Hot Water Heater with Solar - TANK SIZE????

I have an active direct circulating solar hot water system in my Florida home for family use (not for a pool or anything like that) and had it installed around 20 years ago. It consists of an 80 gallon tank and a roof mounted panel, I also added a Little Gray Box timer so in case there's not enough sunlight it will heat the water conventionally at times of high use (morning showers, etc). In the last couple weeks my youngest moved out to his own apartment so it's just me in the house. My tank is nearing it's life expectancy so it's time to replace. My question is should I stick with the 80 gallon tank or go with a smaller tank, like a 50 gallon? I can see if I don't have enough sun to heat the water, the electric means to heat 80 gallons is pretty taxing. But I can also see that on sunny days (which we have a lot of), you'd want to capture as much of that heat as possible. Don't want to get into an 'overheating' condition since it's direct circulating the water in the panel has to be kept under the boiling point and the only way to do that is to circulate cooler water thru, and with a 50 gallon tank it might mean it has to vent steam pressure to prevent panel damage.
With the difference between the 50 gallon 12 year tank and the 80 gallon 9 year around $150, and around $200 difference between the 50 gallon 12 year and the 80 gallon lifetime, my question is should I stick with the 80 gallon? Also don't want to be paying to heat 80 gallons of water electrically on cloudy days.
Any suggestions?
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You need pro advise , maybe alt.energy.homepower I would be thinking to not heat the tank but use it for solar storage, if its near hot enough a cheap small electric 20-30a tankless would be far more efficent, for single use it probably would be all you need. but what about a cloudy week or month.
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Looked at the tankless but are they not more for point of use then whole house? And the ones that are whole house cost more then the 80 gallon lifetime tank.

In west central Florida near the coast and it's rare to have a couple days without enough sunlight to get hot water. And the Gray Box cranks on the electric heater just before dawn so the morning shower is never cold, even with cloudly/stormy days. :O)
Kind of brings up another question. If you have a hot water heater and (for this example) NO solar, if you turn the heater off for most part of the day and then on in time to heat water for an evening shower, does it cost more to heat a cool or cold tank of water then it would to just maintain it all hot all day long???? Kind of like that old AC question, better to leave it on all the time rather then try to cool and hot house when you get home?
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your far better going for the larger tank, it costs not much more and allows storage for cloudy days, a smaler tank would be a resale killer when that time comes.
turning off heat does save a little, but the real improvement for you will be the added efficency of a new tank and its superior foam insulation.
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Just like with heating a house, you save by having it at a lower temp and then bringing it to desired temp when needed. The only exceptions would be if energy rates were different at different times of the day, or if a different energy source were used, eg in winter a heat pump might kick in electric heat to raise the temp quickly. In the case of an electric water heater though, the cost savings of setting it back for say 8 hours isn't going to amount to much at all. In that time, it won't cool very much.

The only real cost factor from an operating standpoint is the amount of electricity it takes to cover the standby heat loss of a 50 gallon tank vs an 80 gallon one. And I would think that could be easily offset or more by the fact that with solar the 80 offers more storage and less likely to need some of the electric to raise the temp.
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There are small electric 15-30a units I have used many times, a friend had one to heat a sink in his garage, flow was good and water hot. You have to do temp measurements of what is avalaible with only sun and after shade with tank off for a few days to realy know what btu is needed, it would be flow and temperatures over a few days. I can say I went from electric high insulated tank to a 460$ 117000 btu Bosch Ng tankless, that cut my electric bill maybe 30 -35$ and only increased gas maybe 7$ so I have a 4 yr payback on Ng Tankless. Ng, Propane, or electric is the cheapest option to operate, small electric may work fine if tank temp is high enough. It depends on how hot you want HW, I only need 96f at the showerhead in summer and maybe100 in winter to shower or 110 out of the tank, if the solar doesnt go below 70f something very small would work. You need to talk to people that use solar and explore options. A whole house electric that heats s single ufe from near 40f incomming water would require maybe 200 amps on its own, but a small unit might only need 20-30a if your solar set up had enough reserve. Then there are cheap Ng and propane tankless, the cheapest might now be 500$ not intalled
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Good idea. A solar tank combined with a whole house tankless sounds like it could be a good solution. You could do it with a smaller tankless because the incoming water from the solar tank will be much higher than typical water from city or well. It would be more economical operating than using an energy source to keep the tank always at 130.
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