Hello. I am not new to this forum but had to join with a new name since I could not login with my old password. I used to be just BRAWLER, and I was active a few years back when i was building my SIPS house which I finished a couple years ago. My current question regards solar hot water. My principal residence is a 70 year old ranch with a no-operable solar water set-up. I have the oputunity to purchase a complete new system minus glycol made by Velux with various components made by various companies. The tanks are bradford white, the pump station is sunmaxx not sure who made the panels. My choice is between a 60 tank with single loop exchanger and passive vent gas backup or 80 gallon single loop with powervent gas backup. I also have a choice of four small flat panels or two bigger flat panels. We are only 3 people in our house and do lots of laundry. My wife takes long , hot showers. i currently have a brand new 50 gas WH. Also would it make sense just to use the new tank as a preheat for my curent small gas WH. As always any advice greatly appreciated.

I should have added that my price for everything will be very low. $2300.00 which is probably 1/3 retail. Which would be the best fit 60=passive vent or 80=powervent. And besides square ft what are the pro=cons for multiple smalle panels. Thanks

The best ROI (other than cooler or shorter showers) is to see if youf wife will accept low flow rates delivered at high pressure. I happily shower at less than 1 GPM and a small booster pump doesn't cost much.

Drain water heat recovery can pay off in some cases.

It's not clear what your goals are here. Is it the desire for even longer showers, lower gas use, or lower utility bills, or some of all of the above?

The price of photovoltaic (PV) solar has continued to fall, and is heavily subsidised in many locations. From a cash point of view you may get better ROI spending the $2300 on PV solar offsetting your power bill rather than offsetting natural gas use. In cooling dominated climates going with PV + a heat pump water heater may have an even better net return, since it also offsets some of the air-conditioning.

PV is simpler, more reliable, and much lower maintenance than solar-thermal. I've been seeing quotes in the $4/watt range and lower in my neighborhood, that's the all-in price (before subsidies). Parts of TX are hitting under $3/watt. With tax credits and other subsidies it's hitting in the $1.50/watt range in parts of CT. If you are similarly priced & subsidised that $2300 buys you about 1.5kwh of grid tied array, which will deliver at least 2000 kwh/year or more (in some places MUCH more) in most of the US lower-48. In my neighborhood that would be worth about $350/year in offset electricity.

The same $350 would buy about 300 therms of natural gas, which is considerably MORE than the total hot water heating use of my 3-person household (including the endless-shower teenager.)

But your gas & electricity rates will differ, as will the levels of subsidy and local PV pricing.

A $600 drainwater heat recovery unit can turn a 50 gallon gas-fired hot water heater into an "endless shower" experience, cutting the fuel use for those showers roughly in half. Doesn't do a thing for tub-filling capacity. You need at least 5' of vertical drain downstream of the shower to take advantage of it. These things are dumb as a box o' rocks, have zero maintenance costs, and an anticipated lifecycle of 40+ years. You can get 'em on the cheap by opening up an account with these folks over the phone to get the wholesale price: http://test.efi.org/sites/default/files/power_pipe.pdf (They are considerably more money when purchased direct from the manufacturer or through that big orange box store.)

Dana, Thank you as always for such a well thought out response. I have not purchased the system yet and after reading your response I probably will not. The Velux labeled tank is a Bradford-White which I have read are problematic. but besides that its a smoking deal. Our lowest gas bills are 29.00 bucs during august which is strickly hot water. Including winter we probably speand something like 400-450 on hot water per year. How much do you think we could expect to ofset this with south facing panels of aprox 54,000 sqcm at a 4/12 pitch at 35% latitude? 4 panels that are 112x114cm with brackets, all the insulated hoses valves , vents,pump station and expantion tank for 2300.00 My house has plumbing from the basement to the roof. If say that PV is a better return i certainly believe you. I just have the oppurtunity to buy this from the local habitat restore and seemed like a no brainer. Glad I posted. Thanks again. Michael

There's no such thing as a "no brainer" when it comes to solar thermal- you have to run the real numbers on the actual system against the actual alternatives. In most US markets solar thermal is a loser against (currently cheap) natural gas in a standard tank type hot water heater, even after substantial subsidies. (A DIY install of a used or surplus system probably won't qualify for any subsidy, but may come in at somewhat lower cost than a fully subsidised system if you discount your sweat-equity heavily.)

Most gas bills come with a monthly charges just for being hooked up to the gas-grid, so your $/MMBTU are higher in the non-heating season. Incoming water temps are lower in winter so it may take more BTU to heat that winter-water, but the $/MMBTU is likely to be lower when you're also using gas for space heating. Your real hot water heating cost are probably less than $300/year, but without reviewing your billing in detail it's hard to really tell.

It's also impossible to estimate what the output of your system will be without SRCC rating on the system and more detailed weather/insolation information. A typical ~55 square foot flat panel system could supply anywhere from 50-100% of your summertime hot water for 3 "typical"users in a high-insolation area, maybe half the mid-winter water heating. For the "endless shower" set there is no saying how much it will offset. (But to be sure, a decently sized drainwater heat exchanger cuts endless shower energy use by half or more.) There are also maintenance and electricity costs to factor in as well.

Solar pre-heating won't reduce the standby loss of tank-type gas hot water heaters either, which are typicially on the order of 700,000-1,000,000 BTU/month. The EF rating of a gas water heater presumes about 63 gallons/day of hot water use, but will be somewhat higher than rated for high-volume users. In general a power-vented unit will do better in this application than an atmospheric-drafted version, since the atmospheric drafted version has somewhat higher convective losses from room air entering at the combustion-air intake of the burner that gets heated by the hot water surrounding the center flue heat exchanger, and rising out the draft-hood &/or up the flue. With power drafted units there is at least some impedance to that flow. Electric hot water heaters (including heat pump water heaters) do not have this issue, and can be made VERY low-loss with insulation.

If the gas water heater rated ~0.60 or something it means that it loses about 40% of the heat going into the water is lost in standby. If pre-heating with the solar, even if 100% of the heat is entering the gas heater is from the solar, it's throwing away 40% of that heat (assuming EF test standard volumes). That cuts into the solar fraction that's actually delivered to the showerheads & taps. Those types of issues are why solar hot water usually works best as stand-alone rather than pre-heat. If using it as pre-heat only it's better to go with a VERY large solar tank, which keeps the pre-heat temperatures low. Low storage temps enhance collector efficiency and lowers the standby loss of the solar tank. High volume in the solar storage tank allows you to store more heat too.

Even a $600 drainwater heat recovery heat exchanger has a pretty low ROI against cheap natural gas for most people, but will be a decent investment for the endless-showering set.

To carry through to some possible results (your numbers will vary), say you spend $360/year on water heating and a solar system reduces that by 60%. So you pay $2300 and save $216/year. Say 10 year straight-line payback. Are you definitely going to have the house for that long? Do other options beat this? For me, PV is about 12 years. In your case, drain water heat recovery might pay back in 6 years. Lower flow shower heads might pay back in a few months.

You also have to factor in the cost of the opportunity loss of the roof top real estate that might have been devoted to photovoltaic solar. In five years it's anticipated that the average rooftop grid tied PV system will come in at or under $1.50/watt. With federal/state/local subsidy it's already at that price point or lower in some locations.

The 55 square feet of solar-thermal foot print COULD be worth about 600 watts of PV which even in not-so-sunny New England would be returning 1.1-1.2 kwh per watt per year. At say, 15 cents/kwh that would be an easy annual upside of $100/year, for an outlay of ~$900, and a miniscule fraction of the maintenance of solar thermal. Leveraged against a heat pump water heater ($1000) and drainwater heat recovery ($600) you'd likely end up with a better raw $/annum return than the theoretical max you'd ever get out of 55' of solar thermal.

Like I posted previoiusly, there is no such thing as a no-brainer- you have to do the math. And in the case of solar thermal, you have to do the system design math too.

Both solar heaters and PV panels are better ground mounted than roof mounted. So one should consider this when looking for property on which to build a home. Yes, if you don't have that option, then you have to live within your roof real estate constraint.