# Mini split efficiency w/ colder outdoor temps...



## DRINKSFORALL (Jan 13, 2015)

I have a Mits mini split systems rated at 21 seer (2 condensors and 3 "head" units) and was wondering at what outdoor temp does it make sense to go back to oil heat (forced hot water) to save $$ ??? My electric rate is .13 per kw (thru Verde electric) and I can get oil for around 2.30 a gal. The Mits can produce heat down to 0 degrees but I believe its efficiency is poor at those temps. leading to higher electric bills. I live in central Mass. where electricity has gone thru the roof the last month. I also supplement w/ pellet stove usage.


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## Highbeam (Jan 13, 2015)

13 cents per kwh is "through the roof"?

As long as the heat pump is running it is making heat at no less than 100% efficiency. It is making less heat of course and may not be able to keep up with demand so the oil boiler will have to kick on. You can use the fuel cost calculator on this site along with your oil boilers efficiency rating to determine how much extra you are paying for that oil heat but you have no choice in the matter. When your heat pump isn't able to keep up then your oil burner will have to make up the difference.

If you introduce a third fuel, pellets then you again need to compare that cost to the oil to see which is cheaper.

Great to have choices.

Is your minisplit a "hyper heat" model?


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## peakbagger (Jan 13, 2015)

Unless there was recent change, hyper heat units only come in a single head unit.


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## DRINKSFORALL (Jan 13, 2015)

Highbeam said:


> 13 cents per kwh is "through the roof"?
> 
> As long as the heat pump is running it is making heat at no less than 100% efficiency. It is making less heat of course and may not be able to keep up with demand so the oil boiler will have to kick on. You can use the fuel cost calculator on this site along with your oil boilers efficiency rating to determine how much extra you are paying for that oil heat but you have no choice in the matter. When your heat pump isn't able to keep up then your oil burner will have to make up the difference.
> 
> ...



Our kw rate at this time last year was .10, now at .16 ( a 60% jump). I pay 12.9 per kwh because I switched to another supplier (Verde). Will try to find fuel calculator - thanks!

My unit is a little over a year old (MSZ GE 15NA). Not sure its hyper heat. I have 2 head units of one condenser and a third off its own condenser. I have gotten heat as low as -1 (last week)


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## woodgeek (Jan 13, 2015)

Ok.  $2.30/gallon, if you assume delivers 100 kBTU, is $23 per million BTUs.  IF your boiler is esp efficient, maybe you get 110kBTUs, and your heat cost is more like $21/MBTU.

1 kWh of elec is 3414 BTUs, so 1MBTU from a resistance heater requires 292 kWhs @13 cents, that is $38 per MBTU.

If the mini is running at some COP factor relative to resistance, its heat cost is $38/COP.  You need to have COP > 38/23 = 1.65 for it to make sense to run it.  Odd are, the mitsu is running above that almost all the time, above 40°F its COP is prob >3, and its heat cost less than half of oil.  By the time it gets so cold that the COP drops that low, both the input power and output BTUs have dropped through the floor, so its not making much difference anyways.

Want a temp?  Let's say below 10°F.

What is your cost per ton of pellets??


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## Highbeam (Jan 13, 2015)

https://www.hearth.com/econtent/index.php/articles/fuel_cost_comparison_calculator/

Here's the calculator. The current low price of oil really makes things wonkey.


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## DRINKSFORALL (Jan 13, 2015)

woodgeek said:


> Ok.  $2.30/gallon, if you assume delivers 100 kBTU, is $23 per million BTUs.  IF your boiler is esp efficient, maybe you get 110kBTUs, and your heat cost is more like $21/MBTU.
> 
> 1 kWh of elec is 3414 BTUs, so 1MBTU from a resistance heater requires 292 kWhs @13 cents, that is $38 per MBTU.
> 
> ...


 Around 269.00 BUT the pellet stove does a poor job of heating rooms beyond the room it is in which is a 20 x 20 great rm.


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## woodgeek (Jan 13, 2015)

According to the calc, your pellets and your oil cost about the same amount per BTU at current prices.


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## Highbeam (Jan 13, 2015)

How much did you pay for that oil? and do you plan to replace it before the price goes up? What I'm getting at is the current price of oil only matters if you buy it one gallon at a time and burn it right then.


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## STIHLY DAN (Jan 13, 2015)

peakbagger said:


> Unless there was recent change, hyper heat units only come in a single head unit.


 
As of 2015 they can now be multiple.


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## AK13 (Jan 13, 2015)

peakbagger said:


> Unless there was recent change, hyper heat units only come in a single head unit.



Yes, there has been a recent change. They just released a multi-port version.


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## STIHLY DAN (Jan 13, 2015)

woodgeek said:


> Ok.  $2.30/gallon, if you assume delivers 100 kBTU, is $23 per million BTUs.  IF your boiler is esp efficient, maybe you get 110kBTUs, and your heat cost is more like $21/MBTU.
> 
> 1 kWh of elec is 3414 BTUs, so 1MBTU from a resistance heater requires 292 kWhs @13 cents, that is $38 per MBTU.
> 
> ...



 An m series at 5* depending on inside unit type, cop is 1.3-1.6


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## Highbeam (Jan 14, 2015)

STIHLY DAN said:


> An m series at 5* depending on inside unit type, cop is 1.3-1.6


 
Though the COP is 1.3-1.6, what % of rated output is being made? Not sure if the m-series is the hyperheat but in general, at low temps you not only have reduced output but also a more expensive output.


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## woodgeek (Jan 14, 2015)

If I were the OP, I would run the oil on its stat, set a couple degrees below the minis stat setting (so the mini is primary, with oil backup).

I'd shut down the pellets altogether (to avoid cleaning), and bet on pellet prices dropping in the future due to cheaper oil.  Then I would stock up on the cheap pellets and burn them when oil got more expensive again.


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## DRINKSFORALL (Jan 14, 2015)

Highbeam said:


> Though the COP is 1.3-1.6, what % of rated output is being made? Not sure if the m-series is the hyperheat but in general, at low temps you not only have reduced output but also a more expensive output.


And thats what I would love to figure out. At what outdoor temp does it makes sense to switch to oil (forced hot water) versus using the 3 Mits HP's??? There must be temperature point where the HP's start pulling way more voltage/ amps to get the job done and become less economical


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## STIHLY DAN (Jan 14, 2015)

Highbeam said:


> Though the COP is 1.3-1.6, what % of rated output is being made? Not sure if the m-series is the hyperheat but in general, at low temps you not only have reduced output but also a more expensive output.



M series at 5*f is 65% capacity. hyper heat is 100%. At -13* hyper heat is 80% capacity.


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## DBCOOPER (Jan 14, 2015)

It would be interesting to see what  the current draw/wattage is at low temperatures compared to a warmer temperature.


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## Highbeam (Jan 14, 2015)

DBCOOPER said:


> It would be interesting to see what  the current draw/wattage is at low temperatures compared to a warmer temperature.


 
It's all here. You know that full rated output is X btu which can be converted to watts. You know that for every watt of energy the heat pump uses you are getting 1.3 -1.6 watts of heat to the space. You know that 5 degrees the M series is at 65% capacity. You know how much a watt costs.

So lets start with X equals 20,000 btu. 65% of 20,000 is 13000 btu. 13000 btu is 3807 watts. Now you are spending between 2928 and 2379 watts to get it due to the COP range.

Okay, now just add your cost per killowatt hour and you have a dollars per watt or btu delivered. Compare that to the cost per btu from your oil burner.


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## Highbeam (Jan 14, 2015)

I wish all heat pumps disclosed % output at various temperatures as well as COP at various temperatures. Surely they know.


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## peakbagger (Jan 14, 2015)

AK13 said:


> Yes, there has been a recent change. They just released a multi-port version.



Great, I know  more than a few folks who have been waiting for a hyper heat version multihead


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## woodgeek (Jan 14, 2015)

Highbeam said:


> I wish all heat pumps disclosed % output at various temperatures as well as COP at various temperatures. Surely they know.



It depends on the outdoor humidity.  Dry air outside == even lower output and COP.


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## Highbeam (Jan 15, 2015)

woodgeek said:


> It depends on the outdoor humidity.  Dry air outside == even lower output and COP.


 
Of course but still, they can disclose the tables and curves at various or at one humidity.


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## woodgeek (Jan 16, 2015)

After long research, I figured out that the output tables that I am using are for 100% outdoor RH and do not include defrost losses.  Completely unrealistic, but as specified by the relevant agency.  Real BTU outputs in dry weather can be 30% lower.


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## peakbagger (Jan 16, 2015)

I haven't instrumented it but I find that in cold damp conditions, I get less heat as the unit is frequently defrosting. Once it clears up the unit may not defrost for hours, the trade off in winter is that  clear skies usually means radiation cooling so the outdoor temp plummets.

 I am heating my entire house today with my 1 ton minisplit. The outdoor temp of 25 deg F and the unit is cycling so I am not even using its full capacity. The sun was out so I expect I was offsetting the power used with solar but it just started snowing so I get to raid the KW piggy bank.


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## woodgeek (Jan 16, 2015)

I'm 25F too, and my 4 ton conventional HP is just treading water this AM.


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