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
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.