I'm working on my never ending project and looking at the air conditioning. It's a 100 year old Victorian, about 18oo sq ft with a new 850 sqft addition. The whole house will have radiant heating and I'm looking at one or two mini-splits (8 total zones). Going with the heat-pump splits will allow me to save the boiler run time for the coldest weather and/or a backup heat source. My concerns are that the rooms are small (225 sqft) and the heat loads are small. Zone 4 My concern is based on Dana1 stating in a previous post the these units don't work well with a heat load under 4500 btu/hr. One of the quote I have is from a Mr. Slim system with 9k heads that will vary between 1700 to 12000 btus on the cooling side and 1800 to 18000 on the heating side. I'm assuming these units are variable out put. Am I reading this right? And would it solve my problem?

It's not a minimum room size, it's a minimum LOAD size.

Run a Manual-J or I=B=R load calculation. If the heating capacity of the mini-split at your 99% outside design temp is more than 1.5x your load at that temp, it's not a really great fit, since the minimum modulated output during the shoulder seasons would be well above the average heat loads during those periods.

If your space heating load for that room at the 99% outside design temp is 4500 BTU/hr @ say +6F (if that's your 99th percentile temperature bin) and the capacity of the unit at +6F is 18,000 BTU/hr at that temp, it means that your shoulder season average load is well below it's minimum modulated output. But the 9000 BTU/hr cold-climate mini-split like the -FH09NA puts out 10,900 BTU/hr @ +6F, not 18K, and it's 10:1 turn down ratio at 47F is not typical. But I'd be a bit surprised if a 225' room would have a heat load anywhere near 4500 BTU/hr unless it's an upper floor dormer or something(?). But say it's heat load at +5F is 4500 BTU/hr with a 70F interior, at 65F delta-T, or 69 BTU/hr per heating degree below 70F. That means it balances with the minimum modulated 1800 BTU/hr at a temp of 70F - (1800/69)= 43F. Any time it's above 43F, it will be cycling on/off, with much bigger room temperature swings than when it's modulating. You can fix that with a wall thermostat (for a price), but it's still less than ideal.

http://www.mitsubishipro.com/media/289469/msz-ge06na%20submittal%20for%20mxz%20multiple%20indoor%20unit%20styles.pdf

In zone 4 your outside design temp is probably in the mid to high teens the twenties, and your 225' room if it's been tightened up and doesn't have a gazillion square feet of single pane glass probably has an even lower heat load than that- perhaps 2700 BTU/hr @ 20F or about 54 BTU/hr per degree F below 70F. The balance point at min-mod on the FH09 is then 70-(1800/54)= 37F. Any time it's over 37F it'll be cycling on/off.

Is that really the solution you're looking for, one that you're willing to drop a couple or three grand on ?

If it's a multi-split that can accommodate a -GE06NA half-ton head and your outside design temps are within the operating range of a compatible compressor it might be a better fit. It maxes out in heating mode at about 7K or so, not sure what IT's min-mod output is, but with multi-splits there is also a minimum load at which the compressor will run.

If the mini-split conditioned rooms are next to one another and can share a mini-ducted solution that might be somewhat better solution than an oversized head in each, since the combined load is higher. A 9RLFCD has a mid-mod-output of 3100 BTU/hr @ +47F, so split between two miniscule load rooms, half the minimum-modulated output is about 1500 BTU/hr, about 15- 20% lower than the 1800 BTU/hr of a FH09.

http://www.fujitsugeneral.com/PDF_06/Submittals/9RLFCD%20Submittal.pdf

But calculate out the room loads (both heating & cooling), and specify the corresponding indoor & outdoor design temps for those loads. We'll see if there's something that makes sense (mini-split or other.)

Is this primarily an air conditioning solution? Does it NEED to cover the heating load efficiently at some low outdoor temp?

The primary use for the splits is on the AC side. The heating side was just a benefit because the cost difference is so little between AC only and Heat pumps. I was looking at using it as "dual fuel" type setup. Heating with the splits during the milder heating periods and saving run time on the radiant system. It's sounding like that idea is out the window. Looks like the splits will the least effective at the time I would want to use the heating the most.

The prime interest is being able to zone the AC. The lady of the house is putting on a sweater at 78 deg and by 68 deg she looks like she heading for Antarctica. I like it cold. So by the time she is comfortable, our house guests are hot. Zoning is most important in the bed rooms. Also, the "sitting room" (her room) and the "media room" (my room) are critical because they will very different temperatures year round.

Here's the heating/cooling loads.

Zone 1 basement
media room 5714/1551
stairs 1519/1405
total 7233/2956


Zone 2 1st floor
Sitting room 2730/2511
bath 122/106
small office 688/398
total 3540/3015

Zone 3 1st floor
Dinning room 1316/503
vestabule 642/417
total 1958/920

Zone 4 1st floor
parlor 3007/1689
powder rm 697/203
total 3704/1892

Zone 5 1st floor
kitchen 2131/1820
laundry 832/417
total 2963/2237


Zone 6 second floor
Master BR 5260/2817
bath 888/317
total 6148/3134

Zone second floor
BR 2 2747/1525

Zone8 second floor
BR 3 3195/2457

Any other technologies that would work better?

Winter Design Conditions
Outside db 13 °F
Inside db 70 °F
Design TD 57 °F


Summer Design Conditions
Outside db 94 °F
Inside db 75 °F
Design TD 19 °F
Daily range M
Relative humidity 50 %
Moisture difference 36 gr/lb


In real life, any room the wife spends time in, will be 75 degF in the winter.

This is the classic problem with micro-zoning, where the individual micro-loads so tiny that all of them are too low to be served individually. With hydronic heating you can get around the limits of boiler modulation with buffer tanks, but there is no analogous solution for heat pumps. You simply have to combine zones, and temperature-balance them as best you can.

You'll have a better shot at temperature balancing reasonably if you zone it by floor which you might be able to pull off with mini-duct cassettes running the ducts between joists, using door-cuts or jump ducts to a common return. It looks like you have about 12k/11k of load on the first floor, and 12K/7k on the second floor, which could be handled with a pair of 3/4 ton or 1-ton mini-duct cassettes, or one of each (depends on vendor).

The basement loads could be handled with a half-ton head on multi-split, or it's own 3/4 ton mini-split.

From a micro-zoned cooling only point of view you could buy a truckload of PTACs , but that would sure look ugly on a Victorian (they don't even look good on a Holiday Inn Express, but they work! :-) ) PTHPs would all be running heat strips rather than the heat pump at +13F, which would be a pretty lousy way to go.

Square pegs and round holes.

We radiate lots of old Victorians and use Unico, properly balanced with one thermostat. The basement gets a dehumidifier.

If you want to live in an old house, you have to face certain compromises.

C'est la vie!

No kidding on the compromises. But I'm up for the challenge. Not that I have much choice because I live here. The choice is doing it right or not. The room zoned splits looked good from the sales point, but not so good when the numbers get crunched. Too expensive of a deal to not be perfect.

I've had an eye on Unico's system since I bought this place 12 years ago. My designer hates them. He thinks they don't work and make too much noise. Any ducted system will make too much noise if it's not installed properly. But he doesn't live here and he's very old school in his beliefs and practices. Noise is a BIG factor in my book. That's one of the appareling things about the mini splits.

On a properly designed/installed Unico, what kind of noise levels can be expected?

Also on the mini splits and multi room zoning, how effective would a head be servicing two rooms through an open door?

I have been "hearing" the noise complaint for years in regard to high velocity HVAC and since I abhore noise, getting plenty on the job sites, I am quite particular about noise that does not contribute to my comfort. This is one of the reasons I replaced the my own Honeywell Wifi with an totally silent EcoBee 3. The relay in the Honeywell did not contribute to my comfort.

Our Unico systems are designed on our Wrightsoft software and are very quiet. We use the newer 2.5" ducts unless space really dictates the use of 2". Either way you will not hear them run unless you are have great hearing and you are still. A refrigerator, normal conversation, or a computer fan are usually louder.

The the most common reason high velocity systems exceed normal noise levels is the propensity for retail customers to take the low bid. An appropriate number of terminals and careful control of velocity will cost more than the "minimum" number used by the "competitive" bidders.

It is all about design.

Unless you are near passive house or net zero construction, most people will not be happy with a mini-split serving more than one room per head.

Does the Unico with the iLink provide heating and cooling or just AC?

Posted By BadgerBoilerMN on 05 May 2015 07:50 PM
I have been "hearing" the noise complaint for years in regard to high velocity HVAC and since I abhore noise, getting plenty on the job sites, I am quite particular about noise that does not contribute to my comfort. This is one of the reasons I replaced the my own Honeywell Wifi with an totally silent EcoBee 3. The relay in the Honeywell did not contribute to my comfort.

Our Unico systems are designed on our Wrightsoft software and are very quiet. We use the newer 2.5" ducts unless space really dictates the use of 2". Either way you will not hear them run unless you are have great hearing and you are still. A refrigerator, normal conversation, or a computer fan are usually louder.

The the most common reason high velocity systems exceed normal noise levels is the propensity for retail customers to take the low bid. An appropriate number of terminals and careful control of velocity will cost more than the "minimum" number used by the "competitive" bidders.

It is all about design.

Unless you are near passive house or net zero construction, most people will not be happy with a mini-split serving more than one room per head.

Which is why picking a heating & cooling solution for that house from a mini-split product line you'd want to zone it with a single mini-DUCT cassette per floor, and not a wall-coil head type. 

The Fujitsu  -xxRLFCD mini-duct cassettes are even more efficient in both heating & cooling mode than the quite respectable Mitsubishi -FExxNA wall coil types of just a few years ago, and have decent capacity down to -5F.  (To my mind that's a product line destined to put GSHP installers in US climate zone 5 back on their heels, since it answers the distribution argument with a cheap pre-packaged low design-risk solution.)  A single 1.5 ton unit can handle the entire heat loads of a 1500-1800' new code-min house in US climate zone 5 locations, and a 1-per floor approach could manage even 3000-4000' houses.

So far that's the only mini-duct cassette series that cuts in colder climates, but both Mitsubishi & Daikin are sure to be upgrading the mini-duct competition in the next couple of years.  So far the Fujitsu RLF units are the only mini-duct mini-splits that have made it onto the Efficiency Vermont subsidy eligible list, with HSPFs in the 11s, and SEERs north of 20. (VT is US climate zone 6.)  That probably won't be true 100 weeks from now- it's a competitive marketplace.

Whether that's a solution that really works in this isn't is hard to divine in web-forum. From the load numbers point of view it's not a problem, but determining whether there is reasonable duct routing & cassette mounting space probably requires a site visit.

Putting a ducted cassette for the second floor is a non-issue. Most of the space is accessible from an open unfinished attic and the small part that isn't, will be demoed down to the studs, so ducting could be added. The first floor would be more of an issue. It is over an unfinished basement so it could be ducted with floor vents. Not my first choice, but doable.

Posted By arcamm on 07 May 2015 04:35 PM
Putting a ducted cassette for the second floor is a non-issue. Most of the space is accessible from an open unfinished attic and the small part that isn't, will be demoed down to the studs, so ducting could be added. The first floor would be more of an issue. It is over an unfinished basement so it could be ducted with floor vents. Not my first choice, but doable.

When installing any ducted system in an attic it's important that both the air handler & ducts be completely inside both the pressure & insulation boundary of the house.   If in a vented attic any duct leakage would drive air infiltration rates well above the natural stack effect or wind infiltration whenever the air handler is running, increasing the load and lowering efficiency. If above the insulation there is direct gain/loss through the duct & air handler walls, increasing the loads and lowering efficiency. 

It's usually possible to build an air sealed insulated service chase with the insulation above the ducts & air handler even in a vented attic, but it's an additional expense.  It's also possible to air-seal the attic and insulate at the roof deck, but that's usually more expensive.

So mini-splits will only work in a couple of my zones. Unico can provide an inverter outside unit, an indoor head and air handlers, but can't zone the air handler. Hi-velocity can zone their air handlers, but there's no local representation. Spacepak is interesting, but their air to water chiller is old technology. So I'm seeing some really expensive band aiding happening here if I'm not careful. Does anybody make small package or wall hung ie for a single room)chilled water fan coil?