Is This Strange Or What?

ncrealestateguyFebruary 2, 2013

I just replaced my air handler and heat pump. The HP is a 19 SEER. I live in Charlotte. The last few nights have gotten really cold for here.
The HP is not kicking into auxilary mode until about 26 or 27 degrees. This has been the case now 3 or4 times. The house is still comfortable as far as heat is concerned. The temp. setting is being held. What is really strange is that at 26 degrees, the auxilary heat was not on. Then when the temp climbed to 27, the auxilary kicked in. At 28, it kicked back off.
It is a two stage compressor and a variable blower. Is it not kicking into auxilary mode because it has such a high efficiency rating? I was always told that they should kick over at about 30 - 35 degrees. I have put a call into the guy that installed it for his take.

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On my Carrier equipment the switch is determined by some settings in the Set Up mode of the Thermostat.

Again, on my equipment I have to enter the Set Up or Service Mode. I recently changed mine from 30 Deg to 25 Deg and I am currently evaluating that change.

    Bookmark   February 2, 2013 at 9:58AM
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Sure you are not going into defrost cycle?

Auxiliary heat and HSPF efficiency have nothing to do with each other.

Without more documented details, I think you are fretting over nothing.


    Bookmark   February 2, 2013 at 10:03AM
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It sounds like you heat pump is, or is similar to a Carrier Infinity 2-stage HP. Tiger is right regarding defrost cycle. Around here those HPs are useable down to at least 10F - minimum! Properly sized, you should not under normal operating conditions ever need your backup except for defrost or compressor lockout.


    Bookmark   February 2, 2013 at 10:33AM
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Yes, it does enter defrost mode. That is not the problem.
My confusion is that it has been said on here and over and over again by the experts that a HP is not efficient below about 30 - 35 degrees. So why is mine staying in HP mode down to 26 or 27?
It is a Bryant (Carrier) Evolution 19 seer 4 ton.

    Bookmark   February 2, 2013 at 3:56PM
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" So why is mine staying in HP mode down to 26 or 27?"

I tried to suggest why in the first post in response to your original post.

What is your thermostat set to as far as the crossover from heat pump to aux heat? 25, as mine is?

    Bookmark   February 2, 2013 at 4:27PM
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It is true that HP BTUs begin to drop off below 30-32 degrees. That is why you need the heat strip to supplement the BTUs lost. But the condenser still is producing heat and generally I don't recommend locking out the condenser until outside temps dip below 10 degrees. One would have to calculate the cost to run the condenser versus the BTUs produced. When COP drops below a 1:1 ratio, then the homeowner is in a losing proposition.. He should note the outside temperature and then lockout the condenser from operation.

For Charlotte, NC I have probably told you more than you need to know.


    Bookmark   February 2, 2013 at 4:38PM
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You aren't having a problem IMO. The unit is maintaining the desired indoor temp without using the auxiliary, which means the heat pump is producing sufficient heat and the auxiliary by definition is not needed.

If it wasn't maintaining the desired indoor temp and the auxiliary wasn't activating, then there'd be a problem.

    Bookmark   February 2, 2013 at 5:43PM
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I was told by the installer that the HP senses when it can not extract an efficient amount of heat and that is how it knows when to go to auxilary heat.
So Saltidawg, you are saying that I can go into the thermostat and set whatever temperature that I want for it to switch over? If this is true, then the manafacturer should know what the most efficient value is and that should be the temp setting for the aux heat to come on, no?

    Bookmark   February 2, 2013 at 10:05PM
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The heat pump does not sense when aux heat is needed. When the thermostat setting can not be maintained and inside temp begins to drop, aux heat will activate to maintain thermostat setting.

Depending on type thermostat one has and it's features, you can lock out aux heat from activation. One can also lock out the HP condenser. This requires an outdoor thermostat sensor.


    Bookmark   February 3, 2013 at 2:43AM
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So why don't manafacturers stae what that optimum crossover temp is?
And it sounds like from the above posts that both the HP and the aux. heat come be on in some degree at the same time. Is that right? Or is the crossover either one or the other?

    Bookmark   February 3, 2013 at 8:29AM
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The so called crossover temp that you refer to varies from home to home because of various factors including insulation and building qualities of home, number of persons living in home, and indoor comfort temperature setting.

I don't believe you have ever identified the model thermostat you have, at least not in this thread. Do you even have outdoor temperature sensor connected to thermostat?

Assuming you have the correct thermostat and outdoor sensor, I would lock out aux heat down to 30 degrees and condenser at or below 15 degrees.

Do you know size of heat strip?

Of course you will need aux heat to temper air on defrost calls.

I personally think you are making too much out of this. What exactly are you attempting to accomplish?


    Bookmark   February 3, 2013 at 9:19AM
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It makes sense to me that if a HP has an outdoor temperature that it makes sense to switch over to the aux heat, then it does not matter where in the Country it is located, what type of insulation is in a home nor how many family members live in the house. The HP is pulling heat out of the outside air, so why would all of these other factors matter at what temp it should call for Aux heat to come on at?
I am attempting to make heads and tails out of conflicting statements that I have been told and that I have read here... that HPs should switch over to aux heat at about 30 - 35 degrees. When the temperature was 26 degrees and mine was still using the HP, it made me question why everyone says that it should have already switched over.
My thermostat is the high end of the Evolution series an does have an outside temp gauge.

    Bookmark   February 3, 2013 at 12:59PM
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Perhaps you should consider a HP course at a community college in your location.

People come on this forum asking for help but sometimes can't accept the good and accurate advice they are given.

Meaning no disrespect but you are one of those persons.


    Bookmark   February 3, 2013 at 1:08PM
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For the OP, the first answer you received was:

"On my Carrier equipment the switch is determined by some settings in the Set Up mode of the Thermostat.

Again, on my equipment I have to enter the Set Up or Service Mode. I recently changed mine from 30 Deg to 25 Deg and I am currently evaluating that change."

Did you check to see what temperature your thermostat is going to cause your heat pump to stop and your Aux heat to takeover.

If it is set for 25 degrees and you want it set differently, do it!

    Bookmark   February 3, 2013 at 1:41PM
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If the Coefficient of Performance (COP) for the heat pump is greater than 1.0 when the temperature is 26 degrees then it is operating more efficiently than the auxiliary heat. However at those temperatures the air coming out of the registers may be cooler than you like. The wind chill affect of the cooler air may make you uncomfortable. Therefore you would want to raise the temperature to where the auxiliary heat comes on to make the air feel warmer.

You stated you feel comfortable with the current set up, but someone else in another house may not. This is why the manufacturers allows you to set to the crossover temperature for the maximum comfortable and efficiency.

    Bookmark   February 3, 2013 at 2:55PM
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I came here looking for an answer as to why it is that evryone has told me, and everything I have read, says that a hp should kick over to aux heat at around 30-35 degrees, even though mine does not. I also asked if there is an optimum temp for this, then why does the manafacturer not divulge this temp.
TD, you have not answered any of my questions. There are people on this forum that authentically like to help people out with their knowledge in a particular field, and then there are those who reply here with snobbish remarks, acting as though their reply is the only answer out there, just to make themselves feel superior.
Meaning no disrespect, but you are on of those persons.

    Bookmark   February 3, 2013 at 2:59PM
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This post was edited by saltidawg on Sun, Feb 3, 13 at 21:33

    Bookmark   February 3, 2013 at 3:23PM
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It is normal and correct for auxiliary and compressor/heat pump to run simultaneously. The definition of auxiliary, aka supplemental heat in that scenario, is that supplemental heat supplements/boosts the output of the heat pump (meaning both run together) when the heat pump can't by itself maintain the desired indoor temperature.

A bare-bones heat pump with a plain-old thermostat and no outdoor temp sensor ... the indoor thermostat is what controls when auxiliary/supplemental triggers by way of the indoor temperature dropping sufficiently below the setpoint. For example, a 2F differential ... if the desired room temp is 72F, and the heat pump can't maintain 72F, and the room temp drops to 70F, then the thermostat triggers the auxiliary to boost the heat pump. This is a self-regulating scenario -- the thermostat automatically triggers the auxiliary any time the indoor temp falls too far below the setpoint (OR if the homeowner manually raises the thermostat setpoint more than 2F at a time). With such a bare-bones system, the heat pump is never locked-out from running. However, the homeowner can himself decide to engage the emergency heat switch on the thermostat to manually shut off the heat pump so that only the auxiliary runs. Emergency heat is normally engaged if the heat pump (compressor or condensor fan) fails and the only source of heat is the auxiliary, but it can be deliberately engaged by the homeowner if/when desired. An example is during wet/rainy/humid weather if the compressor coils are quickly and heavily icing-over between defrost cycles.

Optionally adding an outdoor temp sensor to the system, with a compatible thermostat, allows setting lock-out temps depending on the capabilities of the thermostat control.

1. Auxiliary lock-out. The auxiliary will not run (except it will run during defrost) unless the outdoor temp is lower than the lock-out. This means, for example on a lock-out of 35F, that the auxiliary will not run if the room temp is 2F (or 5F or 10F, or whatever) below the setpoint, unless the outdoor temp is lower than 35F. Some thermostats also may trigger the auxiliary on a time-based algorithm. If the heat pump has been running for a period of time and the setpoint has not been reached, the thermostat "figures" that the heat pump isn't able to keep up, so it triggers the auxiliary to go ahead and reach the setpoint so the system can cycle off. Auxiliary lock-out can avoid running the auxiliary to "top-off" the setpoint unnecessarily, if the indoor temp is not uncomfortable and the heat pump would "catch up" by running a while longer than the thermostat's control algorithm allows.

Understand that auxiliary lock-out does not mean that the auxiliary will always run at outdoor temps lower than the lock-out. It just means that the auxiliary can run if the thermostat calls for it.

2. Heat pump lock-out. The heat pump will not run (meaning *only* the auxiliary runs, in which case it can be referred to as backup heat) when the outdoor temp is lower than the lock-out. This means, for example on a lock-out of 25F, that the auxiliary will run and the heat pump will not run when the outdoor temp is below 25F, no matter what is the indoor room temp in relation to the setpoint.

3. At outdoor temps between 35F and 25F (in the above-described example), the auxiliary and heat pump may run at the same time if the thermostat senses the room temp being sufficiently below the setpoint.

The point of the lock-out options is for the installer (or the homeowner) to garner additional control of and efficiency from the unit based on specific, local conditions such as electric rates vs natural gas rates (if the auxiliary is natural gas). The "correct" lock-out temps for your system depends on specific local conditions, the design, components, capacity, and performance of your system. Your system, not mine or Saltidawg's or anyone else's here.

I have a low-end, builder-grade, 1-speed Carrier heat pump with electric-strip auxiliary. It initially did not have an outdoor temp sensor. Running setbacks on the programmable thermostat when working 12 to 14 hrs per day, I found that the auxiliary would trigger during the last 10 mins or so of the recovery because the thermostat wasn't "happy" that the full setpoint wasn't quite reached at the target recovery time. Letting the heat pump run for maybe 30 minutes longer would finish the recovery, but the thermostat "insisted" on triggering the auxiliary. I had an outdoor sensor added so I could lock-out the auxiliary and thus prevent the unnecessary triggers of auxiliary at end of the setback recovery periods. It's still a low-end, builder-grade, 1-speed system but at least I have control now to prevent the auxiliary from triggering unnecessarily. There's no provision on the thermostat options to set a lock-out temp for the heat pump in favor of auxiliary. FWIW (which isn't much), my auxiliary lock-out is 25F.

    Bookmark   February 3, 2013 at 7:04PM
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they are telling you that the homeowner
determines the settings depending upon
their comfort.

personally, I think you question has been
answered. just not to your satisfaction.
so perhaps your installer can achive what
posting here has not.

is this your first heat pump?
I am still learning about mine,
as this is the first winter with hp.

best of luck.

    Bookmark   February 3, 2013 at 10:46PM
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There are a lot of myths about HPs in NC. I suspect it is pretty common for a HP to not keep up below 30 degrees and that is where the myth comes from. Obviously a good heat pump setup will keep up below 30 if it gets that temperature often in the winter.

Sizing is by cooling here so you would think HPs would do fine to 20 at least but insulation also sucks so that drives things.

What is a little disturbing (no offense) is that presumably the OP is a Realtor and doesn't really understand the most important mechanical system of a house.

I guess that will be offensive. Honestly though, it is a pet peeve of mine. Kind of like typical car dealer salesmen who don't understand their cars.

Given that HPs are pretty common in the Charlotte area, it does make sense to educate yourself on their operation.

It has probably been said but while a HP is less efficient below 30 or so, it still is far more efficient that electric resistance. That changeover is usually around 10 degrees. Most of the time, the HP will not keep up well before that in NC. But if your HP is fancy and new and your insulation good, it could keep up to 10 degrees - if we ever see that again.

    Bookmark   February 4, 2013 at 6:49AM
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THAT is the kind of info I was looking for... Answers and details w/o attitude. I appreciate the time it took.
David, I do consider myself a " mini - expert" on a lot of items regarding real estate. This was one area that I was lacking. So here I am trying to gather info with details... Dadoe did that. One reason why I did not fully understand the details on the associations between the heat pump and the air handler, is because I was being told by mamy people in the profession, very conflicting "facts".
Even if there are things I do not fully understand in my profession, I always pull in experts for my clients. I am an expert on real estate law, negotiations, contract law, and client services. One does not need to be an expert in every detail such as septic, HVAC, electrical. When the need arises, I have relationships with time honored experts to refer my clients to.
Thanks again to those who helped me... even the private messages.

    Bookmark   February 4, 2013 at 7:35AM
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I am upstate in NC from Charlotte. One of my heat pumps has high efficiency propane furnace as the auxiliary. I chose the propane as it can be powered by a portable generator in case of power outage. The dual-fuel setup shuts off the heat pump and switches to the furnace at 28 deg-- they not do operate simultaneously. The furnace is upflow with the coil above. I was told that the hot air from the furnace flowing directly over the coil would cause refrigerant pressure problems if the compressor was running at the same time. The 28 deg was chosen by the contractor who did the final setup of the system. It now has operated 8 years with no problems.
My other heat pump has electric auxiliary. The return air flows over the coil and then over the electric elements. Those can operate simultaneously.

This post was edited by bus_driver on Mon, Feb 4, 13 at 9:14

    Bookmark   February 4, 2013 at 7:59AM
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I agree that when you don't know, you ask the
experts in that specific area. it is just
sound logic to not get yourself in a bad
situation of giving incorrect information.

If there is something I'm not sure of when
dealing with a client..I admit that I'm not sure.
but that I'll find out and get back to them with
the info. people like it when you admit you don't
know everything!

how would one send you a private message OP?
I don't see any info in your profile to allow this.

best of luck.

    Bookmark   February 4, 2013 at 11:53AM
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First I'll say that HPs are rated by the BTUs they produce. This is true for both heating and cooling. But we'll stick to heating.

It is a fact that as the temperature drops, a HP will produce fewer and fewer BTUs. At some temperature point, the amount of electrical power to produce XXXX BTUs is equal to the electrical power of a resistance heater making the same number of BTUs. This point (Coefficient of Performance of 1.0) is known by the manufacturer and differs from model to model. I've looked around for this number (temperature) but I've never found it. But it is less important than other things.

Like TD suggested the amount of BTUs you need to keep a home at a desired temperature will vary due to construction, layout and the other factors he mentioned. Maybe your house is so well insulated that it only needs 1 BTU to maintain 70F on a 30F day (I doubt it). Maybe it needs 20,000BTU. And if it does, and you HP can only make 18,000BTU you're going to get chilly. And that's irrespective of that COP = 1.0 temperature.

But even not knowing that COP = 1.0 temperature, your thermostat 'understands' what's happening. That is, you want 70F and the HP can't keep up (irrespective of what the outdoor temperature happens to be). So it turns on the extra heat (in your case it is probably staged).

The folklore about HPs comes from lower efficiency units and drafty homes. You were on the right track when you suggested that your higher efficiency unit could handle lower temperatures better.

BTW: If efficiency means 'cost', the manufacturer does not know whether you are going to use electricity, propane, natural gas, hydronic, wood or coal for auxiliary heat. Nor would they know the cost of each in your location. Hence having them suggest a temperature for 'switch over' would be useless.

Another thing you might look at: your thermostat may have a setting whereby you tell it to always start in the high stage first below XX* for heat (and above YY* for cool). Since thermostats usually wait 15 minutes before switch stages, this 'jumps ahead' a bit.

    Bookmark   February 4, 2013 at 5:30PM
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another informative and detailed post regarding my questions. I appreciate it.
I am getting a good grasp on how this thing works.

    Bookmark   February 5, 2013 at 6:46AM
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