Hi, I was wondering if I could get some advice on a recently installed Mitsubishi Hyper Heat mini split system. The heat pump is not functioning properly and despite multiple visits our HVAC contractor (a Mitsubishi diamond contractor no less) has not been able to fix the issue - their level of expertise on these units seems very subpar. I'll try to give as much detail as possible... I did hours and hours of research prior to purchase but am not an HVAC tech so please don't jump on me if I don't have terms right.
The system is a 2-ton MXZ outdoor unit with 3 indoor units (two 6K btus in bedrooms, one 12K in living). The units don't maintain heat, sometimes fail to put out any heat, and go through cycles where they enter defrost mode every 20 minutes with no frost on the outdoor coil or change in outdoor temp. A typical day will be, say 23 degrees outside and the unit defrosts over and over, only blowing cool or lukewarm air in between defrost cycles (which last about 15-20 minutes), so the indoor temp drops as low as 52. The unit will then inexplicably run with fan on high giving off good heat for an hour (or 2, or 3, no pattern) raising the temp to near setpoint (70) then enter the hellish defrost cycle loop for another few hours, or a full day... again, there is no pattern, no big change in wind or snow or huge temperature drops outside. This can occur in the morning, afternoon, middle of the night, etc. The unit can, on some days, keep the house warm when it's 5 and snowing outside and struggle when it's 30 and sunny... or vice versa.
Our contractor has added refrigerant after the second visit (system was 2 pounds low), replaced thermistor outside, replaced a board in the outside unit, etc. on third visit. There are no error lights/codes. They also had Mitsubishi rep on the phone for two hours, running different tests and even Mitsubishi had no answers on fourth visit. We've tried all the simple stuff (using a higher setpoint temp, adjusting vanes, running both with and without the iSee sensors active, different fan speeds, etc.).
The home is a two story 1,300 sq. ft. 1910s farmhouse on the Iowa/Minnesota border but with vinyl windows, blown-in attic insulation, blown-in wall insulation, and closed cell foam encapsulated foundation/basement so reasonably tightly wrapped. If the unit were undersized or the house was leaky, wouldn't the unit just blow heat all the time and never reach temperature? Also, a small 1,500 watt Vornado can heat the entire main floor living space to 64 on a 10 degree day, so I know the house can be kept warm. We have no dedicated backup heat source (just some larger space heaters) because from what we were told and the research I did, these units should still put out top level heat down to 4 degrees, and partial heat down to -13. We figured we could supplement on very cold days but having to run backup heat with a top-of-the-line heat pump when it's 20-30 degrees out is absurd. I could have installed more reliable (albeit inefficient) hydronic baseboards for thousands and thousands less.
This has to be an issue that someone with a decent working knowledge of these systems can diagnose. Our contractor up here seems to be out of their league, which is frustrating given the fact that they're supposed to be a certified Mitsu diamond dealer with great reviews. We're at our wit's end... is this normal for these systems (meaning their marketing is absolute BS) or do we have a right to be mad that this contractor can't pinpoint the issue? And what do we try next?
The system is a 2-ton MXZ outdoor unit with 3 indoor units (two 6K btus in bedrooms, one 12K in living). The units don't maintain heat, sometimes fail to put out any heat, and go through cycles where they enter defrost mode every 20 minutes with no frost on the outdoor coil or change in outdoor temp. A typical day will be, say 23 degrees outside and the unit defrosts over and over, only blowing cool or lukewarm air in between defrost cycles (which last about 15-20 minutes), so the indoor temp drops as low as 52. The unit will then inexplicably run with fan on high giving off good heat for an hour (or 2, or 3, no pattern) raising the temp to near setpoint (70) then enter the hellish defrost cycle loop for another few hours, or a full day... again, there is no pattern, no big change in wind or snow or huge temperature drops outside. This can occur in the morning, afternoon, middle of the night, etc. The unit can, on some days, keep the house warm when it's 5 and snowing outside and struggle when it's 30 and sunny... or vice versa.
Our contractor has added refrigerant after the second visit (system was 2 pounds low), replaced thermistor outside, replaced a board in the outside unit, etc. on third visit. There are no error lights/codes. They also had Mitsubishi rep on the phone for two hours, running different tests and even Mitsubishi had no answers on fourth visit. We've tried all the simple stuff (using a higher setpoint temp, adjusting vanes, running both with and without the iSee sensors active, different fan speeds, etc.).
The home is a two story 1,300 sq. ft. 1910s farmhouse on the Iowa/Minnesota border but with vinyl windows, blown-in attic insulation, blown-in wall insulation, and closed cell foam encapsulated foundation/basement so reasonably tightly wrapped. If the unit were undersized or the house was leaky, wouldn't the unit just blow heat all the time and never reach temperature? Also, a small 1,500 watt Vornado can heat the entire main floor living space to 64 on a 10 degree day, so I know the house can be kept warm. We have no dedicated backup heat source (just some larger space heaters) because from what we were told and the research I did, these units should still put out top level heat down to 4 degrees, and partial heat down to -13. We figured we could supplement on very cold days but having to run backup heat with a top-of-the-line heat pump when it's 20-30 degrees out is absurd. I could have installed more reliable (albeit inefficient) hydronic baseboards for thousands and thousands less.
This has to be an issue that someone with a decent working knowledge of these systems can diagnose. Our contractor up here seems to be out of their league, which is frustrating given the fact that they're supposed to be a certified Mitsu diamond dealer with great reviews. We're at our wit's end... is this normal for these systems (meaning their marketing is absolute BS) or do we have a right to be mad that this contractor can't pinpoint the issue? And what do we try next?
