# Thread: Do I need a steam humidifier with a heat pump?

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## Do I need a steam humidifier with a heat pump?

I live in middle Tennessee and we are installing a new heat pump all electric. I was going to install a steam humidifier with my new heat pump, but a contractor told me I did not need a humidifier with a heat pump. He said I only need a humidifier with gas or dual fuel. Is that correct? I was going to get a IAQ thermostat to help control the humidity level.

2. The reason home needs more than a little humidity added in the winter has nothing to do with the heat source. Your contractor doesn't understand what's happening within your home but that's no surprise. I didn't understand for most of my career either. But here's the straight scoop.

1. The warmer air is, the more moisture it can hold.
2. The amount of moisture actually in the air in relation to how much mositure the air can hold at a given temperature is what we call Relative Humidity, (RH).
3. In winter, the outdoor air generally has a RH level of between 35% and 50%. Keep in mind now, thats at outdoor temperatures. To put a number on it, let's say it's 35F outdoors and the air is at 50% RH. If we measured the amount of moisture in the air, say 10-grains, we know that at 35F the air could hold 20-grains because it's only at 50% capacity. Right?
4. Bring that 35F air @ 50% RH into your home and warm it up to 70F and guess what? Now that 70F air could hold 30-grains of moisture, so the RH just fell to 33%, just because we warmed the air. Mind you, it doesn't matter what warms the air, it's just the fact that it warms up that drives the RH down. And so that's where the humidifier comes into play.
5. If the RH of the air coming into the home drops from 50% to 33% because the grains of moisture stayed constant, then adding 5-grains of moisture to the air as it warms would increase the total grains to 15 and since 15 is 1/2 or 50% of the total 30-grains the air can hold at 70F, you've now.

Now that you know what RH is and how the cold air coming in from outdoors is affected, you should be able to see the problem. The problem is that air is escaping from your home, which requires new, cold air to come indoors, where it gets warmed by hour heat pump or fossil fuel system and the RH level drops just because it warmed up and still have the same grains of moisture that it had outdoors at 35F.

My recommendation is to first plug all the leaks in the house. Stop the exfiltration and infiltration of air as much as you can. Then install a humidifier if still needed. Unless you've got a very large home or a very leaky house, you can probably do with much less than a steam humidifier.

3. There is a lot more to learn about your house in regards to humidity. You have to understand the science involved in it-Building Science, or have someone who is trained in evaluating the whole house. This involves insulation levels, air-bariers (aka thermal envelope), internal & external moisture sources. This is the stuff the new legislation for weatherizing homes is about, like Homestar, Energystar, REEP etc... . If your house gets to dry in winter I'd wager you have significant air leakage in your house that is costing you more in energy bills & has you installing oversized equipment to keep up with it.

4. My recommendation is to first plug all the leaks in the house. Stop the exfiltration and infiltration of air as much as you can.

Good post. Was typing mine while you yours. I would add the importance of having a blower door test performed after "plugging all the leaks" due to ensuring that the house still has the minimun ventilation it needs for its & the occupants, especially if there are any non-sealed combustion -especially natual draft- appliances in the home. A too welled-sealed home will/may require mechanical ventilation.

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Thanks skippedover very much for the detailed explanation. Our house was built in 1990 with R19 in the exterior walls and R38 in the attic. I think it is insulated well enough and it is not drafty at all. In fact, we have very reasonable electric bills. Our house is about 2000 sf so it is not a very large house.

The reason I was considering installing a steam humidifier is because in the winter my skin is dry and my nose is extremely dry with occasional nose bleeds. I thought this would help, but I have to admit I don't like the idea of putting "moisture" in a house because I know that can be your worse emeny if the humidity is too high.

When you say I could probably do with a lot less than a steam humidifier, what are your recommendations?

6. There are many good quality by-pass humidifiers on the market that should do the job nicely. I think a steam unit is overkill for a home that's reasonably tight. Aprilaire, General and others make good humidifiers without the expense and complications of steam. In our state it's not uncommon to find old farm houses that the wind sails through, large size and they sometimes need at least one steamer if they're going to try and keep the humidity level reasonable. Your physical needs description certainly sound like dry air. I'd recommend a descent hygrometer so you can measure the humidity constantly. Perhaps you're about to have a birthday or anniversary and could request one as a useful gift?? The humidity level year round should be between about 35% and 50%. I would not recommend anything above 50% as mold can form at 55% and above.

A final thought. While your home may be well insulated, that doesn't mean it doesn't leak air. Insulation is great when there is no air movement but when the air moves, insulation is just a great air filter. As someone else said, the only real way to check for the leak rate is a blower door test. That could/should be done to determine the leak rate of the home and if it is found leaky, help find some places that need to be sealed. First seal the envelope, then insulate the exterior. Recessed can lights are a great leak source in many modern homes. Doesn't matter what floor they're on either. Those ceiling cavities are directly attached to other areas of the home and the air will flow amazingly freely once it gets into the 'skeleton' of the home. If your current HVAC contractor doesn't want to humidify your home, I'm sure there's one out there who's more concerned with your concerns than his own.

7. The air barrier & the insulation should be located together to form a continous & contiguous "thermal envelope", or "building envelope".

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I have a by-pass humidifier installed on my heat pump's air handler.

Using a hot water supply as recommended by the mfr.

I'm wasting a lot of hot water down the drain, though.

I have a separate needle valve right before the solenoid valve to regulate the water flow. I have to tweak the valve and adjust the flow next winter.

Take care.

9. Originally Posted by gary_g
I have a by-pass humidifier installed on my heat pump's air handler.

Using a hot water supply as recommended by the mfr.

I'm wasting a lot of hot water down the drain, though.

I have a separate needle valve right before the solenoid valve to regulate the water flow. I have to tweak the valve and adjust the flow next winter.

Take care.
Gary, you might want to switch to cold water, that is unless you're already having trouble maintaining a good humidity level. The manufacturers tell you to use hot water to get the maximum output form the machine. But cold water evaporates in a dry atmosphere almost as well and at a much lower cost. Also, there are now a couple of humidifiers out there that don't maintain a reservoir of yucky water but also don't drain away excess. They work well and are more environmentally friendly for those sensitive to allowing gallons of water to drain away. FYI.

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I think we have decided on the RUUD heat pump system and I noticed on their website they have the steam humidifier and the bypass humidifier. So either is a choice with our equipment. I thought the steam humidifier was a "newer" choice. I read where you have to replace the humidifier pad with the bypass whereas the steam humidifier seemed more maintenance free, but I don't really know anything about either and I really appreciate you input and knowledge on this subject.

I am going with the 500 series thermostat (which I think is the same thing as the Honeywell HD) and it has a humidity control which should constantly measure and display the humidity level.

Going a step further, someone recommended in our climate to forget the humidifier all together and get either a HRV or ERV, but again I don't know anything about them.

11. amd
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If your house is leaky enough to require a humidifier, it doesn't need an HRV or ERV.

Replacing a conventional or mid efficiency furnace with a high efficiency furnace (combustion air from outside) or wall electric heatpump can eliminate the need for a humidifier. (The same goes for installing a direct vent water heater)

I would replace the hvac system now and monitor the indoor relative humidity for one heating season. 20-35% is desirable depending on the outdoor temperature; levels consistently below 20% in typical winter weather indicate that humidification is required.

*40%+ RH = window condensation

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The reason home needs more than a little humidity added in the winter has nothing to do with the heat source. Your contractor doesn't understand what's happening within your home but that's no surprise. I didn't understand for most of my career either. But here's the straight scoop.

1. The warmer air is, the more moisture it can hold.
2. The amount of moisture actually in the air in relation to how much mositure the air can hold at a given temperature is what we call Relative Humidity, (RH).
3. In winter, the outdoor air generally has a RH level of between 35% and 50%. Keep in mind now, thats at outdoor temperatures. To put a number on it, let's say it's 35F outdoors and the air is at 50% RH. If we measured the amount of moisture in the air, say 10-grains, we know that at 35F the air could hold 20-grains because it's only at 50% capacity. Right?
4. Bring that 35F air @ 50% RH into your home and warm it up to 70F and guess what? Now that 70F air could hold 30-grains of moisture, so the RH just fell to 33%, just because we warmed the air. Mind you, it doesn't matter what warms the air, it's just the fact that it warms up that drives the RH down. And so that's where the humidifier comes into play.
5. If the RH of the air coming into the home drops from 50% to 33% because the grains of moisture stayed constant, then adding 5-grains of moisture to the air as it warms would increase the total grains to 15 and since 15 is 1/2 or 50% of the total 30-grains the air can hold at 70F, you've now.

Now that you know what RH is and how the cold air coming in from outdoors is affected, you should be able to see the problem. The problem is that air is escaping from your home, which requires new, cold air to come indoors, where it gets warmed by hour heat pump or fossil fuel system and the RH level drops just because it warmed up and still have the same grains of moisture that it had outdoors at 35F.

My recommendation is to first plug all the leaks in the house. Stop the exfiltration and infiltration of air as much as you can. Then install a humidifier if still needed. Unless you've got a very large home or a very leaky house, you can probably do with much less than a steam humidifier.
Great post. I am disgusted when I see a house that has a humidty problem and the homeowner is sold a humidifier with their new heating system. More often than not it is one of the 'big name' HVAC companies who charge a premium price for their so-called expertise.

Good point as well about the difference between insulation and a tight building envelope.

Here is an interesting example I came across: The homeowner has a newer house (less than 10 years old) with R50 attic insualtion, R20 walls and decent thermal pane windows, which had condensation issues. They were buying new windows with the hope of solving the condensation problem. I suspected high humidity since the windows were decent, and the blower door test confirmed that the house was extremely tight. Now the new windows (triple pane, low-e, argon) may have helped reduce the moisture level on the glass due to the higher temperature of the glass, but this does not solve the problem of high humidity. When I opened the attic hatch here is what I found:

This is probably happening inside the walls as well. I noticed the house had a humidifier that was set high, I suggested that the humidifier be disconnected, and if necessary a HRV installed if high humidity levels persist.

This is an example of a very tight, poorly ventilated house with many occupants (5 people in about 900 sq. ft), and from what it sounds like, the opposite of what you are experiencing, just wanted to illustrate the other end of the spectrum and an appropriate application for an HRV.
Last edited by jdaley; 04-21-2010 at 12:17 AM. Reason: added more details

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