An HVAC problem that needs a solution

Jaberstein

Here is a problem that is common in some homes.

I built the house I live in. Two stories with a condensing gas furnace in the basement for the 1st floor requirements and also a 2nd condensing gas furnace in the unheated attic for the second floor HVAC, Heat tracing on the furnace condensing drain lines and all ductwork sealed and wrapped with insulation in the attic. This is a common installation in a lot of new homes but I used all hard duct and fittings, but most installations use a lot of flex.

Here is the problem, and this occurs not only on the 2nd floor diffusers, but also on the 2nd floor bathroom exhaust. During the winter months I also have an efficient wood burning fireplace, which can heat the house when in use. However when in use, the 2nd floor system is hardly ever activated. This results in the warm moist air over time rising up into the diffusers and the bathroom exhaust ducts, (even though there are dampers in the fans) which rise up thought the attic and exhaust through the roof. The moisture in the warm air condenses when cooled by the attic air and results in a small amount of condensate running down the duct work and forming on the 2nd floor ceiling and causing water stains. This condensation also develops in the furnace over time and on occasion, has caused the filter to get saturated, frozen due to the below freezing temperatures and collapse, when the furnace finally cycled on.
There is a constant convection of warm moist air rising through the 2nd floor diffusers and up into the duct work and eventually the moisture condenses.

What I think all ductwork located in a unheated attic should have, is something similar to how we piped a dual temp system when downfeeding heating only terminal equipment, which was, the feed came off the top of the main and dropped to the equipment, which prevented thermo siphon of the cold water when the system was in the cooling mode.

There should be a duct fittings that would work like an inverted P trap used in plumbing, which hopefully would trap the warm air in the inverted loop and stop the convection of warm moist air up into the attic duct system, resulting in the condensing of the moisture. This if it works would also stop a lot of heat loss of the warm conditioned air rising up into the attic duct work when the furnace is cycled off.

Anyone have any experience with this problem?

JStar

Have you closed all of the diffusers?

Jaberstein

I have adjusted the dampers in the diffusers in the winter to try to reduce the convection, but I can not close them, as I still use the 2nd floor HVAC system in the winter. I also had this problem in the previous home I owned, which didn't have a fireplace, Actually a lot of home in that development had the same problem. Neighbors were always talking about painting the water stains by the bathroom exhaust and diffusers.

Tinman

What type of duct insulation did you use and what is its R value?

Jaberstein

Stephen
The duct is insulated and all seams taped with
Owens Corning Duct Wrap
1 1/2" thick Type 150
R 4.8 based on average thickness of 1 1/8"

Tinman

This may be a contributing factor? R8 is the minimum that can be used in my area. Its also required to mastic all metal seams or use UL181 foil tape on all metal seams.

ChrisJ

Stop using the forced hot dust and put magnetic covers on the registers in the winter. Problem solved.

Curious why you didn't go with radiant for heat? You could've run an outdoor wood burner with it as well.

Jaberstein

Chris
The home is located in the Pittsburgh area. The system was selected for heating and air conditioning. It is a propane furnace with a heat pump. Radiant would have still required some form of ductwork for A/C. System works excellent except for the convection of air up into the diffusers during the off cycle. All homes with the ductwork system in a unheated attic are seeing this same problem.

Covering up the diffusers is not an option as they are needed for discharging the heated air, and the bathroom exhaust needs to stay active.
Attached is a picture of the ductwork, prior to insulation, all seams sealed with mastic and then metal taped.
Insulated ductwork will only retard the transfer of heat, so regardless of the R value, in time the air inside the ductwork will equalize to the cool attic air, depending on time the furnace is cycled off.

JStar

Simple solution: keep the fan running in the constant ON mode.

Tinman

"Insulated ductwork will only retard the transfer of heat." Only?
Of course that's what it does. And if you have the right R-Value of duct insulation, it will mitigate, if not stop, what you're experiencing now. That's why there are codes and code enforcements. An R4 insulation was code like 20 years ago. 10 years or so ago they bumped it to R6, now its R8.

I couldn't even venture a guess as to how many sheet metal duct systems I've installed in attics where there is zero condensation problems.

I agree with Joe that running the fan continuously will stop the condensation but unless I'm using that continuous fan to clean air or something else, I see it as a bandaid not a solution.

clammy

Just wondering aside from only having a r 6.5 duct wrap are you sure that the installer wrapped the cans . I know i rarely see anyone do it wheather its a straight cooling or heat cool system .Over the years i have installed plenty of heating cooling systems in attics and have not had issues with condensate at the registers .i would take a look at the cieling cans and also maybe check to see if the can is sealed tightly against the sheetrock and not leaking air into the attic space between causing condensation.While your at it check your return box and make sure that it s propely insulated .I have heard of this issue with a c system in attics in which cieling cans where not insulated and caused condensation issues .If its seems that all these homes are having the same issue i would look at some of the finer points of insulating duct work .Peace and good luck clammy

clammy

After a quick re read just wondering are you sure the duct wrap was installed properly ,i ve seen alot of guys pull the wrap tight which in turn does not give you the r valve it s rated at ,many may never realize it but theres a art to every thing including the proper way measure ,cut ,wrap ,over lap duct wrap and tape over lapping vapor seams . I have always stapled ,used a re enforced fiber glass tape for all seams and then bailing wire also left the insulation loose and allowed to re expand to it s rated thickness if not done you ain t getting the insulations rating .My buddies laugh at me when i wonder how these guys insulate ducts with wrap with stapling wire tieing and taping it drives me crazy but usually its comes with the price they didnt pay for the job . i think even with the best insulation pratices one may never get the r valve that the product is rated at thats without putting some extra effort in$.Just some food for thought peace and good luck clammy

Jaberstein

Thanks for the input, but here is the problem, I have run the fan continuously which does stop the problem, but like Stephen, I feel it is a band aid not a solution. Considering I have a log burning efficient fireplace which uses outside combustion air (which a revision to this fireplace gave me the idea for the duct trap) when in use and burning for long periods is when the convection of air into the diffusers and bathroom exhaust is condensing, sometimes I can go for days without the furnace cycling on. This is when the R value of the insulation has little effect. Also this problem also occurs in the bathroom exhaust, which is also insulted.
All the other homes in this development used an octopus fiberboard fitting on the unit discharge and then flex to all the diffusers, how this ever meets any code is beyond me. Low R value and high static on the fan. Being in the business I chose hard duct properly sized.
The efficient fireplace I installed "Fireplace Extrordinair" revised there installation instructions (after I installed mine) and the revision was to the outside combustion air ducting. Originally they indicated to install the outside intake above the fireplace connection and duct down to the fireplace intake, but the revision was to install the outside air intake below the fireplace connection and loop above the fireplace connection creating an inverted loop. This revision came about because there were too many complaints (me included) of the interior of the fireplace being close to the outside temperature when not in use, due to the infiltration of O/A air. With the revision the inverted loop seals this infiltration of cold air and only feeds the fireplace when a delta P occurs due to the burning fire.
I feel the true solution is a duct trap, as, even if you do not have a condensation problem, regardless if you just run the fan continuously or just cycle it there is always a convection of warm conditioned air rising into the diffusers and giving up it heat in the attic. This is wasted energy.

JStar

My only concern about the duct trap is adding an additional friction loss/drop that would completely change your original duct design.

Jaberstein

I agree these extra fittings will have to be considered when calculating the fan static, but most of the duct systems I seen installed with all the flex running through out the attic have to create more way more static than some additional L/R elbows.

JStar

Figure on an extra 10ft of duct loss for each 90 elbow, plus the extra pipe. That can add up quickly. For your drawn heat-trap, you'll be adding an additional 30-40 ft of equivalent duct length. Compare that to a 20% loss for flex duct...

ChrisJ

Everything is a compromise.

You running the fan contentiously is a compromise, not a band-aid. It solves the problem and probably yields more balance.

How do you feel about your door opening not being framed properly? Unless that picture was taken before it was finished? I'm hoping that was the case.

Jaberstein

Chris
If you look at the picture you can see finished southern pine header at correct height for the door openings. Not sure why the builder left the open higher at first, but building was finished to code and inspected.

ChrisJ

Jaberstein said:

Chris
If you look at the picture you can see finished southern pine header at correct height for the door openings. Not sure why the builder left the open higher at first, but building was finished to code and inspected.

Where are the jack studs?
Inspected means nothing unfortunately.

ChrisJ

I realize this is way off topic but it needs to be mentioned.

Fact is, on a non-load bearing wall it doesn't really matter, but what else did they do wrong?

Jaberstein

Chris
I find it funny I am discussing heat loss through duct work and you are talking door openings. Again look at the finished wider opening beyond the opening you are talking about. It has the southern pine head and jack studs installed and it is at a lower height/elevation than the opening in the forefront of the picture. Not sure why the builder did this but the openings are at the same height now, so he must of added a header and jack studs once he no longer needed the higher opening.

KC_Jones

Why on new construction ever put the ducting in the attic? I have seen this topic on many sites and it's always about the problems it creates. It seems to me the solution is don't do it. When did this become a standard practice? Maybe I am the crazy one, but the only advantage I can see is cheaper on installation. Beyond that it just seems like a bunch of potential problems. I could see it maybe on a retrofit in an existing house when people don't want to disturb the building too much, but with a clean slate why?

DrPcFix

You need doubled studs on both sides of the door frame to give some rigidity to the frame.

ChrisJ

Jaberstein said:

Chris
I find it funny I am discussing heat loss through duct work and you are talking door openings. Again look at the finished wider opening beyond the opening you are talking about. It has the southern pine head and jack studs installed and it is at a lower height/elevation than the opening in the forefront of the picture. Not sure why the builder did this but the openings are at the same height now, so he must of added a header and jack studs once he no longer needed the higher opening.

You're right.
But I brought it up because I felt it was a concern.


EDIT : Just checked, and technically jackstuds aren't required if the wall is non-load bearing. It's just preferred.

My apologies.


So back on track,
Aren't you concerned about your trap holding moisture?

Jaberstein

Chris
The only way to find out if this might work is to try it. I think I am going to just install this set up thru the ceiling without connecting it to the duct work and see if it in fact seals or stratify the air and stops the convection. I actually hope this might solve a building pressure equalizing problem.
I notice things that most people never see.

ChrisJ

Jaberstein said:

Chris
The only way to find out if this might work is to try it. I think I am going to just install this set up thru the ceiling without connecting it to the duct work and see if it in fact seals or stratify the air and stops the convection. I actually hope this might solve a building pressure equalizing problem.
I notice things that most people never see.

I'll be honest,
I've had others tell me things wouldn't work and they did.

That said, go for it.

But I still like Jstar's idea to just run the fan.

Jaberstein

KC_Jones,
I think they install the ductwork in the attic because the system works better for maintaining 2nd floor temperature for the air conditioning , as opposed to a basement system that feeds the second floor. Diffusers out of the ceilings, and system is dedicated to the 2nd floor.
The basement system would work if it was zoned 1st and 2nd floors as at night the heat in the attic radiates down thru the ceilings (even with insulation there) and the 1st floor is satisfied and without zoning the homes the ones I lived in were always 5 to 7 degrees warmer than the thermostat setting (located on the 1st floor) . The system was cycled off due to the 1st floor being cooled to temperature setting, but the 2nd floor was warmer due to the heat of the day that had built up in the attic radiating thru the ceilings.
We actually installed a duct system on a roof of a commercial building due to no available space for major duct mains. Troubles were many, rain leaked into the return ducts due to negative pressure and the many individual diffuser feeds thru the roof each required a portal and roof seal. Worst system to ever have.

ChrisJ

Does anyone make a spring loaded checkvalve like damper? That closes completely when no positive pressure behind it but opens fairly easily? Like an outdoor damper for a bathroom fan or clothes drier except much larger and designed to be in a ceiling?

Tinman

Installing ductwork in an unconditioned space is common and a perfectly acceptable option for a 2nd floor zone (often better). If done correctly, you're golden. I've done it in 1400 sq ft homes and 14,000 sq ft homes successfully back when we were doing more sheet metal work.

I would run the fan to correct the problem short term. It's not a solution long term. ChrisJ, why in the world would you run a 1/2hp or 3/4hp PSC motor (I'm guessing) all the time if you don't need to? If you want to for the sake of air filtration, dehumidification, humidification, etc., it makes sense.

Here's how I do it. I'm not saying it's the only way or the very best way on the planet but it works for me. Each and every time. My thing is Hydronics but I know sheet metal. Back in the day, I was a union tinner. Local 73 out of Chicago.

I never line the ductwork. I always wrap it with foil back, fiber glass R8 insulation. When you line, you end up spending more on metal because the liner inside the duct takes up space causing you to increase the duct size. By using the right size liner (R8), your duct is going to be much, much larger. Clammy is right. There is a right way and a wrong way. Do it right. I use solid rectangular trunk line (round is fine) the entire way and solid branch ducts most of the way. The last 5-6' of branch duct before the boot, I use insulated flex (again, R8). I find there's less possibility of condensation that way and I make sure to insulate the boot as well.

I also use to work HVAC Tech Support. If I got this call, my first 2 questions would be:
A. What is the duct insulated with?
B. What is the humidity level in your home while you're getting the condensate?

The door framing looked fine to me. That's all I've got.

JUGHNE

With all sealed combustion appliances and a pretty tight house it sounds like you have just simply too much humidity in the living space.
There are controls for bath fans that auto start for humidify removal. The HW T-stat 8000 series and others have a recir setting for the furnace fan control. Fan runs randomly about 1/3 of the time.

Had problems in tight new houses with condensate running back down all exhaust thru roof venting. Trying to convince people that if they run these fans longer it would help to relieve the humidity in the structure. Just a 30 to 60 minute cheap crank on (tick-tock) timer for fan control would help. But some were concerned about electricity usage.

Maybe for you some type of air to air exchanger is necessary.

It is my notion that to remove humidity (dry out the air) requires expending some energy. Either thru heated humid air vented to the outside and cold dry air brought inside. Or a whole house dehumidifier with compressor running.

The BTU's provided by wood burning are thought to be pretty economical and shedding some to the attic thru duct losses or thru air exhausting are just the cost of saving energy. IMHO
(from an ex wood burning furnace owner)

And concerning the door, if you had a drafty exterior door it would help keep the house dryer.

JStar

ChrisJ said:

Does anyone make a spring loaded checkvalve like damper? That closes completely when no positive pressure behind it but opens fairly easily? Like an outdoor damper for a bathroom fan or clothes drier except much larger and designed to be in a ceiling?

Barometric damper. But, again, it comes at the price of adding to the total airflow loss.