Is Manual J really necessary?
Preparing to line up a heat-pump install.
It’s quite clear that a manual J is the way to go, but seeing as its well out of my wheelhouse (and apparently of most installers too- I’m in central Maine) I’m wondering if circumstances exist where a manual J really isn’t needed.
The situation is an isolated room:
-3 exterior walls
-around 540 sqft
-insulated with an 11 inch double stud wall, R60+ ceiling
-No blower door test, but undoubtedly very well air-sealed
The plan is to install a single mini-split unit to heat the room. Cooling bonus.
It seems the purpose of the heat load calc would be simply to determine the size of the unit. Is that all?
Am I totally misunderstanding that there are basically only 3 sizes of ductless splits to choose from?
I get the mentality here at GBA to test everything and be super accurate, especially in more complicated/whole house situation, but I don’t really understanding what all this data crunching is doing for me in this situation. Am I really likely to find that a manual J would tell me I need a different size unit (9, 12, or 15) than a rougher go at it? Are there other install parameters that I should be tweaking based on a more accurate load-calc?
edit: I am open to (and likely) to play with an online free ‘load-calc’ to get in the ball park. I’m also not opposed to a little pencil work if its worth the time, but as Martin put it, it’s ‘Not recommended’ and not as accurate so…
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If you don’t do a manual J and you let your installer select the equipment they will most likely be sized by the local rule of thumb something like 700 sqft per ton. The rule of thumb will work OK if you house has leaky single pane window, almost no insulation and 20 ACH like most of the house they work on.
Being oversized is more of a problem when cooling than heating. Will you do much cooling?
If we guess your tight insulated room and use a high performance wild guess 1500 sqft per ton at 500 sqft we could guess you need 1/3 of a ton. My guess is you are unlikely to find anything under ¾ ton
Walta
fundamental heat load calcs are basic math
square feet x delta T over r value=BTU
with the insulation you mentioned it will show that the smallest mini split is too large
I think there are 6k btu mini splits available now, if you cannot get a contractor to do a manual J, demand they put in the smallest available or prove why they need bigger
"[If] you let your installer select the equipment they will most likely be sized by the local rule of thumb something like 700 sqft per ton."
Yeah my plan was to have a sense of what I needed (whether it was based on manual J, napkin math, or common sense) and not rely on the installer thumbs.
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"with the insulation you mentioned it will show that the smallest mini split is too large"
Yeah this is essentially why I was scratching my head about the need for an extensive calculation. I figured since we're not ordering up custom sized equipment, I would pretty clearly fall within the category of 'smallest equipment available.' Hence not understanding why I would need to run a manual-J or the like, for this situation.
I guess a follow-up question (for myself to ponder at least) would be whether I should consider options besides inverter driven heat-pumps (I'm zone 6a). My preference is not to install gas and I was trying to avoid adding any more load to the existing oil boiler, but perhaps it would be more economical.
It is attached to an existing structure that is heated with an oil boiler- hydronic baseboard delivery. I did contemplate adding a pipe (new zone) that punches through the shared wall. I would obviously have to consider if the boiler is up for it, but my instinct is it would be...
Thanks for the thoughts
Tyler, I'm also in central Maine (Palermo) and a cold-climate heat pump in our climate zone is an excellent match for your type of structure. While a Manual J calc is always a good idea, it is not currently required by our state code and your situation is about as simple as it gets. Assuming your floor is also well insulated and you have pretty good windows, a 6KBtu cold-climate heat pump is probably all you need. (I like Mitsubishi, installed by Revision Energy or The Breathable Home.) But the only way to know for sure is to do a Manual J calculation. Revision might even do one for you if you ask them to.
Note that people need fresh air and if you don't use a HRV, that adds a large load. Windows also matter. Also account for actual worse case weather creating a significantly higher load than design day.
Thanks Michael, I appreciate hearing your thoughts given your experience and proximity to me. I'm in Gardiner.
Jon, I haven't locked down ventilation strategies yet, but that's good to think about. I was considering either exhaust only or possibly something like a ductless lunos.
I'm in this grey area where the purpose/use of the room will be relatively low load for me (used as shop/studio) but would likely be sold as living space. I've obviously designed more of a full on living space with my build (well insulated and tight), partly because I'll use it a lot in winter, wanted the sound deadening, and so that if it's sold as living space it'll be good to go (wicked efficient). Now I'm debating how to balance that with things like ventilation, heating...
Tyler, I'd recommend Lunos E² ventilators. A single pair will be on the low side of recommended ventilation rates, so you might want two pairs. They aren't inexpensive but they are simple to install, work well and will be more comfortable and energy efficient than an exhaust-only system. Will you be using your studio for activities that would warrant additional ventilation?
Yeah I was eyeing those E^2. They look ideal other than the high $ tag.
It's primarily a wood shop, and I will occasionally be applying finishes, so having good ventilation would probably be prudent. I was toying with the idea of a sort of vent hood set-up in a corner of the shop (enclosed?) kind of treating it like you would a cook-stove in a kitchen. I don't want to make it over complicated or expensive, but do care about my respiratory health...
I won't be using spray finishes or anything like that. Really nasty operations I'll reserve for outside.
I don't know if you have extra some solar gain in the design so extra BTUs to spare, the cheapest in that case is open windows.
There is the WhisperGreen ERV (bath fan with an ERV core built in) that is not that much more than a nice bathroom fan here.
A couple of companies that make through the wall HRV/ERV systems, don't know how they compare as I've never priced it out:
http://www.cyclonerangehoods.com/hrv-and-erv/sv130/
Lunos aren't cheap, but they compare favorably with better quality ducted HRVs. Exhaust-only will always be less expensive initially, but that approach can depressurize your house, depending on airtightness, and you don't know where the makeup air is coming from. But if first costs are most important you can give exhaust-only a try. It's easy enough to retrofit in Lunos units if you decide they're necessary, assuming you have access to run an electrical cable.
https://www.coolcalc.com/ is super easy to use and free for homeowners (if you don't need a printout for the inspector, like $10 if you do). With just one room you'd be done before you re-read this thread :) It will account for ERV flow as well.