ᐅ Is a buffer tank useful for a heat pump?

RotorMotor schrieb:

So you have 170m² (1830 sq ft) of heated floor area with screed.
Assuming 100kg/m² (20.5 lb/sq ft), that’s 17 tons of “buffer mass.”
If you now add 500 liters (130 gallons), that’s about 3% more.

I am familiar with the idea of operating heat pumps in single-family homes without a buffer tank. The buffer tank reduces efficiency slightly. However, it is indisputable that a buffer tank makes the overall heating system more "robust."

The calculation in the quote can be done this way, but the consideration and conclusion are not 100% correct. For one thing, it overlooks that water mixes with water inside the buffer tank, meaning heat transfer within the buffer tank occurs much faster than the heat release from the water through the outer pipes in the screed.

Especially in autumn and spring, a buffer tank can help reduce frequent cycling of the heat pump.

For example: roughly, the heat pump can “charge” about 11.5 kWh into a 1000-liter (260 gallon) buffer tank when heating it from 35 to 45°C (95 to 113°F). By comparison: on a day in February, solar gains may mean many rooms in the house require very little heating—in that case, without a buffer, the heat pump will run much shorter cycles and tend to cycle on and off more frequently.

If the house has photovoltaic panels or uses a variable electricity provider like Tibber, it can be interesting overall to charge the buffer tank during the day or at times with low electricity prices.

Since the existing system apparently experiences very frequent cycling, a buffer tank could at least help reduce cycling, possibly as a combination tank with a stainless steel coil inside for on-demand domestic hot water production. But someone with expertise on site needs to assess this, as remote diagnoses can be quite misleading.

BobRoss schrieb:

...a buffer tank could at least help with cycling issues, possibly as a combined storage tank with a stainless steel coil inside the buffer for on-demand domestic hot water production.

Thank you for the reply!

I also found combined storage tanks interesting. However, according to the German distributor, the heat pump is apparently not "compatible" with combined storage tanks.

I originally thought that with modulating heat pumps using inverter technology, a buffer tank wouldn’t be necessary, but if it solves the issue for us, that’s fine by me. The heat pump is sized at 3–12 kW, which is simply too large for the 6.3 kW heating demand.

BobRoss schrieb:

It is undisputed that a buffer tank makes the entire heating system more "robust."

"Robust" against planning errors? Robust against high consumption, definitely not.

BobRoss schrieb:

You can do the calculation in the quote like that, but the reasoning and conclusion are not 100% correct. For one, it ignores that water mixes within the buffer tank, meaning heat transfer inside the buffer tank happens much faster than the heat release from the water through the heating coil walls into the screed.

"For one," but where is the "for another"?

And the "for one" is actually pretty irrelevant.

BobRoss schrieb:

As an example: roughly, the heat pump can "charge" about 11.5 kWh into a 1000 L (264 gallons) buffer tank when heating it from 35 to 45 degrees Celsius (95 to 113°F).

1000 L (264 gallons) is of course very large. In addition, many heat pumps operate at only half the efficiency at 45 degrees Celsius (113°F) compared to 35 degrees Celsius (95°F).
You could argue that this further reduces cycling, but whether you want to achieve that with double the electricity consumption is highly questionable.

BobRoss schrieb:

If the house has photovoltaic panels or uses a variable electricity provider like Tibber, it might make sense overall to charge the buffer tank during the day or at low-price times.

Who actually does that? Such a complex control system?
The original poster does not even seem able or willing to handle a simple control setup right now.
And what’s the point with half the efficiency?

BobRoss schrieb:

Since the existing system apparently experiences very frequent cycling, a buffer tank could at least help reduce cycling — maybe as a combined tank with a stainless steel coil inside for on-demand domestic hot water. But this needs to be evaluated by someone knowledgeable on site; remote diagnoses can be quite off.

Really bad idea. Halving efficiency permanently in the hope of saving a few cycles during heating.
Besides, you’d first have to clarify what the heat pump can handle — whether it even supports these features like overcharging that you recommend.

@jx7: What have you tried already in terms of settings optimization?
Using time slots?
Tried "time to on next"?
ALL valves open?

RotorMotor schrieb:

The original poster doesn’t even seem able or willing to understand the simple control system.
@jx7: What settings have you already tried optimizing?
Working with time windows?
Time to on next tried?
ALL valves open?

I find the first sentence somewhat inappropriate. I studied mathematics and physics and enjoy delving into the processes involved.

At the moment, I can’t try anything because the heating system is broken.

Would the following be a good strategy?

Since January, we have connected a pool to the heating system. For the pool, I need to specify how many minutes per hour it should be supplied. It was last set to 15 minutes. Of course, I could increase that to 30 minutes and lower the supply temperature for the pool accordingly (for example, reduce it from 35°C (95°F) to 33°C (91°F)). Then the heating system would have a forced pause of 30 minutes every hour, without needing to stop the compressor, meaning that compared to operating without the pool, the heating work would need to be done in half the time. This could compensate for the oversizing.

Or am I mistaken?

jx7 schrieb:

I find the first sentence a bit inappropriate.

Really? I don’t think so.
You started the first thread a year ago reporting that the compressor of your heating system was broken.
So you have had a year to improve the settings,
and also to obtain reasonable alternative quotes.
But the repair is only scheduled for tomorrow?

Another issue is that your threads are very fragmented.
You drop bits of information here and there and then omit details in other threads.

A more systematic approach would definitely be advisable here.

RotorMotor schrieb:

You created the first thread a year ago saying that the compressor in your heating system was broken.
So you’ve had a year to improve the settings.

But the repair is only happening tomorrow now?

That’s exactly the dilemma. For a whole year, I have tried to get:
- the installing company
- the German distributor of Ecoforest
- heating engineers not working with Ecoforest
to reset my heating system so that it cycles less frequently, but no one wanted to do it.

Finally, the heating engineer who connected the pool promised to review the settings, possibly with phone support from Enrgi (the German distributor of Ecoforest). This was commissioned in June and started in September. However, the work was only completed in January. The pool has only been connected since January. The final step, reconfiguring the heat pump, did not happen because now the heating system is broken.

The repair appointment was initially canceled because Enrgi insisted on advance payment and said they would only come once the payment was received. Since the transfer could not be processed that quickly, the appointment was canceled again.

Since the costs are very high:
- Compressor replacement €4,800
- Possibly purchase of a 200-liter (53-gallon) buffer tank for €2,400 plus mixing kit €600 plus 3 temperature sensors €90
- Installation of the buffer tank ??? €
Total around €8,000

we are currently obtaining quotes for a new heat pump:
https://www.hausbau-forum.de/threads/waermepumpe-fuer-5-000-7-500-eur-reparieren-oder-durch-neue-waermepumpe-ersetzen.46862/

This will probably be even more expensive, but we could switch to a larger manufacturer with a better service network and get a new heat pump, this time properly sized, for example 2–6 kW instead of 3–12 kW, with a new 5-year warranty.

With a repair, there would still be a risk that the heating system would cycle frequently even with the buffer tank and that the compressor might fail again in 7–8 years. (Or would a buffer tank almost completely prevent that?)