Heating capacity calculation and heat pump selection

A heat pump, which is actually correctly or minimally undersized, can get into trouble here and, even at the highest modulation level, no longer produce enough heat quantity to compensate for the defrost losses.

But that can only happen at NAT. Because on 99.5% of all days it is significantly warmer and the heat pump is only slightly required. If I think about it... In recent years I can only remember 1-2 times reaching the NAT.

Hello,

unfortunately not, this happens especially during the transitional period. Icing is less critical and frequent at low temperatures because the humidity is low. In autumn and spring at 1-2 degrees outside temperature and 95+% humidity, the machines ice up very often.

Best regards

Hello,

Unfortunately, the time to edit has expired, so here is a new post:



This picture shows the behavior quite well. In the morning hours, the machine is operating at its performance limit.

I don't want to blow this topic out of proportion, just wanted to make the note that defrosting should be considered when choosing the performance level of the air-water heat pump.

Best regards

First of all, thanks to everyone for the many responses!


You really seem to know your stuff, so thank you again especially for the very detailed explanation.

My question about that would be, my heating capacity of about 5.6 kW refers to an outside temperature (NAT) of -12°C. If the mentioned defrosting processes mostly take place at high humidity around the freezing point, the 55/6 has about 7 kW at 0°C and the 75/6 about 10 kW.

That should presumably be enough to cover the heating demand for the house at 0°C, I don’t know the heating load requirement for the house at 0°C, but if it is 5.6 kW at an outside temperature of -12°C, then the available capacity of the 55/6 with 7 kW at 0°C should be more than sufficient?

I would be glad to receive feedback.

Best regards

Hello,

I would actually choose the larger machine. It not only has the reserve to compensate for the extracted energy, but it will also need to defrost much less frequently and operate more often in the low modulation range.

A disadvantage during the transition period may be that it sometimes shuts off earlier (lowest modulation stage reached, yet the excess heat cannot be dissipated) than the smaller machine. However, with appropriate settings, this can be partially counteracted and cycling can be turned into longer pauses/running times.

I read a user report elsewhere that comes close to your conditions:

Approximately 150 m2
Heating load at NAT: 5 kW
Underfloor heating

In this case, the large machine 75/6 is in use and the described problem does not occur, unlike with the small machine 55/6 used by other users. In particular, the number of defrost cycles is much lower than with the small device.

Best regards

Icing is less critical and less frequent at low temperatures because the humidity is low. In autumn and spring, at 1-2 degrees outside temperature and 95+% humidity, the machines ice up very frequently.

Yes, but neither the house nor the heat pump needs much power at those times. That’s what I mean. Then it defrosts briefly and continues heating. During this time, no new building cools down noticeably – not even with defrosting every 30 minutes.