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Here is my heat pump installation with questionable COP in Italy

 
Air Source Heat Pumps (ASHPs)
Last Post by materox345 2 weeks ago
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 materox345
(@materox345)
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Joined: 3 weeks ago
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Topic starter 19/11/2025 11:47 am  

Hi everybody, this is my first post here, I've been watching the podcasts for a while and I hope it's ok to write here even if I'm not in the UK but in Italy, hills around Florence to be precise.

So I had my heat pump installed 4 years ago and my installation was a disaster. My installer was very very new to heatpumps and had no idea how they worked or how to configure it properly. Due to the lack of expertise, I turned to the internet and with a lot of struggles I've been using somewhat succefully for the past 2 years but, as you know, I'm alwasy monitoring and ttrying to see if I can Improve efficency. A little summary of my setup. I have a viessman 100-s 8kwh (more on that later), connected to a 40L buffer, from the buffer I have 2 wilo para pumps (regulated to low speed), one goes to the radiator circuit and the other to a fancoil circuit (hydronic split? I'm not sure how they're are called in english).

My house is old, it was built in 1823 and had several upgrade trough the years, it's about 80 square meters but on 3 floors + small attic (less than 10 sqm) with the inside unit of the heatpump (it's a split model), water cylinder  300L. My roof is insulated and all the windows are double glazed but pretty old (18 years). The walls are all stone and brick (mixed) and avergae about 70cm thick , 80cm in some spots. Now I know perfectly that I need to upgrade the windows, unfortunately because of town regulations I could'nt insulate the walls, and I can't install PV on my roof (crazy stupid italian legislation).

I upgraded radiators (apparently not enough) when I renovated 4 years ago and installed the heatpump, and installed the fancoils, 4 total. In the summer I use the fancoils to cool and they work very well, here it gets very very hot in summer. In winter I use both radiators and fancoils to heath the house. The system is setup trough weather compensation, and I have no thermostat but several temp sensor in each room for monitoring.

After years of trial and error and research I understood the logic behind my veissman HP, and I can change almost all parameters. This model unfortunately is oversized for my house, it's a nominal 8kw but it's more like a 12kw, meaning that the minimum heat it can generate it's around 4kwh. regarding the logic, basically the whole heating cycle is based on the buffer temperature. The heatpump heats the water to bring it to the target (set by weather comp) and it can increase that temperature based on a tolerance that I can change, after many tests I set it to 4 degrees up or down. This way I drastically reduced the on/off as you can see from the graphs, unfortunatly this setup I think it's not great for efficency (no buffer would be better) but I can't do it differently because of the dual circuit. So In a situation like the graphs attached, the heatpump is constantly running at it's minimum which is 25hz or 28% capacity, and it can run for hours.

The delta T in this cased is calculated by the heat pump that regulates it's secondary pump to the buffer to mantain 5K spread constantly. The problem I have is that the COP is pretty low, as you can see. My data is based on Thermal energy (from heatpump data), electrical energy (shelly em that measures also the 2 pumps on the buffer). I can also see the heat pump's COP as a datapoint and of course that one is higher. Last night it was around 4-6C and the cop was under 3. Am I just unlucky that my model has low efficency? I know that lowering supply temperature will of course increase COP, but I feel there is more to it. Any idea is appreciated . Sorry for my english as it isn't my first language. 

I forgot to mention that the target room temperature is achieved and mantained pretty well, but since I live in a moderate climate (maybe it goes below zero a handful of times a year), I was wondering if I can get better COP


This topic was modified 3 weeks ago by materox345
This topic was modified 3 weeks ago by Mars

   
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 materox345
(@materox345)
Active Member Member
Joined: 3 weeks ago
Posts: 3
Topic starter 19/11/2025 2:15 pm  
heat1
heat2


   
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 JamesPa
(@jamespa)
Illustrious Member Moderator
Joined: 3 years ago
Posts: 3676
20/11/2025 9:09 am  

Hi, and welcome to the forum.

 

I cant quite understand whats going on in the charts because the labels arent obvious. 

Cn you clarify which of the lines shows:

Temperature of the water exiting the heat pump flowing to the buffer

Temperature of the water exiting the buffer flowing to the emitters

Temperature of the water returning to the buffer from the emitters

It looks like with an outside temp of ~7 you are running at a flow temp (I cant tell whether measured at the heat pump or at the output of the buffer) of ~40.  Is that a correct interpretation?  Can you tell me what your weather compensation parameters are please?  

 


This post was modified 3 weeks ago 3 times by JamesPa

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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 materox345
(@materox345)
Active Member Member
Joined: 3 weeks ago
Posts: 3
Topic starter 27/11/2025 9:07 am  
pump 7degrees 2
pump 7degrees 2
pump 7degrees

@jamespa Sorry for the late reply, I apologize for the graphs not clear. you are correct, so for example for the past 12hrs or so exterior temperature was between 5 and 7 C, the heatpump maintains a constant delta 5 between what it prduces and the return, and it never stops, it runs at the minimum of 25hz which is 28% of the compressor, and it delivers 4.3 KW of thermal energy (according to the data it spits out), it produces water around 40C and I will say that it can't run any better. I have a shelly that measures the electric consumption of heatpump with all the pumps, including 2 wilo pumps that serve 2 circuits at the same temperature (radiators+fancoils), IF I calculate the COP using this data en the heat generated (data from hetpump), I average a cop of 3.4. The heatpump itself says 4.3 because it doesn't count any pump but only the compressor (that's what veissman says). I'm planning to replace some radiators next year so I can reduce the supply temperature, I'll probably have more on/off but I don't see anything else I can try. my next step is to measure the delta after the buffer to see if I can improve on that side, right now both pumps run at minimum speed. you can see from the new graphs the behaviour. The big "bumps" in the graphs are Hot water preparation, not sure about the small bumps but I don't think it's defrost. to read the charts, blue line in the first graph is power consumption for the whole system (shelly em ), red is supply temperature, yellow return temperature, green buffer temperature, blue in second graph is outside air temperature


This post was modified 2 weeks ago 4 times by materox345

   
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