Can a return plenum be too big?

[jjnet]

Can a return plenum be too big? We can either run three longer 12” metal return ducts down to a two foot high by 21” deep by 24” wide basement return plenum, or run shorter duct into a six foot high plenum. The plenum bottom makes a right angle turn to feed the adjacent air handler from below.

Is there any airflow, mixing or noise penalty for a larger volume return plenum? Our system is an all-Trane zoned 3 ton (1100-1200cfm) XL19i heat pump with matched variable speed TWE air handler.

Thanks for your help. We greatly appreciate the info and time contributions by all the pros on this forum.

[mattwood1024]

yes!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[kim]

That turn before the furnace is worse than an oversized plenum.

Big plenums have low velocity. Low velocity does not matter except at the filter and at the supply registers.

[jjnet]

Thanks for the info Mattwood and Kim.

Per Manual D, the right angle turns will have turning vanes for minimizing resistance. With appropriate vanes, these turns appear to be reasonably accepted for vertical air handler input feeds.

For 1100 to 1200 cfm (two 12" returns from above, one 12" and one 8" return horizontally), is there an optimal plenum size or shape for the roughly 21" x 23" opening at the bottom of our air handler and filter?

Thanks again for your help.

[Advanced Response]

I think your other posts were from 6months ago... So this project is still not completed or is it it was done but now you are having problems and trying to do it correctly again..

You should write a book,, how to do your own hvac in only a yr and then redo it when its not working..

Nothing usefull to say, just wanted to tag along...

As stated in the past,, these calculations should be drawn up by a pro,, as even with a book many things must be designed by knowledge and application of the book knowledge and experience.. There are many many wrong ways to do anything,, however there are a few ways that accomplish the same task effectively.. We get paid to do things cost effectively and to accomplish the task properly..

I enjoyed reading your last epidemic.. Maybe this will be just as good..

J

[jjnet]

I appreciate your comments. There are some tasks that are best completed over a long period of time. Since no professionals had any interest in retrofitting ducting to our house, we are installing the ducts ourselves with their help and guidance. Research, system design and a plan for dismantling parts of the house while living in it were performed over a span of a year and a half. We began the actual work in November, 2004. We’ve spent maybe a week and a half on duct installation, while the rest of our time available between being busy and out of town has been eaten up by deconstruction and reconstruction. Only a small amount of the de/reconstruction remains, so the second half of the duct installation process may proceed at a less glacial pace (or not).

As the variety of experience and opinions on this forum confirms every day, it is prudent at various times for us to seek input from more than just our local contractor. Even repeating the same question a few months apart can be a useful exercise. I prefer to think of this approach as wise on our part rather than see it as you do as just another flurry of desperate cries for help from someone drowning in their own ignorance.

Our system is not complete and so is definitely not failing yet, despite your wishful thinking in that direction. We'll be happy to let you know the ways in which it does fail when we reach that point. Since we literally could not pay a professional to install the ductwork, any eventual shortcomings represent a compromise we have to live with. We are very thankful that this forum exists and are continually amazed at the wealth of time and experience freely given to all. To the extent that the system works, it will be about 50% due to our contractor, 49% to HVAC-Talk and 1% to our diligent attempts to overcome our basic ignorance.

Since we've gotten by just fine for 25 years with a tiny Jotul 602 wood stove and no AC, it's not critical that the system meet anyone else's standards of functional perfection. We and those helping us have done our best to come up with a good system, and if nothing else, when it’s completed we will have new options to explore in cooling and humidifying our home.


[Edited by jjnet on 01-21-2005 at 04:19 AM]

[pstu]

Wow. Hope that goes well. I know the feeling of not being able to find any pro to do some work, and feeling compelled to do it oneself. Despite not *wanting* to be top dog with no experience and little training.

Believe the advice you have received from pros is well beyond my knowledge, and well worth listening to. However I did a major up-sizing project on my own returns, and along the way also asked this board if there was a downside to being too big. In my case the answer also appeared to be no, the results dropped ESP (external static pressure) by 0.3 inches water column. And gave me large enough filter area that high MERV filters don't harm airflow the way they do to most systems.

Best of luck -- P.Student

[dash]

Originally posted by jjnet
Thanks for the info Mattwood and Kim.

Per Manual D, the right angle turns will have turning vanes for minimizing resistance. With appropriate vanes, these turns appear to be reasonably accepted for vertical air handler input feeds.

For 1100 to 1200 cfm (two 12" returns from above, one 12" and one 8" return horizontally), is there an optimal plenum size or shape for the roughly 21" x 23" opening at the bottom of our air handler and filter?

Thanks again for your help.


21X13 max,21X10 min,is what I suggest,based on what you have stated.

I'd look at sizing it so all the ducts you have coming to "tie in" will fit,using a size the will match the return plenum width ,for an easy connection,keeping it as c;lose to square as possible.Square because closer to sq. makes better(less restictive)turns ,with or without vanes.

[dpatty]

What was the reason given for the pro's not wanting to do the work?

[Edited by dpatty on 01-21-2005 at 11:02 PM]

[Gunnery Sergeant Hartman]

Originally posted by dpatty
What was the reason given for the pro's not wnting to do the work?
Since we literally could not pay a professional to install the ductwork,

I think they couldn't get anyone to do install their ductwork because they didn't want to pay them...

[dpatty]

Thats what I was wondering about. If you can afford it I can travel and I dont know many who can't.

[jjnet]

Dash – Thanks VERY much for your guidance. That’s exactly the info we were looking for.

Why did no HVAC pros want to retrofit duct to our house? The short answer is they uniformly said that it would take their experienced crews months rather than days to dismantle and rebuild the house sufficiently to include appropriate ductwork.

They really don’t want to do that kind of extended remodel work just to retrofit ductwork into a house, at least around here. They told us that if we spent the months needed to do this work over a long span of time, they would be happy to sell and install the equipment.

We spent six months trying to see if we could find anyone who would do the work, but could not. I’m sure it’s not hard for you to imagine why: they simply had better things to do with their time. Construction in our area is booming and they’ve got all their plates full with the related work.

Our contractor is being paid a lot a lot of money and is making a substantial profit on us, both for the equipment (Trane XL19i and matched variable speed air handler plus electrostatic filter) and installation. He also makes his 30% margin on all duct supplies we purchase at the local Johnstone Supply.

The long answer?

Our house has never had ductwork and was never designed to have it. Instead it was designed as a building that heats itself via passive solar whenever the sun is out for at least half a day. After several sunny days it retains the heat through 1 to 2 cloudy days with no additional heat needed.

During extended cloudy weather we've heated the 1750 square foot main living area with a small Jotul wood stove. To meet code we also installed baseboard heat, but we rarely use it. If we’re out of town in the winter we’ll leave the baseboard stats set to 50 just to be sure plants, etc. don’t get too cold in an extended stormy period. It almost never comes on.

When we built the house 25 years ago, our area (Durango, CO) had just gone through the wettest and coldest winter on record: many days of -20 to -30 F in town and far colder in the mountains (-60). It snowed every week from 1 to 2 feet. Our passive solar heating worked great in this environment, and our wood stove used only about ½ to 2/3 cord of wood a winter compared to the norm of 5 to 6 cords. Nobody every thought about air conditioning in our development at 7000 to 8000 feet. Who would need it?

For the last 10-15 years the climate has been far warmer and drier. Our super-insulated house keeps the summer heat out well for a couple of weeks. But in extended 90 plus weather, the second floor eventually gets hot and all the insulation begins to work against us. Outside it always drops into the 50’s at night, but once the house has built up substantial thermal mass during the hot days, even with all the windows open every night it takes a week of cooler days to cool back down again.

We’ve also gotten older. Frequent second floor temps in the high 80’s and low 90’s are not fun anymore. And it doesn’t look like the climate is likely to cool down again in our lifetimes. So we decided to look into AC and a full forced-air heat and humidification system to make the house more comfortable and appealing by modern standards. We’ve also finished our 1100 square foot basement and wanted to extend conditioning to that area.

We have no attic and no chase or box areas designed for ducts. We have only our basement ceiling area for running sheet metal ductwork to the basement and first floor rooms. No problem here.

But for the second floor rooms we have a one foot air space between the ceiling and roof insulation (6” insulation adjacent to roof, 12” air space, 3 ½” insulation adjacent to ceilings).

HVAC guys are used to cramped and uncomfortable spaces, but see if you can find one who wants to crawl around in a 12” high steeply pitched roof space surrounded by insulation for days on end. The alternative has been to take a lot of time dismantling a bunch of the house and rebuilding walls and ceilings to include ductwork. This is the part they didn’t want to do and that we’re dumb enough to be doing ourselves.

[kim]

I would have not done the duck work if it was so hard.

You could have used PTACs or Mini-splits or even window shakers. Window shakers can be mounted in the wall like a PTAC to improve the appearence and decrease the noise.

You could have used a high velocity system or even hydronics.

As you can now see. Ductwork is not so simple. You will have to replace some of your duct system to repair it. Increase the size going to the underserved areas. Or get window shakers.

[jjnet]

Thanks for your help Kim. We considered window units for the hottest areas, but whole-house humidification was one of our critical goals. Our main floors are routinely in the 15-25% humidity range with the basement usually about 10% higher. I’ve given up playing my wood guitar except in the basement, because repaired dryness cracks always open up again. There are major problems with some wood furniture as well. We need to keep the house up to at least 35-40%, so we no longer need to worry about wood items self-destructing in our dry climate.

We looked closely at high velocity systems in the first go-round with contractors. It looked great for our house at first, but after talking to some people who’ve had high-velocity installed we had to reconsider. My wife is so sensitive to moving air that if we sit near a ceiling outlet in a restaurant we have to ask to be reseated. She will start shivering in 70 degree air if there is any kind of constant breeze. This is kind of hard for most people (including me) to imagine, but it is a fact for her. The owners of Unico, etc. systems that we corresponded with all complained about noticing strong air movement., We felt it would be unwise to gamble on my wife’s comfort, since the whole point of a modern climate control system is to make you more rather than less comfortable.

This is why we bit the bullet on traditional ductwork, and this is why we’re slowly tearing bits and pieces of our house apart and putting them back together again with duct inside.

[pstu]

Are you using the right word when you say "whole house humidification" in the context of air conditioning? Because I have only heard of AC being capable of DEhumidification. Of course my hot-humid climate in S.Texas leads me to be fixated on de-humidification most of the year.

I am wondering if you are anticipating a central AC to help directly with raising humidity, or if you may be intending to add a humidifier to the forced air system and use that to raise the humidity in your house.

If you haven't already, find a psychrometric calculator program (online or trial-download) and plug in various outside air degree/RH combinations and see how the RH changes when the air is brought to room temperature. I did and it changed my whole understanding of humidity issues:
http://www.google.com/search?hl=en&q=psychrometric+calculator&btnG=Google+Search

Hope this helps -- P.Student

[blazer]

i would be a bit concerned about how supper insulated the house is,,, if its to tight and the loads on the house were not run for that tight of a house you might be headed for a humidity problem,, unit short cycling,,and eventuaally mold,,,and when you are gone for long periods of time that air infiltration helps with an air change over,,,,just a thought,,,,, if it is a concern a good humidistat might do the trick or an erv (energy recovery unit),,

[jjnet]

Perpetual, thanks for your input. In the dry southwest central AC routinely includes whole-house humidification. There are many ways of doing this, and we’ll be using a Carel steam humidifier in the supply plenum. This unit offers simpler and safer maintenance than some of the other options we considered.

Our humidity in the house year-round is normally 15-25%, with dry periods in June and September occasionally dipping into the single digits. This environment is frequently unimaginable for those living in the humid east. Our house is drier than most here because our entire south wall is glass. For mold to ever grow we would have to seriously over-humidify for an extended time. We plan on humidifying only to about 40%, which should not be enough to cause any moisture problems.

Thanks for the link to PsyCalc. I'm going to download it and try it out. It looks like we can definitely learn something from the process.

Blazer, our house was built 25 years ago before the advent of tyvek wrapping. It is extremely well insulated but not as airtight as a modern wrapped home. We also have a wood stove which, when not in use, allows outside air exchange to our main floors. The basement gets plenty of outside air via an optional flue built into our fireplace foundation that is also open to the outdoors.

Because our external walls are 2 x 6 double tongue-and-groove cedar over tarpaper and 1/2” plywood, we are very weather and draft-resistant on windy days, at least compared to an average house around here. We ran HVAC-Calc load calculations for both super-tight and medium-tight house settings and figured we were roughly halfway in between.

So these are the reasons we think we’re OK with regard to over humidification and mold, But we could still be wrong, and we really appreciate anyone’s input that might help us see things we’re missing or misinterpreting.

[pstu]

JJnet, thanks for explaining your position. I should say upfront that it's like another world for me, as I am in the hot-humid S. Texas. Where I am the house RH (relative humidity) rarely gets below 30%. But as Joe Lstiburek likes to say, the laws of physics are the same whether you are in Canada or Mexico.

It's been a real eye-opener to me to understand that the measure I should look at is not really RH in outside air, but grains/cubic foot or its equivalent. Cold air has so little moisture capability that when it is warmed up, its RH is usually lower than my goal for the house. The fact your house has little air infiltration should be good for your winter purposes. In the summer I have to wonder if different rules might apply.

I'm not sure whether you clarified whether your ductwork is inside the building envelope, or outside it. Where I live, ductwork is typically located in an attic ventilated to the outside, and any duct leakage results in some degree of house pressurization or de-pressurization (depending on whether supply or return). As I understand it, 15% leakage is typical unless the installer took pains to reduce it. If your ductwork is inside the building envelope, then you have less to worry about with this matter.

There is a microprocessor gadget named "Smartvent" (a too-common name) which samples air from outside and based on its temperature and humidity measurements, makes a decision whether to begin a longer ventilation session. The maker is Arkansas based and I believe his full attention is toward *reducing* humidity especially in crawl spaces. But the principle of a little computer using rules to "harvest" lower humidity when it is ambient, could be turned upside down possibly for your needs. Wish I could point to a well developed product, but the concept is all I can offer.

Best of luck -- P.Student

[Edited by perpetual_student on 01-28-2005 at 07:57 PM]

[coologic]

Going back to the root of the thread...if a return air plenum can be too big, what are the reasons?

[jjnet]

Thanks for the info, Perpetual. I will check that product out.

Coologic, I'm guessing that the relevant goal is to keep the velocity of the return air above some minimum value. This may be in part to ensure good flow through the filter, particularly if it has a high static pressure drop (like our electrostatic filter does). Please correct me where I’m wrong, those who know.

I was surprised that Dash recommended a maximum of a 21” x 13” plenum size. This equates to 273 sq. in. of cross sectional area. The return ducts feeding into it consist of 3 12” @ 113 sq. in each, plus one 8” @ 50 sq. in. The ducts total 389 sq. in. while the return plenum reduces to 273. Maximum system airflow should be in the 1100-1200 cfm range.

[Shophound]

I'm just curious, and this is probably barn door after horse is missing talk, but did you consider evaporative (swamp) coolers for the home? We have family who live in the hills above Loveland, CO and they use a swamp cooler on those hot, dry days.

But I'm guessing it's year round humidification you're after, whether the system is heating or cooling. One system that does everything has great appeal.

[dash]

Originally posted by jjnet
Thanks for the info, Perpetual. I will check that product out.

Coologic, I'm guessing that the relevant goal is to keep the velocity of the return air above some minimum value. This may be in part to ensure good flow through the filter, particularly if it has a high static pressure drop (like our electrostatic filter does). Please correct me where I’m wrong, those who know.

I was surprised that Dash recommended a maximum of a 21” x 13” plenum size. This equates to 273 sq. in. of cross sectional area. The return ducts feeding into it consist of 3 12” @ 113 sq. in each, plus one 8” @ 50 sq. in. The ducts total 389 sq. in. while the return plenum reduces to 273. Maximum system airflow should be in the 1100-1200 cfm range.


Didn't realize how many and oversized(not a problem,just larger than needed)your return branch duct were.

Since they at around 400fpm(velocity),the plenum could be 20X21.Of course we don't know how many Total Equivalent Feet each run is,nor the size of each grille,or how many supply cfms are delivered to the area each return serves.Plus a whole bunch of other things.

When the velocity changes ,for X to more thean X and then back to X ,ESP increases,not a "big " factor.Most good designs will keep the fpm < 800 to 900,so the 21X13 was based on keeping it at >600 fpm.

As always the best way ,is a Manual D,from ACCA to design the entire duct system.

[pstu]

It wouldn't hurt to give the website of the "SmartVent" product:
http://www.smartvent.net/

They have a couple of papers on that site which look to me to be professional quality. I will state again their focus seems to be on avoiding the harms of too *much* humidity either in a crawlspace or inside the house. I might actually buy this thing in the future, but I understand your problem is exactly the opposite of what it does. Still, it seems that a similar device with the same sensors but different microprocessor programming, could in concept be helpful to you. Possibly a phone call to the guy who makes it, would result in some new insight or ideas.

Best wishes -- P.Student

[coologic]

I know of no technical reason to limit the size of a return plenum, although there are many practical reasons.
The filter will have a maximum velocity but no minimum velocity requirement.

[tomgeer]

Simple, Yes

Have seen returns too big.

by the time you are able to start pulling from House tstat is satisfied.

In this case you have caused a house to become over pressured.

Sorry, no time to go into depth.

[beenthere]

Even though they were proceding slowly.
In the 3 years since this thread was made, they should be done with their return system.

[BigJon3475]

:D

[steve3871m]

Simple, Yes

Have seen returns too big.

by the time you are able to start pulling from House tstat is satisfied.

In this case you have caused a house to become over pressured.

Sorry, no time to go into depth.
Please.... Go into depth and elaborate. I'm interested in your thoughts on how on "oversized" r/a will pressurize the envelope.

[DanW13]

I asked this question on another board about 6 months ago, some of you might know which board but none the less the pro's over on that bd. were all in agreement that there was no such thing as too much return air for your furnace !!! Not sure that really answers your real question on the size of the return, but this tid bit of info may help you in the amount of return air. According to the manual D it says you should basically have return air in all floors of your home not neccessarily in every room but that depends on the layout of your home which would determine where and how many returns you would need. Don't know if you have gone to your local library but they have alot of books availible for some good reads on duct sizing and Manual D.

[beenthere]

Although its hard to have too much return, it is possible.
EG: Original duct design done by Manual D. Supply is installed to the design. Return for what ever reason is installed 40&#37; larger then design calls for. Furnace/blower is a standard PSC. Return static is now reduced enough that the blower can't be slowed enough for proper air flow. Poor dehumidification in cooling, and too low of a temp rise in heating.

[adrianf]

Please.... Go into depth and elaborate. I'm interested in your thoughts on how on "oversized" r/a will pressurize the envelope.

Me too, but I think (hope) he was kidding.

[dash]

Although its hard to have too much return, it is possible.
EG: Original duct design done by Manual D. Supply is installed to the design. Return for what ever reason is installed 40% larger then design calls for. Furnace/blower is a standard PSC. Return static is now reduced enough that the blower can't be slowed enough for proper air flow. Poor dehumidification in cooling, and too low of a temp rise in heating.

Exactly,plus Man. D calls for a low resistance return path from all rooms.