Hello –
Could use some advice –
I want to reduce a 10 foot run of duct work on the return from 15 in. x 8 in. to 10 in. x 8 in. so I can make a smaller loop under the basement stairs without having the ducts hang down so low. Do you think the reduced difference in duct size means much to the balance of the air system ? or not really anything to worry about ?
Thanks for any help + reply –
SA
Replies
Too Much Junk
JH
Yes gas furnace - forced air with AC - I believe the blower has two settings - currently on the lower setting -
Could I use a round duct under the steps - it would fit better -
Thanks for the replies -
SA
Yep, just change your blower setting the higher. Your furnace and AC will be slightly better efficiency also, even with the slight duct restriction.
Round duct fine also.
Nimbers: With the transitions, at 2000CFM, est. duct pressure downstream will drop by 0.015 in of water pressure.
TYpical furnace or AC blower airflow starts to drop more rapidly (for details, look up a blower curve) at about 0.5 to 0.7 in H2O. So, the 0.015 you are adding will not disturb the system too much, esp if you up the blower setting.
return air duct
First let me say that I rely on my long time havc contractor to spec. and size my hvac on any of my projucts. Not my job.
My first reaction upon reading the op was that, no you can't just arbitrarly reduce the size of a R/A duct. Just my gut reaction.
Then you say no problem. Just do this and this and all will be fine. When you do these things to a previously properly designed system, want this effect all of the other return and supply ducts in the system? i.e. the cfm ,velocity etc. Just asking. Educate me.
Educate me.
Caught me in a mood....so, to liven the old place up a little maybe?:
Aint rocket science for something as simple as the OP posted, one 10 ft duct section and he can simply move a plug on his blower for a higher speed. 33% reduction in cross section for only 10 feet. The transitions are a bigger deal.
As for 'educate', simply read up on Bernoulli's equations. Or, one can SIMPLY look up the effects of the OP's simply duct change on tables or an online calculator, then compare to the capabilities to the fan curve for his furnace. Probably could just have told OP to go do that search and look up the fan curve for his blower and do the calcs, but then OP would have missed getting advice to pay out $$ to his local HVAC contractor who is somebody's BIL, eh?
OP is not 'arbitrarily' reducing the size of the duct, OP knew enough to ask first. Did quantify the opinion to OP when I had time with the observation that 3% rise in pressure needed to be produced by the blower was not a big deal.
Since I DIY everything associated with residential, I have never had a contractor for HVAC, unless you count the subs such as HamStd who do airplane environmental control systems that I get technical comments from occasionally when discussing non-professional matters?
On the old BT, often gave the opinion that the best duct system was the biggest you could fit in space available without impacting headroom, storage, etc..
I once complained on the old BT that HD had shorted me 2" on a roll of flooring, which was needed for a pattern match. Got 'advice' from 'flooring contractors' to hire it done next time, etc. and that 'they' always add on an extra 18" to any order. Nice spendthrifts aint they with somebody else's $$, just like most governments, eh?
Some further advice to OP - DO NOT hire anybody to change your duct. Cut a 60deg slope transition and use the cutoff metal for the transition, and overlap all the leftovers. One or two (minimum use) pop rivets to hold everything in place and seal the edges with metal tape (not duct tape). You will be good for 40 years.
BTW, didja know great-grandpa went to grammer school with Danny? <G>
duct work
Dang junkhound. let us know when it's that time of the month. :>)
Another option would be custom duct put together to match that angle.
In Summary
Many thanks to all for the help - I think I'm going to be OK with a slight reduction in vent size - probably I'll go round and up the width to 12 inches instead of 10 inches. . I can also cut a small corner out of the stair stringer to raise up the vent. One person suggested to go over the top of the feed through the bays - but I don't want to kill the ceiling height on the other side of the feed. I will reinforce the stringer if I cut it and just to be certain - a friend at our Parish is HVAC - I'll ask him to calculate the air flow before I start changing anything.
I'm a woodworker - so yes - I will do this myself - it won't be pretty but it will work :>) I did think about a custom duct in a triangle form but the transition with the elbows seems like too much trouble.
Any last follow up by you nice folks is appreciated -
Regards,
SA
It is just as important that business keep out of government as that government keep out of business.
HH
Probably no problem, but is this the main return to the furnace? No other return branches that tie in between this and the furnace?
If there are others, it could happen that the new restriction will cause the air to return more easily in the other branch(es), which, in turn will mean that the supplies that use this new smaller return will not flow air as well as they used to. In other words, the air will use the path of least resistance.
Air Flow
Thanks for the help RD
This is the main return to the furnace -
There is only one other branch return between this proposed alteration and the furnace. A much small size duct that takes air from the corner of the L/R.
Regards,
SA
Duct Work
That was the point I was trying to make but got shot down. It was expained to me in no uncertain terms, that by increasing the speed of the blower moter everything would be ok.
If all of the other ducts / grills / reqisters remain the same, I just cant help but believe that the system will perform the same though out the house as it did before. Anyway, I just thought it was a fair question.
It is a fair question, and it's never a good idea to restrict airflow, as everybody probably knows--that's why the OP asked his Q.
In his case, the air will likely just pick up velocity thru the restricted area of the return, but I am a little suspicious even of the original duct size of 8 x 15--that's not much if his airflow is supposed to be much above 600 CFM and he has the usual amount ductwork.
He can get a pretty fair idea of how much air is moving thru the system by checking the temperature rise when the furnace has been running for about 10 minutes. It the difference between the temperature of the return air and supply air is much over 70 degrees (with a clean filter in place), he would do well to let the furnace have more return air. A temp. rise of about 50 is more like it. The temps should be tested right at the inlet and outlet of the furnace.
nah, not trying to shoot anybody down.. <G> did not intend anything negative, sorry if I gave that impression.....
.. had just finished trying to explain power factor to someone who thought that a no-load motor produced more heat than a loaded motor since amps times volts that was going in with no shaft power going out that all the amps times volts hadda go into heat...... at least they had some concept of conservation of energy , arrrrgh <G>
OPs post to go to a round duct is a good idea and if the transitions are done well, a 12" round duct has almost identical drop over long runs as the original 8x15" rectangular duct, --heh, heh, somethigh about air not liking to get crowded into tight corners, eh?