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I've been having some minor trouble with my ac. When it gets hotter, towards the afternoon, it will lose ground and only be able to cool two degrees higher than what the ac is set to. For example, if it is set to 76, the thermostat will read 78. While it's not unbearable or super uncomfortable, it is a little bit noticeable. It didn't have any trouble the day before maintaining the set temperature, but today it was higher, despite the outside temperature being about the same.

I have changed my filter, put new batteries in the thermostat, made sure the heat pump is clean and free of debris and cleared away any grass or anything.

I would like to fix this but I wanted to ask for advice on my 3 ideas on what to do. Should I...

  1. Replace the thermostat? My partner had trouble with theirs in the winter (it was running all the time) and replacing the thermostat helped there. I was wondering if doing the same would help me.

  2. Call out an ac repair person?

  3. Understand that it is hot and over 90 degrees and this is just how it is and suck it up? (I understand that heat pumps can realistically only cool about 20 degrees and may struggle with the new higher temps in the Summer)

I really appreciate your time and advice.

Thank you

isherwood
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Retrobirdcardboard2223
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  • When it's running (in cooling mode), do you get cold air out of the vents? – Huesmann Jul 27 '23 at 12:28
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    Is it really reaching 2° above what you set? Consistently, even after you change the setpoint or on a warmer/cooler day? For example you tell it to cool to 72°F when it's stable at 78° and set to 76° as in your question - what happens? Because if it's always off by the same amount, regardless of how hard you're asking it to work, it not a simple matter of it not being powerful enough. I suspect the issue is going to be that it can't keep up, and the2° difference isn't actually a constant – Chris H Jul 27 '23 at 12:42
  • It did work the day before with a similar outside temperature, and not yesterday was 80 and today is 90? If it was a fast change in how it worked, then it might change the answers. – crip659 Jul 27 '23 at 12:42
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    ... also, you say the outside temp was about the same, but what about the amount of sun you were getting - it will have to work harder to cool to the same temp if there's a lot of solar gain, compared to a cloudy day – Chris H Jul 27 '23 at 12:43
  • If you have separate storm windows, and they are open so you can use the screens, close them when running the A/C - they slow heat movement through windows when it's hot out and cool in as well as when it's cold out and warm in. Consider more insulation for the house in general, but that's a quick one if it applies. – Ecnerwal Jul 27 '23 at 13:14
  • We had the exact same problem after buying our house. The AC unit was brand new but the temp would creep above the set point in the summer. This continued to happen until we replaced the windows. Not the cheapest solutions (maybe not applicable to you) but it has never happened since. – tnknepp Jul 27 '23 at 15:36
  • How big is your house? The generally accepted formula is 20btu for every square foot with an 8 foot ceiling. So multiple that out, and compare it to how many BTU your AC unit can do. If the BTU you calculate is greater than your AC unit then your AC unit is too small. Only if the temperature sensor in the thermostat is broken will changing it actually help you. – cybernard Jul 27 '23 at 16:21
  • What happens when you set the thermostat 2 degrees lower, to 74? Will the temperature drop to 76 or stay on 78? – Mast Jul 27 '23 at 17:08
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    You set it to 76 and you get 78 - if you set it to 70 do you get 72? – user253751 Jul 27 '23 at 17:50
  • I think the best solution for you is to update your insulation and when you redo your roof add a ridge vent. I added a ridge vent, upgraded the ventilation and put on new siding. My electric bill went down by over 30% this past year. – Gil Jul 28 '23 at 02:55
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    @cybernard any formula like that must be making some hefty assumptions about both insulation and insolation. As a rule of thumb it may apply to a typical house, but we don't know if the OP's house is typical. – Chris H Jul 28 '23 at 12:56
  • I had this problem sometimes in my condo; I'm on the top story and on hot days the roof would get so hot that my AC couldn't remove the heat fast enough. When the HOA had a "cool roof coating" installed (read: the roof was painted white) the problem went away. That might not be an option for you, but in general better insulation would help, as it would reduce the rate at which heat enters your living area. – Jeremy Friesner Jul 28 '23 at 13:18
  • @ChrisH I am sure it has many assumptions, but it establishes a minimum baseline. If you house is leaky then that number only goes up from there. – cybernard Jul 28 '23 at 14:07
  • @cybernard but it's not going to be a minimum baseline. It will be some sort of average house in some sort of conditions regarded as typical at some time in the past. If the OP's situation is only slightly worse than that average, they could easily get the effect seen, whether that's due to poor insulation or other flawed assumptions. For that matter a single assumed value covering the whole USA is daft, unless the peak temperatures in a typical summer, and the insulation in houses are somehow correlated – Chris H Jul 28 '23 at 15:02
  • I answered a somewhat related question a few days ago. This isn't a duplicate to that question, but very similar. https://diy.stackexchange.com/questions/277277/temperature-does-not-drop-to-the-set-thermostat-during-the-day/277328#277328 – computercarguy Jul 28 '23 at 22:16

6 Answers6

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If the system is running (fan is blowing; condenser/compressor outdoors is running) then replacing the thermostat won't help any.

You've done well checking the filter and ensuring the outdoor unit is clean and clear of debris.

One other simple thing you can check is whether the coil above the furnace is freezing. Easiest way to do that is to look at the condensate drain after the system has been running a while. If there are drops or a small drizzle of water exiting then it's working; if the drain is dry then ice on the coil is likely. Ice means blockage of airflow, which means reduced ability to cool.

A system does have its limits; there does come a point at which the system can't cool any more and the indoor temperature will rise together with the outdoor temperature. It's tough to say whether yours is at that point; you'll probably need an on-site service person to evaluate that.

Greg Hill
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    If it's icing up it wouldn't be 78 in the house, it'd much hotter. – Dean MacGregor Jul 27 '23 at 16:26
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    @DeanMacGregor Eventually, yes, but if the icing is only minor at this point then the airflow will be impeded only a small amount and the under-performance would be less severe. – Greg Hill Jul 27 '23 at 16:54
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It depends. If the A/C has a mechanical problem, have it fixed. You'll know soon enough, if it suddenly got worse.

If the A/C is working properly and simply has never been able to adequately cool your house in these very high temperatures we are now experiencing, then you have 3 options.

  • Suffer.
  • Get a larger or supplemental A/C unit.
  • Cool your house with the power of your brain. I.E. learn stuff about thermal mass, energy storage, insulation and air leakage (to see whether this trick is worth doing) and then pre-chill your house so the mass of the house is nice and cold before it starts becoming unbearable. Then you can - well not quite coast like in the video, but certainly reduce the need for the system to work so hard between 4 PM and 10 PM. Also good for the energy grid's stability.
Harper - Reinstate Monica
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  • Unless I missed it, that guy didn't mention what the outside temps are when he's coasting through the day without the inside temp getting higher than 77. I know that where I am (Florida), there's no chance I could coast through the day without my compressor running. – Dean MacGregor Jul 27 '23 at 16:44
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    @Dean yes, it depends on the thermal mass of the building, the insulation, where it is relative to building mass, the air leakiness of the building, and very importantly, solar gain, which depends on shading and roof/wall color. – Harper - Reinstate Monica Jul 27 '23 at 17:29
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It's not the thermostat: it just command call for cool (temp > set-point + hysteresis ) or "stop" if temp < set-point - hysteresis.

So if the AC starts and stops the thermostat is fine.

My ideas are:
• dirty condenser (if you can't see trough, wash it with water, avoid pressure wash, going the opposite direction as air, if you still can't see trough, call a tech and have it properly cleaned)
• dirty evaporator (causes low flow at vents, call AC tech)
• dirty filter (replace it yourself, its cheap and easy, also improves air quality inside)
• low refrigerant (call AC tech)
• undersized unit (deal with it or have the unit replaced with a bigger one)

You did whay you could, call AC tech to check pressures and clean the evaporator.

At last: Check you dont have "economizer" open => these allow air change but waste lots of cool, closing them will keep the cold in.

DDS
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If you have a large external unit, check that the air is blowing in the right direction:

enter image description here

Air is supposed to come out the top of this unit. I've had installers that reversed the fan direction and lowered its efficiency by a very large amount.

Call the installers to fix it since it is their responsibility, but it can also be DIY configured if you are inclined.

Nelson
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    That's a good point! A failed start capacitor for the fan motor can have the same effect: when the power turns on the fan will sit motionless until some little disturbance, like wind, pushes it a little. If it gets pushed and started backward it'll continue running backward. – Greg Hill Jul 27 '23 at 13:04
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You have done the simple checks. Now you need to have the system looked at by a service tech.
What you are describing is system that is low on refrigerant. There are other causes of this behavior. A tech will need to inspect the system to determine what the issue is. You can play detective, but you will still need a tech to fix it. (Special equipment is needed.)

Dug
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Have someone come out to inspect and clean your air ducts. Make sure you go with a legitimate company.

EDIT:

Not sure why the down vote, so here's a link to EnergyStar.gov showing that repairing and cleaning will help.

Devastis Elite
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    Not the downvoter, but it was probably because this is a DIY site and a one-sentence "call a company" response is the *opposite* of what people come to this site for. – maples Jul 27 '23 at 14:58
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    I downvoted you because cleaning ducts has not been shown to improve air quality or the overall performance of a system. The Energy Star you linked to says to ensure your ducts are **sealed** which is a different thing entirely. [Here's the EPA on the topic of cleaning ducts](https://www.epa.gov/sites/default/files/2014-07/documents/airducts.pdf) – Dean MacGregor Jul 27 '23 at 16:30