Author Topic: HVAC Help - old and inefficient ducts  (Read 2462 times)

MrsStubble

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HVAC Help - old and inefficient ducts
« on: May 31, 2015, 09:30:00 PM »
Hi guys, hoping someone can help us out. Our house has really messed up system (heat pump/central air) that is very inefficient.  2 of our 3 bedrooms get too little air to effectively heat or cool, and 1 bedroom (our master unfortunately) gets all force making it way to hot in the winter, and too cold in the summer.  The weird imbalance also makes our house sweat in the summer (water beading on the windows) and makes half of it an ice box in the winter.    Our system is new and properly sized to the house so i'm thinking our ducts are the culprits?  There are lots of 90 degree bends and weird splits in the basement before the ducts even start the turn to head up the walls to the rooms.  We want to look at the duct sizes and bends and try to see if it's causing us to lose airflow, but we can't find the calculations anywhere to figure out what we should have.   Is there a basic calculation we should be looking at or a chart we should use?  Every blog i find says contact a professional but being mustachian diy'ers we'd like to tackle this project ourselves. 

zolotiyeruki

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Re: HVAC Help - old and inefficient ducts
« Reply #1 on: May 31, 2015, 10:30:31 PM »
I've actually tried to do this myself.  It's not a super-simple calculation unless you're an engineer, and even then it's not very straightforward!  If you get a good professional, they'll likely plug it all into a computer program that makes it a lot simpler.

With that caveat, if you do a search for "hvac duct elbow equivalent length", you'll find all sorts of resources that will help you figure out how much air goes to each room.  IIRC an elbow is equivalent to 30 feet of straight ducting, or something like that.

What it will come down to, though, is balancing.  Each duct should have a damper on it, near the main trunk, to adjust the airflow to each vent.  If the handle is parallel to the duct, it's open, and if it's perpendicular, it's closed.  It sounds like you need to make sure the two bedrooms' dampers are fully open, and close off the damper to the master.  It'll probably take some trial and error to get it perfect.  If you want to get it really precise, buy a few $5 digital thermometers from walmart--they're shaped like a raindrop and record the highest and lowest temperatures over the last 24 hours.  Put one in each bedroom, and look at the data after a day, and make adjustments as needed.

Goldielocks

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Re: HVAC Help - old and inefficient ducts
« Reply #2 on: June 01, 2015, 12:40:27 AM »
A lack of ample cold air return (ambient air return) reduces air flow / circulation in the home, and makes a strong differential between rooms.

Do you have large air return registers throughout?  1/2 inch gap below interior doors?   Does it even out when you leave interior doors open?

You could also try installing a ceiling fan in a central area, like top of stairs, or upper hallway.

This is a simple, and often overlooked solution.

Heckler

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Re: HVAC Help - old and inefficient ducts
« Reply #3 on: June 01, 2015, 07:04:46 AM »
First, replace your furnace filter.  If it's clogged, it will reduce the flow significantly.

Then, do this.


What it will come down to, though, is balancing.  Each duct should have a damper on it, near the main trunk, to adjust the airflow to each vent.  If the handle is parallel to the duct, it's open, and if it's perpendicular, it's closed.  It sounds like you need to make sure the two bedrooms' dampers are fully open, and close off the damper to the master.  It'll probably take some trial and error to get it perfect.  If you want to get it really precise, buy a few $5 digital thermometers from walmart--they're shaped like a raindrop and record the highest and lowest temperatures over the last 24 hours.  Put one in each bedroom, and look at the data after a day, and make adjustments as needed.


Sincerely,

A mechanical engineer who would not bother trying to calculate duct work you cannot see.

GatewayTwo

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Re: HVAC Help - old and inefficient ducts
« Reply #4 on: June 10, 2015, 12:44:09 PM »
So... my house (built in 1885) has gone through a few different types of heat before I bought it and so our ducts seem to have the same sort of spaghetti installation method.

What I plan on doing is to check out the ACCA manuals from the library, and make sure our system is correct from the start. 

That said, there are some contractors out there who go by rules of thumb and the like, but in very old (or very new) houses, or houses where you've done some MMM style tinkering with insulation and the like, the rules of thumb are not likely to be accurate).

To start, you'd need Manual J (Residential Load Calculation) to figure out just how much energy your different rooms are taking in (and rejecting).  Manual S (Residential Equipment Sizing) can help you figure out if your HVAC is actually the right size, based on the calculations from Manual J.  Manual D (Residential Duct Systems) can help you get the ducting right. 

As Heckler said, it'd be hard to calculate ducts you cannot see, but a few minutes with a flashlight can help you make some basic assumptions to get enough data for Manual D to be useful.  That, and I think the manuals lay out the calculations in such a way that anyone with the ability to use a spreadsheet reasonably well should be able to do it.

In our house, the problem is mostly that two rooms (Master, my daughter's room) get a TON of sunlight, leaky windows, and have dormers where the insulation is still not the best.  So, hot in the summer, cold in the winter.  Balancing didn't work because the effective duct length was crazy after the 3 or 4 u-turns to get around existing gas / plumbing / electrical / giant steel beams that are in the basement.  A little bit of science (and a lot of sitting and thinking about creative solutions) has helped me to figure out how to re-work the whole thing so that the upstairs bedrooms that need it most are pulling off the plenum where the pressure is the highest.  (and also there are no more U-turns.)

(A mechanical engineer with an intense dislike of hack-job contractors and their "rules of thumb".)

One extra note... if you're going to go through the trouble of re-working all your ducts, make sure to seal them well.  You may not be able to do anything about the bits in the walls already, but just making sure you aren't just pushing all your conditioned air into the basement will also help a fair amount.

Dan_at_Home

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Re: HVAC Help - old and inefficient ducts
« Reply #5 on: June 10, 2015, 07:11:43 PM »
Hi guys, hoping someone can help us out. Our house has really messed up system (heat pump/central air) that is very inefficient.  2 of our 3 bedrooms get too little air to effectively heat or cool, and 1 bedroom (our master unfortunately) gets all force making it way to hot in the winter, and too cold in the summer.  The weird imbalance also makes our house sweat in the summer (water beading on the windows) and makes half of it an ice box in the winter.    Our system is new and properly sized to the house so i'm thinking our ducts are the culprits?  There are lots of 90 degree bends and weird splits in the basement before the ducts even start the turn to head up the walls to the rooms.  We want to look at the duct sizes and bends and try to see if it's causing us to lose airflow, but we can't find the calculations anywhere to figure out what we should have.   Is there a basic calculation we should be looking at or a chart we should use?  Every blog i find says contact a professional but being mustachian diy'ers we'd like to tackle this project ourselves.

Do you have flex ducts coming from the main duct channel from your main unit?  If so, there may be dampers installed from the main channel from the heating unit, there are little knobs that adjust air flow through each channel that usually flows into each room.  I found these in my house (my heat pump is in the attic) and has flex ducts to control the amount of flow to each room.  You can experiment with the adjustments to get it right. 

In the pics attached, I had to temporarily remove a piece of flex duct and you can actually see the dampers in action and see how they adjust (this are pics I took from my attic).  For example, if you have the heat on, and one room is too cold, you adjust the damper to open up for more airflow and if a room is too hot, you adjust it to block more airflow.

 


« Last Edit: June 10, 2015, 07:14:21 PM by Dan_at_Home »

MrsStubble

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Re: HVAC Help - old and inefficient ducts
« Reply #6 on: June 12, 2015, 08:33:02 AM »
Guys thank you so much!!  I took everyones advice and started from the beginning investigating the vents, the filter, etc and I think I found the issue.  It seems we have no dampers... at all.  From the looks of the repairs on the vents it seems a previous owner pulled them out (i'm going to guess b/c of squeaky noises?) and then just put them back together with tape so what is happening is all the air is going to the first available vent from the trunk (which is our master bedroom) and everything else is getting what's left.  So, we're going to do a bit of researching (because there are some other odd things going on in the basement) and get it back into a working order first and see what happens from there.  I actually found a catalog that has the calculations in it (wholesale HVAC) so we'll see what we can figure out and i will report back on progress.   

I can't believe there were no dampers in there... just the little switch on the outside to turn them.  Looks like they were cut off with shears.