ASHRAE Journal - September 2021 - 20

TECHNICAL FEATURE
lowest. Ultimately, in all but the warmest climates, electric
resistance heating will result in higher energy costs.
In some instances, when a building is super insulated
electric resistance heating can make sense. Since very
little heating is required in this situation, the additional
expense of a heat pump may not be worthwhile.
However, this is only the case when heating loads are
extraordinarily low.
Conversion of Existing Buildings
Using heat pumps in new construction is more
straightforward compared to retrofitting heat pumps
into existing buildings. The best time to retrofit an
existing building is when the building is going through
a gut rehab and much of the HVAC system is going to
be replaced. A valid concern when converting existing
buildings to all-electric heating systems is the increase
in electric requirements.
Heating loads, which were previously met through
natural gas piping and infrastructure, are converted
to share the electrical service, which already serves
lighting, plug loads and other building systems. When
designing electrical systems, the code requires conservative
assumptions and that spare capacity be built into
the system. Most buildings operate well below their electrical
capacity. In many cases this buffer is more than
enough to cover the additional electrical load required
by the heat pump. Several recent projects by the authors
of this article have not required electrical service
upgrades when converting the building to all-electric
heating and cooking systems.
Where little additional electrical system capacity is
available, system design options exist such as system efficiency,
energy storage and sharing load with cooling systems.
For commercial buildings converting to a VRF system
or for residential buildings that already have cooling,
converting to a heat pump air conditioner or multizone
split system usually results in little or no increased electrical
loads. In larger buildings, thermal storage coupled
with heat pumps allows for smaller and/or fewer heat
pumps, resulting in smaller peak electrical loads.
In some cases, increases to electrical service will be
required. This can be time-consuming and expensive.
Longer-term planning, coupled with other improvements
to the building such as higher density or higherpaying
tenants, can offset these costs.
20
ASHRAE JOURNAL ashrae.o rg
S E PTEM B E R 2021
TABLE 1 Low temperature limits by equipment type.
EQUIPMENT TYPE
R-32 Heat Pump
R-134a Heat Pump
CO2 Heat Pump
High Efficiency VRF
LOWER TEMPERATURE LIMIT
5°F
-4°F
-20°F
-22°F
TABLE 2 Hours per year at each temperature by city.
CITY HOURS PER YEAR BELOW 5°F HOURS PER YEAR BELOW -20°F
Fairbanks, Alaska
Duluth, Minn.
Helena, Mont.
Minneapolis
Boulder, Colo.
Chicago
Denver
Nashville, Tenn.
New York
Seattle
2,177
676
188
387
120
163
25
130
12
5
Heat Pumps in Low Ambient Conditions
A limiting factor of air-source heat pumps is their
operation at low ambient temperatures. In colder climates
such as the Midwest or Canada, heat pump applications
have historically been limited to more expensive
ground-source heat pumps to ensure they would be able
to operate during the extreme winter cold. However,
heat pump technology has now advanced to the point
where they are able to operate at all but the lowest
temperatures.
Table 1 shows the lower operation limits of heat pump
products currently available on the market. Several companies
are also currently researching and testing new
products with improved low ambient performance. We
expect significant progress in this area in the coming years.
Currently available products are already more than
sufficient in all but the coldest climates. As per Table 2,
outside of Minnesota and Alaska, a properly sized
high-efficiency VRF system should be able to satisfy
all heating requirements. These systems are generally
also available with a backup electric resistance coil for
extreme days. It is, however, important to ensure the
controls are properly calibrated so the electric resistance
coil is not operating when it is not required.
The limitation of low ambient heat pumps is they are
currently limited to fairly small capacities. This is less
of an issue for VRF systems given their modular nature,
http://ashrae.org
ASHRAE Journal - September 2021

Table of Contents for the Digital Edition of ASHRAE Journal - September 2021
