Around -1c outside, i am seeing a COP of around 2.3 with my heatpump. Its more or less only doing space heating, running at a low target temperature of 34c
I have an Eletricial heater that gets preheated water from the heatpump, so its lifting from 34c to 45c ondemand.
According to the DataEnegineering sheet, its performing lower than the numbers published.
You don’t tell us what your compressor speed is doing throughout that display, but to generalise, Vendor performance data is usually based on bench tests at whatever speed equates to the BEP (best efficiency point) for marketing reasons, and is quite probably at a fairly high, fairly constant speed.
Real performance often requires running the compressor at reduced speeds and this can have several consequences from an en efficiency viewpoint. These include:
Non-linearity of non-compressor power consumers, such as evaporator fan(s), circulating pump(s), and controller electronics, and maybe a base heater.
Reduced refrigerant flow at lower speeds causes reduced temperature approaches in the evaporator and condenser, which increases the compressor pressure ratio thus energy consumption.
Some mechanical components may have lower efficiency at lower speeds, including the compressor motor (e.g. winding losses) and its inverter, and the compressor itself (e.g. friction losses, leakage losses).
Defrosts typically pull down the apparent CoP in any case, as heat flow is reversed and (for a few minutes) the instantaneous CoP becomes negative.
These “parasitic” losses can become quite significant at low compressor speeds, and result in an apparent reduction in CoP. If your operation demands that your compressor spends most of its time at say 50% of the design speed, measured CoP reductions of 20-30% are not uncommon. You are running about 35% below Vendor expectation, so given all those defrosts, you may not be doing as badly as you think…
Thanks for the reply - i’ll try and see if i see “compressor speed” somewhere in my system and start logging it.
Would a much larger heatpump - lets say it could invert to minimum 12kW @ 0c(and max 35kw @ 0c) - tend to defrost less, as its not performing close its maximum output? (if the requirement only was 12kW)
alright, i just had a look at the “operation parameters” its running with 75hz (4500rpm) when its doing maximum output. But then again, i dont know anything about the compressors capabilities… (buts is a 16kW model, and in the display it was delivering 16.16kW at 75hz.)
When it is doing regular defrosts (running hard, ambient <5c, humidity >90%) COP of 2 to 3 is about what you are likely to get. Luckily that is not too many days a year. Most heatpump manufacturers ignore defrost when they quote those COP numbers, the fact COP goes negative for a while during defrost is just plain embarassing. Many of them also ignore it when quoting Heatpump maximum power.
I’d be very wary of this - if the heat pump is oversized (at midwinter conditions) it will be even more oversized for spring/autumn conditions, and cycle too frequently for efficient operation or compressor longevity.
Cycling due to defrosts is quite different to cycling due to oversizing. Defrost cycles are just the result of the unit’s self-protection algorithms. These vary from vendor to vendor, and aren’t always publicised, but the main condition that provokes a defrost in my unit (a Samsung) is when the refrigerant entering the evaporator reaches a particular (cold) temperature. As @Dave_Storey quite correctly observes, this happens when the controller has to lower the compressor suction pressure more and more to achieve full evaporation, due either to poor heat transfer in the evaporator (i.e. ice build-up), or to prolonged high demand (more than the evaporator was designed for).
Your heat pump can deliver its nameplate duty, so as long as your house heat loss isn’t more than this at midwinter conditions (unlikely, unless you live in a very large badly insulated house), then further upsizing would be counterproductive. (If you do live in such a place, I’d be inclined to spend my money on better insulation than on a bigger heat pump…)
Thanks for the input - i am in the process of replacing some of the windows(especially those that have a draft…)
However , i think my main issue, is poor reading of the performance tables, i was hoping it would be able to sustain 14kw output around 0c - but its more like 10kw, a significant drop in performances. My heat-loss calculation says the house is rated at 10.5kw at 0c. It really annoys my, that i was not able to re-phrase the correct question while selecting which size of heatpump i needed.
Its fairly new, from 1995, good amount of insulation, but unfortunately not 3 layer windows (it has 2, but without a thermal coating)
Right now, its running with the eletrical backup to keep the house warm.
I have the “luxury” of having 2 houses that’s interconnected heating wise, so when its off-season, it could heat both properties, and when demand is high enough, i could close the connection between the 2 buildings, and have the heatpump running at each building by its own.
So i might creating a cascade with 2x16kw in building 1, and get a 30kw heatpump in building 2.
