Posted By Dana1 on 11 Dec 2017 05:21 PM
ZIP (not area) code 13325, in Lewis County NY, is NOT in US climate 4B, it's climate zone 6A.

https://energycode.pnl.gov/EnergyCodeReqs/?state=New%20York

Yes, even though your 99% outside design temp is in negative single digits, and it sometimes hits double digits there are mini-split solutions that work well here. Odds are pretty good that a 3/4 ton or 1-ton cold climate mini-split running on 15 cent electricity would beat $2.50 propane by quite a bit. The rated HSPFs are 12-14 BTU per watt-hour (12,000-14,000 BTU per kwh), and at a 50F standby temp the average efficiency would be higher than that. The binned hourly mean temperature in January in your location is about +20F, which is a 30F delta from an indoor design temp of 50F. At that temperature-delta most mini-splits will beat their HSPF numbers for efficiency.) But even worst-casing it at an as-used HSPF 10 it would only take ~9kwh to deliver the SOURCE FUEL BTU of a gallon of propane, ~8kwh to deliver the net-heat output of a condensing propane heater per gallon of fuel use, ~7kwh to deliver the gallon output of a typical 80-83% efficiency propane wall furnace. 9 x $0.15= $1.35, so its really half the cost of heating with propane, even using a worse than worst-case efficiency assumption on the mini-split.

If it dwells under -20F in that location for hours or days on end it will be safer to go with Fujitsu if freeze-up is a concern, since the Mitsubishi units will turn off at some low temp (below -18F) but automatically restart when temps rise to -13F. Fujitsu units will just keep on keeping-on at arbitrarily low temperatures. But the he existing electric baseboards could also serve as the hail- Mary backup plan for when the mini-split can't keep up (or fails unexpectedly), or for when a Mitsubishi goes into it's auto-shutdown mode for a few hours.

If any of that glass is single-pane it's worth installing low-E storm windows (or insulating shutters) over them, even assuming 50F standby operation.

Posted By Dana1 on 11 Dec 2017 11:15 PM
With the propane furnace in an unvented crawlspace with naked polystyrene on the walls (no thermal barrier) there is a potential fire/explosion hazard. I'm not sure what current code is for that sort of installation. Propane is heavier than air, and will tend to settle into the lowest enclosed space and hang around there.

On the floor of the crawlspace a couple inches of reclaimed EPS or XPS under a vapor barrier and a ~1.5-2" rat-slab might be in order over the longer term, but that's not the highest priority.

Fireplaces are notorius heat leaks, with 24/365 air leaks until you plug it with something. Plugging the flue with a pellet stove vent would be a good start! Pellet stoves are certainly going to have a lower installed cost than a mini-split, but when you're away for weeks at a time I'm not sure how you're going to manage keeping it fueled and operating when you're not there.

Since there is a pre-existing ducted distribution system, it's possible that you can use a Fujitsu RLFCD type mini-duct cassette in lieu of the (likely oversized) propane furnace. The 1-ton 12RLFCD is good for about 16,000 BTU/hr @ +17F, and still has a tested-rated output at -4F would keep going even at lower temps, and would probably cover the heat load well into negative double digits at an indoor design temp of +50F (your "away" mode.) The 9RLFCD might do it too. You might need/want to hack the ducts to make the runs shorter but higher velocity and better "throw", since the cassette isn't as powerful as a furnace air handler. But if it's all wide open an xxRLS3 type wall coil would do just fine, more efficient, and easier to install.

http://www.fujitsugeneral.com/us/resources/pdf/support/downloads/submittal-sheets/12RLFCD.pdf

http://portal.fujitsugeneral.com/files/catalog/files/9RLFCD.pdf

https://portal.fujitsugeneral.com/files/catalog/files/12RLS3H6.pdf

There are installation & service outfits for mini-splits within a reasonable drive of your place, including a couple of better trained Fujitsu "Elite" contractors within 40 miles of that ZIP code:

http://contractors.fujitsugeneral.com/search/index.cfm?int_radius=100&str_zip=13325&contractor_type=Residential

Mitsubishi won't tell you who is covering your area without sharing some personal info on their contractor-finder site, but apparently none of their better-trained "Diamond" contractors are currently serving that zip code:

http://www.mitsubishicomfort.com/contractors

So, run some heat load numbers, at both 50F indoors and at 70F, at an outside design temp of say, -5F (which is pretty close to your 99% outside design temp.)

If you have a heating history on this place it's possible to estimate the heat as-is heat load by fuel consumption. Exact fill-up dates & quantities during a winter period during which the place was continuously heated would be the most accurate. The methods for the napkin-math (it's OK to use a spreadsheet tool :-) ) are spelled out here:

As a backup (or even primary) heating system it might be better to put a small (<35,000 BTU) wood stove or fireplace insert rather than a pellet stove, on the basis that wood stoves don't need power to operate, whereas mini-splits and pellet stoves do. A wood stove with a bit of thermal mass such as a soapstone or ceramic type would be able to keep a 600' cabin from turning into a sauna. When the local grid goes down in a howling winter storm it makes a difference!

Posted By SciGuy on 12 Dec 2017 11:20 PM
More excellent feedback! Perhaps the locals are attempting to strike terror into our hearts. 

If you have a heating history on this place it's possible to estimate the heat as-is heat load by fuel consumption. Exact fill-up dates & quantities during a winter period during which the place was continuously heated would be the most accurate. The methods for the napkin-math (it's OK to use a spreadsheet tool :-) ) are spelled out here:

It looks like we used 2/80 of the 200 gallon capacity over the past three days to keep the cabin at at ~ 60*F. So that's ~ 5 gallons X 91,333 BTUs/ gallon X .8 (?) /72 hours or about 5100 BTUs per hour to hold the cabin at ~ 60*F. I realize that's too short a time for precision especially in light of the propane tanks gauge granularity but it at least gives and an order of magnitude based on the furnace having an 80% efficient. I'll send my diminutive gal pal into the craw space to get its particulars. The weather has varied from low teens at night to just bumping 32*F today. I bet i can get the approximate heating degree days from a reasonable source. That appear to be in line with your back of the napkin math. I'm timing the furnace duty time as I sit here with it 25*F outside and 60*F inside and see ~4 minutes on and 16 off or about 20% How small did they make those furnaces?

As a backup (or even primary) heating system it might be better to put a small (<35,000 BTU) wood stove or fireplace insert rather than a pellet stove, on the basis that wood stoves don't need power to operate, whereas mini-splits and pellet stoves do. A wood stove with a bit of thermal mass such as a soapstone or ceramic type would be able to keep a 600' cabin from turning into a sauna. When the local grid goes down in a howling winter storm it makes a difference!

A wood stove is attractive for the reason you mentioned but I worry that running one at a low burn long term for the low load would be an invitation for creosote production and chimney fires. Been there, done that and it's way too exciting. Might a small kerosene heater with 10 or 15 gallons in reserve might make for a cost effective power out heat source for a few days of power out?

The chimney on the cabin is external to the living space and is well thermally coupled to that space by the 8' X 5' fire place. If one was attempting to build a way to lose heat from a structure they'd be well served to copy this double flue monstrosity/ If we decide to stay here it's being torn down.

Thanks so much fr you well reason help.

Hugh

Posted By Dana1 on 13 Dec 2017 07:54 PM

Posted By SciGuy on 12 Dec 2017 11:20 PM
More excellent feedback! Perhaps the locals are attempting to strike terror into our hearts. 

If you have a heating history on this place it's possible to estimate the heat as-is heat load by fuel consumption. Exact fill-up dates & quantities during a winter period during which the place was continuously heated would be the most accurate. The methods for the napkin-math (it's OK to use a spreadsheet tool :-) ) are spelled out here:

It looks like we used 2/80 of the 200 gallon capacity over the past three days to keep the cabin at at ~ 60*F. So that's ~ 5 gallons X 91,333 BTUs/ gallon X .8 (?) /72 hours or about 5100 BTUs per hour to hold the cabin at ~ 60*F. I realize that's too short a time for precision especially in light of the propane tanks gauge granularity but it at least gives and an order of magnitude based on the furnace having an 80% efficient. I'll send my diminutive gal pal into the craw space to get its particulars. The weather has varied from low teens at night to just bumping 32*F today. I bet i can get the approximate heating degree days from a reasonable source. That appear to be in line with your back of the napkin math. I'm timing the furnace duty time as I sit here with it 25*F outside and 60*F inside and see ~4 minutes on and 16 off or about 20% How small did they make those furnaces?

As a backup (or even primary) heating system it might be better to put a small (<35,000 BTU) wood stove or fireplace insert rather than a pellet stove, on the basis that wood stoves don't need power to operate, whereas mini-splits and pellet stoves do. A wood stove with a bit of thermal mass such as a soapstone or ceramic type would be able to keep a 600' cabin from turning into a sauna. When the local grid goes down in a howling winter storm it makes a difference!

A wood stove is attractive for the reason you mentioned but I worry that running one at a low burn long term for the low load would be an invitation for creosote production and chimney fires. Been there, done that and it's way too exciting. Might a small kerosene heater with 10 or 15 gallons in reserve might make for a cost effective power out heat source for a few days of power out?

The chimney on the cabin is external to the living space and is well thermally coupled to that space by the 8' X 5' fire place. If one was attempting to build a way to lose heat from a structure they'd be well served to copy this double flue monstrosity/ If we decide to stay here it's being torn down.

Thanks so much fr you well reason help.

Hugh

The temperature outdoors varied by quite a bit over those few days. Don't guess- find a local weather station on degreedays.net, (KNYCONST7 on Highmarket road is probably good enough, but there are several others) and download the daily heating degree-day data using a base temperature about 5F below your indoor temperature (Ef you were keeping the place at 60F, use base 55F.)

Then from the BTU/gallon and furnace efficiency, figure out how many BTUs per heating degree-day were used, and divide by 24 hours to derive the BTUs per degree-hour it takes to heat the house. This is the key linear heating constant by which you can estimate the load at any arbitrary outdoor temperature.

Then note that if you want to heat the place to 70F the heating/cooling balance point will be about 65F, and if the local 99% outside design temp is -5F (about right), it means you have 65F-(-5F) = 70F heating degrees.

So the heat load at -5F will be about (your derived BTU/degree-hour heating constant) x 70F. The heat load at -15F will be (heating constant) x 80F.

The error over time periods as short as three days is pretty high, even IF you had better accuracy on the fuel meter. It's easier to use fill-up dates & volumes, and more accurate over periods of 50 winter days or more.

Posted By Dana1 on 14 Dec 2017 10:53 PM
If you're really designing for an outside design temp of -15F heating only with a single-head mini-split only, you're looking at the Fujitsu 15RLS3YH, which will deliver ~15,000 BTU/hr (max) at -15F (tested for efficiency modulating at 18,000 BTU/hr out at both +17F and +47F for the HSPF ratings):

http://orders.sidharvey.com/IMAGES/specs/ASU15RLS3Y.pdf

But the 12RLS3YH would still cover the load at -5F (which is more likely to be your 99% outside design temp)- you can run the wood stove or pellet stove or turn on a small electric space heater when the temps are expected to hit negative double-digits. One you get rid of that heat-sucking chimney (what a thing of beauty! :-) ) it will be more appropriately sized for the -15F load than the 15RLS3YH too:

http://www.fujitsugeneral.com/us/resources/pdf/support/downloads/submittal-sheets/12RLS3H.pdf

Regarding rooftop foam, how deep are the rafters, again?


I couldn't believe it but I'm now convinced that the rafters are only 2 X 6s which blows my mind. There's no apparent venting at the eves for the cathedral ceiling while there is a small vented attic area so there's potential for there to be a draw of moisture from the house into the ceiling insulation via the easy exit out the mini attic. I'd think it would be an excellent situation to add exterior sheet insulation at the time of reroofing with a metal rood after the monster chimney is torn down.

Thanks again for all the very useful comments.
  
Enjoy the holiday season,

Hugh