ᐅ The dilemma of choosing...

Oh dear, I think the Sandman has already been here.. I’ll try to rephrase the question :-)

Is there a temperature at which the heat pump (air-to-water heat pump) requires more electricity than the heat output it produces, meaning the COP drops below 1? Or in other words, from what temperature onwards is the pump no longer cost-effective?

Also, is there definitely a temperature at which the electric heating element in the pump has to run?

satisfied schrieb:


1. Since I don’t think the additional investment is worthwhile, I am currently leaning against applying for a KfW financing, but maybe I’ll change my mind 🙂

2. At what outdoor temperature does an air-to-water heat pump switch entirely to electric heating?

3. Would a combination of a controlled mechanical ventilation system and an air-to-water heat pump be a good choice (Viessmann offers something like this), and would it make sense to power the air-to-water heat pump with a photovoltaic system?

4. Or is it more economical to rely on a condensing gas boiler and possibly supplement it with a small air-to-water heat pump paired with photovoltaics? (This setup was recommended by my heating engineer)

Good morning!

Regarding 1: Your financial advisor or lender should calculate this for you. Generally, this cannot be answered directly, as it depends on many factors. Any tip given here wouldn’t be particularly reliable.

Regarding 2: You probably mean at what temperature the air-to-water heat pump requires the electric backup heater. This depends on the system itself (manufacturer, model), local conditions (weather), heating and hot water demands, and the house. A small example: At -10°C (14°F) outdoor temperature, the system might still produce a flow temperature of 32°C (90°F) for heating, but producing 57°C (135°F) hot water may be too much. The next day, the dew point may cause the outdoor unit to ice up, and then the backup heater kicks in as well.

My boss has an 8-year-old air-to-water heat pump at his home. From about -3°C (27°F) it switches to the electric backup heater. Today, he regrets choosing this system due to very high electricity costs. His house is similarly well insulated as mine, but his monthly heating costs are almost double compared to mine with a gas condensing boiler. He cannot achieve more than 42°C (108°F) hot water in winter without the backup heater running or the outdoor unit icing. However, much has improved technically since then—modern air-to-water heat pumps are much better! Over the year, a good system now reaches a coefficient of performance (COP) above 3, often between 3.3 and 3.5. Therefore, operating costs are roughly on par with a gas boiler at current gas prices, even if the backup heater sometimes has to operate on a few cold days (roughly between -5°C and -9°C (23°F and 16°F) for heating, and around -5°C (23°F) for hot water is a very rough benchmark).

Regarding 3: Opinions will vary here. a) About photovoltaics: In my view, combining a photovoltaic system with an air-to-water heat pump makes sense (spring, summer, autumn). How much it contributes in winter depends on location, weather, snow cover, etc. Personally, I am a fan of photovoltaic systems with high self-consumption (I’m planning to install one myself), so my view here might not be completely objective.

b) The combination of an air-to-water heat pump with controlled mechanical ventilation is often offered. It is attractive since it saves space. However, I don’t think a combined unit is optimal and would personally choose separate devices (if I had an air-to-water heat pump), but this is a matter of personal preference. On the other hand, I would never want to do without controlled ventilation with heat recovery, but again, that’s personal taste.

Regarding 4: This is certainly a neat and elegant solution that will please technicians. With a small downside: high initial investment! Getting a gas connection (+$2,000), buying the burner (+$3,000), installing both systems and integrating them (not to be underestimated!). Also, the gas meter’s standing charges add to the electricity meter fees (around $180–200/year), which are high fixed costs given likely low gas consumption—you could buy a lot of electricity for that money alone. The heating engineer and electrician will be happy about the several thousand dollars in extra revenue. You will very likely not recoup these additional costs quickly, especially since a modern air-to-water heat pump has a COP over 3; you are interested in annual costs, not a few cold days during Bavarian snowy weather.

My personal advice to you, summarized:

- A good air-to-water heat pump with a high COP from a reputable manufacturer
- A photovoltaic system (possibly small, just for the heat pump), and if you enjoy it, a larger one with a small battery buffer as a bonus (spend your money here rather than on option 4)
- Controlled mechanical ventilation with heat recovery and the air-to-water heat pump as separate systems

Best regards, have a great start to the weekend

Thorsten

@T21150 Where do you need 57°C (135°F) warm water? You can't really shower with that. 45°C (113°F) is more than enough and already quite hot. That way, you save money again :-)

Hi TomTom: I agree with you, I also set the hot water to exactly the 45 degrees Celsius (113°F) you mentioned, and the shower goes up to 37 degrees Celsius (99°F). Only for a full bath do I raise it to 60 degrees Celsius (140°F) with a one-time boost, because otherwise the hot water runs out while filling the large tub. By the way, the 57 degrees Celsius (135°F) was just an example with deliberate exaggeration.

Best regards
Thorsten

Thank you, Thorsten, for the detailed response :-)

Are there any significant advantages or disadvantages to an air-to-water heat pump installed indoors? I've also heard of people who extract air through an open basement window and do not run a duct to the outside. Is that allowed, and does it have any benefits?

Scope of Work = Scope of Work

I just wanted to say that it’s pointless to argue about something you might not even receive or, in any case, don’t know what it is.