Correct adjustment of the heat pump in connection with the wood stove

Hello everyone,

I have a question regarding a clever control of my heat pump. I have the following situation. We have a new building with only underfloor heating and a heat pump from Ait (LWD 70A) & a 10 kW water-driven wood stove + 1000L buffer.

Both systems basically work perfectly. However, I want the optimal setting between the two systems. Goal: To heat/as much as possible with the stove and heat domestic water, etc., and as little as possible with the heat pump.

The control itself is initially simple. If the buffer has a domestic water temperature of 51°C, then the heat pump does not switch on. Unfortunately, it is not so simple with the heating circuit, as it depends on the outside temperature (at least that’s how I understand it). Usually, I fire up the stove around 4 p.m., and it burns until about 10:30 p.m.

When the days were not so cold yet, that was also enough until the same time the next day. For a few weeks now, the situation has been different. And the heat pump runs too much for me. Sure, I could start the fire earlier in the morning, but I am also at work, etc. Regarding the domestic water, I could regulate it relatively easily via the switching times. I block domestic water heating almost all day. I only allow a small time window in which the heat pump may heat if necessary. And it complies with that.

But with the heating circuit, it doesn’t work quite like that. I can also change the switching times in the heating circuit and mixing circuit 1. How should I set this for the heating to make sense? As I understand it, the times for the heating are not blocking times.

If I have forgotten any information or if you don’t understand me, please let me know briefly. Because I am just an amateur.

It usually takes about half an hour until the stove reaches 65°C in the combustion chamber and the pump actually starts. Then it also takes quite a long time (2–3 hours) until the temperature in the heating circuit is built up in the buffer. Domestic water is always heated first. And I do not want the heat pump to start during that time. I actually want to give it only a small time window, such as 11 a.m. to 1 p.m. During this time, it may run if necessary but not outside these times.

I know there is the possibility to block the heat pump via the EVU contact as soon as I turn on the control of my stove. However, for that, I would have to pull a cable into the basement, and I do not really want to do that right now. I think this must surely also be possible via the control of the heat pump.

Thanks to you all in advance & best regards

I actually just want to give her a small time window, like 11 am to 1 pm. During this time she is allowed to run if necessary but not outside these times.

Sorry, I don’t know your heat pump. But with my heat pump, I simply have daily programs for when the heat pump should operate. Daily programs for hot water or daily programs for heating periods. When I say "heat from 12 to 2 pm," then the heat pump only turns on during that time. And heats if necessary.

Yes, I have also thought that I basically give the mixing circuit and heating circuit the same time setting. At the moment it is different. But how is it generally with the water? Does the pump still work when a room thermostat opens?

Hello,

the LWD is a fixed speed heat pump, so it can only be on/off. The control is return-flow controlled and is guided by the outside temperature via the set characteristic curve. The characteristic curve thus defines a target return temperature directly and exclusively dependent on the outside temperature, while the controller tries to adjust the measured return temperature to this target return temperature. If the return flow leaves a defined hysteresis range around the target return temperature, the pump switches on or off. Basically, a heat pump should achieve as long runtimes and accordingly few cycles as possible.

Since you have a mixing circuit, there is presumably a second characteristic curve (depends on the exact hardware configuration of your system). This is supply-flow controlled, so the controller measures the supply flow and compares it to the target within a hysteresis range.

So, if your heat pump switches on, the return flow from the buffer is too low and therefore the temperature in the house or screed and buffer has dropped accordingly. The underfloor heating is slow; when you heat the stove, it initially only warms the room air, and with a time delay the floor and even later the buffer – and thus first the measured return flow. Your heat pump will therefore never noticeably react to these short runtimes of your wood stove.

You have three timer programs:

[*]Domestic hot water: Here you define blocking times, during which the pump does not run. I recommend scheduling the hot water preparation for the afternoon hours and blocking all other times, as it is warmest outside at this time of day (e.g., between 3 pm and 6 pm).
[*]Heat pump heating circuit: These times are release periods, meaning the heat pump is only allowed to run during these times. However, it only starts when other conditions fit as well (outside temperature and return flow).
[*]Underfloor heating circuit: With this program, you control the circulation pump that pumps water from the buffer into the actual underfloor heating.

You can now experiment with these programs. I would not restrict the mixing circuit; it should always circulate water as soon as the supply flow no longer fits, since you want to distribute the heat in the house. The heat pump (heating circuit) itself you can restrict to certain times. Then it will only warm the buffer during these release times. You will have to experiment here to see what fits best because the mixing circuit still extracts heat from the buffer and it should not get too cold. The release times for the heating circuit primarily serve to extend the active pump time (see above, as few cycles as possible). I would probably start with a release only during the daytime hours (e.g., from 6 am to 4 pm), the wood stove runs in the evening, and the house cools down somewhat at night.

Best regards

First of all Thank you very much for the detailed explanation. My understanding was like that too. However, it doesn't quite add up. I will now post the switching times and the heating curves here. By the way, I only have heating circuit 1 and mixing circuit 1. My problem is that the heating circuit heats outside of the defined time between 11 am and 1 pm, and I don't understand that. Regards Thomas

Hello,

for clarity:

[*]Heating circuit: This is the circuit from the heat pump to the buffer tank and back.
[*]Mixing circuit: This is the circuit from the buffer tank to the underfloor heating and back.

The switching times of the heating and mixing circuits control the switch between setback (night mode) and boost (day mode). During the entered times, the heating operates in day mode (boost), otherwise in night mode (setback). By default, 00:00 - 00:00 (no time) is set, which means continuous night mode.

Therefore, you must first set a value for night setback in the settings. If you really want to turn off the heating, the value must be set accordingly high (a maximum of 10 degrees can be set). Then you define the boosts in the switching times.

Best regards