Checking Convection Air Filter

I have a "washable" air filter on my furnace. During high demand when the blowers are running near 24/7 I pull the filter and give it a wash in the bath tub with a mild dish soap solution. The water turns black right away. I've always wondered if I'm doing it too often or not often enough. So, what I've done is put a hole in my filter box that I can put the mano probe into. The probe reading is between the blowers and the filter. With a clean filter I see a .2 on the mano.. As I see it, the mano should rise as the filter collects dust because of where I'm taking the reading.. Does that seem right to you guys?

If you are measuring the positive pressure of the air box, yes, if the filter gets clogged, the pressure should go up.

you could hook both line up on either side of the filter Low and high and measure a Differential too I think.

Interesting idea.
Ironically I changed my filter this afternoon. So after reading this I drilled a hole before and after the filter. I have a handheld digital manometer and checked pressures. With a clean filter it reads even pressure before and -.29 after the filter. I will see how that progresses.

WNY wrote:If you are measuring the positive pressure of the air box, yes, if the filter gets clogged, the pressure should go up.

you could hook both line up on either side of the filter Low and high and measure a Differential too I think.

Sorry to complicate things but actually the .2 I stated above is a negative value since I measure negative pressure on the positive side of the scale. I see that KY speed racer does also. My thinking is that the negative pressure between the filter and convection motors should increase since at that location they are pulling air.

Oh ok, wasn't sure where you were measuring your value at... either way, when it gets clogged, the value should change! LOL

Ky Speedracer wrote:Interesting idea.
Ironically I changed my filter this afternoon. So after reading this I drilled a hole before and after the filter. I have a handheld digital manometer and checked pressures. With a clean filter it reads even pressure before and -.29 after the filter. I will see how that progresses.

That's neat. It tells me a couple things. #1 the filter isn't effecting your air flow, yet. #2 your cold air return volume is a little more restricted than mine OR your blowers are pulling harder than mine for whatever reason (clean squirrel cage or higher RPMs).

WNY wrote:Oh ok, wasn't sure where you were measuring your value at... either way, when it gets clogged, the value should change! LOL

Right right..

Lightning wrote:.

That's neat. It tells me a couple things. #1 the filter isn't effecting your air flow, yet. #2 your cold air return volume is a little more restricted than mine OR your blowers are pulling harder than mine for whatever reason (clean squirrel cage or higher RPMs).

It sounds like you may be using a better grade of filter than I am. I buy cheapies and get them by the box and change them once every three weeks or so. A better filter probably is more restrictive I assume. I wonder if that may explain the variation in negative pressure after the filter.

At work, all our air handlers use two dwyer gauges, one before and after the filter. This is how we know when to change the air filter. I see this setup everyday but never thought of using it with a coal stove. Thanks for sharing!

About 10 days have passed since I washed the filter. I checked the mano reading between the filter and blowers on the cold air side. With a clean filter it read -.2"wc. After 10 days its reading -.25 so I washed the filter. Below is a pic of the mano without the filter installed at all Makes me wonder how much its slowing my air flow,
but running without a filter causes a new set of issues.

Good idea lightning !!! I'm getting the drill out now.

That beats my old method. I used to wait until I had to pick the small filter fibers out of my coffee cup once the filter got sucked into the fan.

Paulie

wow if your blowers can pull that hard you got some serious airflow potential there!

I'm a bit anal about keeping that filter clean since I wish I had more airflow for more even heat distribution when it's really cold out. I've even considered getting bigger blowers.

Just kidding Lee

Actually I have a primary and secondary air filter. I pull the secondary ......... replace it with a new one. ...... then replace the primary with the secondary I pulled out.

I plan to start with two new ones, drill my hole like you suggested and record the readings to develop a base line.

I don't recall the cfm on my blower, but I get plenty of air movement running it at half speed.

Paulie

Interesting. I'm out of town soaking up a little bit of FL sunshine this week but I'll check mine when I get back and see how much it's changed

Lightning wrote:
I'm a bit anal about keeping that filter clean since I wish I had more airflow for more even heat distribution when it's really cold out. I've even considered getting bigger blowers.

I use my oil furnace blower periodically to even out the heat. I have my furnace return lines separated. I made a solid piece of board that replaces the air filter in the oil furnace and blocks the forced air from the supply of the Hotblast from just recirculating and going right back into the cold air return. So the air has to go into the house and then gets pulled into the return duct. The board that replaces the air filter has a large opening/hole cut into it. That opening is covered with a piece of Coro-foam board ( a piece of plastic cardboard. Similar to what's used for political yard signs). That piece is mounted to the other piece with a small piano hinge which allows it to swing one way and it's mounted on the side of the board away from the cold air return to the HotBlast. It's kinda of like a trap door.
When the blowers for HotBlast are running, they hold the trap door closed due to the vacuum created. If I turn the big blower for the oil furnace on, it sucks the trap door open allowing return air from the house to go to the big blower on the oil furnace as well as to the HotBlast effectively circulating probably twice as much air.
I'm not sure if I did a very good job of describing that but It works ver well.