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.