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My HVAC is 14 years old so we’re actually planning on replacing it sometime in the next 24 months. I’ve talked to a couple of HVAC companies already because I

My HVAC is 14 years old so we’re actually planning on replacing it sometime in the next 24 months. I’ve talked to a couple of HVAC companies already because I want more cooling power. When it’s above 100F (which it is for 3+ months of the year) my current system struggles to hold 80F. If we use the oven on a hot day, we may not get the temperature back down for days. Our winters are relatively mild (we don’t get much below 25F). We’re in central Texas. The HVAC companies I’ve spoken to won’t sell me an oversized system. They’ve never really said why, but from my research I think it has to do with mold problems resulting from cooling air with too much moisture in it. It doesn’t seem like a big ask for the thermostat to include humidity in its decision on how hard to run the AC, but apparently that doesn’t exist. Would one of your systems solve any of these problems? I’m not a DIY person, so I don’t really want to buy direct. Do you have installation partners in the Austin, TX area?

buttersbrian 4y ago
Have you considered assessing your home's insulation?
- criddell 4y ago
  We did. We had more insulation blown into our attic (it gets crazy hot up there) and we are now looking for new windows.
  - tguvot 4y ago
    actually i'll advise you to take a look at temperature of your walls. both inside and outside. they have a bunch of thermal mass. during the summer they effectively heat the air inside the house. unless you manage to cool walls somewhat (lets say during night), hvac will have to really fight to keep temperature down. I live in bay area and I installed whole house fan which is typically runs through the night at summer. For most of the cooling season it sufficiently cools air and walls in order to require only a couple hours of hvac a day. additional point, you wrote that you have current system installed 15 years ago. back than design temperatures (i.e. how hvac sized based on outdoor temperature) were different. recent updates sometime bring those temperatures up by 10F or so, what can result in extra half a ton - ton of hvac. you could retain company to make a proper hvac sizing (called manual j calculation). hvac contracts from my expirience won't do it.
    - criddell 4y ago
      Our walls are good but our windows are a weak point. During summer, the night temperature drops to the low 90’s here. It’s A/C all day, every day, for at least 3 months. Any idea why the HVAC guy and just about every source online insists that A/C can only give you a temperature delta of 20F? I know it can’t be true, but that claim is everywhere.
      - tguvot 4y ago
        >Our walls are good but our windows are a weak point. are you sure ? at summer I have walls that are 170F outside (at 2pm) and 80-90F inside. Also, window will pass through heat only during the day. Wall will keep heating your inside air through the night as well. Get some cheap thermal camera - it's very educational. You should also check your ductwork/registers. To see if there are any tears/openings/etc. Also, in case you are using flex duct, if it's not tensioned properly, sagging, etc - there will be a lot of air velocity losses that reduce overall cooling experience. >Any idea why the HVAC guy and just about every source online insists that A/C can only give you a temperature delta of 20F? I know it can’t be true, but that claim is everywhere. Something around refrigerant properties, mechanics and physics, etc. I also spent some time in this rabbit hole
      - sokoloff 4y ago
        There's a lot of poor engineering fundamentals among some HVAC salespeople and some websites as well as some sloppy writing. The delta-T that is correctly cited as being "around a max of 20°F" is the delta in temperature across the evaporator (IOW between the return duct and the supply duct [aka the goes-inta and the goes-outta]). It's not the delta-T between the outside air and inside air. With good enough insulation and/or enough cooling, you can get whatever delta-T you want as is obviously the case with refrigerators and freezers.
        tguvot 4y ago
        >The delta-T that is correctly cited as being "around a max of 20°F" is the delta in temperature across the evaporator (IOW between the return duct and the supply duct [aka the goes-inta and the goes-outta]). well said. i have temp sensor on supply and return and I graph them and delta, to make sure that hvac works within proper parameters. otherwise during heat waves you start thinking that hvac maybe out of shape
tutorialmanager 4y ago
How about just adding a small minisplit with heat. Warm day = use the minisplit. Hot day = existing central air. Very hot= use them both. If one goes down you still have something while you wait for repair. The mini split could also provide some heating when its mildly cold.
- criddell 4y ago
  We had a mini split installed into our garage (a Daikin that’s been great). I’m not sure how we would do that for the house. It’s a forced air system with outlets and return air in every room. I’m not sure how feasible it would be to pipe the output from a split system into that ductwork.
  - turtlebits 4y ago
    Mini splits aren't ducted (generally the air handler is mounted high up on a wall). They are great for spot heating.
    - sokoloff 4y ago
      Ductless mini-splits aren't ducted. (These are the type most are familiar with, the ones with a large air handler up on the wall or in a ceiling cassette.) However, mini-split just means that the condenser and evaporator units are separate ("split") and are smaller capacity ("mini") than a traditional central air system (which are also "split"). There are ducted mini-splits in addition to ductless mini-splits.
brudgers 4y ago
You don't want an over-sized system. They short cycle. An over-sized system can quickly cool to the thermostat setting, but leave the latent heat in the water vapor in the air. So you experience the "feels like" temperature...it's not the heat, as they say. A properly sized system runs long enough to condense water vapor and with it remove the latent heat from the air.
- criddell 4y ago
  The short cycle argument only really applies to older and low end systems that are not variable speed. A variable speed system integrated with a humidifier should be able to slowly bring the temperature down, making sure that the cooling does not get ahead of the dehumidifying. I want an “oversized” system that can do > 20F temperature drops. I know they exist because a lot of commercial spaces have them.
  - deleted 4y ago
    [deleted]
  - brudgers 4y ago
    Short cycling is thermodynamics, not an argument.
    - criddell 4y ago
      As I understand it, short cycling is a problem with air conditioners that cycle on and off. You can get staged compressors, multi-speed compressors, and truly variable speed compressors. A truly variable speed compressor runs continuously and so won’t short cycle.
adanto6840 4y ago
Sounds like you may have had a whole-home energy audit done already -- including a blower door test? If not, I'd highly recommend it; I paid ~$500 for one a few years back which included thermal imaging, blower door tests, airflow measurements & calcs, current/draw measurements for all energy-consuming devices, thermal load calcs, etc. You may find better pricing even -- ours included a detached structure/office, and house is relatively large, multiple HVAC units/solar/pool/servers/etc... In my case, I didn't end up with any super directly-actionable insights -- but it was still very well worth the money IMO. Even just to confirm that we weren't losing/"leaking" energy egregiously. The reality was, our household simply uses a lot of energy - hah. I mention this all because, at the time, we were just about to replace our HVAC units & we had just installed solar + storage. I'm located in Las Vegas -- summers are HOT (!) -- but our units (2x 5-ton Bosch heat pumps) can near-trivially keep us at 70F (or cooler), even on the hottest of days. I try to "push" the thermostats up a bit (we leverage a ToU energy plan), but there are days when the battle isn't worth having & the wife needs it at 69F -- and, even on the hottest Vegas days, we're able to hit it (can be expensive to do so, depending on time of day -- but is totally doable). Edit to add -- Oversizing a system can very easily lead to issues; short-cycling, humidity control, etc. The ideal system will run nearly non-stop on "design" temperature days. See if you can find any highly-rated Bosch installers locally; they'd likely be willing to "oversize" a system since the Bosch units are "variable capacity" (inverter-driven), which means you're less susceptible to hitting some of the common issues/concerns when oversizing traditional systems. Alternatively, the high-end variable-speed options from major brands are generally well-regarded though they can be extremely expensive (the price segmentation, and resultant value propositions, are bordering on dubious IMO).
- criddell 4y ago
  Thanks for the Bosch recommendation. Your system is much larger than ours, but you likely have a lot more square footage (we are ~2100 sq ft). I don’t recall the exact number, but I think we are 2.5 or 3.5 ton. Our system is old enough that it’s just time to replace. It’s the one the builder installed 14 or 15 years ago and it’s fairly low end. What you’ve accomplished in your home is exactly what my installer tells me they can’t do. He told me no matter how large of a system I buy, the system will never be able to pul the indoor temperature down more than about 20F (internet searches for “max ac cooling” seem to back that up). He says a bigger system will get me down to the 20F floor faster, but that’s it. I don’t believe it though because of stories like yours and because commercial spaces around here are very cold on hot days.

buttersbrian 4y ago

Have you considered assessing your home's insulation?

criddell 4y ago
We did. We had more insulation blown into our attic (it gets crazy hot up there) and we are now looking for new windows.
- tguvot 4y ago
  actually i'll advise you to take a look at temperature of your walls. both inside and outside. they have a bunch of thermal mass. during the summer they effectively heat the air inside the house. unless you manage to cool walls somewhat (lets say during night), hvac will have to really fight to keep temperature down. I live in bay area and I installed whole house fan which is typically runs through the night at summer. For most of the cooling season it sufficiently cools air and walls in order to require only a couple hours of hvac a day. additional point, you wrote that you have current system installed 15 years ago. back than design temperatures (i.e. how hvac sized based on outdoor temperature) were different. recent updates sometime bring those temperatures up by 10F or so, what can result in extra half a ton - ton of hvac. you could retain company to make a proper hvac sizing (called manual j calculation). hvac contracts from my expirience won't do it.
  - criddell 4y ago
    Our walls are good but our windows are a weak point. During summer, the night temperature drops to the low 90’s here. It’s A/C all day, every day, for at least 3 months. Any idea why the HVAC guy and just about every source online insists that A/C can only give you a temperature delta of 20F? I know it can’t be true, but that claim is everywhere.
    - tguvot 4y ago
      >Our walls are good but our windows are a weak point. are you sure ? at summer I have walls that are 170F outside (at 2pm) and 80-90F inside. Also, window will pass through heat only during the day. Wall will keep heating your inside air through the night as well. Get some cheap thermal camera - it's very educational. You should also check your ductwork/registers. To see if there are any tears/openings/etc. Also, in case you are using flex duct, if it's not tensioned properly, sagging, etc - there will be a lot of air velocity losses that reduce overall cooling experience. >Any idea why the HVAC guy and just about every source online insists that A/C can only give you a temperature delta of 20F? I know it can’t be true, but that claim is everywhere. Something around refrigerant properties, mechanics and physics, etc. I also spent some time in this rabbit hole
    - sokoloff 4y ago
      There's a lot of poor engineering fundamentals among some HVAC salespeople and some websites as well as some sloppy writing. The delta-T that is correctly cited as being "around a max of 20°F" is the delta in temperature across the evaporator (IOW between the return duct and the supply duct [aka the goes-inta and the goes-outta]). It's not the delta-T between the outside air and inside air. With good enough insulation and/or enough cooling, you can get whatever delta-T you want as is obviously the case with refrigerators and freezers.
      - tguvot 4y ago
        >The delta-T that is correctly cited as being "around a max of 20°F" is the delta in temperature across the evaporator (IOW between the return duct and the supply duct [aka the goes-inta and the goes-outta]). well said. i have temp sensor on supply and return and I graph them and delta, to make sure that hvac works within proper parameters. otherwise during heat waves you start thinking that hvac maybe out of shape

tutorialmanager 4y ago

How about just adding a small minisplit with heat. Warm day = use the minisplit. Hot day = existing central air. Very hot= use them both. If one goes down you still have something while you wait for repair. The mini split could also provide some heating when its mildly cold.

criddell 4y ago
We had a mini split installed into our garage (a Daikin that’s been great). I’m not sure how we would do that for the house. It’s a forced air system with outlets and return air in every room. I’m not sure how feasible it would be to pipe the output from a split system into that ductwork.
- turtlebits 4y ago
  Mini splits aren't ducted (generally the air handler is mounted high up on a wall). They are great for spot heating.
  - sokoloff 4y ago
    Ductless mini-splits aren't ducted. (These are the type most are familiar with, the ones with a large air handler up on the wall or in a ceiling cassette.) However, mini-split just means that the condenser and evaporator units are separate ("split") and are smaller capacity ("mini") than a traditional central air system (which are also "split"). There are ducted mini-splits in addition to ductless mini-splits.

brudgers 4y ago

You don't want an over-sized system. They short cycle. An over-sized system can quickly cool to the thermostat setting, but leave the latent heat in the water vapor in the air. So you experience the "feels like" temperature...it's not the heat, as they say. A properly sized system runs long enough to condense water vapor and with it remove the latent heat from the air.

criddell 4y ago
The short cycle argument only really applies to older and low end systems that are not variable speed. A variable speed system integrated with a humidifier should be able to slowly bring the temperature down, making sure that the cooling does not get ahead of the dehumidifying. I want an “oversized” system that can do > 20F temperature drops. I know they exist because a lot of commercial spaces have them.
- deleted 4y ago
  [deleted]
- brudgers 4y ago
  Short cycling is thermodynamics, not an argument.
  - criddell 4y ago
    As I understand it, short cycling is a problem with air conditioners that cycle on and off. You can get staged compressors, multi-speed compressors, and truly variable speed compressors. A truly variable speed compressor runs continuously and so won’t short cycle.

adanto6840 4y ago

Sounds like you may have had a whole-home energy audit done already -- including a blower door test? If not, I'd highly recommend it; I paid ~$500 for one a few years back which included thermal imaging, blower door tests, airflow measurements & calcs, current/draw measurements for all energy-consuming devices, thermal load calcs, etc. You may find better pricing even -- ours included a detached structure/office, and house is relatively large, multiple HVAC units/solar/pool/servers/etc... In my case, I didn't end up with any super directly-actionable insights -- but it was still very well worth the money IMO. Even just to confirm that we weren't losing/"leaking" energy egregiously. The reality was, our household simply uses a lot of energy - hah. I mention this all because, at the time, we were just about to replace our HVAC units & we had just installed solar + storage. I'm located in Las Vegas -- summers are HOT (!) -- but our units (2x 5-ton Bosch heat pumps) can near-trivially keep us at 70F (or cooler), even on the hottest of days. I try to "push" the thermostats up a bit (we leverage a ToU energy plan), but there are days when the battle isn't worth having & the wife needs it at 69F -- and, even on the hottest Vegas days, we're able to hit it (can be expensive to do so, depending on time of day -- but is totally doable). Edit to add -- Oversizing a system can very easily lead to issues; short-cycling, humidity control, etc. The ideal system will run nearly non-stop on "design" temperature days. See if you can find any highly-rated Bosch installers locally; they'd likely be willing to "oversize" a system since the Bosch units are "variable capacity" (inverter-driven), which means you're less susceptible to hitting some of the common issues/concerns when oversizing traditional systems. Alternatively, the high-end variable-speed options from major brands are generally well-regarded though they can be extremely expensive (the price segmentation, and resultant value propositions, are bordering on dubious IMO).

criddell 4y ago
Thanks for the Bosch recommendation. Your system is much larger than ours, but you likely have a lot more square footage (we are ~2100 sq ft). I don’t recall the exact number, but I think we are 2.5 or 3.5 ton. Our system is old enough that it’s just time to replace. It’s the one the builder installed 14 or 15 years ago and it’s fairly low end. What you’ve accomplished in your home is exactly what my installer tells me they can’t do. He told me no matter how large of a system I buy, the system will never be able to pul the indoor temperature down more than about 20F (internet searches for “max ac cooling” seem to back that up). He says a bigger system will get me down to the 20F floor faster, but that’s it. I don’t believe it though because of stories like yours and because commercial spaces around here are very cold on hot days.