Solar DHW: Deal or no Deal?

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btuser

Minister of Fire
Jan 15, 2009
2,069
Principality of Pontinha
So, I've got a "guy" who bought a setup and now is moving and doesn't want to go through the trouble of installation. Neither the state or federal rebate have been claimed yet, and the system is SRCC certified so it would qualify for both.

30-count evacuated tube aray w/ mounting hardware and adjustable stainless brackets.
80 gallon dual coil indirect. Tank has 10 year warranty, not the best but still a $2,000 tank.
All controlls, valves, timers, sensors, basically an entire setup to install 30 tube solar DHW.

All together the asking price is $2500. After fed+state rebates (and selling my own 2 yr old indirect) the system would come to less than a grand. Payback period would be about 2 years.

Anybody happy with their solar DHW?
 
Don't have one, but I'd roll the dice if a deal like that came my way.
 
Be prepared to live vicariously through me. I'm livin' on the edge!
 
lol, i've got a 1000 sq ft of solar south facing roof ready for the day it makes sense. Until then, it is passive gain through S/SW facing windows with overhangs to shade in the summer.
 
I have flat plates up in Northern NH, they put out more heat but at a lower temp than evacuated tubes. Whenever the average temp is over 50 degrees for a couple of days and the sun is out, I can shut down my hot water maker on my boiler and switch to exclusively SHW. This means I dont have to leave my oil boiler running for six or more months per year just for hot water. In the winter the tubes will most likely need a backup for long stretches of cloudy weahter or when you tubes are buried in snow to 120 F. The Plymouth Regional Energy Coop, does exclusively evacuated tubes, they are a nice fit if you want strictly hot water. There are trade offs for either type, evacuated tubes dont deal well with snow compared to flat plates, the snow take a lot longer to melt off the tubes and they are just about impossible to shovel off with a roof rake. If you do install them, its highly advised to mount them at latitude minus 15 degree (somewhere around 30 degress from vertical to optimize winter collection and minimize summer collection. WIth evacuated tubes, a heat dump is mandatory as they as heat the water to well over boiling, some units have built in protection but the standard warning is that polypro glycol break down faster when its over 225 degrees. Antiscald valves are mandatory on you hot water system and you need a mixing valve that control both high and low temp.

YOu may have some ises with the state rebates as they usually require that the equipment be purchased new after a certain date. Even the federal rebate is dicey as they were supposed to be purchased new by homeowner after a cetain date. The fed doesnt require any paperwork but if you get auditted you need to have it.
 
peakbagger said:
f you do install them, its highly advised to mount them at latitude minus 15 degree (somewhere around 30 degress from vertical to optimize winter collection and minimize summer collection.

Doesn't -15 maximize annual production while +15 maximizes winter production?
 
My plan is maximize btus, regardless of what time of year it may be. After thinking it over, I'll going to ask the "guy" for a business receipt dated April 1st. He's a certified installer for this solar product, so that should cover me. Actually getting the state rebate will be a challenge I'm sure, because the biz will be all over it. At least the federal rebates are a littel easier.

I actually like the look of flat plate collectors, but if I end up going on the roof with one of them I'd have a bear of a time with the install. I'm a little nervous with the longevity of an evacuated tube system, particularly the rubber seal where the tubes connect. I see this as a wear point, whereas a flat panel can be 100% brazed. However, the ease of installation and the abillity to fix one tube if it breaks even out my concerns.
 
btuser said:
My plan is maximize btus, regardless of what time of year it may be. After thinking it over, I'll going to ask the "guy" for a business receipt dated April 1st. He's a certified installer for this solar product, so that should cover me. Actually getting the state rebate will be a challenge I'm sure, because the biz will be all over it. At least the federal rebates are a littel easier.

I actually like the look of flat plate collectors, but if I end up going on the roof with one of them I'd have a bear of a time with the install. I'm a little nervous with the longevity of an evacuated tube system, particularly the rubber seal where the tubes connect. I see this as a wear point, whereas a flat panel can be 100% brazed. However, the ease of installation and the abillity to fix one tube if it breaks even out my concerns.
I have 2- 3 by 8 flat plates, drain back, soft tank and a nyle heat pump hot water heater for back up. The solar works great been up for nearly 20 years, had to take it down to reroof, The differential controller died a few months after it was installed other than that haven't touched it. Its running right now. I have mine mounted at my roof pitch, 45 degrees. You won't get much solar from dec to early feb from my experience. Once the middle of febuary hits though its awesome.
 
Sorryfor the confusion, panel angle isnt very useful unless the reference plane is established. A collector at latitude minus 15 degrees (45-15 =30degrees ) is the amount the panel is tilted back from vertical. If the reference is from horizontal, then the panel angle would be latitude plus 15 degrees (60 degrees) . Basically a steep angle maximizes the surface area that is exposed during the winter when the sun is low. It also tends to retain less snow and when the snow slides down, at least part of the panel is exposed. Generally if you want to maximize the heat year round and have use for hot water during the summer, mounting at your latitude is the best compromise. I had mine at latitude for one winter and between the snow buildup and lower performance in the winter, I tilted my up to roughly 30 degrees prior to the next winter. It made a big difference but it doesnt look real attractive on the roof. I also think that the snow reflection works better with steeply tilted panels in the winter. In the summer I get more than enough hot water even with the steep tilt. I change the tilt angle on my PV (solar electric) panels to account for the seasons, but unless I come up with a couple of leak free rotary joints, tilting a hot water panel piped up with 3/4" copper isnt as easy.

I agree with the prior poster, my collectors dont do much in December and January, a combination of dold outdoor temps and lackof blue skies.
 
Hi btuser, I don't have a solar dhw system, but I stayed at a Holiday Inn last night.... :)

From previous posts, I know that you have high water volume needs. The system you've found sounds pretty good. Evacuated tube collectors are great for very cold, cloudy days but do have a tendency to get snow-covered and won't generate as much hot water as a flat-plat collector of similar area in the summer sun. Some of the reading I've done indicates that a hybrid system is really the ticket - use a flat-plat collector as the first collector feeding a evacuated-tube collector as the second collector. The advantage of this is that they balance each other out over the four seasons. In the summer, the initial flat-plate collector will do a lot of heating and that heat won't be lost as the fluid moves through the evacuated-tube collector (and will heat up even more). In the winter, the flat-plate collector won't be as efficient on a cold and cloudy day, but it will not be as prone to snow coverage and will then heat water which will help snow melt off the evacuated-tube collectors, and the pre-heat to the 2nd stage is helpful in the wintertime, in any case.

I can't remember the source of this info, but it is likely either Solar Today or Home Power.
 
Well the whole project was a bust. I went check out the stuff and it wasn't a dual coil indirect, but an electric hw heater with a coil for solar. Busted. "Oh, I thought you meant dual element". That reduced the value of the package to me by quite a bit so I declined. The tubes were nice and everything was new and in order, but I'd need to use two tanks and I don't have the room. Sad in a way, but I'm also kind of excited because the hunt isn't over!


Everything I've read has shown the tubes to be better/same/worse as the flat plate collectors. In other words I've heard the same argument for both sides used in the same argument, the general main consensus is tubes are better, but not by much and may not be worth it from a financial aspect. I like the idea of a flat plate collector having off the shelf parts, easy to figure and repair, no special vacuum tubes to buy if something happened. But the frosted side of me says smaller footprint, replace one tube instead of the entire glazing, cold climate and cloudy days are about 80% of Southern NH.
 
Think about getting a subscription to Home Power and Solar Today and just read up on the technologies for a year before taking the plunge. There is a lot of industry b.s. out there, but these publications do a pretty good job of sorting through it in a consumer-friendly way. Once every one or two years, Home Power publishes all the info from the SRCC ratings for all the collectors, and how to interpret the data. If you are technically inclined, you can design your own system from this data, and understand all of the tradeoffs between collector types.

The evacuated-tube and flat-plate collectors are different technologies and they each have their pros and cons. For DHW, if you live in the south, a flat-plate is a no brainer. If you live in the far north and never see the sun and it never snows, an evacuated-tube collector is a no-brainer. In between, there are trade-offs to both.

By design, the evacuated-tube collectors lose less heat each time the heating fluid is passed through them - this makes them good for cold climates since they will still heat on a really cold day, and they would also be fine for a southern climate if you didn't mind paying twice as much for a collector system (most people mind). By design, they also gain less heat per square foot of total collector area (i.e. roof area) each pass since they have less total collector area per square foot, but they are able to make use of more diffuse solar gain (i.e. cloudy days).

The fact the the evacuated-tube collectors lose less heat makes them good (more efficient on cold days) but also bad (they won't melt snow off them). It is permits them to be used in very high temperature applications (>140 degree DHW). For instance, some types of air conditioning systems can be run on a commercial scale with very hot 180 degree or so water, and evacuated tube collectors can deliver that.

So, a flat-plate that feeds an evacuated-tube collector is a pretty good all around compromise if you need two collectors.

If I worked for a company that made flat-plates, I would write all sorts of white papers and technical articles about why they are better. Likewise for evacuated-tubes. Many installers likely don't really learn enough about the two technologies to make all of the right decisions for a particular climate and customer, and just standardize on one type for everyone to reduce costs. I ran into this even with some of the good installers I talked to. Sometimes, the installers assume that all customers are very cost-conscious and they default to the least costly solution, even if it means you get no production out of it from November through January (the worst combination of cold and cloudy where I live).
 
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