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Ducted Heat Pumps: what’s the best unit to heat half my house?

Rich Cowen | Posted in Energy Efficiency and Durability on

I have a house in MA with a 2018 Mitsubishi heat pump downstairs, a large ductless unit. It is not used very heavily because the whole house is still heated using gas, from the furnace in the attic.

I am considering replacement of that 18 year old gas furnace, using another  heat pump. It does not have to Mitsubishi, but if a Mitsubishi does the job it may make maintenance a lot easier to procure.  The furnace is way oversized and creates drafts when it runs, it is like a tornado.  So, the house is ready to be updated with a less powerful attic unit. (Currently I use ~90 therms of gas in the coldest month of the year).  I can adjust the duct work to direct more of the heat to the upstairs from my attic unit.

The downstairs unit is 18K, the MSZ-FH18NA2. https://www.ecomfort.com/manuals/mitsubishi-708136bd9a2949a926c55242eb7574b3.pdf

Final bits of info:  My zone is 5 in central Mass. and the HPSF of my current heat pump is an impressive 12.0

 

Looking at the spec sheets online I see the following from Mitsubishi:

SV-KP24NA  (24K unit), SV-KP30NA  (30K)

Any experience with these systems? It does appear that they have an HPSF of only 10.4, lower than my current downstairs unit. Are there other systems coming out from Mitsubishi and other vendors that push the envelope of HPSF rating, or that are cheap to maintain on an annual basis?  My house needs very little cooling so I only care about the heat ratings.

Also in my climate zone (it rarely hits 0F and hits -13F only every 30 years or so) do I need to keep any backup heating source like a wood stove or can I just go all electric?  In about 18 months I plan to add more solar and a battery backup.

Thank you.

 

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Replies

  1. Paul Wiedefeld | | #1

    I have the SVZ-KP24NA (indoor, airhandler) and SUZ-KA24NAHZ (outdoor, hyper heat) and I highly recommend it. The HSPF is a very difficult thing to measure and therefore to compare for modulating heat pump systems. For instance: this combination has a seemingly awful COP of 1.66 at Max 23,000 btus/h at 5 degrees. But if you're at 13,927 btu/h at 5 degrees outdoor, its COP is 3.2 (at 5 degrees!), which is outstanding. With a heat loss so low (90 therms/ winter month - does that include cooking and hot water too?), you'll be modulating nearly the whole time.

    1. Rich Cowen | | #2

      I don't understand the calculation you provided. Are you saying the unit peforms better than spec, or what? In my house each floor is about 1000 sq. ft.

      If you are suggesting I could go with an 18K unit instead I will explore that. When I was considering a gas furnace replacement the tech suggested swapping the 5ton system for a 3 ton, so those HVAC guys will be shocked if we can go down to 1.5 ton. But it is doable as we have spent $11K on insulation. See my GBA posts from 10 years ago and you will get the idea.

      fyi my usage did not include hot water and the cooking load is negligible. I honestly don't know what the monthly power usage is on my Mitsubishi, so I should try to measure that, and report back. My water is heated with electricity, a GE unit that still works and keeps the basement cool and dry.

      1. Paul Wiedefeld | | #4

        I'm saying that "spec" isn't really meaningful here. A low efficiency furnace is roughly 80% efficient at all temps and all loads which is pretty well captured by AFUE, but HPSF is harder to nail down. Basically, no need to wait on a future super heat pump, a part load COP of 3 at 0 degrees outdoor is hard to beat. 24kbtu seems well sized for you.

        1. Rich Cowen | | #5

          ok, that is helpful. So oversizing is still helpful. My only remaining concern is the size of the duct work, did you run into any issue putting the heat pump air handler into existing ductwork which expects a powerful blower motor? I imagine that there will be some redesign of ductwork or additional heads involved if I want the smaller rooms in the downstairs to have any working registers. Ultimately the HVAC guys will tell me what I should do but I want to be educated before I talk to them.

          1. Paul Wiedefeld | | #6

            The Mitsubishi has a powerful blower. The most noticeable difference is that since the heat pump modulates, it's a consistent heat. The furnace was on for maybe 10 minutes per hour, so had to move almost the same quantity of air in 1/6th the time. Slow and steady is better.

  2. Johngfc | | #3

    To make an informed decision you need a more detailed analysis. This will give you a more refined estimate of your heating requirements, which of course will reflect your climate (zone, or preferably more detailed data), square footage, insulation package, windows and glazing, air leakage, etc. If you're heating primarily with a gas furnace, and it's used mostly for heating (or you can estimate cooking/hot water), and you can estimate the efficiency of your furnace, then you could calculate energy use (and thus sizing) and if desired relate this to temperature (given climate changes and warming trends). There are MANY posts on this site about sizing HVAC systems - see e.g. https://www.greenbuildingadvisor.com/article/when-do-i-need-to-perform-a-load-calculation

    What Paul was noting is that heat pumps are much more efficient when they're not running at full capacity. In his example, the efficiency of the unit is almost twice as great when it's running at 60% (14,000/23,000) btu/hr than at 'full speed'. You need to size the system so it meets your maximum load requirement, but it's very rarely going to be running at that full capacity.

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