Mini Split BTU measurements?

[kennyr7]

I’m getting quotes from a couple of HVAC contractors to install a single zone mini split. I believe that I need 24k BTU. The lowest quote (from the guy who appears to be the most competent) is for a Mitsubishi MSZ-FS18NA & MUZ-FS18NA system. The specs give a unit size of 18K, but the specs (and the contractor) go on to say that this system has a cooling capacity of 21K & a heating capacity of 30K.

Is this unit going to perform like an 18K, or something larger?

 

[PoorUB]

Ask for submittals showing the heating and cooling capacity at various temperatures. Then decide if it will do the job you want.

Heat output on mini splits vary greatly between manufacturers and models. Some heat well to 10F, some heat well to -10F.

 

[kennyr7]

The quote states that you get full heating down to 5F & that the minimum outside temp for heating is -13F. Meanwhile, their quote for a 24 single zone 2 head unit states that it will actually provide less heat, 24K, at the same 5F temperature.
The 2 head 24K unit is 30% more $ than the 18K single head.
When I question the rep, he tells me that heating BTUs are measured differently than cooling BTUs, which isn’t what I asked.

 

[yeldogt]

I'm confused. In one place you speak of a single head and then you talk of a two head?

FYI -- you will find charts for each. When doing two heads you have to find the compressor listed and look down the chart until you see the configuration make up you are using .... it will tell you the min/max outputs.

For the singles the outputs will be based on temp. The hyper heats deliver full rated capacity at whatever it is now (it was 5 degrees) ... get the pan heater. At higher temps they put out more heat ... you size based on load and output. If building where it never goes under 25 degrees -- that's how you pick the unit. Same with the AC ..... you have to use the chart.

The Mitsubishi hypers are designed to heat in cold temps. All min splits work in colder temps ... only those designed for cold heating will deliver full output at very cold temps. IE: 18k unit giving you 18k BTU at 5 degrees.

One final item. The multi heads don't have as wide an output -- so you have to look at the charts. Remember -- multi heads are designed to be running and working together to heat/cool a space .... not one "on" and the other "off"

 

[theoldwizard1]

I'm confused. In one place you speak of a single head and then you talk of a two head?

The biggest issue with selecting a heat pump is the square footage you want to condition. At some point you will need an auxiliary fan or better yet 2 air handlers.

Insulation/sealing is ALWAYS a big issue !

 

[kennyr7]

Sorry, I wasn’t being clear. I’ve got a 23’x31’x10’ detached garage. 733 sq ft. Slab on grade. Full attic (unused) above. R19 in the walls & ceiling. Drywall walls & ceiling if that matters.
R17 doors, which will open twice a day max. I’m in Connecticut, so winter nights can be -5F & summers are 100F with high humidity. I’m hoping to keep the space at 65-75 year round.

My HVAC contractor is offering me a 18K single head Mitsubishi Hyper Heat for $7800. He’s convinced that this is all I need. He states & the Mitsubishi literature confirms that the 18K rated unit actually produces the 21K cooling & 30K heating. I don’t understand how it can do that & still be rated at 18K?

The same contractor will also install a 24K 2 headed Mitsubishi Hyper Heat for $11k. This unit is 23.6 K max cooling at 95F & 25K max heating at 5F.

Obviously, I’d rather spend less money, but I don’t want to undersize the equipment & have it not be able to keep up. The online calculators & my discussions here tell me I need 24K, so I don’t understand how unit designated as 18K actually performs like a much larger unit?

 

[yeldogt]

The biggest issue with selecting a heat pump is the square footage you want to condition. At some point you will need an auxiliary fan or better yet 2 air handlers.

Insulation/sealing is ALWAYS a big issue !

In a very tight open space like a garage -- single point center mounted heat source works well. leaks will make any space uncomfortable

 

[yeldogt]

Sorry, I wasn’t being clear. I’ve got a 23’x31’x10’ detached garage. 733 sq ft. Slab on grade. Full attic (unused) above. R19 in the walls & ceiling. Drywall walls & ceiling if that matters.
R17 doors, which will open twice a day max. I’m in Connecticut, so winter nights can be -5F & summers are 100F with high humidity. I’m hoping to keep the space at 65-75 year round.

My HVAC contractor is offering me a 18K single head Mitsubishi Hyper Heat for $7800. He’s convinced that this is all I need. He states & the Mitsubishi literature confirms that the 18K rated unit actually produces the 21K cooling & 30K heating. I don’t understand how it can do that & still be rated at 18K?

The same contractor will also install a 24K 2 headed Mitsubishi Hyper Heat for $11k. This unit is 23.6 K max cooling at 95F & 25K max heating at 5F.

Obviously, I’d rather spend less money, but I don’t want to undersize the equipment & have it not be able to keep up. The online calculators & my discussions here tell me I need 24K, so I don’t understand how unit designated as 18K actually performs like a much larger unit?

You can go on the Mitsubishi site and get the specifications ... seems a tad expensive for the 18k. Is it a complex install?

Same with getting the duel head specification .....

As I said before the output range is based on what temp the unit is operating. It;s going to make more heat at 40 degrees --- but you don't need the heat. Units get sized for the extreme . What do you need on the coldest day and the hottest day -- at whatever your target temp is.

When I size my AC in the mid Atlantic -- I use a target temp of 72. My recent project put a 36k BTU just under the size limit. Most HVAC companies would have said to go with the 48k unit. The newer variable speed units only come full ton. They may use 70 as the target temp and 100 degrees outside. I know from experience with an AC running and spray foam -- I'm not worried about the temp inside going up a bit at 2pm when the sun is the hottest. It's all about humidity and I'm only going to have the system set at 74 anyway.

The same can be done in the winter -- simple electric for those few odd days. With the mini you do have a lot of flexibility as they really ramp up and down ..... maintaining temp the 18k should be fine.

 

[theoldwizard1]

[/QUOTE]

In a very tight open space like a garage -- single point center mounted heat source works well. leaks will make any space uncomfortable

How big of a garage ? 24x24 or 40x60 ?

 

[kennyr7]

You can go on the Mitsubishi site and get the specifications ... seems a tad expensive for the 18k. Is it a complex install?

Same with getting the duel head specification .....

As I said before the output range is based on what temp the unit is operating. It;s going to make more heat at 40 degrees --- but you don't need the heat. Units get sized for the extreme . What do you need on the coldest day and the hottest day -- at whatever your target temp is.

When I size my AC in the mid Atlantic -- I use a target temp of 72. My recent project put a 36k BTU just under the size limit. Most HVAC companies would have said to go with the 48k unit. The newer variable speed units only come full ton. They may use 70 as the target temp and 100 degrees outside. I know from experience with an AC running and spray foam -- I'm not worried about the temp inside going up a bit at 2pm when the sun is the hottest. It's all about humidity and I'm only going to have the system set at 74 anyway.

The same can be done in the winter -- simple electric for those few odd days. With the mini you do have a lot of flexibility as they really ramp up and down ..... maintaining temp the 18k should be fine.

Thanks for your input. It’s a dead simple install, but this is Fairfield County, labor rates are unbelievably high, driven in part by the real estate market.

 

[yeldogt]

How big of a garage ? 24x24 or 40x60 ?
[/QUOTE]

One of my outbuildings is two buildings open to each other. Original garage 24x32 with a newer 28x30 tacked on. I tacked the 28 side of the new building onto the 24' side of the old building .... offset the new about 3' so it did not look like a train car. Both buildings are cathedral with the new building having a higher roof height (make the whole building interesting from the exterior). It's had a propane cabinet heater in the center for years and quite comfortable ... 12k mini split for AC in the 24/32. It's a tight building with few windows and two doors.

 

[yeldogt]

Thanks for your input. It’s a dead simple install, but this is Fairfield County, labor rates are unbelievably high, driven in part by the real estate market.

Mitsubishi has tons of information -- so much that it gets ... too much. That's why I say to go look yourself. Google the outdoor unit and search the heads available. The better head with the eye is really slick .... I hope for that money he is giving you that head. Don;t put it too high and make sure to schedule the filter clean -- it's very easy and the unit has a good filter.

In normal times they should be 5k max .... I get that this is not normal. I really don't think a duel is required -- especially at that price. I'm guilty of going to far with some projects ... with a closed up space the key in the summer is humidity. You have to keep the unit running -- stick it on 78 ..... you will figure that one out. Sometimes 80 is too high and holds too much water ....

I guess no natural gas? How difficult is the electric in the building ?

 

[kennyr7]

Times are sure not normal-electricians here are charging $1700-$2300 day…
No natural gas here, no propane installed on the property & I can’t dig up the slab for radiant, I’m already way over budget.
I’ve just ordered the single head 18K unit.
Many thanks for your input-very much appreciated!

 

[Firebrick43]

I think you will do fine. My 15k Mitsubishi hyper on a very well insulated building does 1200 square feet.

I would have a backup resistance heat, just a plug in space heater, for those occasional -20 weeks that happen every decade or so.

 

[PoorUB]

The Mitsubishi Hyper has weird ratings. It is like they are using a larger unit than advertised, running it 100% for heat and 2/3rds output for cooling.

 

[kennyr7]

I think you will do fine. My 15k Mitsubishi hyper on a very well insulated building does 1200 square feet.

I would have a backup resistance heat, just a plug in space heater, for those occasional -20 weeks that happen every decade or so.

Good idea-thanks.

 

[u3b3rg33k]

The Mitsubishi Hyper has weird ratings. It is like they are using a larger unit than advertised, running it 100% for heat and 2/3rds output for cooling.

they can over-speed the compressor and use things like vapor injection to increase heat output.

also, generally speaking, single head units have better turndown ratios (minimum capacity vs rated capacity) than multi-head units. this is good information to have when you are "oversizing" a minisplit, because it's not the same as installing fixed capacity equipment.

 

[mogandave]

Not sure what the price differential is, but if you've got the dough, the cassette/ceiling split units are much better than the wall mount units, at least for AC.

 

[yeldogt]

Times are sure not normal-electricians here are charging $1700-$2300 day…
No natural gas here, no propane installed on the property & I can’t dig up the slab for radiant, I’m already way over budget.
I’ve just ordered the single head 18K unit.
Many thanks for your input-very much appreciated!

Mini-splits do have a huge operating range ... but, they have a sweet spot for efficiency. So you want to size them for the load and factor the space use as well. I slightly oversized the one for a kitchen in my new project ... when we have people over in the summer I don't want to be w/o the capacity -- people -- big stove -- lots of windows. Now I;m not talking about putting a 24k in a 12k room. it was a 18k in a 15k room.

I think every project is over budget today.

Having had some type of weekend place in PA most of my life -- propane was all I ever had for fuel in the outbuildings. When propane would spike in price and if I was going to be away from the property for a few weeks -- I would turn down the temp a bit to around 60. Having a 4 or 5kw electric heater available was a great way to get a bit more fire power when I would get to the property and want to work in the garage. For a couple hundred dollars they are cheap to purchase and I was only using them for a couple hours.

You will be impressed with the Hyper -- they can really throw out the heat even in very cold temps. Unless the building leaks --- you will be happy