I’d appreciate some opinions on the sizing of the heat pump installed in my new-build apartment in in the Pyrenees.
The apartment is around 79 m², single floor, with roughly 80 m² of underfloor heating and 10 loops. It’s a modern, very well-insulated build at about 1300m altitude, with only one side facing outdoors.
The installed heat pump is a Daikin Altherma 3 ERLA14D2V3 / EBVX16S18DJ6V, so essentially a 14 kW unit.
The design temp per regulations is based on the avg. daily minimum of the coldest month in the last 10 years, which would be -3.5C in my case. My own estimates suggest the design heat loss would be about 1.5-2kW.
The installed heat pump seems very large relative to the actual heating requirement, especially since most of the winter will obviously be at much lower loads than design conditions. It seems a little surprising that Daikin’s official and only importer in the country would recommend something of this size?
The system comes with a 50L buffer installed in serie and I plan to run the open with weather compensation.
The importer explained his choice because it gets “quiet cold” in the valley. But the historical data doesn’t really show that. He explained that the at -10 the output is only 7kW, but even at minimum modulation that output seems way too high for the apartment.
Based on those numbers, would you consider this sizing sufficiently questionable that it would be worth getting an independent second opinion on selection, before I simply wait for winter and see how it performs?
To me doesn’t sound like the best selection of heat pump manufacturer or heat pump model. Sizing done with the finger in the air instead of any real science, by Daikin. Even with a heat loss 2kW at -3.5 at -10 you are still a country mile away from the 7kW at -10 output. If you haven’t bought yet don’t, look elsewhere
Example - We are 192m2 ground floor with an another 30m2 of inside the heated envelope that is not directly heated. We have a 4kW heat pump and we are in NE Scotland and can get down to -9 degC.
The 50L buffer brings nothing to the party except a place to dump the high flow rates required by a 14kW heat pump.
Recommendation, do a proper heat loss calculation based on your apartment, size heat pump based on its output at your outside/inside design temperature. Add a 20% margin for defrosting and DHW (if heating you cylinder that way).
Unfortunately already installed, but I’m planning to take this as reservation to the notary. It’s a new block, and all 20 apartments have been equipped with it.
It’s indeed with DHW included
The more I’m learning about heat pumps the past days the more I’m starting to realise it indeed seems like a rather odd selection
Yes, that sounds very oversized. But there’s not much you can do if it’s already installed. Unfortunately this is very common. A mild oversizing a not a problem but a 7x oversize in your situation does have a significant negative effect on peformance, especailly for Daikin units that don’t have great modulation to with. We wrote up an example about of an oversized Daikin here https://docs.openenergymonitor.org/heatpumpmonitor/oversizing.html#over-sizing-factor-vs-performance-daikin-altherma
Here is a real life example of oversized v correct sizing. In this example I had a 6kW and replaced with a 4 kW. Same house, same plumbing, same temperature internally, same monitoring gear, just a different heat pump. 4 kW installed in January 2026. Both used for heating and cooling. Yellow lines are heat and blue are cooling. The blue circle are CoP
It looks oversized.
Silly question, but is the 14kW unit shared with other apartments?
That is the only reason I can think it may make sense.
I had a similar experience with a heating company in the North West of Spain. The company wanted to initially install a 10kW ASHP.
He had installed a similar unit previously and was familiar with the 10kW unit. When challenged to produce calculations it dropped to 6kW with no calculations, simply because my house was smaller. After obtaining heat balance calculations from a heating engineering and doing my own checks it was closer to 3.0-3.5kW. The company still recommended I buy a 6kW unit, as it was typically the smallest unit for the local climate. The 6kW recommendation was due to uncertainty around DHW heating time and “fat” to cover building and installation and intelligent control issues. I think it also avoided the risk of “winter call outs” if the house was cold over the festive period. Especially, if running with multiple zones and thermostat controls. I was also told the cost of the larger 10kW unit was not so different, so it was better. I wonder if the last point was a factor in supplying a 14 kW ASHP in your apartment. The heating company and builder may be happy with their choices but the customer is covering the life time bill.
I opted for a 4kW option, as the smallest ASHP in the range. It works well for me. It still modulates on a typical cold winters day, but is far better than a 10kW unit.
It is installed, however my purchase contract includes a clause stating that if there is a technical dispute an independent expert acceptable to both parties should give an opinion.
Andorran regulations are also quite specific about heat-pump sizing: it should be based on a heat-loss calculation to the EN12831 satndard, using the prescribed local design temperature, with an additional 0.2 kW per household member for DHW.
So unless the installer can produce calculations showing that these requirements were actually followed, it seems more like a potential failure to comply with regulations
I’m not really looking forward to pursuing all of this, but I guess it’ll be worth it in the long run. I also wonder how much energy is being wasted when this installation has been repeated across 20 apartments, and potentially oversized systems have been installed in many other buildings too
