Best way to use mini split heating heat pump?

[h2ofun]

Okay, since so many are doing things to save electric costs, looking for some guidance.

I live in about 4500 sq foot house, 2 plus levels. Used to have 2 propane multipack systems.
To make a long story short, these were removed and replaced with new mini split heating cooling systems, 10 heads
throughout the house. I love the ability do to real zoning.

But, since my solar is down to like 25% of what it produces in the summer, trying to figure out the best way to run these new mini splits for the least cost, but being able to keep my large home warm. It is 60 now.

With the propane heating, I would only run when needed, so this is how I am currently mentally programmed.

But, with the mini split heat pumps, is this the correct way to use? I have read some opinions that I should just set the temperature, like at 64, and let them run 7/24 to use minimum electricity?

But then I am thinking do they cost more to run in the morning, when my outside temps are 30? Meaning, should I turn off and lets say turn them on at 2 pm, and heat up the house to lets say 70, and then turn them off at 7 pm when I go to bed?

Thanks for any advice on best way to use these.

 

[getakey]

Personally, I would set the temp to what is comfortable in the areas of the home that people are using. Once you get an annual true-up you can adjust your strategy. I think with that many panels, you are in danger of getting a negative annual true up. I suspect you will accumulate credits in the summer and if you don't use those credits up in the winter, you have wasted money

 

[zƬesla]

I'm up north with 5 mini-split heads (2 outside units) and an oil boiler for forced hot water baseboard heating. This is my first winter with solar and Powerwalls. I currently have the mini-splits set between 65-70 with the corresponding baseboard thermostat set 2-3 degress cooler. The system generally works well into the mid-to-high 30s. The mini-splits are rated to -5 but I've noticed they consume a lot when it drops below 20-25. Past two months have been abysmal for solar generation, so I may have to rethink the strategy. It is nice having a much lower oil bill though. Figured this is our trial winter and I'll adjust for next year.

 

[zƬesla]

BTW, with the whole house on the Powerwalls, one of my concerns has been running them down if grid goes out in winter. I want the ability to run the heads in summer for cooling so didn't want them isolated. In process of developing a Raspberry Pi-based gizmo to auto-shut off the heads if grid goes out.

 

[h2ofun]

Personally, I would set the temp to what is comfortable in the areas of the home that people are using. Once you get an annual true-up you can adjust your strategy. I think with that many panels, you are in danger of getting a negative annual true up. I suspect you will accumulate credits in the summer and if you don't use those credits up in the winter, you have wasted money

Yep, I only turn on heads for rooms I use. I keep the doors closed on no used areas

I guess I would rather have a negative true up than have to pay, but, I also am tired of being cold.

That is why I am trying to figure out if best to leave them 7/24 at 64 degrees for the rooms I am using, or turn them on lets say at 3 pm to 70 degrees, and turn off at 7 pm. I just do not know if they use more based of the temperature differential between inside setting and outside air temp.

 

[getakey]

the efficiency goes down as the outside temp goes down. If you don't leave at 64, how cold does your house get. They will have a hard time bringing the house up from say 55 to 70 first thing in the morning. Probably better to have them ccycle a bit all night

 

[h2ofun]

the efficiency goes down as the outside temp goes down. If you don't leave at 64, how cold does your house get. They will have a hard time bringing the house up from say 55 to 70 first thing in the morning. Probably better to have them ccycle a bit all night

My house was 56 this morning, and as you know, been pretty warm in calif so far. That will change in a little over a week. Rain is finally coming.

I have used in morning when cold, and it jumped to like 8kw. So I just keep playing and learning. All the google sites I find say set and leave alone.

 

[zƬesla]

An air gap assessment of the house may be worthwhile if one hasn’t been done.

Keywords: energy audit, blower door test

 

[Kbreezy92]

Similar situation here in regards to solar and mini split setup. I've just installed 24 panels and have 2 powerwalls. I have a 5 ton mini split with 5 heads. I only use 3 heads currently. Weekdays I have it set to 64/65 at 2 pm. 2 heads stop at 10 pm then come back up at 5 am to 65. All three stop at 9 am and the cycle repeats. On weekends I have it set to 64/65 all day long. With this schedule and on the EV-2A programs with PG&E and charging one tesla at home twice a week, I'm pretty much breaking even right now in December.

Also, I did my own energy audit for my home. I was able to find areas such as the garage door and front windows that had air gaps. Fixed those and was able to keep the house more insulated. I would also advise on checking the air gaps on the doors from the rooms you don't use/ run your heads on.

As far as energy usage, I definitely see more usage over the weekends but when days are less cold, it obviously uses less energy. My thinking is you should keep the house warm when you are home and turn off the heads when you are not home.

 

[zƬesla]

My thinking is you should keep the house warm when you are home and turn off the heads when you are not home.

Unless you live some place cold like some of us do where the heatpumps would have no chance to warm a house back up on many winter days if gets shut off for a while and there is no other heat

 

[h2ofun]

7 Tips for Operating Your Mini-Split Heat Pump in the Summer

 

[h2ofun]

Minisplit: Leave it on or turn it off? - GreenBuildingAdvisor

 

[h2ofun]

Okay, over the last few weeks I have been playing with my mini split heat pumps. I have turned them off from 4 to 9 thinking I was saving a bunch of money by not using peak rates. (Until I find it is only 2 cents more per kw). I then played with shutting down at 7 pm when I go to bed and turn back on at 4 am when I get up. And the last few days I set them and left them.

My installers told me the best way to use these heat pumps is set them and leave them. Basically all the google searches say the same. They say if the units are running below max, you save a lot.

So after comparing my hourly pge useage for the above different uses, it seems very clear the cheapest, and best comfort, is to set them and leave them. So for me, the lowest they go is 61, which the wife likes. Since we are having a way way over normal temps so far this fall, I know more electricity will be used once it says cold during the day.

 

[Sealander]

Okay, over the last few weeks I have been playing with my mini split heat pumps. I have turned them off from 4 to 9 thinking I was saving a bunch of money by not using peak rates. (Until I find it is only 2 cents more per kw). I then played with shutting down at 7 pm when I go to bed and turn back on at 4 am when I get up. And the last few days I set them and left them.

My installers told me the best way to use these heat pumps is set them and leave them. Basically all the google searches say the same. They say if the units are running below max, you save a lot.

So after comparing my hourly pge useage for the above different uses, it seems very clear the cheapest, and best comfort, is to set them and leave them. So for me, the lowest they go is 61, which the wife likes. Since we are having a way way over normal temps so far this fall, I know more electricity will be used once it says cold during the day.

Heat pumps are common in our area and our HVAC company has installed hundreds replacing both propane and electric strip heating systems. You have come to the correct conclusion. Heat pumps work like air conditioners in reverse. Cycling the power on and off at different times of the day or changing the temperature more than a few degrees up or down makes them work much harder and consume more energy in the long run. Most mini split systems have variable speed compressors and fans and adjust their speed (and power consumption) to the current conditions so they will just slow down as needed and save energy when they reach the temperature set point.

 

[Dave EV]

Minisplit: Leave it on or turn it off? - GreenBuildingAdvisor

The last comment of this post has the right answer (direct link below):

Minisplit: Leave it on or turn it off? - GreenBuildingAdvisor

Regarding the original question generically:

Whether any particular heat pump set up/down/off makes sense depends on many things:

1) how long the setback is for
2) outside temperature vs time
3) the COP vs load curve of the heat pump
4) the ability to limit heat pump output during catch-up - keep it to <= 100% of rated capacity and COP remains high.
5) time of use rates
6) etc

The amount of comfort reduction (or in some cases increase) depends on insulation, thermal mass, humidity, etc.

If estimates show it's anywhere close, use measurements for the best answer.

And there's more data in the embedded link:
Visualizing mini-split performance data - GreenBuildingAdvisor

My thoughts are that if steady state is the most efficient way to easily keep the house comfortable using the least amount of energy with a variable speed mini-split, that leaves a lot of room for efficiency improvements.

For example, from a pure physics perspective looking at just heating, your outdoor compressor is most efficient when it's warm outside - and do you really need or want the same temperature inside from 2-5 AM when you're sleeping and it's at it's coldest outside? Also, the indoor heat exchanger is also more efficient when pulling in cooler air to warm up - it's able to transfer more heat to the air for a given airflow.

If you're seeing significantly less energy use by running in steady state compared to adjusting setpoints based on actual demand, there's something else going on that should be addressed.

 

[Patrick66]

@DaveEV - You have to remember that peak efficiency of the units is not 100% load, it is usually something closer to 50-70%. So, if you do a setback that causes the unit to run at full load to recover you will be hard pressed to do so at higher efficiency.

Now, if you don’t run the unit below freezing and either let the house temperature drop or use a wood-burning stove for heat, you might come out ahead if it gets 20-30 degrees warmer during the day.

 

[SSedan]

I have Fujitsu minisplits as primary heat in my leaky 1970s 2400sq bi-level near Green Bay.
These function to -28f, but honestly begin to struggle if we spend much time below -15f. We keep the main living space 68-70 bedrooms low to mid 60s.
Kids are little and low 60s everywhere would be cold for them.

 

[h2ofun]

@DaveEV - You have to remember that peak efficiency of the units is not 100% load, it is usually something closer to 50-70%. So, if you do a setback that causes the unit to run at full load to recover you will be hard pressed to do so at higher efficiency.

Now, if you don’t run the unit below freezing and either let the house temperature drop or use a wood-burning stove for heat, you might come out ahead if it gets 20-30 degrees warmer during the day.

Yep, that is what I read/hear is if you make them go to 100% load, takes a lot of electricity, which I have seen looking at my pge hourly data. If is hard for me to reprogram my brain to say having something running is cheaper than turn on and off. But, after looking at the numbers for a while now, trying all the different ways, It all supports what experts have said.

 

[Dave EV]

@DaveEV - You have to remember that peak efficiency of the units is not 100% load, it is usually something closer to 50-70%. So, if you do a setback that causes the unit to run at full load to recover you will be hard pressed to do so at higher efficiency.

I'd love to see some real world data, but the link I posted to above seems to imply that there's only nominal differences from 20-120% rated output and it's pretty flat until you start getting to above rated output. At the very least, you'd like to be able to keep the heat pump from running in any inefficient modes, even if that means taking longer to hit your desired set point.

To do this right you need to keep the heat pump in it's sweet spot, but you're likely not running in the sweet spot if you run it at a constant set point with varying outdoor temps.

Ideally the system would look at the desired temperature window schedule and forecast temperatures and then then automatically pick the most efficient way to run the system.

 

[getakey]

I have Fujitsu minisplits as primary heat in my leaky 1970s 2400sq bi-level near Green Bay.
These function to -28f, but honestly begin to struggle if we spend much time below -15f. We keep the main living space 68-70 bedrooms low to mid 60s.
Kids are little and low 60s everywhere would be cold for them.

Do your mini-splits have electric resistance heating for low temps or are they strictly heat pumps?