What happens to heat-pump COP at low power levels?
I’ve decided to install an air-to-air heat pump for heating the house. Choice of brand is easy enough but have stumbled at choosing the size. I don’t mind spending a little more capital on a bigger unit if it will work better on the tougher days. However, there is the problem of the numbers. The published COP ratings all point to the smaller units being more economical on electricity.
I’ve found one technical manual so far. It has a table of expected COPs at varying outside and inside temperatures. But only at maximum capable power of each model! Which, of course, is a higher wattage for each model up in the family. This means that there is no direct comparison between the models let alone across families or brands.
Now, one might expect that at a given low power output, ie: your typical maintained lounge, that all models in a family might achieve the same COP figure … but is that so?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Oh, in case it wasn't obvious, this question applies to inverter models. Fixed speed compressors only have one power level and have to regularly turn off and on instead.
And, if the answer isn't yes, then a second question emerges - How come the larger models consistently have poorer COP?
Your question is not entirely clear but I will tell you what I know. COP corresponds to a unit operating continuously, no cycling. A 1985 study found that COP drops by 7% from ideal when runtime is 90%, and dropped by 30% when runtime is 20%. So if you oversize the unit, you lose efficiency.
This might be the reason manufacturers are making 2-stage compressors. I just don't know if the benefits of a 2-stage are worth the costs. I am struggling with question as I decide whether to use a single-stage or 2-stage heat pump in my house in Seattle.
You might want to do some research on heat pumps with variable-speed / inverter compressors.
Almost all compressors on the market are single speed, which means that they serve part load by cycling on and off. The efficiency figures quoted are always for steady state operation, but if the unit is cycling frequently then it's performance just after starting comes to dominate the performance picture. The starting performance will be lower than the steady-state performance, but it will depend on the specific product in question, and getting that information is likely to be hard to impossible.
The best way to minimize these lossses, practically speaking, will be to maximize your thermostat's deadband so as to minimize cycling.
I'm not aware of any true variable-speed compressors that are also small. There are "digital scroll compressors" that appear in some small package units (e.g. by Aaon) but these are not variable speed either: rather than cycle off, they engage and disengage the scroll plates which perform the compression which allows them to continue to run but use relatively little energy. You get better temperature control, and maybe somewhat better performance, but they're still not true variable speed units.
Check residential (and light commercial) equipment via AHRI.
For example: Certified Variable-speed Mini-and Multi-split HP
Brent,
I am shopping for a heat pump too and lots of brands advertise small (3-ton) heat pumps that are 2-stage compressors (Lennox, York, Trane). Are you saying these are really not 2 speed compressors?
I also had the idea of having a thermostat with an adjustable "deadband", if what that means is the temperature difference between the set point and the temperature when it comes on. I didn't know if that kind of control was useful or available.
Robert
I'm (In the opening post) referring to inverter driven compressors that adjust compressor speeds, along with the fans, according to temperature requirements. They are the only thing one would buy in the last few years. The old fixed speed units are still in the catalogues but I doubt anyone buys them now. Well, not where I live at least.
I had a look at http://www.ahrinet.org/ but found it to be rather devoid of any info. There was just nothing there except a few references to changing certification standards.
Robert,
We all would need a little more information in order to offer specific advice;
is this a complete system replacement or a retrofit of an existing system?, is the desired method of conditioning distribution via air or hydronic, convective, radiant etc. ?
From another post it seems that you are considering the, possibly, incremental upgrade of the thermal performance of the exterior envelope?
An accurate energy modeling of heating and cooling loads would be dependent on insulation, air infiltration and solar orientation etc.
In terms of equipment one may need to look beyond the ‘big three’ you mention (minus Carrier) and towards manufacturers of variable/inverter compressor models; for example (in alphabetical order) Daikin, Fujitsu, Mitsubishi, Sanyo, and others from regions with high energy costs for efficient and effective systems.
The variable compressor systems will adjust to the size of the load using proprietary software/controls to negate over-sizing/short cycling concerns.
http://www.ahridirectory.org/ahridirectory/pages/vrfhp/defaultSearch.aspx
lists mini/multi splits, there are options for fan coils and air handlers to adapt to existing duct work with some VRV/VRF models etc.
I had a look at that PDF. All the usual info.
You basically just answered yes, that the larger heat pumps perform with a higher COP when running at lower power levels. I'm happy to hear that but is there any documentation to back it up?
Here is another overview/collection of research data:
rhttp://wogone.com/iq/review_of_domestic_heat_pump_cop.pdf
Likely you will have to go though to each manufacturer for performance data over broader temperature ranges?
Nice review. But, being a hot water review, all the heat pumps were fixed speed types that heat a hot water cylinder to set temperature then turns off. It didn't deal with my question.
What happens to COP when a heat pump reduces power level as it reaches set temperature in a living space?
I think I've got my head around why the larger models consistently have poorer COP listed in the specifications. It's because the indoor radiator units are pretty much all the same size and same air flow as it's smaller siblings. There is some variation but nothing like the variation in power output levels across models.
How I've come to understand it is that COP is a function of the internal temperature difference from outdoor unit to indoor unit given fixed surface area and reasonably constant air flows. So, to achieve a higher power output, the vapor temperature of the indoor unit has to be raised higher. Reducing the COP.
So, yeah, I guess that's proof enough for me now. The answer is yes. A heat pump running at reduced power level will have better than spec'd COP.