Heat Pump Question

[Ben]

For those of you who have heat pumps, especially dual systems, what temp is your supplemental heating set to? Mine was set by the HVAC guys to 26 to switch to the propane furnace. The temps here the last few days have been running around high 20s-low 30s from after sunset through late morning-early afternoon. Just above my switchover.

I have noticed my unit just running and running, and going into defrost mode (I know that's a good thing) like crazy. While electricity is dirt cheap here, I'm wondering if it wouldn't be more efficient for me to move the switchover for propane to 29-30deg. As Mike Irwin likes to point out   , heat pump heat can seem cold. Mine, at these temps, puts around 80 deg out the registers, and seems to struggle with that (compared to ~90deg if it's in the 40s, or ~110deg with the gas furnace any old time). So while it's keeping the temps at 69 [Kevin Malone], it's always a "cool" 69 unless I have the wood stove going. Big difference in feeling from waking up on a 15deg morning when the propane furnace has kicked in at 0500 and heated the house for me.

All the charts I see on the interwebz seem to have the temps I'm currently experiencing (with 30deg the "average") as where the lines cross for efficiency vs inefficiency. So I'm smack in the middle of "six of one, half a dozen of the other" right now.  

[MillCreek]

I think mine is set to switch over to the auxiliary electric heating coils at 26 degrees.  When my system makes that switch, the electric power meter spins like a dervish and I really see it in that month's power bill.

[Ben]

I think mine is set to switch over to the auxiliary electric heating coils at 26 degrees.  When my system makes that switch, the electric power meter spins like a dervish and I really see it in that month's power bill.

I'm trying to figure out if my unit has electric coils as well, and if those are used during defrost (when basically the AC is running instead of heat). Because I was curious about the "midnight to six" run time so have been tracking on the Idaho Power site, where I can see my hourly usage, and that timeframe/temp when there would be a lot of defrosting is jumping up in KW like crazy. The furnace should only be using like half the juice of the heat pump, which has me wondering if the HP is using coils instead of propane during defrost cycles.

[K Frame]

If you have a propane secondary it should kick in when defrost mode engages.

Essentially defrost mode for a heat pump is turning on the air conditioning.  It pulls heat out of the house and sends it through the exterior coils to defrost them.

The secondary kicks in to negate the loss of heat. I doubt that you have two secondary heaters - propane and electric.

My heat pump hasn't run in November.

My pellet stove has.

Sent from my SM-G960U using Tapatalk

[K Frame]

So what's the deal, Ben? Have you figured out how your system is set up?

Do you have both electric coils AND the propane cheater heater? I've done some digging around and, while not a common set up, it has been done, especially if the propane is set up to be "stage 2" heating, which it sounds as if yours may be.



And, speaking of heat pumps... Mine hardly ran at all in November. I have it set at 60 degrees to provide a back up to the pellet stove. It ran a bit when I was away for Thanksgiving, but when I got home on Sunday, the house was right at 60 degrees.

I kicked the pellet stove on to level 3, and within an hour it was 75 in the living room and heat was rushing upstairs. By the time I went to bed I had cut the stove back to level 1, it was 78 in the living room, 72 in the front hall, and 75 upstairs.

Good God I love my pellet stove.

[Ben]

I still have no idea. I wouldn't even know how to check for heating strips. I'm leaning towards maybe not, as I've been watching the system closely during these "borderline" temperatures. I've noticed - this morning at 30deg for example - that the system will sometimes switch to propane (even though it's set to do so at 26deg), I guess if the heat pump has been running too long or doing too many defrost cycles and not gaining temperature?

It's supposed to be some kind of smart system. I also noticed the other day, it was 67 inside, again around freezing outside, and I was feeling a bit chilled, so I knocked the temp up to 73. It went straight to aux heat, I guess already figuring that would be too great a burden on the heat pump at that temp?

[Ron]

I still have no idea. I wouldn't even know how to check for heating strips. I'm leaning towards maybe not, as I've been watching the system closely during these "borderline" temperatures. I've noticed - this morning at 30deg for example - that the system will sometimes switch to propane (even though it's set to do so at 26deg), I guess if the heat pump has been running too long or doing too many defrost cycles and not gaining temperature?

It's supposed to be some kind of smart system. I also noticed the other day, it was 67 inside, again around freezing outside, and I was feeling a bit chilled, so I knocked the temp up to 73. It went straight to aux heat, I guess already figuring that would be too great a burden on the heat pump at that temp?

Being a "smart" system the thermostat or system operates as a two stage system most likely. On a normal call for heat (one or two degrees under the set point) stage one is energized. If the temperature drops too low below set point stage two/aux heat/emergency heat is energized.

I doubt you have electric and gas auxillary heat. If you don't have dedicated breakers in your panel for electric heat you probably don't have electric strips.

My knowledge of heat pumps is limited but as mentioned above in another post, if the coil is in defrost or not generating enough heat stage two, aux/emerg heat will be energized.

[K Frame]

"I doubt you have electric and gas auxillary heat. If you don't have dedicated breakers in your panel for electric heat you probably don't have electric strips."

When I bought my house in 1993 I had a 6 SEER Bryant system with electric coil emergency/backup heat.

It was replaced in 1995 with a 12 SEER Trane system with electric coil emergency/backup heat.

In neither case were there separate breakers in my electrical box for the emergency/backup coils -- those breakers were/are on the air handler unit itself with master breakers on the panel to control the entire heat pump/air handler system.

[Ron]

"I doubt you have electric and gas auxillary heat. If you don't have dedicated breakers in your panel for electric heat you probably don't have electric strips."

When I bought my house in 1993 I had a 6 SEER Bryant system with electric coil emergency/backup heat.

It was replaced in 1995 with a 12 SEER Trane system with electric coil emergency/backup heat.

In neither case were there separate breakers in my electrical box for the emergency/backup coils -- those breakers were/are on the air handler unit itself with master breakers on the panel to control the entire heat pump/air handler system.

interesting, i guess in the commercial world the current draws are such the heat coils warrent their own circuit. As mentioned my residential heat pump knowledge is limited.

Have you had a system that employed both gas heat and electric heat as its auxillary? Seems mighty redundent.

[Nick1911]

Heat pumps move less heat and have a lower COP as the difference between inside and outside temperature increases.

Your balance point, the outside temperature at which the heat pump is able to provide just enough heat to keep the building at the setpoint by running nonstop, is particular to each building, as each building has its own unique heat loss/gain characteristics.

Economically, this balance point might be somewhere different.  In the KC metro area, only very high end heatpumps have a chance of competing with natural gas furnaces.  Natural gas is just very inexpensive.

We fairly easily find that out for your setup.  Post the model numbers of your equipment, your per KWhr electric cost, and your local propane cost.  

[K Frame]

'Have you had a system that employed both gas heat and electric heat as its auxillary? Seems mighty redundent.'

No, but I did find a little bit of information on such a set up when poking around online. Or at least what appeared to be some information on a set up like that.

The way it was described was that the propane system wouldn't be "armed" until the exterior temperature dropped to a certain point (I guess sort of like an outdoor reset concept).

Until that happened, the electric unit would provide the supplemental heat. Once that outside temp was reached, the coils would be taken out of the equation.

I agree. That seems as if it's ridiculously redundant

[K Frame]

"Heat pumps move less heat and have a lower COP as the difference between inside and outside temperature increases."

Not sure that I understand this...

I thought COP was purely the amount of usable heat that a unit could deliver at a given outdoor temperature, and that the temperature inside the house had nothing to do with it other than requiring to unit to run shorter or longer depending on the call to temperature.

[Nick1911]

COP is an efficiency metric.  It does not indicate how much heat is moved, only how efficiently that heat is moved.  It's a ratio.  For example, if a unit is operating at a COP of 3, you are getting three units of heat indoors for one unit of input energy.  It's a dimensionless ratio, so you can apply it however you want, 3 watts of heat moved for 1 watt input, BTU's, whatever.

The actual amount of heat moved is a separate thing.  It's dictated by the difference in indoor and outdoor temperature, although for most purposes, it's assumed that the indoor temperature is constant.

The amount of heat a given heat pump can move drops as the indoor/outdoor temperature split increases.  A the same time, the buildings heat loss increases.  Where the two cross each other is the balance point.

To try to make this a little more concrete, let me use some actual information from a real heatpump. (GSZ14040)

Assuming you're trying to keep the house at 70F:
If it's 50F outside, this heat pump moves a lot of heat, 29,920 BTU/hr.  It also does this very efficiently, consuming only 2.31KW, a COP of 3.80.
If it's 30F outside, this heat pump moves a some heat, 19,620 BTU/hr.  It also does this somewhat efficiently, consuming 2.08KW, a COP of 2.77.
If it's 0F outside, this heat pump moves a little heat, 9,290 BTU/hr.  It's not very efficient, consuming 1.74KW, a COP of 1.57.  (still better then heat strips)

Heat loss from the building works the other direction.  A building that needs 20,000 BTU's to stay at 70F inside when it's 50F outside might need 70,000 BTU's when the outside temperature drops to 0F.

[Ben]

We fairly easily find that out for your setup.  Post the model numbers of your equipment, your per KWhr electric cost, and your local propane cost.  

Coincidentally, the propane guy was by yesterday, so current costs are:
Propane: $1.73/gal
Electricity is on a tiered plan and I always make it well into the 2nd tier, but lets average at $0.085.

I can't read this guy's handwriting well, but I'll post the invoice and also a photo from the tag on one part of the unit (don't know if it's AC or furnace). The whole thing is Amana (which I guess is also Goodman?):

[Ron]

Pretty unlikely you have electric heat on top of a newer two stage gas furnace.

Not to mention he didn't write anything down regarding electric heat.

[K Frame]

COP is an efficiency metric.  It does not indicate how much heat is moved, only how efficiently that heat is moved.  It's a ratio.  For example, if a unit is operating at a COP of 3, you are getting three units of heat indoors for one unit of input energy.  It's a dimensionless ratio, so you can apply it however you want, 3 watts of heat moved for 1 watt input, BTU's, whatever.

The actual amount of heat moved is a separate thing.  It's dictated by the difference in indoor and outdoor temperature, although for most purposes, it's assumed that the indoor temperature is constant.

The amount of heat a given heat pump can move drops as the indoor/outdoor temperature split increases.  A the same time, the buildings heat loss increases.  Where the two cross each other is the balance point.

To try to make this a little more concrete, let me use some actual information from a real heatpump. (GSZ14040)

Assuming you're trying to keep the house at 70F:
If it's 50F outside, this heat pump moves a lot of heat, 29,920 BTU/hr.  It also does this very efficiently, consuming only 2.31KW, a COP of 3.80.
If it's 30F outside, this heat pump moves a some heat, 19,620 BTU/hr.  It also does this somewhat efficiently, consuming 2.08KW, a COP of 2.77.
If it's 0F outside, this heat pump moves a little heat, 9,290 BTU/hr.  It's not very efficient, consuming 1.74KW, a COP of 1.57.  (still better then heat strips)

Heat loss from the building works the other direction.  A building that needs 20,000 BTU's to stay at 70F inside when it's 50F outside might need 70,000 BTU's when the outside temperature drops to 0F.

OK, that I understand, and that lines up with what I was saying earlier.

What I'm trying to factor in, though, is this statement: "Heat pumps move less heat and have a lower COP as the difference between inside and outside temperature increases."

When I initially read it, it seemed to say that COP was a factor of the difference between the two temperatures, which would mean that no matter what the outdoor temperature, the COP would be the same if there was a differential of 40 degrees (30 outside, 70 inside or 20 outside 60 inside).

[Nick1911]

Good.  Here’s the technical manual for that heat pump: https://www.alpinehomeair.com/related/Amana%20ASZ16%20Spec%20Sheets.pdf
Here’s the relevant section

The furnace is very efficient, with a rated AFUE of 96%.

There’s a couple ways we can drive at this, but to most clearly lay it out lets ask the question: How much money would it cost to get 100,000 BTU’s from each heat source?

Starting with the propane furnace.  1 gallon of propane contains 91,000 BTU’s of energy.  To get 100,000 btu’s in your building:

100,000 BTU into building / (96%) = 104,100 BTUs burnt.  (1 gallon propane / 91,000 BTUs) = 1.145 gallons.  Total cost for 100,000 btus into the building: $1.98.

For the heatpump, this will change depending on the outdoor temperature.

Lets look at the worst case scenario: electric resistance heat (space heaters, heat strips, baseboard heaters)

To get 100,000BTU’s, you would need to consume 29,291 watts, 29.29KW.  A cost of $2.49.

Here’s how the COP works with a heat pump.  It’s an efficiency ratio.  For a heatpump to deliver 100,000 BTU’s, we still need 29.29KW of heat, but we can get some of that from outside the house with the heat pump.  So, we don’t have to put in a full 29.29 KW.  Using the product documentation, here’s where we get with cost of operation, assuming a 70F indoor temperature:

Outdoor Temp	COP	Cost per 100,000 BTUs
65	4.66	$0.53
55	4.30	$0.58
47	3.95	$0.63
45	3.86	$0.64
40	3.65	$0.68
35	3.43	$0.73
30	3.27	$0.76
25	3.08	$0.81
20	2.89	$0.86
17	2.76	$0.90
15	2.68	$0.93
10	2.45	$1.02
5	2.22	$1.12
0	1.97	$1.26
-5	1.72	$1.45
-10	1.44	$1.73

Therefore, economically, with the cost of energy in your area, your heatpump is always cheaper to operate then propane.
It’s in your best interest to run the heat pump as much as you can, until it becomes cold enough that the heat pump can’t keep up.  Only run propane when you are forced to.

However, propane is cheaper then electric resistance heat.  Run propane before you run space heaters.

[Nick1911]

When I initially read it, it seemed to say that COP was a factor of the difference between the two temperatures, which would mean that no matter what the outdoor temperature, the COP would be the same if there was a differential of 40 degrees (30 outside, 70 inside or 20 outside 60 inside).

This is correct.  It's usually glossed over because people usually want their indoor spaces at a constant temperature. 

The efficiency of heat pumps is governed by the carnot cycle.  What matters here is the difference in temperature between the two sides.

The SEER and EER of an air conditioner/heat pumps are limited by the laws of thermodynamics. The refrigeration process with the maximum possible efficiency is the Carnot cycle. The COP of an air conditioner using the Carnot cycle is:



where T_C is the indoor temperature and T_H is the outdoor temperature. Both temperatures must be measured using a thermodynamic temperature scale based at absolute zero such as Kelvin or Rankine

[K Frame]

"This is correct.  It's usually glossed over because people usually want their indoor spaces at a constant temperature."

Those privileged bastards!

OK... Thinking about this some more, I think I now get what you're saying, and the more I think about it, the more sense it makes...

Correct me if I'm wrong, but...

The COP differential between the two temperature sides comes down (is affected? determined? influenced?) to the outside is limited to what heat it can pull from the outside air, but the inside is limited by how much of that heat can be moved into the building from the interior coil?

I tried reading the Wikipedia entry on the Carnot cycle... yikes.

[K Frame]

"However, propane is cheaper then electric resistance heat.  Run propane before you run space heaters."

And both propane and space heaters are warmer than a freaking heat pump.

Run a pellet stove!

 

[Ben]

Thanks Nick - that was very helpful, especially regarding the space heaters.

One of the things I've been doing is running the wood stove, which nicely warms most of the house. Except, irritatingly, my office, where I spend a good part of my time when indoors, and the master bedroom. When running the wood stove,  I supplement those two rooms with radiant oil heaters for part of the day. Sounds like maybe it's cheaper to run the central propane for a short period instead?

While I appreciate that with my electric costs, the heat pump is much cheaper to operate, I'm still with Mike on the "cold-feeling" heat. When it's in that high 20s - low 30s range outside, I often get a little chill when I feel the "heat" from the heat pump blowing out of the registers. 

[Ron]

Have tried turning your blower on while running the wood stove?

Circulate the air in the house to even the temp in the rooms out a little.

[K Frame]

The fan on my heat pump system is constantly on. It runs 24x7x365 as much to filter the air as to equalize temperatures throughout the house.

You can also put a box fan in your bedroom or office. Set it in the door opening so that it is blowing OUT of the bedroom.

It will move cold air out of the bedroom and pull in warmer air from the hallway. Because of the relative densities, it's easier to move cold air rather than warm air.

[Ben]

Have tried turning your blower on while running the wood stove?

Circulate the air in the house to even the temp in the rooms out a little.

Yeah, for an expensive stove though, that blower is freakin' loud if I'm sitting in the living room and watching tv or reading. Instead, once the stove is pumping heat out really well, I turn on the ceiling fan above it, which distributes the warm air well. The problem is that the office and master bedroom, kinda connected to each other, are past a small hall that somehow is really good about blocking heat distribution.

My heat pump fan is set to auto. I might try setting it to full time on for a while to see if it makes a difference.

[Ron]

My heat pump fan is set to auto. I might try setting it to full time on for a while to see if it makes a difference.

That's what i meant, furnace blower