Furnace/AC selection and Manual J review
I have been working through plans for insulating a 3rd floor walk up attic space and am now considering replacing the 80% gas furnace located up there that serves the second and 3rd floors. We are north of Boston.
Details here: (Summary – total R for insulation and framing is R35.4 roof, R18.9 walls)
We will also likely be ccSPF the rim joist between the second and 3rd floor. 2nd floor is 2008 2×4 construction with R13 fiberglass batts. The house is 27′ x 38′. Vinyl Harvey windows (Harvey rep said U=.33, low e, but couldn’t find spec sheet). I am planning around 50 cfm exhaust only ventilation at this time. I may look to hrv/erv down the road.
I used CoolCalc to do a manual J and tried to make it as accurate as I could (see attached).
Heating BTU 28,681
Cooling BTU 17,013
SHR: 0.82
Does this pass a gut check? The big unknown is how much infiltration we will have. The 3rd floor will be tight and an okay 2nd.
The 3rd floor will be currently used as a storage area, so I am thinking I will adjust the balancing to under condition that space a bit. This will only happen if the duct work can support this rebalancing without meaningful increases in ESP.
What should I be looking at for brand/models of furnace and AC? Should I go with a two stage or modulating setup? An ECM motor seems a must. I understand manual S. I just don’t know what is out there in terms of available equipment. My electric rate is $0.169 kWh and gas is around $0.99 therm.
I have someone coming tomorrow that installs Trane and Bryant systems, so we will see what they offer.
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How many total square feet of conditioned space? It looks like 1500' per floor, is that just the 2 floors (3000 square feet), or three?
For a second opinion try loadcalc.net. Both of those tools tend to oversize a bit, relative to professional tools but comparing the two might uncover something.
A 3 ton Carrier Infinity w/Greenspeed or the Bryant Evolution Extreme equivalent can handle those loads reasonably, with an as-used HPSF greater than 10 BTU/watt-hour averaged over the season. Dialed-in perfectly it can hit an HSPF of 12 or 13. Click on the "Heating Capacities" tab on this page:
http://www.tools.carrier.com/greenspeed/
You're really at a buck a therm, all in, delivered price? (In central MA it's running closer to a buck-fifty this year.)
Assuming an as-used HSPF of 10 every kwh delivers 10,000 BTU/hr so it takes 100 kwh per million BTU (MMBTU) at a cost of ~$17.
Assuming 95% efficiency a condensing gas furnace delivers 95,000 BTU /therm so it takes 10.5 therms/MMBTU (plus air handler power) at a cost of $10.50. That's a pretty big difference, but it doesn't air condition.
D0 you have a condensing water heater? If yes, how big is the burner?
A hydro-air coil air handler with a 1.5 ton cooling coil for AC can handle that too, eg: The FirstCo 18HBQB can deliver 33,400 BTU/hr at an entering water temperature of 140F, and has a 1.5 ton cooling/heat-pump coil.
https://www.firstco.com/documents/ProductDocuments/hbqb718.pdf
The 9K of infiltration load seems a bit high- it implies something like 138cfm of natural air leakage:
138 cfm x 60m = 8280 cf/hr ...
...x 61F x 0.018 BTU/ per cubic foot per degree F= 9091 BTU/hr
If it's pretty tight the reality could easily be less than half that.
The U0.077 implied by the load per square foot in the summary seems a bit low for a 2x4/R13 wall, more typical of a 2x6 wall. Your post above suggests both 2x4/R13 and R18.9, so which is it? (both?)
Thanks for the response, I was still thinking gas furnace, 40k btu is going to be on the big side still.
The load calc is for the upper two floor of a 3 story house. The footprint is 27x38 plus a 4x12 dormer section off the east facing front so 1074 SF/floor.
I think you are correct about the gas pricing being more like $1.50/therm. My Oct. bill was about $0.99, but rates go up in November. This brings gas to $15.75/MMBTU.
Water heater is a 50K BTU power vent.
I was using their "semi tight" setting for infiltration, going to "tight" knocks the infiltration load down to 3509 BTU (22,878 BTU total). I can use blower door ELA as well. Any guess on what I should plug in? The ducts will all be in conditioned space behind knee walls and in the floor between the 2nd and 3rd story. Should duct losses still be counted?
The second floor walls are 2x4 R13 as built in 2008. The 3rd story walls are R25.8 ccSPF/Rockwool with 1.5" ZipR furring (18.9 total at 20% FF). Looks like they use U.091 and U.055 respectively.
138 CFM for the Manual J natural infiltration rate sounds plausible, but the ventilation BTU appears to be ~1/2 of what I get (50 CFM * 61F * 1.08). You should do the math as below:
https://higherlogicdownload.s3.amazonaws.com/ACCA/c6b38bda-2e04-4f93-bd51-7a80525ad936/UploadedImages/Infiltration%20per%20Blower%20Door%20Test%20Oct2016.pdf
The 2 stage Goodman GMEC960303AN (30K in , 28.8K out at high fire) will PROBABLY cover it.
At a buck-fifty the gas vs pretty-good heat pump operating costs are within 10% of each other- an HSPF 13 heat pump will beat the gas burner with a bit of margin margin. It wouldn't be insane to think about two mini-splits, one per floor.
A 3 ton multi-stage, or modulating ducted heat pump (with some amount of auxiliary heat strip to cover the Polar Vortex events) would cover it too, but that's usually more expensive.
Are you re-using existing ducts? If yes, are they zoned by floor (with dampers?) The Fujitsu -xxRLFCD mini-ducted units have pretty good capacity down to -5F outdoors, and enough blower power to use ducts from a previous oversized furnace. (The 1.5 tonner could cover 3/4 of your load on it's own.) HSPFs are in the 11-12 range for that series. They're pretty flexible- can be installed either vertically or horizontally, and would take up less space than a condensing gas furnace. A pair of them might be comparable to the footprint & volume of a 30-40K condensing gas furnace, but they can tuck in behind kneewalls, the tops of closets, or vertically in a 5-8 square foot micro-closet.
As lousy as ratio based rules of thumb are, as a sanity check 28,681 BTU/hr into 2148 square feet comes out to 13.4 BTU/hr per square foot of conditioned, space at a delta-T of 61F is at the center of the gaussian distribution for tight 2x4 construction, which is ~15 BTU/ft^2 @ 0F outside, 70F inside (a 70F delta-T).
15 x 61/70= 13 BTU/hr per square foot , within a few percent of 13.4 BTU/hr per foot.
HVAC hacks in our area often spec equipment at 25 BTU/hr per square foot, which nearly always oversizes by a substantial amount. (I've even run into a few using 35 BTU/ft^2).
Jon R I think you are right about about the ventilation load from cool calc, must be bug, I doubled the ventilation to make it work.
I ran loadcalc.net and am getting similar numbers by paying attention to the actual infiltration rate each program is using. I'll post that in a bit, but I want to circle back to basics and ask what should I be using as outside design temps for Reading, MA.
The 99th percentile temperture bin is somewhere between Lawrence's 0F and Boston's +12F, probably within a degree of Framingham's +6F, but probably warmer (closer to the ocean) than Framingham. Try +7F.
https://articles.extension.org/sites/default/files/7.%20Outdoor_Design_Conditions_508.pdf
https://www.captiveaire.com/CATALOGCONTENT/FANS/SUP_MPU/doc/Winter_Summer_Design_Temps_US.pdf
The generic design temp for Middlesex county is +1F, but that's based on weather data from Lowell, which is substantially cooler than Reading.
https://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/County%20Level%20Design%20Temperature%20Reference%20Guide%20-%202015-06-24.pdf
I am intrigued by the option of a pair of minisplits. I looked, but didn't find an answer to how the Fujitsu -xxRLFCD perform as the temperature dips below 5 deg. Do they continue to operate at reduced capacity or shut down?
How do the -xxRLFCD Fujitsu units perform compared to the Mitsubishi Hyper Heat or the similar Fujitsu low temp models. I looks like the combinations that can be ducted as either single or two zone in 30k BTU range are running HSPF below 10.
The Fujitsu units keep running at any temperature, no matter how cold it gets. Efficiency and capacity fall off with temperature, but it doesn't fall off a cliff at some temperature.
The coldest temp at which it has a specified capacity is -5F, at which point the output is always more than the "rated" cooling capacity eg: The 1.5 tonner is good for 21,600 BTU/hr @ +5F, 19,000 BTU/hr heating @ -5F, the 1-tonner is good for about 17K @ +5F, 15K @ -5F, etc.
The extended temperature heating capacity tables for this series starts on page 15 (PDF pagination):
http://bangorwinsupply.com/wp-content/uploads/2014/02/Single-Ducted-Design-Technical-Manual.pdf
So after running through a bunch of scenarios I have determined that coolcalc.com is a bit buggy if you go back and edit things and it doesn't always update correctly.
Loadcalc.com seems pretty good. After wrapping my head around the load calculation, it turns out it is pretty simple for the heating load. I built a spreadsheet that closely matches loadcalc.com for the heating calculation because it would allow me to use my exact window U value and look at possibly under conditioning my third floor. The third floor will be storage and sometimes a work space.
Heating the 2nd and 3rd floor to 70f with an outside design temp of 6f, I get a heating load of 20961BTU. Dropping the 3rd floor to 50f indoors saves about 2000BTU. The infiltration load is a bit of a guess still (3200 BTU). Cooling load at 86f comes in about 8500BTU.
A Fujitsu ARU18RLF seems like the lowest cost install option and almost covers the load. Is this a reasonable option given the expected load? How much heat strip would I expect to install?
Should I look at any other options with these numbers?
Since there's usually a double-digit percentage in the loadcalc.net numbers the 18RLFCD looks pretty ideal. Interpolating the tables at +5F and a code min 68F indoors it would cover your calculated load with a hint of margin.
Also, since loadcalc doesn't discount from the heat load for power use such as lights/computers/ DVRS/refrigerators, or body heat (0.23K per sleeping human, 0.34 BTU per watt of power use) it probably has more margin than you think.
Daikin makes a similar 1.5 tonner, but it's capacity craps out sooner (better for warmer climates than yours) it's modulation range is narrower, and the blower specs are wimpier.
There are no heat strip options for mini-duct cassettes. If you need auxiliary heating it won't be controlled by or coming from the mini-split.
There is only one temperature control for the 18RLFCD- the room temp is either sensed inside the cassette which tracks the entering air temp (this is the default mode), or it can be programmed to track the temperature at the wired remote (which looks a bit like a thermostat, but only behaves as a thermostat when programmed to use the sensor in the remote.) Controlling the upstairs to a separate setpoint (such as 50F) may be do-able with a zone damper, but I've never seen a mini-duct cassette system set up with zone dampers.
For more money, lower comfort, and lower efficiency a 1.5 ton Mitsubishi MXZ hyper-heating multi-split compressor with a 1-ton MVZ air handler for the 2nd floor zone and a ductless head or mini-duct cassette for the 3rd floor would get you there. The MVZ air handlers won't modulate the way mini-duct cassette will, but there are heat strip options, if that's important to you. (The ductless head won't modulate when married to a multi-split compressor either.)
http://meus1.mylinkdrive.com/files/MXZ-2C20NAHZ_Submittal.pdf
http://meus1.mylinkdrive.com/files/MVZ-A12AA7_For_MXZ_MULTI-ZONE_SYSTEMS_Product_Data_Sheet.pdf
http://meus1.mylinkdrive.com/files/OptionalElectricHeatKit_ProductDataSheet.pdf
The advantage of this approach is the more powerful air handler and zoning by refrigerant valves/lines rather than zone dampers. The down side is n0 modulation, resulting in on/off cycling, somewhat lower as-used efficiency. There is also some amount of refrigerant zones even when only one zone is active- you may not be able to get the third floor all the way down to 50F since it's being partially heated even when "off".
Cost-wise it might be cheaper to install one mini-split per floor than the multi-split solution, to get the clear zoning separation with no interaction:
A 3/4 ton Fujitsu 9RLFCD would cover the 15K calculated load of the 2nd floor with some margin, the 12RLFCD would cover it with quite a bit of margin.
A 1/2 Mitsubishi FH06NA would cover the ~6K load third floor zone, and would be able to modulate down to 1600 BTU/hr for ultra-high seasonal efficiency. For a couple hundred more in equipment cost 3/4 ton FH09 goes that low too, but it's not buying you anything really.
https://nonul.mylinkdrive.com/files/MSZ-FH06NA_MUZ-FH06NA_ProductDataSheet.pdf
https://nonul.mylinkdrive.com/files/MSZ-FH09NA_MUZ-FH09NA_Submittal.pdf
Since "The third floor will be storage and sometimes a work space" you might consider punting on third- install a 1-ton or 1.5 ton mini-duct cassette system for the second floor, and MAYBE a manually operated vane for the third floor duct. It may be a bit silly to add more expense & complexity to the system just to cover the intermittent use of that floor. You can do a LOT of temporary/auxilliary heating with a $50 1500W oil-filled radiator (5000 BTU/hr) type space heater if it isn't being used regularly for work.
Alternatively a half-ton point terminal heat pump (PTHP) with internal heat strip for the third floor would cover it at a lower installed cost than a mini-split. With the wall sleeve you're talking about a grand for one of those, but if the construction work isn't DIY it may be approaching mini-split cost. Most have wall thermostat options these days.
Or maybe even lower cost mini-split such as 1 ton Midea Premier Hyper, which is good for ~6.5 K @ +5F
https://d11fdyfhxcs9cr.cloudfront.net/templates/32505/myimages/midea/mehs-12psh1--mchs-12psh1.pdf
Dana-Thank you!
"Since there's usually a double-digit percentage in the loadcalc.net numbers the 18RLFCD looks pretty ideal"
I think a word got left out, but you are saying loadcalc.net results are high. I corrected for some of this with my own calculations using the correct window u value. Infiltration is still the big unknown and hopefully estimated high.
I am generally thinking along the lines of punting on the 3rd floor for now. Design/rework the existing ducts to work with the .36" static and then add some manually dampered supply and returns for the attic. If we end up using wanting the 3rd floor as proper living space, I can put in a small split system.
The ARU18RLF appears to have a auxiliary heat output control (page 34). I need to find out if the on/off parameters are adjustable as the example they have seems a bit funky. I need to look to see what a duct mounted aux heat setup would be like.
My other option is to connect the furnace in my basement to the 2nd/3rd floor ducts. This would involve a fair bit of duct rework, creating a new vertical chase, and a zone control setup. I am thinking the single upstairs heat pump might be a better option, although it doesn't fix the oversizing of my 1st floor system.
I've attached a plot of my load vs capacity chart for the 18RLFCD.
I should be able to heat both the 2nd and 3rd floor to 68F down to about 7F outside.
"Under conditioning" the 3rd floor and allowing to fall to 40f by closing the "dampers" allows going down to about 2F.
Do the published capacity numbers include defrost cycling?
It will be interesting to see how this works in the real world.
I would keep a few space heaters on hand in case things get unusually cold. I can also encourage air movement up from the gas heated first floor.
The ducting to connect this thing and stay within the statics will also be fun to tie into the existing trunk system. It's going to be some custom fab smooth curving stuff.
I don't hit the min heating load of 3100BTU until the high 50'sF. I haven't actually run the cooling side in detail. With the blower running so much, I am thinking about a fresh air duct in the return with an Aldes constant airflow regulator for ventilation. It seems like it would cover a large portion of the year on the heating/cooling sides. Concentrating the cold/hot intake air in the return would provide some additional efficiency vs diluting it and then conditioning it in a exhaust only format. Some relay setup could kick on my existing exhaust fan when the blower isn't running.
I am also comfortable getting a little more capacity by shutting down ventilation for a bit when it gets really cold.
Thanks for the heads-up on the aux heat control for that unit- I never knew it existed. I've never seen aux heating strips advertised for that series- so it may have to control some other equipment (?).
"...double-digit percentage in the loadcalc.net number.. " should have read "...double-digit percentage MARGIN in the loadcalc.net number...". (I need and editor, eh? :-) )
>Do the published capacity numbers include defrost cycling?
The wet bulb and dry bulb conditions are usually spelled out in the fine print in the capacity tables (it's noton that chart? I didn't re-look), but under saturated humidity conditions you'd have to discount it for ultra-high humidity & low temp. If you go with the 1.5 tonner it won't need auxiliary heat to cover the second floor, even under rime-icing fog type defrost conditions. That type of weather is actually pretty rare at your altitude & location (but happens regularly on top of Mt. Washington NH.) With your load corrections it might not always cover the combined load of both the 2nd + 3rd floor, but are you really going to be working up there at 5AM on the coldest night of the year?
I like your graph, BTW.
So I am updating this with some real world data following installation of ccSPF on the walls and roof deck of the attic space, which sealed up the existing soffit and ridge vents. We are working on getting the batts and furring up, but it has been slow going with kids. With just the ccSPF, we have about R11 on the walls and rim joists and R21 on the roof. Details of the plan are here: https://www.greenbuildingadvisor.com/question/attic-insulation-details
I have some real world load data from our Ecobee logs. The existing furnace is an 80% 100,000 BTU serving the second floor (R13 2x4 walls) and attic. Data was analyzed by calculating the daily total run time and average daily indoor/outdoor delta T. Those numbers were then used to calculate the daily BTU/Hour/Delta T shown in the attached graph. The load is about half what it was prior to the insulation going in, while the attic is now about 40 degrees warmer. I expect finishing the insulation and drywall will drop this another 20% or so.
Even without the insulation finished, the load at a 6F design temperature looks to be about 15k BTU/hour based on the averaged daily numbers and just under 20k BTU/hr based on the 99% (2.5 sigma) outer bound. Finishing the insulation should make a ducted minisplit very viable to heat BOTH the 2nd and 3rd floor.
The load numbers surprised me as I was expecting to be around 27k BTU/hr at 6F based on the current state of construction in a manual J calculation. This is a second floor, so I am suspicious there is a meaningful stack effect contribution from the first floor. The amount of daily variation was also surprising, but could be explained by differences in solar gain and wind. The furnace run time also maxes out around 6 minutes due to oversizing and I am not sure of the resulting efficiency impact. I also haven't accounted for any furnace electrical use or related heat contribution.
I kind of stumbled into my approach of binning daily runtime and average delta T. I don't actually know the best practice for this, so I am open to input. I have attached my data if anyone feels like poking around.
The furnace run time isn't going to have much impact on efficiency until/unless it's short-cycling with 1-minute burns or something.
Your binning methodology is probably going to be more accurate than most load calculation methods, but could be refined if you can also correlate wind speeds & sunshine, which would allow you to tune out the solar gain factor, and add in some wind factor.
Either way it doesn't look like you'll have any problem at all heating both levels with just one Fujitsu. Even the 1-ton would handle a 15KBTU/hr @ +6 load, but the 1.5 tonner is still the preferred size here, with enough margin to handle some serious cold while still less than 1.5x oversized at the 99% load.
Thanks Dana!
Any idea where to get daily wind/solar data?
General plan is to finish up the insulation with at least a month of cold weather to get good data and then finalize the HVAC plan.
I am also thinking about where to put the outdoor unit. My existing AC unit sits on the west side of my house under the eaves. I am thinking I want to move it to the gable end so there is never anything dripping on it. The preferred end would be the north side, which also gets it away from our patio. Is there any meaningful efficiency impact on outdoor location, such as putting it on the south side?
Along the same lines is there a secondary pan heater available that you can put on these?