Trying to figure out proper heat loss calculation for mini split install
I have an all electric 1968 cape cod with electric baseboard heat. I have been looking into either installing duct work with a furnace or installing a mini split or multiple mini split units to provide whole home cooling and virtually whole home heating.
Ive read many articles and questions on here about minisplit installs as well as Martins latest article “Getting the Right Minisplit”. And realize many mistakes or gross thumb errors some HVAC techs. have when figuring out equipment sizes. I’ve had 3 different companies here and two have recommended a pair of Mitsubishi systems each with 3 heads. (They both are recommending a head in every room). The 3rd said a mini split isnt right for my home. No certified Fujitsu installers are in the local area. Both systems combine to about 60k-66k btu heating loads.
Id like to double check their work, but am seriously struggling either using or figuring out how to properly use a Manual J. I tried loadcalc.net, and have looked a little at coolcalc. But I’m new to this and some of the information isn’t making sense. On load calc they are asking for “Outside wall: E &W” and “Outside wall: NE & NW” among other values I don’t understand. Are they asking for the sum of ALL NE and NW sides? Why wouldn’t they just ask for all 4 sides entered?
I’d really like to have a mini split system (or partial system) installed before this winter for my family comfort and my wallet comfort of electric heat.
ANY help on anything would be appreciated.
Thanks,
Nick
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I'll also be interested to see what anyone says--I'm a homeowner in Boston area going through this process myself right now. The first of the three HVAC contractors I spoke to this week started right off by saying he would do a Manual J, and suggested more varied design options than the others did (including one option with a ducted system using a Mitubishi MVZ air handler for the two upstairs bedrooms in our small house)--the others were all, "a head here, a head there, another one over there..."
Having also read Martin's article, I'd be interested in doing the calculation myself to sanity check their proposals, and it seems like Cool Calc would make that possible.
Coolcalc doesn’t appear to do heating loads. Although I haven’t dove into it enough yet. But, the name itself appears that it would only do cooling. I’m more interested in the heating aspect.
Also worth pointing out: Martin wrote on this subject previously!
https://www.greenbuildingadvisor.com/article/how-to-perform-a-heat-loss-calculation-part-1 and https://www.greenbuildingadvisor.com/article/how-to-perform-a-heat-loss-calculation-part-2
I had read that article previously, but thanks. As always, Martins articles are worth reading a time or two. Even if My simple mind cant always comprehend what he’s saying. Lol
Nick
Does your heat happen to be on a separate meter? If it is, you can use that data to perform a load calculation. If you use lots of setbacks, this could be problematic though.
https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new
If you wait until it gets cold you could install a wifi thermstat that logs and use that data to get a very accurate heat load calculation. I am not super familiar with options to run baseboard heaters, but there are options.
loadcalc.net is pretty good. I find coolcalc.com a little buggy and things don't always update properly when you change them.
Loadcalc.net is looking for the sum of the area of all the walls oriented in the listed directions. If you are only concerned with heating, you can just lump everything in one line. Heat load calcs don't include orientation, only the cooling calcs.
If you really want to understand what is going on though, set up a spreadsheet and calculate the U values and surface areas for the windows, doors, walls etc.
That’s an interesting idea, and sounds like it could work. Where were you last winter! I’d like to get it done before this winter. But maybe not.
Of all the walls oriented in the listed direction? So when it says E & W, do I add up both sides of the walls square footage. I.e. the west side, and the east side, add them together, and put the result in type 1?
> Does your heat happen to be on a separate meter?
Even if not, you can buy an energy monitor and move it from circuit to circuit to get per room loads. I'd trust measurements, even for a few days, over an often inaccurate Manual J. Both is even better. Of course measurements need some cooler weather and you need to adjust for degree days.
You can also use Summer electrical data to estimate non space-heating electrical use and subtract it from Winter totals to calculate load (but whole house, not per room).
With inverter heat pumps the need for great sizing accuracy is overblown.
The need for sizing accuracy is overblown? Could you clarify?
Thanks,
Nick
> With inverter heat pumps the need for great sizing accuracy is overblown.
As in if they are somewhat over-sized, there is little comfort downside (and sometimes some upside). Get close and don't worry about it.
>"Get close and don't worry about it."
That's right, but the problem is the average HVAC contractor proposal I've seen in the Boston area aren't even close.
I've also seen HVAC contractor Manual Js in MA that weren't even close. Perhaps the worst I saw was on a high performance house with ~R50 double-studwall cellulose and U0.12 triple panes where EVERY input into the tool was IRC code minimum.
That’s what I’m worried about. I want it to be close, not off by 25-40%. I do worry about getting it too oversized where it can’t modulate in the shoulder seasons and is cycling on/off too often.
Nick,
The reason the Manual J calculators ask for specific walls is that solar heat gain will be different, depending on the wall orientation (and what windows went in those walls).
I used CoolCalc for my Manual J on our 1965 cape in Connecticut, and came up with low 30's kBtu for our heat load. 60k seems very high. Our cape is 1600-1700 sq ft, with 2 bedrooms and a bath upstairs in a full length dormer. We went with four 1:1 mini splits to get the low end modulating for shoulder seasons, and did a 9000 BTU for the living room and kitchen downstairs and a 6000 BTU for each bedroom upstairs. I've been thrilled having almost-central air this summer, and am looking forward to seeing how it heats this winter.
I also thought it was high. Both companies have suggested 15k in the living room and kitchen, 9-11ks in the two downstairs bedrooms, and 12k units in each upstairs bedrooms.
I would estimate my total square footage to be around 2200-2400 sq/ft. I am in the process of drawing up a detailed sketch of my house (I’m too detailed with everything, don’t ask) and I need to do the upstairs yet.
I have had the back of my knee walls sprayfoamed with closed cell foam, and underneath the knee walls. Also, I had both sloped ceilings opened up, sistered 2x2’s on the 2x6 rafters, baffles put against sheeting, sprayed foamed tight, then batts put back in. I need to have some blown in cellulose added to the ceiling yet.
I will never buy a cape cod again!! Haha
>I will never buy a cape cod again!! Haha
Same! Well, we love our land & location so the house is maybe a compromise :)
>I used CoolCalc for my Manual J on our 1965 cape in Connecticut, and came up with low 30's kBtu for our heat load.
Same w/ CoolCalc. LoadCalc.net generated a 50 kBtu estimate. O.O There's been skeptics writing about these load models and I've become more and more a skeptic.
Do you have actual energy usage? I had a winter's worth of LP fuel usage that Dana was kind enough to step me through using for real (non-modeled) calculations. Those calculations came at right under 30 kBtu, which is much closer to the CoolCalc #.
So I went with the 30 kBtu and sized based off of that. Actually, our multi-split (a 3-in-1) is 36 kBtu for the outdoor unit, so yes :sigh: a bit oversized by a 6 kBtu buffer -- BUT not tens of thousands of Btus that the HVAC contractors were wanting to size us on. THREE contractors thought the old 70 kBtu LP furnance was right sized! Hah!!
Our multi-split is half the size of our old furnance, with 12 kBtu heads in 3 rooms. This winter will be the test :)
Two different contractors have recommended two outdoor units totaling 60k btu. Sounds excessive. They both claim that it is high, but it’s to keep the house warm as the temp drops to -5 and the units are now around 70-80% as efficient so they over size by that much.
Right now I’m really contemplating one 36k unit with 12k heads in the living room, kitchen, and upstairs bedroom (18x20). I was thinking of having them install this, evaluate over the winter to see how it does and add another outdoor unit for the two small bedrooms if they get cold.
My wife also doesn’t like the mits mxz...36k hyper heat as they are TALL units and the contractor said would be on a 12” pad for snow drifts. The unit is going to be taller than me!!
>"Two different contractors have recommended two outdoor units totaling 60k btu. Sounds excessive. They both claim that it is high, but it’s to keep the house warm as the temp drops to -5 and the units are now around 70-80% as efficient so they over size by that much."
At -13F the Mitsubishi cold climate units have 70-80% of it's rated CAPACITY, not efficiency. 70% of 60K is 42K, which wouldn't quite cover the load of my load at -13F, but that's a temperature that only comes around once every couple of decades in my ZIP code. (It may become more frequent if Polar Vortex disturbances become more common though.)
>"Right now I’m really contemplating one 36k unit with 12k heads in the living room, kitchen, and upstairs bedroom (18x20)."
Think again! With an open floor plan a 12K head could be appropriate for a living room, but would be crazy oversized for most kitchens. Even a 6K head is usually oversized for a bedroom, but a very lossy bedroom with no insulation and a lot of window might take a 9K head.
When a head is oversized for the load and one or more other zones are more right-sized, be aware that with most multi-splits there is refrigerant running through the coils even when "OFF", when any other zone is active, which can let to overheating/overcooling of the space.
Run the design heat load numbers for those individual rooms, and try hard to not oversize the head by more than 1.5x.
A quick and dirty load calculation for a 2x4 frames house with clear glass double panes (or single panes + storms) would be to measure up the square feet of exterior wall in that room (minus windows and exterior doors) and call it 7BTU/hr per square foot of exterior wall, 35 BTU/hr per square foot of window, and just forget about other losses, which is about where it will be when it's 70F indoors and 0F outdoors. So if the 18' x 20' room is a corner room with 9' ceilings ad has four 12 square foot windows you have window losses of 48 square feet x 35= 1680 BTU/hr of window losses.
Then, 9' x (18' + 20')= 342 square feet of gross wall. Subtracting out the 48' of window leaves 294' of wall, for losses of 294' x 7 BTU/hr = 2058 BTU/hr of wall losses.
Add it up and you're at 3738 BTU/hr. No matter how thin the attic fluff or cool the basement below is, unless you leave a window open there's no way you'd have enough load to outstrip the ~8000-9000, BTU/hr that a typical 6000 BTU/hr ductless head can deliver:
http://meus1.mylinkdrive.com/files/MSZ-FH06NA_For_MXZ_MULTI-ZONE_SYSTEMS_ProductDataSheet.pdf
Still thinking of a 12K head for the bedroom?
There is no subsitute for running the load numbers first!
>"My wife also doesn’t like the mits mxz...36k hyper heat as they are TALL units and the contractor said would be on a 12” pad for snow drifts. The unit is going to be taller than me!!"
That's perhaps the least of the problem- it's probably significantly oversized capacity-wise.
In my area (central MA) it's better to wall-mount it with 4' of clearance to the ground. A mere 12" isn't a "drift", it's less than the average depth of a typical nor'easter.
Thanks for the information Dana. I really don't want to overspend or over estimate the system I need. I have tried using CoolCalc tonight a little more. The downstairs numbers come up with a heating load of 27,569 BTU, but the upstairs (2 large bedrooms and one bath) come up with some crazy 50+k BTU number. I have gone over it twice and can't figure out what is making it so high.
Here is my "professional" sketch of the 1st floor plan. I think id like a head in both bedrooms, LR, and Kitchen. Correct me if that looks wrong. I do get some crazy high winds in this part of Ohio in the winter. I was originally thinking 12k in LR, 12k in kitchen, 6k in each downstairs bedroom. But now I'm thinking of 9k in the LR and kitchen. That would give me 30k for the lower 1st floor. I wouldn't put anything in bath (electric baseboard) and in Laundry room (electric baseboard).
House is 2x4 R13 Batts, 8'celiings, All windows replaced with Sunrise Double Pane, Low E, Sliding door is Pella Triple Pane Low E. 10" Fiberglass Batts in the entire 1st floor ceiling. So the upstairs and downstairs have insulation separating them. I think they didn't finish the upstairs for a long time and left doors closed. I have spray foamed the back of the attic knee walls and underneath them also. I plan on adding Cellulose to the upper floor at some point.
I see that the 6k unit is capable of 8700BTU, but that is at what temp? +5F? When it does get -10 (seems like this happens more and more) what will it be putting out then?
I also get confused on if there are 3 heads hooked to one outside unit, what happens when one is calling for heat, will that head be able to put closer to the 8700BTU's out at say -5 than if all 3 heads are calling for heat at that time? I also can't find the low end modulating numbers for heads and if it is different on a 1:1 mini vs. a multi.
SO MANY OPTIONS!
Andris, what brand and model are all 4 of your units? I’m not sure my wife would like 4 outside units scattered across the landscaping. How do they look?
They are Mitsubishi MUZ-FH06NA's and FH09's - Pics near the bottom here:
https://www.greenbuildingadvisor.com/question/mini-split-indoor-wall-locations-1-5-story-cape
I ran the lines for the downstairs units through the basement and inside the walls so the outside wouldn't become cluttered.
>I'm not sure my wife would like 4 outside units scattered across the landscaping. How do they look?
Ascetic is another reason we went with a multi-split. Do the 1:1 mini-split crowd run all their lines to a central utility spot where all their Army of (3-5) outdoor units are sitting together? Or littered around the foundation at various drops? :)
>I ran the lines for the downstairs units through the basement and inside the walls so the outside wouldn't become cluttered.
Another aesthetic topic: exterior or interior line sets? One contractor was going to do interior runs for us, basically using our closets and basement as utility chases, while another highly recommended exterior, for both reduced labor of the install and maintenance of the lines over time. I went with the later, and their installer did a very professional job of channeling the lines together so it almost looks like real gutter downspouts, and chose the least visible exterior wall to run them; but I can tell the difference, and do NOT like the gutter looking lineset covers now attached to our siding. :(
This ascetic convo seldom comes up in GBA. Usually the 1:1 minisplit pro crowd only talks about the failover scenario, aka. a single exterior multi-split unit going down, but that begs the question: why do most homeowners not install multiple fossil fuel heat furnances? or multiple heat pumps for cooling? How is a mini/multi-split decision different from the risk homeowners have taken for decades now? I wasn't convinced.
It's good to hear your wife thinks about ascetics. She get our heads out of the ground for such things :)
Another great post and topic. One contractor will NOT install linesets on the interior of the house (basement). Said exterior only. That would leave me 3 line hide sets on each end of the house if he did all 6 rooms. Said the interior runs CANT be done. Another one said they do it all the time.
Do these linesets fail?
I really don’t want to be regretting my decision come this winter.
How much does Dana cost if you fly him to your house to design the right system? :)
Thank you everyone for the great replies. I need to keep trying to find an answer for how much, or big of a system I really need. I am in NW Ohio by the way. And in the country with NO windblock to the west. We get some very brutal winds here in the winter.
By most of the posts on this site, I really wish there was better access to Fujitsu equipment.
There is nothing wrong running line-sets inside the house or walls. The line sets do expand and contract a fair bit, so if you run them through a stud, you need to put a bushing to allow the line-set to move freely. For long runs you also need expansion jogs or loops, most manufacturers provide specs for these.
The installer also has be careful not to damage the line insulation. Exposed AC lines will sweat, which you never want inside the house or walls.
The harder part to deal with is the condensate drains, you have to be very careful on running these so that the slope is maintained all the way to the final drain location and no drooping at all along the runs. You can run these into the riser of the washer drain or into a floor drain in the basement.
As for look of multiple boxes outside, I don't think it is too much of an issue as long as it is out of the way. Overall, one or two correctly sized ducted mini splits (1:1, not multi split) is a much better option for most houses than a bunch of wall mounts. HVAC contractors seem to really dislike sizing and installing properly ductwork for these.
>"One contractor will NOT install linesets on the interior of the house (basement). Said exterior only. That would leave me 3 line hide sets on each end of the house if he did all 6 rooms. Said the interior runs CANT be done. Another one said they do it all the time.
Do these linesets fail?"
The linesets fail if you drill into a wall to hang your big screen TV and hit the line, eh? :-)
In MA they get routed indoors fairly often, but when you do it's safer to route them in dedicated
or open chases (such as open ceiling joist bays in a basement) rather than random partition wall cavities.
>"How much does Dana cost if you fly him to your house to design the right system? :)"
I'm not in the system design biz, but if it's in some nice backcountry skiing terrain and you'll put me up for the winter and throw a ton of money at me I'm sure I'll be able to fake it! :-)
>"Ascetic is another reason we went with a multi-split. "
.....
>"This ascetic convo seldom comes up in GBA."
That's because they're only talking about the "aesthetic" aspects, never "ascetic".
An ascetic system would be a woefully undersized system designed to encourage the occupants to meditate on the value of sufferering when it isn't keeping up, independently of how many compressors there are or what the indoor heads/cassettes look like. I'm not sure how a multi-split fixes that, or who would want to install an ascetic system in the first place. :-)
I've drilled into one drain pipe before for the upstairs bath. Oops. I will have the linesets CLEARLY planned out and marked and I will be the only one allowed to hang a picture in the house. Lol
NW OHIO does NOT have good skiing terrain, but MI is only 45 minutes away!
If only my local HVAC guys were as informed as Dana ( or even the “ civilians” contributing here)! For those who can’t find locals to do s good manual j, what about online services? Why couldn’t anyone do it online from information inputted online? Dana, you could do consulting without ever leaving your office ....
Also, everyone should check their state for rebates before making a mini split decision. In NJ for efficient systems you get $1000 per single head system, $2000 per multi and $2000 for ducted systems! This is why I am looking at ducted systems like the new mid static Fujitsu’s . A 12000 btu system should be able to handle three bedrooms with a short duct run....
Nick, what did you end up doing? Pics? Comments? I have a similar cape outside of Boston with plans for heat pumps.
The OP with the Cape is also named Nick as well so I assume you are asking this of him. However, in my case I did a cool calc manual j myself online ( it only costs a few bucks to finalize and print out). It is tedious but if you are careful in inputting the data there seems to be no reason why you can’t get good results. It is not complicated; calculations for cooling seem much more involved than for heating. I decided to go with the ducted Fujitsu’s but will not use much ductwork since the area I am working with is essentially open so the installer suggested that the ducted units would be fine (taking advantage of the larger rebate) operating from a single spot just like a wall hung unit. Thus Manual D calculations are not really necessary for me.
I haven’t pulled the trigger yet because In the middle of this my car finally broke down and so now I am trying to decide on whether I can afford an electric car. When I add up all the electric loads including a 7.7kwh car charger and mini split systems ( I already have heat pump hot water and stove etc) and run it through the NEC load calculators it seems to show my 200 amp system is not large enough! This is surprising so I may need to get an electrician involved to see if my calculations are wrong. But this does remind us that those who want to go all electric need to take service size into consideration and plan all of it together.
Thanks Nick D. I have similar plans for a future EV (had a large solar array just installed) so thank you on that feedback.