Heat Pump Defrost

defrostThere are two types of defrost control. First, there’s the really old electro-mechanical style made up of line-voltage relays, snap-acting temperature sensors and timer motors. We’re not going to talk about these because this stuff is old. Really old. Pittsburgh Steelers winning super Bowl XIV old.
If you run across one of these old geezer heat pumps do everyone a favor and replace it with a new, high efficiency heat pump. They may still run, but the efficiency of these dinosaurs is lousy at best. Remember, there’s a reason “they don’t make ‘em like that anymore” …

Heat pumps manufactured since the late 80’s are way more efficient, due in part to electronic defrost controls.

Relax, I know we said the ‘E’ word, but electronic controls are reliable, efficient, and easy to troubleshoot. If electronic controls cause you more anxiety than going to the dentist then stop here and read “Electronic Control Troubleshooting Tips & Tricks”.
First, defrost controls come in two flavors; intelligent, and dumb as a stump.
Both controls do the same thing, they initiate and terminate the defrost cycle, the only difference is how they decide to initiate and terminate it.
Before we go any further, let’s make sure we understand what the defrost cycle is used for.
The defrost cycle melts frost that forms on the outdoor coil in the heat mode. It does this by running the unit in the cooling mode with the outdoor fan shut off so the coil will heat up quickly and melt the frost. The supplemental (normally electric) heat is turned on to warm up the cool discharge air (remember, the air conditioning is being used to defrost the outdoor coil…I know, it sounds dumb, but it works).
Using defrost mode only when necessary is critical to energy savings and comfort for obvious reasons. If the reasons aren’t obvious, let me explain; it’s cold outside and you’re running the cooling with electric heaters energized. Its kinda like driving your truck with the gas pedal mashed to the floor while stepping on the break to keep from speeding.
We run into a problem in trying to determine if there’s frost on the outside coil or not. Defrost boards don’t actually “know” if there’s frost on the coil or not, they assume there’s frost on the coil. The control assumes there’s frost on the coil based on temperature and time, in that order. The thinking goes like this; water can freeze below 32 degrees, so if the coil has been below 32 degrees for a certain period of time there must be frost on it, right? Well, no. Unfortunately heat pumps don’t have eyes, so the best they can do is guess if the coil is frosted based on time and temperature.
You know that jumper on the board where you select 30, 60, 90, or 120 minutes? That’s the amount of time the unit must run in heat mode after the coil temperature drops below 28 degrees before the unit will shift into defrost mode. Here’s the sequence-
The indoor thermostat calls for heat.
The outdoor coil temperature starts dropping as the unit runs. When the coil temperature drops below the defrost thermostat set-point (typically around 28 degrees) the timer starts counting down from the time selection you made with the jumper.
When the minutes run out, the control board shifts the reversing valve into the cooling mode, de-energizes the condenser fan and energizes the indoor electric heat with its “w” terminal, then starts a fixed 10 minute termination timer.
The defrost control now waits for one of two things to happen; ten minutes to lapse, or the coil temperature sensor to open because the coil is warm (typically around 80 degrees) at which time the control puts the unit back to heat mode by shifting the reversing valve back to the heat position, shutting off the electric heat, and turning on the condenser fan

The process then starts all over again.

The mechanical defrost function is basically the same for all controls. The difference is what temperatures and times they use to initiate and terminate defrost.

Patrick is Zen HVAC’s diagnostic and training guy. Patrick started in the trade the day he left technical school and never looked back. He's served in various technical and training roles in the HVAC industry but specializes in system troubleshooting and diagnostics, retro commissioning, and technical training. His moto: If I can understand it, anybody can. Patrick uses the Zen common sense approach to teach Patrick’s Likes- His Wife, kids and dog. Old pickup trucks. Hiking. The industrial Revolution. Patrick’s Dislikes- Taking work too seriously. Anything unintuitive. Emoticons :( Patrick’s Favorite famous person- Theodore Roosevelt “I am only an average man, But I work harder at it than the average man" Famous Patrick Quote- “Well, that was stupid of me”

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2 comments on “Heat Pump Defrost
  1. This site is like a classroom, except I don’t hate it. lol

  2. Ray Roosa says:

    Great post thank you for sharing.

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