Minisplit performance
Single head mini splits are quite efficient, with SEERS in the 20-30 range.
I’m trying to develop some “number sense” for how inefficient they get when you have multiple heads on a single condenser. For one thing, the manufacturer still reports a single SEER.
Shouldn’t there be 2^n different SEER ratings where n is the number of heads? I asked the manufacturer (Mitsubishi) and they couldn’t explain which SEER rating was the listed on in this scenario.
Some people claim the efficiency isn’t much better than a standard HVAC system.
My house has 2×4 walls and the HVAC guys only seem to sell high velocity systems for my walls. They aren’t energy star certified. Surely a multihead ductless system would still be more efficient than a high velocity system right?
My contractor quoted me a system with two compressors. 4 rooms on one and two on the other. The 4 rooms (2 bedrooms an office and kitchen) are all quite small, with maybe 6k-9k btu per room, and the other one for a much larger living room with 30k btu and a single guest room at 9k.
I’m trying to understand if this design of splitting up the rooms with two condensers makes any sense.
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The SEER isn't a function of the number of heads, but rather the total capacity of the compressor relative to the presumed load, since that determines the duty cycle. The test also presumes sufficient combined capacity of the heads/cassetted to max out the compressor.
The notion that a bedroom could have a cooling load of 9K or a living room at 30K (2.5 tons, for just ONE ROOM!?!) isn't necessarily well founded. Even 6K is on the high side for anything but a top floor bedroom with west facing clear glass windows. A typical load/floor area ratio for whole house is about a ton per 1200-1600', so unless that living room is 3000 square feet (instead of 300') it's unlikely to have a load that high. (The exception might be a living room that is essentially a greenhouse, with mostly glass for the exterior walls & ceiling.)
Before diving in either have a qualified third party (not an HVAC contractor) run an aggressive room by room Manual-J . A RESNET rater or certified professional engineer who runs load calculations as service is far more likely to get it right.
Short of that, use an online tool using the most aggressive input assumptions that could be real, on shading factors, air tightness & R-values. One such tool that I sometimes use for quick single room estimates is loadcalc.net (http://www.loadcalc.net/). It tends to overshoot reality by a bit compared to a pro-tool with finer tuning options, but it's still WAY better than a WAG.
If a room's calculated peak load is less than 4K you don't want to give it a 6-7K head served by a multi-split. When married to a multi-split the heads themselves don't modulate, but will operate on a duty cycle to serve the load, which is quite a bit less efficient than modulating up/down with load. A 3/4 ton modulating mini-ducted mini-split on a dedicated modulating compressor serving 2-4 rooms will run longer cycles and higher efficiency than 2-4 half ton heads on a multisplit.
See: https://www.greenbuildingadvisor.com/article/manual-j-load-calculations-vs-rules-of-thumb
https://www.energyvanguard.com/blog/air-conditioner-sizing-rules-of-thumb-must-die
How do I find such a manual j calculator ? I mean a person not a tool. I'm having trouble doing that in westchester, ny.
A freebie online not-super-accurate version that's easy to use is LoadCalc: http://loadcalc.net/ When using that tool use maximum air-tightness assumptions on the house & ducts, and be aggressive on R-value assumptions too, or it'll oversize. (eg, unless you KNOW the 2x4 bays have something less than R13, assume that they're R13.)
A similar freebie tool (that I haven't personally used ) is CoolCalc: https://www.coolcalc.com/
A Wrightsoft Right-J professional too license runs $474 (and there's a bit of a learning curve.)
http://www.wrightsoft.com/Products/Right-J
There are others.
If you have a heating history on the place you can use a fuel-use based load calculation methods as a sanity check on the accuracy of any online tools.
https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new
Right, i am aware of all of these websites. I would like a third party non installer to size my ductless system. That way, there can be no confusion. For example, you seem knowledgable. Can I hire you to come to my house to size the system?
I'm not in that biz, and don't have a license for the relevant software. Even when advising on projects locally I usually dig up a competent third party to run the load numbers, then review the numbers for sanity-checking with the homeowners.
Running your own numbers using the available freebie tools is a worthwhile start, WAY better than a WAG, and way better than most HVAC contractors will give you. Somebody willing to put 2.5 tons of cooling in a Westchester living room is clearly not doing any kind of analysis whatsoever, not even a lousy rule of thumb. In the hot steamy southern US "a ton per 500 feet" is a commonly used rule of thumb that usually oversize barely insulated very leaky houses with signficant margin. So even using that ridiculous rule of thumb, 2.5 tons x 500'/ton= 1250 square feet, which is probably a wee bit bigger than your actual living room.... (just a guess. ;-) )
There are companies that will do Manual-Js and full system specs remotely via email. (EnergyVanguard out of Decatur GA is one. Allison Bailes from that company has done quite a few guest blogs here.)
To be fair my living room has 16’ ceilings and is about 300sqft. He also intended the unit to cool other rooms on the floor, which normal 10’ ceilings and about 500 sqft to them in total.
Without knowing the R-values, window type/area/orientation I'd hazard that that 500' cooling zone still comes in under one ton. Unless it has a 16' tall west facing wall of single pane glass it's nowhere near 2.5 tons. Running a quick & dirty loadcalc.net would verify that.
For your 500 square foot tall ceiling zone I just ran a quick & dirty loadcalc for White Plains location assuming:
150 square feet of south facing clear glass double pane window area, no roof overhangs
150' north facing window
combined 300' of east & west facing window
150 square feet of south facing 2x4/R13 wall with siding
150 square feet of north facing wall
a combined 300' of east & west facing wall
500' of R19 ceiling
500' of R-0 tile floor
0cfm infiltration
3 occupants
It came up with a cooling load of...
~18,000 BTU/hr cooling @ 75F/50%RH indoors, 87F outdoors (the 1% outside design temp)...
...and...
~19,500 BTU/hr heating @ 7oF indoors, 12F outdoors (the 99% outside design temp),
That's probably more glass area than the zone really has. A 25 x 20' footprint (500 square feet) 16' tall for half of it has about the same total square footage of gross wall area (window + wall), but probably less than 50% of the gross wall area is window. The R19 assumption is probably crummier ceiling R too.
Bumping the cooling outdoor design temp to 95F and heating design temp to 0F it still comes up with 19.5K BTU/hr of cooling load, 23.4K of heating load.
Your real window area & real load numbers are almost certainly smaller than that, but go ahead and measure stuff up room by room, run it through loadcalc room by room, and keep track by writing the cooling & heating load numbers (using 87F cooling, 12F heating for design temps) on a sketch of the floor plan.
Then bear in mind in most cases this tool will overstate the actual loads by a double-digit percentage. There is no need to oversize a ductless head for numbers derived with loadcalc. Even undersizing it by 15% would be preferable to oversizing by 15% from loadcalc's numbers.