Critique my Mini Split placement options
Hi all,
I am down to crunch time and getting close to pulling the trigger on HVAC strategy. Before I “bite the bullet”, I want one last pep talk. I think I have the upstairs figured out, but I’m concerned exactly as to where to place my heads.
I have posted two mini split layouts below. Let me know what you think. I have included the room to room BTU loads as well.
The house is going to be tight. We have taped all the OSB and two layers of exterior foam. The only weak spot will be lots of double-hung windows. Haven’t decided on a ventilation strategy, it depends on the blower door tests.
From purely a BTU standpoint, I feel comfortable (rightly or wrongly) that a single ductless heat pump could get the job done. Something like a Fujitsu 12RLS3 should work. The problem is the size of the house. I’ve read anecdotal reports that you shouldn’t have less than 1 unit for ever 1100 sq feet. I’d be twice that amount. The house is 70 feet from conditioned end to conditioned end. That’s too much, I’m sure, for one unit to service, regardless of whether you put it at one end or put it in the center.
So the option I am leaning towards is to place two smaller units, on separate compressors to maximize efficiency and lower the minimum modulating output, somewhere in the house to share the load. I was thinking about two 9RLS3 placed fairly close to the center, pointed out towards the ends. Locating them in the middle of the house puts them in the two rooms that have the highest loads as well as the rooms that will get the most use – – the kitchen and the den. This makes sense to me. Also, during the shoulder seasons, I could turn one off completely and let the other one heat/cool the whole floor, as long as it can provide the heat and cool to all the distant rooms. The only thing that concerns me about this idea is that I have a large 10′ opening between the kitchen and the den, and I wonder if the kitchen unit will affect the den unit, and vice versa. The way the floor plan is set up, the one in the den would be pointed completely away from the kitchen unit, but the kitchen unit would not be able to be pointed completely opposite of the den unit (see floor plans below). Still, the units would be 25′ apart, with the kitchen unit at a 90* angle to the 10′ opening, and I could angle the unit so it blows away from the opening. Or, should I move the den unit so it doesn’t have the same potential to be hit by the kitchen unit, but then there may be less airflow to the guest bath and bedroom?
Also, I am leaning towards the Fujitsu units, but wondering if I should consider two Mitsubishi FH09NA. The Fujitsus have a little more upside potential in case I have been too aggressive on my Manual J calc’s, but the Mitsus have more downside capability for the shoulder seasons.
Thanks in advance.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Clay,
Unless I missed it, you forgot to tell us your climate zone or geographical location. "Lots of double-hung windows" are less of a problem in Zone 4 than in Zone 7.
Whether or not you are satisfied with this approach depends in part on the expectations of your family. You can expect room-to-room temperature variations with this approach, especially in cold climate zones.
You haven't indicated the cardinal directions. If any of the rooms without a minisplit head faces west, the room could easily overheat on summer afternoons -- especially if it has several windows.
Finally, the location of the minisplit head in your kitchen/dining room could easily lead to comfort complaints, since you will be blowing air on the people sitting at the dining room table.
.
I am in climate zone 4A. The front door (bottom) faces SouthEast.
If I were to put the unit on the opposite wall, blowing towards the back of the house, would the smaller room behind it get any heat/cool?
Would it be better to put the unit above the door between the room to the left (mudroom) and the kitchen? The downside to this is that it would push the heat/cool into the big room, which would already have a unit. Or would you put a unit in the mudroom blowing at the door to the kitchen? The only concern would be weather the heat/cool could get to the bottom left room.
Or, should I give up on the plans for ductless units and go with ducted units?
Martin,
I kind of ignored your warning about expecting room-to-room temperature variations. Are you saying that I will experience this because the floorplan is not conducive to two ductless units, or just where I have planned to place the units?
Obviously the safer bet is to go with ducted units? I was hoping to go ductless to save on the labor costs and load losses/gains from ducted units, but if I have rooms that are too hot or cold compared to the rest of the house the savings won't be worth it.
Clay,
With ductless minisplits, room-to-room temperature differences can be expected. A room without a ductless head cannot be expected to be at the same temperature as a room with a head, especially if doors are kept closed. These temperature differences are most noticeable on extreme weather days -- very hot days or very cold days.
Is this a problem? It depends entirely on the flexibility of the family members. I heat a poorly designed house with a single wood stove in my living room. For the most part, all of my family members are happy with this system -- but your family may be different.
Thanks. It is that unknown about the temperature differences that has me worried. If we were talking about a degree or two I don't think it would matter. But if we are talking about 5 or 8 degrees, plus humidity differences, that might be a problem. The doors for the first floor should mostly be open. In fact, the only real doors (other than the ones to the outside) would be the doors to the bathrooms and the door to the guest bedroom.
The room of biggest concern is the SW side room of the kitchen (utility room?), which has a real enough heat load, but limited convective air flow exchange with the kitchen, followed by the south corner room (dining room?), with similarly limited flow paths.
With the large doorway between the big room and the living room (the only room identified with a label in the drawings), placing the head directly across from the doorway (option 2) should work just fine.
The north room (bedroom?) and the bathroom next to it won't get much flow, but the peak cooling loads are miniscule, and the heating loads small. Just a pair of sleeping adult humans would cover roughly half the peak heating load of the north room, (and if it's a bedroom most people prefer sleeping in a slightly cooler room anyway.)
Even a pair of FH06 (each is good for 10,700 BTU/hr @ +17F, which is probably close to your outside design temperature ) can probably handle this configuration without much trouble, but a pair FH09s wouldn't be insane. The blower cfm is the same on both models- you don't get any more air "throw" with the 09, but you do get another 1500 BTU/hr of heating capacity, not that you really need it.
With the isolated rooms in play you may have to run some numbers for the inspectors to prove that even those rooms can be maintained at +68F without local auxilliary heating in the room with the mini-splits you have (even if it means cranking them up to 78F when it's really cold out.)
A single 1.5 ton Fujitsu 18RLFCD mini-ducted unit would cover the entire load with some margin, if you can find a competent duct designer, and have some place to run the ducts.
What concerns me about a ducted unit (or units), is whether or not the space is too spread out for just one unit, and whether two units sharing the load would then be oversized for the job.
If I feel that the cooling load for this space is somewhere in the neighborhood of 10-12k BTUs, then two 9k ducted heads would only be able to modulate down to 6.2k. However, if I had two ducted units, then if the cooling load got down to less than 6k I could always turn one off and run on just one head down to 3.1k. But turning a ducted head off would not be as beneficial because the air mixing really would be limited.
Or am I over-stressing about this?
Thanks Dana. I was typing that last response during your response. Glad to see you think the 18RLFCD could handle the distances, assuming I have someone who can design the ducts the right way.
Yes, the room off the kitchen to the left is a mudroom/utility room. There is a door there but it might not even have one, and if it does it will left open most all of the time. But you are right, that has a pretty decent heat/cool load. The room below the kitchen is indeed the dining room, and the doorway between the kitchen and this room is bigger than it is drawn. It is actually 42", if that helps. If the option #2 is best for the right-side unit (at the back of the house pointed towards the living room), what are your thoughts about the left-side unit. I hadn't thought about Martin's comment about air blowing on those eating at that table.
Hey Dana (or anyone else out there),
Would you agree that a single 18RLFCD is better than two ASU9RLF running on different outdoor units? My HVAC guy is worried about covering the heating load (his calculations are higher than mine), which the two 9k units would do better than the single 18k (16k x2 units @ 14F vs 21k), but I worry about only being able to modulate down to 6.2K (3.1k x2) on the cooling load. I'm thinking my total cooling load is going to be something a little less than 10k. Is 6.2k a low enough modulation, or should I hold strong to the 3.1K of the 18k?
You are correct that the minimum modulation of a single 18RLF is the same 3100 BTU/hr @ +47F as each 9RLF, so there will be less cycling overall if you went with the 1.5 tonner.
But with a pair of 9RLF you'll have better temperature/load tracking on the zones for better room-to-room temperature balance, and the duct runs would likely be shorter, and you have the option of outright turning one of them off during the shoulder seasons (but WOULD you turn one off? Maybe...)
If you go with two, lay it out as an easterly zone and westerly zone, since the SW facing windows will have dramatically different solar gains than the SE windows during the morning and afternoon periods. Since the big room has no SW or SE exposure, you may want to feed it a register from each, but put the zone thermostats somewhere other than the big room. (Say, the westerly thermostat in the kitchen, the easterly in the living room.
With a 1% whole-house cooling load less than 10K don't sweat any efficiency hit from cycling- you're still using very little power (less than a half-ton window shaker) and you may be able to cool just fine with only the westerly zone running if you keep the doors to the other rooms open, only turning on the other zone only on the stickiest of days in "DRY" mode if it's too sticky in the bedroom to sleep. (That's probably what I'd do. ) That way you'd have enough load to keep one modulating most of the time, and if you only needed to run the other for an hour or 10 per week to sleep comfortably it's no big deal.
For completeness, what is the estimated heat load @ +47F, after correcting for the oddball U-factors discussed in your other thread?
Thanks Dana. Interesting idea about overlapping the zones a bit in the big room. That would allow the main living spaces (the kitchen and the den) to be served through the shoulder seasons on one system if needed, letting the less-used spaces (that is a guest bedroom) go a little cooler or warmer than normal until the 2nd unit needs to be turned on.
A quick re-calc on my manual J looks like the heat load at +47F is a tad above 7k BTU/hr. See attached. I updated the cooling loads to figure in potential duct losses, so my heat/cool loads are a tad higher than they were when I posted the floorplan above, but the cooling loads are still less than 10k BTU/hr.