Fujitsu Unit Tripping Breaker
Hi all,
We are finally enjoying our new ski house in the White Mountains of NH, and just want to say thanks for all the help along the way. What a learning curve! Overall, the new home building process, with attention to energy efficiency was super frustrating, but had some triumphs as well, including a blower score below 1.0 ACH/50pa.
We chose to go with an HVAC system designed using a Fujitsu ducted system, 36LMAH1M. The system was installed in the summer and performed really well, once I got the wired controller set correctly. Recently, we’ve started to see cold weather, and although it appears to handle the heat load fine so far (temps down to 15 overnight, so far), we’ve had a few nuisance trips from the outdoor unit. This issue happened first when a short-term rental guest was staying there in Oct, and the temps first started to dip. There was a language and distance barrier (I’m a couple hours away) for us and the guest, but it appeared the unit had to be reset at the breaker a number of times before it would restart, but then worked fine for a month an a half. This weekend, I was there Friday, and all was fine, although I noted the unit was substantially frosted over. The temps dropped to the teens, and all was fine in the morning (still frosted), but after being out for the day, when I came home it had shut off, again, tripping the breaker. I noticed that the unit was fully frosted over. I was able to trick the thermostat into thinking it was too warm, and resetting the unit to cooling mode, which worked to defrost the unit finally, and heating operation started to work again. That lasted a day, and since leaving yesterday the unit worked fine all night, then went offline per the app, so assuming it tripped again.
The submittal sheet refers to a max breaker size of 40A, but I see that the electrician installed a 20A. Could this issue be as simple as an improperly sized breaker? Is the unit trying to switch to defrost causing the over current condition? Or are the naysayers right, that heatpumps just don’t work up there in the cold north?
Thanks for any input…
Jim
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Replies
According to that data sheet, your unit can pull over 30amps in heating mode -- so yes a 40amp breaker would be appropriate. I would not change the breaker until you verify you have wire that can handle 40amps.
Heat pumps go to full power in defrost mode, so that is probably why you are tripping. The electrician installed a "standard" AC power which is 20A, you need more. Based on the datasheet 30A service would do but I would bump it up to 40A and never worry about it.
There are also usually settings for extended defrost. The unit should never be freezing up like that. Bit of frost build up is fine but should be mostly clear.
A 3 ton will pull easily 3 kWh when it gets cold. Especially if indoor fan is on high. So yes- a 20a breaker will get tripped all the time.
This particular unit will run defrost at 2.2kwh at 32f ambient
Constant load is "kw", not "kWh". I see that mixed up all the time. They are very different units, equivalent to "gallons per minute" in the case of kw, and "gallons" (no time unit) in the case of kwh. What you're saying is a bit like saying "I can fit 5 gallons per minute in my 5 gallon bucket", which doesn't make any sense.
3kw alone won't trip a 20A breaker when running a 240v load, since 3kw is only about 15.6A, even assuming a 0.8 power factor. You're not even up over the 80% rule limit for a 20A breaker doing that.
The datasheet does say to use a 40A circuit though, which would be good for ~7kw at 240V when allowing for the 80% rule. With a 40A circuit, which should be using 8 gauge wire, the OP shouldn't have any more problems with this unit.
Bill
… which part of “a 3 ton will pull 3kwh” is wrong? Now be careful here… all I am saying is that a 3 ton can and will in colder weather pull 3 kw per hour (kWh) which is absolutely correct :)
Also a “3 kWh” unit will trip a 20 amp breaker because that is 3 kw per hour which in reality means in the beginning it will pull well over that (and trip the breaker) and towards the end it will run well under that… you know any mini split will start hard and then taper off as the cycle progresses. So for illustration purposes a 3kwh consumption will 4kw the first 20 mins, then 3kw the next 20 mins and 2kw the last 20 for total of 3kwh…
"which part of “a 3 ton will pull 3kwh” is wrong? "
That mix of units makes no sense. There is no time involved, so it "pulls" 3 kwh over what period of time? An hour? A minute? If it's over an hour, it is "pulling" 3 kw. If it's over a minute, then it's "pulling" 180 kw to use 3 kwh, since "kwh" is a unit OVER AN HOUR.
Examples:
"My pickup truck can haul 500 pounds of brick per hour". Sure, but how far did you go? Across the parking lot? All the way home? The statement doesn't make sense, because you didn't specify HOW FAR you hauled that brick.
"my well pump can pump 18 gallons". OK, but can it pump 18 gallons ONLY, then it burns up and dies? Can it pump 18 gallons PER DAY? Can it pump 18 gallons PER HOUR? Usually this unit would be gallons PER MINUTE.
This all reminds me of a chemistry lab a long time ago in college. We all came to class one day, and the professor wrote a number (maybe 17, but it was a long time ago) on the board, then stared at the class... For quite a while. We thought maybe that 17% was the class average on the last lab? He then smacked the board, glared at us, and yelled (this was in his German accent): "WHAT IS!? IS COWS?! IS HOUSES?! UNITS, UNITS, UNITS!!!"
Anyway, I'll always remember that, and UNITS DO MATTER.
Saying it "pulls 3kwh" doesn't tell you anything at all about the actual load of the unit, because you have no time unit involved. If the unit uses 3kw, then it uses (assuming unity power factor and 240v supply), 12.5 amps, which will NOT trip a 20A breaker and will NOT be a code issue with the 80% rule. There is no need to assume anything that way. Your own example you just gave though, implies the actual load varies over an hour, which is why saying the "load" is "3kwh" doesn't make any sense. Loads are in RATES, such as "3 kw", which is like "gallons per minute". A kilowatt hour is a unit of VOLUME, like gallons, a fixed amount.
The big issue I see with confusing those two units is with wind power. I see all the time mixups like "this wind farm supplies 3 MW". OK, but how often? Generating production is usually given in megawatt hours PER YEAR, so 3MWh over a year would be a 342 watt generator running 24x7, or a 3 MW generator running for ONE HOUR PER YEAR. If the AVERAGE rate of production was 3MW, then you'd get 26,280 megawatt hours PER YEAR. It's a huge difference, very significant, and very important to understand. It's easy to manipulate production numbers by mixing up those two units, which I see done intentionally suprisingly frequently in the media.
Bill
You can’t be serious. “3kwh” does not indicate 3kw over what period of time?
Again- a 3kw unit will not trip a 20 amp breaker at 240. Correct. But a 3kwh unit will trip it because as I said it will be 4kw for 20 mins, 3kw for 20 mins and 2kw for 20 mins… for a total of 3kwh. That is why I didn’t say “3kw unit will trip a 20a breaker”, but I said “a 3kwh will”.
I will leave it at that.
Merry Christmas :)
I am serious, because you imply that the unit will then run 1 hour and shut off. It will not run 15 minutes. It will not run 3 hours. It will run EXACTLY ONE HOUR and that's it.
Loads are not given in kwh. Always in kw. Because the operational time, the run time, of the load is not defined when saying the amount of power the unit will draw while running. You will never see datasheets saying that a device needs a circuit sized for "3kwh", for example. It will say the circuit needs to be sized for a given number of amps, and amps * volts gives kw (roughly), not kwh.
And yes, this distinction is important when sizing circuits.
Bill
Thanks for the responses! Had the electrician out today, and he replaced the breaker AND wiring, and we're back up and running. That's all the detail I received, but will get eyes on in the next couple days and see what's changed. We're supposed to see some real cold this weekend, so we'll see how it performs.
Update: 40A breaker and 8 ga wiring installed, and the unit is humming along nicely. 17 deg outside and 70 inside. Thanks again for the reassuring help!