[jotato]

> Sounds like it was undersized for your house (unless your doing something crazy like trying to hold 80+).

It was set to 69 and held steady at 67 running for 8.5 hours. It wasn't until the temperature outside broke 36 that the inside temp slowly started to climb. It is a 3ton system for a 2500sqft home. From what I have read it should be enough. I think the problem is that it is an older heat pump and just doesn't perform well below 35.

> FWIW I was able to get efficiency charts and such at various temperatures from my salesperson, but I gather most don't bother.

I think this is my point? This was here when I moved it. I shouldn't have to go to a salesperson. I should be able to look it up in the specs or even the manual. At a minimum the manufacturer should provide it when asked.

[throw0101a]

> It was set to 69 and held steady at 67 running for 8.5 hours. It wasn't until the temperature outside broke 36 that the inside temp slowly started to climb.

This may be normal. Standard practice, per ASHRAE, is to design for 99% of days, and for the remaining 1% (~4 days per year) the system may have to run 24/7 (either for heating or cooling):

* https://www.energyvanguard.com/blog/we-are-the-99-design-tem...

* https://www.ba-inc.com/hvac-systems-what-is-a-design-day/

This allows for equipment that is "right-sized", as issues can arise (including premature death of equipment from short-cycling) if they are oversized:

* https://www.energyvanguard.com/blog/why-an-oversized-air-con...

* https://www.cooltoday.com/blog/3-problems-caused-by-an-overs...

* https://bryantlincoln.com/understanding-the-risks-of-oversiz...

By designing for 99% of the time, your gear should be just enough to handle things.

[jotato]

I live in the pacific northwest. Winter are low 30s at night for 3 months out of the year :( Granted, highs are mid-high 40s so it works fine during the day 90% of the time

[thsksbd]

If the unit is sized correctly for the appropriate climate, then its broken. There's no reason the system cant deal with 0C, the COP should still be well above 1.

Get the refrigerant level checked by someone that didn't install the system.

For what its worth this is why I don't like to rely on heat pumps. Furnaces are really primitive, simple devices: little more than a heat exchanger and a blower. A heat pump has a hermetically sealed volatile liquid undergoing phase changes by a compressor with seals.

They're cool. They're efficient. They're great.., but they're also far more complicated and we're living in an era of cheaply built consumer goods

[throw0101a]

30F is hardly anything.

Mitsubishis can work down to -13F/-25C and still have a COP of 2.08:

* https://www.mitsubishicomfort.com/articles/keep-warm-this-wi...

* https://mylinkdrive.com/viewPdf?srcUrl=http://enter.mehvac.c...

Heat pumps that look like standard ACs often can run at 100% capacity down to 5F and still have a COP>2 (1=resistive heating).

Also worth perhaps looking into how leaky your house is:

* https://www.energy.gov/energysaver/blower-door-tests

and then after that maybe insulation.