Outside, I have a LENNOX 13hpx-036-230-17 compressor: Outside Unit Outside Unit specifications Outside Unit safety label Outside Unit vacuum protector label

Inside, the air handler is a LENNOX CBX25UH-036-230-1: Inside Unit

The air handler lists 36,000 BTUH cooling capacity, with r410a at 450psi, cust part # 79w36. I have electric backup heating. (I believe two radiant coils because there are two large (30 and 60A) double pole breakers.)

Inside Unit specifications Inside Unit electric heat specs

I have a few questions as a homeowner to help protect the equipment, and run it as efficiently as possible:

  1. Is there any minimum outdoor temperature for operating this unit as a heat-pump, below which damage is significantly more likely to occur?
  2. Is there any minimum outdoor temperature for operating this unit as a heatpump, below which is it less than 100% efficient?

i.e. I know that radiant/resistive heating strips are 100% efficient at creating heat from electricity, and heat-pumps are something like 300-500% efficient because they move more heat from outside than they produce themselves. I know that heat pump efficiency decreases as outdoor temperature drops.

  1. Is there some sort of formula or table that shows the relation between temperature and efficiency?

  2. Is the Heat-pump's heating capacity the same as the listed cooling capacity? (36kbtuh) What is a good way to measure this if I want to monitor its proper operation over time?

  3. When should I expect it to enter defrost mode? (Is it based on a timer, temperature sensor, weight sensor, or something else?)

  4. Do you recommend any sort of awning above the outside unit to keep snow and freezing precipitation off of it? Or any shielding from nearby dryer exhausts?

  5. Are the indoor fans in systems like these of negligible amperage, and simply thrown on to one of the electric heating element breakers? Or is my fan likely the 30 A breaker and I have a single coil?

I would like to use my system as long as it will last, as efficiently as possible.


I got the original installer to come on site and help answer these questions. They showed me how to deduce what coil package was installed: decoded coil chart

They said that my unit will sense if it is too cold to run, and shut down on its own, but my climate basically never gets that cold. Also, that my unit is always >100% efficient. Even if there's no heat to pump from outside to inside, it will pump its own waste-heat inside.

They said my unit ships with a 90 minute defrost cycle, and they adjust it to 60 or 30 minutes. I have bought and will be installing a 'demand defrost' system from Mike MacFarland/EnergyDocs: Mike MacFarland heat pump demand defrost system This will radically reduce the number of times my system defrosts, extending its life, and reducing wasted energy. The $135 cost should pay back in 2-4 years depending on how cold my winters are.

My installer also indicated it was common practice to put the fan on the smaller of the two breakers, as it is considered negligible power draw.

In my main panel, I have two double pole breakers for the furnace/air handler. 60A for stage 1 resistive heat, 30A for stage 2. Then an additional 30A double pole for the compressor outside.

  • Keeping the ice off your out door unit will help it also keeping clean of lawn clippings and leaves is important. The systems I have installed had separate breakers for the heat coils and control/fans . The last thing if you live in a costal area regularly washing down the outside area will help prevent damage from corrosion
    – Ed Beal
    Commented Jan 24, 2016 at 16:12
  • Did you ever get an answer regarding the temperature at which the heat pump shuts itself down and relies on heat strips? I have a Lennox with similar questions, ours seems to stop around 35 degrees, which is absurd, and running up our power bill.
    – trpt4him
    Commented Dec 11, 2016 at 14:03
  • My understanding is there is no lower temp limit. The colder outside gets, the lower the efficiency of the heat pump gets, but the efficiency never goes below 100% and nearly all of its waste heat is dissipated indoors. I installed a switch to block my thermostat from calling for heat strips and I'm staying warm just fine. Commented Dec 11, 2016 at 22:44

2 Answers 2

  1. No. You may risk damaging your unit by running it in cooling mode when it is too cold out (although some systems have safeties around this), but heating in low temperatures should not damage the system.
  2. No. A correctly functioning heat pump should not ever drop below 100% efficiency.
  3. This is dependent upon the specific heat pump model. Often, manufacturers provide charts/graphs describing the efficiency at different temperatures, but I do not see one available for yours. Two data points are specified; at 47f it is rated at a COP of 3.5 (or 350% electrical efficiency), and 17F a COP of 2.42.
  4. No, but the heating capacity is often close, in mild weather. Yours is rated at 33,400 BTU/h at 47f and 21,400 BTU/h at 17f. Measuring this requires two temperature sensors (before and after the coil) and an accurate airflow measurement.
  5. Your system defrosts on a timer, after 30/60/90 minutes of compressor run time, depending on a jumper setting. (this is less efficient than smarter systems that detect increased air pressure from from frost blocking air movement through the coils).
  6. Protecting the unit from falling ice from eaves, and from accumulation of snow, is a good idea. However, make sure to leave ample space so that the airflow isn't restricted.
  7. It says on the side that your blower draws up to 2.3 amps.
  • 1
    Adding to point 7, to 30 amp breaker is probably for the condenser. The 60 amp breaker is for your air handler, and would include the heat strips, blower, and control circuitry. That chart should have a mark on it to indicate which heat package is installed, or written on the unit somewhere. If it doesn't, the contractor that installed it should come back and do that. Without that marking, it wouldn't pass inspection where I live.
    – longneck
    Commented Jan 25, 2016 at 19:50
  • Well, somehow it passed final without any indication of what coil package was installed. So I'll get them back out to figure this out for sure. I have a breaker marked AC that goes to the condenser outside, and then two furnace breakers (60 and 30A) Under heavy continuous load only the 60A heats up and even then well within limits. I just couldn't imagine a 30A breaker for the fan, so I had thought it was a second coil. They're going to have to come back to fix a duct problem anyway.... Commented Jan 26, 2016 at 2:36
  • How hard is it to aquire a 'demand defrost' sensor and control board? I'm qualified to make the electrical modification. What's the best place to order one from? I'd prefer one made by the same manufacturer, as opposed to 3rd party. Commented Jan 26, 2016 at 3:06
  • @BillyC. The best I can point you at offhand would be this blog post: energyvanguard.com/blog-building-science-HERS-BPI/bid/74297/…
    – Zhentar
    Commented Jan 27, 2016 at 19:56

If the heat pump has an oporational temp limit it will control itself. Be careful with an awning, it could cause discharge air to recirculate. Not good. Keep the filters clean and don't use much night set-back

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