Heating Quandry

Brad White_40

A few thoughts:
1. The system is not likely balanced, summer or winter. If the calculated air quantities are fine overall, they are obviously not going where they should. Your office, cold in winter and hot in summer is a prime example. By definition, that does not sound balanced to me.

2. Part of balancing (the sine qua non of balancing) is duct sealing. Do you know how well that was done? I specify Class A sealing which means every longitiudinal and lateral seam and joint. Brush on mastic such as Hardcast Iron Grip. I strive for zero but may settle for 2 percent leakage. In addition to the Class A standard, I also seal any leakage that is "audible and/or sensible to the hand". Hear it and/or feel it, it gets sealed. Period. Critical installations I will pressure test the duct too.

(Conversely, I am even more stringent on water systems -no leakage at all, how cruel of me to insist... :) But when water leaks- you can see them and the world knows too.)

I am bullish on duct sealing. Want to know why? Air is an expensive way to move heat. It is an insulator (low specific heat really) so you have to move lots of volume to do the same work as say, oh, water... The other part is the laws of physics as applied to fans. If I want to speed up my fan to get just another 10 percent volume, I have to spend about another 30 percent in horsepower. What an uphill climb that is! And 10 percent leakage is considered "good" in some circles. You may have more, perhaps much more. HP varies by the cube of the volume. A lot of effort at the fan for a little benefit at the outlet.

Say your problem system is 1200 cfm with 15 percent leakage, 180 cfm lost to the attic and maybe outside. Your office requires say 200 cfm but is getting 100 on a good day. You could be losing about twice what you need to make your office comfortable.

The long duct run leads me to think leakage, all those joints add up. Each is a leakage opportunity. Balancing still is a factor of course, but without sealing you are "chasing outlets you cannot control".

3. How well insulated are the ducts? Are they in a heated attic or are they in an attic vented to the outside? Insulation of ducts in an attic want to be the equivalent of R-11 or better. Model energy codes say R-8 minimum. Part of this is for condensation protection but the other is just plain radiant and conductine heat loss (or gain).

Another part of this is, if the return is not sealed and is in the attic, it will draw in ambient air, hot or cold which will reduce available net capacity to the space. The first rooms served still get first dibs on what is supplied so they may seem fine. We have not touched on energy costs now have we? Forget BTU's lost alone, the fan horsepower you are spending could be doing so much more if leakage is the issue.


Corrective Program (To Do it Right)

1. Identify tightness of ductwork by pressure test. (Blank off supply and return registers with plastic film and duct tape (yes duct tape!). A blower, even a shop vac, and a calibrated orifice is used to maintain the duct at a given pressure and to measure the outflow at that pressure. That is your leakage rate essentially. (Short synopsis).

2. Seal the ductwork if this is warranted. You can strip the duct of insulation and do it manually from the outside. Under pressure you can find the leaks better if not obvious, but seal under zero or negative pressure only. As a retrofit application, a company called "Aeroseal" has a product that is suspended in air and seaks leaks, sealing them from the inside. Brookhaven National Labs (I think) helped develop it for the Dept. of Energy. While I have specified it in the past, I have not seen it finally installed.

Upon sealing, re-test. Re-seal until it meets the target standard. Then insulate the ducts scrupulously only after sealing. Use rigid board on the bottom so it does not sag, or secure with stick-clips.

3. Balancing- How did the contractor do it? Did he measure airflow at the outlets and with what instruments? Or was it "by hand, lick and a promise"? To even do it by hand takes hours because you have to meausure the temperatures in each room. It takes two hours between adjustments for the rooms to come to equilibrium.

(You yourself could do this by the way. Can you get to the dampers? And do they lock in place?)

Properly done, balancing is an iterative process: You start with all dampers open and adjust the fan to the lowest speed that will deliver air to the furthest outlet as a percentage of the whole volume delivered. Then you move back toward the fan (pressure decays moving away from the fan) and close down branches upstream of your furthest room. Each branch is checked as you go. Once all is done, a volume re-check is made as will be any adjustments. (Each time you adjust the margin of error is reduced so subsequent adjustments do not affect the other outlets as much.)

Measurement of the air outlets or inlets is done at least with a pitot tube and manometer in small systems and outlets, and with a flow hood in larger outlets that are surrounded by wall and ceiling surface. (I like Shortridge myself. But these are not inexpensive and are not something you will buy for yourself unless you really, really get into this...) If you want to do it yourself, Dwyer Instruments has some devices that may be affordable for you.

Think of the duct system as one big long balloon filled with air. Let more air out near where the air goes in, there is less for the others. Squeeze it one place, air goes to other places. The point of balancing is to have each branch think that *IT* is the longest branch. That is what dampers do.

I did not get into automatic or shift dampers because I think I gave you enough to go on :)


Best,

Brad

LWise

System unbalanced

I have a 2 year old two story home with a dual outside unit for heat and air for the downstairs and separate furnace and air for the upstairs. The lower floor system works perfectly. The upper floor which is approximately 1500 sq ft does not. The furnace is located in the attic and is at the far East end of the home. The thermostat is located on the wall by the main return which is about 12 ft from the West end of the home. My problem is that when I set the thermostat at 68 the master bedroom is too cold, if I set it at 69 the bedroom is too hot. My office which is on the far West corner and shares the wall with the thermostat wall is 3 degrees colder than the thermostat area. I can stand in the office doorway and reach around the corner and touch the thermostat.

I have had the company that installed the system check it out and they tell me that it is working fine. They supposedly balanced the system by opening and closing dampers in the upstairs ducts. In the summer, I have the opposite problem, I cannot get the upstairs cool enough and the office is about 8 degrees hotter than the rest of the upstairs. The system is now out of warranty so I have no support unless I want to pay for it. I have a friend in the business and He advised me that the air is sufficient for the home but that putting the furnace at the far end could be the problem. There are two returns upstairs, one a 21" is in a small hallway by the thermostat where there are two bedrooms a bath and my office. There is another 12" return in the master bedroom. The only other factor I can add is that my office and the thermostat wall is above the garage which has an insulated ceiling.

I am considering moving the thermostat to the area of the return in the master but I am open to suggestions.

Plumdog_2

Everything Brad has said; and less

A typical attic or crawlspace forced air "system" is the builders choice of low cost installs. Typically, one size flex duct is snaked to each of the register boots, and one boot is installed in each room regardless of heat loss/gain considerations for that room. This results in severe imbalance of airflow. Frequently, the flex duct will fall off at the connections and just lie up in the blown-in insulation; heating the outdoors and keeping snow off the roof. I have seen flex duct fill up with condensate and fall to the cieling, creating a p-trap of sorts and preventing airflow.
You should get a light and inspect the ductwork in the attic; or get an expert to look it over.

Brad White_40

Actually, Plumdog, you said more...

I forgot about flex duct. The use of it on a job anywhere becomes a license to do illegal acts with ductwork under the age of 18....Hard duct where you can and limit flex duct to straight runs not over 5 feet long.

That's my rap-
Good call, P-Doggy!

Ron Schroeder

But Brad

Flex Duct comes in 25' legnths, that means I have to leave a coil somewhere if its only a 5 foot run. Tongue firmly planted in ckeek.

Brad White_40

And cheek stuffed with flex duct too I might add!

The key to over-sized flex duct lengths is easily solved by a box cutter. The TSA has mine so now I use my teeth....

:O)>

LWise

reply

Thanks for the info! Indeed the system is flex duct. I have a friend that retired from the business who installed commercial hard duct work. he has no fondness for flex duct and thinks it is the most inefficient way to move air. I think he is right! I still can't figure out why the master is hot at 69 and cold at 68. How can a 1 degree change in the thermostat result in a 8 degree swing in the room temperature? I changed the thermostat with the same results. I will however take your advice and check all of the joints before it gets too hot up there.

Thanks
Les