWh) => consumption 2900 kWh/year => $203/year
Air-source heat pump, ($0.20/kWh) => consumption 686 kWh/year => $137/year
Ground-source heat pump, ($0.20/kWh) => consumption 533 kWh/year => $107/year
With a moderate, realistic coverage of necessity (10%) by the solar thermal system for domestic hot water, the following “savings effects” result:
Gas condensing boiler: $20/year
Air-source heat pump: $14/year
Ground-source heat pump: $11/year
With an investment of $5,000…7,000 for the solar thermal domestic hot water system, the following payback periods result:
Gas condensing boiler: 300 years
Air-source heat pump: 429 years
Ground-source heat pump: 545 years
Who would consider installing a solar thermal system for domestic hot water with these numbers, especially when financed externally, and without considering depreciation, electricity consumption for control systems and pumps until then ;-)
Naive or optimistic people may hardly understand this, especially when bought as a general contractor (GC) or main contractor (MC) package. Intelligent builders definitely see this differently and strive for independent, objective advice, planning, and dimensioning, even if it costs a few dollars!
Best regards.
Hi
This assumes that the hot water storage tank regularly cools down overnight and then needs to be reheated by the heating system in the morning.
I believe that with a well-insulated tank and no or nighttime switched-off circulation, this situation doesn’t occur very often. Especially not during the warm season, when the tank is heavily heated around midday and maintained at a high temperature until the evening.
What charging temperatures can be expected at the storage tank around midday? I guess well above 60°C (140°F).
For me, it will probably be the case that one of us showers in the morning during the week and the other more around midday, with both tending to shower around midday on weekends. Hopefully, this will raise the efficiency well above 10%. When asking around, everyone I know who uses solar thermal systems for hot water says that the heating system is completely off during the warm season, yet they still have good hot water temperatures in the morning for showering.
I won’t be able to share my own experience until next year.
This assumes that the hot water storage tank regularly cools down overnight and then needs to be reheated by the heating system in the morning.
I believe that with a well-insulated tank and no or nighttime switched-off circulation, this situation doesn’t occur very often. Especially not during the warm season, when the tank is heavily heated around midday and maintained at a high temperature until the evening.
What charging temperatures can be expected at the storage tank around midday? I guess well above 60°C (140°F).
For me, it will probably be the case that one of us showers in the morning during the week and the other more around midday, with both tending to shower around midday on weekends. Hopefully, this will raise the efficiency well above 10%. When asking around, everyone I know who uses solar thermal systems for hot water says that the heating system is completely off during the warm season, yet they still have good hot water temperatures in the morning for showering.
I won’t be able to share my own experience until next year.
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