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.
M
motorradsilke12 Jun 2022 08:39BungaSeppel schrieb:
Yes, you're right, that's correct. So what does that mean overall? As far as I know, gas currently costs less than one-third of electricity. Do heat pumps really produce more than three times as much heat as gas heaters at any operating point (and if so, which one) when comparing their energy input? That’s not the case everywhere. We pay around 20 cents per kWh for electricity for the heat pump. My son, in the same town, pays 11 cents per kWh for gas.
If you can even still get a gas contract.
It really comes down to the supply temperature and energy costs. The rest can be calculated. With my underfloor heating, the supply temperature is still too high. I would probably achieve an annual performance factor of about 3.
However, I would need 4 to be on par with gas. I have a heating load of 8 kW at natural gas temperature. The small Geisha unit wouldn’t be sufficient again.
At the moment, the investment would be too high to be economical.
By the way, it wasn’t economical for me five years ago either; otherwise, I would have installed underfloor heating back then. Well, so be it.
However, I would need 4 to be on par with gas. I have a heating load of 8 kW at natural gas temperature. The small Geisha unit wouldn’t be sufficient again.
At the moment, the investment would be too high to be economical.
By the way, it wasn’t economical for me five years ago either; otherwise, I would have installed underfloor heating back then. Well, so be it.
D
Deliverer12 Jun 2022 17:47There is no need to immediately remove existing gas heating systems. There simply are not enough sanitary units and heat pumps available.
What matters is that from NOW ON, no new gas heating systems are installed in new buildings or during any upcoming heating system replacements. It does not matter at all what is the most cost-effective option at a given time.
We can discuss (though it won’t change anything...) whether we need to be climate neutral by 2035, 2040, or 2045. But we cannot dispute THAT we have to be.
So make sure to set the course correctly!
What matters is that from NOW ON, no new gas heating systems are installed in new buildings or during any upcoming heating system replacements. It does not matter at all what is the most cost-effective option at a given time.
We can discuss (though it won’t change anything...) whether we need to be climate neutral by 2035, 2040, or 2045. But we cannot dispute THAT we have to be.
So make sure to set the course correctly!
B
BungaSeppel13 Jun 2022 10:57Okay, thank you all very much for your input here.
I think I will try to involve an engineering firm in the renovation planning to design all the energy efficiency measures and underfloor heating for the best possible annual performance factor.
For now, I will continue heating with the existing gas boiler, which is still well maintained, and at some point, when it becomes financially worthwhile (for example, if the old gas heating breaks down) and after gaining experience over one or two winters regarding how warm the house stays and how our heating habits stabilize, I will switch systems!
I think I will try to involve an engineering firm in the renovation planning to design all the energy efficiency measures and underfloor heating for the best possible annual performance factor.
For now, I will continue heating with the existing gas boiler, which is still well maintained, and at some point, when it becomes financially worthwhile (for example, if the old gas heating breaks down) and after gaining experience over one or two winters regarding how warm the house stays and how our heating habits stabilize, I will switch systems!
D
Deliverer13 Jun 2022 18:29Yep, taking some time and planning well certainly doesn’t hurt. If defects start to accumulate or there is no more gas, then switch.
Deliverer schrieb:
At first, it feels strange, but as soon as a house has windows (i.e., not >200 kWh/sqm), partial heating, night setback, and on-off heating are not just very uncomfortable, they actually consume MORE energy. You don’t have to take my word for it; you can look it up in various studies yourself.
This is mainly (and especially for heat pumps) because this type of heating behavior requires a higher supply temperature. The higher it is, the less efficient the heating system runs, especially heat pumps.
If you have a thermal building envelope (house), you should maintain the entire envelope as evenly as possible, 24/7, at your comfort temperature. Or preferably lower continuously if you want to save energy, but not fluctuating back and forth.
You already recognized one reason: with radiant heating and a properly sized heat pump, you can't just quickly raise a room temperature by 5°C (9°F). It takes a whole day. Second point: if you have many cold rooms and only heat the living area, you not only have to put in the energy actually needed for the room but also compensate for the adjacent cold walls. So, you need even higher supply temperatures to keep that one room reasonably comfortable. And this actually unnecessary overheating of the heating water creates more losses than you could ever save. This immediately makes me think of a comparison: the on-off behavior is somewhat like constantly accelerating and coasting in low gear with a combustion engine vehicle instead of maintaining a steady speed at low RPM.
Regarding heat pumps: from what I’ve heard increasingly in conversations with neighbors in similar houses, they experience about 80-100% higher consumption with the heating installer’s default settings. We keep it around 23°C (73°F) room temperature. So there’s still some room for optimization with the settings.
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