What is your preferred AC head-pressure control?

Pete_18

I would like to add a head-pressure / low ambient control to my Unico w/ Lennox XC13 so that my AC can function better in low ambient conditions. Looking at the Hoffman 814-10EH and ICM 326HN. Does anyone have experience with either to make a recommendation (or an alternate product?) I would prefer a control capable of slowing down my single stage condenser fan when necessary and not just cycling it off/on.

clammy

I ve used the icm in the past with good results ,there nice espically with a motor settings for ball or sleeve bearings and a theramister for sensing temp .I vast improvement over cycling the fan on pressure which i cannot stand on a c .The coils end up getting racked from expansion and the fans get baked stick with the icm you ll be happy peace and good luck clammy

Techman

Curious, why does this system run in lo ambients?

Pete_18

Very rarely but we have some wild temperature swings where it can be 80s during the day and 50-60 overnight. Mostly common scenario is that I have the AC setpoint at 69, and the house temp all of a sudden hits 70, but the temp outside has dropped in to the low 60s and the problem is that the system can't handle bringing it down 1 degree in this situation, so the air handler runs non-stop because the thermostat is calling for AC but not getting the house to 69 and it keeps pushing out air that is not cold until the temp in the house finally comes down on its own. In the mean while, the condenser fails to operate properly in these conditions. I would rather not have to worry about turning off my AC any time the outside temperature is low enough that the AC coming on to bring the house temp down by 1 degree ends up in a problem. This is worth a little money to add in a control to not have to worry about this nuisance.

Empire_2

Have you had the system checked recently? The reason I ask is that 1 or 3 * should not bee a problem especially with temps dipping outside. 50 and above is within the design of most equipment. ICM is my choice period. If your a tec, you have already checked that, sorry......:-)

Mike T.

Pete_18

Yes, it has been checked out numerous times and serviced / cleaned annually, this is why I feel confident at this point knowing that the solution to this nuisance is a head-pressure control. I wish my AC could run at 50-58F outside, but it cannot. In the 50-60F range outside, the condenser does not run properly and is unable to generate cold air to take my house from 70 to 69F or 71 to 69F. The only other suggestion I have had before doing this was to check the CFM to make sure I am getting 250CFM per ton, I have not asked if this was done and I don't know how to check myself.

Empire_2

I'd have to hit the books again, but the Unico's are not 250 cfm/ton but 450cfm/ton. This is a hi velocity unit as in small supply diffusers? I do believe that your suction is lower than normal, but head should be somewhat the same. I'm sure someone in here will chime in. It's been a while since I have worked on the unico and the like. Don't want to lead you in the wrong direction.

Mike T.

Pete_18

Confirming 250cfm/ton across the coil was a direct recommendation from Unico, so I assume that is accurate. I don't know how much cfm it is pulling as the AC company never told me (yes - high velocity)

Don_197

I really like the ICM.............it attempts to hold your condensing temp at 105 deg............which is awesome (as long as you locate the sensor in the recommended area) If you have a sleeve bearing motor.............you can't ramp the fan down too far however.......the bearings won't get the needed lubrication.........and you'll tear the motor up.

Techman

Pete, why can't you do what?

You didn't complain about about discomfort on a 95* day. So, lets figure this out, you must have a minimum CondTemp of 95*f

Pete_18

The system works perfectly any time it is > 60 and change outside, even on days when it is 100F out. It's only in situations where the outside temp is significantly lower than the inside that I have an issue. (Sorry for being dumb, I don't know how to measure the cfm).

Harvey Ramer

Is your lineset length, size and elevation all within the correct parameters? Might want to have that investgated.

What is the air temp difference across the coil? Should be 24 to 26 degree temp drop.

Larry_52

Open a window?

Pete_18

Funny. This isn't about my desire to bring the house down 1 degree when it is cold out, it is about my desire to not have the AC kick on and have the air handler run non-stop and freeze up because the condenser can't run properly. If I had some magic feature of my T-Stat to not allow it to even call for AC when the outside temp was < x degrees F, that would be another option, but my T-Stat is not that sophisticated.

pecmsg

You don’t need a head pressure control you need the system looked at. High Velocity systems run higher CFM's

Pete_18

Thanks for the feedback. I will have the AC company double check the actual cfm I am getting before spending money on the head pressure control. Over the years they have checked everything else out many times and fixed/resolved many other problems.

I did confirm with Unico's documentation what their Support team had told me that 250 cfm per ton is correct and you want a min of 6-8 outlets per ton. I'd have to count again, but I believe I have 26 or 27 outlets. My return is a split 20x25 (2nd Floor) and 14x20 (3rd Floor) .

The system has been checked (and re-checked) from end-to-end over the years, so I kind of expected the head pressure control for this one scenario was going to be necessary if I wanted to resolve this nuisance, I was primarily looking for recommendations on which one and it sounds like people like the ICM, but I will take your advice and have them double check the cfm and any other cause of the problems cooling with outside temps in the 50s/very low 60s.

Techman

Pete is correct about the CFM .These HiV systems run in the "refrigeration" range, so they use lower CFM to get very cold/frosting/freezing evap coils.They even have a freeze/frost stat. Is there a SightGlassMoistureIndicator in the system? Should be.

Pete_18

Outside at the XC13 condenser I have a Sight Glass if this is what you are asking about (on the refrigerant line). I don't recall seeing anything like that over near the Unico, but it's upstairs in a very tight space in my attic.

Ironman

It could be that the freeze stat is turning the compressor off because the suction pressure is dropping too low when the ambient temp is low. If that IS the case, then the ICM 326 should correct the problem.

I would confirm that the freeze stat is functioning properly before installing a head pressure control. If you temporarily bypass it and the system runs okay below 60* ambient, then it's the freeze stat that's turning the cond. unit off.

250 com per ton is the correct air flow for Unico. If the unit has back up heat, air flow can be easily calculated while running that. If not, then a manometer and a blower performance chart are needed.

Pete_18

Thanks Ironman, I am fairly confident that your (could be) analysis is exactly what is happening simply because I have watched the system for a long time and had it torn apart over the years to deal with and fix the other problems it had. I don't think I would have a clue how to bypass the freeze stat myself though and getting the AC company out to play around when it's cold out will likely cost a lot more money (for their time) than just paying them to put in the ICM326 which is going to be needed in the end anyway since your analysis is spot on.

No heat, AC only on this system.

Silly question, let's say I did figure out how to bypass the freeze stat to stop the condenser from turning off in this low ambient situation, what will be the impact to the system of doing this as a test? Does it run just long enough to cool down and shut off properly since the condenser isn't being forced off and then you are fully confident that the head pressure control is the proper solution? I assume it's going to eventually freeze up if i'm bypassing the control trying to prevent that.

Techman

The freezezstat is there for a reason. It protects the compressor. Freezing/frosted coils is normal w/Hi V . You can bypass the SAFETY control as a test ,sure, but you better be watching it. ironman does make sense.

Nick W

My Unico system has been running problem-free now for 12 seasons with an ICM 326H. The only problem is the noise.

Pete_18

Glad to hear it Nick. The noise from hi-velocity doesn't bother us, only this silly situation where in lower ambient conditions if the T-Stat calls for AC to pull the house down by 1 - 1.5 degrees, the freeze stat eventually kicks in and it just creates a mess where the condenser cycles and the temp never cools down properly leaving the freeze stat triggered and the air handler running non stop. ICM326HN is on order as of this morning, appreciate everyone's help as I worked through this.

Ironman

Pete said:

Thanks Ironman, I am fairly confident that your (could be) analysis is exactly what is happening simply because I have watched the system for a long time and had it torn apart over the years to deal with and fix the other problems it had. I don't think I would have a clue how to bypass the freeze stat myself though and getting the AC company out to play around when it's cold out will likely cost a lot more money (for their time) than just paying them to put in the ICM326 which is going to be needed in the end anyway since your analysis is spot on.

No heat, AC only on this system.

Silly question, let's say I did figure out how to bypass the freeze stat to stop the condenser from turning off in this low ambient situation, what will be the impact to the system of doing this as a test? Does it run just long enough to cool down and shut off properly since the condenser isn't being forced off and then you are fully confident that the head pressure control is the proper solution? I assume it's going to eventually freeze up if i'm bypassing the control trying to prevent that.

Simply jumping the wires to the freeze stat will bypass it. And, obviously, you don't want to leave it that way for long.

Nick W

My comment above about the noise meant the noise generated by the head pressure control when it is working and slowing down the fan. It is an obnoxious 60-cycle hum that is louder than the condensing unit itself.

Ironman

Nick W said:

My comment above about the noise meant the noise generated by the head pressure control when it is working and slowing down the fan. It is an obnoxious 60-cycle hum that is louder than the condensing unit itself.

The noise is probably from the condenser fan motor itself. The settings on the 326 may need adjusting to a higher minimum rpm or the motor may have worn bearings which will cause poling.

Ironman

@Pete
A note of caution: installing the 326 requires locating the sensor in the correct place on the condenser coil. The instructions say place it 1/3rd of the way down in the coil, but that can be vague and misleading with multi-pass coils. Gauges and an electronic thermometer should be used to confirm it's properly located or else your head pressure may be too high or too low.

Pete_18

Unfortunately for me my AC company sent a less experienced tech out to do the ICM 326 install. It took him 2 hours to install it because he was not clear on how to wire it, not because he was making sure it was functioning properly (probe reading right temperature, checking pressure, or whatever else you would do).

To make a long story short, it is now wired (hopefully correctly), but I am not sure it is sensing the right temperature (he didn't check it in any way). I say that only because the system is shutting off the condenser completely after a few minutes even with the fan speed variable. Hopefully he didn't break anything else in the process, but I have no way of knowing that until it warms up outside. He did wire it "Backwards" initially causing the fan to not kick on and then it initially turned off.

It may be time to find a new AC company which has more experienced techs, which is a bummer because I had referred others to this company previously and was very pleased with their work on the initial install and follow up.

After the install, we did witness it go to variable speed and he made it go to full speed (by covering the condenser with plastic wrap), so it obviously senses enough of a temperature change to do that. I am not sure if the coil is getting down below 70F today (outside temp is unfortunately only 49F) which is required for it to keep the condenser on but shut off the fan. I have never seen one of these in operation before, so I don't know if today is a day where you would expect it to totally turn off the fan or not.

Today is way below any day I would normally run AC, but after he completed the install, I ran in today as a test to see if it worked. After calling for AC, the condenser ran without fan for a few seconds, fan came on and then went to variable *(as expected) and it ran in in variable for maybe a few minutes before the system either kicked it off for low/high pressure/freeze stat (no idea which one kicked it off), but it doesn't appear to have been the ICM which kicked it off since the condenser actually shut off. The only thing I can easily tweak on the ICM is the cut out speed, I had originally set it as the recommendation of ICM to be in the middle of the sleeve bearing range and tweaked it slightly lower to see if it would make a difference (still in the range but lower than half way). I did not try to make it higher, perhaps it is too low at the half way speed (and now even lower).

I have not checked the probe with a multimeter to see what it reads both running/not running and I have not yet used my laser temperature gun to see if I can get a useful reading off the spot in the coil he chose. He did go ~1/3 down (by eye) and inserted the probe through the coil itself vs. mounting it on the liquid line.

Any thoughts on what to do next is greatly appreciated. Wish they had sent out a more experienced tech and just crossing my fingers that the system will actually function properly when it warms up again in a few days.

ChrisJ

This is actually the same reason I had planned on doing a low ambient kit if I ever install A/C in the house. Many times I go home after a hot day and want to drop the temp in the house, but it's already in the 40s or 50s out and it's so humid it's foggy out.

My window A/Cs complain, I assume from slugging and who knows what else, but I ignore it. I'd rather not put a split unit through this kind of abuse for obvious reasons.

The way things are right now the chances of this happening anytime soon are slim but I'm glad to see I'm not alone.

I have even considered making an aluminum flap to restrict airflow through the condensers on cool days hoping for improvement, but time hasn't allowed any such experimentation.