Fujitsu 12RLS3H in -25°F weather?
matthewberger94
| Posted in Energy Efficiency and Durability on
Does anyone have experience with running a Fujitsu 12RLS3H in -25°F weather, 5-6 hours a day for 8 weeks? I live in zone 7.
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Replies
Matt,
Where exactly do you live? Even in Climate Zone 7, I find it hard to believe that you have really experienced the conditions you describe.
If you do get -25°F temperatures between midnight and 6:00 a.m. every night for two solid months -- an unlikely description of the weather in Climate Zone 7 -- it's rare for such cold nighttime temperatures to occur during two months of cloudy weather. What is likely to happen is that the sun will come up, providing solar gain through your south-facing windows.
Moreover, if it's -25°F at 6:00 a.m., the temperature is quite likely to rise to -5°F by 1:00 p.m. So your Fujitsu unit will have many hours during the day to bring your house back up to temperature after such a cold night.
Response to Martin (Response #1),
I live in Wahpeton, ND. It's not that cold every night for 2 months, it's cold but not that cold, sorry for over exaggerating. The average daily low in January was -6F in 2014, the coldest day was January 2, @ -26F.
Here is some more climate data for Wahpeton in the year 2014. https://weatherspark.com/history/29837/2014/Wahpeton-North-Dakota-United-States
I really would like to only buy 1 unit and locate it centrally in a 1080 SQFT house. Slab would be R-34, Walls would be R-48, Roof would be R-86. At an ACH of 3.0, the heating load is 16,500 BTU HR for Minneapolis, MN (don't have climate data from Meteonorm for Wahpeton) Expecting that to rise once I buy the data set and run the numbers in PHPP
I'm using 3.0 ACH because of reading an article by Joe Lstiburek on how that is an easy number to achieve after sealing up all the BIG holes.
What would a safe number of BTU/HR heating demand be if I'm looking to use the 15RLS3H (not the 12RLS3H) as my only heating source? It has a heating capacity of 18,000 BTU.
Matt,
The Fujitsu 12RLS3H has a nominal heating capacity of 16,000 Btu/h, so it's going to work for you on all but the coldest nights. I suggest that you put in a few lengths of electric-resistance baseboard to use on the coldest nights, and I think you will be fine.
The min-modulation of the 15RLS3H is the same at the 12RLS3H, and delivers about 15,000 BTU/hr @ -15F, which is pretty close to your 99% outside design temp. Fargo's 99% design temp is -17F and they're north of you. St, Cloud MN's design temp is -14F, Minneapolis is only -11F. see: http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Outdoor_Design_Conditions_508.pdf
When you state, "At an ACH of 3.0, the heating load is 16,500 BTU HR for Minneapolis, MN..." what are the indoor & outdoor design temperatures you are using? What tool are you using for the calculation (PHPP)? Is 3ACH the ventilation rate, or the tested air leakage @ 50 pascals (and thus failing PHPP limits)?
The difference in cost between the 15RLS3 and the 12RLS3 is less than the installed cost of a couple of electric baseboard heaters, and with the same minimum modulation there's no reason NOT to go with the bigger unit. It probably WILL cover your load at -15F or even quite a bit lower. That 16.5K @ -15F seems high for the house you are describing, unless you have a quite a lot of window area.
Response to Martin #3
Martin,
I've thought about putting in some electric-resistance baseboard heaters to back up a mini-split system but that is one of the reasons I messaged this boards, I don't want to deal with furniture or beds butting up against them. I'm just leery that the RLS3H models will be able to handle my climate. So if you know of anyone that has installed them up here and uses it throughout the winter to heat their house, that would be the best confirmation that I'm in the clear.
Response to Dana #4
Dana,
What is min-modulation?
Where is it stated the 15RLS3H has a heating capacity of 15,000 BTU/hr @ -15F? I see that the heating capacity is 18,000 BTU/hr on the spec sheet?http://www.fujitsugeneral.com/PDF_06/Submittals/15RLS3HSubmittal.pdf
The indoor and outdoor design temps are 70, -8 and -14. I'm using PHPP. The ventilation rate was figured by using BSC new standard. The infiltration rate is 3.0 ACH @ 50 pascals. I'm using that PHPP as a personal guide, not trying to achieve certification. I'm using 3.0 ACH because of reading an article by Joe Lstiburek, say that it is really easy to get to 3.0 ACH once all the BIG holes are sealed. I'm going to seal like it is going to passive although, just wanting to know worse case scenario right now. May only need a 9RLS3H once the blower door test is done.
For the electric-baseboards I totally understand. Window area is 74.5 SF South (6.8% of Total SF), 0 SF North, 11 SF East, and 18.5 SF West. The windows I plugged into the PHPP are Inline Fiberglass windows out of Canada. Don't know if my numbers are correct though. When I compare them to the ones which come preloaded in the PHPP software, they seem very far off. There is a comparison between my numbers and the certified windows in an attachment. Just sent out for quotes, hopefully they come within budget but I don't think so. Know of any others that would be cheaper with just about as good of quality?
Also I haven't done any thermal bridging calculations in the PHPP, I don't know where to even start for them. Are they that complicated and worth the time and effort or are they not important?
Matt,
If you don't like electric-resistance baseboard, buy a couple of portable electric-resistance heaters and store them in a closet. You'll only have to bring them out one or two days a year.
Martin,
Thanks that will work with me.
"min-modulation" is "minimum modulation".
The linked to sheet is the HSPF test submittal page, not a complete specification for the product. The HSPF test requires that the minimum-modulation level at 47F be tested, and that number IS in the submittal pages.
In the "Capacity" section there is line titled "Min - Max Heating", which is tested at +47F. The 15RLS3H can modulate between 3100-23,900 BTU/hr at an outdoor temp of +47F, and an indoor temp of 70F.
The "Nominal Heating" is the modulation level at which it's efficiency is tested at both +17F and +47F. (Thus the unit has to be able to deliver at least that amount of heat into a 70F room at an outdoor dry bulb temp of +17F.) Most mini-splits will be able to deliver more than the specified nominal, even at +17F, but not a whole lot more. Maxed-out the 15RLS3H will deliver about 20 KBTU/hr @ +17F, but at a lower efficiency than the HSPF numbers indicate.
The HPSF test does not require testing or specifying capacity at temps below +17F, but those capacity numbers are in the extended temperature capacity tables, usually with a range of indoor & outdoor temperatures listed.
I don't have a link to the capacity tables for the RLS3H series but they are the same or nearly the same as the RLS2H series (it's somewhat less efficient predecessor.) The heating capacity tables for the older 15RLS2H can be found on page 16 (PDF pagination) here:
http://bangorwinsupply.com/wp-content/uploads/2014/02/RLS2H-Design-Technical-Manual.pdf
According to the table, at a dry bulb outdoor temp of -15F, and an indoor temp of 70F they are specifying it will deliver 15.0 KBTU/hour, but at a room temp of 60F it will deliver 15.8 KBTU/hr, and at 65F it makes 15.4 KBTU/hr.
So at 68F (the code specified minimum requirement at the 99th percentile temperature bin) it can do at LEAST, 15K, but not a lot more than that.
An infiltration rate of 3ACH/50 is literally 5 times (500%) of the maximum leakage allowed for PHPP. What is the heat load calculated to be at zero infiltration? Hitting 3 ACH/50 is more than a stripe on the floor, but it's not much of a hurdle. It's pretty common for simple shaped house designs to hit in the 1 ACH/50 range when it's being built with air-tightness in mind, and under 0.5ACH/50 for the air-sealing-obsessed.
The thermal bridging matters, and must be factored in to come up with consistent results. How you get to a nominal R48 can matter a lot. Building a 2x12 studwall 24" o.c. with R38 batts and R10 foam on the interior or exterior to get to R48 would have a LOT more thermal bridging than a 2x4 16" o.c. double-studwall with a 13" of cellulose between the gypsum and the sheathing, since the R1.2/inch wood is about 20-22% of the total wall area in a 24" o.c. studwall.