ᐅ Assessment of the Heating Concept for a KfW 70 Energy-Efficient House

Bieber0815 schrieb:
V ~ r³, A ~ r². V/A grows faster than A. Heat loss relative to the volume approaches zero for an infinitely large tank (this is the theoretical limit).

Not exactly, V/A increases with r (as you correctly calculated), but A also increases with r² (as you mentioned yourself), which means heat losses increase linearly with r.
So, doubling the radius gives you four times the volume, but also double the loss. The ratio improves, but the absolute loss is still greater. (Assuming a cylinder, roughly estimating with the side surface area much larger than the end caps)

An infinitely large tank unfortunately also has an infinite surface area to lose heat through, and how to compensate for infinitely large energy losses… well XD

Bieber0815 schrieb:
Yes, that is true, but in my opinion, the interesting factor is the losses relative to the stored heat. The source term in the energy balance (the heat supplied electrically by the heat pump, which should be independent of the surface-to-volume ratio) is in my opinion irrelevant.

It does matter, because the absolute losses are larger. If you store 500 liters (132 gallons) but only need 250 liters (66 gallons), the unused 250 liters unfortunately also cool down and have to be reheated. So despite better efficiency, you lose more energy in absolute terms. The calculation only works if you actually use the full 500 liters more frequently. For the balance, what counts in the end is (energy input) / (used hot water), and with higher absolute losses you have to use more energy.

For example, increasing the storage from 200 liters (53 gallons) to 500 liters (132 gallons) requires about a 1.6 times larger radius, resulting in approximately 1.6 times the energy loss for 2.5 times the volume of water. This additional energy is simply lost. No ifs or buts, even without considering “the energy loss per liter is lower.” If the tank cools down faster than you can use the hot water, then a somewhat smaller tank would definitely be more efficient.
If you calculate based on the height of the cylinder instead of the radius, the advantage disappears anyway because the surface area also nearly doubles when doubling the volume (which is often the real case, since usually you can’t place a small, wide tank due to space constraints).

Bieber0815 schrieb:
Does your heat pump have a suitable interface or do you have to DIY it? Are there solutions for engineers (for example in Excel)?

There is both an interface and the need for some DIY. For example, I prefer a DIY solution for electricity consumption rather than relying solely on the heat pump’s data.
I’m not sure what exactly you want to have in Excel?

To be honest, I’m not sure how storage works with heat pump systems. Certainly, the size of the storage tank must be tailored to the system (house), and there is probably some kind of optimum.

Nevertheless, regarding the point above, here is a comparison. If you buy two mulled wines—one served in a 200 mL (7 fl oz) glass and the other divided into ten 20 mL (0.7 fl oz) shot glasses—and assuming the starting temperature is the same for all, with the diameter-to-height ratio of all glasses (both large and small) being equal, as well as the heat transfer coefficient of all glasses being the same, then the mulled wine in the large glass will stay warm longer than the wine distributed in the smaller glasses. Would you agree with that?

Whether or not I understand 1) if a storage tank is even useful at all (BeHaElJa?), and 2) if yes, what size it should be (and for what purpose: domestic hot water or heating buffer), is, of course, another matter entirely.

BeHaElJa schrieb:
Heat pumps and buffer tanks are not necessarily the best combination. From personal experience: the house is so slow to respond that you can’t just quickly adjust the temperature up or down

As far as I know, the buffer tank is not intended to compensate for temperature fluctuations inside the house. Rather, the heat pump heats during the day (when outdoor temperatures are higher) and rests at night (when outdoor temperatures are lower). Heat is drawn continuously—at night from the buffer tank, which is then replenished during the day. This is a simplified example, but it’s how I understand the role of the buffer tank in a heat pump system.

Bieber0815 schrieb:

Still, regarding the above point, a comparison: If you buy two mulled wines—one served in a 200-mL (7 fl oz) glass and the other divided into ten 20-mL (0.7 fl oz) shot glasses—and assuming the starting temperature is the same in all glasses, the diameter-to-height ratio of all glasses (both large and small) is equal, and the heat transfer coefficient is also the same for all glasses, then the mulled wine in the large glass stays warm longer than the wine distributed in the smaller glasses. Do you agree with this?

Yes, I agree with that. Now imagine drinking 40 mL (1.4 fl oz) of mulled wine, and the next morning the remaining 160 mL (5.6 fl oz) in the large glass is cold and no longer tastes good. If you had only ordered 2 shot glasses of mulled wine, you would have saved overall.
Regarding storage tanks, I quickly calculated a real case with the sizes of actual tanks: 500 L (132 gallons) with a height of 15.7 dm (62 inches) and radius of 3.25 dm (13 inches), and 200 L (53 gallons) with a height of 11 dm (43 inches) and radius of 2.5 dm (10 inches). The larger tank has 2.5 times the usable volume and 1.8 times the surface area. It can potentially retain heat about 38% longer, but it also loses 1.8 times more energy in the same period compared to the smaller tank. I think the difficulty lies in distinguishing between heat and thermal energy. A bigger tank keeps the heat longer but experiences greater absolute energy losses. This means any oversizing increases operating costs.

For heating with underfloor heating, a buffer tank is not necessary. The screed stores far more energy than a mere 500 L (132 gallons) of water.
If you charge the tank to the flow temperature of the heating system, the energy obviously isn’t enough to heat through the night when it’s cold (and it’s definitely too low for domestic hot water). If you heat the tank very hot using an air-to-water heat pump to store energy for the night, the system’s efficiency drops significantly because the temperature difference is much too high. In that case, the heat pump runs more efficiently when connected directly to the heating circuit, even if it’s 10°C (18°F) colder at night in winter than during the day, because the flow temperature of the underfloor heating is certainly more than 10°C (18°F) lower than the tank’s required charging temperature (underfloor heating maxes at about 35°C (95°F), while the tank needs to be heated well above 50°C (122°F) to maintain the flow temperature throughout the night).

Saruss schrieb:
And now you drink 40 mL (1.4 fl oz) of mulled wine, and the remaining 160 mL (5.4 fl oz) in the large glass are cold and no longer taste good the next morning. If you only ordered two shot glasses of mulled wine, you’ve saved overall.

It seems we are talking past each other a bit. You are overlooking that the buffer tank is also supplied with heat; the heating system keeps it at temperature and compensates for losses. On another point, you are right: If I only want or need two shot glasses (2 x 20 mL (0.7 fl oz)), then I will only order those two and not five times that amount. (Here it’s just a matter of choosing the optimal size of the buffer tank; it will certainly depend on the circumstances …). If I want the drink to stay warm as long as possible, I would prefer one glass of 40 mL (1.4 fl oz) rather than two glasses of 20 mL (0.7 fl oz). Explanation: see above.

Saruss schrieb:
For underfloor heating, you don’t need a buffer tank. [...]

Thank you, that makes sense to me! I looked again at the brochure; the Rotex buffer tank is primarily for domestic hot water but can also be used to support the heating system. What actually happens will probably depend on the control system, which I am naturally not familiar with.

Source: Rotex brochure

Bieber0815 schrieb:
It seems like we’re talking past each other a bit.

I think so too about the “talking past each other.” You sometimes talk about heat as temperature, while I’m referring to heat as energy.

Bieber0815 schrieb:
You’re neglecting that heat is also added to the storage; the heating system keeps it at the right temperature and compensates for losses.

But that’s exactly what you’re neglecting! The losses, as I calculated above, actually increase with a larger storage tank! A storage tank that is 2.5 times bigger requires 1.8 times the amount of heat energy to compensate for losses and maintain the temperature!

Bieber0815 schrieb:
On another point, you’re right: If I only want or need 2 shot glasses (2 x 20 mL), then I would order just those two and not five times the amount. (Here it’s all about choosing the optimal tank size, which will certainly depend on the circumstances …). If I want the liquor to stay warm for as long as possible, I’d prefer one (!) glass of 40 mL rather than two (!) glasses of 20 mL. Explanation: see above.

That may be so. But the metaphor doesn’t account for the fact that the heat pump can supply hot water at the same efficiency anytime.
The larger tank keeps the temperature (careful, not to be confused!) longer but pays for this with a significantly larger heat energy loss!
If it’s still unclear to you why large tanks don’t really make economic sense, I can calculate the exact energy amounts for heating, including times, losses, and temperatures.

Bieber0815 schrieb:
Thanks, that makes sense to me! I looked again at the brochure; the Rotex tank is primarily for hot water but can also be used to support heating. What actually happens then probably depends on the control system, which I don’t know.

The “problem” is always that you lose energy when storing hot water. This means, with underfloor heating and a heat pump, using the storage tank for heating is generally less efficient than directly using the heat pump.
The only exception might be short, small cold spikes where you use heat from the tank instead of the electric heater (which rarely happens). But if the cold spike lasts longer, or if you want domestic hot water, efficiency is lost again because the storage has to be reheated at the same source temperature.
Another issue is the “stratification” of hot water in the tank (hot on top, cooler at the bottom). I can imagine that heating operation disturbs this stratification, causing the tank to be recharged more frequently than when used only for domestic hot water.