Vacuum indicator/gauge...

[Externet]

Hi.

The intake manifold in my automobile has a vacuum indicator that signals when to replace the air filter. Even my vacuum cleaner has one to signal the flow trough the collecting bag is impaired.

 

I want to install an indicator to tell me when my central airconditioner / furnace needs filter replacement/cleaning.

 

What would be a reasonable figure for the amount of vacuum at the HVAC blower intake to consider it restricted, in need of attention ?

[Chrillis]

Im not sure exactly what you are asking? Are you asking what pressure? If so, you shouldn't notice a terrible amount of pressure differential between each aide of the filter between a clean filter and a dirty filter. That being said you can install a spring based pressure valve with a diaphragm. Dialing in the spring type to get the proper "k" value is going to take some trial and error.

As an engineer. The question is "how do i get the most out of each filter" and the answer is buy a washable filter. Wash it as much as you want. One time cost.

Chrillis, ME

[sethoflagos]

The book answer is to seek and follow the manufacturer's advice in the matter.

 

Since the pressure drop across an air filter is typically less than normal fluctuations in barometric pressure (oto a few inches water gauge) it's common practice to measure the differential pressure across the filter rather than rely on either upstream or downstream absolute pressure measurement. Rigging up a manometer from plastic tube with some dyed water is a simple enough DIY job for an HVAC air filter, and quite accurate enough.

 

A commonly used rule of thumb guide is to clean or change out the filter when the dP at full air flow is double the dP when new.

 

The only caveat I can think of is that for low pressure fans (fan dP is not much higher than filter dP) fan air flow can vary quite appreciably with filter condition. To compensate for this, an ammeter in the fan circuit gives a fairly linear measure of changes in air flow. In this case, you can change out based on a doubling of dP/I.