ᐅ Heat Pump Experiences – Your Assessment?

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:

• 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.

The efficiency of a heat pump decreases as the supply temperature rises, and a poorly insulated house will require a relatively high supply temperature.

If my considerations above are incorrect, and the heat pump somewhere within the well-insulated area becomes more cost-effective than the gas boiler (due to some nonlinearity in the efficiency of the heat pump or gas), is there any general benchmark at which point it can be said that a heat pump roughly pays off?

As of now, gas will always be cheaper than a heat pump based on your assumptions. Of course, this could change at some point with rising gas prices...

BungaSeppel schrieb:

…but as I said, I first want to clarify my knowledge gaps or the misunderstanding I seem to have.

Could it be that your misunderstanding lies in how a heat pump works? A heat pump doesn’t use the electrical energy directly for heating, but instead uses it to extract heat energy from the environment, transfer it into the house, and raise it to a temperature suitable for heating. It does this so efficiently that the heat output is approximately 3 to 5 times higher than the electrical input.

The energy content of gas, on the other hand, is converted directly into heat through combustion.

At least, that’s how I understand it.

hanse987 schrieb:

The efficiency of a heat pump decreases as the flow temperature increases, and a poorly insulated house requires a relatively high flow temperature.

Yes, I understand that much already. But what exactly does that mean? Is there a point where the heat pump overall outperforms the gas heating system? Because the latter doesn’t improve just because the flow temperature is low (in a gas/underfloor heating combination, I can also set a low flow temperature for the gas system)? And if so, where is that point?

seat88 schrieb:

"If my reasoning above is wrong, and the heat pump somewhere within the well-insulated area becomes more cost-effective than the gas boiler (due to some non-linearity in the efficiency of the heat pump or gas) – is there a general guideline indicating from which point a heat pump roughly pays off?"

As it stands now: gas will always be cheaper than a heat pump according to your calculation. Of course, this could change if gas prices rise at some point....

Okay! But if that’s the case: why would anyone install a heat pump at all? Is it solely based on the speculation that later on, when the gas price is so high that the calculation shifts, there will no longer be any subsidies?