A few months ago I wrote about air-source heat pumps (ASHPs) in cold climates, and I promised more info on how to select the right systems and get the best performance. Below are some things we’ve learned from our work with ASHPs in the Northeast; much of this is based on the results from a study supported by the DOE Building America program. To be clear, we’re talking about inverter-driven (variable-speed) heat pumps in residential applications during heating season. Cooling is certainly important also, but we’ve been more focused on the heating performance, especially at lower temperatures.
1. Size the system properly. This is nothing new for mechanical systems, but the issue is a bit more confusing for variable-speed heat pumps. Some have claimed that oversizing variable-speed ASHPs is not a big deal. Compressors are much more efficient at lower frequencies, you still have the full coil area for heat exchange at low speed, and so (the argument goes) it’s almost better to oversize a variable-speed heat pump so the compressor is at lower speed more of the time. And this makes sense to a point, but in our experience we’ve found there are two other factors that come into play:
A. Cycling at warmer temperatures. This issue is pretty well understood, even by people who claim some oversizing is not a big deal. Most ASHP compressors can’t modulate down to speeds below 20-30%, so there are times when this 20-30% capacity will be result in too much heat. In order to limit overheating, the heat pump will have to turn off entirely and start to cycle. And when these heat pumps cycle, they are NOT efficient.
We saw this at several homes in our study. Site #2, for example, was a Passive House with a ~5,000 Btu/h design load has a 1-ton heat pump. In general the heat pump was more efficient at higher outdoor air temperatures, but – because of cycling – this heat pump’s efficiency started to drop as outdoor air temperatures rose above 40°F or so. The heating loads in this home at these warmer temperatures are really small, so this may not have had a big impact on seasonal efficiency, but it’s definitely noticeable.
B. Indoor fan speed. I believe this is a much more significant issue than cycling at mild temperatures, though it may be primarily a concern for ductless heat pumps. When a heat pump is oversized, the fan speed of the indoor fan coil is typically in low speed most (or all) of the time. In our study, these oversized systems with consistent “low” fan speeds had lower efficiencies. Fan coils can be manually set to higher speeds, but with larger equipment this may lead to noise or comfort issues. Better to get the proper capacity for the space.
There’s a lot published elsewhere on how to size heating and cooling systems. ACCA Manual J is the industry standard, and ASHRAE Fundamentals guidelines provide similar results. Sizing heat pumps is trickier than other systems as both heating and cooling capacities must be considered, but oversizing – especially of ductless heat pumps – may result in lower efficiency.
2. Pick good, cold-climate equipment. The typical metric for heat pump efficiency – HSPF – was developed decades ago when heat pumps were more prevalent further south. In colder climates, I look more closely at listed capacities and efficiencies at very cold temperatures – at or below the design temperature for the location. One big challenge is that different manufacturers present this info differently, and some don’t provide it at all.
NEEP has tried to address this issue by developing a cold-climate performance specification and listing of heat pumps that meet this spec. There’s a database on NEEP’s site with performance parameters on well over 100 cold-climate ASHPs. This can be a great resource for identifying good cold-climate products, but keep in mind most of this info comes from manufacturers – there’s not (yet) a rigorous, independent procedure to evaluate cold-climate performance.
3. Consider low-wall fan coils or ducted systems. One of the striking trends in our study was very high return air temperatures with ductless heat pumps. Perhaps this shouldn’t be surprising, and ductless fan coils are typically located very high on walls – often only a few inches from the ceiling. As warm air rises (and the heat pumps are sources of warm air) it’s not surprising that return temperatures were high (high 70’s°F were typical and well over 80°F was quite common).
IR imaging of this ductless fan coil (very soon after it turned on) shows there’s no short circuiting – i.e. hot supply air going directly up to the return. But the ceiling is already starting to warm up, and it’s not surprising that return air temperatures are significantly higher than average room temp.
Both capacity and efficiency drop dramatically with high return temperatures, and this is likely one of the major reasons for relatively low efficiency in the systems we’ve studied. A hopeful development, I think, is the growing number of low-wall or floor fan coils from several manufacturers. For heating dominated climates, I think these fan coils may make a lot of sense. I’ve not tested any, but I suspect they’ll have much lower return temperatures and, therefore, better efficiencies. Ducted heat pumps (with decent design) can also avoid the high return air issue.
Ductless floor (or low-wall) fan coils may offer higher heating efficiencies, but we haven’t yet seen many installed (or tested them). This is an example image from a Fujitsu brochure.
Coming next, recommendations on locating outdoor units, installation concerns, and O&M issues.