Mitsubishi Hyper heat cold weather BTU output

[Jim Sweazey]

New construction, high performance house. We are planning on using Mini- splits with a masonry wood stove backup.
The short version of my question is- where can I get the cold weather BTU output for a Model MXZ-3C30NAHZ2? I'll attach a file that is an example of what I'm looking for.
----------------------------------------------------------------------------------------------------------------------------
The long version is a more detailed description of the issue if anyone wants to read and comment.

Zone 6A New construction 7500 HDD.

We are questioning if our heating plan will cover our needs. The energy modeling, I did with Remdesign (.5 ACH) shows Peak heating load at 22 kbtu/hr. The manual j that was done shows heat load at 29,000.

The current plan is to have a 30K BTU Mitsubishi hyper heat Mini Split condenser with a 12K ceiling cassette (main living area), 12K Ducted (two bedrooms from a common upstairs hall), and a 9K wall unit (master bedroom). We have a large Tulikivi “fireplace” that would provide heat when the outside air is too cold for the Mini Split. We also put hydronic tubing in the basement floor and plan on the same for three tiled areas on the main level. We did not buy the boiler and manifold and only put the pex in for resale, and as a backup plan if needed.

So far, we are disappointed with the heat given off by the wood. The Tulikivi, by design, holds a lot of heat and radiates it out for a long time. My research says it should give off 10,000 BTU with one load and 20,000 with two loads per day. We haven’t given it an accurate test. The house starts out very cold and doesn’t have a ceiling, I need to get better at building the fires, and we should have more continuous fires. That said, the best we have done is radiant heat from the fire while its burning (an hour or two) and 160-degree surface temperature, on the stove, that only is felt if touched.

My question/concern is, does it sound like this plan will handle the cold Minnesota weather? I have found charts that show the BTU percentage efficiency as the ambient air temperature drops but am unable to find a source for this information for the models I will install. Does anyone know where this can be found? The additional heat I’m considering is to run pex to some areas of the house without connecting. If the current plan fails, I could install a toe-kick unit for the open kitchen/ living room and baseboard in a few other places. I think I could provide the hot water with an additional propane water heater. The current plan is to be all electric.

There are so many variables. I feel the most difficult to predict is how air tight the house will end up. My contractor keeps saying, that the HVAC is getting too complicated and thinks we should just install a conventional forced air system. My goal was to have the house so efficient that this wouldn’t be necessary and could rely on the benefit of zoned conditioning and wood burning.

Sorry this is so long but it’s hard to explain in a few words.

Thanks in advance,
Jim

 

[Dana]

Most Manual-Js will overestimate measured reality by a bit, even those that done "by the book" with aggressive assumptions.

The H2i Mitsubishis will definitely handle MN weather, with the caveat that at some point when its in the -20s F most will automatically turn off, and won't re-start until it warms up to -15F or so. The shut down temp in the spec is -18F, but treat that as a maximum- there are many field observations of them continuing to work at temps below -20F. (I haven't dug that detail out of the MXZ-3C30NAHZ2 though- perhaps they have changed that "feature" with the -NAHZ2 s. Consult with Mitsubishi.)

Note, those capacity numbers are "Heating capacity without any flost", which in Japanese-English really means "...without any defrost". If it's -13F with super high rime-icing humidity outside the defrost cycles will take a sizable bite out of the capacity cookie, but most of the time it won't.

If you have the Tulikivi backing it up (and you remember fire it and keep it stoked several hours in advance when the temps are predicted to go that low) there shouldn't be any issues with it keeping the place warm enough, but note, a load of 10,000 BTUs of wood needs three burns per hour to average 30,000 BTU per HOUR. Your house needs way more than 20,000 BTU per DAY. If your firebox is really that small you may want to invest in additional backup.