For the record, and too late for this matter:
Booster circuits are optional for heat-pumps.
They have no utility in mild climates, those that never get below 45 degrees Fahrenheit. Such places do exist, just not anywhere close by.
The industry standard is to provide 0,1,2 and perhaps 3 booster modules for a given heat-pump scheme.
Here in Sacramento, a single booster is more than enough to handle occasional dips below 32 degrees.
When running in heat-pump mode (reverse air conditioning) the exterior coils evaporate refrigerant -- even down below 20 degrees. This does not cause massive ice dams because the air is already dry -- naturally. Other factors can intrude, however. Any, local, saturated air would cause ice to form over the coils. A good design addresses this, one way or another.
Because of the limited size of the coils, when the refrigerant is evaporating at 20 degrees, the exterior air ejected by the unit is still quite a bit warmer... say 10
to 13 degrees. This temperature 'head' represents thermal resistance during the fluid to fluid heat transfer. (Air and liquid refrigerant are both deemed to be fluids.
This temperature head, coupled to the physics of suitable refrigerants, means that it's no longer possible to extract heat from the outside air once it falls below 30ish degrees.
If your weather is like Sacramento, then a single booster will get you over the hump -- if the grid can handle it. (Mass adoption of heat-pumps would overload any grid during a severe cold snap.)
The greater portion of the lower 48 American states can get by with just two boosters. Both are typically twins, and can be retro-fitted at any time. Their slot is typically exposed -- on the same side as the air filter.
Some, not all, designs permit a third booster coil assembly. This, when added, provides BTUs to take you through the worst storms. However, your e-meter will be spinning.
If you've never seen what a booster looks like: think of an oversized toaster grid/ ancient e-overn. Some strongly resemble blade-style server computers: they can be shoved into place almost as fast.
These boosters do present an issue to electricians during the build. The disco has to be sized to allow for them. Further, protection is also needed for the rest of the machine. what ever is done must not impede HVAC service (filters, boosters, etc.) while still attaining NEC standards. That can really require clever work as the HVAC industry thinks we're miracle workers.
Last edited by Tesla; 01/24/14 07:02 PM.
