Humidistat rewire help

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Yar02169

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Hello!

I have a whole house humidifier, and the humidistat has seen better days. The plastic strip thingie that determines the humidity has failed, so the unit sits in the extra dry house air.

The humidifier is an old Sears unit, that works well. The humidistat mounts into the air return, I have the contact closed, so the humidifier works whenever the furnace runs now, better to have the humidity than a dry mouth.

Here's my issue: The current humidistat uses line voltage, and when needed, the switch closes and the humidifier turns on. A replacement line voltage humidistat is quite expensive, so I'm figuring to go with a 24V humidistat. That's where I get a little foggy in how to do it.

I know I would take the Line In to a transformer, down to 24V for the humidistat. The humidistat would do it's job, and I assume run a signal to a relay that would turn on the humidifier.

Now, what do I need for a relay? The box store has a transformer, so that's not an issue.
Can I use the line feed to the transformer over to the In on the relay? That would allow the Out to power the humidifier.
I guess it depends on the relay, but how would I go about mounting this set up? A new question is, it there an all-in-one unit that would keep me from re-inventing the wheel?

Thanks in advance,
Ray
 

Dana

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Humidifiers are a solution problem, and have a record of causing mold issues in older houses.

The better solution to dry wintertime conditioned space air is to air-seal the house to less than 5 air exchanges at 50 pascals pressure (ACH/50, a standard blower door test number.) Current code-max in MA under energy stretch codes is 3ACH/50. Air sealing work is subsidized by MassSave in most of MA, and is one of the most cost-effective energy measures you could ever take (even without the subsidy), with a big ROI on heating energy use, and a big ROI on comfort.

With a house sufficiently tightened you would need to actively ventilate to get the house BELOW 30% RH @ 68F in winter, and if it creeps up to 40% RH @ 68F it's wise to run the kitchen & bath exhaust fans more religiously to bring it down. Installing a vacancy detector switch on the bath fan set with a 30 minute time out is often all that it takes. If that's too drying (not likely) you can adjust the time out preset appropriately.

Monitor the RH & temp in a few rooms (not the bathroom) with a cheap $10 AcuRites. Don't sweat the RH numbers if they're higher than 40% at temps below 68F, but pay attention if it's above 35% at higher room temps even during cold snaps.

The health & comfort zone for humans is 30-50% RH. But for houses in this climate it's better to hew toward the low end of that range from half-past December through half-past March, since any 35%+ / 70F air leaking from the conditioned space into walls will deliver moisture, adsorbed & collecting in the sheathing whenever the outdoor temps are below 40F. Then in spring when the temps warm up and the sheathing moisture is released into the wall cavities mold conditions can occur.

In an air-leaky house with a humidifier, the moisture you're putting into the air with a humidifier ends up accumulating in materials along the exfiltration path, at much larger volumes than you get with tighter houses, making it an even higher risk. With dry interior air the mold risk is low, even in leaky houses, but if you intentionally raise it with a humidifer, the mold risk soars.

So, rather than monkeying around and spending time/money/energy fixing the solution-problem, band-aid, try fixing the actual problem, which is the high air infiltration rate.

FWIW: My over-ventilated office is running 16%RH @ 70F right now, and it's lip-cracking hand-drying dry-sinus misery. My sorta-tightened house in Worcester runs in the mid-30s for RH even during cold snaps, higher if people don't use the kitchen or bath fans regularly, and it's MUCH more comfortable. I've long since scrapped the humidifier a previous owner installed on the hydro-air zone, which was more of a hazard than a help.
 

DougB

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Using a relay: Relay has a 24 volt coil - normally open. Wire the 'hot' 120v wire to one terminal of the switching relay - the other terminal to the humidifier. When the relay is energized, this will supply 120v to the humidifier. The neutral is wired direct to the humidifier.

Connect the primary side of the 24v transformer to the hot wire on the furnace fan - this way the transformer will only be powered when the fan is on. Connect the other primary to the transformer neutral.

Now for the secondary on the transformer: Connect the neutral secondary to the relay coil. The other secondary - will run to the humidistat - this is the 'switch'. The return wire from the humidistat connects to the other terminal of the relay coil.

If you Google 24 volt switching relay SPST N/O AC you will find them for $18 - $25 check Grainger.
 
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