On cool mornings, which unit is cheaper to run?

[Jakemedic]

Hello,
In these unprecedented times, I need some help desiding which heater to use. Normally it would be natural gas, but it’s price has increased 25% already since last winter and it really isn’t even heating season yet. I have installed both a Mr. Cool mini split (for ac purposes) and a Mr Heater natural gas overhead furnace. I am paying 1.35 per therm for natural gas and .12 cents per kWh. The Mr. Heater is about 80% efficient while the Mr. Cool is not very efficient with heating Efficiency of 10 SER. (From the yellow label). When it gets really cold out, there is no way to use the heat pump as it only goes down to 12 degrees if my memory serves me correctly.

This morning it was 22 degrees outside and inside my shop was 49 degrees. My shop is very well insulated with R19 in the walls with minimal thermal breaks, and R49 in the attic. Which would be the best to bring my shop to sweatshirt temp of 60 degrees?

 

[u2slow]

I would use fuel to get it to temperature, and the heat pump to maintain. Jmho.

 

[jlv03]

From 12 on up, use the heat pump to maintain a temp and the gas heater to bring up the temp. Below 12 default to gas.

At least that would be my thoughts, comfort wise.

 

[Jakemedic]

Thank you for your reply! This morning I turned on the mini split mostly cause I haven’t cleaned the overhead furnace yet. Maybe this afternoon I will get up and blow it out for the season. Thanks again!

 

[danski0224]

If you really want to know, you need to run the numbers through an app that will give you the theoretical and economical balance points for your system.

Anything else is just a guess.

You will need a bit of info to get the numbers.

My theoretical balance point is 14 degrees, economic is 32, based on my utility costs a couple of weeks ago.

 

[u2slow]

You have to consider wear & tear on the heat pump too... making it work at the borderline of it's ability, it may not last long.

I heat with wood and a heat-pump at the house so I have a similar 'dual' scenario.

The shop isn't sealed up yet, so I just have to dress warmer as the temperature drops. (Cheapest solution, lol)

 

[kaymccampbell]

Although the heat pumps are rated to 12 or so degrees, the ones I've seen in operation are not really efficient much lower than the high 20's. They really struggle in that low range. That may have changed in the last few years.

I'm surprised you lose so much heat. My shop, with similar insulation, stays around 56-57 in 20 degree weather, just from the cast off heat of the equipment running. The heat rarely fires unless I kick the thermostat. This morning, with the temps around freezing it was 67 down there. Are your big doors insulated?

 

[Jakemedic]

Although the heat pumps are rated to 12 or so degrees, the ones I've seen in operation are not really efficient much lower than the high 20's. They really struggle in that low range. That may have changed in the last few years.

I'm surprised you lose so much heat. My shop, with similar insulation, stays around 56-57 in 20 degree weather, just from the cast off heat of the equipment running. The heat rarely fires unless I kick the thermostat. This morning, with the temps around freezing it was 67 down there. Are your big doors insulated?

Yea my overhead door is R19. I think the reason it went so low is that I had my exhaust fan open. I closed it today, see what tomorrow brings. The door over the exhaust fan is also insulated to R19.

 

[yeldogt]

Hello,
In these unprecedented times, I need some help desiding which heater to use. Normally it would be natural gas, but it’s price has increased 25% already since last winter and it really isn’t even heating season yet. I have installed both a Mr. Cool mini split (for ac purposes) and a Mr Heater natural gas overhead furnace. I am paying 1.35 per therm for natural gas and .12 cents per kWh. The Mr. Heater is about 80% efficient while the Mr. Cool is not very efficient with heating Efficiency of 10 SER. (From the yellow label). When it gets really cold out, there is no way to use the heat pump as it only goes down to 12 degrees if my memory serves me correctly.

This morning it was 22 degrees outside and inside my shop was 49 degrees. My shop is very well insulated with R19 in the walls with minimal thermal breaks, and R49 in the attic. Which would be the best to bring my shop to sweatshirt temp of 60 degrees?

Heat pumps typically rate in COP's ... do you have a 10 HSPF (that the heat) SEER is cooling. 10 HSPF gets you close to a 3 COP

What you need to do is see what COP is listed at different temps -- this will drop as the temps drop. Remember --- ratings in cold weather are for different things. One is telling you if it will work at a given temp and the other is telling you what is the output. So when you see -- works down to 5 degrees ...That's what it will do. Look at the chart to see the output as the temps drop. If it says gives full output at 5 degrees -- that is telling you that a 18k btu unit will still provide 18k of heat at 5 degrees. the hyper heat type have pan heaters and other items to make them work in colder temps.

What you need to to is find a fuel converter online and punch in your electric rate and the gas rate. On the electric side you use 100 percent and the gas side you use 80 percent. That will give you the straight electric cost as if you were using electric resistance. here is the magic of the heatpump -- divide the electric cost by the COP. In the case of 3 -- the cost is 1/3 of the resistance. That's what you compare.

my little converter only uses 85% for natural .. cost for million BTU is $15.88. Straight electric is $35.17. COP of 2 gets you to $17.58 .. so the cost of COP at 2 is darn close to that 80% furnace. Anything above is going to win. My guess is it's going to have to be quite cold for the gas to finally win. You have cheap electric -- all of mine is over 20.

A heat pump will always be cheaper than straight electric

 

[Jakemedic]

Heat pumps typically rate in COP's ... do you have a 10 HSPF (that the heat) SEER is cooling. 10 HSPF gets you close to a 3 COP

What you need to do is see what COP is listed at different temps -- this will drop as the temps drop. Remember --- ratings in cold weather are for different things. One is telling you if it will work at a given temp and the other is telling you what is the output. So when you see -- works down to 5 degrees ...That's what it will do. Look at the chart to see the output as the temps drop. If it says gives full output at 5 degrees -- that is telling you that a 18k btu unit will still provide 18k of heat at 5 degrees. the hyper heat type have pan heaters and other items to make them work in colder temps.

What you need to to is find a fuel converter online and punch in your electric rate and the gas rate. On the electric side you use 100 percent and the gas side you use 80 percent. That will give you the straight electric cost as if you were using electric resistance. here is the magic of the heatpump -- divide the electric cost by the COP. In the case of 3 -- the cost is 1/3 of the resistance. That's what you compare.

my little converter only uses 85% for natural .. cost for million BTU is $15.88. Straight electric is $35.17. COP of 2 gets you to $17.58 .. so the cost of COP at 2 is darn close to that 80% furnace. Anything above is going to win. My guess is it's going to have to be quite cold for the gas to finally win. You have cheap electric -- all of mine is over 20.

A heat pump will always be cheaper than straight electric

My seer rating is 20 for my ac part of the mini split. I appreciate you taking the time to figure this out for me. The mini split is much quieter than the overhead furnace. But it appears to take longer to warm up the shop than the furnace. I am just blessed that I had the resources to put both in. I’m so glad I did. The insurance company wouldn’t let me put a wood drying, unless I left out the overhead door. That would have been nice for heating and burning my mistakes. Thanks again!

 

[Jackfre]

I was unaware that the Mr Cool units were only 10 seer. My 10 & 12 yr old 12 & 15 units are 23 & 25 seer and todays models are in the mid-30’s. Jake, that isn’t a knock on you, but for those considering buying a mini-split they need to compare the seer/hspf ratings per model/manuf. To our OP, you have options. I would run the HP until it didn’t maintain temp. What size is it? what is really going to determine what you run is your comfort in the space and you are covered with both fuels. Congrats

 

[Jakemedic]

 

[yeldogt]

My seer rating is 20 for my ac part of the mini split. I appreciate you taking the time to figure this out for me. The mini split is much quieter than the overhead furnace. But it appears to take longer to warm up the shop than the furnace. I am just blessed that I had the resources to put both in. I’m so glad I did. The insurance company wouldn’t let me put a wood drying, unless I left out the overhead door. That would have been nice for heating and burning my mistakes. Thanks again!

There should be numbers in the manuals for the other ratings.

You gas heater could be putting out 60k or 80k BTU .... or more. What is the mr cool? An 18k BTU units is doing just that .. 18k.

60k at 80% is 2.5 times more heat

 

[Jakemedic]

This is my sticker from my mini split. Does a great job of cooling my shop for sure. Never thought of using it before for heat, until now. Natural gas prices have skyrocketed.

 

[yeldogt]

This is my sticker from my mini split. Does a great job of cooling my shop for sure. Never thought of using it before for heat, until now. Natural gas prices have skyrocketed.

should be graph with curve --- what size is it

 

[Jakemedic]

18,000 btu I’ll have a look in the manual to see if there is a graph with a curve. Thanks!

 

[dcg9381]

There's a curve. From my own experience - and there is no data behind this, my 24k Daikins don't work work a darn below 35 degrees. And they cycle power to defrost the condenser fins. Judging from my electricity bills (.11/Kwh) and lack of heat in the shop, they are NOT the way to go when it's cool. It'd likely be different with a Mitsu hyper-heat unit.

I heat with propane space heaters when it approaches freezing.. Frankly I do the same thing in the house as those heat pumps are less efficient than the Daikin units.

With swings of 50% or more on LPG in the last year, it very well could change the economics though...

 

[dcg9381]

What is the mr cool? An 18k BTU units is doing just that .. 18k.

Look at the performance curve in the cold. It's no going to be 18k. It could very well be 9k or less below freezing on some of these units.

 

[yeldogt]

Look at the performance curve in the cold. It's no going to be 18k. It could very well be 9k or less below freezing on some of these units.

I don't have a Mr. cool --- the last non hyper I did was an LG back on 06 .. not sure they even had them at the time.

 

[danski0224]

My seer rating is 20 for my ac part of the mini split. I appreciate you taking the time to figure this out for me. The mini split is much quieter than the overhead furnace. But it appears to take longer to warm up the shop than the furnace. I am just blessed that I had the resources to put both in. I’m so glad I did. The insurance company wouldn’t let me put a wood drying, unless I left out the overhead door. That would have been nice for heating and burning my mistakes. Thanks again!

Heat pump air discharge temperature is lower than fossil fuel heat

 

[PoorUB]

The 80% heater at $1.35 NG costs about $17 per million BTU

The Mrcool, $0.12 per KWh about $12 per million.

You should be able to run the Mrcool to around 5F before it will be about the same cost as the unit heater, but it might not put out enough heat to warm up the building. heat output drops off badly below 17F outdoor temp.

I would probably stop useing the Mrcool around 15F.

Specs are here.
https://mrcool.com/wp-content/dox_repo/mc-diy-es-ss-en-01.pdf

 

[Kermit987]

If you truly want to know, you should run the numbers through a program that will provide you with your system's theoretical and financial balance points.
To raise the temperature, I would use fuel, and to keep it there, I would use a heat pump.

 

[u3b3rg33k]

Look at the performance curve in the cold. It's no going to be 18k. It could very well be 9k or less below freezing on some of these units.

most mini's are inverter units, so they can speed up to provide the same BTU output at lower outdoor temps.

fancy models like hyperheat will have things like vapor injection, to increase the deliverable BTU over a simpler unit.

https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=2641&context=icec

Thermodynamically the VI technology offers significant advantages in applications where temperature lift is high (e.g. water heating, space heating and refrigeration), and relatively smaller benefits in applications such as residential a/c where efficiency standards are based on tests conducted at very low temperature lift conditions. This could explain why VI technology is more widely known and used in residential applications in Europe and Asia, compared to the US where the residential market is focused almost exclusively on air conditioning applications.

 

[PoorUB]

most mini's are inverter units, so they can speed up to provide the same BTU output at lower outdoor temps.

fancy models like hyperheat will have things like vapor injection, to increase the deliverable BTU over a simpler unit.

https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=2641&context=icec

Um, all heat pumps the BTU output for heat drops of at some point when the outdoor temp goes down. Being an inverter unit or not, it still happens.

 

[u3b3rg33k]

Um, all heat pumps the BTU output for heat drops of at some point when the outdoor temp goes down. Being an inverter unit or not, it still happens.

"at some point" yeah. sure. there's a big difference between a unit that stops being useful at 40F and a unit that provides nameplate BTU down to 5F.

not sure what the point of this reply was, tbh.

 

[dcg9381]

"at some point" yeah. sure. there's a big difference between a unit that stops being useful at 40F and a unit that provides nameplate BTU down to 5F.

not sure what the point of this reply was, tbh.

I think it's an important point for those that wouldn't otherwise know. You install the Daikins that I did in Michigan, you're going to be very disappointed in heating performance.

 

[PoorUB]

"at some point" yeah. sure. there's a big difference between a unit that stops being useful at 40F and a unit that provides nameplate BTU down to 5F.

not sure what the point of this reply was, tbh.

Reread your post and think about it.

 

[Sumboodie]

1 kWh = 3,413 Btu = .0341 therms

So $0.12 of power is equal to about $0.04 of natural gas. If your heat pump is 3x more efficient than the gas heater, than it'd be a money saver.

 

[Sumboodie]

Yea my overhead door is R19. I think the reason it went so low is that I had my exhaust fan open. I closed it today, see what tomorrow brings. The door over the exhaust fan is also insulated to R19.

3" door?

 

[u3b3rg33k]

I think it's an important point for those that wouldn't otherwise know. You install the Daikins that I did in Michigan, you're going to be very disappointed in heating performance.

that's something that the installer should be understanding and pointing out. not every homeowner should need to be an expert on every appliance they buy.

Virtually all minisplits are inverter units, and will adjust their motor speed to provide the required/nameplate BTU, but plenty of split systems (traditional AC style) are still just an AC running backwards, and will have a pronounced drop in delivered air temp/BTU as outside temps drop.

due to the the higher SEER/HSPF requirements, there are budget lines that have inverter options, some don't even make noise about it. I have a 5 ton at work that serves as a MAU/economizer. It's a concord branded unit, but guess who's name is on the circuit board? yep, LG.

This is nice because it's very easy for an inverter unit to have "extra" features like low ambient cooling, or doing things like "not freezing the coil with low inlet air temps" by reducing compressor output. It was marginally more expensive than a straight cooling unit, and when you factor in not having to buy and install an NG furnace as well, it was actually cheaper overall.

 

[mike93lx]

Yea my overhead door is R19. I think the reason it went so low is that I had my exhaust fan open. I closed it today, see what tomorrow brings. The door over the exhaust fan is also insulated to R19.

You sure about the door being r19? That would be exceptionally well insulated

 

[dcg9381]

that's something that the installer should be understanding and pointing out. not every homeowner should need to be an expert on every appliance they buy.

Installer? You're talking to him. Number of people I talked to when purchasing a unit online? Zero. Course, that's my responsibility. We've got quite a few mini-split self installs going on within GJ. That's why I think the point that u3b3rg33k made is important. I don't think I made a mistake by buying a unit that has a big heating capacity drop. 99.8% of the use of this unit in my state is for cooling.... But there is some pretty clear confusion if some users think an 18k unit puts out 18k of heat in all circumstances.

 

[u3b3rg33k]

Installer? You're talking to him. Number of people I talked to when purchasing a unit online? Zero. Course, that's my responsibility. We've got quite a few mini-split self installs going on within GJ. That's why I think the point that u3b3rg33k made is important. I don't think I made a mistake. 99.8% of the use of this unit in my state is for cooling.... But there is some pretty clear confusion if some users think an 18k unit puts out 18k of heat in all circumstances.

normally oversizing is bad. for minisplits I don't think it is. I look at the minimum output (single head units usually win this over multi-splits by a significant margin), and go "is that reasonable?" any excess on sizing just gets you to comfort faster, then drops down into the efficient operation region to hold temp.

my garage is getting a 24k mini. my 5kW (17kBTU) electric heater will get me into the mid 60s in single digit weather. the mini will do it either faster, or for a whole lot less $$$, even in cold weather (NG is not happening). I don't really need it warmer than that out there.

another fun thing to notice is a lot of minisplits use the same indoor/outdoor coils, and just put in a bigger compressor /spec the RPM higher for the bigger BTU units. you'll see a tradeoff on SEER/HSPF as the BTU's go up, while the product line is dimensionally identical otherwise.

 

[Jakemedic]

You sure about the door being r19? That would be exceptionally well insulated

Yes a special order door. 2 1/4” thick. Actually I just looked and it is R 18.4 I opted for a 10’ wide 8’ tall door. Kept the price somewhat reasonable. It still was nearly 1000.00 over 2 years ago and the same door is 1600.00 now.

 

[mike93lx]

Yes a special order door. 2 1/4” thick. I opted for a 10’ wide 8’ tall door. Kept the price somewhat reasonable. It still was nearly 1000.00 over 2 years ago.

Wow

 

[fitter30]

Do u want to wait x time or 1/2 x time to get warm turn both on then turn one off. Set temperature back everything loses temp walls, all the stuff in there. All depends how comply u want to be.

 

[yeldogt]

Will say this again. Working at 5 degrees and providing full output at 5 degrees is not the same.

Hyper heat mini-splits are designed to work in cold climates .. they will provide full output down to the rated temp. The older ones were 5 degree many now are -15 degrees F . Have to read the graph .... they will still operate at a lower temp than this full output outside temp .... but, will provide lower output (BTU's of heat)

Non hyper units are designed for warmer climates --- again you have to read the graph and understand the output at different outside temps. Some can freeze at low temps because of how the water comes off them in defrost and how they defrost.

Hyper units typically have greater condenser coil size and pan heaters -- etc.

heat-pumps are always going to be cheaper to run vs resistance.

It's often the case that a heatpump sized for AC will not be large enough to heat a space in a cold climate -- it still may be cheaper to run than something else ...... so run them both. People who don't maintain temps are often not happy with heatpumps because they don't give thought to how they work. A 18k heatpump may be able to keep a space warm but it's only got 18k of you start out cold. Many gas heaters are oversized and can be 80k or more

 

[PoorUB]

Will say this again. Working at 5 degrees and providing full output at 5 degrees is not the same

This is a point that people keep missing.

A friend on mine has a Bosch heat pump for his home. Everything I have read says they run down to zero F. My buddy was asking me when to do change over. I finally found an output chart and it falls off badly at 15F so he set it at either 15F or 20F. A far cry from zero!

With heat pumps the out put fall off as it gets colder and you need more BTU to heat the space so it is a can't win situation. Many heat pumps drop off badly right below freezing. A few will do a decent job heating to -20F.

 

[Copymutt]

Guys, its time to cut the ties to energy suppliers as much as possible.
Wood, Solar, Recapture are not tied to political agendas. Works for me.

 

[yeldogt]

This is a point that people keep missing.

A friend on mine has a Bosch heat pump for his home. Everything I have read says they run down to zero F. My buddy was asking me when to do change over. I finally found an output chart and it falls off badly at 15F so he set it at either 15F or 20F. A far cry from zero!

With heat pumps the out put fall off as it gets colder and you need more BTU to heat the space so it is a can't win situation. Many heat pumps drop off badly right below freezing. A few will do a decent job heating to -20F.

The Bosch splits are a nice unit and are great for retrofitting .... my brother did two when he bought a place in SC. Not going to be a problem there as no real cold weather

My Carrier split is good to 2 degrees full output .... Forget what my Mitsubishi mini-splits are -- think 5 degrees.