undersized ducting
i have a 60k btu gas furnace, and i don't have enough supply ducting to allow for the required CFM. when the fan is on the factory fan speed of 5, the temperature rise across plenum's is likely too high (65-70ish, when the data plate says 30-60) but of course there is less static pressure and the heat is less noisy. when the fan is turned the whole way up to 8, the temperature rise drops to a more reasonable ~55 ; still high but within spec. but then of course, the static pressure is high and it's very noisy. in other words, until i can add more ducts, i'm scr3w3d one way or another.
my question is, until i can add more ducts, am i better off with the high fan speed? better temp rise but more static pressure? my logic is that i'd rather put more strain on the blower than the HX. If i drop fan speed / pressure down, the HX ends up seeing more heat. I understand that neither is ideal and the most proper fix is a smaller system or bigger ductwork, i'm just talking about until then. which is the lesser evil
Comments
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First you need to explain how furnace and ductwork met. They knew they weren't a match for each other but fell in love anyway? Are they newlyweds, or have they been together a while?
What make and model furnace? Natural gas or LP?
Can you post pics of the supply and return plenums and their sizes?
Actual heat rise numbers are needed. Gas pressure readings would be good too. What if the furnace is over fired? Or under fired? Actual static pressure would be awesome.
Air filter is clean? High efficiency air cleaner. MERV#?
Is there an AC evaporator at the supply?
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oversized equipment on undersized ducts.
lower the equipment BTU/h and or increase the duct CFM capacity.
there is no 3rd choice.
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newly installed natural gas furnace where there used to be an even larger oil furnace. new furnace is 60k btu run tru by trane a951. i can post pics soon. with fan on highest speed, temp rise is about 55 (plate says should be 30-60) and total ESP is about .88" (plate says max is .7"). i won't touch the gas valve myself but i've clocked the meter coming in a 1000 BTU per minute or 60k BTU per hour so i'd say firing correctly. brand new filter, merv 8. yes, cased coil above the HX
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New undamaged cased coil?
Any way you can draw a complete duct layout?
What's the sq ft of the heated space? The 40K wouldn't cut it?
Without facts, I "feel" it might not be duct related, unless the old furnace was blowing like a Chinook wind.
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Picture is worth a thousand words. Shots of the furnace and ducting around it.
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little bit older , but undamaged , cased coil. it's 6 duct lines , each one coming directly off the plenum. i would describe it as "home run" / "spider web" / "octopus" style ; there is no trunk line. here's pics for you and @Kaos . house is about 1000sf, was told need the 60k btu because of the climate (Boston) harsh winters. if it's not duct related, what do you think it is then?
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Is that round duct the only return to the trunk? Can you do a tally of the supply duct and sizes.
The issue is your furnace is oversized by about a factor of 2x to 3x but that ship has sailed. The old one was probably running at very high temps.
Might be good to pop off the panel by the AC coil and take peak at the bottom of it. Those can get clogged over time.
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The ductwork is pretty horrendous. The labeling on the combustion air intake and flue pipe is comical, is that DWV pipe on the exhaust? The coil on top of the furnace is a problem, it needs a transition. You can certainly lower the static pressure by redoing that ugly ductwork. 60K for 1000 square feet? Was a load calculation performed to determine the size of the furnace? You can try down firing the furnace but honestly there's no simple fix, your installing contractor just did a very poor quality job on everything.
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Also they mixed up the pipes. The exhaust needs to be CPVC not ABS.
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Well, there it all is. If I was the furnace, I'd ask for a divorce.
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there is also a large make-up return air plenum in the basement. the AC coil was cleaned over the summer
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the labeling is a requirement in my state, not sure what's so comical about it. yes a load calc was done, i also got about 5 quotes and literally every company was suggesting a 60k furnace. it gets very cold here so it's more necessary although still probably a little oversized. i think ultimately as you allude to, i just need more ductwork (i'd rather not downfire it and make it run lean). in the interim like i mentioned, i have gotten temp rise down to 55 which is acceptable & within spec. just that static is very high now
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the exhaust is PVC, by the way. it says right on the pipe, i just looked. will post a pic if you don't believe me
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why lol
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What was the static pressure readings, I’ll bet 1.0 or higher.
That duct system is atrocious.
If a manual “J” said smaller then go smaller!1 -
The black pipe connected to the white pipe does not appear to be PVC. It's looks like ABS or DWV.
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I noticed a second flue pipe. Do you have an atmospheric vent water heater in the basement? You really don't want to be pulling any return air from the basement. That's against code for a very good reason. Was this job DIY? I'm really getting the impression that this was never inspected.
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Why? Because it's never going to work right as installed. There's a good reason to follow ASHRAE manual J, manual S and manual D. Nothing you have has been done correctly. It's really hard to fix something when everything was done wrong.
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System 1738 Black PVC is UL 1738 certified for flue gas exhaust venting. Despite being widely used, standard white stuff is not rated for exhaust venting.
Edit: See Section 12.5 of NFPA 54/ANSI Z223.1 National Fuel Gas Code (NFGC)0 -
@PC7060 Interesting. Around me rated pipe is white for low temp and grey for high time.
@camiarrobino Get a differential pressure gage good for 1" wg. Dwyer stuff is cheap, digital ones from Amazon are not great as they are mostly for up to 3psi so not enough resolution.
Measure the pressure drop along your system.
-ambient air to return
-differential across air filter
-differential across furnace
-differential across AC coil
-supply plenum to ambient
This will tell you where the losses are. Once you have that, you can have a game plan of how to fix it.
60kBTU in a 1000sqft is nonsense. I was heating a 1500sqft uninsulated brick house + finished basement in zone 5 with 40k furnace and even that was oversized.
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Kaos: “Interesting. Around me rated pipe is white for low temp and grey for high time”
@Kaos - Not sure where you are located but products seems to vary by region. Duravent PolyPro is common in Northeast US and is a grey materialThe Canadian standard for exhaust gas venting required 90°C rated CPVC distributed under “System 636” trademark. I think it is also grey.
Seems like anything goes here in Virginia so use of unapproved white DWV PVC for furnace/boiler exhaust venting is typical.
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i'm not sure how many times i have to tell you, it says PVC RIGHT ON THE PIPE. No, there is nothing atmospheric vented in the basement i have no idea what you're looking at. there's a direct vent tankless WH which isn't even pictured here
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i measured about .3" before the filter, and .5" after the filter. so .2" drop across filter? regarding between the furnace and coil, i'm not exactly sure where to drill hole in the cabin to measure that?
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also, no, this was not a DIY job, permits were pulled and it was inspected. not to sound like a jerk, but you've already been proven wrong on both the pipe material of the black flue, and that there was another atmospheric vent flue in the area. maybe you're seeing the plugged up vent holes on the chimney where there USED TO BE atmospheric vented equipment, but as the plugged up status would indicate, not anymore. so, maybe stop jumping the gun so much here trashing someone else's setup / trying to make an issue out of things that are not
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.2 across that filter is normal
What’s the TESP.
Again I’ll bet off the charts!
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the last reading that i would need is between the coil and furnace, correct? if so, where should i drill that hole?
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@camiarrobino Please be respectful, people that have seen a lot of bad installs are trying to help you out.
Understandable not wanting to drill hole near a refrigerant coil, that can definitely end bad. Find a screw there and take it out. Get something like a blow gun tip or ball inflator needle and put it on your diff gauge's hose and use hte screw hole for the reading.
Yes, delta across the coil and plenum to ambient. What are you using for the readings?
0.3" on the return is not great but not terrible. You should be able to cut down to about 0.1 by adding some extra returns.
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search for
T oral
E external
S tatic
P pressure
that duct work sucks!!’1 -
i carefully drilled hole in the top right of the furnace cabin box.
my TESP is 1.2" which i know is crazy high. .5" on return and .7" on supply. so overall the supply is fu(k!ng me more than the return? when i drop the fan speed setting by 1, it drops to .8" total , which still high but a lot more reasonable. i do plan to add another supply soon. the temp rise is around 55ish, so i think i should probably keep the speed where it is for now, i don't want that number getting higher
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i've got it now. yes i'm aware on the ductwork - old house, old ductwork. used to be even worse (no insulation) before i added some insulation which raised DT on both heat & cool and stopped the ducts from sweating in the humid summer. despite all this, it does heat and cool the house well so at least i have that
my TESP is 1.2" which i know is crazy high. .5" on return and .7" on supply. when i drop the fan speed setting 1, it drops to .8" , which still high but a lot more reasonable. i do plan to add another supply soon. the temp rise is around 55ish, so i think i should probably keep the speed where it is for now, i don't want that number getting higher
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Yes, you need to fix one side, supply is more restrictive so a good start.
You can use proper top takeoffs for each, these significantly reduce the EL. Also check registers. Lot of old house have 2x12 ones, once you put a grill on them, these only flow about 1/3 of a 4x12.
Low hanging fruit would be to replace the flex with hard pipe and change the take offs.
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