Omnistat2 on HeatPump w/HeatStrip

[toddr]

The past few weeks, it's been a little cool in our area!  The past few days, I've noticed that one of our heat pumps seems to be running all the time.  In the past three days, it's ran +/-48 hours.  Below is some quick info of what I'm seeing (bullet version; with questions to follow.
 
Quick Info

• New House, first winter; not sure what normal is!  Previous houses were smaller/single electric-only heat pumps.
• 2 Heat Pumps (2 stage) with backup heat strips (3T upstairs/ 2T downstairs)
• 1 Heat Pump (2 stage) with backup dual fuel/gas (3T downstairs)
• 1 ER Temp/Humidity Sensor

 
Conditions

• Night temps have been 25-35F
• Day temps < 40F
• Thermostats are setpoints are 68/73 for downstairs; 69/74 for upstairs.

 
Unit(s) Performance

• Upstairs heat pump is up stairs and doesn't kick on that often...heat rises. Register heat is around 86F. (single arrow beside temp on thermostat)
• Downstairs dual fuel system is switching properly as it gets colder.  Register heat is around 110F (on gas).
• Downstairs 2T heat pump is running all the time.  Single stage register heat around 83F. (single arrow beside temp on thermostat)
• Note:  When I force the heat strips to come on, by increasing the temp >2 degrees; second stage registers heat around 110F (noted double arrows beside temp on thermostat).
• This past summer, the house seemed to cool fine and as needed.  No issues with air flow, etc.

 
As I understand HVAC, the first stage heat pump should only heat about 15-25F above internal temps, maybe lower under very cold conditions.  Based on this, my units seem to be working properly (68F + 20F = 88F +/-Cold factor).  
 
My question:
1)  Does the thermostat tell the heat strips to kick on or is that a function within the unit?
1b)  if function of unit, should it have an external temp sensor in the heat pump itself?
1b)  if thermostat, where is that setting in the omnistat? I've caught myself going into every menu, but could have very well overlooked it.
1)  With the two electric-only heat pumps, should the heat strips kick on if it's below 35 degrees; like the gas switch over?  They seem to only kick on when there is >2F heating difference 
2)  Other than energy consumption, is it normal for a heat pump to run almost all the time in this cold weather?
 

 

[RAL]

To me, it sounds like the heat pumps are operating properly.  86 degree temp at the air ducts is pretty normal when the compressor is running.  110 is normal when the back up heat is on.
 
With the slightly warm air that a heat pump produces, it's good at maintaining air temperature, but not so good at raising it more than a couple of degrees in a reasonable amount of time. That's why you see the heat strips kick in when you try set the thermostat up by more than 2 degrees.
 
The thermostat determines when to activate each stage and/or the back up heat. If the thermostat doesn't see a sufficient rise in temperature after some amount of time, or if the temperature is too far below the set point,  it will activate the next stage or the back up heat, if necessary.  
 
Heat pumps work well with warmer outside air temperatures, but even at very cold temperatures, it can still extract heat from the outside air, just not as efficiently.  As the air temperature drops below 20 degrees F or so, the coefficient of performance becomes 1, and the heat pump is no more efficient than electric resistance heat.  Still, it is able to heat the air.  But since your house will be losing more heat to the very cold outside temperature, the heat pump may not be able to keep up, and the back up heat has to kick in.
 
If you have a gas furnace for back up heat, it is probably cheaper to heat with just the furnace at some point.  Where that is depends on the efficiency of your system, the price of electricity and the price of natural gas in your area.
 
I used to have a Carrier heat pump with electric heat strips as back up, and even when the heat strips were on, the compressor still ran - both were working to heat the house.
 
I now have a Carrier heat pump with a propane furnace, and the Carrier thermostat allows me to set lock out temperatures below which the compressor won't run, or above which the furnace won't run.   But I wouldn't expect to find the ability to control that on the Omnistat (but I don't own one, so can't say for sure).
 
How much your compressor should run depends on the size of your system, the outside temperature, and the insulation of your house.
 
Depending on how the system is sized vs the heat loss of your house, what you see may be normal.  If it's able to keep your house warm on the coldest days for your area, then it sounds like it is doing ok.  If it ran 48 hours out of the last 72, you still have some margin and it could deal with even colder days.
 
Here, it was 6 degrees last night, and my compressor was running quite a bit!

 

[Madcodger]

RAL's answer is excellent. This sounds normal for an air exchange heat pump and is the reason we installed our first Nest (better control over when the backup strips kicked on). Many other stats can likely exert similar control, but no idea about the Omnipro. We have found that having the heat pump run is of course more expensive than not having them do so, but nothing compared to the dreaded backup heat.

My only suggestion is to have an energy audit done, to find better ways to save that heat. If you do so, have a REAL one done, by a BPI / Resnet certified auditor, and not some moron from a company that just wants to sell you something (windows, insulation, etc.). And having a "newish" house is no guarantee that you don't have problems. Many builders often specialize in maximizing the transfer of money from your account to theirs, more than they specialize in minimizing the transfer of heat out of your home. Expect to pay $300 - $600 depending on your location, make sure a blower door test with smoke sticks is involved, and understand that the thermal camera is often more for show than of real benefit, especially if you don't have some significant difference between inside and outside temps when it is done. So, I suspect your heat pumps are fine, but your house has some energy leaks.

 

[ano]

Many builders often specialize in maximizing the transfer of money from your account to theirs, more than they specialize in minimizing the transfer of heat out of your home.

That is definitely true.

 

[toddr]

Thanks for the reassurance.  At first I thought something was wrong, but when I did some info digging, it looked like it was actually running fine. I guess the one thing that concerned me was the motor running and potentially wearing out.  But realize that it's probably harder on the motor during startups/shutdowns than constant running.  Our builder, for the most part, was good.  I was at the lot 1 or 2 times every day, usually working on things during night/weekends.  
 
Thanks for all the feedback!

 

[StarTrekDoors]

You didn't mention where you are located, but changing the setpoints to lower numbers may also help.  The Omnistats seem to do a good job of managing temperature in a multistage environment, as your installation notes above would suggest, but you can take it a step further by implementing some degree of control over your air handlers.  Most of what you described were guesses so verification with the manufacturer could allow you to customize your air handling for great efficiency and perhaps less run time on the HP.  I would want to know if the Omnistats are reading temp correctly, have they been customized to run shorter/longer duration, what does the HP manufacturer publish for efficiency, is gas kicking in late, and of course, the excellent suggestion above for an energy audit by a truly reputable and certified (certified does not mean reputable).
 
So what region and country are you located?
 
Also, if you builder is not your best friend and you didn't personally pick out the heat pumps and equipment, you probably got builder grade equipment. Being onsite to watch the subcontracted crew doesn't mean the builder shopped around to find you the most efficient equipment or hired an installer that could optimize your installation, so running a lot, while it's common, may not be ideal if anything if "off".
 
The motor running all the time is wearing out
 

• Thermostats are setpoints are 68/73 for downstairs; 69/74 for upstairs.

 

[Madcodger]

Also, no offense intended here, but if you don't have a good understanding of what makes a house thermally efficient, you have no real idea as to whether your house is as efficient as you would wish. For almost all homeowners, a "good contractor" is one they LIKE, for reasons that have nothing to do with skills or performance.

It's like having a physician that has great "bedside manner" but a sketchy skillset. Many of us have seen an older relative or friend who "just loves" Dr. ___ because they listened closely, joked around with them, and wrote any prescription they wanted. But their actual care was a mess. The same is so often true of builders. Cleaning up a jobsite, having clean cut crews, and patiently working through things with a homeowner are awesome. But they have no direct correlation to how thermal bridging, air sealing, insulating, etc., have been done.