I currently have three water heaters in use (so technically two of them are hot water heaters!).  I valved them so that I can have cold water flow into them from any order (or exclude a tank, or tanks, altogether).  The first tank is an 75 gallon Schuco tank hooked up to 2 flat plate solar collectors.  The second tank is a non-powered 50 gallon tank connected to my 6 ton ground source heat pump.  The third tank is a 50 gallon direct fired oil water heater. 

In the summer the water goes into the geo tank, where the basement warms it up.  The high side of the compressor doesn't get hot enough to kick on the desuperheater pump (cool ground temperature and minimal A/C use with mild Connecticut summers).  Water then goes into the solar tank, where on a hot day I'll have a tank of 130+ degree water.  After a sunny November day today the tank is at 120. It then goes into the oil water heater.  That tank is set to around 127, because when it's set any lower the hot water flowing into it messes with the thermostat and can lead to chillier than preferred showers. 

In the winter I have water go into the solar tank, which flows into the geo tank, and finally the oil water heater.  Once it gets cold the heat pump has no trouble making a tank of 135 degree water. 

There are currently 2 people in the household, and the fixtures are low flow.  I probably average 30 to 40 gallons a day of hot water usage.  The oil tank has run for 36 hours this year, which is about 7 minutes a day.  Each time it runs it runs for around that amount of time.  My guess is that most of this is recovering the heat lost from storage. 

I'm guessing 80% of the time the solar or geo tank has hot enough water that I don't need to burn any oil.  Ideally I'd have some motorized valve system that knows which tank is the hottest, and draws the water from that, but it seems like it won't ever pay for itself.  With the current setup I'm thinking the oil tank is just keeping hot water hot, but I do need some sort of finishing tank.  I was wondering if folks think I should keep the oil tank (I might go through 40 gallons of oil a year, so if I fill up my 2 330 gallon tanks I won't need to order oil for 17 years!), get an instant electric tank, or some other option. 

If I keep the oil tank (costs me nothing until the oil tank runs dry), any recommendations on what to replace the oil tank with when it dies?  I'll probably take out one of the oil tanks, as I can't imagine oil can sit around forever. 

Have you considered a Heat Pump Hot Water heater?

Also consider adding insulation to the oil fired tank. If it's mostly standby losses, a small electric tankless might be most efficient as the final stage.

Posted By jonr on 04 Nov 2016 12:10 PM
Also consider adding insulation to the oil fired tank. If it's mostly standby losses, a small electric tankless might be most efficient as the final stage.

Or a cheap 40 gallon electric tank, which doesn't need the fat wiring and big breaker. An electric tankless uses only modestly fewer kwh, but even a small one is a serious amp-hog.

In many markets grid electricity is cheaper or comparable to heating hot water with an oil water heater.

Posted By ChrisJ on 04 Nov 2016 11:47 AM
Have you considered a Heat Pump Hot Water heater?

I thought about it, but figured since most of my losses were probably standby losses that it would make more sense to get rid of the third tank. 

A heat pump water heater only makes sense if it's your primary water heater. In your case the heavy lifting is already being done by the solar thermal and the 6 ton GSHP.

A standard electric tank has truly miniscule standby loss relative to your typical center-flue oil-burner like a Bock or Everhot, and it's really cheap & low-instantaneous power compared to even the smallest electric tankless. Even the raw cost of energy for electricity is cheaper than an ~80-85% efficiency oil burner in some markets (but not yours, at least this year) , and increasingly greener too. Even at the current minimum-legal EF 0.95 a standard electric hot water heater is going to be the best bang/buck- there would be negative payback for "upgrading" to an EF 0.98 tankless, due to the much higher initial installed cost, and the very small amount of overall power used even with the EF 0.95 tank in your system, let alone the miniscule power use difference.

It will take about the same floor area as the oil-burner it replaces, but you'll at least have freed up the square footage of the oil tank.

The 40 gallons of oil burned at 80-85% efficiency is about 4.5 million BTU (MMBTU), but at least half of that is standby loss, so the amount of water needed to heat the water (as opposed to store it) is probably ~2.5 MMBTU. At $2/gallon that's $80. For delivering 2.5 MMBTU that's $32/MMBTU.

A kwh of electricity delivers 3412 BTU of heat, but at EF 0.95 the net would only be 0.95 x 3412 BTU= ~3240 BTU. For 1 MMBTU it would then take 1,000,000/3240 = 310 kwh, which at $ 0.20/kwh would cost $60 /MMBTU, or roughly twice the cost of heating it with the oil burner at this year's oil & electricity pricing. So with an electric tank you're looking at $150/year instead of $80/year, that's it. Going to an electric tankess would save you about $5/year.

Spending the difference in installation cost on upsized rooftop PV would have a MUCH bigger return on investment.

Oil prices have nowhere to go but up, whereas (despite recent volatility related to natural gas pipeline capacity) electricity pricing is going to be moderated over the medium & long term by the ramping up of zero-marginal cost wind & solar power (which have driven electricity costs in sunny-windy Texas down, even as demand there has risen.) Not that you were considering another oil-burner.

If you are ~100% standby losses (looks possible), then it's $80/year for oil vs ~$0/year for electric tankless and ~$40/year for electric tank. You might try bypassing the oil heater to see how close you are.

An electric tank EF of .95 is not accurate in a mostly standby case.

Given a household of only 2 people and the hardware you already have it is difficult to imagine anything making sense for a finishing heater other than a cheap 40 or 50 gallon standard storage electric resistance water heater.

While I agree with the basic math suggesting that half the fuel use / heat input of a conventionally fired center flue storage tank water heater is lost up the flue owing to off cycle standby losses, in this case the loss ratio may be much higher owing to the fact that inlet water temperature to the oil fired finishing tank is most often elevtaed by one of the other two technologies - solar and geo desuper.

A storage electric tank with lower standby losses (no center flue) would make much more sense as a finishing tank (and, if I've read the post correctly, allow all the oil tank infrastructure to be done away with)