Digital humidistat R-C resistance too low

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catour
Posts: 13
Joined: Sun Dec 31, 2017 11:16 pm

Digital humidistat R-C resistance too low

Post by catour » Mon Jan 13, 2020 10:15 pm

Hi guys, I've just bought a Honeywell HumidiPRO humidistat with an outdoor temp. sensor to mitigate condensation on the windows. The unit requires 24 VAC on its R-C terminals, so I connected my furnace R to the thermostat R and furnace C to the thermostat C. However, when I powered up the furnace, the thermostat didn't react and a 3 amps fuse near the furnace control board tripped within a 1-2 seconds, so I disconnected everything and measured again R-C voltage of the furnace (it's 27.5v btw, should I be worried it's so far from 24v?), and then I found that the resistance between R and C terminals of the humidistat was just 0.3 Ohm, does it mean they got shorted internally or I'm missing something? If they are shorted, that would explain why the fuse tripped.
Just in case, here is the unit manual https://customer.honeywell.com/resource ... 794EFS.pdf.

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emtnut
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Joined: Mon Mar 02, 2015 7:21 am
Location: Canada

Re: Digital humidistat R-C resistance too low

Post by emtnut » Tue Jan 14, 2020 10:15 am

catour wrote:
Mon Jan 13, 2020 10:15 pm
Hi guys, I've just bought a Honeywell HumidiPRO humidistat with an outdoor temp. sensor to mitigate condensation on the windows. The unit requires 24 VAC on its R-C terminals, so I connected my furnace R to the thermostat R and furnace C to the thermostat C. However, when I powered up the furnace, the thermostat didn't react and a 3 amps fuse near the furnace control board tripped within a 1-2 seconds, so I disconnected everything and measured again R-C voltage of the furnace (it's 27.5v btw, should I be worried it's so far from 24v?), and then I found that the resistance between R and C terminals of the humidistat was just 0.3 Ohm, does it mean they got shorted internally or I'm missing something? If they are shorted, that would explain why the fuse tripped.
Just in case, here is the unit manual https://customer.honeywell.com/resource ... 794EFS.pdf.
That RC measurement seems off ! Possibly it was bad out of the box.
Or, something funny with your wiring (sometimes one cable from the furnace gets split out to another cable, and if someone didn't get the colors right, it could have blown something )

I'd start with a call to the manufacturer, see what they suggest.

Honeywell customer care 1-800-468-1502


In the meantime, you could measure the RC wires (not connected) and verify you have 24Vac
(note, the 27v is normal. It will go down a little as you load the transformer, but even 27V at the tstat is fine.)
~~ Ford Ford Ford Ford Ford Ford :mrgreen: ~~

catour
Posts: 13
Joined: Sun Dec 31, 2017 11:16 pm

Re: Digital humidistat R-C resistance too low

Post by catour » Tue Jan 14, 2020 7:15 pm

I am almost sure that the unit arrived broken, bought it on Amazon an new, but the box was re-sealed and on the back panel of the unit I found marks from screws, i.e. it had been attached to a wall before. I'm going to return it, just wanted to ask whether I was doing something obviously wrong.