Heat pump efficiency as outdoor temps drop

[vtbuster]

I just got my first heat pump (Lennox XP16) last summer and I've heard that they get less efficient when the temps drop. Is there any kind of standard temp where it's more efficient to run the aux heat? I got a visionpro 8000 tstat and was curious about what temp I should (if any) set the compressor and aux heat lockouts to?

[big johnson]

That setting should have been done when your contractor installed the unit. Is your aux heat energizing too soon?

[vtbuster]

It hasn't gotten that cold yet, and to my knowledge the aux heat hasn't kicked in at all. When they installed the system, it didn't have the outdoor sensor for the tstat yet. Now that I've got it, I'm curious where that efficiency break-point is between heat pump and electric-coil heat.

[gary_g]

It hasn't gotten that cold yet, and to my knowledge the aux heat hasn't kicked in at all. When they installed the system, it didn't have the outdoor sensor for the tstat yet. Now that I've got it, I'm curious where that efficiency break-point is between heat pump and electric-coil heat.

The efficiency of the heat pump is greater than electric strips whenever the C.O.P. (Coefficient of Performance) is greater than 1.

Here is some tech data for my 14 SEER Goodman 3-ton heat pump:

Ambient
Temp--------COP--------BTU's

55F----------4.04-------38,800
45F----------3.64-------33,500
35F----------3.25-------28,700
25F----------2.82-------24,900
15F----------2.46-------20,800
5F-----------2.06-------16,600
0F-----------1.84-------14,400
-5F----------1.61-------12,300
-10F---------1.35-------10,100

Note how the output btu's drop with decreasing ambient temperature, but the COP is >1 down to below 0F.

[vtbuster]

Thanks, this is exactly the kind of info I was looking for. Do you know where/how to find COP info on specific models? Is that something that the manufacturers are required to publish?

[gary_g]

Thanks, this is exactly the kind of info I was looking for. Do you know where/how to find COP info on specific models? Is that something that the manufacturers are required to publish?

Goodman provides outstanding technical operating data on their website. Most HVAC manufacturers do not provide this data to the public. You are going to have to contact Lennox or your installer to get this info.

The numbers that I provided should be in the ballpark with your Lennox.

When you have resistance electric strips in your air handler, like I do, you always want to run the heat pump because it is more efficient than strip heat. When the thermostat senses that the heat pump cannot maintain setpoint, it will bring on the aux electric strips in conjunction with the heat pump. Dual-fuel set-ups (heat pump w/gas furnace back-up) are systems that the heat pump is typically locked out at a certain minimum outdoor temperature.

[vtbuster]

When the thermostat senses that the heat pump cannot maintain setpoint, it will bring on the aux electric strips in conjunction with the heat pump.

How does that tstat sense that the heat pump can't keep up? Is it something like x degrees lower than setpoint for y minutes? Thanks for your help.

[gary_g]

How does that tstat sense that the heat pump can't keep up? Is it something like x degrees lower than setpoint for y minutes? Thanks for your help.

Yes, the t-stat knows that the heat pump has been running for "x" minutes and cannot maintain setpoint. It then cycles the aux strips on/off (along with the running the heat pump) to reach and maintain setpoint.

[beenthere]

When to lock out the heat pump on a HP with electric aux heat, varies with what type of defrost control the HP has. And your areas winter humidity levels.

A high humidity level, a HP with time and temp defrost, you may want to lock it out at a COP of 1.5, average humidity levels perhaps 1.3

If it has on demand defrost, 1.3 in high humidity areas, and 1.1 in low humidity areas.

[vtbuster]

Ok, looks like I have no cause to be concerned. If my Lennox XP 16 has a COP table that even closely resembles the one below, I won't need to worry until we get below 0 F. Since Raleigh NC's lowest temp on record is -9, I'll sleep well knowing that my setup is running as efficiently as possible.

[segfault]

Yes, the t-stat knows that the heat pump has been running for "x" minutes and cannot maintain setpoint. It then cycles the aux strips on/off (along with the running the heat pump) to reach and maintain setpoint.

I guess it depends on the thermostat. On my heat pump's thermostat, there are two stages of heat and one stage of cool. The second stage of heat kicks in immediately approximately 2-3 degrees below the setpoint. There is no delay before the second stage kicks in. If you move the setpoint up more than a couple of degrees, it will energize the aux heat. This is why people tell you to "set and forget" a heat pump during the winter.

[vtbuster]

Does anyone know how the visionpro 8000 determines when to kick in the aux heat? I'm afraid that it will when my schedule switches from night time 62 degrees to daytime 70 degrees.

[beenthere]

With adaptive recovery enabled. It will still use the aux heat, but not as much. It will start recovery earlier, and moniter its progress.
With adaptive recovery disabled, it will bring on the aux heat right away.

If you have an outdoor temp senser, you can lock out aux until its really needed.

[vtbuster]

What you wrote makes perfect sense. I have adaptive recovery turned off.

My outdoor temp sensor is on order. After I ordered it I was curious how necessary it was. Aux heat lockout above 40 degrees will make it a worthwhile purchase. Thanks for your help.

[gary_g]

I have adaptive recovery turned off.


Why??

[vtbuster]

Good question - for some reason, even after countless screw-ups, I took the installers advice and disabled it. Is the group concensus that adaptive recovery is a good thing?

[adamk]

Don't know which vision pro 8000 thermstat you have, but the 8320 allows an aux lockout down to 40 degrees. The 8321 allows an aux lockout down to 5? degrees.

You would only need the outdoor tstat if you wanted it lower. E.g. you have a 8320 and want to lockout the aux until 25 degrees.

Adam

[vtbuster]

I have the 8321 t-stat and the outdoor sensor will be here tomorrow. Is it ok to use an unused pair from the tstat wire for the outdoor sensor?

[mlock]

You need the adaptive recovery "on" if you're going to use a night set back, or the heat strips will come on immediatly with the increased day time set point, and thus negate any set back savings.

[gp_wa]

Is the group concensus that adaptive recovery is a good thing?Adaptive recovery works great for me.

[gary_g]

Adaptive recovery works great for me.

I second that notion. I have been using Adaptive Intelligent Recovery for years (I have a heat pump with aux electric strips, and use a setback of 3 degrees).

[beenthere]

They recomend you use another stat wire, so as mot to have interference. With temp sensing.

[Alden_Sloe]

Adaptive recovery works great for me.

I'm still fighting mine. It insists on turning on a full hour before the wake time and starting the heatpump in 2nd stage heat. Runs on 2nd stage for about 20 minutes and then kicks back to 1st stage pretty consistently bringing the tstat temp up early (4 degree setback).

Problem is stage 2 (2 stage HP) jumps the bedroom temp up 4 degrees before the tstat temperature rises 2 degrees. Then when it goes back to stage one the bedroom (two outside wall plus lots of glass) drops back three degrees to lag the tstat by a degree. Besides the noise from the increased air movement on 2nd stage (I admit, I'm already spoiled rotten by how silent stage 1 operation is :o ) the temperature swings have me throwing the covers off and on an hour before I needed to get up. During the sleep period the fan is set to ON and rarely if ever do we get 2nd stage heat; the bedroom and the main tstat track within one degree. During occupied time in the evening the fan is set to AUTO and the bedroom is consistently 2 degrees cooler when we go to bed, perfect.

The system has had almost 2 weeks to "learn". I have an outdoor temp sensor on order but I doubt this is going to change the behavior. I also have a remote indoor sensor which I haven't deployed yet. FWIW the "aggressiveness" for the temp control is left at the default 2 setting and once the set point is reached I've rarely if ever seen it call for 2nd stage. Once at the set point the displayed temperature on the tstat and the set point are perfectly in sync 100% of the time (never once in two weeks have I seen these differ even one degree).

Bottom line, if it's going to start an hour early bringing the house up to the set point it seems the intelligent thing to do would be to start in first stage and switch to stage two only when it determines that the rate of temperature isn't going to be sufficient to reach the set point in time. Nothing I've been able to read here or in the [pitiful] documentation explains it.

[beenthere]

Alden.

You can't keep blaming your thermostat because of your bad duct design(well, you can, it just won't accomplish anything).

Its your duct system that is causing the over heat problem in the bedroom.

Fix that, and you'll be happy with teh stat.

[Alden_Sloe]

Alden.

You can't keep blaming your thermostat because of your bad duct design(well, you can, it just won't accomplish anything).

I respectfully disagree. Fix what? It performs flawlessly as long as the HP is called on to perform in a rational manor. Temperatures throughout the house are consistent. They vary from the ends of the house to the central living area where the thermostat lives but the differential is consistent with the purpose of the rooms. Even if I changed the ducts so that the master bedroom was a mirror of the main living area where the tstat lives starting up the recovery in stage 2 an hour ahead of the set point would be just plain stupid.

[beenthere]

If you fixed the duct work to the bedroom/bedrooms which ever the case is.
And the other end of teh house.
The stat would operate differently. Because the area its in would now warm up quicker.

It bases its start time on how long it has to run to maintain the set back stat.

First stage gives you relatively even room temps, because your duct work isn't undersized for that CFM. Since it gets loud in second stage, your ductsystem is undersized, or the unit is oversized.

[Alden_Sloe]

If you fixed the duct work to the bedroom/bedrooms which ever the case is. And the other end of the house. The stat would operate differently. Because the area its in would now warm up quicker.

It bases its start time on how long it has to run to maintain the set back stat.


No, it starts an hour ahead of setback in stage 2. It never operates in stage 2 other than recovery (only 4 degrees). You'd think that after 2 weeks the "intelligency" would start to realize that that's overkill. It does turn off stage 20 minutes into is "recovery" stage which to me says it's realized the overkill.

Me thinks the "intelligence" has no concept of a 2 stage HP. Starting an hour in advance would be great if it started in stage 1. It would come pretty darn close to the set point at the set time. The IAQ has showen zero "adaptive intelligence" and I'm loath to change the duct work the architect worked out 50 years ago and is working just fine if you operate the equipment in a rational mannor.

The tstat has no concept of what's going on in the bedrooms. It only sees the main living area which is remarkably consistent. It's dropping the ball big time on this so I fail to see how giving it more data to chew on will improve things. I'm certainly not going to change a perfectly functional (in heat mode) duct system.

[gp_wa]

The IAQ has showen zero "adaptive intelligence"Yeah, the IAQ is garbage. Rip it out, box it up, and send it to me. I'll dispose of it properly. :D

[Alden_Sloe]

Yeah, the IAQ is garbage. Rip it out, box it up, and send it to me. I'll dispose of it properly. :D

What do you have to trade? It's not garbage and I've never said so. Intelligent? that's open to debate. The tstat has some great features but it also has some major intellectual flaws. Besides the [lack of] control functions consider the non user selectable display functionality.

[gp_wa]

What do you have to trade?A lovingly used 8320 :p

[beenthere]

Your right, it only knows that the area its sensing temp in, is requiring long run times in first stage to maintain a low temp.
So it goes to second stage for recovery to the higher temp.

Your 50 year old duct work never had a VS blower hooked to it before, that is capable of moving nominal rated air flow.

A PSC blower would probably be quieter. Because it would not be able to move as much air.

PS: Is your heat pump a Goodman. Goodman heat pumps handle 2 stage operation somewhat differently then other brands.

[Alden_Sloe]

Your right, it only knows that the area its sensing temp in,
A PSC blower would probably be quieter. Because it would not be able to move as much air.

PS: Is your heat pump a Goodman. Goodman heat pumps handle 2 stage operation somewhat differently then other brands.

The furnace and HP are both York. The HP is the 8t and the "furnace" is the 8PV 80% operating so far only as an air handler. The furnace has a PSC blower. The PSC blower is one thing I was able to discern was well worth the advertising hype.

The air noise is secondary. Like I said, the 1st stage has spoiled me to silent operation.

[vtbuster]

Just out of curiousity, what is the temp going from and to? ie what is your nighttime temp and morning/wake temp that the tstat is trying to achieve?

[beenthere]

Is it possible. That the stat is starting the HP infirst stage. And after 15 minutes, the stat determines its not recovering quick enough. Then brings on teh second stage, and then the higher noise is waking you, making you belive it started in second stage.

Also. Do you have have them enable Hot Heat pump.
At outdoor temps below 50°F, when it goes to second stage, the blower will continue to run in its first stage speed for 10 minutes, then it will check the liquid line temp and determine if it needs to increase fan speed or not.

[Alden_Sloe]

Just out of curiousity, what is the temp going from and to? ie what is your nighttime temp and morning/wake temp that the tstat is trying to achieve?

Night time set back is 62, same as daytime away. Morning wake and evening occupied temps are both programmed at 66. Once at temp I'd swear the tstat is stuck because it never varies even one degree from the set point.



Is it possible. That the stat is starting the HP infirst stage. And after 15 minutes, the stat determines its not recovering quick enough.
Nope, been awake enough mornings when it kicks on to know for a fact that it's starting in 2nd stage. This is a new toy and I'm monitoring it pretty closely :cool: OK, given the discourse below I need to back up on this statement. The fan is ON meaning it runs on low constantly during the night. I think the circulation speed is less than low stage heat but I'm not sure :confused: Both are barely noticeable. I haven't been staring at the tstat or holding my hand on the register so it's possible the system is starting even earlier than I'd thought but not kicking the blower up until 10 minutes of HP operation in stage 2. It's still starting WAY earlier than necessary but I can understand how the tstat would be totally confused as I don't believe it has any concept of "Hot Heat Technology".


Also. Do you have have them enable Hot Heat pump.
At outdoor temps below 50°F, when it goes to second stage, the blower will continue to run in its first stage speed for 10 minutes, then it will check the liquid line temp and determine if it needs to increase fan speed or not.
I was lead to believe Hot Heat was enabled but I haven't opened up the cover on the HP to check. I can't say I have much faith in the installing company. No problem with them coming back to fix things at no charge but the laundry list has gotten pretty long. :(

Temperatures have been above 50 during the day and below 50 at night. I have noticed the system kick on the higher airspeed a couple of times during the day/evening for no apparent reason. The Hot Heat feature would be a good explanation. In general the cycle times have been right around 10 minutes so it's entirely plausible that the HP is always running in 2nd stage but with the fan on low. I will have to investigate :confused:

[beenthere]

You'll want to make sure they have it wired properly.

Some techs get confused when they have more then 5 or 6 wires to connect. :(

[vtbuster]

Speaking of confused techs.... mine never hooked up the L contact of my visionpro 8321 to the L contact of my 2 stage Lennox heatpump. Is it necessary and am I missing out on anything? In his defense, all 8 tstat wires are already in use.

[beenthere]

No. Its just teh error fault indicator. To tell you there is a problem.
Not required for operation though.