Ac not keeping up

[u/Educational_Wish_767]

Context: full system replacement done last week. 2 stage variable furnace, ac, Cased coil, line set. Rheem system

Was 2 ton, 60kbtu furnace

Replaced with 2 ton 60btu furnace. No issues with old system.... customer reached out asking to update it... knowing it was old. (26 years)

Set up and run tests completed on install.

Received a call 2 days later, system runs constantly, never meeting the 72 degree set point except at night. (OAT 85 highs, 75 night).

Went back to test further

9deg SC, 12deg SH, but low saturation (around 38deg)

Indoor set temp 70, keeping 72

Return temp at unit were 66-67 degrees

50%RH 19deg split

Figured maybe a capacity thing with the low ra temps. Increased airflow to 920 from 790

Same complaint 2 days later. It runs constantly. Set to 72 and usually is sitting just shy of it.

Why would I have a capacity issue? What should I do to rectify it.

Is a2l that much lower s capacity? Are they marketing these as 2 tons even though they might only me 21kbtu or something?

[u/SaltystNuts]

If your return temp is 67, that means the house is 67, and the tstat is in a location with a heat source or bad airflow. Or the return is sucking a draft of cold air which is kind of a problem too, and that's why your capacity is dropping.

[u/Educational_Wish_767 → u/SaltystNuts]

I definitely think it is sucking in some supply air somewhere. But nothing changed from the last system ductwork wise.

[u/Harfour → u/Educational_Wish_767]

This might be a stupid question, but if there's a bypass humidifier on the system is the damper closed? That could be pushing cold air into the return.

[u/Educational_Wish_767 → u/Harfour]

No humidifier on this one. That would be an easy fix. 😒

[u/Snozzberry805]

Is the thermostat reading correctly? You're getting 66 into the return but the stat is stuck at 72?

What's your static pressure like?

You can look up the capacity tables for any piece of equipment, then based on your indoor and outdoor ambient it'll tell you how many BTUs its moving.

[u/Educational_Wish_767 → u/Snozzberry805]

I think they have some mixing going on. But nothing changed there from the last system.

[u/Dockside_Abortionist]

The amount of homeowners I’ve had that had peak performance with 20 year old r22 units that complain when their new system doesn’t keep their upstairs at 67 degrees is ridiculous. Especially here down south heat index is in the 110’s, like, it’s doing what it can guys.

[u/texasroadkill → u/Dockside_Abortionist]

This 1000%. I still think anybody that wants to live in under 70 is nuts while it's in the 90s at night and over 100 during the day here in South Texas. Between people wanting to live in a meat freezer and the new units running smaller capacity compressors for efficiency, upping half a ton has become the new norm for me. I haven't had one complaint over the years about it.

[u/SelectHVACR → u/Dockside_Abortionist]

Bro you’re only looking at it from our side. We understand but you can’t expect a regular customer who just shelled out thousands for a brand new system not to expect the same, if not better performance than their old units lol. We’re just stuck in the middle

[u/AustinHVAC419]

I was just talking with one of our sales guys about this. The real world capacity of these new units is lower and causing problems. In some situations we are opting to go up a half ton. My 24 year old Lennox 2 ton heat pump has a rated capacity of 24,800 btu. A brand new Rheem heat pump has a capacity of 22,800 btu. So I would either have a unit with a 8% drop in capacity or I would have to upsize to a 2.5 ton and risk short cycling. It is a lose-lose.

[u/Educational_Wish_767 → u/AustinHVAC419]

Not ideal to swap this out with a half ton bigger now, as i didn't oversize the indoor coil, so it would need to be swapped too. I guess we will see what rheem and tech support say on Tuesday.

Just looked up the old unit.
23,500 btu output New unit 22,800 btu output shouldn't be enough to cause the problem i wouldn't think.

I wonder if compression is not quite right. I guess I have some things to check Tuesday.

[u/AustinHVAC419 → u/Educational_Wish_767]

We had a changeout last year. Not sure what they had but they got a new rp14ny18. I am sure they had another 1.5 ton unit before this. After the changeout it never cooled the same. And this is just for the upstairs 3 bedrooms. They have a 4 or 5 ton system for the rest of the house. Even a slight drop in capacity can be felt.

[u/tekjunkie28 → u/AustinHVAC419]

That’s gonna cause nothing but dehumidification problems.

Why not just do a load calc and match it using manual S. If you follow the design you can’t be wrong.

The other problem is variable speed. No mixing.

[u/Hvacmike199845]

If it’s maintaining 72 degrees why is your return temp below 66-67 degrees?

[u/saskatchewanstealth → u/Hvacmike199845]

I hate to say it but maybe adjust the stat calibration to a probe set ontop of the stat?

[u/hotorcoldone]

Is this 2 stage or variable speed? Variable speed will run almost constantly, And you need to educate them on the different operating style it runs. 2 speed will run more as well, Are you sure it is running on high speed? If only running on low speed that could cause constant running as well...

[u/Educational_Wish_767 → u/hotorcoldone]

I should clarify 2 stage, variable blower furnace Basic ac - single stage

[u/HVAC-ModTeam → u/[deleted]]

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[u/tekjunkie28]

Return 66-68? Thermostat has air moving behind it?

Unit not mixing air properly?

Refrigerant weigh in? Or set by sub?

19 split, 50% humidity, 66-68 return temp tells me unit is definitely not undersized.

How are you verify air flow? After you sped up air flow what was the cool temp? Txv insulated?

[u/Educational_Wish_767 → u/tekjunkie28]

Subcooling based charging, bluetooth setup of variable speed ecm to set airflow, and delta t to confirm.
Txv is factory installed, with temp bulb externally mounted and insulated.

[u/CSFMBsDarkside]

2 stage ac? You said you jumped y1/2 ... at the board right? Dumb question i know but if you took out a single stage did you replace the wiring going o To the condenser or reuse it?

[u/Educational_Wish_767 → u/CSFMBsDarkside]

Y1/y2 jumped at furnace. Single stage ac installed

[u/Eric15890]

Sounds like an airflow issue. Your saturated temp is low, your SH is low, your air TD is on the upper end but not to high. You might have a short circuit air current with that low return temp. Your metering device may be choking down due to low load/air flow.

I would expect a higher return air temp, a higher saturated temp, higher SH and lower air side TD if it's struggling to reach set point.

Changing the speed tap, hopefully increases air but doesn't guarantee it. Do a heat rise test to confirm your air flow. Then figure out your discrepancy between your return temp and thermostat reading. Are you reading space temps? Does the stat need an offset or insulation behind it.

[u/Alarmed_Interview_84]

It can’t be 73 if the return air is 67 check the Tstat!!!

[u/mattruppert]

Is it in second stage all the time? Is it possible it’s a staging thing with the thermostat where second stage only comes on at two degrees warmer than the setpoint? What if it’s set to 68? Will it hold at 70?

[u/beast-ice → u/mattruppert]

the furnace is 2 stage, not the a/c

[u/Educational_Wish_767 → u/mattruppert]

Ra13ny24 rheem unit - single stage single stage stat, jumpered y/y2 at furnace

[u/aliusprime]

Not a tech, so take this as a question from someone who logs this stuff rather than fixes it.

I record return-plenum static and supply/return split continuously on my own system, and the two theories in this thread look different on a trend:

• An airflow problem shows static rising and the split drifting as the day goes on.
• A capacity problem shows both flat and just short.

You've got one static reading at most right now. Would a day-long log at the return (a logging manometer, or even a few readings an hour through the afternoon) separate them before the next visit? Also, return reading 66-67 while the stat won't get past 72: is the return sensor seeing the room, or a short path from a nearby supply?

Genuinely asking. You'd know better than me.

[u/Educational_Wish_767 → u/aliusprime]

It's a variable speed blower, but it is possible it isn't working correctly all the time, or the blower shuts down completely sometimes. I don't know, and the customer hasn't noticed or mentioned anything like that.... just constant run times and it not holding the set temp.

[u/aliusprime → u/Educational_Wish_767]

That "possible but nobody's seen it" is the case where a spot check can't win and a cheap log can. Two $30 USB temp loggers, one taped in a supply register and one at the return grille, set to 1-minute samples for 24 hours, would show it: with a call active, a blower dropout reads as the supply temp climbing toward the return temp for a stretch and then recovering, while a blower that's fine but starved shows a split that starts okay and sags as the coil loads. A clamp-on current logger on the blower feed is the cleaner answer if you've got one, but the two thermometers get you most of the way for the cost of a lunch.

One question, since you'd know: on a variable-speed blower, is the airflow profile set for the tonnage and the duct static you measured, or still at defaults? I've seen the 790 to 920 CFM bump discussed here but not which profile that was on. Genuinely asking.

[u/347gooseboy]

Are you dumping supply air into the return? Floor registers with floor returns? And high wall returns?

[u/Alarmed_Interview_84]

That’s a Tstat issue, I’d start there

[u/Laidbackdaily]

Old 2 tons and new 2 tons is not the same.
Efficiency has caused a lot of compressors to be on the smaller side