Solar collector t&p valve excessive tripping

Discussion in 'Solar and Geothermal Water Heating Forum' started by joe smith 100, May 12, 2009.

  1. protech

    protech Master plumber

    Messages:
    51
    Location:
    Central Fl
    See here: http://www.fsec.ucf.edu/en/industry...ual/documents/Section_2_SolarWaterHeating.pdf

    Go to the section entitled "1. Active Direct Systems"

    Look at "Figure 2 Additional system components" to see a basic drawing of an active-direct system (A type of open loop system)

  2. protech

    protech Master plumber

    Messages:
    51
    Location:
    Central Fl
    You'll notice that a pressure only relief valve is used up on the panel while a T/P relief valve is used on the tank. This is FSEC here. These guys are at the for front of solar.
  3. flamefix

    flamefix New Member

    Messages:
    71
    Location:
    Exeter, England
    So no Temperature relief valve on the collector then as all agreed to.

    This active system I just don't like the idea of domestic cold water supply circulating around a panel into the tank and then on to my shower or tap personally. I can understand having the collector direct to the store but I'd rather have my hot water indirect. I think I would have a plate to plate heat exchanger connected top to bottom of the store with a pump. Triggered by a flow switch on the cold inlet side which when a hot tap is opened causes the heat from the store to heat the hot water. Or as I said earlier on the solar side, and keep the store as the tank of hot water. But I am digressing rather from the topic.

    you can see why I was confused by the words active..
    http://www.solar-trade.org.uk/solarenergy/howwork.htm

    What's the typical install cost of one of these active systems if I may ask?
    Last edited: Sep 6, 2009
  4. protech

    protech Master plumber

    Messages:
    51
    Location:
    Central Fl
    About the lowest you can get one here before you get your rebates and incentives back is $3500 for the smallest system under the most ideal circumstances. Most people will end up paying in the mid 4 thousands for a system sized for 2-4 people installed on a single story shingle roof. Bigger systems, multiple stories and tile roofing all make the price go up. The high end of the spectrum would be $8000ish.

    This is of course for active-direct systems. Indirect systems cost even more and put out less heat (size for size). If you live south of Ocala Fl there is no need to have an indirect system unless you are both paranoid about freezing and like to spend extra money for no reason. If you do live north of that point, then I would recommend an indirect system.


  5. Alphacarina

    Alphacarina New Member

    Messages:
    26
    Location:
    Biloxi, MS
    Another good reason we use it is because it's more efficient . . . . we get more hot water

    So long as freezing isn't a problem (or can be dealt with) there's no real reason to use an indirect loop

    Don
  6. flamefix

    flamefix New Member

    Messages:
    71
    Location:
    Exeter, England
    But in a southerly latitude efficiency is the least of your problems surely? and You can't get more hot water you can only get it quicker albeit if you draw it off and it reheats quicker you can get more but you are limiting your store temperature to 120F correct? With an indirect sealed system you could run the store higher and mix it down to 120F thereby increasing the amount of hot water you can draw off anyway.

    I can accept the no need to have an internal tank arrangement and have a system like this for example..
    I'll not link direct to them but you can work it out as I am not advocating their product just citing it as an example.

    The cost of something like that is far cheaper more in the region of $1500

    3*w.eliotsolar.com/solar_hot_water.htm
  7. Alphacarina

    Alphacarina New Member

    Messages:
    26
    Location:
    Biloxi, MS
    No - My tank temperature will go as high as 150 degrees on a really good day . . . . but 125 to 135 is more typical

    I have a single 4 by 10 AET panel feeding two Sears Best 55 gallon electric water heaters for storage. I have a T&P valve on each tank and one at the top of the collector

    For 9 or 10 months of the year, solar provides 100% of my hot water and only in the dead of winter is any electricity applied . . . . and even then, it's just to the top of the supply tank (the one with the thermal mixing valve on it) and only for 30 minutes in the morning before the sun comes up so that we have the normal temperature water for morning showers - The electricity is only used to raise the temp of the top half of the supply tank up to 120 degrees, which is what the mixing valve is adjusted for

    I like having 110 gallons of really hot water . . . . it doesn't cool off much overnight and we can take a half hour shower in the morning, knowing it's basically all FREE

    Don
    Last edited: Sep 7, 2009
  8. protech

    protech Master plumber

    Messages:
    51
    Location:
    Central Fl
    I have a similar setup as Alphacarina. I have a 4'x10' flat plate collector and an 80 gallon 5 port solar tank with 3 dip tubes. I installed a kilowatt hour meter on my old electric 40 gallon tank for a year before I installed my solar setup. it was costing me $55 and some change every month to feed electricity into that heater and that was back when rates were at $0.12 per KWA. I've had the solar setup on the same meter now for about 2 years and it's running at 95% solar fraction. That also includes the energy used to circulate the water. I've cut my water heating costs by 95%.
  9. flamefix

    flamefix New Member

    Messages:
    71
    Location:
    Exeter, England
    But Don you'll easily acheive that with an indirect system. In fact the systems I install can reach 80 -85c before limits cut in those limits can cut in at 60c or 50c what ever you wish.

    However Jimbo from this site informed me the following in a separate post "perhaps the other thing which weighs heavliy on our procedures is the well know law firm of Dewey, Cheatham, and Howe. ALL water heater manufacturers, and many of our plumbing codes, SPECIFY that water heaters should NOT be set higher than 120ºF ( about 50ºC) for the reason that if someone gets scalded, the plumber and the manufacturer will get sued." that doesn't sit with solar systems achieving higher temps than this.
    However as Long as you have a thermostatic blending valve then upper store temps don't matter within reason.

    I'm not trying to Diss your installation or your system or antagonise you, I just don't follow your logic for having a system soley based on efficiency, and then not to store the heat at higher upper temperatures than you currently do possible with a fully stratified store and have it direct connected to your cold water mains supply. This side of the pond we simply would not be allowed to.

    Given your solar radiation levels at your latitudes then the efficiencies you may gain from a direct system is to me fractions if at all over the indirect system.

    Plus I suspect that an evacuated tube design would provide you more hot water in your winter season also.
  10. protech

    protech Master plumber

    Messages:
    51
    Location:
    Central Fl
    It very simple. Per dollar invested an indirect system puts out less heat than a direct system. If I install a 40 square foot collector ran direct and you install the same size collector indirect with both tanks containing the same volume and dimensions, my system will put out about 10% more heat than yours will. It also does not require the heat transfer fluid to be replaced every 4-5 years. In short, if you live in an area that rarely freezes, it makes good economic sense to install a direct system. I know from talking with the folks on the navitron forums that to get the same output over in the UK you end up spending more than twice what we do in Florida. Some of that is due to exchange rates and some due to our southern latitude, but it's also because we run direct flat plates and you tend to run vac tubes indirect.

  11. protech

    protech Master plumber

    Messages:
    51
    Location:
    Central Fl
    For a southern latitude, and per dollar invested, that statment would be incorrect.

  12. Alphacarina

    Alphacarina New Member

    Messages:
    26
    Location:
    Biloxi, MS
    Yeah, my system is over 3 years old now and I suspect it's about paid for itself already . . . . I got a whopping tax writeoff back when I installed it - I put in new high efficiency windows that same year and I wound up not paying any tax at all!!

    I guess the main reason that I went with a direct system is because it was MUCH cheaper and MUCH simpler . . . . and because we have water with next to no mineral content - So why not?? To buy a 'correct' 82 gallon solar tank was just a ton of money and then a bunch more for shipping. I bought all my big parts locally so there were no shipping charges. I don't recall the exact number, but I have a little less than $2500 in the entire system, and after 3 + years of operation, I'm VERY pleased with it - Assuming I'm saving the about the same $50 per month you mentioned, they've already saved me $1800 or so . . . . and I got about a $1500 writeoff, so I'm way ahead already

    My tanks are standard water heaters with a full 3 inches of foam insulation. When I was designing the system, I had planned on installing another foot of fiberglass insulation around them . . . . . but, when they are full of 150 degree water, the outside cover of the tank still feels cool to the touch, so they aren't radiating away much heat

    To be honest, I don't understand why millions more people don't have solar water heating installed - Of all the 'green' energy concepts out there, it's the most refined and perfected of them all and it also has the quickest payback of about anything else you can do

    Don
    Last edited: Sep 7, 2009
  13. flamefix

    flamefix New Member

    Messages:
    71
    Location:
    Exeter, England
    A 300litre twin coil solar cylinder stainless steel cylinder 2.1m tall and 5m2 of flat serpentine coiled high efficiency plate would cost £1442 +£2300 =£3742 = $5987 This is branded european produced with all the pumps and controls. That's full list. Vaillant/Viessmann product. Others would be cheaper maybe 20-25% and cheaper again if you look at the chinese product, maybe 30- 33%

    You'd be able to get that for upto 40% trade/ internet prices off = $3592 plus installation and pipe between plate and cylinder.

    So then plus your rebates perhaps not much more than you're spending just so you know.


    The thrust of the point I am discussing is the direct connection between the cold water provision and your store coupled to the collectors. We couldn't do that here water regs would prevent that even with backflow prevention.
  14. protech

    protech Master plumber

    Messages:
    51
    Location:
    Central Fl
    To get the same output, one would only spend about $4000 in florida before rebates and incentives. That price is for everything. Materials, installation, permit.

    Do you see what I mean about vac tubes not being economical in Florida?
  15. Mikey

    Mikey Aspiring Old Fart, EE, computer & networking geek

    Messages:
    2,714
    Location:
    Central Florida
    Hot is better

    There was a discussion in another thread arguing that hot water should be stored at 140°F or higher to ensure various disease-causing organisms are killed, rather than just suppressed. The most notable is probably Legionella.

    • At 60 °C (140 °F) - Legionella dies instantly - pasteurisation occurs.
    • At 55 °C (131 °F) - 95% die
    • 50 to 55 °C (122 to 131 °F) - Can survive but do not multiply
    • 35 to 46 °C (95 to 115 °F) - Ideal growth range
    In Quebec, a study of 211 homes (178 electric water heaters, 33 oil or gas water heaters) found Legionella contamination in 40% of electric water heaters. No water heaters using fossil fuels were contaminated. The authors concluded that, because of design variables, use of an electric water heater was the most significant factor leading to Legionella contamination in hot water in the home.

    Many references support the concept of keeping water hot in the water heater, and using tempering valves downstream to present non-scalding temperatures at fixtures.
  16. Alphacarina

    Alphacarina New Member

    Messages:
    26
    Location:
    Biloxi, MS
    If you have a modern, high tech, very well insulated tank, then keeping water stored at 140 degrees continuously might only cost you a few bucks a month, over keeping it at the lowest temperature which will satisfy your needs . . . . typically 110 to 115 degrees - But if you don't have modern equipment, the heat lost to the ambient will be large and you'll be spending lots of extra money just to be sure all water borne disease microbes are killed - One more reason to buy the highest end, best insulated water heater available when the time comes . . . . especially if you're ever planning on anything solar

    I guess it doesn't cost me anything extra, as my tanks typically get to at least 140 degrees a time or two each week without me doing anything. Needless to say, I have a mixing valve set to 115 to make sure none of that very hot water ever shows up at any tap

    Don
  17. protech

    protech Master plumber

    Messages:
    51
    Location:
    Central Fl
    I'm not sure where you are going with all of this as most solar heaters produce water above 150F. I'm lost.........

  18. protech

    protech Master plumber

    Messages:
    51
    Location:
    Central Fl
    And what exactly is the problem with a direct system again?

  19. flamefix

    flamefix New Member

    Messages:
    71
    Location:
    Exeter, England
    I am guessing he is referring to the storage of that water thereafter where at the temperatures listed legionellae can occur. And also it could occur in the collector itself during periods when it doesn't reach the higher temperatures.

    Well my understanding is that your cold water domestic supply is directly connected to the solar circuit, Which could result in back flow in the cold water supply main, and regardless of a check valve this would not be permitted here. The minimum backflow prevention that would be suitable would probably be a an air gap as formed from a header tank, with a ball cock valve arrangement.
    But then in your direct system utilising that method would mean the water pressure would be at the gravity head supply of the header tank not mains water pressure.

    There was mention of heat loss from from storage tanks at higher temperatures and I can tell you the calculated heat loss from a storage vessel used here is in the region of 1.8 to 2.4kW /24hrs from memory.

    Could you clarify the above as I thought the second statement taken from and earlier posting referred to an active system and the first statement refers to a indirect in which case aren't the two similarly priced? cheers
  20. protech

    protech Master plumber

    Messages:
    51
    Location:
    Central Fl
    #1 Legionellae can not grow in the collector or the tank because the longest amount of time the collector will go without reaching temps above 150f is about a week.

    #2 What is it that your governing body is worried about that is going to back flow from the collector? In an active-direct system it's just potable water in the collector loop. We already discussed above that bacteria cannot grow in the collector loop because of the high temps. How is it any different than having a two story house with a hot water circulation loop?

    #3 I'm not sure about the heat loss with the type of tank I install. I've never calculated it. I know that the tank temps can reach 200F and remain above 120F for about a week from experience. It's frequent that I get a service call about a week after the start of some gloomy weather (which is very infrequent here). The reason for this is because the homeowner will turn the circulation cut out temp up to high and the over heated water will trip the high limit thermodisk on the electrical backup heating. After a few months go by, we will get a spat of gloomy/rainy weather and it takes 3-5 days for the homeowner the run out of stored solar heat. That's including usage too, not just stand by losses.

    #4 No, they aren't similar in price. The direct system is far cheaper (all things considered). You quoted $5987 just for the materials. I quoted $4000 for materials AND installation. Your system is twice the cost. How is that similar?


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