Regarding sizing of heat pump HVAC units
I was looking to have my traditional HVAC system replaced with a ductless system by Mitsubishi. I will be doing this as part of a significant overhaul of my home where I will significantly increase its air tightness, insulation, and replace all of its windows and doors.
My home currently has a 3-ton ducted system with almost no insulation for 1350sqft and it will be approximately 1840sqft when completed. I would like to ideally get a 0.5-1ACH for airtightness while the windows will be most likely be Alpens with a U0.18 rating (have a question coming soon about this as well). As for insulation, the quantity of insulation is still being determined and planned out but hopefully, it too will substantially exceed that of a normal house.
I had the local Mitsubishi dealer provide come out and they recommended having a system that is 5 tons if using only one unit. In fact, they preferred to split with 2 outdoor units at 3 tons each. Given my current conditions, this recommendation made me wonder if the sizing of the new ductless system is overkill, especially given how much more insulation I will have going forward and how airtight the home will be but I honestly do not know a lot about this stuff.
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Your instincts are correct- 5 tons of ductless for a < 2000' house is usually extreme overkill. Soliciting proposals from ductless installers almost always ends up somewhere between ridiculously oversized to grotesquely oversized. When you get into things like better than code windows and air tightness the typical HVAC installer's eyes glaze over, or they just nod and use the same crappy BTU per square foot rule of thumb their grandfather taught them.
The way to get it right is to run a room by room, zone by zone load calculation, or hire a competent third party who is NOT an HVAC installer to run those numbers for you. The most commonly used load calculation method is ACCA Manual-J. There are a few freebie online tools based on Manual-J that are sufficiently user-friendly to ballpark it (eg LoadCalc.net, Coolcalc.com) if you subtract out the air infiltration loads (why do they always overestimate?) but for heating loads an old-school I=B=R method is usually good enough for sizing the equipment, and is easy to set up using standard computer spreadsheet tools. See:
https://www.greenbuildingadvisor.com/article/how-to-perform-a-heat-loss-calculation-part-1
For outside design temperature purposes, where are you located?
My home is located in Dublin, CA (SF Bay Area) in climate zone 3C I believe.
Dana, thank you for the advice on the calcs - I also checked with my T-24 consultant and he can prepare the heating and cooling calcs for me, but the link is still useful so I understand what the calcs mean. Should I be looking at other systems/brands of heat-pump?
Also, I am not redoing my subfloor and so I discounted radiant heating options but I just found out that I could still do it if needed. Is radiant heating better than a heat pump for heating efficiency?
> Should I be looking at other systems/brands of heat-pump?
Yes. If re-using the existing ducts for some zones, Fujitsu's mid-static ARUxxRGLX series mini-duct cassettes have pretty good modulation ranges and ridiculously high cooling COPs. The compressors are comparable to those used in true cold-climate mini-splits, which gives them better turn down ratios and higher efficiency. Since your latent loads are mostly negative they can run in a super-efficient cooling mode 99% of the time during the cooling season.
There are others. When you have the heating and cooling load numbers in hand, go shopping for alternatives on the NORTHEAST ENERGY EFFICIENCY PARTNERSHIPS database. While this is a compilation of mostly cold-climate air source heat pumps, it's still usuful for comparing apples to apples at Dublin's 25F & 93F 99% & 1% outside design temps (https://articles.extension.org/sites/default/files/7.%20Outdoor_Design_Conditions_508.pdf). Using the search function and using the +5F slider to bracket heat pumps that deliver your design heat load BTUs or a bit less would find products that fill the bill. The list can be narrowed by selecting for ducting configuration (compact ducted, central ducted, ductless, etc), and further narrowed by vendor:
https://ashp.neep.org/#!/product_list/
A typical reasonably tight 2 x 4 framed house will usually have a design heat load on the order of 10 BTU/hr per square foot @ 25F, and in a relatively dry climate a cooling load of about a ton per 1500' @ 93F (with substantial error bars around those numbers, but not 2x or 0.5x.) So for an 1840' house you'd probably be looking at less 20,000 BTU/hr for both heating and cooling, something like 1.5-2 tons if using a highly efficient cold climate type compressor, not 3 tons, and not 5 tons. Looking at that mid-static Fujitsu series, the AOU/ARU18RGLX delivers 21,500BTU/hr of cooling @ 95F, and 19,800 BTU/hr of heating @ +17F (more at +25F):
https://ashp.neep.org/#!/product/25348
The 2-tonner in that series can deliver 29,000 BTU/hr of cooling @ +95F, and 24,800 BTU/hr heating @ +17F:
https://ashp.neep.org/#!/product/25350
Note that the minimum modulation at +47F is 5400 BTU/hr for either one, so there isn't going to be a huge penalty in efficiency or comfort for upsizing a bit, but pumping up to the 2.5 tonner would be a mistake unless really necessary, since it's minimum modulation level is 9200 BTU/hr @ 47F, which is probably more than your load at that temperature, so it will be forced into cycling on/off taking a hit comfort and efficiency:
https://ashp.neep.org/#!/product/25351
(Note the 27,650 BTU/hr @ +5F of that 2.5 tonner. That is enough heat pump to cover the load of many tight 2x4 houses with 1840' of conditioned houses even down to 0F, and almost all 1840' IRC 2018 code min houses even into sub-zero territory.)
This house in Berkeley is heated and cooled with a single 1.5 ton Fujistu AOU/ARU18RLFCD (with a less powerful blower):
https://www.greentechmedia.com/articles/read/what-does-it-take-to-electrify-everything-in-your-home#gs.le8ni9
Larry Waters (707-342-1981) is the guy who designed that system is from the East Bay- it may be worth getting a proposal from his company.
There are other equipment options out there- get the room by room, zone by zone numbers first.
>Is radiant heating better than a heat pump for heating efficiency?
If the ducts are well designed & implemented, fully inside the pressure and insulation boundary, and the radiant is provided by a reversible air source heat pump chiller the efficiencies are comparable. But if it's an electric boiler running the floor the heat pump is WAY more efficient (about 4x in your neighborhood).
Dana, Wow! that is a detailed response - thank you very much.
I will dive into it this evening after work and I will probably have a lot of questions; on the first pass, a lot of it went over my head...
Take the time to read/watch Nate Adams' take on the importance of NOT oversizing the HVAC for attaining maximal comfort, and why the smallest equipment that actually covers the load is the RIGHT equipment:
http://www.natethehousewhisperer.com/home-comfort-101.html
http://www.natethehousewhisperer.com/hvac-101.html
http://www.natethehousewhisperer.com/hvac-102.html
Nate was an insulation contractor fed up with how state/city sponsored efficiency programs were being run, and decided to go into the "comfort retrofitting" business in the Cleveland Ohio area:
https://www.greenbuildingadvisor.com/article/how-an-efficiency-program-killed-my-business
https://www.greenbuildingadvisor.com/article/hard-truths-of-home-performance
His projects still involve a LOT of air sealing and insulation retrofit work, but also includes yarding out the ridiculously oversized mechanical systems that are too big to be able to provide comfort, then monitoring the houses and replacement systems in some some cases for years after the fact to dial it in to HVAC comfort & efficiency nirvana. Check out some of the extensively documented case study project links on his blog's home page:
http://www.natethehousewhisperer.com/blog
(scroll down a bit to the large font "Case Studies")
If it's not already there, spending even half the amount of the ridiculous 5 ton proposal on really nailing the air sealing and insulation details you can almost certainly get this place within range a of 1.5-2 ton modulating heat pump. On this project (on a ~1700' house) he specified a 3 ton Carrier Greenspeed heat pump, and when the contractor showed up with a 4 tonner he made them take it back:
http://energysmartohio.com/case_studies/1917-net-zero-ready/
The Greenspeeds only have a 2.5:1 turn down ratio and have to be sized pretty close to get much modulating comfort out of them. Their capacity falls off with faster than the better Fujistu or Mitsubishi units too (which makes them not necessarily the best choice for Cleveland, but would work great in Dublin CA) but the smallest in the series is a 2-tonner, which may still be too big for your loads. This handy online tool shows the capacity across temperature with differing air handler options if you click on the "Heating Capacities" tab. With most air handler options the 2-ton 24VNA24A**30 puts out 26-27,000 BTU/hr @ +25F, which may be substantially above your design load:
http://www.tools.carrier.com/greenspeed/
A 2 ton Greenspeed is an option that would cover it, but if your load is under 20K there are probably better options out, given the narrow turn-down ratio.
https://ashp.neep.org/#!/product/29393
Sometimes the sizing is driven by a flawed approach to zoning. If you think you need 8-10 heads you'll inevitably end up with massive oversizing.
In actuality not every room needs a unit. Low static units serving adjacent rooms with minimal ductwork is often a superior solution. This is not a compromise if the zoning is done methodically. 2 or 3 zones often makes sense but 6-10 mini split zones in a regular size home is often indicative of a poor approach.
Some good points are driven home in this podcast.
https://www.greenbuildingadvisor.com/article/multi-zone-heat-pump-issue