HVAC/R and Air Conditioning - Why survive SHTF if it gets hot

[Pretend To Imagine]

I figured I could share my knowledge of over 23 years in the trade to help people work on their own equipment. You will need to have the tools to take accurate and safe electrical readings, temperatures of both the air streams of your equipment, and line temperatures.

In my area there isn’t as much need for heat as the rest of the country, so I don’t have as much experience with gas heat, but I am familiar with it.

There is a lot of basic maintenance that will save you lots of trouble. Change your air filters regularly, and make sure no air is bypassing them. Keep your outdoor unit clean, keep your drain clean. Keep plants and other obstructions away from equipment, and make sure your dog doesn’t piss on it. (Seriously it will destroy the unit)

There is a fair amount of repairs you could do yourself, but keep in mind some diagnosis has to be done on live electricity with equipment running. Electricity and spinning mechanical parts can obviously injure or kill you. Also gas heat can kill everyone in your house. Use CO detectors.

I’m here for any questions regarding your equipment. I’m also generally a DIY person and have done significant electrical wiring and plumbing projects for my house as well as appliance repair.

 

[Null]

Is it true that most HVAC units are oversized for the buildings they're installed in?

 

[Pretend To Imagine]

Is it true that most HVAC units are oversized for the buildings they're installed in?

Yes and in a humid climate that is not a good thing.

 

[Null]

Yes and in a humid climate that is not a good thing.

I've heard that. Why? Does the compressor kicking on and off leave moist air to stagnate more? How do you convince a contractor to not give you whatever they have in stock?

 

[Grub]

I've heard that. Why? Does the compressor kicking on and off leave moist air to stagnate more? How do you convince a contractor to not give you whatever they have in stock?

If the unit's over sized it will short cycle and will not run long enough to remove the humidity. Unit oversizing isn't quite as bad as undersized ducting though especially for acs and heatpumps. Undersized ducting will kill the fan motor on the indoor unit and cause the unit to run inefficiently.

 

[Lurker]

I've heard that. Why? Does the compressor kicking on and off leave moist air to stagnate more? How do you convince a contractor to not give you whatever they have in stock?

husband is from 3 generations of hvac

the short answer is: you can't!

basically with an oversized unit, the air gets cold too fast and it's unable to fully draw out the moisture, leaving you with a a cold damp room.

 

[1Tonka_Truck]

How do you convince a contractor to not give you whatever they have in stock?

Most residential systems are oversized because it's easier than doing any design work. I can count on one hand the number of house plans I've seen with any amount mechanical design. That, and math is hard. Same reasons ducts wind up undersized for the oversized systems, especially on retrofit.

 

[Pretend To Imagine]

I've heard that. Why? Does the compressor kicking on and off leave moist air to stagnate more? How do you convince a contractor to not give you whatever they have in stock?

The longer the run time the more humidity is removed. All newer equipment is worse at humidity remover than older equipment. I’m in a humid subtropical climate, and we are actually needing dedicated dehumidification more and more to supplement the AC units.

How do you convince a contractor to not give you whatever they have in stock?

Hire a reputable 3rd party to do an actual load calculation.

 

[Grub]

That, and math is hard. Same reasons ducts wind up undersized for the oversized systems, especially on retrofit.

Sometimes customers just don't want to pay. Especially on retrofits where redoing ducting can get expensive fast depending on how the ducting was ran originally and where the unit is located. We've taken out a few oil furnaces and swapped them for heat pumps and customers just either couldn't afford or didn't want to pay to have the ducting changed but were insistent on getting a heat pump. Oil furnace ducting is awful for heat pumps. 5 inch branches, tiny fucking main trunks and plenums on the supply side and if you've got any kind of return you're lucky. Those systems always seem to have issues.

We had one recent one where the blower motor on the air handler burnt out in less than a year. We warned the customer multiple times before, during and after install his ducting wasn't adequate. He didn't want to pay. We ended up going back, swapping the motor and luckily, it was an old house with t-bar ceilings in the basement so we managed to run some extra ducting into the rooms and so far no more callbacks. The air handler sure does whistle on the return side though. Sounds like a tea kettle.

 

[Pretend To Imagine]

Sometimes customers just don't want to pay. Especially on retrofits where redoing ducting can get expensive fast depending on how the ducting was ran originally and where the unit is located. We've taken out a few oil furnaces and swapped them for heat pumps and customers just either couldn't afford or didn't want to pay to have the ducting changed but were insistent on getting a heat pump. Oil furnace ducting is awful for heat pumps. 5 inch branches, tiny fucking main trunks and plenums on the supply side and if you've got any kind of return you're lucky. Those systems always seem to have issues.

We had one recent one where the blower motor on the air handler burnt out in less than a year. We warned the customer multiple times before, during and after install his ducting wasn't adequate. He didn't want to pay. We ended up going back, swapping the motor and luckily, it was an old house with t-bar ceilings in the basement so we managed to run some extra ducting into the rooms and so far no more callbacks. The air handler sure does whistle on the return side though. Sounds like a tea kettle.

It’s a common issue in my market as well. Very common to see an undersized filter causing 0.6” pressure drop when clean, already exceeding design static pressure of a unit before even attaching ducts

 

[Grub]

It’s a common issue in my market as well. Very common to see an undersized filter causing 0.6” pressure drop when clean, already exceeding design static pressure of a unit before even attaching ducts

Yeah we learned 4" filters are pretty much a no go for the units we install no matter how good the ducting is, even on new installs with properly sized ducting and equipment and not just one brand all the central heat pumps we install are like that. Heat pumps really need a lot of air. It's the most important thing for them to work properly. Especially return air. Even older gas furnace ducting is sometimes not good enough just because they don't have enough return ducting or it's too small. We pretty much always, unless we just can't, add more return air whenever we swap a heat pump in where something else was.

 

[Pretend To Imagine]

Yeah we learned 4" filters are pretty much a no go for the units we install no matter how good the ducting is, even on new installs with properly sized ducting and equipment and not just one brand all the central heat pumps we install are like that. Heat pumps really need a lot of air. It's the most important thing for them to work properly. Especially return air. Even older gas furnace ducting is sometimes not good enough just because they don't have enough return ducting or it's too small. We pretty much always, unless we just can't, add more return air whenever we swap a heat pump in where something else was.

A 4” filter is gonna have less pressure drop than a 1” filter of the same type.

I’m running one 20x20x2 MERV 8 on my 3 ton heat pump and am able to run the blower on the low speed setting and get the correct airflow.

Before I installed the 2” filter I was short on airflow with a 20x20x1 MERV 8

 

[Grub]

A 4” filter is gonna have less pressure drop than a 1” filter of the same type.

I’m running one 20x20x2 MERV 8 on my 3 ton heat pump and am able to run the blower on the low speed setting and get the correct airflow.

Before I installed the 2” filter I was short on airflow with a 20x20x1 MERV 8

Well, after a few units in a row that wouldn't stop tripping the limit switch with a 4" filter and fixing the problem by swapping to a 1" filter we decided to just not with the 4"

 

[Hans Kammler]

Extremely based thread. I'll throw my 2 cents in here as well, being experienced in these matters. If you own a home in the Deep South, the very humid parts, here are three things you need to look out for.

1: Does your system run on R22? (I.E. is it old?)

2: Does your system have metal ductwork anywhere in the attic (plenums, ducts?)

3: Is your attic insulation sufficient?

For #1, if your system is old, it very likely was designed and sized correctly for R22. As long as your unit doesn't leak, cools the house and chugs along, you're fine. Your unit should be maintaining a 20-25 degree temperature gradient cooler than the outside air. If it's 98 degrees and your home is 74 inside, your system is doing great, especially if it's old. Be ever vigilant in changing filters, rinsing off the outside unit twice a summer, and peeking your head into the air handler to ensure it's not freezing up. If you have an R22 system, run that sucker into the ground and get every last penny you can out of it.

For #2, if you have metal ductwork in the South, it likely works fine for R22 (old) systems. HOWEVER, if you replace your unit, you MUST replace the ductwork with fiberglass ductboard. The only metal parts of your ductwork on a new R-410A system should be the plenum collars and the vent boots! If you replace your units and do not replace the metal ductwork, you will create a moist disaster in your attic. I have personally seen old metal ductwork dump gallons of condensation onto the ceiling of a cheap homeowner who didn't want to pay for his ductwork replacement. R-410A systems blow colder air and exacerbate condensation issues! It is critically important to find an HVAC contractor who knows this, and knows how to design and hand-build fiberglass ductboard systems. If you have an old system, and the contractor says "Ah nah man you can re-use your old metal ductwork, there's nothing to worry about!" - RUN! Find someone who knows what the fuck they're doing. This is particularly important in the South and not so much in flyover or Northern states. Where I live, it's so humid that even the plenum collars sweat if the ductboard plenum is not designed and built perfectly. I spent a lot of time figuring out exactly how to get it right - don't gamble on someone who tells you your metal ductwork from 1978 is "sufficient". IT'S NOT!

For #3, attic insulation can have a major effect on the humidity and comfort of your home. Oftentimes, homes down here have blown-in cotton insulation or fiberglass batts that are just barely sufficient. If you have money to blow, I highly recommend Icynene closed-cell-foam insulation applied to the inside of the roof. I have personally seen this bring attic temperatures down from 100+ degrees to a nice cool 75 degrees after application. This, plus fiberglass batts above the ceiling, gives you the absolute best insulation and moisture barrier you can ask for. You must also ensure your attic is properly vented - the more ventilated, the better. Eave vents, soffit vents, and a ridge cap vent are ideal in these extremely hot zones. An un-vented attic can reach 140 degrees down here - I've been in them. The ideal comfort temperature is 75 degrees at 50% humidity, however, this is difficult to achieve in our area. With a properly insulated attic, you can run your A/C down to 60 degrees even during the hottest months of the summer.

In summary - if you have an old system, run it till it breaks - and make sure it is actually broken! It could just need a capacitor. If you have metal ductwork, make sure it's replaced with a properly designed fiberglass ductboard system when you upgrade your unit! And finally, look into improving your attic insulation, there is nothing like this closed-cell-foam insulation, it is truly magic shit.

Thanks for reading anons. Stay cool.

 

[Hans Kammler]

I've heard that. Why? Does the compressor kicking on and off leave moist air to stagnate more? How do you convince a contractor to not give you whatever they have in stock?

If you open your refrigerator, you usually can't see the foggy cool air come out, or at least very little. But if you open the freezer, you see big clouds of frosty air. This principle also applies to HVAC. A cooling surface (the air handler coil) must be a certain temperature to capture humidity and condense the water into the drain pain. This ideal coil temperature is achieved through proper airflow and proper refrigerant charge within the unit. Additionally, HVAC units are built in a range of sizes generally corresponding to a home's square footage. The different sizes require different dimensions of ductwork - any duct that's too small for a system will sweat condensation profusely in the attic. If you put a 5 ton unit on a house that needs a 1.5 ton, it will cool, but it will barely dehumidify the air. If left to run constantly, the interior of the home will condense water all over the walls and turn into a moldy disaster. In effect, a system that's too big acts more like a malfunctioning freezer than a refrigerator, and a humid cold house feels infinitely worse than a dry house at 78.

The contractor MUST know how to size the unit to the house. They should be considering the insulation present in the home, the windows, the average heat load of the area, the duct sizes, the way the ductwork is run, the quality of the duct connections including leakage, the performance of the unit previously installed and the overall tightness of the home's envelope in regards to air leakage.

It's not so much that the short cycling leaves moisture in the air, though it does, but that the unit, when running, isn't really removing humidity at all when it's so oversized. In fact, it can add to the humidity issue and make it much worse. HVAC is an art form and as a homeowner you've got to find the autistic savant that really loves the science of putting together perfect systems.

 

[Pretend To Imagine]

Saying R410a systems blow colder air is simply not true.

The issue you will have with older duct systems is they weren’t sealed properly or insulation has degraded.

R410A is getting phased out this year. Going forward it will be R32 and R454B for comfort cooling.

 

[Pretend To Imagine]

Well, after a few units in a row that wouldn't stop tripping the limit switch with a 4" filter and fixing the problem by swapping to a 1" filter we decided to just not with the 4"

What is the MERV rating of each? If you have a super restrictive 4” filter compared to a 1” spun fiberglass filter that doesn’t catch anything then that will be true.

If we are talking same MERV pleated filters a 4” one has way more surface area compared to a 1” and therefore less pressure drop.

Have you measured the pressure drop of the filter and the total external static pressure of the furnace or air handler?

 

[Anstiv]

Is it true that most HVAC units are oversized for the buildings they're installed in?

You should checkout Technology Connections video on HVAC system sizing. It's really comprehensive.

You can calculate the size of your HVAC system yourself as long as you know the size of your house and the design conditions (the coldest and hottest the outside will get in a given year in your area). You can also take into account if your house is well insulated or not, but that's not too important. Get multiple quotes on HVAC and tell them you insist on getting a system of roughly that size. I haven't gone through this process yet but it is on my list of things to do within the next couple years, and this is my plan.

https://youtu.be/DTsQjiPlksA?si=jkW_GpDJR5kou46-

 

[Pretend To Imagine]

My phone isn’t letting me reply to the specific part of the post regarding ducts sweating, but here is a good video on the topic.

https://youtu.be/LYKqGQozW8c?si=xR63HmexIauLsgLb

 

[Grub]

What is the MERV rating of each? If you have a super restrictive 4” filter compared to a 1” spun fiberglass filter that doesn’t catch anything then that will be true.

If we are talking same MERV pleated filters a 4” one has way more surface area compared to a 1” and therefore less pressure drop.

Have you measured the pressure drop of the filter and the total external static pressure of the furnace or air handler?

Yeah that probably should be done and we have the tools to do it but but you try explaining that to a crazy diabetes addled old man who's response to anything is 'I have 45 years of experience blah blah blah'.