Whole-house bypass humidifier question

[dwasifar]

I have a dual-zoned home HVAC system (two furnaces, two AC units). Each furnace has a bypass whole-house humidifier. Originally these humidifiers were Honeywell HE220, but those eventually failed and were replaced with HE240A units (not at the same time, but within a year of each other).

The difference between the old and new units is that the HE240A includes a pressure switch. One tube from the pressure switch leads to the humidifier body and the other to the cold air return, so the switch can detect a pressure difference and allow the water valve solenoid to open.

I noticed the house getting mighty dry, so I did some troubleshooting. The pads and drains were bone dry, with dry calcification in the drains suggesting there hadn't been water through them in quite a while. I eventually determined neither pressure switch was operating. When I bypassed the pressure switches, the solenoids opened and the humidifiers started operating.

In this installation, 24v to each humidifier comes off the "HUMI" tab on the furnace's control board. Since that tab is only high when the furnace is blowing, it looks to me like the pressure switches are checking for a condition that will never occur (humidifier energized but furnace not operating). So if I understand it correctly, I can leave them bypassed.

Or am I missing something important?

 

[fitter30]

Hoses on switch. Hoses have a grommet for duct to hose seal? If that doesnt work just hook up one line leave the other disconnected. Is there a damper in the flex duct line?
Switch hook up
Black supply side
grey return side

 

[FredWanaker]

If the humidifier is installed on the supply duct (as is recommended), the pressure switch
needs to have the tubing fed to the return duct. If the humidifier is installed on the return duct,
the pressure switch needs to have the tubing fed to the supply duct.

Manual he240-he280-honeywell-humidifier-installation-and-owners-manual.pdf . Part number appears to be IS30132-5972

 

[dwasifar]

Hoses on switch. Hoses have a grommet for duct to hose seal? If that doesnt work just hook up one line leave the other disconnected. Is there a damper in the flex duct line?
Switch hook up
Black supply side
grey return side

Yes, there is a grommet, and the tubes are correctly installed, but my question is not "How do I get this switch working again?" Rather, it's "Why would I need this switch at all?"

The point of the switch is to turn off voltage to the solenoid when there's no airflow, right? But the furnace mainboard controls the humidifier and only provides voltage to it when the blower is running. So if there's no airflow, that means the blower isn't running, and there's no voltage to the humidifier anyway. So the pressure switch, as far as I can tell, is redundant in this situation and might as well be bypassed.

I can see why the pressure switch would be needed if you were powering the humidifier with a plain old transformer. But that's not the case here.

 

[PoorUB]

Just bypass the pressure switch The HUMI terminals are hot only when the fan is running.

 

[dwasifar]

Just bypass the pressure switch The HUMI terminals are hot only when the fan is running.

That's what I thought.

 

[FredWanaker]

But the furnace mainboard controls the humidifier and only provides voltage to it when the blower is running.

that is not how the manual I read spec'd it be installed. It is designed to have its own humidity controller, and its own transformer. It needs to know when it the air handler is running, and it uses differential pressure between return and supply. The pressure hose has to be on the correct side too depending whether the unit is installed in return or supply. I posted a link to the manual. Good luck.

 

[fitter30]

that is not how the manual I read spec'd it be installed. It is designed to have its own humidity controller, and its own transformer. It needs to know when it the air handler is running, and it uses differential pressure between return and supply. The pressure hose has to be on the correct side too depending whether the unit is installed in return or supply. I posted a link to the manual. Good luck.

Most furnaces built in the last 10 years come with a humidifier output 24volts that come out of the main control board when blowers running. To adjust rh there is a damper that adjust airflow over the evaporator pad.

 

[PoorUB]

Most furnaces built in the last 10 years come with a humidifier output 24volts that come out of the main control board when blowers running. To adjust rh there is a damper that adjust airflow over the evaporator pad.

Umm, no.
Yes the furnace will control power to the humidifier, but you still need a humidistat to control how much humidity you have in the house. You can not accurately control humidity with just a damper.
You should use a humidistat that offsets with outdoor temps to control moisture on your windows. As the temp drops outside the humidity in the house drops too.

 

[csi123]

Yes you definitely need a humidistat unless you enjoy mold growing all over your house. The damper is not for controlling the humidity. It is for closing it off in the summer when the AC is running.