NA — Whole House Humidifier

[JGoodish]

After many years of procrastination, I’d like to finally install a humidifier on my forced-air furnace. It seems that Aprilaire is the most commonly used brand, and I was planning to use the Aprilaire 700 until I realized that it wouldn’t fit. In fact, I’m not sure what *will* fit, whether I should put it on the return or supply (concerned with reducing supply pressure), and how to route the ducting (assuming a bypass humidifier). Picture attached, but between the steel beam on one side of the supply duct and the water heater on the other, there isn’t much room on either side.

Any thoughts and suggestions appreciated.

EDIT: The furnace is a basic Concord which the builder installed. The 6” duct off of the supply in the picture leads to a register in the kitchen.

 

[Capt. Geoffrey Thorpe]

The unit is mounted on the return, and a duct runs to the supply.

 

[sarangan]

After many years of procrastination, I’d like to finally install a humidifier on my forced-air furnace. It seems that Aprilaire is the most commonly used brand, and I was planning to use the Aprilaire 700 until I realized that it wouldn’t fit. In fact, I’m not sure what *will* fit, whether I should put it on the return or supply (concerned with reducing supply pressure), and how to route the ducting (assuming a bypass humidifier). Picture attached, but between the steel beam on one side of the supply duct and the water heater on the other, there isn’t much room on either side.

Any thoughts and suggestions appreciated.

EDIT: The furnace is a basic Concord which the builder installed. The 6” duct off of the supply in the picture leads to a register in the kitchen.

My old system had the humidifier on the supply side. It worked great. Despite my concerns, the contractors mounted the new humidifier on the return side. Now it doesn't work. I am running hot water to the unit, but the rise in humidity is barely noticeable. Cold air just can't pull enough moisture from the unit. Even though the bypass air is coming from the hot side, it just isn't enough to compensate for the cold air in the return ducts.

 

[JGoodish]

My old system had the humidifier on the supply side. It worked great. Despite my concerns, the contractors mounted the new humidifier on the return side. Now it doesn't work. I am running hot water to the unit, but the rise in humidity is barely noticeable. Cold air just can't pull enough moisture from the unit. Even though the bypass air is coming from the hot side, it just isn't enough to compensate for the cold air in the return ducts.

I guess the question would be: how cold is the return air? In our house, even the cold air likely doesn’t vary more than a few degrees (maybe less) so I can’t imagine in having a large impact on effectiveness. I would prefer to have a powered humidifier on the supply side, but there is a steel beam in the way which doesn’t provide enough clearance to mount it above the evaporator coil. I’ve seen some installations which mounted the humidifier on the evaporator coil plenum (right next to the coil), but I’m not sure if that’s a good idea or not.

Also, running the duct for a bypass model behind the furnace (as shown in post #2) is a good idea, but there isn’t enough clearance to do that, either.

I thought about a steam humidifier. They are expensive, use expensive cartridges, and lots of electricity, so it would seem that they are not ideal from a cost perspective.

 

[sarangan]

I guess the question would be: how cold is the return air? In our house, even the cold air likely doesn’t vary more than a few degrees (maybe less) so I can’t imagine in having a large impact on effectiveness. I would prefer to have a powered humidifier on the supply side, but there is a steel beam in the way which doesn’t provide enough clearance to mount it above the evaporator coil. I’ve seen some installations which mounted the humidifier on the evaporator coil plenum (right next to the coil), but I’m not sure if that’s a good idea or not.

Also, running the duct for a bypass model behind the furnace (as shown in post #2) is a good idea, but there isn’t enough clearance to do that, either.

I thought about a steam humidifier. They are expensive, use expensive cartridges, and lots of electricity, so it would seem that they are not ideal from a cost perspective.

The cold side will be at the same temperature as your house. But the hot side will be significantly hotter, not just a few degrees warmer. Put your hand on the supply duct and see. My guess is it will be something like 120deg F. If your house is at 70deg F, that is a 50deg F difference. The only reason I agreed to mount it on the cold side was because on a vertical furnace any water leaks will drip into the burner and cause problems. When mounted on the cold side, it will just drip on the floor.

 

[JGoodish]

The cold side will be at the same temperature as your house. But the hot side will be significantly hotter, not just a few degrees warmer. Put your hand on the supply duct and see. My guess is it will be something like 120deg F. If your house is at 70deg F, that is a 50deg F difference. The only reason I agreed to mount it on the cold side was because on a vertical furnace any water leaks will drip into the burner and cause problems. When mounted on the cold side, it will just drip on the floor.

Yes, but that shouldn’t matter because when the air hits the house it’s going to be close to what the thermostat is set at, and contain the same amount of moisture.

I’m not sure that there is a high risk of water leaking inside the plenum from a panel humidifier; even if the solenoid fails open, water should pass over the panel and out the drain. The exception may be if you have a humidifier model without a drain.

 

[JOhnH]

This isn't going to be helpful, but if I want to humidify my house, I open a window.

Now a good DE-humidifier would come in handy.

 

[JGoodish]

This isn't going to be helpful, but if I want to humidify my house, I open a window.

Unfortunately, that is not a solution available to those of us who live in places where it snows.

 

[sarangan]

Yes, but that shouldn’t matter because when the air hits the house it’s going to be close to what the thermostat is set at, and contain the same amount of moisture.

I’m not sure that there is a high risk of water leaking inside the plenum from a panel humidifier; even if the solenoid fails open, water should pass over the panel and out the drain. The exception may be if you have a humidifier model without a drain.

Well not quite. The bypass style humidifiers humidify only a small portion of the air that flows through the furnace. The average increase in RH is probably in the range of 10%. That's why it is important to have very hot air flowing through it (as well as a hot water supply). If the air temperature is 120F and you raise its RH to 10%, by the time it cools down in the house, the RH will be 45%. On the other hand, if you install the humidifier on the cold side, you will be humidifying 70F air to 10% RH, and that is not going to translate to a higher humidity in the house. This is exactly my situation now.

I've had issues with water leaks. Calcium build up in the pad can cause clogs and spray water in other directions. The water fittings can come lose over time, since they are subjected to vibrations of the furnace and heat. That's been my experience.

 

[catmandu]

If you can figure out how, we REALLY liked the Aprilaire 800. We put it on the “main” zone of a four zone setup on our 4600 sqft house, and it provided good humidity everywhere. Changed the cartridge once each year, very reliable.

 

[JGoodish]

Well not quite. The bypass style humidifiers humidify only a small portion of the air that flows through the furnace. The average increase in RH is probably in the range of 10%. That's why it is important to have very hot air flowing through it (as well as a hot water supply). If the air temperature is 120F and you raise its RH to 10%, by the time it cools down in the house, the RH will be 45%. On the other hand, if you install the humidifier on the cold side, you will be humidifying 70F air to 10% RH, and that is not going to translate to a higher humidity in the house. This is exactly my situation now.

I've had issues with water leaks. Calcium build up in the pad can cause clogs and spray water in other directions. The water fittings can come lose over time, since they are subjected to vibrations of the furnace and heat. That's been my experience.

I'm not sure that I follow. A bypass humidifier installed on either the hot or cold side will have the same hot air flowing through the bypass duct and media. I understand the possibility that hot air over a larger surface area on the supply side may result in a greater increase in RH through the bypass, but you're only trying to increase the RH to an appropriate level for the target temperature (and the cold side isn't likely to be anywhere close to 50F delta from that target), so as long as enough moisture is added to the slightly colder return air this should be attainable unless you have a very drafty house. Powered systems may be more efficient and possibly better performers, but the issue in my case is that a powered system (at least the Aprilaire one) won't fit on the supply-side plenum because there is a steel beam running 6" from the top of the supply ducting above the evaporator coil which blocks the unit from protruding beyond that point. That is why I'm looking for alternatives or installation possibilities that I'm missing.

As for the leak risk, I suspect that it is minimal if the system is properly installed and maintained. I suspect that the only way to really eliminate this risk is to use a steam system, where the water source isn't attached to the furnace plenum at all.

 

[Albany Tom]

Do you have hard water? If no, then no problem. If yes, then maybe a problem. With hard water, some humidifiers will leave white dust all over everything.

The second data point may be irrelevant, but when I worked on a datacenter HVAC upgrade project around 2010, the engineers said that they hate doing humidifiers, because they leak. When they use them, they always use steam, because of the concern over things growing in the water of other systems. After the legionnaires thing, the engineers don't like standing water anywhere. No idea if big commercial systems are anything like home systems.

Third data point, no idea what the material of the heat exchanger is, but cold dry air is less corrosive than wet air. So without knowing any better, I'd want to add the moisture after the HX.

 

[weilke]

Make sure its installed in a way that the water can't drip back into the air-handler once it starts leaking. Replacing a few feet of corroded ducts vs. a $5000 piece of equipment.