American Standard heat pump w/ aux gas heat

[stealle]

My new home construction is almost done. I’ve got the American Standard Platinum 17 heat pump with auxiliary gas heat. It was my understanding the heat pump would heat my home until it got so cold it couldn’t do so efficiently then the auxiliary gas heat would heat my home. So I was thinking one or the other would be running. It’s been bitter cold here in central Illinois. Temps well below zero. This morning it was -11f. My outdoor AC/heat pump fan was spinning away at 76%. I’m not sure if the aux gas is running at the same time. Tried to open the unit to see if there are any flames, but opening the door trips the unit and shuts everything down. The house is nice and warm. I just want to know everything is running properly. So...

1. Should the heat pump be running at sub zero temps?

2. Can the heat pump and aux heat work together at the same time or is it just one or the other?

 

[Dana]

With most dual-fuel heat pump systems it's either/or. Heat pumps with auxiliary resistance heating strips can run both simultaneously.

At -10F most heat pumps will still be delivering a COP (coefficient of performance) of more than 1, but it'll be under 2. Auxiliary heating strips alone (without the heat pump running) would have a COP=1, just like electric baseboards etc.

If the heat pump is able to keep the place at temp without auxiliary heating at -11F it's almost certainly sub-optimally oversized for your seasonal heating load, and will be doing a lot of on/off cycling during the shoulder seasons, or even during more normal winter temps.

 

[Sponsor]

 

[stealle]

With most dual-fuel heat pump systems it's either/or. Heat pumps with auxiliary resistance heating strips can run both simultaneously.

At -10F most heat pumps will still be delivering a COP (coefficient of performance) of more than 1, but it'll be under 2. Auxiliary heating strips alone (without the heat pump running) would have a COP=1, just like electric baseboards etc.

If the heat pump is able to keep the place at temp without auxiliary heating at -11F it's almost certainly sub-optimally oversized for your seasonal heating load, and will be doing a lot of on/off cycling during the shoulder seasons, or even during more normal winter temps.

Thanks for the info! I don’t think I need to worry about the system being oversized. The Platinum 18 (i mistakenly said 17 in my first post) is a variable speed system and advertised as an “always on” system. I’ve found that to be the case even when the weather was mild. The compressor speed as shown on the thermostat seems to always be between 10-97%. I’ve never seen it off or 0%.

 

[Dana]

The Platinum 18 only has a bit more than a 2:1 turn down ratio, at most 2.5: 1 between it's minimum and maximum capacity at any outdoor temperature. To get the most out of them you really DO have to pay attention to oversizing to hit their HSPF numbers. See the min & max capacity graphs starting on page 10 in the linked document. Just because it's always on doesn't mean the compressor isn't cycling on/off. I have no idea what the thermostat display means when it says 10%- but whatever it really means it's certainly NOT 10% of it's maximum BTU-rate capacity.

Better class ductless and ducted minisplits have even larger turn down ratios, and it's possible to sub-optimally oversize them too.