Does any know the technical aspect of if/when a heat pump becomes so inefficient (outside temps), the heat strips become as efficient (shorter run times)?

I'm aware that nothing cost more than direct conversion of electricity to heat, but the dismal temp differential, and the long run times for the heat pump and house fan have me wondering. We have used heat pumps since 1984, so very familiar with their operation (and so glad we have a great wood stove to supplement the heat pump).

Way back when I worked for Lennox, the lab guys told me that even at -40F, the HP was still more efficient than straight electric heat.

The ductless systems are a high efficiency heat pump that produce heat at below freezing temps. I have had heat pumps since 1985 and used a fireplace to help heat the house. I know they have gotten much more efficient in recent years. I would install ductless today because they run the condenser and inside unit from a single 120v line to the condenser that feeds the inside unit.

Oops, the answer to your ? Is the units have gotten much more efficient in recent years. So it depends on the unit.

I had temps of 17 degrees the last couple of days and the heat pump kept me plenty warm. I have my strips disabled. It's always more efficient than running 15kw of strips forever.

I think the bigger issue is you only have so much capacity so when it's super cold you need the electric heat to make up the difference, so to speak.

Interesting topic. I just had to have a breaker replaced that feeds on of our HVAC units. We were down to 15 in B/CS. I had 2 HVAC techs come by over a couple of days. One emphatically recommended going to EM Heat at anything below 30. The other was just as emphatic that you could leave the heat pump in standard heat mode and that once the units run for 16 minutes the heat strips activate as a back-up to heat the house. So that tech never goes to EM mode. Do with that information what you will!

Just because a tech can fix a system doesn't mean he has a clue as to how it really works.

Appreciate the snarky comment, very constructive. That's why I finished my post with the chosen last sentence.

dr_boogs said:

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Interesting topic. I just had to have a breaker replaced that feeds on of our HVAC units. We were down to 15 in B/CS. I had 2 HVAC techs come by over a couple of days. One emphatically recommended going to EM Heat at anything below 30. The other was just as emphatic that you could leave the heat pump in standard heat mode and that once the units run for 16 minutes the heat strips activate as a back-up to heat the house. So that tech never goes to EM mode. Do with that information what you will!

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First tech didn't know what he was talking about. Regular heat mode on the thermostat will automatically kick on the strip heaters when needed to supplement the heat pump, usually around 30 degrees. If you put it on emergency heat mode the heat pump will not turn on, only the strip heaters.

Quote:

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Does any know the technical aspect of if/when a heat pump becomes so inefficient (outside temps), the heat strips become as efficient (shorter run times)?

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Short answer is usually strip heaters start supplementing the heat output of the heat pump around 30 deg. F and surpass the heat output of the heat pump compared to electricity used around 20 deg. F. You'll likely need both the heat pump and strip heaters running at the same time below 20 deg to keep your house warm if they were sized for Texas weather.


The lower the outside temperature the less efficient a heat pump is as there is less heat to exchange to transfer into the house. The electric strip heaters supplement the heat pump when needed. Electric strip heaters are expensive to run compared to most other heat sources, they use a lot of electricity for the heat they provide. I was told by my dad that the cost vs heat output to run the heat pump compared to the strip heaters equalizes around 20 deg F. Most correctly sized systems for Texas are going to need to run both the heat pump and strip heaters once the temperature gets below 20 deg.

This is all based on what I remember working for my dad in high school. It's based on the units he installed and how he calculated capacity for each home. He owned a residential HVAC installation company for 30+ years. He was an early adopter of the heat pump and Lennox's largest installer of heat pumps in the Houston area for several years.

On top of all of this, we are assuming a system is set up properly. These settings are all user (or tech) definable. I have a Daikin Communicating system with EWC Board. I can lock out my heat pump at just about any temperature I want. I can also disable my heat strips completely outside of emergency mode. My system uses low speed heat pump depending on demand, but will upstage to high speed heat pump if needed. If it still needs more, it will upstage again and bring on heat strips at 90% capacity. I currently have it set for 95%.

I really like my heat pump. My second hvac system is standard straight electric. It's a 2 ton. I can definitely tell from my electric bill when that thing has been churning the 10kw strips. Burns way more power than my 5 ton heat pump.