Questions re undersized return ducts and locking out High Heat

Hogan

Hi I am new to the forum.  Hoping to get advice or confirmation of my thoughts.

We have a 3000 sqft 2 story home with basement.  20yrs old but pretty tight with air sealing etc.  Furnace and AC in basement and another in attic.  

Basement furnace used to be very oversized 120,000btu single stage condensing and died after 10yrs.   Replacement furnace was recommended as 100/66 2 stage condensing.   It works fine and probably 95% of the time on Low Heat.   I decided to run some tests and notice that it trips the limit on High Heat after about 5 mins. I have since been doing investigating and reading and testing. 

-used a manometer to dial down gas from 3.7 to 3.2.   Did not fix issue

-tested using cheap fiberglass filter instead of merv7 pleated, did not fix

-used a higher motor speed tap and did not fix

-took off old whole house humidifier that I wasn't using anyway as it seemed that hot air was entering into return air.   Cooled return air a bit but didn't fix issue

I ran a test with the blower motor door open and it could run without hitting limit.   Temp at first and closest register was around 128-129 steady while under operation with normal ductwork it would creep up to 135, 36, 37 and the limit would trip. 

I was going to buy static pressure probes for the manometer but decided it is somewhat irrelevant as I believe I just proved my suspicion that this is an issue of insufficient return air.   It does seem we have somewhat limited number of duct returns so the system is likely imbalanced.  

So I guess my question is, at this point I just locked out High Stage by disconnecting W2 at the control board.   Beyond the fact that I feel like they sold me an oversized furnace, and I can address that whenever it is time to change furnace again, is there anything else to check or do?    I thought about opening up a vent in the return duct right at the furnace but the problem there is adding that return suction will just be trying to suck air past 2 doors...utility room and then basement door too.  When I had the blower door off you could definitely feel it at the basement door. 

I don't want to have someone ripping drywall etc to add more return ducts just so I can tell myself my furnace can go to High Fire when it generally never needs to anyway....so was just making sure I wasn't overlooking any other simpler explanations or fixes

Thanks

STEVEusaPA

It's probably still overfired by a factor of 2. Static pressure measurements, both supply and return would help diagnose where you would want to fix ductwork.
Best bet would be leave it on low fire. When it gives up the ghost, do proper calculation to see how many btu's are actually needed, and what you can deliver with the existing ductwork.
Taking the blower door off doesn't tell you much. But leaving the blower door on and checking the CAZ is more important.

SuperTech

I see this type of problem all the time unfortunately.  The only thing you can do besides adding additional returns is to modify the return ductwork at the furnace. Sometimes the addition of a coffin box return and radius elbow with turning vanes will make a difference.  I've also modified the ductwork so the returns use both sides of the furnace for better airflow and lower static pressure.  

You are probably on the right track but you need to make sure the supply ductwork is adequate as well. Also make sure the blower wheel is clean as well as the secondary heat exchanger and evaporator coil if you have A/C.

pecmsg

Is it the return, supply or both thats the issue?

You need a Total External Static pressure reading to tell.
https://www.youtube.com/watch?v=6uMqw69XkRw

Hogan

Thanks for the above

@Steve when you say overfired by 2x you mean this furnace could probably be a 50-60K BTu max size or something.     When I just got new AC units (Bosch) I had them replace my attic furnace with a smaller one (65k/45k 2 stage) and I believe that one runs all the time on low stage too.    The blower in the current one I'm talking about is just too big for the ductwork

@SuperTech yes when we were buying the house our inspector flagged ductwork.  The hack builders had that monster 120k furnace along with a 5 ton AC in basement haha.   And feeding that was a 24x10 return duct along the ceiling that fed into a single 20x8 down into the side of the furnace.   Looking that up, 24x10 could flow 1320 CFM and 20x8 only 750.   They added a second 20x8 right beside it coming off the ceiling duct (to work around the PVC exhaust and AC copper lines).   We had them swap in a smaller AC too.  Hacks probably counted basement airspace in their calcs, if they did calcs at all. 

@pecmsg I ordered some static probes yesterday but then figured the test is irrelevant now if I am not going to undertake ripping up the house to add ducts.   And I thought that showing it ran with the blower door open "proves" that it is running at excessive static pressure and the return side is by definition a culprit since it ran when I relieved the return side pressure by opening the blower door.

Hogan

Well that's weird, I typed a response and posted and it was there and then disappeared

Basically I was saying that I can still buy static probes and run the test but since I have minimal return ducts AND the unit stayed running on High with the blower door open I would think that return pressure and flow is certainly a good part of the problem.   And since I am not wanting to tear up the house to add more returns then I'm sort of stuck with this too-large blower for the ductwork at hand. 

Original ducts when we were buying the house were even worse and it was obvious the builder used a hack AC friend because they had a 24x10 return duct on ceiling going to a 20x8 single down duct to side entry on a 120k gigantic furnace blower with a 5 ton AC coil.   Inspector flagged and we got a HVAC friend in to look and they agreed all wrong.   Had builder swap in a small AC and they also added a second 20x8 down duct.   From what I can see 20x8 can flow 750 CFM and 24x10 1320.   So still my big blower here that can run at 1500 or 1750 or 2000 on the higher taps is too big for the ducting

SuperTech

You wouldn't have to tear up anything to improve the ductwork at the furnace.  Enlarging the return ductwork connection at the furnace, using a coffin box, using both sides of the furnace as well as the bottom, adding radius elbows and turning vanes can make a big difference in reducing static pressure. You just need someone who knows how to make proper ductwork and custom sheet metal fittings and transitions. I recommend trying to find a real pro, it's not easy to fix a hack job.

Hogan

SuperTech said:

You wouldn't have to tear up anything to improve the ductwork at the furnace.  Enlarging the return ductwork connection at the furnace, using a coffin box, using both sides of the furnace as well as the bottom, adding radius elbows and turning vanes can make a big difference in reducing static pressure. You just need someone who knows how to make proper ductwork and custom sheet metal fittings and transitions. I recommend trying to find a real pro, it's not easy to fix a hack job.

Ok I understand

Correct me if I'm wrong but if the main trunk coming into furnace room and covered by finished basement elsewhere is 24x10 doesn't that effectively already limit flow to 1300ish CFM?   And since they have 2 downward returns coming off that for 750 CFM each then it is the ceiling part that is a constraint and so adding even more space around the furnace may not help?   Unfortunately the furnace is up against the wall so both sides would not work.   Could potentially raise it up to include side and bottom.

One consideration I have is that if I am really still oversized on the equipment, how much money to spend on trying to get more breathing for a High Stage situation that I don't really even need, and whenever the NEXT furnace gets needed then will be downsizing and so ducting may work fine for smaller unit even on high fire.   Attic unit used to trip out all the time unless I used a cheap fiberglass filter.   Now with the new 2 stage overall smaller unit with more advanced blower motor it runs fine with a Merv 7 pleated

pedmec

First thing you should do is set the gas pressure back to 3.7 at high fire. Condensing furnaces and especially two stage furnaces are very sensitive to adjustments to the gas pressures. Never play with gas pressure adjustments without having a combustion analyzer watching what effect it is having flue products in the system. I have been doing combustion testing on gas fired boilers and furnaces for 25 years and you would not believe the carbon monoxide numbers that they produce when not running at the required pressures. it would seem that you would be fine in that you have more excess air but what you do is change the burner pattern and it does the opposite of what you would expect. i have done multiple experiments on 90+ systems and this is what normally happens.

You stated that you replaced the furnace but you did not say whether you changed the A-coil. your coil could be dirty if you are still using the old A-coil.

Did you go thru all the dip switch settings and set them to the proper air flow to match the system?

SuperTech

You have to take a bunch of static pressure measurements to see exactly what is affecting airflow.  Airflow and static pressure doesn't like sharp 90⁰ bends. Post some pictures of your furnace and the ductwork near it, I bet you have room for improvement. I agree with the post above, any changes in gas pressure should be checked with a digital combustion analyzer even if you stay within the manufacturer specified range. Without proper testing you are only guessing. A condensing furnace needs 150 CFM of airflow for every 10,000 btus of input.  You might be able to downfire the furnace and lower static pressure but these changes need to be verified with a complete combustion analysis. This is usually not something that 99% of DIYers are capable of. 

Hogan

@pedmec

Why would I go back to 3.7 if the stated placard range is 3.2 to 3.7 and I thought general wisdom was set it around 3.5?     I can readjust for sure not to be bumping on the lower end of the range.   But kinda moot point if I'm going to leave W2 locked out....will never go on High Fire.  I understand what you're saying about combustion check though

This is a 2013 era furnace so has speed taps and not dip switches....but yes I tried different taps on High and none of them fixed the issue.  On lowest Red tap and low fire I get nice air movement and the manual says that is around 950-1000 CFM.   1250 and certainly 1500 on the next taps are really pushing (and pulling) it for my ductwork.   I can get some probes and do the measurement but I can just tell that pressures are high. 

Hogan

SuperTech said:

You have to take a bunch of static pressure measurements to see exactly what is affecting airflow.  Airflow and static pressure doesn't like sharp 90⁰ bends. Post some pictures of your furnace and the ductwork near it, I bet you have room for improvement. I agree with the post above, any changes in gas pressure should be checked with a digital combustion analyzer even if you stay within the manufacturer specified range. Without proper testing you are only guessing. A condensing furnace needs 150 CFM of airflow for every 10,000 btus of input.  You might be able to downfire the furnace and lower static pressure but these changes need to be verified with a complete combustion analysis. This is usually not something that 99% of DIYers are capable of. 

For sure there are 90 degree bends.  There is a 24x10 duct along the ceiling and then 2 20x8 ducts (are those the "coffin box" someone referred to?) coming down out of that, and then another hard 90 at the floor into the side of the furnace, and on the up flow supply on top of the furnace it goes straight up and then does another 90 square turn into another large square ceiling duct.   So I totally agree this has a lot of hard 90s just around the furnace.  I will see if I can post a picture later.  

I guess I will do a pressure test just so we can see how much we are talking here

As I look around on basement and first floor I can count up all the supply registers vs return registers.   I can't tell for sure but assume many of the supply registers are coming off a couple shared round supply ducts that seem to be 8 inch diameter.   On the returns there are only 3 ducts on the first floor and they are the type where there is a rectangular duct in the wall and then 2 registers that open off that same duct, one near the floor and one up near the ceiling.   They don't open or close.   The effect is that you feel more airflow sucking at the floor than you do at the ceiling register and in any case you are beholden to the volume of that duct.    In the basement there is only One return duct in the ceiling.  It looks large and square like a box in the ceiling but I can see that it just turns into a round duct right there to travel through the ceiling and it appears to be 8 inches diameter.   It must attach to that 24x10 ceiling box duct closer to the furnace as Im sure 2 of the other 3 return ducts do.   The final return duct comes on the other side and plugs into the top of the return duct where it turns down to go from the ceiling to the floor at the furnace and is 8 inches diameter. 

Hogan

Also any tips on running the static test?   Can I do it through the limit switch hole or do I need to drill a hole in the back of the furnace case at the very top above the heat exchanger but below the coil

Hogan

And also I still think this is a duct sizing issue.  I will take off a register and measure those rectangular in wall returns but eyeballing a table I was looking at, I will bet those thin rectangular ducts only can flow a couple hundred CFM and I only have 3 of them on first floor and 1 in basement.   So of course when you try to run a big blower at a 1500 CFM setting it's like sucking through a straw

Hogan

Hogan

So this is the crappy return ducts I have.   I have 3 of these on main floor.  In basement I have a single return that starts in a square box but that enters to a round duct probably 6 inches diameter.  

The one in the picture above is 9 inches wide max by 2.75 inches and of course rounded too.  I am looking at duct charts and while I don't see this exact size, they show a 3 1/2 by 10 as 120 CFM.   So this is not even that large.    So let's say 100.   3 of those.  And 8 inch round is 200.   So we are talking 500 CFM of return here.  I don't really even need to take static tests yeah?   It seems pretty obvious I am under-ducted for having a furnace blower of this size.   And thus why it seems to run fine on the 900-ish CFM tap (the red, slowest)

pecmsg

You cant just use the size of the duct also have to calculate the transition to the main duct and any bends. that give you total equivalent length.

You need static pressure readings!

Hogan

@pecmsg

Yes I understand.   I guess what I'm saying is it should be pretty obvious that by STARTING WITH such a small amount of ductwork, I will have flow issues (which I already can sense).   Meaning, if a small little return duct like that is specced at around 100cfm and I have 3 of them, it's not like there is some magical way that the hidden in wall transition will make it flow 500 CFM.   

Basically I know I have flow issues so it's just a matter of how bad they are.   I will try to get a static probe to see

EBEBRATT-Ed

@Hogan

Forget the static pressure for now if all you have are those 3 ovals returns your way undersized. Removing the grills may help temporally but you don't want to suck the cat into the returns or any plastic bags etc.

Is you furnace 100K input on high fire or is that the output? How many tons of AC?

to move 1500 cfm you need a minimum of the equal vent of a 24 x 8 (larger is better?.

Can you find a location above or around the furnace to cut in a large floor register if that would not be objectional?

Hogan

EBEBRATT-Ed said:

@Hogan Forget the static pressure for now if all you have are those 3 ovals returns your way undersized. Removing the grills may help temporally but you don't want to suck the cat into the returns or any plastic bags etc. Is you furnace 100K input on high fire or is that the output? How many tons of AC? to move 1500 cfm you need a minimum of the equal vent of a 24 x 8 (larger is better?. Can you find a location above or around the furnace to cut in a large floor register if that would not be objectional?

Right that is what I was trying to say

I ordered a static probe anyway just for curiosity but it's sort of like the 300 pound person looking to use a computerized body fat percentage....we know up front that the number is high haha

To be clear I have 3 of those ovals on first floor.  There is also one return duct in basement ceiling that is prob 8 inch round.   The silly thing is each oval has 2 return vents attached to that same duct, one at ceiling and one at floor.  Obviously the floor one pulls most of the air.    I assume in that setup one might have had closeable registers so you could switch from drawing warm air from ceiling in AC usage and drawing cold air from floor in heating usage but my home just has plain registers.  Knucklehead HVAC installers when the house was built.  

It's a 100k high efficiency unit but remember on low fire it is only 66k which is how I'm running it now that I locked out High Fire.  I think I'm just gonna live with it that way.    Honestly this house probably never needed 100k heating anyway.   I shake my head even more that originally they put in a 120k single stage.   I honestly don't know how that unit didn't constantly trip the limit before it died.  

We have a 3 ton AC but it is a new Bosch inverter that apparently can adjust itself as necessary based on the cooling load.   But there again that unit says maximum 1200 CFM on the coil to avoid water blow by and so basically that is using one of the first 2 taps on this blower.   The 3rd 4th and 5th taps on this blower (1500, 1750 and 2000 basically) are all way too big for my ducts

EBEBRATT-Ed

@Hogan 1200 cfm is right for a 3 ton that is why I asked 400cfm/ton is what is needed for cooling to work right. If your cfm is lower than that on cooling you will ice up the evaporator. On some jobs you can go as low as 300/ton but that can cause issues.

On heating you need to stay within the temperature rise stamped on the units name plate for the heat exchanger to last. Overheating and repeated limit trips are NG. And on the other end too much air flow can cause condensation on the hX and cause failure.

You need to find out the amount of air you are moving to avoid issues. Assuming your moving 1200 because your on that tap is not good. I would increase the return duct size you are very short on air flow.

When you on low fire with 1200 cfm I would expect the temp rise to be around 48 degrees. Your going to need more than 1200 to run the heat on high fire you would need around 1600 cfm on the heat.

CFM= btu output of furnace/ (1.08 x TD)

CFM= btu output of furnace/ 1,08 x 55 assuming 125 supply and 70 return

1600 CFM=96000/(1.08 x 55)

Hogan

EBEBRATT-Ed said:

@Hogan 1200 cfm is right for a 3 ton that is why I asked 400cfm/ton is what is needed for cooling to work right. If your cfm is lower than that on cooling you will ice up the evaporator. On some jobs you can go as low as 300/ton but that can cause issues. On heating you need to stay within the temperature rise stamped on the units name plate for the heat exchanger to last. Overheating and repeated limit trips are NG. And on the other end too much air flow can cause condensation on the hX and cause failure. You need to find out the amount of air you are moving to avoid issues. Assuming your moving 1200 because your on that tap is not good. I would increase the return duct size you are very short on air flow. When you on low fire with 1200 cfm I would expect the temp rise to be around 48 degrees. Your going to need more than 1200 to run the heat on high fire you would need around 1600 cfm on the heat. CFM= btu output of furnace/ (1.08 x TD) CFM= btu output of furnace/ 1,08 x 55 assuming 125 supply and 70 return 1600 CFM=96000/(1.08 x 55)

On low fire I am on lowest tap which is more like 900-950 CFM.  Temp rise on low fire is around 55. 

I just locked out High Fire for now

I would prefer not having them come in and tear up walls and ceilings for this.   If my basement was unfinished this would be a piece of cake to address but it is fully finished. 

I'm getting a static probe and will see what the readings are just to see how bad it is.   Too bad those return ducts can't use more of the space.   Maybe that rounded design is supposed to be better for airflow vs rectangular 

Hogan

OK I finally had a chance to get my measurements. They're kinda ugly, but what I suspected given those small return ducts....

ON LOWEST SPEED TAP (RED):
Above Furnace, Below AC Coil: +.33
After Filter, Before Fan (with cheap Merv 2 fiberglass filter): -.46
After Filter, Before Fan (with 1 inch Merv 7 pleated filter): -.62
Before Filter (fiberglass): -.36

So - I assume -.36 is kinda high for pre-filter. I didn't measure above the AC coil but can. I assume it will drop around .10 to .15 to around .25-ish.

ON 2nd LOWEST TAP (ORANGE):
Above Furnace, Below AC Coil: +.43
After Filter, Before Fan (with cheap Merv 2 fiberglass filter): -.61
After Filter, Before Fan (with 1 inch Merv 7 pleated filter): -.78
Before Filter (fiberglass): -.48

ON MIDDLE TAP (BLUE) (3 of 5):
Above Furnace, Below AC Coil: +.53
After Filter, Before Fan (with cheap Merv 2 fiberglass filter): -.73
After Filter, Before Fan (with 1 inch Merv 7 pleated filter): -.93
Before Filter (fiberglass): -.59


SOOOOOO ugly yeah? My takeaways are that this system is underducted. And I guess I should just use a cheap fiberglass filter to avoid adding the extra .16 WC of pressure coming from that 1 inch MERV 7 pleated filter yeah? Better to lose some fine filtration but lower the pressure and increase airflow yeah? Are there any 1 inch filters that can pass air more like .1 WC like that cheap fiberglass yet still have better MERV filtration? (presumably with a lot more pleats than mine has)?

Any other advice beyond "you could have someone come out and pay them a lot of money to rip up the house and add some more ducts, or otherwise just stay running on Low Fire and Lowest Speed Tap and in the future, probably get a smaller furnace"?

jesmed1

We have this same problem on our new church furnace. Static pressure over 2x the limit specs by MFR, because the installers put in too big a furnace for the the small return duct cross section. The return can't be resized due to space limitations.

Our only recourse was to lock it in low fire mode and install the "rock catcher" fiberglass filter.

It took me months to locate the root cause and convince the furnace installer that they installed too big a furnace for the fixed return size. They eventually agreed after I showed them the static pressure readings from my digital manometer. But by then it was too late to fix anything.

We did discuss installing turning vanes in the undersized return plenum that has a sharp 90 turn, but that probably still wouldn't have helped enough, so we didn't pursue it.

So now we have a furnace running at 40% of its rating. I feel your pain.

Hogan

jesmed1 said:

We have this same problem on our new church furnace. Static pressure over 2x the limit specs by MFR, because the installers put in too big a furnace for the the small return duct cross section. The return can't be resized due to space limitations.

Our only recourse was to lock it in low fire mode and install the "rock catcher" fiberglass filter.

It took me months to locate the root cause and convince the furnace installer that they installed too big a furnace for the fixed return size. They eventually agreed after I showed them the static pressure readings from my digital manometer. But by then it was too late to fix anything.

We did discuss installing turning vanes in the undersized return plenum that has a sharp 90 turn, but that probably still wouldn't have helped enough, so we didn't pursue it.

So now we have a furnace running at 40% of its rating. I feel your pain.

The funny thing is the original furnace was even more of a hackjob install....120k btu single stage, even larger blower. No wonder that unit died within 10yrs. I then used a family friend HVAC guy for the replacement and he has a very successful business, but here again he just downsized it somewhat with a finger in the wind.

I need to pull out the manual again to eyeball the CFM numbers and see if it's worth it to step up one notch from Red to Orange and take the higher pressure (and presumably still higher CFM) or just leave it on lowest Red tap and call it "best I can do for now"

Can't believe the homebuilder installed so few return ducts for the amount of supply ducts. Hack

jesmed1

My biggest surprise was that even our HVAC guy didn't really know how to use a ductulator correctly to calculate CFM. After I pointed out the undersized return plenum, he plugged in the dimensions of the plenum but failed to apply the "effective length" factor of like 60 for a bottom return plenum with a sharp 90 turn, so his static number for the plenum was way too low. I was shocked that I, a homeowner engineer, had to correct him.

Once I realized that they hadn't bothered to do a basic CFM calculation before installing the furnace, and that if they had, it probably would have been wrong anyway, that confirmed to me they were careless, inept, or both.

Another shocker was the other HVAC tech (his brother) who didn't know that the max allowed total static for the furnace was 0.5" WC. When he asked me where I got that from, I had to show him...on the rating plate.

Hogan

jesmed1 said:

My biggest surprise was that even our HVAC guy didn't really know how to use a ductulator correctly to calculate CFM. After I pointed out the undersized return plenum, he plugged in the dimensions of the plenum but failed to apply the "effective length" factor of like 60 for a bottom return plenum with a sharp 90 turn, so his static number for the plenum was way too low. I was shocked that I, a homeowner engineer, had to correct him.

Once I realized that they hadn't bothered to do a basic CFM calculation before installing the furnace, and that if they had, it probably would have been wrong anyway, that confirmed to me they were careless, inept, or both.

Another shocker was the other HVAC tech (his brother) who didn't know that the max allowed total static for the furnace was 0.5" WC. When he asked me where I got that from, I had to show him...on the rating plate.

yep I have been consistently disappointed with the HVAC folks I have found over the years. At the time of our first furnace replacement 10yrs ago, of the oversized 120k single stage monster, I had a guy out from one of the well-known and highly rated places around here. I started stumping him with some basic questions about what was the difference between the "good, better, best" equipment options he was presenting to me. This was just based on some very early poking around on the internet and some logical questions about different features that were presented on the bullet points from his own sales sheet. "What is XYZ on this model vs ABC for that other model and why would I want that or not want that feature?" Ummm, Deer in the Headlights followed by some incoherent answer like "because that one is better". Ahh I see, makes perfect sense, Good Better Best, no explanation as to why Best is better than Better or Good, but I should just decide how much I want to pay and whether I have been a good boy and deserve Best, or Better, or only Good (???)

I've found this buying cars too. The salesman was incorrectly noting different features and trim levels. It's like dude, if you're a Toyota salesman or a Honda salesman, take the time to at least learn about the current Toyotas or Hondas you are supposed to be selling every day as your full time job.

jesmed1

It seems a common problem is installers who don't bother to do sizing (heat loss or CFM) calculations and just replace whatever was there with a new unit of the same size, or make educated guesses. Sometimes that works, sometimes it doesn't.

bburd

If this were my house I would figure out some relatively painless, low cost way to add some return airflow capacity. Of course if the furnace uses indoor air for combustion, you cannot take return air from the furnace room.

Hogan

bburd said:

If this were my house I would figure out some relatively painless, low cost way to add some return airflow capacity. Of course if the furnace uses indoor air for combustion, you cannot take return air from the furnace room.

Furnace is high efficiency so uses outside air via a PVC vent and then exhausts thru a second PVC vent

I do have a powervent gas water heater in that same basement utility room. May be OK because the powervent is well sealed onto the top of the water heater but I didn't want to be drawing too much suction from that utility room in case it would be somehow drafting the gas water heater exhaust and broadcasting it around my house thru the furnace. There is potential good spot for a return in the corner of the basement where the return duct comes across the ceiling, but unfortunately a very powerful furnace vent is right there too so it feels like I would just be sucking in that freshest, strongest first heat vent on the supply trunk and sucking right back in as return air heading back to the furnace. Sort of defeats the longer supply and return flow.

pecmsg

jesmed1 said:

It seems a common problem is installers who don't bother to do sizing (heat loss or CFM) calculations and just replace whatever was there with a new unit of the same size, or make educated guesses. Sometimes that works, sometimes it doesn't.

The installers are  minor issue. 

The salesman are the biggest issue. 

bburd

Hogan said:

There is potential good spot for a return in the corner of the basement where the return duct comes across the ceiling, but unfortunately a very powerful furnace vent is right there too so it feels like I would just be sucking in that freshest, strongest first heat vent on the supply trunk and sucking right back in as return air heading back to the furnace. Sort of defeats the longer supply and return flow.

Can you cut a floor grill in above that duct, then cut a hole in the top of the duct and seal off the rest of the joist space with sheet metal? Then you can draw air from the floor above. You want this relatively far from the furnace to avoid airflow noise, or with a couple of duct turns in between if closer. 

Zman

Best practice would have been to design the supply for 0.1" WC and the return for 0.05" WC. Fans hate restricted returns, especially when there is a 90 before the return.
Is there a way to let the return breath a little? Maybe cut a grill in near the furnace and let it pull some air through the space?
For the filter, it is a bit counter-intuitive but thicker filters have less resistance than thinner ones due to the increased surface area. Can you modify the filter box to fit a bigger filter?

EBEBRATT-Ed

What @Zman said is correct.

Hogan

Zman said:

Best practice would have been to design the supply for 0.1" WC and the return for 0.05" WC. Fans hate restricted returns, especially when there is a 90 before the return. Is there a way to let the return breath a little? Maybe cut a grill in near the furnace and let it pull some air through the space? For the filter, it is a bit counter-intuitive but thicker filters have less resistance than thinner ones due to the increased surface area. Can you modify the filter box to fit a bigger filter?

I understand re 4 inch filters having more flow than a 1 inch filter but the box is just a 1 inch.  The downward vertical duct is right beside the furnace so would have to be moved outward to allow room to add a 4 inch filter.    I think the tiny return ducts (3 of them in that 9 x 5 oval coming from first floor plus one 6 or 8 inch round in the basement and thats it!) is definitely a constraint. 

I have thought about cutting a grill near the furnace but worried on a couple things 1) that utility room also has a Powervent gas water heater and 2) given the imbalance with the airflow I am worried that adding a large sucking vent right at the furnace will mean I'm getting negative pressure at the closed door at the top of the basement stairs and also the door to the utility room too.  I guess I could just leave the utility room door open or cut a vent into that door or something.    Actually I just got rid of the bypass humidifier that was on there and crossing between supply and return air, so I could just take off the sheet metal patch that I put over that connection and see how much that might lower the return air pressure.   But is it still against code to have a return air being pulled from a room that has a Powervent water heater?

If I just cut in a grill on the return trunk in the utility room (again subject to the water heater question) how big?   

Zman

Your furnace is not making the building pressure positive or negative to the outdoors so the power vent "shouldn't" affect the furnace. That being said, where is the water heater combustion air coming from? I don't like appliances that are competing for combustion air to be shoved in a closet together. Is your furnace pulling CA from the outdoors? Is it possible to make your water heater pull it from the outdoors?

Hogan

bburd said:

Hogan said:

There is potential good spot for a return in the corner of the basement where the return duct comes across the ceiling, but unfortunately a very powerful furnace vent is right there too so it feels like I would just be sucking in that freshest, strongest first heat vent on the supply trunk and sucking right back in as return air heading back to the furnace. Sort of defeats the longer supply and return flow.

Can you cut a floor grill in above that duct, then cut a hole in the top of the duct and seal off the rest of the joist space with sheet metal? Then you can draw air from the floor above. You want this relatively far from the furnace to avoid airflow noise, or with a couple of duct turns in between if closer. 

Possibly but above the furnace is kitchen and family room with hardwood floors etc.  I worry about boogering that up but also imagine a floor duct would be horribly dirty with dog hair etc being sucked down all the time.  Fully finished basement makes it more cumbersome.  

bburd

Hogan said:

bburd said:

Hogan said:

There is potential good spot for a return in the corner of the basement where the return duct comes across the ceiling, but unfortunately a very powerful furnace vent is right there too so it feels like I would just be sucking in that freshest, strongest first heat vent on the supply trunk and sucking right back in as return air heading back to the furnace. Sort of defeats the longer supply and return flow.

Can you cut a floor grill in above that duct, then cut a hole in the top of the duct and seal off the rest of the joist space with sheet metal? Then you can draw air from the floor above. You want this relatively far from the furnace to avoid airflow noise, or with a couple of duct turns in between if closer. 

Possibly but above the furnace is kitchen and family room with hardwood floors etc.  I worry about boogering that up but also imagine a floor duct would be horribly dirty with dog hair etc being sucked down all the time.  Fully finished basement makes it more cumbersome.  

You could hire someone to do it if you're not comfortable cutting into the floor. I have done this, it's not that difficult. Floor return grills are quite common, the furnace filter takes care of the dirt and pet hair.

ratio

pecmsg said:

jesmed1 said:

It seems a common problem is installers who don't bother to do sizing (heat loss or CFM) calculations and just replace whatever was there with a new unit of the same size, or make educated guesses. Sometimes that works, sometimes it doesn't.

The installers are  minor issue. 

The salesman are the biggest issue. 

What's the difference between a used car salescritter and an HVAC salescritter? The used car salescritter knows they're lying to you.

The sad fact is, if you know all the lipstick colors you'll do fine in sales. This seems to be true nearly everywhere.

Hogan

ratio said:

jesmed1 said:

It seems a common problem is installers who don't bother to do sizing (heat loss or CFM) calculations and just replace whatever was there with a new unit of the same size, or make educated guesses. Sometimes that works, sometimes it doesn't.

The installers are  minor issue. 

The salesman are the biggest issue. 

What's the difference between a used car salescritter and an HVAC salescritter? The used car salescritter knows they're lying to you.

The sad fact is, if you know all the lipstick colors you'll do fine in sales. This seems to be true nearly everywhere.

Ahh I see
So with smaller places the contractor does the quoting and also leads the installation.   That person is potentially more experienced.   But in a larger operation they have sales guys running around selling the furnaces and AC units and then another guy or guys come to install.   And the more experienced owner may be overseeing it all but not getting as close to the work.  I think that was the type of sales guy that was in my kitchen with his nice pictures and Good Better Best.    Basically Good was a single stage unit, Better was a 2 Stage and Best was a Modulating.  Fair enough but he wasn't able to articulate why I needed a Modulating unit and of course the sizes he was recommending were ultimately wrong.  I assume what happens is they just eyeball the size of the house and just "know" the general size, and don't bother to see that my house is newer construction and tight sealed and also has crappy return ducting

Hogan

Guys am I correct to assume it is better to stick at a static of 0.79 (low speed tap, fiberglass rock catcher filter) which shows 955 CFM according to manual, vs the orange speed tap which would increase static pressure to 1.04 (still with rock catcher filter) and would indicate cfm of approx 1075 interpolating on the table?    My assumption is 1.04 WC vs 0.79 is much harder on the blower motor and not worth working it so much harder just to add another 120 CFM if the house is warm etc.    Agree?