Best place to measure supply & return static, and supply air temp, in this system?
aunsafe2015
| Posted in Mechanicals on
I’m seeking advice on the best place to measure static pressure and supply air temperature in the HVAC system shown in the pictures (Trane XV18 heat pump). As you can probably tell from the pictures, the flex duct was poorly designed and installed, and I’m trying to make some improvements to it, and I’d like to be able to measure any improvement.
On the supply side picture, the red dot is about 10″ from the air handler and is where I was planning to insert the supply static pressure probe. The blue dot is several inches past the red dot and is where I was planning to measure supply air temperature.
On the return side picture, I was planning to insert the return static pressure probe at the red dot.
Are the proposed locations reasonable places to measure static pressure and supply air temp? Or are there places on the plenums that would be significantly more accurate? I don’t need perfection… just want to be reasonably close to accurate numbers.
Thanks in advance!
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Replies
Your locations are good. Generally you can measure the pressure anywhere in the plenum.
You should also check the pressure in the return plenum. I've had one system where a single fitting in the return was responsible for 1/3 the pressure loss of the whole ducting.
Thanks for the response. When you say, "you should also check the pressure in the return plenum," do you mean something other than what I showed in my second picture of the return plenum? I was planning to measure the static pressure in the return plenum at a point very close to the filter, as shown by the red dot in the picture. Is that a good place to measure in the return plenum, or should I move a few inches further away from the filter?
Very strange and disturbing photos, is this unit in an attic with all the ducts going up not down to the occupied spaces? What is the R value of the insulation under the attic floor? Is attic storage so valuable that you would double the length of your duct work to keep the floor open? Why is the unit way over by the wall instead of being located in the center of the space? I am always upset to see equipment and ductwork in unconditioned spaces.
While the pressure numbers you get will be interesting how will knowing them help you decide what you want to do?
I do think the temperature deferential number from the supply and return is one of the most important things you can know about your system.
Walta
Yep, it’s a walk up attic. That’s why the HVAC is over on the side. The builder assumed the owner would finish the attic at some point and wanted to maximize finishable space.
I’m not the original owner. And I didn’t know much about HVAC or duct work when I bought this house. The duct work is an absolute disaster. It’s so bad in some places that I think the installer must have affirmatively been trying to do a bad job.
The good thing is, is that there is a lot of low hanging fruit and easy improvements I can make, which is why I want to take before and after measurements. I don’t think I’ll be able to turn it into something good, but hopefully I can take it from “terrible” to merely “bad.”
If you want to accurately measure the pressure delta across the blower, then you can't include the filter in the path. But it's fine if you just want relative measurements.
Just want relative measurements. So you think the place I plan to measure return static is ok for that? Would it be better a few inches further into the plenum away from the filter? Or is right next to filter as pictured ok?
That pressure measurement location for the return is fine. I thought that it was for temperature, thus my earlier suggestions.
Generally, you can take pressure measurement anywhere in the plenum as it will be the same. You'll only start loosing pressure after the takeoff fittings for the first set of ductopus runs.
It is sometime good to check the pressure loss across your filter as well. This only an issue if the filter box was not sized correctly or if someone installed a hepa filter for a system not designed for one.
Quick update:
Initial SP measurements were 0.45" WC supply, 0.14 return (keep in mind that does not include filter, coil, or heat strips). The air handler itself actually has a way of calculating static pressure, and it was giving a TESP reading of 0.76" with the fan speed set to 100% (dry coil).
I added some wide straps to improve the way some of the ductwork was hanging and remove compression points. I also replaced a sharp 90 degree kink in a section of 12" duct work with a rigid metal elbow. I also took a particularly nasty section of 8" duct that was basically being crushed by a wall and overtightened straps, and I completely replaced that section of duct and made sure it was hung properly, not crushed, and pulled as tight as I could reasonably get it.
End result? Supply side static measurement dropped from 0.45" to 0.29", and the value calculated by my air handler dropped from 0.76" to 0.59". Air velocity coming out of the vents at the rooms serviced by the ducts I improved increased by 20% or more (I don't know how to measure air volume, so I just used a cheap anemometer to measure velocity). (I didn't measure return static again, yet, because I didn't make any changes to the return ducts. Those are in much better shape--though still bad--than the supply ducts.)
Pretty happy with that result. Though increasingly depressed how bad the original installation job was. If I could get that significant an improvement with a few hours of work as an average DIY homeowner, I can only imagine it would have only taken the installing contractor a few extra minutes to have simply done the job correctly in the first place.
As for the workmanship if you do not find and big leaks it was better than most installs.
One has to wonder how much better it would work if the ducts were 30-50% shorter with 180° less bends.
Did you log the temp differential and motor wattage?
Walta