GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Audio Play Icon Headphones Icon Plus Icon Minus Icon Check Icon Print Icon Picture icon Single Arrow Icon Double Arrow Icon Hamburger Icon TV Icon Close Icon Sorted Hamburger/Search Icon

Community and Q&A

Heat pump strategy

Tim Koelker | Posted in Mechanicals on

I posted the following question on HVAC Talk and only received a few comments mostly preferring traditional heat pumps, even if oversized. I’m thinking folks here may have different ideas. Your thoughts???

Hoping those here can provide some guidance on a heating / cooling strategy for a new construction home

I’m looking to build in climate zone 4C or 5. The house plans I like are open floor plan single level ranch styles of approximately 1800-2000 square feet I plan to insulate above code with major effort in air sealing to get below 2 ACH/50. Heat loss calcs come in approximately 24K BTU with cooling at 16K BTU. I would plan to build on an insulated slab and keep ducts inside the conditioned space with potentially two zones. My wife has vetoed any ductless wall units. We assume natural gas will not be available. We will provide adequate ventilation.

A few alternatives I’ve thought about:

a) Conventional but high efficiency heat pump – something like a 2 ton Carrier Greenspeed 20
b) Conventional single speed heat pump – low cost but perhaps adequate due to low load
c) Mitsubishi or Daikin inverter mini split with 2 ton air handler (MVZ or FTQ) – are these better or worse for modulating than the Greenspeed option?
d) Two 1 ton single zone ducted mini splits – provide redundancy for breakdowns and zoning, split the flow from the ducted unit with short duct runs to rooms, modulates
e) Multi zone mini split with two ducted inside units – split flow as in d)

What are your thoughts on cost, efficiency, comfort, etc? Is new construction on a path to using ducted minisplits in the future vs conventional systems?

Thanks

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. User avatar
    Dana Dorsett | | #1

    What are the 99% outside design temperatures in the locations you are looking to build? You may be within the capacity of a 1.5 ton Fujitsu, depending on the design temps.

    A 24,000 BTU/hr heat load for a 2000' house is 12 BTU/ft, which is about where code-min 2x6/R20 houses come in at 0F outside, 70F inside. A better-than-code house would normally come in quite a bit lower than that unless it has a LOT of glass or you have high ventilation requirements without benefit of heat recovery.

    In most locations you will find 5-10 contractors willing to install traditional ducted heat pumps for every contractor familiar with & comfortable designing ducts for mini-duct cassettes, which have lower static pressure to work with.

    The turn-down ratio of the MVZ is about the same as the Greenspeed (about 2.5/1) but nowhere near as good as Fujitsu 18RLFCD, (about an 8/1). The turn down ratio of the Daikin FTQs is only 2/1. Both the MVZ and Greenspeed are likely to come in 1.5-2x more expensive than an 18RLFCD too. From a comfort point of view having it cruising along at low speed modulating nearly all the time is preferable to a 2.5 / 1 turn down, which will cycle a lot during the shoulder seasons.

    http://portal.fujitsugeneral.com/files/catalog/files/18RLFCD1.pdf

    If you're looking to break it into zones it you may find a 2-ton/3 zone multi-split and a mixture of ceiling cassettes and mini-duct cassettes works better than all mini-duct. But without a floor plan and the room-by-room, zone-by-zone load numbers it's hard to guess.

  2. Tim Koelker | | #2

    Dana,

    Since we're still way preliminary and haven't settled on a floor plan - or state for that matter, I ran a couple hypotheticals on loadcalc.net using Spokane as a proxy. I may be calculating glass or other dimensions incorrectly too. I'll have a pro run Manual J when we get further along.

    The bigger question for me is on the strategy of traditional heat pump, mini with higher volume/pressure air handler and traditional ducts, two single zone ducted (or cassette) mini splits or a multi split.

    Since you're in favor of the Fujitsu, and if we make the assumption that the floor plan works out better with two indoor units and less ducting, does it make more sense to have two separate outdoor units or go with a multi split? It seems that the cost of the multi zone outdoor unit is high enough that it would be about the same as with two separate systems. Not sure on efficiency.

  3. User avatar
    Dana Dorsett | | #3

    Until you have a floor plan and heating/cooling loads this discussion is WAY premature.

    If your heat load at +7F (Spokane's 99% outside design temp) is 15,000 BTU/hr or less you won't get much modulation out of a 2-ton Greenspeed. The max capacity of the 2-ton Greenspeed @ +7F is about 21,000 BTU/hr, but it's minimum output at +25F is about 10,500 BTU/hr, which is roughly your load if you are at 15,000 BTU/hr @ +7F. With that solution any time it's above 25F it would only cycle on/off, not modulate. As it happens, 25F is roughly the mean temperature for the month of January in Spokane, so even during the coldest month of the year it would would be cycling quite a bit, and practically never modulate during the spring or fall:

    https://weatherspark.com/averages/30356/Spokane-Washington-United-States

    There's not much point to a modulating system that is oversized to where it can't modulate, and the "right sizing" window is pretty narrow for systems with turn down ratios of 2.5:1 or less.

    A load of 15,000 BTU/hr for an 1800' house is a ratio of over 8 BTU/hr per square foot, R7.5/foot for an 2000' house and,that is roughly where you would be in a tight rancher if you went with R25-R30 whole-wall walls (say 2x6/R20 + R10-ish continuous foam), R65-70 cellulose in a trussed attic, and a not-ridiculous amount of U-0.25 window, and heat recovery ventilation. There are ways to screw that up of course, (by insisting on 27 corners or something) but you don't have to.

    So, don't fixate on the solution until you have an AGGRESSIVE Manual-J in front of you based on your REAL U-factors, not some HVAC contractors' pro-forma Manual-J that somehow ends up with all code-min assumptions with code-max air leakage, ASHRAE 62.2. ventilation rates with no heat recovery, and no credit for plug loads or warm blooded occupants, etc. But if you're really looking at a "pretty good, better than code" house odds are your loads will be lower than optimal for a GreenSpeed, but might still be OK for a 1-ton or 1.5 ton MVZ.

    When looking at multiple mini-splits vs. multi-split, pull the submittal sheets and look at the minimum heating output @ +47F. If the multi-split's minimum output is less than the sum of the multiple mini-splits it may work out better than 2+ compressors. But it often falls in the other direction if it's only two zones, two compressors. At 4 zones the minimum modulation of a multi-split is usually lower than four separate mini-splits, but it varies by manufacturer & series.

  4. Derrick Krienert | | #4

    Very interesting discussion. Is there any scenario where electric mini-splits are more cost-effective than natural gas? I would love to install (2) mini-splits with high turn down ratios, but with our utility rates ($0.75/therm, $0.09/kWH), I don't know whether switching to electric would be cost-effective in a zone 5 climate.

    If a home were to go all-electric like in the OP's situation by installing a HPWH, there would be the additional savings for the NG fixed costs ($17/mo for our utility), but then any savings on the NG fixed costs also have to offset the purchase price ($1,000) & operating expense for the HPWH.

  5. D Dorsett | | #5

    A right sized mini-split with HSPF of 12 delivers 12,000 BTU/kwh, so normalizing to 1-therm (100,000 BTU) that's 8.33 kwh, which at 9 cents costs 75 cents, in a zone IV location (not to be confused with US climate zone 4) Find yourself on this map:

    http://www.fsec.ucf.edu/en/publications/html/FSEC-PF-413-04/images/Figure5_lg.gif

    Burning 75 cent/therm gas to deliver a therm of heat into the house take more than a therm of fuel. In a 95% furnace it takes 1/0.95= 1.05 therms at a cost of 79 cents, plus the electricity used to run the air handler & controls.

    For you it's basically a wash, but you also get high efficiency AC out of the deal.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |