How to turn on the fan, when humidifier starts?

doofusFebruary 4, 2013

Hello!

We have a fairly vanilla thermostat, that controls heat (hot water) in winter and central AC in summer.

In addition, there is a humidifier to inject moisture into the AC ducts. The trouble is, humidifier has its own control, but, to have an effect, needs the fan running.

In summer the fan (in "Auto" position) starts together with the AC, but is not affected by humidifier's controls.

Is it possible to alter the existing wiring (both the humidistat and the thermostat are located next to each other) so that fan would kick-in automatically, when either the AC or the humidifier turn on?

Currently we have to keep the humidistat set to 0 most of the time and turn both the fan and the humidifier on and off together manually (by setting humidistat to the desired humidity and the fan -- to "On"). Obviously, this is suboptimal... Leaving the fan running at all times in winter is a little too noisy.

Thanks for advice!

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weedmeister

I think that sometimes you can put a 'sail' switch in series with the humidifier. This is something you install in the vent so that when the fan is active, the humidifier has power.

    Bookmark   February 4, 2013 at 4:37PM
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doofus

This is something you install in the vent so that when the fan is active, the humidifier has power.

I think, I need it the other way around -- because the fan is never on during winter (when AC is not in use).

Currently, AC turns on the fan. I'd like for humidifier to also be turning it on.

    Bookmark   February 4, 2013 at 4:44PM
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mike_home

You need a thermostat measures humidity. It can be programmed to turn on the humidifier and fan whenever there is a call to humidify. The Honeywell Visionpro IAQ thermostat is one example. Take a look at the installation manual.

Here is a link that might be useful: Honeywell Visionpro IAQ thermostat

    Bookmark   February 4, 2013 at 8:34PM
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ionized_gw

How in the world did you get a humidifier installed in a cooling-only system? Can any of the pros here say that this humidifier will work properly with no furnace running to provide arid air at the humidifier?

    Bookmark   February 5, 2013 at 7:50PM
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mike_home

The OP is running the fan in the air handler during the winter in order to circulate air through the humidifier. The set up works but there is no control of the fan by the humidstat. He would like the humidstat to control both the fan and the humidifier.

    Bookmark   February 6, 2013 at 10:02AM
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ionized_gw

Are you planning on doing this yourself? With and old-school furnace It should be easy for any competent service pro to install a relay to run the blower with the humidistat is running. Not being a pro, I would not know where to start with modern, electronic controls.

I question whether a humidistat will work properly without a furnace running. If air needs humidity, assume 30% RH in the house, the RH in the hot duct at the furnace is going to be less than 5%. There is a big difference in evap rate.

    Bookmark   February 6, 2013 at 10:51AM
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doofus

How in the world did you get a humidifier installed in a cooling-only system? Can any of the pros here say that this humidifier will work properly with no furnace running to provide arid air at the humidifier?

Why would not it? Though the hot-water heating does not dry the air as a hot-air system heating would, the relative humidity of heated cold air is still lower.

So, it is not as dry, but one's lips may still chap overnight.

    Bookmark   February 6, 2013 at 1:53PM
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ionized_gw

Because, as I pointed out in my last post, an in-duct humidifier typically has air going by that is over 100 F. The relative humidity is in the single digits. Water evaporates much faster at 5% RH than at 30%.

How does hot water heating dry the air less than forced air? RH is the measure so only absolute humidity and temp go in to that. All other things being equal, how the air is heated has no impact.

    Bookmark   February 6, 2013 at 2:19PM
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mike_home

Ionized,

What you say is true, but you can still raise the humidity of the house with room temperature air. In a typical furnace set up the humidifier can be installed either on the supply side or the return side. If it is on the return side, the air would be at room temperature. Even if the humidifier was on the supply side, there will be times the humidstat is calling for humidity without a call for heat. In this case the air would be at room temperature.

A forced air system does not dry out the air more than a hot water system. The relative humidity decreases regardless as to how the air is heated.

    Bookmark   February 6, 2013 at 2:47PM
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ionized_gw

OK, so the humidifier has to run a lot longer on the supply side, or with no heat, but it will work adequately. I was concerned about the magnitude of the effect.

The idea that a forced air system dries more than steam or hydronic heat is a common one. The simple answer is that they don't, inherently. There are at least two slightly more complicated answers that might make it different. If the forced air heat is blowing on YOU, it might feel more dry. In addition to that, a leaky forced air ducts outside of the house envelope tend to bring in dry outside air which would dry the house more.

    Bookmark   February 6, 2013 at 3:06PM
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doug_gb

This is the dumbest thing I've heard of - a humidifier in an A/C plenum.

Additionally blowing ambient temperature air is going to feel really cold.

Get a couple of portable power humidifiers and be done with it.

    Bookmark   February 7, 2013 at 12:43PM
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doofus

This is the dumbest thing I've heard of - a humidifier in an A/C plenum.

Honestly? You never heard anything dumber? Seriously? Or are you just using one of the most beaten-up and overused metaphors (second, perhaps, only to the "touch base") to begin your forum posting with an insult? Ok...
Additionally blowing ambient temperature air is going to feel really cold.

Not any colder, than from a portable humidifier, right?
Get a couple of portable power humidifiers and be done with it.

We already have the humidifier installed. It works. It is Ok. The only problem is the one I'm describing -- the fan must be either running continuously or turned on manually. It is mildly inconvenient, but does not justify throwing the existing device out and buying new one(s).

In addition, the advantages our existing system has over portable humidifiers are:

  1. No additional space is required in the living areas.
  2. No need to refill the water. Ever.
    Bookmark   February 7, 2013 at 1:26PM
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doofus

You need a thermostat measures humidity. It can be programmed to turn on the humidifier and fan whenever there is a call to humidify. The Honeywell Visionpro IAQ thermostat is one example. Take a look at the installation manual.

Here is a link that might be useful: Honeywell Visionpro IAQ thermostat

Thanks, Mike. Our current thermostat is much simpler -- the Honeywell TH4110D. On top of it sits the humidistat -- Honeywell H8908A.

I was hoping, I could just run an additional little wire from humidistat to the fan-controlling connection (labeled "G") on the thermostat. But if that's not going to work (or will be turning on the humidifier whenever AC is turned on in summer), and I need to replace the existing units with a single one, will I not have to run any additional wires? The installation diagram for the model you suggested looks a lot hairier :-)

    Bookmark   February 7, 2013 at 1:48PM
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doug_gb

There are a couple of factors in the solution. How many low voltage transformers do you have for the humidifier, furnance and AC? I'm thinking you have one for each.

If so, you can accomplish it with a 24 volt relay. Hook the relay to the low voltage humidistat circuit so it's energized when the humidistat is calling for humidity. Take the Rc (cooling low volt) into one side of the relay. Then take the output of the relay and connect it to the G terminal on the TH4110.

To test, before you actually decide to do this: with a small piece of wire jumper the G and Rc terminals - the fan should run.

    Bookmark   February 7, 2013 at 2:30PM
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mike_home

I think Doug's suggestion will work but I would want to make sure you don't short Rc and Rh connections when the thermostat is set to heat in the winter. The G terminal may be energized by one or the other depending on the heat and cool switch. If there a voltage drop between the transformers you may smoke the relay.

The IAQ thermostat would be nicer. These are meant to be installed by professionals, but the instructions are clear as to how to install it.

    Bookmark   February 7, 2013 at 2:58PM
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doug_gb

How about this: Leave the fan set to on - in the basement cut the wire going to the fan relay - attach this to the NO contacts on a 24 volt relay - which is wired to the Humidistat control circuit - when the humidistat calls for humidity, the relay is energized and the fan will then run.

In the summer, you would have to jumper this relay to work without the Humidistat.

    Bookmark   February 7, 2013 at 3:38PM
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