many digital tstats DESIGNED FOR HP USE automatically switch to Emer heat. some call it Aux, some say Emergency.

BTW, it is NOT just for if the HP fails. it is for when HP cannot perform well anymore due to low ambient temps or to bring the indoor temps back up quickly. most all tstats will kick inthe aux source if the desired temp is 2-3 degrees or more than current temps. try kickiing your heat temp up to 5 degrees warmer than teh tstat is reading right now. it should kick in teh aux, though it may ALSO run the HP.

your stat should also kick in the aux during the defrost cycle. it HAS to do this because to defrost a HP you switch it back to AC mode and it blows cold air in the house. the aux is used to bring this back up to heated air so you don't get blasted periodically!

OK, so if a heat pump *fails* lets say froze up or the compressor locks up during the winter and it's 40F outside, the HP should be running but can't provide the heat that the thermostat is set for, in this case 68F, the system automatically turns on the gas aux? What shuts down the HP? I think I understand a HP normally runs and switches between heat and cold to provide conditioned air to a home. My question is when the HP fails and cannot provide heat for the home I assume the gas aux turns on but what shuts the HP down? I'm confused on the term "emergency heat".

zeke- in my system the auxilliary heat is a heater pack that is seperately powered and sits above the output of the HP blower fan. the tstat will call for this aux heat (turns on the heater packs) when the desired temp and the actual temp are more than 3 or 4 degrees apart. I also thought that the heat pump itself ccould cause the Aux heat to turn on when it senses a problem.

Inside your heat pump is a computer controller that takes inputs from the tstat as well as sesnors inside the unit. if the compressor freezes or some other issue is sensed - then I believe the heat pump itself will shut the compressor down and kick on Aux heat.

I had this happen to me with one of my geothermal heat pumps last winter when the HP computer sensed that the compressor was working to hard. It tuned out to be a loop field problem (they had the wrong mixture of anti freeze in the line). So when the tstat would call for the HP to turn on, the blower would start, then the compressor would start, then about 60 seconds later the compressor would turn off and the Aux heat would come on. I think the HP computer made all of that happen - pretty sure it wasn't the tstat.

You are lucky to have the gas backup heat. I have electric heater packs and I absolutely hate to see the darn red light come on - because I know that I have a 10 kilowatt toaster running and it is very expensive. The good news is that when the system is working correctly, it can get to zero outside and the Aux heat won't come on.

David - do all heat pumps have a defrost cycle? I always assumed that when Aux heat came on it was either really cold outside OR there was a problem. How can you tell if the system is in defrost mode?

All heat pumps defrost far as I've ever known. There is normally no indication on the thermostat of the defrost cycle when it occurs as it's a normal part of the operation. Typically when in defrost, the compressor runs but the outside condensor fan shuts off. Under the right weather conditions, steam may be seen coming from the condensor unit.

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During defrost the heat pump switches to a/c mode and turns on the heat strips to prevent a "cold blast." Emergency heat and auxiliary heat aren't technically the same thing, but usually this involves the heat strips. Emergency heat is heat strips only. Auxiliary heat is heat pump and heat strips when the heat pump can't keep up or has to catch up from a differential. If you have gas furnace backup, it's a different ballgame. I wouldn't want the heat pump running with the gas furnace as the same time, though this may happen during the defrost cycle (not 100% sure how it's usually setup).

I have a Honeywell Vision series T-stat with an outdoor temp sensor that controls the Heat Pump also. When it gets below 20 degrees outside the T-stat shuts down the heat pump and goes to staging the Aux heat, in my case two stages of heat strips. There are relay units that can be added to a HP to do the same thing but they whole assembly is outside in the elements. I chose to just have a temp probe outside under the deck so there are no extra relays or circuitry than the minimum for the HP. I can also just look at the T-stat when I want to know how miserable it is out.
Another bonus to a t-stat with outdoor temp is if you run your temps on a time schedule it can figure approximately how much run time is needed to get the temp to the set point by past memory as most can, but can also look at the outside temp to calculate when to start to ramp up the temp.
If the temp outside is 30 degrees and the house is 65 and the set point is 70 at 5pm it might take 1.5hrs to raise the temp to that point. If a few days later it is 50 degrees out and it has to bring the temp up the t-stat can wait and start bringing up the temp say an hour before 5. rather than 1.5hrs. I didn't think it would make a noticeable difference until I swapped from an energy star t-stat to one with outdoor temp. I really only purchased to get the control feature so it will keep the HP from running when its too cold because I would forget and let it run all night when it was 0 out and I know that is not the best on things.

Usually the only difference in set up is with a HP and gas backup, there is usually a plenum stat that is enacted so that when a HP goes into defrost (AC mode) the gas furnace fires to resolve the cold air issuers but with additional circuitry only will allow the plenum to rise to about 100-110 degrees by opening the 2nd stage (furnace burner) via the plenum stat (usually a snap disc).

Many complex HP's and AHU's have the extra wires to accomplish if none present by adding simple plenum F&L inserted and only using limit side to cut burner during activation via defrost signal/cycle.

Without this additional safety, a gas furnace and over boil the refrigerant causing havoc on the compressor that is looking for that temp difference.

If it wasnÂt covered, switching on emergency heat just shifts the auxiliary heat to the first stage and locks out HP operation. Depending on the stat, auxiliary heat is usually 1-2.5 degrees behind 1 stage which is the HP.

"Without this additional safety, a gas furnace and over boil the refrigerant causing havoc on the compressor that is looking for that temp difference."

That's what I was thinking. I wasn't quite sure how they would handle the furnace's operation. I was concerned with pressures, too.