when does emergency/backup heat kick in

[hobbit]

This is kind of a general question and the specifics for particular
brands/units might be all over the place. I'm in New England, where
we have ice storms and weird "snowpocalypse" events and power
failures from a half-second to a week, and it would seem to me that
if a heat pump with an outdoor unit and an indoor unit *equipped with
heat strips* loses functionality in its compressor side, that the air
handler should be able to at least heat the house a little from the
resistance strips on its own. The question: are most systems able
to do this, once they're configured to know that they have the
resistance heat capability? Will they revert to that "emergency"
operation if the outdoor unit has some showstopper problem?
This would seem like essential functionality, because a system
might be keeping an unoccupied house from freezing down and
we wouldn't want an issue in the greater complexity of the heat
pump completely disable something that could still run and
provide heat in a simpler [but more expensive, sure] way when
needed -- at least until the main system gets fixed.

I ask because I was trying to experiment with this on the Daikin
last night, outdoor unit powered down, ambient around 60 and
indoor still around 70 but I pushed the setpoint way up to see if it
would do the right thing with the installed 3 kw heater. It just sullenly
sat there with its "U4" error and didn't do squat. What would you
expect the right behavior to be?

_H*

[Shophound]

Maybe your Daikin locks out the heat strips when outdoor ambients are in the 60's.

[hobbit]

I did have that thought ... except that OAT can't be measured if the
outdoor unit doesn't have power anyway. The scenario I'm thinking
is indoor AHU powered, outdoor unit dead ... I should still have heating
capability.

I don't remember any particular statement of upper limits on heat
strip operation, but I'll have to scan my documentation again...

_H*

[Shophound]

Outdoor unit may not need power if there is a pressure switch involved. Dunno.

[hewitt]

Sounds like program t stat however have a tech come out

[XcelTech]

I don't know what Daikin system you have so that makes it hard to tell you what it should and shouldn't do. However, a split heat pump system is often(not always) setup for emergency heat in one of two ways.

1.) the thermostat is set for heat and emergency heat. When the operator wants heat they can switch the thermostat to either heat or emergency heat. Heat will be your heat pump and emergency will be your heat strips.

2.) a thermostat will be installed in the outside system. The 24v circuit will use this OAT to decide if the OAT is to low for the type of refrigerant to properly change state. If the temp is to low the thermostat will switch to your electric heat strips and shut down the outside unit.

Either system can be easily change to other by a properly trained and experienced HVAC technician with good knowledge of the type of refrigerants physical attributes, solid understanding of electrical engineering, and a few electrical parts.

[hobbit]

That would all likely be true *IF* this rig had a traditional stat.
It doesn't. Daikin's big thing WRT control is two-wire data
links -- one two-wire shot to the stat, and another one from the
AHU to the outdoor unit. It runs a fairly simple data-over-power
protocol, but of course that's all proprietary and not a whole
lot of information about programming is forthcoming from
Dakin's manuals, even if you know how to read the bad
translations from Japanese [and I do, having had a fair bit
of experience digging through auto service info]. So it's
not as simple as applying 24V to a "W2" lead for that
happy little "COP = 1" glow from the air handler...

My install tech isn't any help on stuff like this, it took
him a half-hour of messing around to figure out how to
inform the AHU that it's got aux heat in the first place.
That's all set right now, I just need to know how to make
that work as a true "emergency" backup if the outdoor
unit is lunched [or simply off].

_H*