Mini Split, Three options, and a lot of confusion.

[Diyohmy]

Hi, I have a 1500 square foot house. The setup is pretty simple, in the front we have two large rooms, one being and open-concept living room slash kitchen which is connected to a large family room by a 4-foot opening with no door. A hallway leads back to our 3 smallish bedrooms, all 10 x 10 to 10 x 12.

We had three HVAC companies give us bids. All three were quite nice and informative, but all three gave quite conflicting information as to the what's and why's of our potential best solution. Worth noting is that we went in thinking ductless.

Option one was a mini split in every room, 5 total. An issue with this one is that window placement on our master bedroom prevents install on an exterior wall.

Option 2 was a mini split in both large rooms, and a small ducted unit in our attic feeding three bedrooms.

The third guy liked option 2 but actually went left field with his recommendation. He recommended a traditional gas furnace and electric AC unit with ductwork in our attic. The furnace could go in our garage or attic, but my wife wouldn't want it in the garage. She's also concerned about putting gas into our attic for safety reasons, although neither of us know if that fear is warranted.

A friend of ours recently did option 2 but it's really too soon to have solid feedback. The third guy did a solid job of selling me on the merits of a traditional ducted system, even though he also does mini splits. Price-wise they are all similar enough not to take any off the table

I've been reading my share on the net trying to sort out these ideas, but most sites always seem a bit like propaganda for whatever they happen to sell. I've had great luck on this forum asking for advice before, and would love to hear any thoughts on the matter.

 

[Dana]

You are right to steer well away from the contractor with the split-AC + gas furnace proposal. There are no gas furnaces (except possibly the smaller versions of the Dettsons) that can be right sized to a sub 20,000 BTU/hr heat load. There are several heat pump solutions, both mini-splits and big air handler types, but putting any of that equipment in the attic is a mistake, adding parasitic load with duct gains/losses, and increases the amount of air-handler driven outdoor air infiltration.

A mini-split per room is universally a case of GROTESQUE oversizing that will lead to lower efficiency and lower comfort! It's even worse if it's on a multi-split compressor, since the individual heads don't modulate, and will always be just cycling on/off at a ridiculously low duty cycle when the head is oversized for the room load. Learn from this guy's misery with oversized multi-splits.

To get the most comfort and best efficiency out of modulating equipment it has to be sized to the load that it actually modulates, at least most of the time.

Most 1500' houses in Portland OR could be heated and cooled with a single 1.5 ton Fujitsu -18RLFCD mini-ducted unit, or perhaps one wall-coil type sized optimally for the largest most open zone, and an appropriately sized mini-ducted system for the rest of the house, or perhaps a pair of 3/4 ton units. You probably don't need even 2 tons of compressor to heat and cool the whole place. The 1.5 ton Fujitsu delivers 21,600 BTU/hr @ +17F (AHRI- it's max can be higher than that). Unless you have the crummiest 1500' house in Portland with all leaky aluminum framed single pane window (and no storm windows) your heat load @ +17F is probably less than that.

To get to the optimal solution and to be able to rule out the total dogs requires doing an AGGRESSIVE (per the manual) room-by-room Manual-J heating & cooling load calculation. Ideally you would have a professional engineer or RESNET rater or some other qualified energy nerd running professional Manual-J tools for this, but NOT an HVAC pro. Hire somebody who makes their reputation and money on the quality of their numbers for this part of it, not somebody who makes their money primarily on installing and servicing the equipment. Only 1 out of 20 HVAC contractors run the numbers at all, and most of those that do use egregiously conservative assumptions about R values and air tightness and end up with load numbers more than 1.5x reality, then size up from there.

If you want to give it a shot with an online tool, loadcalc will oversize it by some even if you use the tightest lowest-infiltration assumptions, and most aggressive R-value assumptions that are even remotely realistic. If you're sloppy it'll oversize by 100%, if you're really aggressive it'll be less than 1.5x, but probably still more than 1.2x of what an aggressive Manual-J using a pro-tool would deliver. Give it a shot, room by room, and build yourself a room by room spreadsheet.

If you have a heating history on the place, you can run a fuel-use load calculation to bracket the likely whole-house load using (wintertime only) fuel use against local weather history between the meter reading dates. If you room by room numbers add up to about the same for the whole house as the fuel use calculation it's an indication that your at least close.

I doubt ANY of the 100 square foot bedrooms has a design load more than 1200 BTU/hr @ +25F (the approximate 99% outside design temp for Portland), which is well below even the minimum modulated output of even the best half-ton mini-split.

This house in Berkeley CA is heated and cooled with a 1.5 ton mini-ducted Fujitsu, which is overkill for the heating load, but somewhat less oversized for the cooling load (but still oversized.) With no basement to route ducts in they made a mini-closet "mechanical room" next to the front door that looked like this:

Note how they soffited the duct runs below the ceiling level to avoid penetrating the air & pressure boundary at the attic. The return is just one big grille under the mini-duct cassette.

Vertical mounts like that are a Fujitsu-only proposition- all other vendors require horizontal mounts, which can be more awkward to deal with. Eve nif you put it in the basement it takes less space and is easier to service, swap filters, etc. if it's vertically mounted. Fujitsu's also have much better specs on their mini-duct cassettes than the competition, which allows higher impedance flex duct or much longer hard-piped ducts to still work.

The one real environmental disaster they perpetrated in the Berkeley CA house was to use a tankless electric water heater, which quadruples their peak power draw. Local code required upgrading the power service anyway, but an electric tankless really an abuse of the distribution grid, to be avoided whenever possible.

You may have to sign up for a (free) trial prescription to read these, but GBA covers mini-split issues fairly often, both in the blogs and the (public, not behind paywall) community forum pages:

http://www.greenbuildingadvisor.com/articles/dept/musings/bruce-harley-s-minisplit-tips

http://www.greenbuildingadvisor.com/articles/dept/musings/rules-thumb-ductless-minisplits

http://www.greenbuildingadvisor.com/articles/dept/musings/how-buy-ductless-minisplit

 

[Sponsor]

 

[Jorgebaloy]

In a multi-zone or multi-split system, more than one indoor unit is connected to a single outdoor unit. It allows you to set different temperatures for different areas of your home.

A multi-zone system costs you less than buying a number of separate single-zone systems to cover the entire home. It also saves you the outdoor space.

However, the installation process is a bit more complicated, due to piping requirements.

 

[Dana]

In multi-zone systems the individual wall coils/heads do not modulate at all- they operate at a single speed alternating between full capacity or off. Since the compressor does not have an infinitely variable continuous modulation range, zones that are off still have refrigerant running through them whenever at least one zone is operating, whether the blower is running or not.

This excess refrigerant flow sometimes leads to over-cooling or over heating in rooms that have low heating/cooling loads, leading to lower comfort, and poor temperature control.

Most mini-splits sold in the US will modulate within a range when the head is served by a "inverter drive" compressor unit but the range available is also not infinite, and varies by make & model. The minimum modulated output of many mini-splits is higher than actual design condition loads, which forces it into on-off cycling at minimum capacity rather than modulating, tracking the load. While this is more efficient than the multi-zone models turning on/off at maximum capacity, it's still nowhere near the efficiency that the heat pump is capable of when right-sized for the load and adjusting it's speed up/down in response to the immediate cooling or heating load and running nearly continuously. When one sets the blower speed of a multi-split head to a lower speed there is even higher refrigerant flows in the zones that are nominally "off", and it can even cause the compressor to cycle on/off which cuts into efficiency even more.

This is why it usually makes more sense from an efficiency / comfort / cost point of view to use a modulating ducted mini-split rather than installing multiple oversized mini-splits or an oversized multi-split head in every room whenever that is possible.

In my area the installed cost of multiple mini-splits is usually (but not always) cheaper than 3-5 head multi-splits of similar performance, but people don't always have the space for very many outdoor compressor units (or object to multiple compressors on aesthetic grounds), and opt for the lower efficiency and lower comfort of non-modulating multi-split heads.

 

[Jacobsond]

I have 2000sf house. I had installed 2 units. Panasonic heatpumps.2ton main floor 1.5 upstairs. Both modulating. Cools the house well,but frankly a little oversized. Heat the house well into december. Then again I can heat with them late feb. Im in ND so dec and jan temps are normally below zero and they struggle when it gets below 10.The regular furnace (hot water heat) does a better job. A head in every room seems really like overkill to me.

 

[Jorgebaloy]

The best weather for heat pumps to operate in is warm or moderate. This is because it allows them to easily extract the surrounding heat and convert it into energy efficiently and effectively. Plus, the fact that they can be used as heaters and/or air conditioners means that even if you live in a hot country, you may discover that a heat pump is a good investment.

It should be noted that some heat pumps work better in colder climates than others. The reason for this is that during the colder weather in winter months, heat pumps that have exposed piping will need to work a lot harder to provide your home with heat than they would during the milder autumn weather.

If the heat pump system is unable to get enough warmth from the air, then a supplementary system will be needed instead. This can lead to an increase in your energy bills for a short period of time each year. For some, this can become an inconvenience as the point of purchasing the heat pump was to save on energy bills as well as to use a greener energy source.