Humidifier Bypass Duct Placement Help Needed

[Michigan Ray]

Hi,
I'm installing the Aprilaire 400A bypass humidifier on the supply side of my downflow forced air furnace without air conditioning. The main return duct is capped off measuring 14 x 20 x 4' 6" high directly over the furnace. The air filter is installed in the middle of the 4' 6" duct (2' 3") due to the return gradually reducing to the size of the furnace. I plan on installing the 6" bypass duct in the lower section of the main return. This places the 6" bypass duct between the furnace filter and the furnace. The h'stat is automatic which needs to be installed a min. of 6" above the bypass duct. If I have to install the bypass duct duct "prior" to the air filter the h'stat will be considerably high. I want to make sure this install is done properly.
I have a couple questions;
1) Is it ok to install the 6" bypass between the air filter and furnace? I believe that I read somewhere that it needs to be installed "before" the air filter and furnace.
2) If installed between the air filter and the furnace, would the bypass duct introduce dust/dirt into the furnace, fan, etc.?

Thank you in advance for your help, Ray

 

[Dana]

If it's not too late to send the thing back, spend the money on air-sealing the house and the ducts.

Low indoor air humidity in winter is a symptom of excessive ventilation rates. Anything over 35% RH @ 70F indoors in winter is bad for your house, and often increases the mold-spore levels in the house in spring when all of that stored moisture in the studs & sheathing warms up and turns the wood into a mold smorgasbord.

A reasonably tight house with more than one occupant will never drop below 30% RH, even during the arctic cold snap dry air conditions.

 

[Sponsor]

 

[Michigan Ray]

Hi,
Actually, the lack of humidity is mainly a symptom of dry forced air from a heating system. A humidifier is a must not an option. Ray

 

[LLigetfa]

Actually the low humidity is from cold dry outdoor air infiltrating. A heating system cannot dry air. Air is dried by cooling, not heating.

Anyway, should not the humidifier be placed on the cold return side? Every one I ever dealt with was on the cold return duct.

 

[Dana]

That's the myth, but it's patently false on the raw physics of it:

The only way to remove moisture from the air is by running it over chilled surfaces below the dew point of the room air. At 35% RH/70F the dew point is about 41F. A heating system doesn't remove moisture from the air- it's heating it up, not cooling it down. Where in the heating system ducts or furnace are the temps running well below 41F? This is why you get condensation out of air conditioners, and not furnaces.

But the myth has continued to be believed due to the less broadly known unintended ventilation issue: Forced hot air with unbalanced leaky ducts, in an air leaky house will drive excess AIR INFILTRATION. The air-handler induces pressure differences between rooms with at least some of the return flow taking "the great outdoors" as the return path. The outdoor air in winter has a dew point well below 41F (it can't be higher than the outdoor temp). That unintended return path is an unintended high ventilation rate displacing the conditioned space air with much drier outdoor air.

The solution is to seal both the house AND the ducts (both supply and return) to get the ventilation rates under better control.

There are existence proofs of this everywhere, even cool dry Saskatchewan. You can cheap out and just install a humidifier to bring the comfort levels up desite the leaky house, but it's false economy. Those air leaks are also chewing up a large fraction of the fuel and air-handler power use, and delivering less heat to the house than the nameplate BTU output numbers imply, due to this induced extra heat load that doesn't appear in the Manual-J load calc.

It's also been true in my first-hand experience. When I moved into my current house it had a humidifer on the forced air system, and unless it was on the room air would hover between 15-20% RH in winter. Once I fixed the largest air leaks- attic spaces behind kneewalls in the upstairs, and the foundation sill & band joist in the basement, along with a bunch of other random points the interior RH now only hits 30% after several successive days where the daily high is in single digits. Most of the winter it's running 35-36%, and when it goes higher than that I leave the bathroom fan on until it drops back to 35%. (When I eventually install automatic ventilation system I'll run it under DE-humidistat control in winter.) I decommissioned the humidifier five years ago and never looked back, and have never seen the interior RH drop below 25%, not below 30% in the past 2 years after further air-sealing.

(edit: apparently cross-posted with LLigetfa's correct assessment.)

 

[Michigan Ray]

Humidifiers are best ran with hot water and/or on the supply side.

 

[LLigetfa]

You are just full of preconceived notions. The bypass from the hot side provides the heat. Otherwise you would be pushing humidity into the cold side.

 

[Michigan Ray]

Not to argue, but I guess everything I have read from the pro's is wrong..... go figure. Not to mention my own experiance with/without humidification. Everyone is entitled to their own opinion. I just disagree with yours.

 

[Jadnashua]

The humidifier bypass main unit is typically installed on the main supply duct - after the fan and in the hot air. Some air goes through the humidifier to the low pressure side (i.e., the return). Now, there are other models that use an internal fan to blow humidified air into the main duct, but the idea of the by-pass models is their simplicity - the furnace's blower moves the air, and in the bypass models, the air flow is from the heated air, through the humidifier, to the return - i.e., it bypasses the normal outlet. In some ways, this is also good, as evaporation cools the air stream, and in a bypass unit, it gets a chance to be reheated as it goes through the furnace heat exchanger again. http://www.aprilaire.com/themes/aa/en/manuals/AA-Humidifier-Owners.pdf explains it all, if you're interested.

IOW, they all don't work the same...

 

[Michigan Ray]

Hi,
Thanks for the post in explaining the humidification process. Hopefully this will help others understand the need for humidification. I understand the importance of how your home is built, insulated, etc., but here in the northern/midwest states a humidifier is almost alwys needed to maintain proper RH. Thanks again, Ray

 

[LLigetfa]

I understand the need for a minimum level of humidity for human comfort as I'm sure Dana does, but you are treating the symptom rather than the cause. You are loosing humidity because of dry air infiltration, not because your heating system is drying out the air. Stop the leaks and you then will have the opposite problem, that being getting rid of excess humidity.

With dry air infiltrating the house, you also have the same amount of humid air exfiltrating. That is rotting your house. Gallons of water will condense in the walls and ceilings of the house as the humid air moves to the cold side of the insulation.

 

[Michigan Ray]

Wow!! You're right....my house does leak. I don't know of any house that is air tight, how on earth would we breathe? Thank goodness with all my preconceived notions I have never sufficated as well as have my insulation, walls & ceilings collapse due to gallons of water and rot caused by my humidifiers. "Finally after home ownership for over 40 years I'm becoming educated." Do some research and ask the pro's. Education is bliss....sincerely, Ray

 

[Jadnashua]

Wow!! You're right....my house does leak. I don't know of any house that is air tight, how on earth would we breathe? Thank goodness with all my preconceived notions I have never sufficated as well as have my insulation, walls & ceilings collapse due to gallons of water and rot caused by my humidifiers. "Finally after home ownership for over 40 years I'm becoming educated." Do some research and ask the pro's. Education is bliss....sincerely, Ray

Fact...in Canada, where they have a much higher standard for sealing the house, most of them (the newer ones at least) have an HVAC heat recovery system to help remove excess moisture and bring in fresh air. You shouldn't be so sarcastic...the leakier the house is, the dryer is will be in the heating season. So, as was said, adding moisture is required if you don't or can't seal things up, but if you do seal the house well, you may then need to get fresh air in and extract some moisture in the same situation.

 

[LLigetfa]

Thank goodness with all my preconceived notions I have never sufficated as well as have my insulation, walls & ceilings collapse due to gallons of water and rot caused by my humidifiers.

Now you are contradicting yourself. You would never suffocate in your leaky house.

Do some research and ask the pro's. Education is bliss....

I did my share of research into R2000 building practices when I built my house. It is almost as airtight as it can be and needs dehumidification, not the opposite. That is why I have HRV, to remove moisture and bring in fresh air.

BTW, Dana is a well respected "Pro" on this forum. Too bad you don't take his advice either.

 

[Michigan Ray]

Like I said "Education is bliss" Thank's for all your thoughts, opinions and clearing things up. BTW my original post still remains unanswered. Ray

 

[Jadnashua]

Unless the manufacturer specifies, I don't think it really makes a difference...the air that is bypassing has already been through the filter.

 

[Scup]

I am a newbie here and do not know squat, but that is not going to stop me from putting in my two cents.

Warm air can contain more water in its gaseous state than cold air. That same warm air if contained will have water condense out of it if cooled or brought outside say where it is cold. The reverse is also true, bring cold air into your home and it will suck up moisture. Far too many modern homes are sealed too tight and really bad things can happen because of such. You can get sicker than dog real quick if certain products like new carpets, or questionable insulation is installed. Because fire places, oil burners, wood stoves, wood furnaces, etc. need a source of air to operate properly I am really surprised that nobody even mentioned this.

When your heating appliance is lit off, say an oil burner is operating; gases are coming out of your chimney or side shot (active exhaust draft). What goes out must be replaced with something or your home will have a negative pressure created within its walls. If you would seal off your home 100 percent, your fire place, wood stove, and furnace, could not possibly work properly. I am not saying that you have to open a window, although that would work, but what is recommended for say a simple wood stove is an air vent to the outside that terminates somewhere near the intake of your appliance of the same size as its flue pipe. Practically, since some homes are not sealed that well, the previous statement might be somewhat of overkill and smaller size intake can be used since the outside air can usually find cracks and crevices to sneak into the house.

I was amaze at the accuracy of a cheap relative humidity meter that I had purchased by Duracraft. I calibrated it in a sealed plastic bag for which I exactly knew what the relative humidity was. Perhaps some could, but I cannot detect small differences in relative humidity hence I need an accurate meter. It is claimed that the comfort zone should be somewhere between 30 to 50 %. This of course depends on what a particular person or family desires. I have sinus problems and find that I am more comfortable at the higher end. Right now the relative humidity is exactly 44% sitting in front of my computer and that is exactly where I want it.

I decided to incapacitate the humidifier in my Five in One Williamson installation from day one since I was concerned by the method they used to place moisture into the house. They dribbled water into a metal grid contained within the unit and circulated air over the grid. Everything was controlled by a humidistat, and everything worked but the humidistat had no idea of what the humidity was in the blower area and duck work. I paid a fortune to have the duck work installed and was in no mood to potentially shorten its life or have it replaced. I now used a simple but large evaporator type of standalone unit that holds around 20 gallons of water, has a filter in it that catches crap that probably would create havoc with my sinuses, and I can add ounce or two of an antibacterial solution since even mold would probably prefer to grow in my sinuses than in my home. The calibration knob on the unit is way out of whack but repeatable and observing my meter there in no problem in properly controlling it.

Never purchase an ultrasonic humidifier unless you like to see white dust on everything.

Can you home be damaged by too much humidity; the answer is yes but I think I had better give an explanation. Suppose the contractor who built your home was a jerk. It happens! Instead of placing a proper vapor barrier on the inside of your insulation he places it on the outside because it is easier and will not get in the way of the roughing in phase of construction. Even worse, 4 mill poly is really very cheap so he uses that. I feel very sorry for anyone under these circumstances. The moist air in your home will now condense on the cold poly during the cold months. This water (no longer vapor but real wet water) is now within your walls screwing up your insulation, studs, framing, rusting your metal electrical boxes, inviting wood eating creatures, and who knows what else.

I really wish Tom Sawyer would chirp in as you really need have to a guy from Maine that understands what heat, cold, and moisture, can do and how to control it. As I said, I have my beliefs, and could be wrong, but these are the things I believe to be true unless someone convinces me otherwise.

Sorry Ray, but I could not answer your question, but hopefully I have given you something to think about.

 

[LLigetfa]

Far too many modern homes are sealed too tight and really bad things can happen because of such. You can get sicker than dog real quick if certain products like new carpets, or questionable insulation is installed. Because fire places, oil burners, wood stoves, wood furnaces, etc. need a source of air to operate properly I am really surprised that nobody even mentioned this.

Modern well sealed homes already factor for that by providing a HRV. In modern homes, the appliances often have their own dedicated make-up air vent. By code in most places, a general make-up air vent is required to be run to the cold return duct.

When your heating appliance is lit off, say an oil burner is operating; gases are coming out of your chimney or side shot (active exhaust draft). What goes out must be replaced with something or your home will have a negative pressure created within its walls. If you would seal off your home 100 percent, your fire place, wood stove, and furnace, could not possibly work properly.

It is nearly impossible to seal a home 100%. Most homes will have some degree of negative pressure. There is the stack affect that places this Neutral Pressure Plane (NPP) somewhere between the basement and the attic. Where the NPP falls depends on the balance of sealing against infiltration and exfiltration. The classic mistake many folks make is to seal where they feel cold air infiltrating but overlook sealing where the warm air is exfiltrating. It is harder to detect where warm air leaks out.

I am not saying that you have to open a window, although that would work, but what is recommended for say a simple wood stove is an air vent to the outside that terminates somewhere near the intake of your appliance of the same size as its flue pipe. Practically, since some homes are not sealed that well, the previous statement might be somewhat of overkill and smaller size intake can be used since the outside air can usually find cracks and crevices to sneak into the house.

The dedicated Outside Air Kit (OAK) on a woodstove is mandated by code in some jurisdictions. My woodstove has an OAK. The stove would only be affected by the NPP when the stove door is opened. Above the NPP, there would be positive pressure ensuring no smoke enters the house. Below the NPP, the OAK can actually push smoke into the house when the stove door is opened.

Can you home be damaged by too much humidity; the answer is yes but I think I had better give an explanation. Suppose the contractor who built your home was a jerk. It happens! Instead of placing a proper vapor barrier on the inside of your insulation he places it on the outside because it is easier and will not get in the way of the roughing in phase of construction.

You don't need to have poly on the cold side to have condensation. Moisture laden warm air exfiltrating to the cold side will reach the dew point and condense in the insulation and/or on the inside surface of the sheathing. The poly would just exacerbate it as air movement that would eventually help to dry the wetness would be inhibited.

 

[Jadnashua]

The most efficient burners (heating, either water or air) provide their own combustion air. But, there are common appliances in your house that do exhaust air, and therefore, need some makeup air: bathroom exhaust fan, dryer, stove vent, etc. A huge user of air is a fireplace, and many woodstoves (although some provide outside air for combustion). The net energy effect of a traditional fireplace is probably negative...it sucks in more outside cold air than it provides heat, but those effects may be felt in other rooms outside of the radiation from the thing.

No house is perfectly sealed, but the newer ones are pretty close. But, to remain healthy, they have an engineered leak installed - one that can prevent negative pressure in the house, and extract some of the conditioning you've done to the air (both heating or cooling), to minimize their energy use. This sort of thing is common, from at least what I read, in new construction in Canada, and some other places around. ANd, it's a really good idea anywhere.

Uncontrolled air leaks into the house (or out of it) waste energy. Interior air quality will suffer if you seal too well and do not provide a proper fresh air intake and exchange, hopefully, well engineered. Those leaks do tend to make the house less comfortable. Much better to manage it. A well sealed house will probably not need a humidifier - in fact, it may need to expel some moisture. Depends on the number of people, how long and hot of a shower they each take, the amount of cooking, etc. going on.

 

[Scup]

LLigetfa mentioned things that I have never even heard of! I did note he is from Canada and it does not surprised me that Canada would have issued far more code than here in Connecticut. Someone in CT can go into Preston Trading Post, purchase a very nice wood stove, bring it home, install it, and may or may not be breaking some code somewhere, but I would doubt if anyone would even take notice unless he kills himself, has a chimney fire (fire department is called in), or the tax assessor pays him a visit and sees it.


Question:

What is a HRV?

I have never even heard about a Neutral Pressure Plane? Other being a very impressive sounding engineering term, keep in mind that I am only an electrical engineer and have no understanding of why I have know where the NPP is in my home. I have no attic so what would be the consequences? Does that make the NPR move upward or downward and which way would be better. Goodness, if something bad is going to happen if my NPP is off its proper mark, please tell me what horrible thing to expect, and if it would be possible to get my NPP back to where it should be?

You are right on the mark when you claim that most do not properly seal off hot air leaks, but here in CT we did have a free service in which a representative would come to your home and present you with an infrared pictures taken of the outside of your home. You could very clearly see where you had hot air leaks. I am not joking here, I really do not know where this service went or if it is still done, but I have never met anyone who has had it done.

I own a dedicated air kit (OAK). Unfortunately, I did not know it even had a name when I figured out I needed one and had to make it up myself. I did have a couple of problems with its operation though; the first mistake made was I did not place a screen on the outside of the feed pipe, so mice used it as a giant sliding pond right into my basement. The second problem is I had forgotten just how high the snow could fall here in Ct during a bad winter and it buried the input pipe. The final problem was I had placed the input way to close to my side shot (a force draft blower for my oil fired furnace) and was sucking on some exhaust fumes along with the cold air to provide oxygen to my wood furnace. It did not take me very long to fix the above problems.

In this section you do mention NPP again. This time I am quite serious and am not joking around and really have no idea of what you are talking about. I live in a modified A frame. My wood fired furnace is in the basement. The bottom of its door is about 20 inches from the concrete floor. My furnace does not throw smoke into our living quarters unless something is very wrong like a neglected or missed chimney cleaning.

Perhaps what I think you mean by an OAK just might not be what I have assembled. Since I have no idea of what you are talking about let me explain what I have assembled. My wood furnace and oil burner are so close to each other I could place my hands on both of them at the same time. I have run a pipe to the outside to bring air in for either the wood or oil furnace. The pipe terminates between the two furnaces. Only one furnace will be used at any given time. There has never been a need to run both simultaneously and come to think of it, I have no idea of what would happen if I tried to run both but I suspect it would work out fine if I used a larger air intake pipe. Whenever I run one of the appliances, cold air is definitely finding its way in from the outside. Very easy to tell since all one has to do is place his hand an inch or two from the inside edge of the pipe. Since this pipe is not directly connected to the either appliance (not 100% dedicated to either but probably comes pretty close since I only run one at a time) it is impossible for the air intake pipe to bring smoke into the house. The side shot is fairly directive and now is nowhere near the air intake and the top of wood furnace stack is 35 feet from the air intake (that is what happens in an A frame). I have no idea where the NPP is, or care, because as long as one of my appliances is operating, exhaust fumes will be leaving my home via the side shot or chimney and the draft created by either insures there is a negative pressure. In addition, I do not get smoke into the house when I fuel the wood furnace (door is open) although I do suspect the efficiency is down in the mud when I am refueling it.

When it comes to damaging your home by improper use of a vapor barrier I did present an extreme case so Ray could easily understand my explanation. I never claimed that is the only method in existence.

I am not sure why you took such an obvious offense at my post but if it makes you happy then be my guest! I would be apologetic if I knew what I had said that upset you! Please understand that what you have for code in Canada is probably tenfold what we have in the states. Not saying what you have is better or worse, since I have not bothered to study either one. Since this post was to help RAY out, I realized I have diverged way too far from helping him out and so have you. In America we have always had the freedom to do what we may including killing ourselves through stupidity. However, we are catching up very quickly, we now have paralleled your medical care system, have extremely restrictive gun control laws, our lawn mowers will not work unless we hold on to them, and we cannot paint the bottom of our boats with tin based antifouling paints. Although in Connecticut, we can still purchase a 30,000 BTU kerosene heater (made in China) from Amazon.com, bring it into our living rooms, light it off, not open a window, and be found dead of oxygen starvation the following morning.