Static pressure

[jeff520]

What are considered to be the acceptable ranges of negative pressure at the inlet to the air handler (i.e. after the filter in the return plenum) and the positive pressure at the start of the distribution ducts?

How much additional pressure drop at the inlet to the air handler would indicate that it was time to change the filter in the return?

Assume a system that is actually moving 1200 cfm of conditioned air.

[jeff520]

Do any of the professionals ever measure this?

What would they be looking for?

[Pascone10]

static pressure on a system depends on many variables and cant be given in a general answer. different filters are more restrictive than others while different coils have different pressure drops. the system has to be designed with these things in mind. beginning of a duct system a .2" is ok to do a good design. now as far as changing filters depending on restriction. most homeowners change the filters by manufactures recommendations. and once again the restriction all depends on how the system was designed using manuel D. i hope i answered your questions. let me know. what are you looking to do?

[jeff520]

I'm establishing a baseline for my recently installed system. The house was built in the mid 50's and had evaporative cooling. (This is Arizona, and that was normal for the time). The evaporative cooler moves more than 10,000 cfm and the ductwork was designed for that, so the supply ducting looks good from the refrigeration demand of only 1200 cfm.

I measure a static pressure of between 0.02 and 0.03 inches of water in the trunk immediately after the coil.

Return was another story. There is no ductwork - the furnace sits on a false floor in a closet and the return was originally a 6X 18 inch register in the bottom of the closet door. Marginal for the original furnace, way too small for the new refrigeration air handler. The installer increased this register to 8 X 20 (still not enough!) and I have since built a false floor and put an 8 X 24 in the door of the adjacent closet, which now becomes an extension of the furnace closet plenum. I have also made two short 3 X 12 ducts to expand the plenum into an adjacent room. That is the very best that can be done to increase the return duct area and I now measure a pressure of 0.19 between the fan and the Clean Effects filter (or .37 if I insert a pleated prefilter as well)

So total static pressure is a high of 0.40 with a clean 1" pleated filter in place. That seems reasonable to me since the specs for the furnace cover a static pressure range from 0.1 to 0.9 inches.

I was asking here to find out what sort of numbers the professionals typically see, and what they might aim for when they have a chance to build a complete system including the duct work. Keep in mind that my installer felt the job was done with just over one square foot of return register, so this is not a question I would expect to get a good answer to from them!

My apologies to the moderators if you feel this is too close to DIY - I see it as a purely educational question and not directly pertinent to any carpentry I choose to do around the house.

[beenthere]

As a rule. You shoot for the units TESP rating. Usually.5"

Seldom manage to get it exact.
Your TESP is wrong, because you have to measure the static between the A/C coil and the furnace. The coil is external to the furnace.

Your coil could add anywhere from .1 to .3 to the supply esp you measured.

[dash]

I'm a little confuse ,you mention a one inch filter and also a Clean Effects filter.

Assuming you have the Clean Effects,my gues from seeing them is that by the time you see much rise in static ,it's way past the time to clean it.It appears it would capture very same particles on it's core,once the core is covered ,I 'd think it would not be an effective filter,but the static may not have risen very much.This would be the same as an EAC,plates are dirty ,no rise in static.

Just my guess from looking at them and reading about them.

[jeff520]

Call me paranoid, but I have still to develop confidence in the Clean Effects ability to get all the particulate matter out of the airflow and to hold it securely until the filter cells are cleaned.

During construction I did see some large particles of sawdust and wallboard that were mechanically caught on the inlet side of the Clean Effects cell structure but i saw no evidence of slightly smaller particles caught within the cells themselves. The cells do get dirty, so Clean Effects is doing something, but I feel a lot more secure knowing that particles about the size of a grain of sand will certainly get caught and held by the pleated first stage of filtering.

The 1" pleated filter is intended to catch the larger material and then the Clean Effects should catch the rest.

Since my maintenance contract does not cover cleaning the indoor coil I really want to do all I can to make sure it stays clean and does not become the final filter before the supply ducts.

Also, there is no easy way for the homeowner to know that the Clean Effects is really working. At least I know the prefilter is not dependent on electronics to perform its function so it is a good independent form of redundancy.

[air2spare]

Since my maintenance contract does not cover cleaning the indoor coil I really want to do all I can to make sure it stays clean and does not become the final filter before the supply ducts.

Also, there is no easy way for the homeowner to know that the Clean Effects is really working. At least I know the prefilter is not dependent on electronics to perform its function so it is a good independent form of redundancy.



That's why it's hard to beat a good ole MERV 10 or better media filter ;)

[gary wilson]

I'm establishing a baseline for my recently installed system. The house was built in the mid 50's and had evaporative cooling. (This is Arizona, and that was normal for the time). The evaporative cooler moves more than 10,000 cfm and the ductwork was designed for that, so the supply ducting looks good from the refrigeration demand of only 1200 cfm.

I measure a static pressure of between 0.02 and 0.03 inches of water in the trunk immediately after the coil.

Return was another story. There is no ductwork - the furnace sits on a false floor in a closet and the return was originally a 6X 18 inch register in the bottom of the closet door. Marginal for the original furnace, way too small for the new refrigeration air handler. The installer increased this register to 8 X 20 (still not enough!) and I have since built a false floor and put an 8 X 24 in the door of the adjacent closet, which now becomes an extension of the furnace closet plenum. I have also made two short 3 X 12 ducts to expand the plenum into an adjacent room. That is the very best that can be done to increase the return duct area and I now measure a pressure of 0.19 between the fan and the Clean Effects filter (or .37 if I insert a pleated prefilter as well)

So total static pressure is a high of 0.40 with a clean 1" pleated filter in place. That seems reasonable to me since the specs for the furnace cover a static pressure range from 0.1 to 0.9 inches.

I was asking here to find out what sort of numbers the professionals typically see, and what they might aim for when they have a chance to build a complete system including the duct work. Keep in mind that my installer felt the job was done with just over one square foot of return register, so this is not a question I would expect to get a good answer to from them!

My apologies to the moderators if you feel this is too close to DIY - I see it as a purely educational question and not directly pertinent to any carpentry I choose to do around the house.

Is this a furnace or an air handler? I'm a simple New Englander with a hydronics background, so I'm wondering why you have a 10,000 cfm duct system with 1200 cfm. Anyway, your supply pressure seems very low. Makes sense from what you indicated. Anyway, you mentioned AHU and Furnace...There is a difference... AHu's have the coil 'within' the esp. There may be differing opinions on what esp is and what tesp is. If it's an ahu, and you see the rating on the nameplate, you're looking at esp, and the coil does not get counted in becasue it's 'internal'. But, like we all know, if the coil gets gooped up we know you 'won't' see that on esp. It's a silly play on numbers. I could blab on, am i confused? Please state your appliance.

[jeff520]

Gary,
My system is a Trane 80% gas furnace with an AC evaporator coil above it. It feeds into ductwork that was installed in the mid 50's to handle the original furnace (inside the house) and an evaporative cooler on the roof.

Just as we don't see snow shovels or power cords hanging off the front of cars, you may never have met an evaporative cooler. It is a large blower that sucks in outside air through a pad made of wood shavings. The pad is kept wet by a continuous stream of water pumped from the floor of the device. Excess water drips off the bottom of the pad and the water level is maintained by the same sort of ballcock arrangement found in the average toilet tank.

The hot desert air is cooled when it passes through the pad, and thousands (literally) of cfm of warm damp air enter the home. This is preferable to hundred degree very dry air supposedly.

That is why my antique ductwork is built to handle ten times the airflow it now has to carry for the AC system. One of the selling points for AC in this area is derision of the evaporative cooler, claiming it is like a giant vacuum cleaner and that your home is the bag. There is some truth to this because there is a lot of dust in the air here.

I am interested in monitoring the performance of the whole system and that is why I wanted to know some typical numbers for static pressure. I will be chasing my contractor to get the figures from Trane for the pressure drop across the coil for different air flows. That would be a very quick and easy way to monitor relative CFM performance and also long term fouling of the coil itself.

[gary wilson]

OK, Thanks for the explanation. What is the reading after the furnace and before the coil? and what is the reading after the filter and before the blower? I confused myself when you mentioned 'air handler'.

10,000 cfm? What did that return sound like before this new furnace went in?

Do you have a psc or ecm motor?

As far as how effectice your Clean effects is, you may want to find someone with an Air Advice unit to check out the air quality.