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I recently bought a house with really nice hardwood floors, but the builder didn't put any thought into installing a humidifier system. I'm now looking into my options and steam humidifiers seem to be the highest output solution, albeit overkill. Unfortunately, the only power source I have available in the furnace room is on a 15A breaker shared with two bathrooms, but my tinkering instincts won't accept defeat so quickly.

I'm thinking that I could use a UPS with sufficient output power to support the 7A steam humidifier. The trick is that I would use a current switch to sense when the humidifier is on and subsequently turn off the power source to the UPS (with a relay) and when the humidifier is off the UPS will reconnect and start charging. My theory is that the no load charging current of a UPS is always lower than the maximum load current. Someone correct me if I'm wrong.

The question is: How do I determine a UPS's charging current spec? If this is low enough to put on the 15A circuit then I'm good to go. If it's still too high then I have to accept defeat... unless someone has another clever solution. Given the construction of the building I cannot run a new 20A drop to this room.

Assumptions: Yes, I know, this isn't what a UPS is meant for.. but that isn't a part of the question. Also, I'll probably go through a set of batteries in a year and I'm OK with that.

ChrisF
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Jake
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  • Are you unable to swap the breaker with a higher rated one? –  Dec 03 '12 at 01:48
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    That would be like putting a bigger fuse in a car. The wires are only rated for so much and to keep things safe the breakers have to stay as is. Otherwise, the wires could get hot, melt, or burn and we don't want that for the holidays! :) – Jake Dec 03 '12 at 01:55
  • I guess I should have put a disclaimer if his wiring would allow it. 20A isn't uncommon AFAIK, and might allow him to get by. Otherwise I'd just run little romex. Not too hard. –  Dec 03 '12 at 02:01
  • No worries. I would rather run the wire, but I would have to do some serious demolition to make that happen. – Jake Dec 03 '12 at 02:04
  • How long will that humidifier run? How often it'll run? You have a 120V, right? –  Dec 03 '12 at 02:11
  • It is 120V. That's a tricky question, and probably the crux of the whole problem. I would guess that it may run for 10 minutes twice a day nominally, but on a dry day it may run longer. Of course, this would deplete the battery. I could put a time limited relay on the UPS output but that's getting complicated... – Jake Dec 03 '12 at 02:18
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    *"How do I determine a UPS's charging current spec?"* - A Kill-a-Watt power meter would give an answer. You need to verify that the humidifier can cope with square or semi-sinusoidal (or whatever) waves if you don't put up the big bucks for a UPS that does output pure sine-wave AC. @lexvegas - 20amp circuits are not common, and have special outlets to indicate that higher current capacity. –  Dec 03 '12 at 02:28
  • @sawdust good point on the Kill-a-Watt. I'll check that out on my *other* UPS. Yes, the current switch is not rated for 7A, but I should clarify that I was planning on using an appropriately rated relay to switch the humidifier. – Jake Dec 03 '12 at 02:47
  • If it wasn't obvious already, you'd need to modify the UPS to deal with this situation. Most are designed to complain *very loudly* when they're powering devices on battery. –  Dec 03 '12 at 02:59
  • I was hoping someone at superuser would have expert knowledge on UPS power requirements but @MaQleod is probably right. – Jake Dec 03 '12 at 02:59
  • @Charles Yes, with something like an APC Smart UPS I can disable the alarm. – Jake Dec 03 '12 at 03:00
  • Typical UPSes take several hours to charge but drain in 12 minutes or so. So the charging current is going to much, much less than the full load current. –  Dec 03 '12 at 03:52
  • I think you're way out in left field. If you have a regular forced air furnace see if you can't install a bypass humidifier. They draw virtually no current. Or, just put the steam humidifier on the 15A circuit. Two bathrooms, unless they include electric heaters of some sort, will draw a max of about 6A. –  Dec 03 '12 at 04:01
  • +1 for a quirky 'just might work' power management solution. The UPS should have a current rating right on the nameplate. It would be better to somehow prevent the UPS from just passing power through when there is a load, and have it always charging the battery. Maybe put a cheap little inverter on the battery from the ups and a cheap little charger on it too. OTOH, you should have good reason to waste all that lead. –  Dec 03 '12 at 06:49
  • @DanielRHicks - *"unless they include electric heaters of some sort"* - Umm, I think the most common one is called a (handheld) "hair dryer", and many are rated over 1000 watts. –  Dec 03 '12 at 07:10
  • Is the 15A bathroom circuit for lighting, or does it feed receptacles as well? – Tester101 Dec 03 '12 at 12:06
  • A bathroom should have at least one dedicted 20amp breaker for a GFCI outlet. If you already have two bathrooms completely on the same circuit then it would not be a good idea to run a UPS on this circuit as well. – maple_shaft Dec 03 '12 at 13:36
  • @sawdust - 1200 watts. I had to make it a mandatory limit to not run more than one at a time, the breakers just can't handle it. – Fiasco Labs Dec 03 '12 at 15:21
  • I'll have to double check the bathroom breaker configuration but I think only the light fixtures are shared. – Jake Dec 03 '12 at 18:19
  • I should have clarified in the question that a bypass unit is somewhat difficult to install given that I have a downdraft unit in a cramped closet. Regardless, that's probably what I will do. – Jake Dec 03 '12 at 18:26
  • Instead of a bypass unit you can install a "power humidifier" that has a fan (but no heater) -- total power drain under 100 watts. This is about the same price as the steam unit. If you buy the AprilAire unit you can supply it with hot water, allowing it to work effectively on the cold plenum. (Other brands do not allow hot water and only work on the warm plenum.) – Hot Licks Dec 04 '12 at 16:20

2 Answers2

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No.

You definitely want to put the humidifier on its own circuit, or a circuit that is not shared with two powder/bath rooms. You could look for a convenience receptacle in the basement and determine if it's on the same circuit. If not, you can probably come out of that for the humidifier. If the humidifier pulls it's full 7A rating when it starts, you probably want to avoid having it share a lighting circuit, as the lights could dim when it turns on. That can cause premature wear on ballasts and greatly reduce the life of any incandescent lamps you might have in use. Take care to know exactly what else is on any circuit you find in the neighborhood.

NEC requires wiring devices, such as the relays you're talking about to be listed and approved. Assembling something to cut the power to the UPS when the humidifier kicks in using all listed and approved parts does not constitute a listed and approved device. Your approach, while very creative gets a little too far into the "Rube Goldberg" brand of tinkering.

My advice is, have an electrician install a single use receptacle for the humidifier on its own 15A circuit, even if that's slightly overkill. If your panel will not accommodate this, and tandem breakers aren't a possibility, then consider increasing the size of your panel and / or service.

Two bathrooms on a single 15A circuit is problematic enough, it sounds like you need to bite the bullet and get a few additional circuits where they're needed.

Tim Post
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  • Definitely a +1 for the last two paragraphs alone. Overkill is good where power supplies are concerned. – Rory Alsop Dec 03 '12 at 09:36
  • Excellent technical answer, particularly the NEC standard... I was afraid of that. I will look into my options but I don't think I will be able to install another drop. Thanks! – Jake Dec 03 '12 at 18:27
  • Is this a new house? As of 1999 electrical code mandates a separate 20amp circuit for each bathroom! – Bryce Dec 03 '12 at 20:19
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You may be able to solve this problem with a subpanel. The wiring to the furnace is probably beefy already. Feed the subpanel with the cable that currently goes to the furnace. Add a breaker in the subpanel for the new circuit to the furnace. Add a circuit for the humidifier.

To know for sure if you can do this, we'd need more details:

  • The size of the breaker(s) feeding the furnace

  • The size of the conductors feeding the furnace

  • The load ratings on the furnace

If you're lucky, the electrician may have picked a larger wire than required, perhaps to protect the blower motor.

Jay Bazuzi
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  • Also, if you do get an electrician to run a new circuit for your humidifier, have him install a small subpanel in the furnace room. That will make it easy to add new circuits in the future. – Jay Bazuzi Dec 03 '12 at 15:23