ᐅ Comparison of Different Heating Systems

Under optimal conditions, Legionella bacteria can double approximately every 3 hours. This means that from a single Legionella, a colony of 65,000 bacteria can develop within 48 hours.

Well, when I go on vacation, I prefer to maintain 60 degrees Celsius (140°F). Running a few liters of water through a 300-liter (79 gallons) or larger water storage tank in the basement is by far not sufficient as a precaution.

You can try to justify it, but I am not a fan of these lukewarm water storage units because of this issue.

When using the heating element for the Legionella control program, the temperature is always maintained at 60 degrees Celsius (140°F). So that means hundreds of extra euros per year.

Nutshell schrieb:

Under optimal conditions … Legionella bacteria can double approximately every 3 hours. This means that one Legionella cell can multiply into a colony of 65,000 within 48 hours….

Yep. But that requires stagnant water at around 35 degrees Celsius (95°F). In a single-family home with about 50 degrees Celsius (122°F) in the hot water tank and daily use, the risk is practically nonexistent.

Nutshell schrieb:

Well, when I go on vacation, I prefer to keep it at 60 degrees Celsius (140°F). Running a few liters through a 300-liter (79 gallons) or larger hot water tank in the basement is by far not sufficient as a measure.

The temperature in the hot water tank doesn’t change during a vacation either, so the Legionella control program continues. Where exactly is the problem?

Nutshell schrieb:

You can sugarcoat it all you want… I’m not a fan of these lukewarm storage tanks because of this issue.

So you’re not a fan of standard practice and everyday reality (installed hundreds of thousands of times every year) or the future, where heat pumps will be standard anyway. That’s your choice, but you’ll be like a rare breed wearing double safety belts. And you don’t trust scientific facts from the Robert Koch Institute or studies on Legionella infections in single-family homes (under 100 cases per year with an average occupant age over 60, likely also not heat pump users)? Then size your heating system accordingly and spend extra money on high-temperature hot water production. I’m not trying to convince you otherwise, but other readers might find this thread useful to form their own opinion.

Nutshell schrieb:

Using the heating element for the Legionella program keeps the water at 60 degrees Celsius (140°F) all the time. That means hundreds of dollars extra per year.

How do you calculate hundreds of dollars extra? My estimate:
- In almost all regions in Germany, there is no deep winter from March to November, so the heating element isn’t needed by default.
- Then there are a few weeks (not the entire country) where daytime temperatures average between -10 and -20 degrees Celsius (14°F to -4°F).
- Finally, we are talking about raising the water temperature for the Legionella program from 50 degrees Celsius (122°F) (normal hot water temp) to 60 degrees Celsius (140°F) once a week. With a typical 3kW heating element, this amounts to weekly costs of about 50 cents.
- From my own heat pump experience: I have never used the heating element in winter except for defrosting the system, which is specific to my heat pump manufacturer.

Of course, you can turn everything into a problem. But then you leave a factual path. And that’s exactly what you’re doing. As I said, do what you think is right. You won’t be doing anything wrong or harmful. It’s just not necessarily required.

When you return from your summer holiday—usually in countries where worrying about water quality would be much more worthwhile—you could run the legionella program and sadistically boil and eliminate your newly grown colony along with its reigning queen. Then, continue using the water as usual in everyday life as if nothing had happened. This does not cost hundreds of euros.

If Legionella is truly the only issue, he could simply install a hygienic storage tank or a domestic water station. These only reduce efficiency by a few percent and have no problems with Legionella.

You could also simply run a circulation pump for a few hours each day (mine consumes 7 watts) and control it with a timer via the air-to-water heat pump. It is true that Legionella bacteria only multiply in stagnant water. If the water is constantly moving, they do not develop. Therefore, maintaining 60°C (140°F) is not necessarily required.

Based on our recent analyses (I work professionally with water circuits), bacterial growth is no longer detectable above 50°C (122°F).

For about 50€ (around $55), you can have your water sample tested at a local laboratory as a private individual if you want to be on the safe side. I do this annually and have excellent water quality.

This discussion about legionella somehow reminds me of the asbestos panic. It feels abstract, exaggerated, mostly not really relevant, and fairly easy to manage in private households.

Now, since heat pumps don’t have chimneys, the “black mascot” shows up once a year at single-family homes to carry out the government-mandated legionella inspection for 75 euros plus lab fees and travel costs.