Musings on Water Heaters

I'm coming up on 10 years in this house. This is my third water heater. The original one failed about 6 years ago by springing a (thankfully small) leak that may have been triggered by a combination of me not knowing that my water is a little acidic if not treated properly (I have a tank of carbonate that has to be replenished as the water dissolves it and it wasn't replenished for the first couple of years I lived here), plus shutting it off for week for vacation and letting everything contract as it cooled. Since it was a matter of needing an emergency replacement so we'd have hot water again, I didn't have a lot of time to evaluate options, and ended up with basically the same thing I had - propane, power-vent, and at least efficient enough that it could exhaust through a PVC pipe rather than a traditional flue, but pretty much the only example of such that the plumber could get their hands on from existing warehouse stock. 

I've been periodically looking at options for the eventual replacement that reduced the rate at which I consumed propane, because in addition to its contribution of greenhouse gases, propane is expensive. Not as bad fuel oil (aka diesel), but still significantly more expensive than natural gas. This went from background research project to slightly more urgent when the current tank celebrated the end of its warranty by starting to drool from the bottom late last year. 

Some of the options I considered and some thoughts about them:

• Tankless - still propane, but notionally more efficient because you're not storing a tank full of water. Tankless hot water heaters deal in the number of degrees that they can heat water at a given flow rate, meaning that things like your water inlet temperatures matter a lot, as well as how you use hot water. I'm on a well, and so my inlet temps can be fairly cold in the wintertime, meaning that I would need something that can handle ~80-90°F temp rise at typical flow rate, and let's just say that my showerheads are "probably" not in their original 2.5gpm configuration, so fairly high BTUs, or I'd need to put 2 units in series. I also have a furnace-mounted humidifier that can run independently of the furnace and uses hot water, which means the demand is long-duration at very low flow rate. This would mean either the flow rate would be too low to trigger the hot water heater at all, meaning that the humidifier would be much less effective because it was using cold water, or it'd run the hot water heater way too often and waste energy even if it's good at throttling back to match flow rate demand. Basically, it's a really good demand for a tank full of already hot water. 
• Add-on heat pumps (option one, option two)- these are basically a standalone air to water heat pump that you plumb into a circulation loop so that they are adding heat to your existing hot water tank. In my application it's sort of the equivalent to a hybrid heat system like I am using for my main furnace- heat pump when it's efficient, propane when it's not. This is obviously more attractive if you have an existing tank with a fair amount of life left such that it's not a great plan to replace it, but my problem was that the two options I linked to above were the only ones I could find, and they were either discontinued or otherwise unavailable when I would have been looking to do this. In retrospect, I'm glad they were since I ended up having to replace the tank anyway, and a purpose-built heat pump water heater is way better at this than the bolt-on would be.
• A desuperheater - This is a secondary heat exchanger that pulls the excess heat out of the refrigerant on a heat pump before it's sent outside. These are pretty common on ground source (aka geothermal) heat pumps, but I found this one that is intended to be added to regular air source systems. I exchanged some email with the company and we jointly concluded it's not a good fit for my setup due to the installation requirements. First, the unit has to be mounted higher than the compressor right near the outdoor unit, and if you want it to run year-round, it has to be somewhere that doesn't freeze, meaning I couldn't mount mine outside. My heat pump outdoor units are on the exact opposite side of the house from my hot water heater, so that means a new plumbing loop, and because the ground level is only a foot or two below the basement ceiling, I'd have to mount it in my living room, instead of in my basement. I asked if I could mount it below the compressor, and apparently that causes problems, so while this may be an interesting option for some folks, it's not for me. 
• A more efficient storage hot water heater - traditional tank-style, but best possible efficiency.
• Condensing gas hot water heaters, where they use a secondary heat exchanger to pull enough heat out of the exhaust that some of it actually condenses, are less common than their furnace counterparts, but are at least available. They're quite expensive, and the increase in efficiency (98%) vs what I had (high-80-something%) felt like diminishing returns.
• Heat pump water heaters - these come in two main types: a split system with the heat pump part that mounts in a different location than the storage tank, and all-in-ones that basically just look like a taller electric water heater. The split systems give you additional flexibility in terms of where you source your heat and sink your cooling/dehumidification, along with the noise and power run, and some of them skip the backup heating elements in favor of a more heavily insulated tank and higher capacity heat pump. They can be put outside where the weather goes below freezing, but they're better suited for places where that's not the case, both in terms of effectiveness and in terms of how they keep themselves from freezing up when they're not actively heating your water. My hot water heater closet backs up to my garage, so in theory I could have put a split unit there, but my garage doesn't always stay above freezing either, nor does it have a lot of spare heat in the winter. Since the all-in-one units also have the usual 4kW heating elements as a fall-back, they require the same 30A 240v circuit as a regular electric water heater, which I had to add because I was switching types.  

I ended up going with this Rheem Performance Platinum model, or at least the Home Depot equivalent (there are some very minor differences between the Harry Homeowner version and their pro-series that is only available through a plumber/plumbing supply). I went with the HD version because it was in stock, while the pro series one was quoting lead times measured in months. Between the leak and the looming end of the tax year for rebate purposes, I went with expediency. It's considerably taller than my old water heater, and has slightly larger capacity - the old one was 60 gallon, this one is 74, although it is considered an 80-gallon due to the size.

Water heaters have an efficiency measure (UEF) that tells you how much of each unit of energy is going to the actual water heating vs being wasted in exhaust gas, etc. The best gas ones are .90-something, electric are 1, but the number can actually be greater than 100%. In this case, the one I bought is rated at 4, because the electricity to drive the heat pump nets roughly 4x the heat output of running a resistance coil of the same draw. I can't do a direct comparison because I didn't have a traditional electric water heater before, but electricity around here is below national averages and it's definitely an improvement over propane. There are also some asterisks on that efficiency number, because during the heating season, I'm pulling that heat and humidity out of space that I already paid to heat and humidify, albeit via a similarly-efficient heat pump, but the balancing factor is that during cooling season, I'm getting some "free" additional cooling. 

My hot water heater closet is already ventilated (louvered door and two wall registers) because of the former unit's combustion air requirement, so my initial installation just had it pulling air from that room, and it worked fine, but probably was not as efficient due to the resulting low temperature of that room. 

But both intake and exhaust can be ducted, so very shortly after installation, I took advantage of the fact that my basement is all suspended ceiling and I have a utility room where my network switch and two servers are happily converting electrons to heat 24x7 and set it up to pull air from there instead. Exhaust still stays un-ducted in the water heater closet for simplicity, and because the intake and exhaust shrouds are both sold separately, the maximum acceptable ducting length is cumulative between the two sides, and the exhaust shroud is almost $200. Either way, the water heater is now pulling the warmest air available (that room stays in the neighborhood of 5-10°F warmer than the rest of the basement) and the closest I have to indoor waste heat.

I considered running a ducting loop out to the garage so that it would be partially cooled in the summer, but since that involves some pretty large wall perforations (they recommend 8" duct), and would also require some relay logic to ensure that I don't do this when the garage is below the minimum operating temperature for the heat pump (40°F) I'm not sure how much benefit this would provide unless I at some point stop having as much waste heat in the utility room. 

As with many appliances in our Internet of Shite future, There's An App for That. It tells me when the water heater is running, what mode, how much hot water is available, and allows me to manage all of the settings including setting schedules, set temperature, getting cute with geofencing, and keeping track of power usage. So far, according to the usage stats, I'm averaging about 24 kWh of electricity per week, though there are some posts online implying that the power usage meter in this thing is not all that accurate. Using the rule of thumb I'm finding other places of a 3-4 hour runtime per day at 4kW, this seems well under an equivalent electric water heater, and now that the coldest part of the season is behind me, my propane demand drops to zero until next winter, so I'm happy with it. I have not run out of hot water, so it seems to be managing demand pretty well. 

Tesla Safety Score is Frustrating

Lots of digital ink has been spilled about Tesla's Full Self Driving beta, so I won't retread it here, except to say that I was hoping to be able to talk about my experience using it by now, and I can't, because I haven't. I have growing annoyance about the fact that I paid a substantial premium for a set of features I have yet to be able to fully use on my car, nearly 18 months into ownership. I do use Autopilot extensively, and some of its features were only available if you opted for the FSD upgrade, so it's not a completely wasted investment, but it's also been 6+ months since I requested access to the beta without getting it. 

What "requesting access to the beta" actually means is that I agreed to let Tesla grade my driving to prove that I'm a safe enough driver to be a good beta tester. It's sort of like one of those insurance company nanny widgets that you plug into the OBDII port to get a discount because you drive fewer miles or more safely than the average driver in your area. The FSD beta was initially opened to drivers with a perfect 100/100 safety score, and supposedly Tesla decrements the minimum acceptable number periodically when they want to let another tranche of people into the beta. I'm seeing things online that they're at 98 or 97 right now. But I still have no FSD beta access, so let's review what this means in real-life driving situations. 

Tesla's Safety Score is a daily score that contributes to a 30-day rolling average, which is the figure that Tesla looks at to determine who is a safe enough driver to be eligible for FSD beta. The cynic in me thinks that this is actually Tesla's way of making sure that those with more consistently complex driving environments don't get access to FSD and end up having a bad experience when the limitations of the relatively stupid robot that people are expecting to drive like a human are exposed. Anyway, my safety score tends to be in the 96-98 range. I've managed to get it to 99 periodically, but apparently not for long enough to qualify for the beta. There are 5 categories, and each is graded as green/yellow/red (increases your score/improving this will increase your score/decreases your score) based on your performance against an opaque figure Tesla calls "fleet median". I call it opaque because I don't know if it's the entire Tesla fleet or just those actively trying to qualify for FSD beta but not in the program yet, or if it includes those using FSD, or what geographic area is included - US only vs worldwide, etc. The app says "Median value for the Safety Factor based on the vehicles used to calibrate the safety score" which makes me think maybe it's none of those, and instead a carefully-selected control group of people who are also capable of consistently driving like the EPA range test group - the safest, most conservative few percent of drivers in the easiest traffic environments. Below is each category, and some thoughts about why I am where I am.

• Forward Collision Warnings (per 1k mi)
• This cares a lot about closing speed vs speed of the vehicle in front of you, and whether you're still holding the accelerator rather than braking to adjust, but it's a bit aggressive. I would be willing to own it if this was a situation where I wasn't paying attention and almost rear-ended someone, but that's not what happens in my case. Almost every one of these dings I've had are times where the warning triggers as I'm reacting to whatever set it off, which is frustrating, because I'm being graded on my reaction time rather than how safely I'm driving. Plus this one often goes hand-in-hand with a ding in the next category, so often it's a double-whammy. Given the amount that I drive, I almost always have one of these in the 30 day history, but usually not more than 1.
• Hard Braking (>0.3g)
• Assuming that you're not driving aggressively (accelerating hard and then braking hard) and are trying to drive smoothly, there are two main ways that this gets triggered, and in my opinion both are a bit unfair to use as an indicator of unsafe driving, since in most cases, it's exactly the opposite. Either you're braking hard to adjust for sudden traffic slowdowns, someone who does something you're not expecting, an obstacle in the road, etc. or you're trying to use regen braking/one pedal driving as much as possible and get caught by the fact that (as I discussed in a previous post) the regen braking level is not consistent and have to apply the friction brakes to slow down faster. This doesn't count when Autopilot is active, so there's another incentive to use Autopilot as much as possible. 
• Aggressive Turning (>0.4g)
• These cars have a low center of gravity, so they handle pretty well. Take advantage of that with even slightly spirited driving, and this metric is toast. So if we resign ourselves to having No Fun in the twisties, the other big trigger for this is when you are using Autopilot on secondary roads and it requires intervention - you're more likely to have to provide an aggressive, last-minute steering input to correct for Autopilot failing to turn aggressively enough on its own to properly follow the lane, or misjudging which lane it's supposed to be in as it crosses an intersection, etc. This would be a lot more fair if it didn't include incidents where autopilot was disengaged by the maneuver, but it's clear from my scores that this isn't what happens. 
• Unsafe Following Distance (appears proportional to speed, but no quantitative metric is given)
• This one is most frequently triggered when you're dealing with multi-lane roads and have to get into another lane because you need to turn or exit or pass, or if someone cuts you off. City traffic means aggressive drivers that will take an appropriate following distance as an opening to get in front of you, and the need to occasionally do the same to other drivers to force your way into a given lane. Consistent use of Autopilot helps, but Autopilot tends to be very conservative when dealing with requested lane changes in heavy traffic and so sometimes you have to override it to get where you need to be in time to make the desired maneuver. 
• Forced Autopilot Disengagements (if you're not holding the wheel for long enough that Autopilot shuts itself off)
• This is the only one I've had a perfect score on the entire time, because to trigger this, you have to have been not been applying sufficient pressure to the steering wheel for the better part of 60 seconds, then ignore both a visual and auditory warning. It's not difficult to stay on top of this one, and arguably this is a really good metric to disqualify people from participating in the beta because they're demonstrably not willing to properly supervise the robotic student driver to keep it from doing something stupid or unsafe. 

I see some things online indicating that there are ways to game the system. I'm not really interested in doing that, because I'm more interested in seeing whether it's actually possible to qualify in an area full of traffic, aggressive drivers and difficult situations if you don't cheat. Also while it's annoying that I still don't have access, this sort of reminds me of the reasons why I always stayed a release behind current when I had a Mac - there is definitely benefit to letting other people rush in on release day to be the unpaid beta testers and find all the broken stuff that shouldn't have escaped internal testing so that the system sucks less by the time I use it.