Would a heat pump in cooling mode with a very dirty air filter be most likely to fault on high pressure or low pressure.

A very dirty filter can cause a low pressure fault in the cooling mode and a high pressure fault in the heating mode.

Bergy

Low pressure.

Dirty filter reduces airflow across evap. At all but the driest dewpoints the lowered evap temp will result in icing. Icing (frosting) is sneaky in that it progresses, and its progress reduces airflow still more until it is completely blocked.

Frost (as opposed to solid ice) is a very good insulator, so an frosted evap will exhibit refrigerant temps way below freezing. The effect on the high side will be both greater subcooling and lower pressure and compressor amps. A fixed orifice system with a recip compressor may fail from compressor liquid slugging. Scrolls are more tolerant but can overheat since low evap pressure means reduced refrigerant density and thus heat capacity and conductivity. Refrigerant flow cools compressors.

There is an interesting relationship between poor filtration and coils / blower blades - in cooling mode the wet evap grabs dust and gets blocked by mud. In heating mode, the dry evap lets dust pass through and coat the next thing in line, the blower blades. I have heard that a mere 1/16" crud accumulation on blower blades reduces airflow by 28%

Hope this helps - please point out any flaws or ambiguities

Posted By engineer on 05 Oct 2010 09:41 PM
I have heard that a mere 1/16" crud accumulation on blower blades reduces airflow by 28%

What is the recommended method of cleaning the blower?

thanks guys

The only way I know of is to pull the blower out of the unit, brush each fin with a nylon brush and finish with compressed air cleaning.

If there is a simpler and faster way I would LOVE to know!

Bergy

Posted By geome on 05 Oct 2010 10:32 PM

Posted By engineer on 05 Oct 2010 09:41 PM
I have heard that a mere 1/16" crud accumulation on blower blades reduces airflow by 28%

What is the recommended method of cleaning the blower?

I have always pulled the entire blower cage, taken the wheel off the motor, used a coil cleaner solution and washed the housing and fan followed by compressed air to dry it. Most geo units I've seen are relatively clean compared to some air/air units in rental homes where the filter is changed every 5 years whether it is needed or not.

Years ago I ran my blower wheel through the dishwasher...something only a SINGLE man can do. It got it squeaky clean but the thing was never quite balanced after that experience.

Posted By engineer on 06 Oct 2010 06:58 PM
...something only a SINGLE man can do.

If your wife asks you to move the transmission so that
she can take a bath, you just might be a redneck.

Would a shop vac with a soft bristle brush attachment do any good, or would the crud not make this method very effective?

Doubt it...I found a fairly stiff three inch nylon brush at Johnstone Supply that works pretty good. I've even had to use a long, skinny flateblade to get the worst of it off before using the brush.

Bergy

Agree.

The stuff encrusts the blower blades almost like a stiff dirty plastic or wax. The dishwasher likely loosened it via a combination of heat, caustic detergent and high velocity water jets. No shop vac brush attachment can come anywhere near that combined force and effect.