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Community and Q&A

Heating options to replace aging gas furnace in CA CZ16 (CZ4B/CZ6B)

JoshAt | Posted in Mechanicals on

Hi all.

I have a 15 year old furnace that wasn’t well taken care of and is coming up on it’s last legs.  House is 1650sqft, 1.5 story with cathedral ceilings, built in ’73 and is not well air sealed: 0.6 ACH on our last blower door test. Work is being done to hopefully bring that down to ~0.4 but we have a T&G roof which has some natural limitations.

I’m not overly worried about summer: the cathedral ceilings have skylights and a large fan mounted under them which do a great job at keeping the place cool. Winter on the other hand gets cold. We’re in CZ16 so nothing crazy but we did hit 0 a couple of times this year.

I would just replace it with another high efficiency gas furnace as the ducting is already all in place but here’s the rub: the furnace is in the crawl space and it is _extremely_ tight. 24″ from soil to the bottom of the joists. It’s currently mounted next to a sump pump, the piping of which has failed several times during the previous owners tenure, flooding the blower cavity (thus the 15 year replacement..). That problem is resolved, hopefully, but the space is very, very tight to work in.

Which brings me to my question: what other alternatives do I have for heating this house that aren’t a furnace in the crawl space? Outdoor heat pump/reverse cycle AC? Outdoor gas furnace? Suck it up and pay someone big $$ to work in that tight space and replace it?

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  1. user-2310254 | | #1


    There are ducted mini split heat pumps that have "hyper heat" condensers. That might work, but putting any type of air handler in a ventilated crawl space is probably not a good idea. Would you consider sealing the crawl (a heavy duty liner with insulation on the foundation walls)? I'm replacing an old ducted system similar to yours with a Mitsubishi hyper heat. Because the house has two wacky zones, the design includes two air handlers, a branch circuit controller, and single three ton outdoor unit.

    This gives me heat and air, which is essential for CZ3A. Do you have drywall behind the T&G ceiling?

    Also... That air leakage rate doesn't sound right. A .6 ACH/50 measurement get you into passive house territory. Maybe you mean 6 ACH/50.

    And... The California climate zones don't get used much on this site. Can you look up your IECC location ( and edit your post?

  2. JoshAt | | #2

    I would love to encapsulate the crawl space but it's so tight everyone has told me that it can't be done and the one company that said it could quoted me $22k, so I'm taking that as a "it probably can't be done right". The existing furnace spent it's life in this ventilated crawlspace that previously did not even have a vapor barrier, with it's air intake pointed down at the air. Optimally any solution I come up with wouldn't involve that crawl space as maintenance down there is a real pain and significantly increases costs. I'll do it if I have to -- and I might with 200" of snow in December alone -- but I'd like to try to avoid it!

    The upstairs is an A-Frame shape with one bedroom at either end and open loft in the middle. The furnace works hard to try to warm the big loft and living space which usually keeps the upstairs bedrooms at a pretty much perfect temperature and that zone never runs. Perhaps that situation makes mini splits make more sense: I'll take a better look.

    No drywall behind the T&G. I'm not entirely sure what's in that cavity but it has some leakage near the peaks, less so all round which makes me think there's an attempt at an air barrier there.

    I think you are correct on the 6ACH. I'm waiting on the written report so was going off memory.

    Hah.. I said CZ16 because I always struggle to find it on the IECC maps. I live in Lake Tahoe which is probably CZ4 on that map but I live on the edges, so between CZ4B/CZ6B on the map.

    Naively it feels like my options are:
    - Replace the furnace like-for-like
    - Install mini-splits, remove the ducting (& maybe encapsulate the crawlspace with all the extra work space not taken up by ducts)
    - Replace the furnace with an outdoor heat pump/AC and connect up the ducting.

  3. user-6623302 | | #3

    Where is the duct work, under the house, insulated? How do you make your hot water? Where is the heater. Is there an attic, attic insulation?

  4. JoshAt | | #4

    Duct work is fiberglass insulated flex duct that runs in the crawlspace, hanging from the floor joists (not resting on the ground). Second floor ducting comes up via some chases in closets. Hot water heater is a gas tankless hot water heater mounted to an exterior wall in the corner of the laundry on the first floor (inside).

    No attic. 25ft vaulted/cathedral ceilings.

  5. user-6623302 | | #5

    Some ideas:
    Build a small addition/stand alone boiler room. Use air handlers in the house connected to the boiler with insulated pipe like they use for a wood boiler. Move your water heater as well? Many options for air handlers and boiler. High volocity air may be a good choice.

    Direct vented gas heaters. In wall furnace, floor furnace, or stand alone heater.

    Both ideas gets almost everything out of the crawl space. This will make it easier when you decide to do something to insulate under the floor.

  6. user-2310254 | | #6


    Have you considered hiring an HVAC engineer? I think this approach will deliver a better and more appropriate system.

  7. JoshAt | | #7

    Thanks Jonathan, lots to think about.

    Steve, absolutely, that's my next step. I just like to try to be informed a little before I hire someone else so I can make best use of their time!

  8. user-6623302 | | #8

    Sounds like you have a vacation home that you are trying to, as we say here, winterize. We have a lot of which were upgraded in the 1960s, 70s, and 80s. Now they knock them down and build new. I would suggest that a long term plan is important. You don't want to do something that gets in the way of something else.

  9. JoshAt | | #9

    Close! Former vacation home that is now just home... turns out being in the woods really appealed to us :)

    Yes, I'm trying to long term plan and that's why getting everything out of the crawlspace (in spite of the higher cost and effort) seems worthwhile to me.

    I think I'm going to get a local engineering companies input (they've done some title 24 work for me in the past) and then find some local Mitsubishi experts to see what a dual zone mini-split would look like for me. The appeal of moving the mechanical, perhaps side mounted for snow, outside and freeing up all of the ducting is pretty compelling. Ground coverage here is a no-go: the TRPA prohibit any expanded ground coverage for water management purposes.

    We'll likely spray foam the floor and rim joists vs encapsulate due to the difficulty to try to get this ACH under control.

    Thanks all for the input!

  10. user-6623302 | | #10

    There is frequent discussion on this site about converting covered porch/three season rooms into fully incorporated living space. Would be worth a search for details for your floor renovation.

  11. walta100 | | #11

    Is there any reason you could not excavate the crawlspace yourself?

    If you are in CZ4 it seems like the bottom of the footing would be about 36 inches below grade you would want to be sure not to disturb them or any other load bearing column.

    Get yourself a short shove and 20 5 gallon buckets and get to work.


  12. user-6623302 | | #12

    How about excavating a portion of the crawlspace for a conditioned boiler room. They do something like that for coastal homes which are up on tall posts.

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