First Month with Ductless Heat Pump-Thoughts & Questions

Alright, I had been debating getting a ductless system for about two years now (lived in this house for over three) due to the heating issues I've had in the past. We live in a 2100 sq.ft split level that had baseboard and a woodstove in the closed off basement when we bought it. The first winter we owned the home, heating was futile. The woodstove in the basement couldn't due much since there was no direct access to the living area above, not to mention the fiberglass batting in the basement ceiling.

The next winter, we removed the batting, and built a set of stairs into the basement from the kitchen above (picture taking a circular saw to your hardwood flooring). I also purchased an Englander NC-30 stove to replace the smoke dragon that we had. That made a HUGE difference, but we still needed to supplement upstairs with an Edenpure, that to me, wasn't very efficient and still couldn't keep the temps higher than about 66 on average.

Our lowest living level was remodeled the year we bought it, and we built a stone fireplace with a Regency P90 propane fireplace. That heats the ~500sq.ft family room more than adequelty. It was out main floor (the kitchen/gym/living room) that was struggling to stay warm. So after much research, we went with a local Mitsubishi Diamond installer, and purchased a Mitsubishi MXZ-2B20NA with MSZ-GE09NA and MSZ-GE12NA evaporators. These were to service our ~900 sq. ft main floor.

They were placed in the following locations:



Here's a spec sheet copy from the unit showing guaranteed operating temps:



We had the unit installed on October 10th which is about the start of our billing/metering cycle for our electric bill, so it was time great. We just received our bill the other day, and we're using WAY less electricity than this time last year even though there were two more days in the billing cycle compared to last year, and our average temps were 1 degree lower than last year. Take a look:



My impressions? So far, it's doing a good job holding the room temp to about 70-72 depending on outdoor temps. It's quiet, it's efficient, and I'm happy overall. When the outdoor temps are in the mid 30s-40s, this thing keeps the house TOASTY.

The problems? Well, for one, the unit temp as denoted on the thermometer is never accurate to the actual temps being put out by the unit. For instance, I placed a thermometer right next to the vent on the 12K unit which had the remote set to 74 degrees. The HIGHEST the temp reading on the thermometer was 72. Most of the time it's around 70-71. This discrepancy gets larger with colder temps. For instance, this morning, it was 27 degrees outside. I had the unit at 74 degrees. The thermometer read 70, and the T-stat in the room for the other heating system read 68!

I have been in contact with the installer, and he told me to change the fan speed from auto to a pre-set constant speed like 2 on the 1-5 scale. That helped, but I also worry about the efficiency of this practice. It seems odd to me that for a unit rated down to -4, that it would be struggling so much in the mid-20s. Could there possibly be an underlying issue here?

Maybe the system isn't fully charged? I've checked the outdoor unit for excessive pooling or signs or defrosting not working, but all looks well. Could the unit be undersized? Thoughts?

Well I heat my upstairs floor, around 600SF with one 12,000 btu/hr inverter heat pump quite nicely. In the mid 20s I can get up to 80F if I wanted. Mine has and outdoor rating down to 5F. Mitsubishi's are nice units, your unit should have no problems getting above 72F at those outside temps provided your 900SF is properly insulated and not open to additional square footage. You can check the delt T across the indoor coil to confirm proper operation. Take an air temp reading of the air entering the indoor coil, then take a reading of the air being blown out by the unit, insert probe 1" into the louvre in the middle of the the unit. There should be about a 20F difference between the 2 readings. That 20F is not set in stone and will vary based on air temp of the room, outdoor air temp, and blower speed, as well as compressor speed if the unit is an inverter model. If the temp difference is less than 12F I would likely think the unit has a problem. If less than 5-10F there is a problem, unless the compressor is running at low speed if its an inverter unit. Refrigerant leaks can be common on these units due to the flare fittings, sometimes the installers don't get the flare just right. These units cannot be topped off with refrigerant, the gas must be recovered and then a new charge weighed in, this of course is done after fixing any leaks, your dealer can use an electronic sniffer to leak check the unit if you think this may be a problem. These units are very charge sensitive and they must be charged to within +- 1/2 ounce unless the manufacturer specifies otherwise. How many total btus/hr is the unit rated for? Is this unit an inverter model with the variable speed compressor? Was a load calculation done to determine the proper size for the unit? As far as the temp differences, they all do this, If I want my upstair to stay at 70 I need to set it to 67-68 to accomplish this, no biggie there, I just figure out what works. Hope this helps.

Thanks for the report. RE your observations....the first spec sheet is not saying it can carry your load down to those temps, just that it will operate down to those temps and provide some useful heat. If/when your BTU load exceeds what they can supply, the tstat will mo longer be satisfied, the units will run flat out, and the rooms will drop below the tstat setting. Or perhaps I don't understand your issue??

I was thinking of getting a mini split or two. You guys sound like you like them so far?

I love mine, but you need to do a load calc on the area you intend to heat/cool with it. Don't go by rules like 12,000btus/500sqft. Insulation, windows, solar gain, etc play a huge role in determining what size is needed. You can download load calc software online and play around with it, some may be free and some are low cost. With an inverter unit you can oversize and still be very efficient because the compressor is variable speed, it can adjust to match the load. If you undersize then you are hosed because you can't get more from the unit than what it's rated at. If a load calc says you need 18,000 btus/hr at your locations design temp, then go with a 24,000 btus/hr inverter model. Just don't go overboard with oversizing, or your unit will be turning on and off alot and that defeats the purpose of a variable speed compressor. You want that compressor running all the time, varying it's speed as needed to get the most efficiency. By oversizing a little you have that extra margin that you may need during an unusually cold night.

Here is why your problem is occurring. Your indoor unit is located on an outside wall.The indoor unit is talking in air from near that wall, and that air is colder than what is in the middle of the room, or on another interior wall. The units temp sensor senses the air temp as it enters the unit. So the unit thinks the room itself is colder that what it is. There is no way around the issue, just know what you need to set it to be comfortable, and take comfort in knowing that your heatpump is in fact working as designed. Hope this helps.

Thanks for the insight on a few things. I'll try to address them as best as I can.

First, the operating range. I'm aware that's not assuming full rated Btu/hr output at those temps. The point was that at an outdoor temps of ~25 degrees, this unit should not be struggling to maintain temps inside at 70 degrees.

I don't have the manual in front of me here at work, but I found specs from the Mitsubishi which oddly are different than mine in terms of operating temps (states 6 degrees F at the lower limit). At 47 degrees, it has a max rated output of 22,000 Btu/hr and 12,500 btu/hr at 17 degrees.

I am slightly concerned now that it is undersized. I didn't actually do a formal load calculation, but I had the installer come out and we talked about the room layout, insulation level, and the fact that there are two walls of windows in this room as well. In the end, this was the unit he speced out for the situation. The compressor is sitting on a platform under my deck (has about 3 ft of vertical clearance), and one evaporator is mounted on a wall shared with the garage, and the other evaporator is on an exterior wall.

I'm a bit confused about having to lower the temp levels to maintain a higher temp, unless, as was stated above, the place where the unit is drawing air in from is cooler/warmer.

In some sense, all ASHPs are undersized. Paying for a unit big enough to carry your whole load on the coldest hour of the year is seldom cost effective. I have one unit carrying my whole house, 'over-sized' it a 'ton' relative to what the installer wanted, and it maxes out (and call back up to help) around 22°F.

If I understand...you are using yours to heat a part of your house, with wood backup? Does your house really only need 15 kBTU/h at 25°F? When the stove is rolling, are those thermostats at setpoint?

I haven't fired my stove this season except for one day during Sandy. I wish I could answer the need for 15K btu/hr rating, but I can't, and alas, it's probably why I'm experiencing the issues I am. I just know it's not right to have to crank this unit to 74 to get 68 room temps when it's only in the mid 20s outside. If you look at the floorplan I posted, it's a very open concept. The stairs leading to the upstairs are almost in the line of sight of the 12,000K unit, so I imagine that some heat is escaping to the upstairs (the upstairs is not as cool as it was in winter's past), so it may be trying to heat a larger area than the ~900 sq.ft area.

Sounds like you are undersized as you are trying to heat more than 900sqft, remember heat is rising up those stairs. As with all ASHP's as the outdoor temp drops so does your units ability to provide that heat just when you need it most. This is why a load calc is so important to determine your heating requirements, oversized a little no big deal, undersized a little and it becomes a big deal. Have you talked to the dealer yet? If they speced it and it doesn't keep up maybe they will work with you on a solution.

I have been in touch with him on a few occasions. I haven't approached him about it being a sizing issue yet, so I have no idea what to expect.

There really isn't a 'right size'. For any given size, there is a temp where it will max out. Increasing the size by a good deal might only drop the balance point 5 degrees or so. The installer should have told you a design balance point and walked you through what to expect (re when you would require backup).

Yes but this is true of any heating system. When I looked at the performance curve of these mitshibishi units I was not impressed. At 5F it had lost 60% of it's capacity. The unit I have which is a cheapy chinese unit puts out 15,000btu/hr at 47F and 11,500btu/hr at 5F. I think the OP's true problem is that open stairway, he's really trying to heat more than 900sqft, how much more I don't know. You do make a great point of balance point, ASHP's are the only heat source whos heating output drops as the outdoor temp drops, this can make sizing really hard. And yes at some point a backup heat source will be needed to keep up. Most ductless minisplits do not have backup resistance elements, so the OP will have to use something else for backup heat when the temp really goes down.

I agree with the above. The unit is prob sized to heat ~900 sq ft to a reasonable balance point (like the low 20s), and is struggling due to heat running to the upstairs. Either the upstairs will need to be cut off, or some supplementary heating (like the woodstove or baseboard) will be required.

The unit is clearly undersized relative the OP's expectation, and I would blame the installer for that.

Wow that was weird, my old screen name came back up in the post above, I had re-signed up because my email address had changed. Can the mods create just one ID for me. JDenyer232 and JDenyer236 are the same. Thanks and sorry for the confusion.

Thanks for the input everyone. I had believed that the unit was trying to push heat upstairs since the temps in the hallway upstairs in the back bedroom were much more comfortable than in winter's past (especially these few mornings lately when we're in the mid 20s). I suppose at this point, I have two options. Live with it. Or try to get a bigger unit in place, which I doubt the dealer will be willing to do (and would it truly be worth it)?

I don't mind supplementing with the NC-30, as I love cutting and burning wood. The problem is I also put this system in with the hopes that when we sell it in a few years, that those who might be interested in buying would be more inclined to buy seeing an alternative heating system if they have no interest in burning wood. The way it stands now, they wouldn't be able to maintain 70+ degree indoor temps on the main floor through a PA winter on this system alone.

IIRC you have a variety of conventional heating systems including a propane furnace, and baseboard in addition to the mini. The mini is likely far and away the cheapest. As a mini-fan, I think one solution is to put another mini upstairs. A budget solution would be to put a little baseboard in the level 2 space, and wire that and the mini into a two-stage thermostat. IOW, the mini gets called first, and the baseboard gets called only when the tstat senses the mini is not keeping up. Would solve the existing comfort issue, and provide a cost effective 'conventional solution' at resale. When you ran the wood stove, presumably the mini would be able to keep up, and the baseboards would never get called.

Depending on your insulation/airsealing status, you could have that work done for less than a new mini, and then you might be all set. When I had my HP put in, it maxed out ~30°F. After airsealing my attic, that dropped to 22°F. IN my climate that made a huge difference (from the January average temp, to the January average low temp).

So, if you are not tight yet, airsealing and/or baseboards might get you to a happy place.

I think Woodgeek has a great solution for you here. I like the budget solution he has proposed, seeing that you seem interested in selling in the future, you may not see a return on an investment of another ductless minisplit being installed. This thread is exactly why I'm a huge fan of proper load calcs being done, that way you know if the unit will carry the load and what is needed for backup heat when dealing with heatpumps. When getting proposals from dealers ask for a manual J calc, if they don't know what that is then find another dealer. The manual J really doesn't apply to pellet stoves as most pellet stoves are installed as supplemental heat to lower your primary fuel usage and pellet stoves are really more like space heaters, unless your floor plan is totally open. Although a manual J can certainly be used to size a pellet stove should you choose.