Efficient low temperature conventional air source heat pumps
ranson
| Posted in Mechanicals on
Hi,
Can anyone point to conventional air source heat pumps (not mini-split) that are highly efficient at low temperature? I’m looking for something that will work year round in Rochester, NY, zone 5. (7°F manual J design conditions, but it can hit -5°F worst case.) I’m in the middle of a house re-design, so my old manual J is out of date, but I expect the heating load to be 20-30kbtu/h at 7F.
Thanks,
John
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
I am sorry for not being helpful, but what is a "conventional" air source heat pump in USA? I though mini-split is a conventional one?
John,
The Lennox XP14 is an air-source heat pump with specs listed down to -15 degrees F.
Heating capacity is 11,000 Btuh at 5 degrees F and 5,800 Btuh at -15 degrees F.
https://www.greenbuildingadvisor.com/sites/default/files/XP14_expanded_pg3.pdf
Of course, your house may need more heat than this unit provides.
Davor,
In the U.S., most homes with heat pumps have complicated duct systems. In many cases, one duct system sends hot air to each room, while a second return-air duct system pulls air from one to four locations back to the appliance. The ducts are usually large.
A unit called an air handler contains filters and a big fan. The air handler sends the air into the ductwork. These forced-air systems don't look anything like the minisplits that are common in Europe and Asia.
John,
You might consider the Trane XV20i. See the image below.
Source of image:
https://aceee.org/files/proceedings/2016/data/papers/1_700.pdf
.
They have a spreadsheet here of units which meet their cold climate spec:
http://www.neep.org/initiatives/high-efficiency-products/emerging-technologies/ashp/cold-climate-air-source-heat-pump
The spreadsheet is interesting, but it has a lot of numbers that don't make sense to me. A number of units have almost identical rated and maximum capacities, but the COP at the two values will be markedly different.
For instance:
Rated: 37kbu/h, 4.52 COP
Maximum 37.4kbu/h, 2.4 COP
Are there really heat pumps where a 1% change in output drops the COP by 1/2?
--John
JOHN RANSON:: With modulating systems the "rated" capacity is the modulated output at which the unit was tested for efficiency at +47F outdoor temperature, 70F indoors. The maximum capacity at +17F has to be at least as much as "rated" level at which it's efficiency was tested at +47F, but it can also be higher.
At +17F the COP at the rated modulation level will be dramatically lower than the COP at +47F.
Thank you Martin.
Sorry for slightly hijacking thread, but I was actually very interested in a ducted system here, but nobody does it. Distribution is a problem with minisplit in a house with multiple rooms.
Hi Dana,
I should have been more clear. They have columns for rated capacity at 17F and COP at rated capacity at 17F. I'm seeing these discrepancies at a single temperature.
Ah, I think I found the problem. It looks like they have bad data in the "Input Power at Rated Capacity 17F" column. It's identical to the "Input Power at Rated Capacity 47F" column.
I have a Rheem RP17 with the backup strips locked out when the temp is over 8°F. This heat pump did almost all the work last week when it got down to 0. The sales literature calls the unit a 3 stage unit but in fact the compressor runs at one of 5 different speeds.
Walta