High static across clean coil

Vanisle

Hello. Just checked static on 19 year old 3.5 ton heat pump air handler. Results are 0.2 return, 1.0 across coil, less than 0.1 supply. Label indicates 0.4 test static.
Thoroughly cleaned both sides of coil with coil cleaner and water rinse. Retested: same result(coil did not appear to be dirty before cleaning). This was on the high speed blower setting. Tested on low speed. Got 0.9. Removed the filter and got 0.75. Any ideas on this mystery? House is heated adequately. Blower is in clean condition.

GW

.4 test static —that’s external. You’re testing the drop on the coil which is a different matter. 

You’re at .3 external- seems good 

jesmed1

How close to the coil are you placing your probes? If there's a sharp turn in the ductwork either on the return or supply side of the coil and you're placing one of your probes in that sharp turn, some of the static pressure you're measuring across the coil is actually due to the turbulence caused by the sharp turn in the duct and not the coil itself.

We had a similar problem at our church where the static pressure was too high because of a 90-degree return just under the furnace where the return duct was too small, with a sharp 90-degree bend with no turning vanes. If you have something like that just before/after your coil it will cause a drastic increase in static pressure.

GW

TEST—- maybe it’s actually TESP?? 

One in the same in any case when in comes to air handlers 

total external static pressure 

all of these ratings are “external” to the appliance 

Vanisle

Thanks for the replies. I made a mistake in the return air static. There was a significant gap allowing air to get into the filter area which reduced the static to 0.2. When I sealed that up the pressure was 0.4.

GW said:

.4 test static —that’s external. You’re testing the drop on the coil which is a different matter. 

You’re at .3 external- seems good 

What do you think about a return static of .35 and supply static of .1? The original ducts are in 50 year old house with 5 inch round ducts to the 13 registers. When i took the blower door off the static only climbed to 0.3 supply. This doesn't seem normal to me. It should be way higher i would think

pedmec

Your doing something wrong with your testing. Removing the filter should not cause the supply static pressure to decrease. Your measuring after the supply blower. removing the filter will drop the return static pressure but you will get a little increase in the supply external static pressure due to the little increase in air flow you get by removing the air filter. Not reduce the supply static as indicated in your original post. These measurements all rely on the proper placement of your probes. External Static pressure is the resistance to airflow. Your measuring the supply ductwork resistance to the amount of air your trying to put into the supply. Add more air to the supply increases the external static pressure. Return external static pressure is the same, just on the suction side of the blower.

Vanisle

jesmed1 said:

How close to the coil are you placing your probes? If there's a sharp turn in the ductwork either on the return or supply side of the coil and you're placing one of your probes in that sharp turn, some of the static pressure you're measuring across the coil is actually due to the turbulence caused by the sharp turn in the duct and not the coil itself.

We had a similar problem at our church where the static pressure was too high because of a 90-degree return just under the furnace where the return duct was too small, with a sharp 90-degree bend with no turning vanes. If you have something like that just before/after your coil it will cause a drastic increase in static pressure.

pedmec said:

Your doing something wrong with your testing. Removing the filter should not cause the supply static pressure to decrease. Your measuring after the supply blower. removing the filter will drop the return static pressure but you will get a little increase in the supply external static pressure due to the little increase in air flow you get by removing the air filter. Not reduce the supply static as indicated in your original post. These measurements all rely on the proper placement of your probes. External Static pressure is the resistance to airflow. Your measuring the supply ductwork resistance to the amount of air your trying to put into the supply. Add more air to the supply increases the external static pressure. Return external static pressure is the same, just on the suction side of the blower.

Vanisle

This is where I put the probe. A few inches above the top of the air handler in the supply plenum and well below the two trunk ducts. Static was .1 in this location with filter installed. I thought this seemed low for a 3.5 ton air handler. Out of curiosity I removed the blower door thereby removing all return static. The supply static went up to .3. Would it not jump off the scale normally in this situation, or is this an expected thing: the .2 increase in supply static with door off blower? I'm wondering if there might be some other problem with supply duct air loss or maybe blower under performance.

jesmed1

You would expect supply static to go up somewhat when the blower door is removed because airflow increases, but it won't go "off scale" because you still have the coil producing significant resistance.

I  don't think you have blower "under performance" because if it were under performing you wouldn't be able to get such a high total static. Our church total static was also 1.3, mainly because the return plenum was undersized, but the blower was fine.

Can you show a pic of where you're putting the probe in the return?

Vanisle

The pic is in the post above. It shows up on my screen. Let me know if it doesn't on yours. This is a heat pump air handler so opening the blower door bypasses the returns, filter and coil. Thanks for the help!

Vanisle

The total external static would be .35 return plus .1 supply so only .45. If i'm not mistaken that is what is measured when people say your air handler should be .5 ideally. Doesn't include coil or filter unless it specifies somewhere. Don't know how often it would specify that the filter is included since that makes zero sense to me. Normally I would be happy with this result since .45 is a good number but this house has ducts that are fifty years old and were built for an oil furnace so undersized. Its also a 3.5 ton air handler servicing 14 or fifteen 4x10 registers so I would expect supply static to be .4 or more measured in the supply plenum. I'm not a tech just a homeowner who is looking at a replacement heat pump and some of these things need to be resolved before jump into a new system. Contractors who will quote may be able to help but around here, they are all busy with installs with all the government money and might not be willing to spend to much time on this.

jesmed1

It sounds like you're measuring static in the right places, but just for reference here are some diagrams showing where to measure for different systems.

https://energystar.gov/sites/default/files/specs/National%20Comfort%20Institute%20-%20Measure%20and%20Interpret%20Static%20Pressures.pdf

Static pressure in the supply and returns don't seem excessive, but the pressure drop across the coil still seems high. You should be able to find the pressure drop spec for your specific coil if you know the mfr. Here are some tips for checking the coil.

https://www.contractingbusiness.com/service/article/20870514/hvac-service-five-minute-dirty-coil-test

GW

Vanisle said:

Thanks for the replies. I made a mistake in the return air static. There was a significant gap allowing air to get into the filter area which reduced the static to 0.2. When I sealed that up the pressure was 0.4.

GW said:

.4 test static —that’s external. You’re testing the drop on the coil which is a different matter. 

You’re at .3 external- seems good 

What do you think about a return static of .35 and supply static of .1? The original ducts are in 50 year old house with 5 inch round ducts to the 13 registers. When i took the blower door off the static only climbed to 0.3 supply. This doesn't seem normal to me. It should be way higher i would think

You're doing pretty well with those numbers. Some people who live in glass houses might say 'you should never be over .1" on the supply of the return'. But in the real world it's gonna be common

GW

Also---a gap in the return would mean that the coil would see more air, which would mean more of a drop. I bet your drop on the coil is less now that have corrected the gap.