Ahhh! That does make more sense now. Sunward's schematic showed the mixing valve on the solar output but you would use less electricity if it were on the electric output. With 160 gallons of storage I would probably never see the meter turn. Thanks for the tip.
BTW, I already spent $2100 on PV panels. I found 6 new Evergreen 215's on Craigslist. I'm waiting until spring to install them on a shed I'm building. That gives me all winter to find the right grid-tie inverters. Hopefully Enphase will make one to operate with a lower voltage. I think their lowest one now is 24v.

-----Original Message-----
From: Vermont Skiing Discussion and Snow Reports [mailto:[log in to unmask]] On Behalf Of Dana Dorsett
Sent: Tuesday, October 18, 2011 11:20 AM
To: [log in to unmask]
Subject: Re: [SKIVT-L] OT Solar Hot water

>One question though: Why would you set the mixing valve on the solar
slightly lower than the
>thermostat on the electric tank? My thought was to set it the same or
slightly higher so that
>the electric would only come on when needed.

Maybe I wasn't clear- the mixing valve should be on the output of the elecric tank, and the feed from the solar store into the electric tank should be at full temp, whatever that happens to be, undiluted by the cold water.  If you insulate the electric tank well the standby loss of the higher temp feed won't be any worse than if that water was in the solar tank, but it ensures a minimum of power use on the electric tank.  You set the mixing valve to the lowest temp that actually meets your needs, since the lower the temp of the water abandoned in the distribution plumbing at the end of a draw, the lower the net loss.

Typically you'd need 115F water to end up with 110-112F tub-filling water through an anti-scald mixer with 40F water feeding the cold side of the mixer.  You need to keep the storage temps above 120F to limit the growth of Legionella culture- they won't grow at 120F+, but they won't die. It takes 140F over extended periods to actively kill Legionella, but the higher increases the standby losses by quite a bit, so you don't want to be using electricity to get there. There will be times when the solar water temps peak at 140F or higher, so 120-125F for the electric backup setting is just fine.

If you haven't already, go ahead and swap out the sacrificial anode on the electric tank (ever 7 years whether it needs it or not- put a tag on it with both the date it was replaced and the date it should be done again.)  An electric tank will go pretty much forever if you swap the anodes regularly as it keeps corrosion at a minimum. it's usually a sub-$35 part.  The only other thing that's likely to fail is the heating elements, and they too ar swappable on the cheap. Elements typcially die from thermal cycling from turning on & off.  Usually the upper fails first, which comes on first during recovery and is sometimes the only element that fires. With solar providing most of the water heating energy the elements will see far fewer cycles than when they're providing 100% of the energy.  Even if the sucker is already 10-12 years old, if the anode hasn't failed yet you should be able to get another decade (or even two) out of it, with very little in maintenance, save maybe an element swap at some point.

As a side note: Something often lost in the tankless discussions is the standby power and freeze-control power used by gas/propane tankless heaters.
It varies a lot between units, and can sometimes approach the standby power use of an electric tank heater(!).  So in a low duty-cycle high-standby application such as solar backup the standby power of a gas-fired tankless deserves some scrutiny.

Second side note: At the current subsidized cost of photovoltaics there's a financial argument in this scenario for spending the extra coupla grand you might have spent on a tankless and putting it into PV.  The unsubsidised lifecycle cost of PV has already dropped below 10cents/kwh, but with subsidy we're talking sub-5-cent electricity in some situations.  With reasonably open shading factors a half-kilowatt array will return over 600kwh/year in most of New England, and can (depending on subsidy) cost similar money as tankless HW heater.

>Would it make sense to tap into it for fall and spring radiant floor

It depends a bit on just how much excess heat you have, and the type/efficiency of your heating system. Installing radiant (even with a DIY
staple-up) isn't cheap, and it could potentially take a couple of forevers to break even on fuel savings. Unless you have a very tight and highly insulated house the money might be better spent elsewhere, from an ROI perspective.  If it turns out you need heat dumping in the spring/fall to keep the storage temps bounded, setting up a small supplemental-heating zone using low temp panel radiators (eg: ) would be cheaper & easier to implement than radiant floor, and still pretty cushy (if not quite as cushy as toasty-warm floors.)  To meet code it would have to have a heat exchanger isolating the heating side from the potable side, but in some instances "open" systems would be allowed, provided that there is a guaranteed minimum duty cycle on the heating loop to avoid stagnation issues, which could end up been a net loss.


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