ᐅ Heating System Design (Ground-Source Heat Pump) – Experiences and Tips

Hello everyone,

we are building a single-family house with a local general contractor, and I have some concerns about whether the heating system is being properly designed or if they are just installing “what they have always done.” Maybe some of you can help with my following questions:

First, the key details of the house:
- approx. 170 sqm (1830 sqft) living area,
- KfW55 standard,
- postal code 48xxx,
- solid construction, brick-faced (2-layer),
- ground-source heat pump with underfloor heating (deep drilling),
- 2 adults, 2 (small) children,
- decision on photovoltaic system yes/no by external provider is still pending (9.75 kW peak and probably with at least a small battery if installed).


For the heating system, after several inquiries I received the information that presumably (design by the contractor’s energy consultant is apparently not yet finalized) the following system will be installed:
- Viessmann Vitocal 300-G in the 5.7 kW version (the heat pump),
- 200-liter (53-gallon) buffer tank,
- 300-liter (79-gallon) domestic hot water tank.


My questions:
(1) How large should the buffer tank for the underfloor heating be? Is it even necessary? There are 200 liters planned...

(2) According to the general contractor, the buffer tank is especially important if I get a photovoltaic system. I don’t understand why? Wouldn’t it be less important then?

(3) What size domestic hot water tank is reasonable for 4 people? Is 300 liters necessary, or is an integrated 220-liter (58-gallon) tank like in the Viessmann 333-G sufficient? I think it is otherwise similar in performance...

(4) Does a separate domestic hot water tank (Vitocell) outside the heat pump unit make sense from a maintenance perspective? I heard from Viessmann that replacing the integrated tank is quite complex and time-consuming, and that the integrated tank is also significantly (500–1000€) more expensive… How long does such a tank last, and how long does a heat pump usually last? Are there any experiences?


Thank you very much in advance!

Hi,

Thomas7 schrieb:

(1) How large should the buffer tank for the underfloor heating be? Is it even necessary? 200 liters are planned...

The heating buffer should be exactly 0 liters. The heat pump should directly supply the underfloor heating without a hydraulic separator (ERR), or only in individual rooms at most. Buffer tanks and hydraulic separators are the most common efficiency killers for heat pumps.

Thomas7 schrieb:

(2) According to the main contractor, the buffer tank is especially important if I get a photovoltaic system. I don’t understand why? Wouldn’t it be less important then?

Your main contractor either has little knowledge of how to operate a heat pump properly or just wants to make money. It’s always claimed that you can store heat in the buffer tank when you have excess photovoltaic power. But if you look at the possible temperature difference, the water volume in the tank, the heat capacity of water, and how the heat pump’s efficiency depends on the flow temperature, you will realize this is nonsense. You can store a much larger amount of heat in your screed (floor slab) and building mass with a 3°C (5°F) temperature increase without killing the heat pump’s efficiency. You won’t even notice this 3°C (5°F) increase in the flow temperature thanks to the building’s thermal inertia.

Thomas7 schrieb:

(3) What size of hot water tank is suitable for 4 people? Do you need the 300 liters or is an integrated 220-liter (like the Viessmann 333-G) enough? I believe the latter is otherwise similar in performance...

That’s hard to say without knowing your habits. With high-flow rain showers or full baths combined with simultaneous showers, 220 liters will probably not be sufficient. It also depends on the type of hot water storage—freshwater station, hygienic tank, or standard storage tank. For the first two types, even 300 liters may not be enough.

Thomas7 schrieb:

(4) From a maintenance perspective, does it make sense to have a hot water tank as a separate Vitocell (not integrated into the heat pump unit)? I heard from Viessmann that replacing the integrated tank is time-consuming and the integrated tank is also quite a bit more expensive (500–1000€). How long does a tank normally last and how long a heat pump? Are there any experiences?

Basically, I think it’s best if the components can be replaced independently. That way, you avoid being locked into one manufacturer if you want to upgrade the heat pump later but the tank is still fine.

Thomas7 schrieb:

- Decision on photovoltaic system yes/no by an external provider is still pending (9.75 kWp and probably at least a small battery if chosen)

• As long as you don’t have a KfW40+ (or equivalent) house and KfW funding, a battery is not cost-effective. Storage costs are higher than feed-in and buyback.
• With this system size, you could cover about 35% of your heating energy demand, which is quite attractive. So, go ahead and put the system on the roof. As long as the scaffolding is still up and the electrician is already on site, it’s cheapest to install it then. The price should be well below 1200 €/kWp. There is a very good specialized forum for this topic where you will get great advice. Just search for photovoltaics forum.

One more tip:
With solid construction, always plan for a building climate control system (BKA). It’s cost-effective, increases comfort, improves cooling, and boosts heat pump efficiency. Similar applies to wall heating in the bathroom.

Otherwise: Have you already applied for BAFA funding?

Best regards,
Nika

Hello Nika,

thank you very much for the detailed reply.

The heating buffer tank should be exactly 0 liters. The heat pump should directly supply the underfloor heating without a domestic hot water buffer tank (DHW buffer), or at most in individual rooms. Buffer tanks and DHW buffers are the most common efficiency killers for a heat pump.

According to the building specifications, we have an adjustable thermostat in every room. I assume that means I can, for example, set the bedroom temperature a bit cooler, but if I generally leave it as is and do not adjust it every day, then in the context of buffer tank yes/no this doesn’t yet count as a DHW buffer, and the buffer tank would essentially only be needed if I constantly regulate rooms warmer or cooler and expect the heat capacity to be available quickly? Am I understanding this correctly?

So, for efficiency, it is important to maintain a relatively constant heat flow without constant fluctuations?

Furthermore, I read (article on Wärmepumpe.de) that heat pumps require a minimum constant flow rate or something similar. What is this about and why would I then possibly need a buffer tank (that’s how I understood it)?

Your general contractor either has little knowledge of how to properly operate a heat pump or just wants to make money. It is always claimed that you can store heat in the buffer tank when there is surplus photovoltaics. But if you consider the possible temperature delta, the water volume in the buffer tank, the heat capacity of water, and the dependence of the heat pump’s efficiency on the supply temperature, you will know that it makes no sense. In your screed (and the building’s thermal mass), you can store a significantly larger amount of heat with a 3°C (37°F) temperature increase without reducing the heat pump’s efficiency. You won’t even notice the 3°C rise in supply temperature because of the inertia of the building mass.

I don’t think they just want to make money; I suspect they just have little knowledge :-/

It is hard to say without knowing your habits. With rain showers with high flow rates or full baths plus simultaneous showering, 220 liters (58 gallons) might be difficult. It also depends on the type of hot water tank. Fresh water station, hygienic storage or conventional tank? For the first two, even 300 liters (79 gallons) will probably not be enough.

Okay.

• As long as you don’t have a KfW40+ house and KfW subsidies, a battery is not cost-effective. The storage costs are then higher than feeding in plus buying back electricity.

I am not sure about that. I am building a KfW55 house. According to all calculations (5 quotes plus my own worst-case scenario), it almost comes down to the same with or without a ~5.1 kWh battery. Self-consumption increases, feed-in decreases, and the difference between the two scenarios is 3600€ in the first 10 years (battery warranty period). The battery costs me 3450€. Since the battery lifetime is given as 20 years (probably optimistic, of course), things shift clearly in favor of the battery the longer it lasts.

I also know all the forum opinions that batteries are not worthwhile and all the sales promises that they are. The problem is that with the decreasing Renewable Energy Act remuneration, rising electricity prices and (still) falling battery prices, the balance is increasingly moving in favor of batteries, while the forum opinions remain unchanged...

Otherwise: BAFA subsidy already applied for?

Yes.

Regards,
Thomas

Thomas7 schrieb:

According to the building specifications, we have an adjustable thermostat in every room. I assume this means I can, for example, set the bedroom temperature a bit cooler. But if I usually leave it at a set value and don’t adjust it up and down daily, does that still count as ERR in the buffer context, yes or no? And the buffer is essentially only needed if I constantly adjust rooms warmer or cooler and expect the thermal capacity to respond quickly? Am I understanding this correctly?

So, for efficiency, it is important to maintain a relatively constant heat flow without frequent fluctuations?

I also read (article on Wärmepumpe . de) that heat pumps require a continuous minimum flow rate or something similar. What is this about, and why would I possibly need a buffer tank for this (as I understood it)?

They are usually controlled with thermal actuators. These only have open/close positions and are commonly PWM controlled. This means the thermostat basically tells the actuator to be closed for 7 minutes and open for 8 minutes within the next 15 minutes. This leads to

• The possible flow rate in the heating system not being constant
• The flow rate regularly falling below the minimum flow rate that the heat pump requires
• The hydraulic balancing (if done at all) being ineffective, because the flow rates of the individual heating circuits never really match each other anyway

To compensate for this mess, an expensive heating buffer tank is needed, which further complicates the system. Basically, you end up installing two expensive components that increase the heating system cost by about 2000€ without making it more efficient or comfortable. A good builder knowledgeable about heat pumps would advise against both.

Regarding setting temperatures...
This is done via hydraulic/thermal balancing. Different room temperatures in a new build are practically unrealistic. Therefore, it is regulated via the heat pump’s heating curve (which should be set as low as possible). And if you do want the bedroom about 1K cooler, you can either simply reduce the flow on the heating circuit with a valve or install ERR only for the bedroom and bathroom. But ERR for the whole house is just a waste of money—both in investment and operation. So, I recommend wiring every room but leaving out the thermostats and the heating buffer.

For further reference, I also suggest another post of mine on this topic
https://www.hausbau-forum.de/threads/heizlastberechnung-10-3kw-lwwp-mit-9-5kw-ausreichend.34063/post-390515

Thomas7 schrieb:

I’m not sure about that. I’m building to KfW55 standard. According to all calculations (5 offers + my own worst-case calculation), the result is almost the same with or without about 5.1 kWh battery. Self-consumption increases, feed-in decreases, and the difference between both scenarios is about 3600€ over the first 10 years (battery warranty period). The battery costs me 3450€. Since the battery’s lifetime is stated as 20 years (probably optimistic, of course), the longer it lasts, the more it favors the battery financially.

I have a battery (gifted through KfW). With current battery prices and electricity costs, it’s more economical to feed electricity into the grid and buy it back later. This costs a maximum of 20 cents/kWh (kilo­watt-hour). Storing electricity yourself costs significantly more. This also excludes VAT on energy consumption. Additionally, with your relatively small 9.75 kWp system, there probably won’t be much surplus for the battery in winter. This means it will frequently need to be recharged from the grid with expensive electricity to maintain the charge. I have a 22 kWp system with a 6 kWh battery myself. Even with 22 kWp, it isn’t regularly charged during winter, and if I had paid for it myself, depreciation over 20 years would exceed the savings.

If batteries become cheaper in five years and electricity prices possibly rise, you can always retrofit a battery later. Just plan for a hybrid inverter from the start, and when storage costs fall below the difference between feed-in and purchase tariffs, retrofit the battery. I calculated when this would be financially viable, considering VAT, storage losses, battery cost degression, etc. To compete with the current difference between feed-in tariffs and consumption prices, batteries need to become at least twice as cheap. Also, if you plan to get an electric vehicle in about five years, that effectively gives you your battery storage.

I’m not trying to convince you to forgo the battery for now—there’s nothing in it for me. You are welcome to get it if you are convinced. But there are better ways to invest your money when building a house.

Regards,
Nika

gnika77 schrieb:

That will cost you a maximum of 20 cents per kWh. Storing the electricity yourself is significantly more expensive.

Exactly, it’s actually quite simple. If you calculate a storage system to be cost-effective, you’ve made a miscalculation.