Carrier Greenspeed report #1
I’d posted several questions and wanted to provide an update – we are 24 hours into our new Carrier Greenspeed, 5-ton. We live in a 1950 house in CZ4. We’ve performed substantial (but nowhere near deep) air sealing and insulating. Our heating loads are somewhere around 68,000 BTUH, or a bit higher per manual J.
BEFORE
1994 168,000 BTU oil furnace w/4-ton AC, updated coil
We would employ major setbacks to keep costs down. 60* during weekdays, average 68* when home, crank to 71* to heat for showers. The house was incredibly uneven in comfort, because most of the heat was noty coming through the ducts but instead being radiated by a 168k BTUH beast in the basement! Unless the heat was blaring, the house was far from comfortable, with many rooms being very uncomfortable.
AFTER (Day 1)
We are stunned by the comfort in every room of our home! First 12 hours of ownership, outdoor temps ranged from 30-34*, and I was experimented with manual lockout of heat strips. Because we’d had no heat in the house for 2 days during install, heat gain was slow – without heat strips, we were gaining about 1 degree every hour or so in the mod 60s*. Eventually we turned on heat strips and let the machine control, and the temp rose 2 degrees quickly. I’ll continue to test but strongly suspect that without setbacks, the unit can carry the load into the 20s, before needing heat strips.
The air at the registers is hot – we were both concerned that this would be an issue. But the exact opposite is the case – we are much more comfortable in every room of the house than we were before. We haven’t experimented enough to decide our preferred temperature yet (we’ll probably select 69* as my wife likes it warm), but overall our first experience with a heat pump is just remarkable.
In terms of bad outcomes, a few 65 year old registers painted many times over were slightly but not very much louder due to the increased airflow, and we might get around to replacing them. But I believe the constant airflow is actually a reason why the house is so much more comfortable than before.
We went with the Greenspeed after great due diligence – we went with the best installer and company – we also looked at the Trane and Lennox with variable speed compressors.
I’ll post back about energy efficiency and cost savings. Based on our first 24 hours, our hope is that this will be our unit for many years. I’m hoping for 50% savings on heating and cooling, but it is hard to believe that we this much comfort can save money too.
Feel free to ask any questions.
Thanks to everyone who offered feedback during my research.
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Replies
In my opinion it is best the set the temp and not change it unless you will be away for several days.
Try keeping the strips locked out and find out how much cold the heat pump can handle.
I am just down the river from you and my heat pump was set to lockout at 8°F. My strips ran on low for 3 hours this year. I am going to try 1° next time it gets that cold.
Do not be surprised when you find the strips do run during the defrost cycles. Yes the system will make some strange sounds while defrosting. I had 57 defrost cycles in the last 2 weeks with strip run time of 2 hours.
What is the temp of the air exiting the registers and your set point?
Walta
Thanks Walta. I agree with your suggestions and will continue to experiment to see "how low can it go."
Do you also have a Greenspeed? I'm amazed your HP was carrying full load as low as 8*! That is the economic balance point, but what about comfort? What is your indoor air temp?
One thing that I liked about the Greenspeed is that I can lock out the heat strips for heating, but not for defrosting. I'm sure this feature is to make the system idiot-proof, which in my case is a nice feature to have.
Air at registers certainly >100*, but that is only a guess. What kind of system do you have, and what is the air temp? What tool do I need to measure?
And last, as long as I'm asking: are some new grills better than others, to replace the older heavily painted grills and reduce the mild whistle?
>"The air at the registers is hot – we were both concerned that this would be an issue. But the exact opposite is the case – we are much more comfortable in every room of the house than we were before."
During the winter when the compressor is running at mid-speed or higher the temperature at the registers is the "warm summer breeze" effect. It's during the shoulder seasons when it's idling along at low speed with the compressor cycling on/off that the air at the registers is most tepid.
The GreenSpeed still has substantial capacity at temps as low as 8°, but it's not a cold climate type compressor. At -5F it's capacity is less that 2/3 what it delivers at 25F, and at -10F it's less than half.
"During the winter when the compressor is running at mid-speed or higher the temperature at the registers is the "warm summer breeze" effect. It's during the shoulder seasons when it's idling along at low speed with the compressor cycling on/off that the air at the registers is most tepid."
Aha. So, we will learn more over the next few months! Any suggestions to mitigate this effect?
Lowering the air handler speed to it's minimum will deliver higher exit air temp. I don't know enough about the controls for the GreenSpeed to say it that's easy or hard to do.
Thanks Dana.
Yesterday we learned that even at mild temperatures, heat strips are very expensive! Yesterday was 34-39* outside, and we left the house and experimented a 2* setback without locking out the heat strips. When we returned, I manually increased the temp 2*. This morning, I checked yesterday's KwH usage: $9 for heat pump and an unsustainable $14.50 for heat strips, and they likely didn't even run very long!
My system is a RHEEM it is not the top of their line system It the next to the top most of the hardware is the same they crippled it with software so it operates at only 3 speeds 1700, 2800 and 6800 RPMs. My unit warms the air 12-14° on low 14-16° medium and 20-25° on high. With the set point of 69° I mostly see low 80s as it mostly runs on low.
With my system the variable speed fan is an automated function the computer changes it to get the air flow needed for maximum efficiency. If there are user controls they are well hidden.
Again I encourage you the stop playing with setbacks and just let the system run. I see it as part of learning to live with a heat pump. I was very surprised to find that I felt cold at bedtime and hot when I woke up. I had thought it was all about the 15° set back I was use to at the old house.
Walta
In regards to temperature setbacks, my experience & simulation results are different. I have a 3 ton american std heat pump and air handler. I do use a setback at night. I also use a ramp up in the morning to minimize the use of the heat strip and stage 2 of the heat pump. The tstat allows several time periods so I increment the temperature by 1 or 2 degrees every half hour, then I turn it up another degree when I get up. Not for everyone, but a better tstat logic could handle this smoothly. I only seems to get strip heat running when it's around 5F or colder. My american std thermostat logs run times and I had 4.3, 8.8, and 3.3 hrs of strip heat in Dec, Jan, and Feb respectively. Some of that was associated with defrost cycles.
Just for grins I just turned up the stat by 2 degrees, it's 31F outside and the heat pump went to 100%, but no heat strip.
Before the house was built, I setup an energyplus simulation and ran scenarios both with nightime setback and without and the cases with setback showed lower energy use.
>"...I setup an energyplus simulation and ran scenarios both with nightime setback and without and the cases with setback showed lower energy use."
With modulating heat pumps like the GreenSpeed it will almost certainly use more energy to go with setbacks, since (like mini-splits) the heat pump itself (not counting the heat strip) will be more efficient when modulating at part load than at full speed.
Dana, My system is modulating, and the sim was setup that way.
I understand what you're saying, and on the surface the increased efficiency of the part load HP would seem to trump the reduced heat loss with a setback. But, unless its very cold, the heat pump doesn't have to run full speed to warm up the house. Can you point to studies that prove your point? The thermal capacitance of the house and the outside temperature when the heat pump is running play into this too. My house may be different than typical since our sun tempered house picks up quite a bit of energy from the sun on a typical day.
>"Can you point to studies that prove your point?"
An in-situ empirical study of a high-R house developments in MA all heated with modulating ductless showed that the worst energy use performance relative to the simulations for those houses were at the houses where they kept bumping the temperature up & down:
https://www.buildingscience.com/sites/default/files/migrate/pdf/BA-1407_Long-Term_Monitoring_Mini-Splits_Northeast_v2.pdf
I have a new Carrier 25VNA4 heat pump. I live in CZ4A. I have it set so that the backup NG furnace does not come on at all, not even for defrost. I will leave it that way until I determine the balance point and perhaps make adjustments after that. Just curious if anyone besides me is bothered by the intense recovery back to set point after defrost. My set point is 67 and sometimes after defrost the room temperature drops to 66. This causes 100% compressor capacity and high blower speeds to get back to 67. I set maximum blower speed to 1480 CFM and maximum compressor capacity to 50%, but this does not seem to control capacity and blower speed coming out of defrost. Is there any reason it needs to be this way? This is a 3-ton unit. Any suggestions? Thanks.
You have 2 balance points one is mechanical and the other economic.
At the mechanical balance point the heat pump can no longer provide more BTUs than the house is losing at that temperature.
The economic balance point is the temperature when the gas furnace cost less to produce a BTU than the heat pump.
Your choice to disable the gas heat in the defrost mode is not one every one would make. You have chosen to sacrifice your comfort to avoid the use of the backup heat. My backup heat for defrost is about 3 minutes per defrost cycle that happens about once every 4 hours.
Walta
So far, the sacrifice to comfort has been less than with the use of gas for defrost. I'm not sure I can describe what happened with gas, except to say it blew cold air for a long time. I'm sure this is a thermostat adjustment but I have not figured out how to adjust it. I hate having the technician out for everything, especially during the pandemic. I often feel like I know more than he does about the thermostat and I wait long periods for him to complete his call to Carrier. If only I could communicate with him or Carrier over the phone. However, that will never happen.
I appreciate your comment about the economical balance point and I will have to investigate that further. I always thought it would be at a colder temperature than the mechanical balance point being an 80% efficiency furnace. However, I know that may not be correct. My cost for electricity is relatively inexpensive at $0.165 per kWh, and cost for gas is $1.54 per therm. These cost include all other additional expenses. At these costs, I have determines that the cost per BTU is about a third less for electricity. The other concern I have with gas is that it take a while for it to heat up the furnace before it actually starts to run. My impression is that with the heat pump reversing to defrost, it is done with the process before the gas blower would kick in. However, I do not know for sure if this is accurate. I have estimated, but not timed it. I estimate that the defrost last about 2 minutes. If the heat pump start the defrost just after the compressor turns off, it is not a problem and with an outside temperature of 23 degrees (F) my room temperature stays at set point. If it starts a while after the compressor stops, it loses about 1 degree. However, to recover the one degree, the compressor goes to 1oo% and the blower to high for quite a while and I find that bothersome comfort wise and sound wise. Thanks greatly for your reply! Please let me know if you have any other thoughts or suggestions.
Walter, my statement that the cost per BTU is about a third less for electricity is based on the the amount of BTUs made available by the heat pump which has a COP of 3.66 and the furnace being only 80% efficient. This comes to $13.21 per million BTUs for the heat pump and $19.30 per million BTUs for the furnace.
> 67 and sometimes after defrost the room temperature drops to 66. This causes 100% compressor capacity
1 degree isn't a big deal requiring 100% output. I'd look into a way to adjust the system for a milder reaction.
This is my first heat pump. I have played with the Infinity thermostat quite a bit to try to correct this. It doesn't seem to make any sense to me either. If you haven't already, read my reply to Walter above. It might provide a little more information. Carrier provides very minimal instructions with the thermostat. It kind of turns into trying various changes to see what might work but I get a little concerned that something like lowing the maximum blower speed could have serious detrimental consequences since I have no idea what is the appropriate setting. Doing so decreased the cycling but did nothing for the defrost recovery issue. However, I appreciate your comment. At least now I know I am not the only one thinking this isn't right. If I don't get the answer here at GBA to this issue, I will probably call the technician again. Thanks!