Your pet peeve, technical or otherwise.

[james_s]

I'm sure the efficiency is not helped by the fact that engine speed (thus compressor speed) varies without any relation to needed cooling capacity.

I've often wondered how AC compressors handle the varying input RPM's, which can have a 10:1 dynamic range in a gasoline engine (diesels are about 5:1 or 6:1 max). This has been a problem since AC was first introduced in cars back in the, what, 70's? Back then it would have had to be a purely mechanical solution. I wonder if they just put a pressure regulator on the output and short-circuit the unnecessary flow back to tank like most fuel systems. To get adequate cooling while idling means the compressor would be massively oversized given that typical engine operation at cruising speeds is ~2000 RPM. Typical idles are about 1/3rd of that.

The older ones don't really do anything special, they are oversized at high engine RPM and don't perform as well at idle, being optimized for best efficiency at typical cruise RPM. With a fixed orifice system you just piss refrigerant into the evaporator at a rate dependent on the pressure delta between the high and low side and any refrigerant that doesn't boil off flows into the receiver where it is gradually fed back to the compressor. With a TXV the charge is metered into the evaporator as needed so excess refrigerant will back up into the condenser. You can fit the entire charge into the condenser and plumbing between the compressor and evaporator so if the compressor is spinning really fast you'll just see the suction side pressure drop as more and more of the refrigerant is sitting in the high side.

[james_s]

I can barely hear (or feel) when the engine on my hybrid CR-V turns on. It's remarkable. Now I suppose that's not fair to compare the hybrid system with a standard gasoline-only car, as there might be a slight lag while the gas motor starts up in the latter. In the hybrid, the batteries can move the vehicle until the engine starts and the transition between the two is seamless.

As for battery life, well, I live in the Sonoran Desert, and replacing car batteries every two years is just standard.

Hybrid systems are different, they have a large brushless motor/generator that can start the engine almost instantly and even move the car on their own. Some of the better start/stop systems use the same type of motor/generator as both the starter and alternator. Many of these systems though use a conventional starter motor, it sounds just like a regular car starting and the starter motor is not powerful enough to propel the car. There is a very noticeable clack as the bendix engages the flywheel teeth.

[IDEngineer]

Residential. I have put in a couple of forced air furnaces recently that had ECM blowers

Yep, I know furnaces have had variable speed blowers for a while now. But I wasn't aware that residential compressor/condenser units (the big, outdoor, mostly empty box with the fan and lots of radiator surface area) had variable speed motors in either application (fan or compressor).

[james_s]

Residential. I have put in a couple of forced air furnaces recently that had ECM blowers

Yep, I know furnaces have had variable speed blowers for a while now. But I wasn't aware that residential compressor/condenser units (the big, outdoor, mostly empty box with the fan and lots of radiator surface area) had variable speed motors in either application (fan or compressor).

This is an example of one: https://hvacdirect.com/3-ton-20-seer-goodman-high-efficiency-ac-with-inverter-technology-gvxc200361.html

Note the price of this 20 SEER inverter unit is about 4.5x that of a 13 SEER single speed system of the same capacity.

[IDEngineer]

Note the price of this 20 SEER inverter unit is about 4.5x that of a 13 SEER single speed system of the same capacity.

Yep... as with everything, the cost-benefit curve goes asymptotic.  Just like ICE's and turbine engines. Squeezing out that last little bit of efficiency adds a lot of complexity and a lot of expense. That's why I'm surprised the payback works on smallish residential units like your example.

[james_s]

I don't know if the payback works, but they are available for those who want the best. I suppose in a hot climate with expensive electricity you might come out ahead but I haven't done the math. Certainly where I am with a mild climate and relatively cheap electricity it wouldn't make economic sense. They do have performance and noise level benefits though so I guess if a person has the money it could make sense.

That said, I could install one of these high end things myself for about the same price as hiring a company to come out and put in one of the cheap ones.

[IDEngineer]

I don't know if the payback works, but they are available for those who want the best. I suppose in a hot climate with expensive electricity you might come out ahead but I haven't done the math. Certainly where I am with a mild climate and relatively cheap electricity it wouldn't make economic sense. They do have performance and noise level benefits though so I guess if a person has the money it could make sense.

I think you nailed it on both counts. Our HVAC company's rep, while visiting here to quote, said "We carry the highest efficiency units for those people who put more emphasis on energy savings than money." That message is pretty clear.

Here in the Pacific Northwest, we might run our AC for a few days during a 2-3 week period. And we have relatively inexpensive hydropower. So the math definitely does NOT pencil out, the most efficient units don't make sense unless your goal is to absolutely positively minimize your "footprint" - in which case why are you buying AC at all?

That said, I could install one of these high end things myself for about the same price as hiring a company to come out and put in one of the cheap ones.

We normally do all of our own work too, and I came very close to doing this myself. But I eventually chose to go with a friend at an HVAC company because I don't have the refrigerant equipment, don't have any experience using it, and therefore wanted someone else I could call if things didn't work properly. I still did my own electrical, pad prep, permitting, etc. Just not the copper lineset and charging.

Generally I find that for the same amount of money as hiring out a job I can buy all of the associated tools and then I'm set forever, but I doubted I'd need the gas equipment that often. That last rule, however, has left me with a lot of "unusual" tools. Most people don't own things like rotohammers, concrete coring bits, and full-size Case 580SK loader-backhoes. But once you DO own such things it's amazing how you can no longer live without them... the projects just appear like magic!

[Cubdriver]

I've always looked at projects as an excuse to buy additional tools, and you're absolutely right - once you have them, you find plenty of additional uses further down the line.  Unfortunately I've not yet done anything that could justify my buying a loader/backhoe, but don't think I wouldn't like to!

-Pat

[james_s]

The first time I put in a split system I did it myself because it was not long after I bought my house and I was broke. I made a vacuum pump out of a surplus rotary compressor for a window AC unit and I bought the manifold and other tools on ebay. The equipment was scratch & dent stuff I got for a fraction the retail price from a local ebay seller. In total I spent less than $1600 installing a high efficiency condensing furnace and 3 ton heat pump and that system is still working to this day. Over the years word got around and friends started asking me to install systems for them, I acquired better tools over time because I could just ask them to buy me a tool needed in exchange for doing the work. The attitude I have gotten from various people in the HVAC industry further motivated me to keep doing it myself, and I vowed never to give them a dime. I've gotten pretty good at it and I think most would be hard pressed to identify one of my installs as a DIY job. In fact I've seen "professional" installs I thought were much sloppier.

One thing that can tip things in favor of the highest efficiency system is if it's a heat pump rather than A/C. When you use the same system for both heating and cooling the higher efficiency can pay off more quickly.

[IDEngineer]

I've gotten pretty good at it and I think most would be hard pressed to identify one of my installs as a DIY job. In fact I've seen "professional" installs I thought were much sloppier.

I share your opinions. Much respect for your DIY. Someday I'll share the project that caused us to buy the loader/backhoe... it involves lots of two-ton blocks of concrete.

One thing that can tip things in favor of the highest efficiency system is if it's a heat pump rather than A/C. When you use the same system for both heating and cooling the higher efficiency can pay off more quickly.

I had the same thought. Using a single device to work both directions could definitely help things pencil out. In our case we replaced a 25YO natgas furnace with a more efficient natgas unit, plus the aforementioned "traditional" AC system, as both natgas and electricity are relatively inexpensive here. Heating is a much larger consideration here in the foothills of the Rocky Mountains at 48 north latitude so we optimized for that.

Ironically, we made the decision to install AC and only afterwards decided to update the furnace. While the old unit was still working, it had always been a bit undersized and the effort to install the AC coil in it would have to be duplicated if it failed anytime soon. So we decided to buy some insurance against EOL furnace failure and lower our natgas consumption in the process.

[james_s]

Yeah once you're installing an AC coil you've done about 90% of the work required to replace the furnace so unless the furnace is relatively new and in good shape I'd probably replace it at the same time too.

[IDEngineer]

25 years old and undersized, meaning it had been working hard its whole life. Replaced!

[jmelson]

As for battery life, well, I live in the Sonoran Desert, and replacing car batteries every two years is just standard.

I just replaced the hybrid battery in my 2009 Civic Hybrid.  I had it replaced under warranty at 68K miles, Honda did a software mod to improve battery life, and the 2nd battery went bad at 185K miles.  But, that one was on ME!  I ended up getting a rebuilt pack from Bumblebee batteries, and it is working very well.
Jon

[james_s]

25 years old and undersized, meaning it had been working hard its whole life. Replaced!

It's rare to encounter an undersized furnace, usually they're oversized. Properly sized it should run almost continuously in the coldest weather, which in the PWN doesn't happen very often. Too big and they short cycle which is not particularly good for comfort or for the heat exchangers.

[IDEngineer]

It's rare to encounter an undersized furnace, usually they're oversized. Properly sized it should run almost continuously in the coldest weather, which in the PWN doesn't happen very often. Too big and they short cycle which is not particularly good for comfort or for the heat exchangers.

In addition to being undersized, the original HVAC contractor for this house (long before we owned it) did a TERRIBLE installation job. Their worst offense was horrific intake restriction... there were only two intakes for the entire house and both were achieved by drilling a few one-inch holes through the bottom 2x4 in interior walls, then sticking a grille on the drywall. Total cross sectional area wasn't even close to the 16x25 filter surface area at the blower intake. This starved the blower for intake air for the first several years of its life.

Starving the intake is one of the worst things you can do to any pump, moving any medium.

When I saw what had been done, I cut a huge rectangular opening in the return ducting as a stop-gap measure. Later, when we remodeled the lower level, *I* designed entirely new ductwork and had them relocate the furnace too. The HVAC guys weren't used to homeowners designing their own ductwork but they did what I asked, including large-radius curves instead of sharp corners on both supply and return ducts to minimize turbulence and improve efficiency, and having TWO return paths from separate sections of the house, each with its own separate 16x25 filter (the blower now draws from both sides of the furnace enclosure, then exhausts upward into the firebox). When the AC was recently installed the new HVAC guys, with no prompting from me, commented very positively on the ductwork. "Someone was really thinking when they did this." 

Part of the decision to proactively replace the furnace was because I knew the poor thing had strained under insufficient airflow for years before we showed up. That would have caused excessive heat in the firebox with all of its associated ills, which cannot have lengthened its service life.

[james_s]

Insufficient return air ducting is also something I've encountered a lot. It surprised me given the relative simplicity of the return side but when I have encountered excessive static pressure the return side has almost always been the worst offender. Low static pressures are especially important for heat pumps and modern high efficiency condensing furnaces, they're designed to move more air and have much lower bonnet temperatures than older furnaces so they need larger ducting.

[jmelson]

Either way the main issue for me is that they are annoying, the starting and stopping engine is jarring to me, and the ones that use a conventional starter are noisy. The fuel savings are negligible and the whole thing is political more than anything and a way to game the way emissions are measured.

I have a 2009 Honda Civic Hybrid.  It has a CVT and the start/stop is about as seamless as anything I have ever experienced.  You take your foot off the brake, and the engine is instantly (and pretty silently) started by the traction motor.  The transmission engages a "start clutch" like a wet pack motorcycle clutch under computer control, and then engages it further when you press on the "gas" pedal.  The only thing that is annoying with it, is if you have to back up a hill, the start clutch shudders a bit.
The only other issue is if you come to a stop in winter, and the engine is barely warm enough to stop, then while sitting, it may need to restart.
You have to kep your foot FIRMLY on the brake under these conditions, or the car may start to roll before you can react.  They ought to give a warning beep before the restart.
Jon

[jmelson]

I ran an experiment in our new Toyota Sienna Hybrid:

I have a Honda Civic Hybrid (2009) that has a dual compressor.  There is a belt-driven scroll compressor in the same housing as an electrically-driven compressor.  The power train computer can select to run either depending on need, but it runs the electric compressor when the engine is stopped.
Jon

[Alex Eisenhut]

Why do split ACs have a drip pan under the evaporator .... but no easy way to access the drain when it clogs?

Yesterday a sudden waterfall came out of my wall, I had to take the housing off the unit and reach in to see the gunk blocking the drain. It was fixed in 30 seconds but now I can't get the housing back on.

Why isn't there a hole in the casing or a brush already inside that you can manually jiggle without having to take the cover off?

[IDEngineer]

Welcome to the omnipresent reality of unserviceable product design. I try very hard to keep in-field service in mind for my designs, often at belittlement from my peers. Too many Engineers have never actually had to WORK on things in the real world. If you've been on your back in a pool of oil, reaching up into the dark greasy confines of some machine to access a poorly placed control module or filter or whatever, with accumulated dirt and leaves and mouse poop falling into your face, those experiences stay with you and improve your own later designs. "You know, an access panel would have made this so much faster and easier and cleaner."

[Bassman59]

The only other issue is if you come to a stop in winter, and the engine is barely warm enough to stop, then while sitting, it may need to restart.
You have to kep your foot FIRMLY on the brake under these conditions, or the car may start to roll before you can react.  They ought to give a warning beep before the restart.

The owner's manual for my 2021 CR-V hybrid is pretty clear about the possibility of "creeping" if you take your foot off of the brake pedal. It really feels like the car has a standard gasoline engine and automatic transmission.

Thing is, my other cars have manual transmissions, so this whole "brake" thing is alien to me.

[Bassman59]

As for battery life, well, I live in the Sonoran Desert, and replacing car batteries every two years is just standard.

I just replaced the hybrid battery in my 2009 Civic Hybrid.  I had it replaced under warranty at 68K miles, Honda did a software mod to improve battery life, and the 2nd battery went bad at 185K miles.  But, that one was on ME!  I ended up getting a rebuilt pack from Bumblebee batteries, and it is working very well.
Jon

Just to clarify -- the batteries I mentioned are the 12 V batteries, not the main hybrid drive pack batteries.

[james_s]

I have a 2009 Honda Civic Hybrid.  It has a CVT and the start/stop is about as seamless as anything I have ever experienced.  You take your foot off the brake, and the engine is instantly (and pretty silently) started by the traction motor.  The transmission engages a "start clutch" like a wet pack motorcycle clutch under computer control, and then engages it further when you press on the "gas" pedal.  The only thing that is annoying with it, is if you have to back up a hill, the start clutch shudders a bit.
The only other issue is if you come to a stop in winter, and the engine is barely warm enough to stop, then while sitting, it may need to restart.
You have to kep your foot FIRMLY on the brake under these conditions, or the car may start to roll before you can react.  They ought to give a warning beep before the restart.
Jon

I'll clarify once again that I'm NOT talking about hybrids. I'm talking about conventional gasoline powered vehicles that automatically stop the engine when you stop the car. They are NOT as seamless as a hybrid because they lack the large battery and electric motor that can get the car moving, they just have a conventional engine and starter motor so there is a lag as the engine starts up.

[james_s]

Why do split ACs have a drip pan under the evaporator .... but no easy way to access the drain when it clogs?

Yesterday a sudden waterfall came out of my wall, I had to take the housing off the unit and reach in to see the gunk blocking the drain. It was fixed in 30 seconds but now I can't get the housing back on.

Why isn't there a hole in the casing or a brush already inside that you can manually jiggle without having to take the cover off?

I know it can happen but I've never encountered a clogged one. My evaporator is uncased and is mounted in the supply plenum, the cover over the hole I cut to install it is easily removable if I need to access it. Cased coils normally have a cover that can be removed but it's often tricky. In more recent installs I've done I used cleanout elbows in strategic locations on the condensate drain to make cleaning easier.

[IDEngineer]

I'm talking about conventional gasoline powered vehicles that automatically stop the engine when you stop the car. They are NOT as seamless as a hybrid because they lack the large battery and electric motor that can get the car moving, they just have a conventional engine and starter motor so there is a lag as the engine starts up.

I'll second this. Earlier in this thread I joined the complaints about these auto-stop engines, and I too meant traditional ICE engines that behave this way. We now have a brand new hybrid vehicle (our first) which obviously stop-starts its engine frequently and I will concede it manages that cycle much, much better for all the reasons already described above.