Door manufacturers are notorious for exaggerating their doors r values, as there is no check in place in the industry to ensure honesty. They will measure the r value in the center of one panel rather than the whole assembly.
On cool mornings, which unit is cheaper to run?
- Thread starter Jakemedic
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American Locomotive
Well-known member
Modern mini-splits (especially hyper heat unit) have very high COPs, and are honestly even competitive with natural gas at moderate temperature (40+ or so).
Here's a spreadsheet I put together for my friend, who had propane heat and was considering mini-splits (which he then installed). The numbers are all relative to the price of propane.
Here's a reworked version with the numbers you provided (still using propane as a benchmark reference), assuming your minisplit has a COP of 3 (Use the "Mitsubishi Minisplit @ 17 to see COP of 3 numbers). I'm not sure how low your minisplit can maintain a COP of 3 for, but it appears that it's just about equivalent to natural gas in terms of overall cost - at least in the 30-40*F range.
Here's a spreadsheet I put together for my friend, who had propane heat and was considering mini-splits (which he then installed). The numbers are all relative to the price of propane.
Here's a reworked version with the numbers you provided (still using propane as a benchmark reference), assuming your minisplit has a COP of 3 (Use the "Mitsubishi Minisplit @ 17 to see COP of 3 numbers). I'm not sure how low your minisplit can maintain a COP of 3 for, but it appears that it's just about equivalent to natural gas in terms of overall cost - at least in the 30-40*F range.
dcg9381
Well-known member
I knew one of you guys could do the math!
You didn't like my info in post 21??
Also your question asks about cool mornings. Your mini split is less money to run down to about 15F, then the efficiency drops off. At 30F or 40F is will be even more efficiant.
dcg9381
Well-known member
Educate me on how you determine efficiency at low temp, because it varies substantially. You mentioned "I'm not sure how low your minisplit can maintain a COP of 3 for" - Some of these units I've seen publish a BTU/ambient temp curve. Is that taken into account here?
(Not being critical, asking a legit question)
the manufacturer should post a curve that shows the outputs
Yes, A good chart will have watts consumed, and BTU out at various temps.
Even a poor chart will show the BTU out and various temps. When the BTU out drops, it usuall goes pretty fast. Like falling off and 15F to maybe half output buy 5F. Most people will just change over to the other heat sourse at the start of when the BTU falls off.
strutaeng
Well-known member
Disclaimer: I'm certainly not an HVAC guy at all.
My house has a plain Jane 4 ton natural gas furnace. We did a addition to our house and added a 2 ton mini split with heat pump. I'm new to heat pumps.
On my little 25x25 garage I've got an inverter window shaker and ran gas for our clothes dryer... decided to run a larger line and installed a Mr. Heater shop heater. I only run the garage AC or heater if I really need to be in there working on something during extreme heat or cold.
I've wondered this: Aside from operation costs to run each, which one is more expensive to operate from a initial cost, expected useful life and maintenance costs? That would be really helpful to know. I have no idea on this...and that's assuming you already have service set-up for each system.
My house has a plain Jane 4 ton natural gas furnace. We did a addition to our house and added a 2 ton mini split with heat pump. I'm new to heat pumps.
On my little 25x25 garage I've got an inverter window shaker and ran gas for our clothes dryer... decided to run a larger line and installed a Mr. Heater shop heater. I only run the garage AC or heater if I really need to be in there working on something during extreme heat or cold.
I've wondered this: Aside from operation costs to run each, which one is more expensive to operate from a initial cost, expected useful life and maintenance costs? That would be really helpful to know. I have no idea on this...and that's assuming you already have service set-up for each system.
Captain Spaulding
Well-known member
True, but wood is nearly impossible because of insurance, and hazy skies almost every day make solar much less useful in many areas of the country. I have 17KW solar and the output drops off dramatically in sync with the heating season here.
IMO, if you get 10 years out of an inverter minisplit with minimal service you are doing well.
A gas shop heater will run for 15-20 years and very likely may never need service.
The mini split may be cheaper to run, but then you need to figure in replacement costs. If you hire it out that can be several thousand dollars. Divide that cost over severl years and look at your heat and AC bill and replacement. It might not justify it.
I have said it many times, you either pay the contractor, or you pay the utility, there is no free lunch. Higher efficiency equipment will save you money on your utility bill, but it will need more service. The higher efficiency equipment is probably a better way to go, less money in the long run, but within reason. I don't advocate buying the highest efficiency equipment you can find, but go one or two steps up from the basic equipment. If the minimum for an air conditioner is 14 SEER, go with the 16 SEER. You will never see a return on 20+ SEER.
I get a kick out of gas furnaces. 95% is pretty typical. Someone has a 98% furnace available. Now I will admit I don't know the pricing, but come on, it is 2%! If it is $500 more it will never payout in the long run. I pay around $800 for natural gas a heating season. That comes to $16 a year savings. It would take 30 years to come out on it assuming $500 more for the equipment.
Denwood
Well-known member
On the door topic (likely your largest heat loss), you might want to check my thread here on door seals:
www.garagejournal.com
Our commercial overhead doors here are typically using 2" insulated panels, but in my case, the door seals were a large loss. After sorting the issue, I show in the thread the difference after proper air sealing and a massive drop in radiant heat run times for that zone. I'd agree 100% with everyone here that the overall cheapest way to manage heat is to use a large temperature setback when you're not in the shop, bump with gas, and maintain with your heat pump if it is capable. You should take a good look at the temperature/performance chart for that heat pump to confirm the numbers.
Garage door seals - $$$ flying from your wallet?
This is a repost of one of my thread posts, however I figure it's worth some discussion here in the heating/cooling area. I learned something this week about the value of garage door seals. The space in question is at my commercial site, but the garage door there is typical (albeit) larger to...
www.garagejournal.com
Our commercial overhead doors here are typically using 2" insulated panels, but in my case, the door seals were a large loss. After sorting the issue, I show in the thread the difference after proper air sealing and a massive drop in radiant heat run times for that zone. I'd agree 100% with everyone here that the overall cheapest way to manage heat is to use a large temperature setback when you're not in the shop, bump with gas, and maintain with your heat pump if it is capable. You should take a good look at the temperature/performance chart for that heat pump to confirm the numbers.
All about BTU ..... How many and at what cost. Heat pumps are AC units working in reverse. In a hot part of the country where you need big AC units the heating needs are low and the BTU outputs of a heat pump are fine.
My place in SC is about equal what it needs in the summer for AC and winter -- so heat pump is great. But just like AC on a hot day .. you don't expect a property sized unit to drop the temp in an hour on a hot day.
Someone with a 36k BTU heat pump for AC and a 110k furnace for heat in the winter ... Sure the heater will bring the space up to temp faster if you drop it down. The heater has 3x the BTU's .... it's a question of cost for each type of fuel.
My place in PA has the heat pump on now -- it's much cheaper running the heat-pump in this weather vs the propane furnace or boiler. The unit is only 36k and the heat load at design temps show the building at around 50k ... so the heat pump does not have the BTU's to get the job done ... but -- it's still cheaper to have the heat pump giving what it can and the rest being taken care of by something else. That's not always possible. I run the radiant floor in the winter .... and use the heat pump to balance things out the way I like
I suppose the point I'm making is for the crowd that wants the temp jacked (comfy) for time they spend in their garage, and the rest of the time is frost protection.
A sizable gas heater can make it warm quick. The 'right size' heat pump will take hours. How many BTU can you afford, vs how long do you want to wait?
A sizable gas heater can make it warm quick. The 'right size' heat pump will take hours. How many BTU can you afford, vs how long do you want to wait?
dcg9381
Well-known member
With our Dakins, it becomes pretty obvious at about freezing... Same thing with our crappy residential heat pumps (given they have electrical heat, assuming the grid stays up). I've seen 20 degrees maybe once in central Texas in the last 30-40 years - when the power was out. Course, when it became "obvious" it was too late to do anything about it! I'm prepared this year with 400 gallons of propane and 3 portible 20 - 30k BTU heaters.
there's almost no reason not to get the biggest minisplit you can get, if you want that. most have significant turndown ratios, generally down to 5-9K BTU. so you can get the 36k BTU unit, even if you only need the 12K unit for cooling. once you're at temp, it'll drop capacity to load-match.
Depending on the model a 36k will only go down to 1/3 in ideal conditions ...the 12k will do 2500c and 3700h. In humid climates proper sizing matters more. There is no question that VS allows more flexibility but sizing still matters.
People who install mini-splits can't say enough about how well they work but then don't understand that multi-speed and full variable speed traditional equipment is the same idea. The same installers will say -- too complicated or not going to really do anything ?
I've seen insane prices from some installers (FU pricing?) that don't seem in-line with reality. also worth noting that most multi splits have much higher minimum outputs (e.g. double) than single headed systems. I think that's for oil return, but it might be for some other reason. It's not like they tell you why on the spec sheets.
Typically with a multi head the compressor is naturally going to be larger -- so a 30 or 36 compressor is only going to be able to go down so low. There are reasons to use a multi-head and reasons not to ... they work best when used in "zones" where all the heads are on. You want the capacity shared at all times since even when a head is off there has to be refrigerant flowing to it
I'm not talking about minimum BTU going up as the units get bigger, I'm talking turndown ratio. often, the multi-splits have an inferior turndown ratio to a single headed machine.
as for heat output falling off at low temps, this unit is basically on spec down to -13F:
single head machine: 18k rated capacity, 17kBTU at -13F, minimum capacity 5,150BTU. about 3:1 turndown
Naturally COP will fall but it's getting 94% rated output at -13F...
as for heat output falling off at low temps, this unit is basically on spec down to -13F:
single head machine: 18k rated capacity, 17kBTU at -13F, minimum capacity 5,150BTU. about 3:1 turndown
Naturally COP will fall but it's getting 94% rated output at -13F...