Advice on the Sizing of this Mitsubishi Heat Pump
I am installing Mitsubishi HVAC SYSTEM in new 4000 sqft house – 2 story and finished basement. We plan to use 2 x 36k outdoor unit.
does rain effect the out door unit performance? should I cover the top of the unit?
Can you please review below information and suggest if this system is properly sized and energy efficient? I am open to other ideas also.
Location: Secaucus, NJ
Building: single family, 2 story with open concept finished basement
Foundation: ICF basement foundation with R-26 in wall and R15 In the Slab
Framing: 2×6 stud wall with ZIP-R6 sheathing + R-23 rockwool insulation
Attic: vented, 2nd floor ceiling insulation-R38
we are planning to install air handler in the attic and then insulate around it to keep it in conditioned space.
Windows- double pan fiberglass, Windows U-factor is 0.31 and .29 SHGC
Should I install ERV or HRV for this location? installed standalone or integrate into air handlers?
Please see attached pdf and image for manual j calculations and zoning layout.
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Please see attached pdf and image for manual j calculations and zoning layout.
Heating load for an ICF basement where most of the wall is below grade strikes me as rather high. The bedroom loads strike me as quite high for a house with R-6 exterior insulation and R-23. A full manual J should show the average U value for the entire shell.
Even if the manual J is accurate, the outdoor units are oversized. I also don't know why you'd need two units in an open floor basement. Just seems like a waste of equipment.
Correction
outdoor units are 36k and 42k
I need to make a decision on this soon. Any help is appreciated
thanks
I agree with Patrick, those loads look high considering your insulation plan. The units are oversized even for the given loads and the indoor units are going to cycle a lot. In a multi-zone setup, your outdoor unit can only modulate down to roughly the rated capacity of the indoor unit that is calling for heating/cooling. If I recall correctly, the PEAD units do have a good turn-down ratio, but again only in single zone setup.
With respect to ventilation, I would go with an ERV. It will temper the humidity relative to an HRV. I am about 20 miles from you and my RH is currently 40% and has only moved a couple of percentage points from that level during the winter. You will see higher numbers in the summer and strategies to deal with that include turning down the ventilation rate (get an IAQ monitor before doing that) and employing dehumidification if needed. I maxed out at 55% this summer, which was less than what if would have been with an HRV and it was still very comfortable. Frankly, the worst days are when temps are in the 60's and it is raining outside. A stand-alone dehumidifier or running your heat pump in dry mode is a way to deal with that.
Thanks Jonathan and Patrick reply
This load calculations were perform buy Rookie HVAC contactor who is working for my GC.
I will get another Load calculation done by another Licensed HVAC Engineer and repost it here. I also had doubts about the results.
As far as the Mitsubishi mini-split ducted and non-ducted combined zoning setup, what combinations is more efficient and provide comfort?
Does Mitsubishi heat pump maintain capacity on rainy days?
thanks
>"This load calculations were perform buy Rookie HVAC contactor who is working for my GC."
Load calculations even from experienced HVAC contractors are usually full of errors. To be reasonably accurate all the input assumptions have to be on the aggressive side, but it's human nature for HVAC contractors to err to the conservative side and oversize the system(s) out of fear that it might not keep up.
A professional engineer or some other third party whose reputation is made on the accuracy of their numbers should be the ones running the load calculations.
Oversizing a modulating heat pump is bad for efficiency, and worse for comfort. The "ductless head in every room" approach is an invitation to poor performance on both fronts- consider combining nearby doored off smaller-load rooms into zones to be served by right-sized (for the combined load) mini-duct cassettes.
Even if you assume the newbie's calculations are correct, the total heat required (both floors combined) is ~44,000 BTU/hr, which at NJ type design temps could be covered by a single 3 ton hyper heating MXZ, and they're specifying a 3 ton MXZ PLUS another 3.5 ton N, simply to be able to accommodate the number of zones (?). This is a bad approach. For the first floor they're specifying two zones, a 1.5 ton air handler, and a 3/4 ton ceiling cassette, 2.25 tons total, yet they're specifying a 3.5 ton MXZ for that zone? (Kyu, main pata nain!) Even a 2 ton MXZ is way overkill for the 18000 BTU/hr load, and the 1.5 ton MXZ-2C20NAHZ2 would be more appropriate from a capacity point of view, but the 2 tonner may still be needed if the cassette sizes really need to be that big. (You can hang more cassette capacity than the compressor is rated for, but only by a certain amount.)
https://ashp.neep.org/#!/product/29024
https://ashp.neep.org/#!/product/26170
A 3.5 tonner is simply ridiculous.
Similarly, a 2.5 ton 3-zone MXZ-3C30NAHZ2 more than covers the calculated load for the second floor:
https://ashp.neep.org/#!/product/26173
So why did the spec a 3-tonner there?
>"Does Mitsubishi heat pump maintain capacity on rainy days?
Rain has no effect on capacity. Very low outdoor temperatures do affect capacity however, but the nominal "rated" heating output of Mitsubishi H2i "hyper heating" systems is still available at +5F/-15C, and ~75% or more is still available at -13F/-25C.
Dana
do you speak hindi?(Kyu, main pata nain!)
i think there specifying 42k heatpump because it's combined with basement load which is 15k + 1st floor load 18k is combined.
once i have another load calculation i will post it here.
>"do you speak hindi?"
Ha- kuch, likken acha na. Meri patni mahat bollywood fan hain. She learns the dance moves, I figure out the lyrics. :-)
Most people I know named Patel are Gujarati speakers, but I get lost pretty fast after the "Aapne kem che?". They seem to have a better handle on Hindi than I do, so I figured you might too.
>"i think there specifying 42k heatpump because it's combined with basement load which is 15k + 1st floor load 18k is combined."
I missed that on the first glance. 15K seems like a LOT for a basement, assuming the foundation walls are insulated, or maybe it's a walk-out?
Specifying a pair of MLZ-09s, each of which is good for 12K in heating mode is overkill even for a 15K load. They may have done that for heat distribution reasons, but a single KD15 ducted cassette is good for 18K, and would offer more distribution options. (But it would take longer to install due to the ducts.)
http://meus1.mylinkdrive.com/viewPdf?srcUrl=http://enter.mehvac.com.s3.amazonaws.com/DAMRoot/Original/10006\M_SUBMITTAL_SEZ-KD15NA4_FOR_MULTI-ZONE_SYSTEMS-en.pdf
A KD12 (or maybe even a KD09) would likely cover the real basement load.
Dana,
Good to know you understand my language.
Can you please reply to my Post #27 and the review the revised load calculations? I am trying to reduce 2nd floor load by bringing duct work and units into the conditioned space with spray foam into roof rafters.
So far Jonathan and Patrick suggested to keep 1:1 ratio with indoor unit and out door units. I will also post more options to consider.
thanks
HVAC contractors unfamiliar with heat-pumps and/0r well insulated homes tend to be very conservative with their Manual J calcs. Their logic is usually to oversize because they don't want the customer complaining that it is too hot or cold with a system that is undersized.
In general, ducted is more efficient than non-ducted and more expensive. You can combine ducted and non-ducted indoor units on a single outdoor unit and Mitsubishi has a list of allowable combinations on their website. Personally, I find ducted systems more comfortable and I am fine with a slight loss in efficiency for greater comfort. The reason I say this is twofold: 1) a properly designed and installed ducted system will generate virtually no noticeable noise and 2) a ducted system allows you to combine small loads together to get to a properly sized unit. Both of these points are compelling reasons for using a ducted system in bedrooms. In large open areas, a wall mount will work fine, especially if you set it and forget it, but they are not silent, which may or may not be an issue. Some folks have posted about units like Midea that can modulate down to accommodate very low loads in bedrooms, but as far as I know, Fujitsu, Mitsubishi, Daiken do not. The smallest Mitsu unit the last time I checked was a 6k unit and that was actually a software de-limited 9k unit according to a Mitsubishi engineer who said would result in very poor dehumidification.
While ducting may work well for bedrooms, the issue has been dealing with the static pressure for long duct runs. I was contemplating a PEAD unit, but the runs I was looking at generated too much static pressure. I lucked-out that Fujitsu came out with a medium-static unit that fit the bill.
A Mitsubishi unit will perform fine in the rain, the only issue may be dehumidification on days when you don't really need heating or cooling. Dry mode will work for those instances. Some units are better than others and you always have the option of using a dehumidifier. In our climate, this is not a big issue IMO.
Here are few additional points/observations that you may find helpful:
- 1:1 setups (1 outdoor unit or for each indoor unit) are more efficient that multi's
- If you do need to go with a multi unit, branch boxes don't typically work well in super insulated homes. If you do need to go the multi route, you are better off with an outdoor unit that has the ports on board (this was told to me by a Mitsubishi tech who was charged with trying to find out why their units with brach boxes were performing well in drafty homes, but poorly in super insulated homes)
- For wall mounts, get a unit that can sense the room temperature from something besides the intake on the unit itself. My Fujitsu wall mounts cannot sense the room temp with the remotes and the units are too high on the wall so they are shutting off before they should. I now need to get the wired "remote" to fix that.
- As far as pricing goes, I found that Mitsubishi was the most expensive, then Daikin that Fujitsu.
- For my basement and 1st floor, I went with a combination of wall mounts and a slim duct. In retrospect, I should have done that with a ducted, medium static unit.
Jonathan
Thanks for detailed explanation. I should have Load calculation result by Tuesday.
Once I have i will post it here.
I think Mitsubishi now have mid static PEAD units as well.
0.2 in.wc and 0.6 in.wc static pressure.
https://www.mitsubishicomfort.com/products/indoor-units/horizontal-ducted/compare
Does manual J and D provide required Duct static pressure in the results?
I like keep all 3 floor heating and cooling separate if possible. Open basement will be used mostly for kids to play in the weekends, 1st floor used mostly during daytime, and second floor will be occupied mostly for nights.
Harikrushn,
Manual J will give you heating/cooling loads and the required CFM to handle those loads, Manual S will size the equipment based on Manual J, and Manual D is for duct design which will include static pressure. I would think the mid static PEAD should work in your case.
I am conditioning each floor separately like you are planning. My garage (unconditioned) and basement are on the same level. Garage ceiling is insulated as the family room is above it, the basement ceiling is not. If I keep the basement unit off, I do have heat loss from the first floor to the basement. Not uncomfortable but the first unit floor will run more frequently to keep the set temp. If I had a redo, I would have insulated the basement ceiling. I do have a HPWH which kicks out low-50's air when running and the ERV is also dumping air that is roughly 6 degrees less than the ambient air so the basement temp is running in the low 60's with heat off.
I do have an open concept first floor, but I have doors on each side of the foyer so I can close it off which helps to keep the first floor heat from escaping to the 2nd floor. In the summer time, I keep those doors open and the 2nd floor bonus room unit keeps the 1st floor cool.
Jonathan,
I have insulation between floor with R-38 fiberglass batt. See attached.
Basement and 1st floor is completely closed with door.
1st floor and 2nd floor is open at the stairs and i have 2 velux skylight ( 22x72 each) on the roof ceiling under the stairs.
Also
As per plan, Attic is vented and insulation is R-38 batt in between 2nd floor ceiling Joist (2x10) -16" on center.
Hvac equipment and duct-work are also in the attic. To save energy and increase efficiency, i have 3 options :
1) Spray foam insulation ( close cell) in between roof rafter under the roof sheathing with 1" of gap between roof sheath and insulation for venting
2) insulate between 2nd floor ceiling joist and create small conditioned space for hvac equipment and most of the duct work.
3) keep Hvac equipment in the laundry room and duct-work in the attic with double insulation and proper air sealing
Please let me know your thought on this or propose another idea for better efficiency.
thank you
>"1) Spray foam insulation ( close cell) in between roof rafter under the roof sheathing with 1" of gap between roof sheath and insulation for venting"
How are you planning to establish the 1" gap with a spray foam solution? With closed cell foam it's fine to apply it directly to the underside of the roof deck (no vent gap).
Dana,
thank you for your replies.
I thinking of using vent baffles that create 1" gap. But if it's not require then I wont use it.
I thought it will help prevent any moisture problems.
https://www.lowes.com/pd/proVent-Provent-14-in-x-48-in-Rafter-Vents/3012018
In unvented attic with closed cell spray foam between rafters, do i need to have supply and return grill in the attic? if yes, then is it safe for indoor air quality?
thank you
Harikrushn,
The laundry room is a good centralized location for the PEAD. If you could live with some soffits for the ductwork, you could save yourself the hassle of building a conditioned, unvented attic.
With respect to skylights, I have a Wasco Skymax (now owned by Velux) in my foyer. I also have 2 Velux skylights in 2nd floor bonus room with a cathedral ceiling. Skylights are energy hogs. In hindsight I would have skipped the Velux, they don't really add that much to that room. The Wasco skylight lightens up the foyer and is a nice architectural feature. I am not sure what I could have done to reduce the energy penalty besides paying the uncharge for Lamilux skylight.
Dana and Jonathan,
Please see attached load calculation report.
Basement: 1412 sqft , 15942 Heating BTUH, 7091 Cooling BTUH
1st floor: 1304 sqft, 19324 Heating BTUH, 18839 Cooling BTUH
2nd floor: 1723sqft, 27312 Heating BTUH, 25573 Cooling BTUH
Can you please review and suggest Mitsubishi ducted or/and noducted units?
Once again, the basement numbers are making absolutely no sense whatsoever. If you well and truly have an R-26 ICF basement, and (since it's a basement) most of that wall is below grade, your heat loss for the wall component of the basement should be nowhere even close to what they've modeled.
Seems like the 2nd floor got bigger and the heating BTUH seems a bit high, especially since it is a second floor (heated from below?). As Dana has pointed out previously, the nameplate and actual capacity of the Mitsubishi units can differ and substantially - so take his advice on always checking the specs. I have a 3 zone (15k, 12k, 9k) ducted with a single MXZ 30 hyper heat (junction box in unit) for 2,100 sq feet (and the second floor just under 700 sq ft is largely over an "unheated" garage where garage ceiling is spray foam/batt) - your 2nd floor is probably in part over a garage as the sq ft is larger than your 1st floor?
We had -10 F for about 8 hours at night (no sun) with wind (including gusts to almost 30 mph) and all rooms remained at 65 F and above. Heat load calculation for the entire house was around 27,000 (Montana), ACH 50 < 3.0 (guessing around 1.5 after improvements) and we have way too many windows (6 patio sliders on the first floor).
PBP1,
Thanks for your reply. Information you provided is good reference point for me to compare with my load calc results.
What is your wall assembly type and R-value of it?
Your 2800 sqft house load calculation is around 27k vs. my 2nd floor alone is 27k.
one of the bedroom/bath/closet is over the garage but I am also planning to spray foam garage ceiling.
No problem, though I apologize, I must clarify, the total is 2100 sq ft (1st floor around 1400 and 2nd floor around 700). The walls are 2x6 with closed cell foam and batts (flash n batt) along with the lid (specified was around R26 walls and around R49 ceiling, though they were generous on the foam), so you can see, it's not impressive and no continuous exterior foam (might have done differently today). All windows are 0.3U or less. The house is mainly on piers, with flash n batt floors. And, I have big thermal bridges to the outside too, 6x12 beams both 1st and 2nd floors that run interior to exterior. Many "sins" but trying to make up for it elsewhere (looking at thin film solar). Max power usage on the heat pump in a very cold month is around 1,100 kW (with total kW for everything under 1,500 kW in such very cold months, Sept is around 300 kW total). I think a low ACH 50 might help - as did painting my south facing garage door gray (was white before the paint jobs were finished).
thank you
Wow, this is worse than the 1st Manual J. Dana is the expert on this, but I noticed that 62% of your heat loss is from infiltration and ventilation. You will have a tight house with controlled mechanical ventilation, yet the Manual J shows infiltration of 3,225 cfm at 50 Pa. IIRC, 3 ACH50 is equal to 25cfm50/100sf, so based on your square footage, even if you were just code minimum 3 ACH50, your infiltration CFM should only be 1,088.
He calculated at 5 ACH50. This seems high to me compare to how tight house are built.
With ICF basement, Zip-r6 + R23 wall insulation, Sprayfoam insulation at the critical part of the house( garage ceiling, at rim board joints and attic) , what is the best ACH50 i can achieve?
You will be closer to .5 than 5. Tightness really comes down air sealing and you could have the best insulation plan, but if the subs do a bad job of taping the Zip or windows, you have lots of penetrations for venting, then you won't be as tight as you would expect. Assuming you have a good contractor, HPWH and dryer, recirculating central vacuum and hood vent, awning, casement or tilt n turn windows, then 1 ACH should be achievable if not better.
The good thing about tightness is that if you start with a good plan, you have the opportunity to address any issues before it is too late. This is accomplished by doing a blower door test after you complete your air barrier (I would include the spray foam in this especially in the basement rim joist and all penetrations for electrical/plumbing are in and sealed), but before installing air permeable insulation. If you don't achieve the ACH you were hoping for, you can find the leaks and address them then. When you are satisfied with the ACH level, then you can revise the Manual J, S and D to confirm that you have spec'd the correct equipment.
So far Framing is completed, waiting for windows to arrive 1st week of March. Until then I was hoping to finalize HVAC system.
You can get most of the HVAC design done before the blower door test. The results of the test may require a change in the size of the unit or some modification in the duct work, but usually the duct work is not fabricated until the job is ready to start and the supply houses keep an inventory of the heat pump components so the time from ordering to delivery is very minimal. The hard part is determining zoning, location of the units, duct paths, and selecting the installer.
Thank you guys for your support. I was able to get revised load calculation base on your inputs.
revised load calculations ( also attached):
Basement: 1304 sqft , 4230 Heating BTUH, 3032 Cooling BTUH ( 129 cfm)
1st floor: 1412 sqft, 14584 Heating BTUH, 16924 Cooling BTUH ( 866 cfm )
2nd floor: 1718 sqft, 21664 Heating BTUH, 24446 Cooling BTUH ( 1098 cfm)
2 ach50 were assumed instead of 5 previously
Basement load significantly reduced after he corrected his mistakes. he specified R-15 wall with 2x4 stud and cmu block wall instead of R26- icf wall and he also forgot to include R-15 slab in the previous report.
Please review and let me know if it can be improved further.
Based on the load can you please suggest which type units can be used for each floor.
should each floor have separate outdoor units ? or combine basement and 1st floor and keep 2nd floor unit by itself?
Mid static ceiling unit or multi position air handler?
would it be better to separate 2nd floor into 2 zones ( master suits and rest of 2nd floor)?
thank you
The basement numbers looks more realistic now.
Your 2nd floor numbers are being driven primarily by the duct losses and glazing. Keeping those ducts within the conditioned envelope and going with a lower u-factor window would help those numbers quite a bit. I can give you a data point from my own experience last night that may provide some sense of how much. I had one 12 RGLFX running last night. It was set to 67 degrees, heating 3 bedrooms and part of the attic (actual room temp was 69 degrees). My wall construction is R-12 Roxul exterior, 2x6 framing with DP cellulose, my widows are u-factor of .13 and my ducts are within the conditioned envelope. I have more glazing, so maybe that is a wash with your slightly higher u-factor windows leaving the ducts as the major difference. At 6am, it was 22 degrees and my unit was drawing a steady 800 watts which is about 40% of max power input at those parameters. I think we saw a low of 12 degrees this year which is 2 degrees below my design temp and although I don't have the usage data that day I can say that the unit had no problem keeping the set temp.
My point is that, except for the location of the ducts, your 2nd floor appears to be similar to my 3 bedrooms + attic and my 12 RGLX is adequately sized to heat the space even at 2 degrees below the design temp. Note that at my design temp of 14 degrees, 17.4 kBtu's is the max capacity of my unit.
For me, the importance of getting the unit sized correctly has less to do with efficiency than it does with comfort. The forced air in my previous house was constantly cycling and noisy which I found very uncomfortable. These mid-static units provide an even temperature, they are silent and as an added bonus they are very efficient.
It would be interesting to see what your Manual J would look like at 1 ACH, .13 u-factor windows and no duct losses.
Jonathan
your response in encouraging.
If I spray foam in-between the rafters and bring the duct work into the conditioned space do I need to have supply and return vent in the attic as well? what is your R-value for the roof?
Unfortunately we already ordered the windows otherwise I would consider going to triple pane glass. Also I doubt we can achieve 1 ACH or lower knowing my contractor skill level.
Deleted
I have external insulation on my roof, 4" of polyiso and DP cellulose between the rafters. It has been a few years, but I recall the code requirement for unvented attics without exterior insulation in our zone was for R-15 of CCSP against the sheathing and then the balance can be air-permeable insulation. If you decide to add exterior insulation on the roof, the ratio of exterior to total r-value has to be at least 30% in our zone. I have supply and return for heating/cooling and ventilation in my attic. Mine is considered habitable, although not sure if that makes a difference. Temp and humidity in my attic is basically the same as the 2nd floor.
My attic height is only 4ft at the center.
Please see attached zoning options which I am considering
Are these units sized assuming you will continue to have the duct losses?
I just matched the calculated load.
I would suggest you keep your 2nd floor units and ductwork within the conditioned envelope of your 2nd floor and leave your attic vented. For the master, I would place the unit in the ceiling of the laundry room, have a supply on the wall between the master and laundry and also provide a supply to the master bath taking a duct directly across the hallway, tight to the master bed door and terminated with a register above the linen closet. For the return, I would build a soffit along the side wall of the master bedroom and place the register in the far corner of the room. These should not be large ducts, so maybe the soffit is 12" square max. You can put some cove molding on it and add some LED light in the cove to make it an architectural feature.
For the other 2 bedrooms, I would place the unit is either of the 2 closets and then duct it from there.
I would run this by your HVAC engineer and if he thinks it would work, have him redo the Manual J and then see what the loads look like. My guess is you could do this with a single mid static unit that will modulate down to 3kBtu's, or 2 units if your feel you need to zone the master separate from the other 2 bedrooms.
Thanks Jonathan I can look into that option. I got quote for Spray foam insulation in the attic rafter today ( $14000 for R-38 Icynene closed cell). After seeing this quote, I should bring the units and ductwork into 2nd floor ceiling, but it's tough to layout without lots of drop ceiling.
My 2nd floor ceiling is 8.5 feet, should have kept 9'ft.
Thankfully the units are not very tall. My Fujitsu slim duct version is only 8". Figure a couple of inches to hang from the ceiling joists and wrap in insulation for sound attenuation and it is about 12" total. The ducts should be no more. If you are going to give up ceiling height somewhere, laundry rooms and closets are great places to do so. The ceiling height in my mudroom is 7.5' and it feels fine, especially since it is well lit. If you had to drop the ceiling in the family room or kitchen that would be problematic.
Jonathan,
Tomorrow I will go to the site and figure out the layout if it will work out because framing is already completed. I may have to modify it slightly.
If i may ask, what is average monthly electric Bill for your house since we are in same area, have similar size house, and installing similar system? or you can tell me the range.
thanks
I have solar and so far am net zero so my bill is "pre-paid" with the upfront cost of the solar. For the 12-months ended 2/7/21 which was the one-year anniversary of the solar install, I generated just under 12.6k kWH's and I used 11.2k kWH's. Yesterday I was actually net positive by .3kWH's generating 37.6 and using 37.3. Today, I generated .1 and will use around 40kWH. I have high solar gain windows which adds quite a bit of welcome solar gain on cold winter days. I also burn wood in an EPA certified fireplace when temps are below 40's.
My previous house which was built in 2007 is less than 2 miles from my current house and slightly smaller used 57k kWH in 2016. I did the analysis then to help me model my new house. I think the 57k vs 11.2k kWH's is an apples to apples comparison of 2007 code vs 2020 near PH build energy savings.
Here is picture of the tray ceiling in my dining room. There are not any ducts in there, but if needed to I could have sized it to fit. The cove molding on the top will have LED lights (on my list of things to)
Jonathan
thanks for sharing your details.
If I encapsulate the attic and bring ductwork into the conditioned space, my engineering was able to reduce the 2nd floor load down to 6K so total is about 18k for second floor.
If I hide the slim ducted units under the sealing with soffit, wouldn't it be hard to service or replace if need to?
You will need to have access to unit and you should make the access opening large enough to allow the unit to be removed if needed. A dropped ceiling with acoustical ceiling tiles or a soffit with an access panel will work. I prefer the access panel because you can get ones that accept a drywall insert so all you see is the outline of the opening.
Jonathan
I did look into mounting the slim ducted unit in the ceiling. For master suit, it work out very well but for other 2 bedroom it's creates complications.
I am thinking of just insulate the roof with Spray foam and bring in the duct work into the attic.
Can you please share your ERV setup and brand ? is integrated into ducts or it's setup with it's on supply and exhaust. ?
thanks