I have a 4 bedroom home (built after 2000) in MA. The first floor and finished basement are heated by a 90K BTU FHA natural gas furnace and a 3-ton AC unit. The 2nd floor also has a 82K BTU FHA furnace and a 3-ton AC unit. I have solar panels (18.7kw) providing 98% of my electricity. I am thinking of installing a ductless mini-split system for the 2nd floor. Ideally, this would entirely replace the FHA/AC system. On the 2nd floor, we use heat in the master bedroom (231 sq. ft), master bath (90 sq. ft), the kid's room (168 sq. ft.), hallway (100 sq.ft) with the remainder space (915 sq. ft.) used occasionally. The attic has R30 on the floor, R19 on rafters w/radiant wrap. We like the temps to be between 68-72F. How should I size the ductless system? Would it be adequate for heating? How much electricity in kWh would the system use per day for heating? for cooling? Should I keep the 1st floor FHA as a backup or should that too be switched to mini split? The 1st floor has a mostly open floor plan but with three rooms (two of which have no doors). What else should I think about?

The size of your minispits should be based on your heat loss; that's the only way to determine if they'll do what you want them to do. Start with a blower door test/energy audit and find out where the leaks are in your house and fix them. Have your HVAC company calculate the heat loss of each floor, and size the units.

We built grid tied homes that run primarily on solar power, and use minisplits for heating and cooling; they're fantastic, but each unit is fairly small in comparison to the average load of a code house. While it may not make sense for you to add insulation, you can make a substantial difference in your heating load by simply keeping the heat you make, inside the house. Most homes are very leaky and therefor hard and expensive to heat and difficult to cool. It also makes a huge amount of sense to insulate the concrete walls in your basement. (8" of concrete=R1).

Done well, minisplits might cut your heating bill by half or more and increase your comfort. But much of the improvement will be gained not by adding units, but by tightening your house. I converted my old house (2500 SF) to three 12,000 btu miniplits and tossed out my FHA oil burner. Two of my three units work great; one struggles in very cold weather due to significant infiltration and therefore higher heat loss near that unit. I'm obviously not done upgrading the house, but even so we're seeing significant savings and we have a much more comfortable house.

The oversizing factors on your existing equipment is beyond ridiculous, well into the LUDICROUS range, no matter how big the 4 bedroom house is.

Typical square foot per ton cooling ratios in MA are north of a ton per 1500', many are north of a ton per 2000'. The exceptions are houses with expansive west facing "sunset view" windows, which are impossible to shade with overhangs and get a lot of late in the day solar gain.

Space heating load ratios for tight houses with 2x6/R19 framing and code-min low-E windows are typically in the 12-14 BTU/hr per square foot @ 0F outdoors (not counting the basement floor area which has a much lower ratio), 68F indoors, in an aggressive (per the manual) Manual-J load calculation. Most locations in MA have 99% outside design temps north of 0F, some even in double-digits.

You have 180,000 BTU/hr of space heating, which might make sense if the house is bigger than 10,000 square feet.

You have 6 tons of cooling compressor, which might make sense in a house bigger than 8000 square feet.

I'm gonna hazard your house is between 2500-3500 square feet, not counting the basement.

The HSPF efficiency specs will usually be met if the mini-splits are properly sized for their loads. The units of HSPF are BTUs per watt-hour, electricity is billed in kwh. So an HSPF of 10.5 would deliver 10.5 BTU per watt hour, which is 10,500 BTU/kwh.

The capacity of a mini-split varies with outdoor temperature. The specified AHRI capacity is the modulated output level at which it is tested for efficiency. Since it's tested at both +17F and +47F it means it puts out at LEAST that much at +17F (but it might be able to deliver more at that temperature. But at +6F or +12F (or whatever your outside design temperature is) it's max capacity will be less than what it is at +17F. To specify it as the sole heating source requires looking up the extended temperature capacity tables for the unit to determine it's capacity at your desired indoor and outdoor design temperatures.

The very smallest mini-split heads are ridiculously oversized for the load of an individual bedroom, yet a lot of installers will recommend one head per room separate zoning. A typical 250 square foot bedroom will have a design heating load under 4000 BTU/hr even for a top floor bedroom with 3 exterior walls. Most will be under 2500 BTU/hr which is below the minimum modulation rate of most mini-splits. (A notable exception being Mitsubishi's FH06NA and FH09NA). The result is that they never modulate, which delivers a HEAVY hit to the as-used efficiency. To hit their HSPF numbers they have to be modulating at least some of the time rather than cycling on/off. Multi-split solutions with more than one head on a single compressor aren't even fully modulating, and the minimum output of the compressor is usually roughly the maximum output of a half-ton ductless head.

When the systems are never modulating, always cycling, the as-used seasonal average HSPF will be ~7-8.5 BTU/watt-hour. But even that is usually cheaper heat than 85% efficiency oil, or 95% efficiency propane.

Hire a registered P.E. or a RESNET rater to run an AGGRESSIVE (per the manual) room by room Manual-J heating & cooling calculation on your place, and stress that you want the most aggressive assumptions that are still "reasonable". Use the real published U-factors for the windows & doors (if you have them), and assume that the house is so tight that it needs mechanical ventilation (which might be true now, but can probably be made that tight cost-effectively.) Then and only then is it possible to figure out the most optimal heating/cooling solutions.

Do NOT let HVAC contractors run the load numbers. While some are competent to do that well, most will botch it royally, and many will just make something up on the spot based on their "experience" (or how far behind they are on their boat payments, mayhaps?). Even if it costs $500-1000 for an engineer to run the room by room numbers, it'll usually save that much in equipment cost up front, and every year thereafter in lower electricity bills.

It may turn out that a right-sized ducted cold-climate mini-split using the same heating ducts is the "right" solution. A 3 ton PCA-A36KA4 + PUZ-HA36NHA4 Mitsubishi hyper-heating split system can deliver 38,000 BTU/hr @ +5F:

http://meus1.mylinkdrive.com/files/PCA-A36KA4-PUZ-HA36NHA4_(-BS)_Submittal.pdf

As it happens +5F is the 99% outside design temperature for where I live in MA, and the heat load of my 1920s antique sub-code 2400' house + 1600' of insulated basement is just shy of 38,000 BTU/hr @ +5F. With the heat strip backup to cover the additional load at PolarVortex negative double digit lows that COULD be a 100% solution for me.

They make smaller versions as well, but avoid oversizing even if you have to use a non "hyper heating" version. The large air handler versions have small turn-down ratios, and it's easy to end up bringing an HSPF 11 system like the aforementioned down to HSPF 8 as-used performance with oversizing. A right sized HSPF 10 version that needs more heat strip to cover the load at the 99% outside design temperature can still beat an oversized HPSF 11 unit than never really modulates. It's important to look at the minimum modulated output of any mini-split at +47F and compare it to the heat load of it's zone/room at +47F. The "single head per room" solutions almost never modulate even at the average January temperature for the location, even when they are all separate, fully modulating mini-splits.

Thanks. Half of my walkout basement is finished. The other half is unfinished (no windows, no exterior doors). I suppose I could put up insulation board on the concrete walls on the unfinished side.

Thank you for this detailed response! My house is 3300 sq. ft. not including the basement. Not sure if that includes the bonus room over the 2 car garage. I don't like the wall mounted units. I'd like to with units embedded in the ceiling or wall. The new Mitsubishi units fit between joists so perhaps that is the solution. > It may turn out that a right-sized ducted cold-climate mini-split using the same heating ducts is the "right" solution. A 3 ton PCA-A36KA4 + PUZ-HA36NHA4 Mitsubishi hyper-heating split system can deliver 38,000 BTU/hr @ +5F: Currently, the master bathroom is a bit cold in the winter for showers and the bonus room (over garage) is also cold in winter (hot in summer.) I'd like to address these two issues. We also like to have a comfortable hallway temperature (kids like to play there, etc...) So if we do ducted plus add a zone for the bonus room and perhaps master bath, would that be best?

Where you put the units relate directly to the heat loss in those locations. Finished spaces above garages for instance are generally very poorly air sealed and therefore very hard to heat; attics are typically also very poorly air sealed. Installing indoor units above drywall (basically in the attic) without air sealing around them seems sketchy at best. For best results with heat pumps, you'll need to determine the exact issues in each area and fix them. If you prefer to not do that and simply throw new heating units in hoping that will fix the problems, put in as man as you can.

Run the room by room heat loss numbers. There is really no way to shoot from the hip on this and get both high comfort and high efficiency.

While high-mount wall coils are the most common, there are also floor mount models that are more appropriate for heating dominated climate (they take in the cooler air near the floor rather than the warm air near the ceiling, which is good for efficeincy), but also ceiling cassettes, mini-duct cassettes etc. Floor mount coils all look roughly like this:

http://www.greenbuildingadvisor.com/sites/default/files/imagecache/GBA_Rotator_Image/Fujitsu-floor-minisplit-ROTATOR.jpg

Mitsubishi's new ceiling cassettes designed for 16" o.c. joists look like this:

https://ahrexpo.com/wp-content/uploads/2018/01/New-MLZ-Series-One-Way-Ceiling-Cassette.pdf

Mini-duct cassettes use fairly standard register grilles, sometimes smaller than those used for ridiculously oversized systems, sometimes designed for long throw (to provide good mixing while avoiding long duct lengths) etc.

There are plenty of options, but without the load numbers you can shoot yourself in the wallet and STILL end up uncomfortable.

To sanity check the whole-house heat load calculations, run a fuel-use based load calculation using both base 60F & base 65F weather data (assuming you keep the house 65-70F indoors most of the time), as detailed here:

http://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/out-old-new

BTW: If line breaks in your post aren't showing up, try using Chrome or Firefox for a web-browser.

Installers in my area are quoting very high install costs for ductless (3-4x the product), told me that a Manual J load calc required an engineer and wasn't necessary, and have been steering me toward 9k+ wall-units. Furthermore, they tell me that the system cannot be used for primary heat. Therefore, I'm also considering a hybrid heat pump and furnace. It's looking like a much more cost-effective option. Thoughts?

There are a number of builders in MA and New England who routinley install minisplits in their new homes as the primary or sole heating source, so the idea that they "cannot be used for primary heat" is ridiculous, and in fact, false. But it's possible that contractor installs heating systems in poorly built leaky homes where they would not work well for that purpose. If you have a very leaky house, this may be the case for you.

Posted By cleangreen on 02 May 2018 07:57 PM
Installers in my area are quoting very high install costs for ductless (3-4x the product), told me that a Manual J load calc required an engineer and wasn't necessary, and have been steering me toward 9k+ wall-units. Furthermore, they tell me that the system cannot be used for primary heat. Therefore, I'm also considering a hybrid heat pump and furnace. It's looking like a much more cost-effective option. Thoughts?

So, be specific, where IS your area? (got a ZIP?) Mitsubishi has a regional design center in Southborough MA- it' might be worth asking them for some competent installers in your area. But only after doing your homework.

In competitive bidding five years ago three 1.5 ton Mitsubishi wall-blob style cold climate mini-splits werew installed in deep energy retrofit on 3-family in Worcester for about $13K. (Total capacity @ +5F of about 70KBTU/hr, which that house didn't really need.) That's $4.3K per head less than $3K per ton.

Less than 2 years ago I was involved in a project where in competitive bidding they installed 4 tons/6 zones of Fujitsu multi-splits for about $15K on Martha's Vineyard. That's $3.8K/ton.

A bit over a year ago a coworker in Arlington had a pair of FH12NA and an FH09NA (three separate units) installed at his 1800' circa 1920 house for about $12K (about $4.4K/ton).

All of these prices are the invoiced cost. MassSave typically has rebate subsidies that will apply in most locations in MA. In all of those houses mini-splits are the primary heat (the ONLY heat in the MV and Worcester houses.

9K+ wall units are fine, if they are commensurate with the actual loads, which is why you need the room by room load calculations. The guy in Arlingon's 3/4 tonner was ridiculously oversized for the load of the kitchen, and could have been despensed with but for the fact that his wife insisted on the air conditioning in the kitchen.

The 4 tons of multi-split on MV was a bit oversized for the 3000' house, and would have been better off with one 2-ton multi-split and a 1.5 ton mini-duct cassette, but got over-sold on the notion of micro-zoning each bedroom with half-ton heads. The heating & cooling loads of the individual bedrooms were less than 2000 BTU/hr each, and without ducts the bathroom and walk-in closet went completely unheated, noticeably so during cold weather.

The individual apartment loads of the 3- family were all in the 10-13,000 BTU/hr range. The owner would have been better off with 1-tonners (or 3/4 tonners for two, a 1-tonner for the top floor) but he wasn't familiar enough with the products and balked, opted to up-size "just in case". The minimum output of the installed FE18s at 47F is 8,000 BTU/hr, so they really don't modulate much with load, which is bad for efficiency. But the loads and power use are so small it's in "who cares?" territory for the tenants- still cheaper utilities than most apartments in Worcester.

To get what you want/need you have to do most of the research yourself, figure out what it takes, then SPECIFY the equipment and put it up for competitive bid. If you're going to HVAC contractors and soliciting proposals you'll get the kind of ridiculous oversizing and pricing you seem to be getting. On the MV projects I saw proposals for a 4 ton ducted Carrier Greenspeed + 1 ton Daikin for over $30K (yeah, right, like any of THAT makes sense for that house!) On the co-worker's initial solicitations for proposals there were several that came in over $20K, some with equipment that was inappropriate for the climate, all of which were oversized for the loads by at least 2x. In both of those cases I insisted that they get a professional third party Manual-J. The Manual-J in MV ended up costing ~$800, and was very well done, the one in Arlington was a few hundred less, pretty sloppy in some details, but not terrible, way better than a WAG.

A pair of right-sized MVZ or PVA series air handlers on hyper heating compressors to replace your two ridiculously oversized hot air systems might still be the right solution. With your oversized hot air the cycle times are so low it becomes a contributing factor for some rooms being too cold. With right sized (and better still, MODULATING systems like the PVA/PUZ systems) the run times are long, the temps don't overshoot, and there's always a warm summer breeze out of the registers when it's cold out, rather than the hot flash followed by the chill you almost certainly get with dual 90KBTU hot air furnaces.

Have you run the fuel use load calculations yet?

The hardware for the PVA/PUZ systems are more expensive per ton, but you'd only need two of them if using the same ducts as the forces hot air. The non-hyper heating versions come as small as 1-ton, which might be enough to manage the first floor depending on where the load numbers come in. But a 1.5 tonner or 2 ton are available too.

http://meus1.mylinkdrive.com/files/PVA-A12AA7___PUZ-A12NKA7-BS_Product_Data_Sheet-en.pdf

http://meus1.mylinkdrive.com/files/PVA-A18AA7___PUZ-A18NKA7-BS_Product_Data_Sheet-en.pdf

http://meus1.mylinkdrive.com/files/PVA-A24AA7___PUZ-A24NHA7-BS_Product_Data_Sheet-en.pdf (hyper heating)

http://meus1.mylinkdrive.com/files/PVA-A24AA7___PUY-A24NHA7-BS_Product_Data_Sheet-en.pdf (not hyper heating)

The MVZs are cheaper and non-modulating, but can work with just a single 3-4 ton multi-split compressor (hyper heating or non)

https://resource.gemaire.com/is/content/Watscocom/Gemaire/mitsubishi_mvz-a12aa4_article_1436343373152_en_ss.pdf?fmt=pdf

A couple of MVZ A24AA4s or -AA7s on an MXC-8C47 compressor could heat most homes in MA with quite a bit of margin, but that's probably more compressor & air handler than you need.

http://meus1.mylinkdrive.com/files/MXZ-8C48NAHZ_Submittal.pdf

All of these systems have heat-strip options for covering the additional load at extreme low temps.

The key to getting the efficiency out of them (modulating or not) is right-sizing them for the actual loads, even if you have to use heat strips to cover the load when it's 10F cooler than your 99th percentile temperature bin.

BTW: So called "dual fuel" hybrids generally suck, on both comfort and as-used efficiency, especially when oversized on the fossil burner end. Most are single speed on the heat pump, cycling both the air handler and compressor a lot during the shoulder seasons, and are usually married to an oversized fossil burner to ensure you get the hot-flash followed by chill effect when it's really cold out.

A right sized high HSPF modulating heat pump with just enough heat strip to cover the additional load at 10F below your 99% outside design will still be leveraging most of the heat from the heat pump even when heat strips are needed, and will run long nearly continuous cycles (which is good for both efficiency & comfort) most of the season.

If you want to take a shot at a quick & dirty DIY Manual-J, there are worse tools than loadcalc, which will at least ball-park it for you:

http://loadcalc.net/

To get closer to reality you have to use aggressive assumptions- make sure you select the tightest infiltration option, highest R and lowest window U-factor that's even remotely reasonable as a guess, and even then it usually oversizes by more than 10% than what a proper Manual-J on a pro tool would deliver. You can use a fuel-use load calc as the sanity check to know when you're getting close.

I bought the Mitsubishi unit new on Ebay and installed it myself. A new unit will come precharged and if your good with tools, as most of you are, you will have no problem with the installation. The installation instructions are excellent. Mine has been trouble free and well worth the money.

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