I built a well-insulated house this fall and after learning about minisplits on this site, we decided to heat our house with 3 minisplits. I can write more about this at a future date, but for now I’m having trouble with one of them and am trying to debug it to get as much information as I can before approaching the HVAC tech that installed it. We are in climate zone 7, so we have a lot of cold nights here (we’ve been below -22F several times this winter).
2 of our minisplits are Fujitsu 15RLS3H, which have been working just fine, no problem heating our house.
The minisplit we are having trouble with is a Mitsubishi MSZ-FH09NA, which is heating a smaller zone of our house which has a 5,300 BTU/hr design heat loss at -12F (our 99% design temp). The base pan heater was installed. The reason we went with the Mitsubishi here, rather than the Fujitsu, is that the Mitsubishi is rated for a capacity of about 6,800 BTU/hr even at these cold temperatures, which is sufficient for this zone, and as Dana has pointed out in this forum, the Mitsubishi can throttle down to a low-end of 1,600 BTU/hr, whereas the Fujitsu can only go down to 3,100 BTU/hr before cycling, so the Mitsubishi should be a lot better fit for this zone when the temperatures are more mild.
So, that’s the background, now here’s the problem…
I’ve been measuring things to confirm that the Mitsubishi is not functioning as it should. Today it is 34F outside here. I set the Mitsubishi to the highest setpoint, 88F. I set the fan to speed 3 and let it go for a while, then measured the temperature of the airflow coming out. It was blowing 85F air, and the incoming air was 70F. According to the engineering sheet for this minisplit, fan speed 3 is 225 CFM, and at my elevation BTU/hr is close to 1.0 * CFM * dT, so that would put the output at 3,375 BTU/hr. I changed to fan speed 2 which is rated for 167 CFM and got a temperature output of 92F, so that would put the output at 3,674 BTU/hr.
Someone please let me know if this is an inappropriate way to calculate the BTU/hr output, but as a point of reference I did this on my Fujitsu a couple weeks ago (it was 5F outside) and got:
– Fan speed 4 (547 CFM): 120F output (78F input) – dT: 42F — 22,974 BTU/hr output
– Fan speed 3 (459 CFM): 131F output (78F input) – dT: 53F — 24,327 BTU/hr output
At 5F the design manual for the Fujitsu’s show 19,900 BTU/hr at 5F outside, 75F inside, so it would seem that this experiment achieved a close result, not factoring in defrost cycles.
So, back to the Mitsubishi, the engineering sheet shows a maximum heat output of 12,000 BTU/hr, so we are far short of that. I measured with a power meter that the unit was drawing about 1150W during this time, which would be equivalent to 3,900 BTU/hr of resistive heat, so it would appear that the COP of the unit is less than 1 right now.
So it is clear that something is wrong, but what? The lineset is a little longer than the 25 feet precharged, so they were supposed to add about 2 oz of refrigerant. If they forgot to do that, would it affect things this drastically?
I measured the hot line of the lineset at about 177F while it was running.
If anyone has any insight into what needs to be investigated more, or any more tests I should run, before I approach the HVAC tech with my findings, I’d really appreciate it! Thanks.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part