Heating design (brine-water heat pump) experiences / tips

Hello everyone,

we are building a single-family house with a local general contractor and I am a bit worried whether he is correctly designing the heating system or just installing "what he has always done." Maybe some of you can tell me something about my following questions:

First, the basic data of the house:
- approx. 170 sqm living area,
- KfW55,
- postal code 48xxx
- solid, clinker brick (2-shell)
- brine-water 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 kWp and probably if then with at least a small battery)


For the heating system, after several inquiries I received the statement that probably (design is apparently not yet completed by the energy consultant of the general contractor) the following system will be installed:
- Viessmann Vitocal 300-G in the 5.7 kW version (the heat pump)
- 200 liter buffer storage tank
- 300 liter domestic hot water storage tank


My questions:
(1) How large does the buffer storage tank for the underfloor heating need to be? Is it needed at all? There are 200 liters planned...

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

(3) How large a hot water storage tank is reasonable for 4 people? Do you need 300 liters or is an integrated 220 liters like in the Viessmann 333-G sufficient? I believe it is otherwise identical in terms of performance...

(4) Does a hot water storage tank as a separate Vitocell (not integrated into the heat pump unit) make sense from a maintenance perspective? I have heard from Viessmann that replacing the integrated storage tank is complex in terms of labor time and that the integrated storage tank is also quite a bit (500-1000€) more expensive... how long does such a storage tank last and how long does a heat pump last? Are there any experiences?


Thanks a lot in advance!

Hi,

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

The heating buffer should be exactly 0l in size. The heat pump should load the underfloor heating directly without ERR, or at most in individual rooms. Heating buffers and ERR are the most common efficiency killers with a heat pump.

(2) According to the general contractor, the buffer tank is especially important if I get photovoltaics. I don’t understand why? Isn’t it actually less important then?

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 during photovoltaic surplus. But if you look at the possible temperature delta, the amount of water in the buffer, the heat capacity of water, and the dependency of the heat pump’s efficiency on the flow temperature, then you will know that it is nonsense. In your screed (and the building mass) you can store a significantly larger amount of heat with a 3K temperature increase without killing the heat pump’s efficiency. You won’t even notice the 3K increase in flow temperature due to the inertia of the building mass.

(3) How large a hot water tank is sensible for 4 people? Do you need the 300 liters or is an integrated 220-liter one like in the Viessmann 333-G sufficient? I believe it is otherwise similar in performance...

That is hard to say without knowing your habits. With rain showers with high flow rate or full baths and simultaneous showers, the 220 liters will probably be difficult. It also depends on the type of hot water tank. Fresh water station, hygiene tank, or regular tank? For the first two, even 300 liters will probably not be enough.

(4) Does a hot water tank as a separate Vitocell (not integrated into the heat pump unit) make sense from a maintenance perspective? I heard from Viessmann that replacement of integrated tanks is complex in terms of labor time 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 last? Are there any experiences?

Basically, I am of the opinion that the components should be exchangeable independently of each other as much as possible. Then you don’t have a lock-in effect with one manufacturer, in case you want a newer heat pump but the tank is still fine.

- Decision on photovoltaics system yes/no by external provider is still pending (9.75 kWp and presumably, if at all, with at least a small battery))

[*]As long as you don’t have a KfW40+ house and KfW funding, a battery is unprofitable. The storage costs are then higher than feed-in plus repurchase.
[*]With this system size, you could cover about 35% of your heating energy needs. So definitely attractive. That means, the system should go on the roof right away. As long as the scaffolding is up and the electrician has to do his stuff anyway, it is cheapest. The price should then be well below 1200€/kWp. But there is a very good specialized forum for this. They can help you well on this topic. Just google photovoltaics forum.

One more tip:
Always plan a BKA with solid construction. It is cost-effective, increases comfort, improves cooling, and increases the efficiency of the heat pump. The same applies to wall heating in the bathroom.

Otherwise: Have you already applied for BAFA funding?

Regards Nika

Hello Nika,

thank you very much for the detailed answer.

The heating buffer should be exactly 0l in size. The heat pump should directly charge the underfloor heating without ERR, or at most in individual rooms. Heating buffer and ERR are the most common efficiency killers with a heat pump

So according to the building description, we have a controllable thermostat in every room. I guess I can, for example, set the temperature a bit cooler in the bedroom, but if I usually leave it like that and don’t turn it up and down every day, does that still not count as ERR in the context of the buffer yes/no, and the buffer is basically only needed at most if I keep adjusting rooms warmer or cooler and expect the heat capacity to be quickly available? Do I understand that correctly?

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

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

Your general contractor either has little idea 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 with photovoltaic surplus. But if you look at the possible temperature delta, the amount of water in the buffer, the heat capacity of water, and the dependence of the heat pump’s efficiency on the forward flow temperature, then you will know that this is nonsense. In your screed (and the building mass) you can store significantly more heat at a 3K temperature increase without killing the heat pump’s efficiency. You won’t even notice the increase in the forward flow temperature by these 3K due to the building mass inertia.

I don’t think he wants to make money, I suspect rather little expertise :-/

It’s hard to say without knowing your habits. With rain showers with high flow rate or a full bath and simultaneous showering, it will probably be difficult with 220 liters. It also depends on the type of hot water tank. Fresh water station, hygiene tank or normal tank? With the first two, even 300 l will probably not be enough.

Okay.

[*]As long as you do not have a KfW40+ house and KfW funding, a battery is unprofitable. The storage costs are then higher than feed-in + repurchase

I am not sure about that. I am building KfW55. According to all calculations (5 offers + my own worst-case calculation), it comes out almost the same with or without ~5.1 kWh battery. The self-consumption increases, feed-in decreases, and the difference between both scenarios is €3600 in the first 10 years (battery warranty period). The battery costs me €3450. Since the battery lifetime is specified as 20 years (probably exaggerated, of course), the balance shifts entirely in favor of the battery the longer it lasts.

I also know all the forum opinions that batteries are not worth it and all the sales promises that they are. The problem is that with declining Renewable Energy Act compensation, rising electricity prices, and (still) falling battery prices, it increasingly shifts in favor of the battery, and the opinions in the forums nevertheless remain...

Otherwise: BAFA funding already applied for?

Yes.

Regards,
Thomas

According to the construction description, we have an adjustable thermostat in every room. I assume that this allows me, for example, to set the temperature in the bedroom a bit cooler. But if I generally leave it like that and don’t turn it up and down every day, does that still not count as ERR in the buffer context yes/no, and the buffer is basically only needed if I am constantly making rooms warmer or cooler and expect the heat capacity to be available quickly? Do I understand that correctly?
So for efficiency, it is important to create a relatively constant heat flow without constant fluctuations?
Furthermore, I have read (article on Wärmepumpe.de) that heat pumps require a steady minimum flow rate or something like that. What is that about and why might I need a buffer storage because of it (that’s how I understood it)?

It is usually controlled with thermal actuators. These only know on/off and are usually PWM controlled. This means the thermostat basically tells the actuator, within the next 15 minutes you are closed for 7 minutes and open for 8 minutes. This leads to

[*]The volume flow possible in the heating system is not constant
[*]The possible volume flow regularly falls below the minimum flow required by the heat pump
[*]The hydraulic balancing (if any was done at all) is rubbish because the volume flows of the individual heating circuits never match each other anyway

And to compensate for this mess, an expensive heating buffer is needed, which further messes up the whole system. Basically, you install two expensive things that immediately make the heating system about €2000 more expensive without making it more efficient or more comfortable. A good heating installer who knows heat pumps would advise against both.
As for setting the temperature...
You do that via hydraulic/thermal balancing. Different room temperatures in new buildings are anyway illusory. Therefore, you regulate this via the heating curve of the heat pump (which should be set as low as possible). And if you do want the bedroom to be 1K colder, then you can either simply throttle the heating circuit at the Tacosetter or actually install ERR for the bedroom and bathroom. But ERR throughout the entire house is just a waste of money, both in investment and operation. Therefore, have each room wired, but leave out the thermostats and the heating buffer.
Otherwise, I refer you to another post of mine on this topic

I’m not sure about that. I am building KfW55. According to all calculations (5 offers + my own worst-case calculation) it almost comes down to the same with or without ~5.1 kWh battery. Self-consumption increases, feed-in decreases, and the difference between both scenarios is €3600 in the first 10 years (battery warranty period). The battery costs me €3450. Since the battery’s lifespan is stated as 20 years (probably exaggerated, obviously), this shifts the balance in favor of the battery the longer it lasts.

I have a storage battery (given free by KfW ). At current battery prices and electricity costs, it is more profitable to feed electricity into the grid and then buy it back. That costs you a maximum of 20ct/kWh. Storing electricity yourself costs significantly more. And I haven’t even factored in VAT on energy consumption. Also, with your “meager” 9.75 kWp, not much will be left for the battery in winter. That means it will regularly have to be recharged with expensive grid electricity for maintenance charging. I myself have 22 kWp and a 6 kWh battery. Despite the 22 kWp, it is not regularly charged in winter, and if I had paid for it myself, the depreciation over 20 years would be higher than the savings.
If batteries become cheaper in 5 years and electricity (perhaps) more expensive, you can still retrofit the battery later. Just plan a hybrid inverter and as soon as storage costs are lower than the difference between feed-in and remuneration, retrofit it easily. I once calculated when it makes sense, taking into account VAT, storage losses, battery degression, etc. To keep up with the current difference between remuneration and purchase, batteries still have to be at least twice as cheap. Also, if you perhaps get an electric car in 5 years anyway, you have your storage.
I don’t have to convince you to (initially) do without the battery. I have no benefit from it. You can gladly get it if you are convinced by it. But there are better ways to spend your money reasonably when building a house.
Regards, Nika

It costs you a maximum of 20ct/kWh. Storing the electricity yourself costs significantly more.

Exactly, it's actually quite simple. If you calculate a storage system in a way that it pays off, you have miscalculated.