Fluctuating Head pressure on ac unit?

RayWohlfarth

We replaced a coil and condensing unit for a customers home. The old system was an r22 and the new is R410a. We flushed the old line set with the R11 flush and blew it out with nitrogen. We triple evacuated it and weighed in the refrigerant using the manufacturers recommendations. The line set is close to 100 feet. the coil has a txv. The system runs good with a 40 degree coil temp but the sub cooling is only 2. the manufacturer suggests 10. We added a bit more gas and the coil temp stayed the same but the head pressure started going crazy. It would be around 300 psi and then spike up close to 500 After recovering the refrigerant down too the original level, the system is running but the sub cooling is only 2-3. Any suggestions or is this ok?
Thanks

Matt_67

Did you check the superheat at the coil by chance?  Also curious what the ambient temp was and if it’s a micro channel condenser?

RayWohlfarth

@mattmia2 Did not check the superheat The Manufacturer said to use sub cooling because of txv it is a micro channel condenser

Matt_67

It’s definitely correct to charge using sub cooling, but superheat will tell a lot about how the system is actually functioning.  I’ve seen some odd things with micro channel coils but not specifically what you have going on.  How long did it run before the gas was removed?  I’m wondering if the expansion valve closing was causing the spike.  It would be good to check that the txv bulb is attached well if you haven’t done that already.

ChrisJ

Yeah, you really need to know what the superheat is.  It's always important just with a TXV you go by subcooling to charge because you expect the superheat to be controlled by the txv.  But you don't know what's going on unless you check it.  

How is the txv installed?  Is the bulb installed correctly?   Is the TXV the proper size?  Is it an adjustable txv?

After rereading it sounds like the superheat was likely too low and the valve was trying to keep it reasonable.  

EBEBRATT-Ed

I have seen units do that and run fine. The fact that the head rose when you added gas is telling you the condenser is full because when you added more gas you flooded the condenser and drove the head up.

I would check the superheat but a TXV can't make high head pressure. The TXV is feeding based on your 40 degree evaporator.

The only other thing I can think of (and this would be true of only microchannel coils) is if one or a couple of the channels is blocked.

Maybe there is enough room to get temp readings on the individual microchannel

STEVEusaPA

How much of that 100' line set is running thru an attic?

clammy

On longer line set the sub cooling should be checked at the liquid line in the attic being a decent amount of sub cooling occurs as the liquid stacks in line it tends to get additional sub cooling which when taking your readings at the unit will tend to make you over charge . With micro channel they seem extremely sensitive and seem to be over charged pretty easy . I personally stay away from the micro channel being I be replaced a few that where only a few years out of warranty when they stated leaking at the bottom .i love the idea and from units I ve worked on they seem to not take much of a charge being they condense liquid extremely well which is there up side but the down side is that the bottom 1/ 3 of the coil does not have the same volume as w a copper condenser cool which I feel may be why they over charge so easy . I know from the micro I have charged that I really had a wait for my pressures to level out before adding even the tiniest amount of refrigerant as said they tend to go from on the money to over charged in no time flat . Peace and good luck clammy

RayWohlfarth

Thanks everyone I will be going out today. Now the customer called and said the coil is frozen Will be interesting to see what the solution is

EBEBRATT-Ed

Microchannel coils hold little refrigerant. That is why the MFGs state you can't pump down a condensing unit with a microchannel coil

ChrisJ

EBEBRATT-Ed said:

Microchannel coils hold little refrigerant. That is why the MFGs state you can't pump down a condensing unit with a microchannel coil

Many manufacturers claim you can't pump down a unit with a scroll compressor either which is completely B.S. if done carefully.

I know little to nothing about microchannel, so I have no idea other than often when a manufacturer claims something it's to cover their butt under all conditions.

ChrisJ

RayWohlfarth said:

Thanks everyone I will be going out today. Now the customer called and said the coil is frozen Will be interesting to see what the solution is

Pictures of the TXV bulb installation and please get us SH and SC numbers.

Ironman

I’ve seen as little as 3oz. over charge cause a 3ton micro channel condensing unit’s head pressure to go bonkers just like what you’re experiencing.

Does the manufacturer allow for a 100’ line set with that specific model? As others have mentioned, a long line set going through a hot wall or attic can change the sub cooling. Check it at the c/u and the indoor unit and see if there’s any difference.

Also: what’s your static pressure and cfm per ton? Micro channel coils are sensitive to having proper air flow.

Personally, I think that they’re junk technology and I avoid selling and installing them.

Is this a heat pump or straight a/c? Make and model?

RayWohlfarth

Well here is the followup I checked the static pressure of the fan and found the air flow for the 3 ton coil was less than 900 cfm. After opening some closed dampers on the return and replacing the filter, the airflow increased to 1,300 cfm. After that, the pressures seemed to be more stable with a 40 degree coil temperature. We were chased away by a rain storm before we could check superheat and sub cooling. Thanks for all your help

ChrisJ

RayWohlfarth said:

Well here is the followup I checked the static pressure of the fan and found the air flow for the 3 ton coil was less than 900 cfm. After opening some closed dampers on the return and replacing the filter, the airflow increased to 1,300 cfm. After that, the pressures seemed to be more stable with a 40 degree coil temperature. We were chased away by a rain storm before we could check superheat and sub cooling. Thanks for all your help

I wouldn't allow the customer to run that unit until you can confirm the SH is reasonable.

RayWohlfarth

We checked it today The SH is 9 and the sub cooling is 10

ChrisJ

RayWohlfarth said:

We checked it today The SH is 9 and the sub cooling is 10

Personally,
I feel that's good.