Static Pressure in PSG Caddy Plenum

[Jonathan Stroble]

Hello,

I have owned a PSG Caddy for the past two winters and recently had the plenum and some ducting rebuilt after moving the furnace to a different location in the basement.

Through much trial and error I have learned techniques to operate the furnace during the coldest winter months so that the house temperature tops out at around 68 degrees Fahrenheit; but, the process seems very labor-intensive and the early-morning temperature frequently drops below 60 degrees Fahrenheit and requires several hours to recover (with the help of electric space heaters in the main living area). I suspect that part of the challenge is in the ductwork and plenum configuration.

The owner's manual has a trouble-shooting topic for "Furnace burns well, warm air plenum is very hot but there is not much heat coming out in the room." One suggested cause is "Low static pressure," and the suggested fix is to "Rearrange the ducting to respect the minimum static pressure of 0.20 in. w.c." Elsewhere in the manual, there are tables suggesting an optimal static pressure range of 0.20" w.c. (min.) to 0.50" w.c. (max.). (In one place it says "0.20 to 0.05," but I suspect that the second number is a typo because everywhere else it says 0.5.)

I purchased a manometer on Amazon.com ("PYLE Meters PDMM01 Digital Manometer with 11 Units of Measure"), drilled a hole dead-center in the side of my plenum, screwed in a small metal nozzle that came with the manometer, and attached a 4" length of plastic tubing also included with the manometer. I chose the location based on the diagram shown in the manual, which I have attached here.

With various blower speed settings chosen, the highest plenum pressure I measured was 0.14". My questions for the forum (at last) are: How likely does it seem that the plenum static pressure is a significant factor in heating efficiency? And, how might I remedy the problem in order to get the plenum static pressure above 0.20" wc?

As an additional data point, I wonder if my plenum might be too tall... could that cause low static pressure? It's 31.75 inches tall, which is about 2/3 the height of the furnace itself. (And about 24.5" x 28.75" square, corresponding to the width/depth dimensions of the furnace.) It seems noticeably taller than the plenum that it replaced, even after the contractor shortened it at my request to ensure at least 6" of clearance to the floor joists above it.

Thanks for reading all of this! Happy to provide more information on request.

 

[brenndatomu]

How likely does it seem that the plenum static pressure is a significant factor in heating efficiency?

Very I'd say

And, how might I remedy the problem in order to get the plenum static pressure above 0.20" wc?

Raise the blower speed or partially close some duct or register dampers until your heat is balanced throughout the house the way you want it and the SP is still met, can take a bit of time to set up.

As an additional data point, I wonder if my plenum might be too tall...

I don't think that will affect the SP

I'm not sure how much difference it makes, but I always hear the HVAC techs talk about using a pitot tube in the plenum. Also, I think it depends which way the tube faces in relation to the airflow and whether the low side of the manometer is reading room pressure or return plenum pressure.
Maybe someone with a bit more expertise than me will jump in here, summer is slow season here ya know. I'm interested to see what others say, I am playing with my systems SPs right now myself

 

[DoubleB]

Jonathan Stroble said: ↑
How likely does it seem that the plenum static pressure is a significant factor in heating efficiency?
Very I'd say

I'd also factor a few additional things (and I'm no expert, by the way). I'd say the static pressure might be a significant factor. I didn't see you mention if your plenum is excessively hot (although you got to that part of the troubleshooting in the manual). First I'd confirm if your plenum is too hot. I don't know that there's a rule, but if it's over 130F with a high fan speed, then I suspect you don't have enough airflow.

However, since the highest SP you achieve is 0.14 inH20 with the highest fan speed (if you are measuring it properly), you ought to have plenty of airflow to carry heat out of the Caddy and into the plenum. Then, is your concern that some rooms are hot and others are cold? If so, then try adjusting the dampers or registers for individual rooms as @brenndatomu said.

Also, if you go to bed at 68 with the Caddy cranking and awake below 60, and it takes a long time to recover, that sounds to me like a leaky, underinsulated, and/or really large house (I sometimes have similar results with my leaky/underinsulated house).

For perspective, I only have 0.10 inH20 SP in my supply plenum, with the low fan speed. I could put it on high speed and achieve 0.20, but I don't feel any need to because the 0.10 at low speed means that I have larger ducts and more airflow than a system with 0.20 at low speed. And if 0.20 at low speed is good enough for SBI, then my 0.10 at low speed is even better cooling for the furnace. In other words, at 0.10 my plenum temps are around 115F (with the damper shut), which I would argue is no where close to overheating the Tundra or failing to remove the heat the fire produces.

I also doubt that plenum height matters. However, I congratulate you on enforcing the requirements to the contractor (don't get me started on my experiences with that). Try positioning your tube head-on into the airflow, and then perpendicular (sideways) with the airflow. Does the measurement change much? If not, then I don't think the velocity of the air contributes much to your measurement and I think either direction should be a decent representation.

Again, this is just what has worked well for me, I'm not an HVAC tech, so take it as you like.

Good luck!

 

[heaterman]

Think of duct static as being equivalent to the pressure from your faucet.
Higher pressure provides more flow of whatever medium is being moved.

 

[STIHLY DAN]

Think of duct static as being equivalent to the pressure from your faucet.
Higher pressure provides more flow of whatever medium is being moved.

Higher pressure means its moving faster, not necessarily more flow (*volume)

 

[brenndatomu]

Higher pressure means its moving faster, not necessarily more flow (*volume)

Right, because if you squeeze back a couple dampers, the air speeds through the remaining gap, that raises the pressure, but reduces the overall volume a bit. Very easy to see on a blower curve chart
Is static pressure test properly done with the low side of the meter in the return duct or just sampling room pressure?

 

[DoubleB]

Is static pressure test properly done with the low side of the meter in the return duct or just sampling room pressure?

My understanding is the purpose of static pressure is to help even out flow to different rooms, therefore I'm pretty sure static pressure compares the supply plenum to the room pressure.

I understand that the total pressure rise through the furnace blower will be higher once you add pressure drop through the cold return duct and filter.

 

[STIHLY DAN]

Is static pressure test properly done with the low side of the meter in the return duct or just sampling room pressure?

Sampling room pressure. In the return would be the differential. Pressure is tricky, you don't want to much air going through the coil, but you don't want too little either. Also you don't want the air passing through to fast not being able to pick up thr heat.