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Douglas, HVAC Technician
Category: HVAC
Satisfied Customers: 3089
Experience:  Manufacture Rep for Major Brand, Technical Trainer, NATE
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Trane Whole House Humidifier Not Working

Customer Question

Along with a newly installed Trane high efficiency heat pump, I had a Trane whole house humidifier installed. The humidifier has been tested by the installer to make sure it has positive air pressure flowing into it, and that the water is coming on a running through the approximately 12"x12" evaporator screen whenever the heat comes on. The problem is that in a relatively small (1300 sq ft) condo, the humidity is almost completely unaffected by this unit. The difference between turning it completely off and turning it on at the max setting, is only a difference of 2 or 3 % humidity. With the unit set at 40%, the humidity in the house only goes up to about 25%. This has been verified with 2 different humidity monitors.

After several visits the Trane installer is saying there is nothing else they can do other than maybe switching it to run off of hot water rather than cold water to try and get it to evaporate the water faster. But would seem to be ridiculously inefficient and expensive to run water out of the home's hot water heater each time the heat comes on. One other thing to note is that this is a downdraft configuration (the unit is installed upside down and is in a closet on the loft level of the condo). So the positive pressure is being tapped from the output of the bottom of the unit via an 8" flexible duct. This is being run back up to the top of the unit where the humidifier is installed. And the humidified air is presumably then being pushed back down through the heating coils of the unit and out through the output duct work.

HELP !!!

Additional Note - The following model numbers are on the installation receipt, but I don't have the humidifier Model number. It does however have a "TRANE" logo on the outer plastic housing.

Heat Pump

Air Handler

Backophealir ???
Submitted: 5 years ago.
Category: HVAC
Expert:  Douglas replied 5 years ago.
Hi, I'm Douglas and I am here to help you resolve problems with your heating and cooling equipment.

This may not be the greatest news. but this is an evaporative type humidifier. The way it works is hot air blows across the pad, hot air has a high capacity for moisture and will be very dry, it absorbes the moisture. The problem here is it is installed on a heat pump that produces luke warm air, this luke warm air does not have the capacity to suck up the moisture like a gas furnace hot air would.

If your humidifier for instance was rated for say 15 gallons of water into the air per day, that would be with a supply air temperature of 120 degrees pretty much continuously at 100 degrees, the capacity of that humidifier may be half or less. If the supply air temperature is 90, even less.

Evaporative humidifers are also at the mercy of the heating run time. If the heat pump is not on, and just the fan is on, then all the water just goes down the drain. Nothing gets picked up. If the heat pump runs for only say 10 hours (collectively) throughout the course of the day, then you may be lucky to get 1/4 of the humidifiers rated output at best.

Some tecncians believe that using hot water will improve this, however this is marginal at best because hot water does not have any influence on the air that is passing over it, 120 degree water does not boil into steam or evaporate any quicker (at least not significantly).

What he can do is remove that humidifier and install a steam humidifier. They are certainly more costly but the steam humidifiers are designed to push water into the air, rather than absorb it.

I strongly discourage contractors from selling evaporative humidifiers with heat pumps without completely explaining this to their customers. Its alot to explain the Psycrometric chart and why or why not air will absorb moisture but give me a minute and I will show you some comparisons so you can see why.

Expert:  Douglas replied 5 years ago.
Lets say your home is 70 degrees with 35% relative humidity for arguments sake.

That means the air has 38.13 grains of moisture per pound of air.

When I pass that over a wet panel, it will not likely pick up any moisture, it would be no better than just setting a bowl of water in the middle of the floor and blowing a fan across it. In fact it will only cool the air.

Now if I heat that same air up to 90 degrees, the humidity at that temperature (relative humidity) will be 18% RH. Dryer but not exactly parched. If wil pick up a little moisture.

If I heat that same air to 120 degrees the relative humidity of that 120 degree air is only 7% and it would suck up alot of moisture.

Think of it like a towl. A damp towl will not suck up as much mosture as a dry towl will.

Relative humidity is just that relative to the temperature.

If I cooled that 70 degree air to say 50 degrees, the humidity would be over 70%.

Again, I dont know if that explains it but it should make a little sense as to why I dont suggest evaporative humidifiers on heat pumps.

Here is a response on one humidifers FAQ sites to the question will a humidifier work on my heat pump...

"Model 400 humidifiers can be installed on heat pump systems. However, due to the fact that heat pumps deliver lower temperature air to the home than gas furnaces, evaporation will be approximately 60% of rated capacity (Note: With other Aprilaire models, hot water can be used instead of cold water to maximize evaporation. However, due to the nature of the wicking water panel in the Model 400, hot water provides no extra benefit). As such, your dealer will need to take the size and age of your home into consideration to ensure the Model 400 will provide satisfactory comfort and protection through adequate humidification"

Now I think they are being liberal with the heat output of the heat pump by giving it 60% capacity.

Another quote from a Humidifier application traning manual.

"Humidifier output is usually based on 120° duct air temperature. Older units have been tested at 140° duct temperature. Hence, warmer air has more energy resulting in higher humidifier output. So when do evaporative humidifiers perform at there rated capacity? When the forced air furnace is in the heating cycle and is discharging 120° air. During any other operating cycle without 120° air the evaporative pad type humidifier is performing well below the rated capacity because no energy is available to change the water to a vapor. Note evaporative pad humidifiers are rated using 120° duct air for 24 hours. Actual daily output from the humidifier will vary greatly based on the furnace operating cycles. HVAC systems discharging air temperature below 120° will not have the output, for example a heat pump. Remember the energy is needed to absorb the water into the air. Heat pump systems require other humidifier technologies that can create their own source of energy, such as steam systems."
Douglas and other HVAC Specialists are ready to help you
Customer: replied 5 years ago.
Not sure if the reply I sent the first time went through...

This is frustrating because I actually asked the installer to use a steam unit, but he talked me out of it saying that the evaporative unit was cheaper and would work fine.

In addition to steam units, which obviously have to generate enough heat to evaporate water with electricity... Is there such thing as a "fogging" unit? Like some of the home table top humidifiers you see that create a cool water fog? And if so, is this a viable option too?

Lastly... Can you recommend a few brand/model units that would probably work in my configuration that will run off of a water line rather than a floor model that would need to be refilled manually?
Expert:  Douglas replied 5 years ago.
I am not aware of any fogging units, those are actually called atomizers, sort of like the little pump purfume bottles, it breaks the water up into a fine mist. The problem with those is they can rust out your ductwork. I used to have an old furnace in a house that someone tried to fashion one into the duct system... I had to replace most of the duct.

As far as steam units go, Aprilarie makes one, so does Honeywell and there are even a few others out there.

I like the aprilaire 800...

See this lInk

Keep in mind steam humidifiers require a dedicated circuit and some need 240v to operate. This of course increases the installation costs but in the end it works unlike the situation you are in. I am sorry he didnt give you your request, I am not sure why he would, unless he simply doesnt understand how they work. He really should check with his disitributor rather than shoot from the cuff when it comes to applications. I am sure it at this point he probably thinks the humidifier is defective or you arent operating it properly as opposed to realizing its simply misapplied.

I would ask him to refund the humidifier portion or apply it towards putting a proper steam unit in. If he's not had experience with steam, maybe he can sub someone who is.