Heat pump: buffer tank, capacity and modulation

Hello again,

in the meantime, I have dug very deeply into my documents again and even found something that can be interpreted as room-by-room heating load calculation:



Do I understand correctly that the 684 W in the hallway or the 696 W in the kitchen are the heating loads per room? When I add everything up, I get:



So 9.7 kW altogether, is that realistic?

Best regards,

Andreas

Looks plausible. Because it is planned with 37°C flow temperature, which of course also requires more heating output. Even 10/15cm is definitely not optimal but not a big deal now.

Do you also have the flow rates? On the ground floor, it's 443l/h. That will definitely be in the range of the heat pumps from 1200l/h upwards. The goal is actually a low temperature difference, which is stated as 8.4°C in your case? That is quite ambitious for a heat pump.

This can be recalculated again and also checked to see if it was designed for the correct NAT. Especially if the insulation is increased anyway. Doesn't cost the earth at the engineering office.

I’m not involved in the funding programs at all anymore. Is all this urgent? Otherwise, you can really try to increase the flow rate over the winter to minimize the temperature difference and then see how far the heating curve can be reduced.

After you have already been living in the house for several years and know your actual gas consumption, I would always prefer that for the heat pump sizing over a calculation. So 6-7 kW is sufficient. If you are diligent, get the performance curves/diagrams of the heat pump from the manufacturer. Then you can optimally select the L/W heat pump. It is important that it can modulate down as far as possible. A heating buffer and ERR have no place in the system of a new building with underfloor heating and only cause higher investment and operating costs. The only case where a buffer would be acceptable for you is a small, serial buffer in the return flow for the defrosting process.

I still don't quite understand why one should completely do without ERR. For example, I don't always want to heat my bedroom. The same goes for the office on the days when I work from home. The bathrooms, on the other hand, maybe a bit more than the other rooms. What is the advantage if one still refrains from ERR?

I still don’t quite understand why one should completely do without ERR. For example, I don’t always want to heat my bedroom. The same goes for the office on the days I work from home. The bathrooms maybe a bit more than the other rooms. What’s the advantage if you do without ERR anyway?

The advantage is that you set the heating correctly right away and avoid unnecessary components. The temperature is directly controlled via the flow or correctly set (and via the design of the underfloor heating in new buildings). If the temperatures are correctly set (hydraulic and thermal balancing), you don’t need ERR anymore, because everything is fine. By the way, not heating the home office on individual days is energetically nonsensical and especially not sensible with underfloor heating. In new buildings/well-insulated houses it is hardly feasible anyway. As a figurative explanation using the example of a car: The heat pump is the engine, the brake is the ERR. So you’re driving at full throttle and have your foot on the gas pedal. At the same time, you notice you are going too fast. But instead of taking your foot off the gas, you press the brake at the same time.

I still don't fully understand why one should completely do without ERR. For example, I don't always want to heat my bedroom. The same goes for the office on the days I work from home. The bathrooms maybe a bit more than the other rooms. What is the advantage of doing without ERR anyway?

There is the mantra to completely do without it. But it's not a law. You already have ERR anyway, so the point of cost savings on acquisition does not apply. I would also always recommend ERRs for bedrooms and other secondary rooms that you sometimes want to keep cooler.

The setting of the base temperature (21 in living rooms, 22-24 in the bathroom) should ideally not be done via the ERRs, but simply by the heating only warming the room automatically to the requested temperature. However, this only works well with a low supply temperature of around 30°. At 37+ it becomes difficult, because without regulating by the ERR, heat will continue to be delivered to the room even if it already has more than the required temperature, for example due to sunlight from outside.

Don't panic: A heat pump also works with ERRs and buffer storage and also with 37° supply temperature (at -10° outside). But in your case I would leave out the buffer storage. If you reduce the temperature spread of the heat pump from 37/29 to 35/31, there is also enough flow for the heat pump, with the same heat output, and the heat pump will be happy.

Regarding heating load: Looks like the correct calculation. I can't say whether the values are accurate, but they should be. Why your consumption is lower than fitting to the heating load cannot be said from afar and with the available documents. With additional insulation on top, the heating load will then not be above 9 kW. Even in the current state a 9 kW pump would suffice. Many 9 kW models can modulate as far down or almost as far down as the 5-7 kW ones. They are just the same technology with different power electronics.

About the energy consultant: 8 kW is the KfW maximum value. It makes sense for corresponding properties with complete renovation to a KfW efficiency house. You are right that this is completely exaggerated in your case. And why does the energy consultant sell/install heat pumps??