Hello everyone,
After reading here for a while, I am now starting my first thread because I recently became a homeowner. I am currently thinking about basic energy-efficient renovations and have some questions about the fundamental purpose of heat pumps, which I have not really found answers to despite extensive searching and reading here. I have a basic understanding of thermodynamics but no technical knowledge about heating systems. However, I keep coming across statements about heat pumps that contradict my basic understanding of thermodynamics. Threads here or on Google are usually too detailed, and even a conversation with an energy consultant didn’t help much, as everything basically ended with "it depends."
My thoughts and what I know or understand:
What I don’t understand:
I want to exclude aspects like “independence from Russian gas,” “gas will eventually be more expensive than electricity,” and photovoltaics from this discussion for now. I understand those points, but as I said, I first want to clear up my knowledge gaps or logical errors that I apparently have.
After reading here for a while, I am now starting my first thread because I recently became a homeowner. I am currently thinking about basic energy-efficient renovations and have some questions about the fundamental purpose of heat pumps, which I have not really found answers to despite extensive searching and reading here. I have a basic understanding of thermodynamics but no technical knowledge about heating systems. However, I keep coming across statements about heat pumps that contradict my basic understanding of thermodynamics. Threads here or on Google are usually too detailed, and even a conversation with an energy consultant didn’t help much, as everything basically ended with "it depends."
My thoughts and what I know or understand:
- To keep a house at a constant temperature, a certain amount of energy is needed, which is lost through the walls. This energy must be supplied to the house by the heating system. The better the insulation, the lower the heat loss, and therefore, the less energy the heating system has to provide.
- Gas is generally (at least until 06/22) cheaper per kilowatt-hour than electricity.
- If a heat pump and a gas boiler have roughly the same efficiency in heating water, then each kilowatt of heat from one kilowatt-hour of energy source (electricity or gas) should generally be cheaper with the gas boiler at first.
- Heat pumps need underfloor or other surface heating systems for good efficiency, while gas boilers can work with any heating system. Installing underfloor heating does not necessarily mean switching to a heat pump, but it does open the potential for that later without much extra effort.
What I don’t understand:
- In discussions where someone plans to install a heat pump, there is always a warning that if the house has poor insulation, the heat pump will have a negative impact on heating costs. But shouldn’t it always do that? According to my reasoning above, even in a well-insulated house, the gas boiler should be cheaper because gas as an energy source is still less expensive for the small amount of energy needed.
- If my reasoning is wrong, and a heat pump does become more cost-effective than a gas boiler in a well-insulated house (due to some non-linearity in the efficiency of the heat pump or gas boiler) — is there any rule of thumb or benchmark to say roughly from which point a heat pump makes sense (for example, below “100 kWh/(m2*a)” or something similar)?
I want to exclude aspects like “independence from Russian gas,” “gas will eventually be more expensive than electricity,” and photovoltaics from this discussion for now. I understand those points, but as I said, I first want to clear up my knowledge gaps or logical errors that I apparently have.
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BungaSeppel11 Jun 2022 14:35k-man2021 schrieb:
Could it be that your misunderstanding lies in how a heat pump works? A heat pump does not use the electrical energy supplied directly for heating but rather to extract thermal energy from the environment, transfer it into the building, and raise it to a level suitable for heating. It does this so efficiently that the heating energy output is approximately 3 to 5 times the electrical energy input.
The energy content of gas is converted directly into heat through combustion.
At least, that is how I understand it.Yes, you are right, that is correct. So what does this mean overall? As far as I know, gas still costs less than one-third of electricity at the moment. Do heat pumps produce more than three times as much heat as gas heating systems at any operating point (and if so, which one?), based on their energy input?K
k-man202111 Jun 2022 15:30If the entire heat pump heating system is well designed, the average coefficient of performance (annual performance factor) is usually well above 3. Whether a heat pump is cost-effective for your specific situation can possibly be determined by a heating engineer or energy consultant.
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BungaSeppel11 Jun 2022 16:07Okay, I think I’m starting to understand better after looking up the seasonal performance factor. If it’s above 3, the concept basically compensates for the higher electricity costs (as long as these are three times higher).
What does this number mainly depend on? Assuming a typical, modern air-to-air heat pump and continuous use of modern underfloor heating, is there any rough way to express this based on the building’s “energy demand,” which is usually given on energy performance certificates in kWh/(m²*a) (kWh/(ft²*year))?
Something like, “100 corresponds to a seasonal performance factor of 3, 50 to 4, 150 to 2”?
Thanks in advance for the input so far!
What does this number mainly depend on? Assuming a typical, modern air-to-air heat pump and continuous use of modern underfloor heating, is there any rough way to express this based on the building’s “energy demand,” which is usually given on energy performance certificates in kWh/(m²*a) (kWh/(ft²*year))?
Something like, “100 corresponds to a seasonal performance factor of 3, 50 to 4, 150 to 2”?
Thanks in advance for the input so far!
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Deliverer11 Jun 2022 17:30This largely depends on the choice of heat pump, plumbing, and heating surfaces.
If these three factors are well selected, you can achieve between 4 and 5 in any reasonably insulated house (roof and windows). A value of three is basically the worst case with a new heat pump. That means something went wrong. Often it is oversized, has setback controls enabled, includes buffer tanks, or active individual room control... typical plumbing overengineering.
With gas, you must also factor in the gas connection, basic fees, chimney sweeper, and maintenance costs. The efficiency of a gas boiler is rarely at 90%, so you have to take that into account as well. All these extra costs don’t apply with a heat pump. I believe there are only a few old contracts left that can economically compete with a proper heat pump installation. New gas contracts are hard to get anyway, so calculating this is basically pointless.
For the record, one should definitely avoid burning gas, although no one has really done that yet.
P.S. The figures you mentioned above are easily achievable with radiators. With underfloor heating or other surface heating systems, you can add about 1.
If these three factors are well selected, you can achieve between 4 and 5 in any reasonably insulated house (roof and windows). A value of three is basically the worst case with a new heat pump. That means something went wrong. Often it is oversized, has setback controls enabled, includes buffer tanks, or active individual room control... typical plumbing overengineering.
With gas, you must also factor in the gas connection, basic fees, chimney sweeper, and maintenance costs. The efficiency of a gas boiler is rarely at 90%, so you have to take that into account as well. All these extra costs don’t apply with a heat pump. I believe there are only a few old contracts left that can economically compete with a proper heat pump installation. New gas contracts are hard to get anyway, so calculating this is basically pointless.
For the record, one should definitely avoid burning gas, although no one has really done that yet.
P.S. The figures you mentioned above are easily achievable with radiators. With underfloor heating or other surface heating systems, you can add about 1.
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BungaSeppel11 Jun 2022 22:52Deliverer schrieb:
This depends entirely on the choice of heat pump, plumbing, and heating surfaces.
If these three factors are well chosen, you can achieve something between 4 and 5 in any reasonably insulated house (roof and windows). A coefficient of performance (COP) of 3 is practically the worst case for a new heat pump. That means something went wrong. Often it’s oversized, night setback modes are enabled, buffer tanks are installed, or individual room controls are active… Basically, plumbing overengineering.Great, thanks very much for the tips so far! A few detailed questions on this:- By choice of heat pump, do you simply mean the correct size? Or is it just a quality issue (investing more money = better coefficient of performance)?
- What are the disadvantages of night setback and individual room control? Our current heating behavior is mostly “leave the room = turn off the heating.” I understand this wouldn’t work with underfloor heating, but it would be unfortunate if we now have to run the heating all year long just to achieve a good efficiency and still end up using significantly more energy overall compared to our current behavior.
Ps: The numbers mentioned above are easily achievable with radiators. With surface heating systems you can add 1 to the coefficient of performance.So you mean a heat demand of 100 corresponds to a COP of 3, and with underfloor heating even 4? How accurately can energy consultants usually predict this number? Are there often unpleasant surprises?Thanks and best regards
D
Deliverer11 Jun 2022 23:14By "proper heat pump," I mainly mean the correct size. Another great aspect is an environmentally friendly refrigerant, and by far the best choice here is R290. It is the most efficient, cost-effective, and environmentally friendly refrigerant on the market.
It may feel strange at first, but as soon as a house has windows (meaning it does not exceed 200 kWh/m² (64,000 BTU/ft²) energy use), partial heating, night setback, and on-off heating are not only very uncomfortable but actually consume MORE energy. You don’t have to take my word for it—you can easily verify this in various studies.
This is mainly because, especially for heat pumps, such heating behavior requires a higher supply water temperature. The higher this temperature, the less efficient the heating system runs, particularly heat pumps.
If you have a thermal envelope (house), you should keep the entire envelope as evenly heated as possible, 24/7, at your comfort temperature. Alternatively, you can keep it permanently lower if you want to save energy, but not fluctuating back and forth.
You’ve already noticed one reason: with underfloor heating and a properly sized heat pump, you cannot just quickly raise a room’s temperature by 5°C (9°F). It takes a whole day. Secondly, if you only heat the living area and leave many rooms cold, you not only have to supply the energy needed to heat the room itself but also compensate for the adjacent cold walls. This means providing even higher supply temperatures to maintain a comfortable temperature in that one room. This unnecessary overheating of the heating water causes more losses than you will ever save.
So: once you have a condensing boiler or a heat pump, set the desired room temperature directly at the valve (radiator or underfloor heating) via hydraulic balancing, lower the supply water temperature as much as possible, and let the system run!
Regarding your last point about the numbers: those are very rough estimates. You can’t really generalize like that; I just wanted to put it into some perspective.
Energy consultants usually mostly focus on securing subsidies. If you want reliable figures, you need a proper engineering firm. They will calculate your heating load and underfloor heating sizing.
Only with this information can you—or a plumbing professional experienced with heat pumps—select the right heat pump and possibly estimate the annual performance factor.
But even if everything is done correctly, and then you start limiting the heat pump with individual room controls and night setbacks, the annual performance factor will quickly drop again.
Many factors influence this, and by far the most important one is the one you are already starting to influence: your knowledge about your future heating system.
It may feel strange at first, but as soon as a house has windows (meaning it does not exceed 200 kWh/m² (64,000 BTU/ft²) energy use), partial heating, night setback, and on-off heating are not only very uncomfortable but actually consume MORE energy. You don’t have to take my word for it—you can easily verify this in various studies.
This is mainly because, especially for heat pumps, such heating behavior requires a higher supply water temperature. The higher this temperature, the less efficient the heating system runs, particularly heat pumps.
If you have a thermal envelope (house), you should keep the entire envelope as evenly heated as possible, 24/7, at your comfort temperature. Alternatively, you can keep it permanently lower if you want to save energy, but not fluctuating back and forth.
You’ve already noticed one reason: with underfloor heating and a properly sized heat pump, you cannot just quickly raise a room’s temperature by 5°C (9°F). It takes a whole day. Secondly, if you only heat the living area and leave many rooms cold, you not only have to supply the energy needed to heat the room itself but also compensate for the adjacent cold walls. This means providing even higher supply temperatures to maintain a comfortable temperature in that one room. This unnecessary overheating of the heating water causes more losses than you will ever save.
So: once you have a condensing boiler or a heat pump, set the desired room temperature directly at the valve (radiator or underfloor heating) via hydraulic balancing, lower the supply water temperature as much as possible, and let the system run!
Regarding your last point about the numbers: those are very rough estimates. You can’t really generalize like that; I just wanted to put it into some perspective.
Energy consultants usually mostly focus on securing subsidies. If you want reliable figures, you need a proper engineering firm. They will calculate your heating load and underfloor heating sizing.
Only with this information can you—or a plumbing professional experienced with heat pumps—select the right heat pump and possibly estimate the annual performance factor.
But even if everything is done correctly, and then you start limiting the heat pump with individual room controls and night setbacks, the annual performance factor will quickly drop again.
Many factors influence this, and by far the most important one is the one you are already starting to influence: your knowledge about your future heating system.
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