Heat Pump Icing

Eugene Silberstein 3 · September 2005

Awesome Bill!

The temperature of the return air right at the evaporator coil is 100 degrees.

Here's my point...

When you are calculating the delta-t across the evaporator coil, you need to take the delta-t ACROSS THE EVAPORATOR COIL, NOT THE WHOLE AIR DISTRIBUTION SYSTEM!

Taking readings at return or supply grills may be convenient, but they are not as useful as temperature readings taken right before and right after the cooling coil.

The system that I presented to you was working perfectly as far as the refrigeration circuit was concerned. That was, of course, until we added tons of refrigerant to the system.

The only problem with the system was that the flexible return duct had separated from the collar on the return plenum.

The 80 degreee supply air was perfect given the 100 degree attic (return) air temperature.

Awesome job, guys!

Until the next one.

Eugene Silberstein 3 · September 2005

What do you think?

Sorry about the topic of HEAT PUMP ICING. This post is not about heat pumps, nor icing coils.

You are on a no-cooling call. The system is a straight-cooling unit with a fixed bore metering device. The air handler is in the attic and the condensing unit is in the back yard.

You take a reading at the return grill in the bedroom hallway and the temperature is 85 degrees. You take a temperature reading at a supply register and you find the temperature of the supply air is 80 degrees.

You calculate the 5-degree delta-t across the evaporator coil and surmise that the refrigerant charge is low. To be sure, you measure the condenser subccoling and the evaporator superheat. The evaporator superheat is 15 degrees and the condenser subcooling is 20 degrees.

You decide that the system is short of refrigerant, so you go to the truck and get your tank of R-22. You add refrigerant to the system. The superheat drops to 3 degrees and the condenser subcooling rises to 35 degrees. The temperature of the supply air is now 79 degrees, while the return air temperature remains at 85 degrees.

Since the supply air is cooling a little better (1 degree cooler), you decide to add more refrigerant. The evaporator superheat is now zero degrees and the condenser subcooling is now 45 degrees. The temperature of the supply air remains constant at 79 degrees and the return air remains constant at 85 degrees.

Considering the information provided, what do you think is the most likely cause for the system malfunction? What would you do if you were the follow-up technician?

don_156 · September 2005

I would

go get my recovery bottle and start recovering all that freon I just put in the unit.

Then before I get started I would first check the fan motor
to see if its working properly,then I would check the ductwork...then I would properly ask the customer why the filter has not been change,and then if I still can not figure why their no load on the coil,I would pick up the phone and call the professor.

Jeff Lawrence_24 · September 2005

Don's right

I'd look at the fan and see if it's running and then check the evap coil....

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Eugene Silberstein 3 · September 2005

Definitely

The system is definitely overcharged now.... It wasn't before.

Let's assume that the blower in the air handler is operating fine.

What would you check on the ductwork, Don? You are definitely getting warm.

Eugene Silberstein 3 · September 2005

Let's say the blower is operating and the coil is clean....

If the filter was dirty, the delta-t across the evaporator coil would be high. The same holds true for a defective blower or a dirty evaporator coil.

Hmmmmmmmmmmmmmmmmmmmmmmm... Quack quack

don_156 · September 2005

I would

> The system is definitely overcharged now.... It

> wasn't before.

>

> Let's assume that the blower

> in the air handler is operating fine.

>

> What

> would you check on the ductwork, Don? You are

> definitely getting warm.

don_156 · September 2005

I would

first check my delta t at the airhandler and see if the reason for the low delta t was because of lack of insulation on my ducts or air leaks.

However if I may say that sometime low delta t could be from the blower speed being set to high as well.

thp_7 · September 2005

Big air

To much fan speed!

thp_7 · September 2005

Big air

Sounds like your average 2ton a-c installed on a 100,000btu 90%. In short to much air flow across the A coil.

don_156 · September 2005

You have

> Sounds like your average 2ton a-c installed on a

> 100,000btu 90%. In short to much air flow across

> the A coil.

Eugene Silberstein 3 · September 2005

MMMMaybe

But, too much airflow across the coil would still result in a lower supply temperature...

Very good try and thanks for playing.

thp_7 · September 2005

Whats your suction #

don_156 · September 2005

You have

to watch the Professor when he ask his question thp,he can
be very tricky.

But thats a good thing!

Eugene Silberstein 3 · September 2005

After...

After adding all of that refrigerant, our suction pressure is absolutely through the roof, so you better have your DOT approved recovery tank handy!

soooooooooo.... what are we looking for in the duct system?

bill_61 · September 2005

return temp where???

you said you took your return temp from the hallway return grille??? whats the return temp in the ductwork right at the air handler, punch a hole in the ductwork and take it, i doubt you would even have to remove much insulation because there probably isn't any and the attic is a sweltering 120 degrees making the return temp close to 100 degrees...that would be a good delta t if your supply air is 80

thp_7 · September 2005

The easy way to spot this

would have been seeing that you would have had to had a suction pressure in the 100# range with all other things normal.

Heat Pump Icing

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