Need advice re: indirect and electric hwt

[bradley888]

Currently my hot water is from an Aero A-30 indirect hot water tank. Since it is heated through my boiler, it's great in the winter because I'm running the boiler anyways. However in the summer, I need to keep the boiler running to have hot water. So I have also installed an electric hwt. I have valves installed enabling me to close off the indirect or electric tank depending on the season. I plan on running the electric during the summer and the indirect during the winter. My question is, when I'm running the electric in the summer should I drain down the indirect and leave it empty until I want to use it for the winter. And vice versa for the electric tank when I shut it down for the winter. Should I be worried about any oxidation forming inside the tank if it is just sitting there for 6mths with no water in it? I figure that the tank will not completely drain down and there will still be residual water inside it, which is leading to my concern about rust. Any help and advice would be greatly appreciated.

[jadnashua]

An alternative hookup method would have the indirect as a feed-through holding tank for the electric WH, then no stale water that might grow things, and especially if the incoming water is cold, it would draw heat from the room, and warm the water going into the electric.

[bradley888]

I originally thought of plumbing it that way, but I want to be able to shut off my electric hwt in the winter time. If I use the indirect as a holding tank in the summer time to feed the electric, then in the winter time, the indirect will be heating the water, and if I shut off the electric hwt, then the indirect will be feeding the electric tank. Since the electric will be off, the water will cool down before it is called for. That is the main reason why I hooked it up differently, otherwise i would have to keep the electric running in the winter as well. Do you think that the electric would still keep the water relatively hot even if it was shut off and was fed by the indirect?

[Dana]

Standby losses on electric tanks are fairly low, and can be made almost arbitrarily low with more insulation, so plumbing them in series, and leaving the electric tank ON (but with it's thermostat set lower than the aquastat on the indirect) is probably the best of all worlds. Your hot water will never be colder than the electric tank's thermostat, but as long as your hot water volume has reasonable daily volumes during the heating-season, the duty-cycle on the electric tank will be next to nothing (if not actually zero, which it might be). Then, when you turn the boiler off for the season you don't have to do anything more- the electric tank will still be on.

If you want to cut down on the electric tank's standby losses even further (to ensure it NEVER fires during the heating season and doesn't add much to the standby losses of the indirect+ electric since they'll both be hot during the heating season), 2" of foam insulation (bead-board styrofoam or XPS pink/blue/green board) under the bottom and on the top, and an R11-R13 fiberglass wrap around the rest approximately doubles the insulation of a typical tank, cutting it's standby loss in half.

While you're hacking away on the plumbing, depending on your drain & boiler/water-heater layout it might make $en$e to spring for a drainwater heat exchanger on the main shower to the house to preheat the cold water feeding both the water-heater chain & the cold feed to at least the shower, if not the whole house. They recover ~50% of the heat used during a shower, which improves the apparent capacity &/or recovery time of the tank. (This works for extending hot water heating capacity for showers, but not much for tub baths, since the drain flow has to occur simultaneously with the potable water flow to get the maximum heat-transfer back into the incoming water). Electricity in most places isn't cheap, and oil, well... it's all over the place, but I don't expect it to stay at current prices for the next decade (do you?)

These suckers cost ~$500-700, but are a reasonable DIY project, and the return for 3-4 shower/day use is worth on the order of ~100 gallons of oil/year (if all your HW was from oil, burned at 85% efficiency) or ~3000kwh/year (if all HW heating is electric.) It'll vary based on your incoming water temp from the street & the amount of shower-water actually used. In high-priced electricity markets like CT or NY, payback is typically under 3 years for all-electric water heating situations. Some people with electric tanks install them just to be able to take back-to-back showers with a little something left over for the 3rd in line.

See:

independent performance testing


one vendors' installation instructions