Help please – Did I make a huge heat pump mistake?!

Hi,
I may have made a very expensive mistake, but I’m hoping someone might have ideas for things to try. I’ve got the system running full bore and the warmest I can get the house is 74°. I’m in Albuquerque, NM and it’s 45° high today and sunny at around 3:30pm.
Our home is about 1900 sq feet and is a single story 1960’s era adobe with 8.3 foot ceilings (no basement or attic). More detailed specs on house at bottom of post.
Old system: single stage bryant natgas furnace w/ 89k BTU output + 5 ton AC
New heat pump: Mitsubishi MXZ-SM48NAMHZ2 and PVFY-P48NAMU-E1 air handler (both 4 ton)
Middle of this summer, I began looking for engineering companies that would do a manual J on an older adobe home and struggled to find anyone (who didn’t also install HVAC systems). I did a rough manual J using hvac.betterbuiltnw.com, but had trouble figuring out the R value for adobe so the numbers didn’t feel solid. Articles here on GBA suggested using therms of gas burned per month to calculate our heat demand during cold months (our furnace was our only gas appliance for over a year): https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler I also tried timing how long the furnace was running at 5:30am when it was 20°F outside on a few different mornings (it ran for ~24 minutes per hour, so about 35.6k BTU per hour on relatively cold mornings).
After looking at experiences of people in semi-similar climates retrofitting older homes of similar sizes using this mitsubishi system (and talking about how well it had worked), I assumed my experience would be similar.
I then proceeded to request quotes from a wide variety of local HVAC installers for the above system. Most pushed for a dual fuel or warm climate heat pump, but I found one mid-sized contractor (that has a good reputation) and they were willing to do the install with the equipment I had specced.
Last night was our first evening with the system. The low outdoor temp was about 16° and the thermostat could only get up to 66 or 67° during the night. Over the course of today, a chilly (yet sunny) day with a high temp of 45°, the highest I could get the thermostat to was barely 74° (i.e. system running full blast continuously, since install was finished yesterday eve).
The installer noted that the air handler fan came with a very small slight dent in it and that he had ordered a replacement fan/housing that he would install as soon as it arrived. He said it made a slight noise, but did not cause a ton of vibration or anything that he thought was a major concern in the meantime. Otherwise, there were no notes from the installer.
After looking around at possible issues, it seems like our ducts are likely too small/long to handle this system. But before I jump to conclusions, I figured I would see what people thought – is there a simple solution I’m overlooking? Or some next steps I could take to start digging myself out of this awful hole?
Any thoughts/suggestions would be so so greatly appreciated. Please let me know if I can provide more info.
**Home Overview**
Region: Albuquerque, NM (zone 4B)
Home: ~1900 sq ft of heated space, built in 1964, adobe, single story, no basement or crawlspace, brick floor on sand with air ducts running under the brick, air ducts sealed last year, double pane vinyl windows (cheap, previous owner did them), 8.3′ ceilings, flat roof with 4.5″ polyiso foam and white TPO membrane – roof is: interior wood T&G, then polyiso foam, then TPO exterior).
Edit: One other thing I noticed is that as soon as the sun started shining this morning, the system did two defrost cycles in very quick succession (within about 20-30 mins of each other). I do not know if this is a clue, but figured I’d mention it.
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Replies
"After looking around at possible issues, it seems like our ducts are likely too small/long to handle this system."
This could well be true. The formula for heat transferred by air is (CFM)*(DeltaT)*(1.03), where CFM is air flow in cubic feet per minute and DeltaT is the difference in temperature between the air and room temperature. I believe that Mitsubishi is rated for a DeltaT of 36F. A gas furnace is probably going to be providing 60-70F. So if the ductwork was properly sized for the old furnace it could well be too small for the heat pump.
The first step would be to verify that this is indeed the problem. A HVAC contractor will have tools to measure air flow and pressure within the ducts, you should be getting at least 1300 CFM for 4 tons. If the ductwork is too small the pressure in the ducts will build up which creates resistance, the manual for the air handler will have a maximum pressure that it can face and deliver its rated output. That measurement should tell you if the ducts are in fact undersized.
Having ducts in the slab is not helping things in terms of energy efficacy. Without insulation bellow the slab, you are heating the dirt under your house which is a pretty good conductor of heat.
The static pressure on these air handlers is programmable. You can usually adjust it in the installer menu. Default is usually 0.5" you can bump it up to 0.8" to get some more oomph out of your blower if the ducts are too small.
Defrost cycle in the morning is normal as you get higher humidity in the morning.
In terms of equipment running properly, quick check is to measure the delta T across the coil with something like a digital meat thermomometer. With the thermostat cranked, you should see about a 35F delta across the coil.
I don't understand the issue. Why would you want to heat your home to over 74 degrees? I would think that when it is very cold out, at or below the coldest design temp, the system might max slightly below a standard max of 72 degrees for heating. For cooling it might go slightly over the standard 78 when it is very hot.
I would not expect a properly sized system to go much beyond those temps at the coldest/hottest exterior temps...
When you have insulated or poorly insulated walls and other surfaces, your body heat radiates to them and makes you feel cold. Raising the air temperature is one workaround. Making the interior surfaces closer to room temperature is more effective but not always possible. (I live in a 200yo house in Maine, still in the early stages of energy upgrades, so I experience this for half the year.)
Thank you so much @DCcontrarian and @Akos. I've followed up with contractor and they're asking the tech what the static pressure was set to. Will ask about measuring airflow if the handler is already at highest static pressure setting.
@user-5946022 – the temperature at the center of the house is significantly warmer than at the ends (and our bedroom is on one end). The warmest I could get it at 4pm (warmest time of day here) was 74° at the thermostat (center of the house) on a day that was 45° high / 20° low outdoors. In middle of a cold night, the thermostat (center of the house) tops out closer to 69° and our bedroom feels closer to 64° (though I haven't measured it).
Suggest measuring not only the temps across the coil but also the discharge temps at a nearby register as well as further away in your BR. I'm betting Akos is spot on and the main issue is the floor location of the ductwork. Even with normal 35 delta across coil, your duct conductive heat losses to ground may be sucking out a high(er) proportion of the delivered heat, and without warming the slab enough to contribute via radiant mechanism.
Thanks for all the input and support thus far!
Update: About a week ago contractor was able to replace the dented blower and blower housing and noted that he had left the static pressure at the default 0.5" setting, due to the original dented blower being off balance and not wanting to push it too hard. When replacing the blower, he updated the switch to the high (0.8") static pressure setting and this has made a significant difference throughout the house. We had a string of cold days/nights, including a 6° morning. Generally, the house has stayed plenty warm at night – within a degree of our 70° setpoint, throughout the nights, including on the 6° morning.
The only room that is still significantly lower temperature the rest of the house is the primary bedroom, has quite a few exposed walls, windows, and northern exposure.
I took a bunch of temperature measurements for reference (see below).
One question: After combing through the MXZ-SM48NAMHZ2 manual, I noticed that it says it is feasible to add up to 130% of the capacity of the outdoor air handler in total capacity of indoor units. In other words, up to 14,400BTUh worth of indoor splits could be added. I would like to look into adding a .5 or .75 ton indoor mini split in the bedroom, but I'm uncertain how much this would draw down the capacity of the existing system (if any)? The mini-split would be roughly 35 feet away from the system (outdoor unit is on roof, immediately above the air handler – so currently maybe 5-10 feet of coolant line, one way). From what I've read, the additional coolant line seems like it would impact the overall capacity by low single digit percentage points, but I don't know if the additional unit itself would also add to this reduction or not?
More generally, I'm curious if anyone has thoughts on system expansion by adding a mini split: caveats, things to check out first, etc?
Thanks again!
**Temperatures captured with digital instant read food thermometer**
Time / day: 1:30pm, 34° outside temp, clear and sunny
Set point: 72°
Temp on thermostat: 72°
Air before coil: 72°
Air after coil: could not figure out how to measure
Hall register: 98° (right across hall from closet with air handler)
Register in dining room: 86° (one extreme end of house, southern exposure)
Register in primary bedroom: 87° (other extreme of house, northern + western exposure)
Ambient room temps: Hall 74°, Dining room 73°, Bedroom 68°
I think when you add a ducted unit to the SM the max oversizing is 100%. Most likely it would still work if you add a 6k wall unit, but I would not test it.
If you need more heat, get a cheap plug in panel heater. The operating cost of that will not be all that much, so the ROI on the extra wall mount will probably be never.
The better use of your dollars is to get the ducting out of the slab. If you are doing any interior re-model, move any ducts that you can into interior bulkheads.
@Akos – thanks so much for the additional advice! We ordered a panel heater for the room and I do think that will help. I'm planning to look into feasibility of adding a mini-split to the 100% utilized capacity, just in case it is sanctioned by Mitsubishi and there's some opportunity to do it in future.
I wish we could get ducting out of slab, but I don't think we have anywhere to put it. It's brick floor atop sand, 100 inches of vertical (living) space, then wood beams and tongue and groove ceiling, with foam insulation and roof membrane directly on top of that. It's a pretty simple setup :|