Solar Hot Water / Heating...

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #62
    Re: Solar Hot Water / Heating...

    Also, from what I have read, the last solar hot water fad crashed badly after massive government funding first brought out new companies (good and bad) that installed product then went under after the funding was pulled. And left a very bad taste with consumers because of poorly designed/installed units with zero support.

    Virtually killed solar hot water for 30 years. I remember the high pressure sales men coming to my parents house pushing this stuff (in a heavily wooded valley with a marine layer of fog 4 days out of 6--where solar made no sense ever). I still walk by installations every day from those times that have not worked for two+ decades.

    And, by the way, where do you think all of that "free government money" comes from?

    Anyway, back to your question--I would go by price per BTU (or kWhr) of heat you get from that unit. It appears to be a modified form of a painted black hot water tank set behind glass. Cheap to build, cannot use in areas with freezing weather, and not too efficient at gathering heat. However, there are no large hot water storage tanks in the house and no electric pumps or antifreeze required--so should be low maintenance.

    I would probably stay away from it as it would not efficiently recover. For example, use 1/2 of the hot water in the panel. Now, 1/2 of the water pipes are cold--absorbing heat--but the other 1/2 are already hot and either not absorbing heat or actually cooling down a bit and heating the cold water in the other 1/2 of the collector.

    It may work fine--but unless it was very inexpensive ($$/BTU), I am not sure that I would use it on my home (I have natural gas hot water--so take my experience--none--with a grain of salt).

    And, don't fall into the trap of expecting to use extra solar PV electricity for hot water heating... It is way too expensive. Rougly it costs you about $0.50 per kW for solar hot water panels vs ~$5.00 a watt for Solar PV Electric Panels. Plus, you only need about 1/4 the roof space for solar hot water panels of similar Watt rating (solar hot water is much more efficient than solar PV). Yes, it will work--but it is not cost effective.

    Lastly, I would think twice about using small electric hot water tanks at faucets. It is a nice way of getting instant hot water at a faucet--but you will have to heat up a gallon or two of cold water electrically every time you turn the faucet on (first use after pipes cool). You will be paying for the electric heating of your hot water--when, instead, you wanted to use solar heat whenever possible (to save costs and energy).

    The better place to install the backup hot water is near (or with) the solar hot water storage tank(s). One method is to use the top element/thermostat of a standard electric water heater connected to a solar hot water collector/pump system. If the solar has the tank warm, the electric heat stays off. If enough hot water is withdrawn, then the top element turns on to make up for the lack of sufficient solar heated water.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • System2
    System2 Posts: 6,290 admin
    #63
    Re: Solar Hot Water / Heating...

    Didn't mean to get off on politics - just looking in my crystal ball.

    So, let's check this out. In Tucson, summer heat is unbearable - right now it hits 100 degrees daily (in the shade). Just lay anything made of metal out in the sun for a few minutes and you can't touch it. So, I would think recovery would not take too long in the summer - so the electric heaters would be off.

    In the winter or periods of heavy use, what will it cost to heat 2.5 gallons of water? 1200 watts for 10 minutes looks like 1.2 kw per hour divided by 6 (60 minutes divided by 10 minutes) equals .2 kw to take one shower. I think Tucson electric rates are $.11 per kw, so $.11 x .2 = $.022 to take a shower. What am I missing here? It seems to me that constantly heating a big 40 -80 gallon tank can't be cheaper regardless of the gas or electric power source. I am not convinced that gas tankless heaters are the answer either, given the purchase price, need for frequent cleaning to remove scale, and complicated computer controls.

    If $.022 per use is correct, then figure 4 showers per day and 3 kitchen sink uses per day - that equals 7 recovery cycles total X $.022 X 30 days per month = $4.62 per month for water heating. The laundry would have to run only off the solar hot water heater (I only use hot water for white clothes.) Figured as a PV panel load when the solar water heater is not used, then we have .2 kw x 7 uses = 1.4 kw per day (assuming absolutely no help from the solar hot water heater - which is unlikely). Is that anexcessive daily drain on a typical grid tie, net metered setup?

    Thanks.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #64
    Re: Solar Hot Water / Heating...

    tucsonjwt,
    if you install all of this, let us know how well it works out for you.
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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #65
    Re: Solar Hot Water / Heating...
    In the winter or periods of heavy use, what will it cost to heat 2.5 gallons of water? 1200 watts for 10 minutes looks like 1.2 kw per hour divided by 6 (60 minutes divided by 10 minutes) equals .2 kw to take one shower. I think Tucson electric rates are $.11 per kw, so $.11 x .2 = $.022 to take a shower. What am I missing here?

    Assuming 95% efficiency, it takes 2.5 gal x 8.33 lbs/gal x 60 F / 95% = 1,315 BTU / 3412 BTU/kWh = 0.38 kWh. At $0.11 / kWh, it’ll apparently cost ~4.2 cents to heat the 2.5 gals of water for the single shower.

    Without taking sides on the issues, I think part of the concern is that the real “cost” of supplying the energy – i.e., the environmental cost, both near and far – is far more the 4.2 cents involved with the energy production and distribution.

    Finally, I was going to include a gentle reminder about respecting the forum’s rules re political posts. However, that “Rules” topic seems to have been deleted from the “Announcements” section, so I can’t cite that topic. Nonetheless, the intro to this "Energy Use and Conservation" area does still provide some guidance.

    It's my view that one of the great features of this forum has been that we’ve done a very good job of objectively focusing on technical issues and solutions. We’ve also done a very good job of staying out of political issues and away from personal attacks. I hope we keep it that way.

    Regards to all,
    Jim / crewzer
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  • System2
    System2 Posts: 6,290 admin
    #66
    Re: Solar Hot Water / Heating...

    Crewzer - I actually timed the recovery of the 2.5 gallon heaters, and it is exactly 10 minutes for the entire 2.5 gallon tank. So, I don't see why any other calculations would be necessary - just 10 minutes worth of electricity at $.11 per kw. At the rate of 1.2 kw per hour (1200 watt consuming unit), I would think I could multiply $.11 x 1.2 and get $.132 per hour, then divide that by 6 (60 minutes per hour divide by 10 minutes = 6). $.132 divided by 6 = $.022 per use. Multiply by 7 "uses" per day = $.154 per day - assuming no help from the solar water heater. Is this calculation wrong?

    The real question is how much load would be required from solar panels instead of grid electricity and is this load practical?

    Drawing at the rate of 1.2 kw per hour (1200 watt consuming units), I get:

    .167 hours x 1.2 kw rate of load = .20 kw per unit per use X 7 uses per day = 1.4 kw per day from solar panels. Since this would be a grid tie system, would I have to add for system inefficiency loss?

    So, how much PV panel capacity would it take to power my 2.5 gallon water heaters 7 times per day? That's really the only question.

    If 1.4 kw per day is not much draw for a PV system (assuming that my calculations are correct), then maybe I should skip the solar heater altogether and just run the heaters off the grid, with the PV panels paying the cost of electricity to run the heaters. I am guessing about a net $2500 to install the solar water heater (after tax breaks). Would this equal the cost of installing more PV panels to generate 1.4 kw per day? I think we get about 6 hours of good sunlight in Tucson, so is that 1.4 divided by 6 or about 234 watts of panels? I think I actually prefer the PV heating of the water, since there is less plumbing, holes in the roof, chance of freezing (yes, it does get below freezing a few time per year in Tucson.)

    Thanks for the input.

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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #67
    Re: Solar Hot Water / Heating...
    Is this calculation wrong?

    Tusconjwt,

    I'm afraid the answer is "yes". For the purposes of this discussion, the energy (kWh) for which you pay $0.11/kWh is the mathematical product of power and time. You've got the time component (10 minutes, or 1/6 hour), but how much power is drawn by the heater? My calculation above suggests the power requirement is 0.38 kWh / 1/6 hour = 2.3 kW.

    Assuming ~12% cell efficiency, PV generated power is really not a particularly cost-effective way to heat water. Solar water heaters are much more efficient (~40% to 60% or so), and they currently qualify for a federal tax incentive that's in addition to and separate from the PV credit. 8-)

    HTH,
    Jim / crewzer

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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #68
    Re: Solar Hot Water / Heating...

    "Is this calculation wrong?"

    try thinking in terms of btus. a btu is the raising of 1lb of water 1 degree f in one hour. it matters not that you'd do this in a shorter time as it still takes the same amount of heat energy and thusly the same amount of electricity to do it. assuming you'd raise it from an ambient water temp of about 70 degrees to about 105 degrees, this is a 35 degrees difference. now there's 8lbs per gallon if i remember right so that is 8x2.5=20lbs of water. multiplying them out gives us 35x20=700btus. each watt/hr of electricity is 3.413btu/hr. 700/3.413=205.1watts. at $.11/kwh x .2051=$.02256 confirmed.
    jim,
    i believe you may have divided when you were supposed to multiply. 1/6 x 1.2kwh = .2kwh x $.11/kwh = $.022. it is only running for 10 minutes in that hour to replenish so it is 1/6th of an hour times the total rate of consumption you'd see if for an hour of 1.2kwh, which is 200watthrs.
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  • System2
    System2 Posts: 6,290 admin
    #69
    Re: Solar Hot Water / Heating...

    I am aware that heating water with electricity, especially with PV generated electricty, is not cost effective.

    Question 1: How many total watts does a 1200 watt device consume if it operates for 10 minutes?

    Question 2: How much PV panel does $2500 buy? (My net out of pocket on a solar water heater is $2500.)

    Question 3: Will that $2500 worth of PV panels power my little water heaters? (Guessing 1.2 to 1.4 kw per day).

    I will hook up the kill-a-watt meter to the 2.5 gal water heater in my trailer and let it cycle through 24 hours, then report the actual watt consumption. This will include the slight reheating that occurs during the day to keep 120 degrees in the tank.

    I am now leaning toward forget the solar water heater if I can get a PV for about the same price - the PV panel should also generate net metered electricity for general household electricity use when the sun is shining - something a solar hot water heater won't do. Remember, many months in Tucson are blazing hot, so I would just get a lot hot water that is not used, as opposed to some electricity that I can use. The sun even shines most of the time in the winter.

    I think I need to demonstrate to you that these little heaters require tiny amounts of electricity compared to the big storage tanks or even the "tankless" gizmos. We should forget about the heating water aspect of these things and just look at the power consumption of the devices - like "how big a PV panel do I need to run a refrigerator consuming 1.4 kw per day?"

    Thanks for the feedback - I am learning - slowly.



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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #70
    Re: Solar Hot Water / Heating...
    Question 1: How many total watts does a 1200 watt device consume if it operates for 10 minutes?

    A 1200 Watt device operating for 10 minutes "consumes" 1200 W x 1/6 hour = 200 Wh of energy, or 0.2 kWh.
    Question 2: How much PV panel does $2500 buy? (My net out of pocket on a solar water heater is $2500.)

    ~450 W or so... See: http://store.solar-electric.com/hiposopa.html
    Question 3: Will that $2500 worth of PV panels power my little water heaters? (Guessing 1.2 to 1.4 kw per day).

    Assuming 5 "hours" of full sun (December daily average in Tuscon) and 60% overall system efficiency, the 450 W array could produce as much as 450 W x 5 hours/day x 60% = 1.35 kWh/day.

    HTH,
    Jim / crewzer
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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #71
    Re: Solar Hot Water / Heating...
    jim,
    i believe you may have divided when you were supposed to multiply. 1/6 x 1.2kwh = .2kwh x $.11/kwh = $.022. it is only running for 10 minutes in that hour to replenish so it is 1/6th of an hour times the total rate of consumption you'd see if for an hour of 1.2kwh, which is 200watthrs.

    niel,

    Nahhh... I believe it's correct. My original estimate was based on a 60 degree F temp increase (i.e., from 60 F to 120 F). A 2,300 W heater running for 10 minutes would "consume" 2300 W x 1/6 hour = 383 Wh, or 1,308 BTU's, of energy which, assuming 95% efficiency, is about what's required to increase the temperature of 20.825 lbs (8 gallons) of water by 60 degrees F.
    a btu is the raising of 1lb of water 1 degree f in one hour.

    Not exactly. A BTU is the amount of energy required to raise one lb. of water one degree F.
    each watt/hr of electricity is 3.413btu/hr.

    Energy is Watt-hour (Wh), not watt/hr. Each Wh is 3.412 BTU's, and each Watt is 3.412 BTU/hr. :wink:

    I'm tired!!
    Jim / crewzer
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  • System2
    System2 Posts: 6,290 admin
    #72
    Re: Solar Hot Water / Heating...

    Jim,

    Thanks for the answers.

    The heaters are only 1200 watts each, not 2300 watts each.

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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #73
    Re: Solar Hot Water / Heating...
    keep 120 degrees in the tank.  ...The heaters are only 1200 watts each

    Tusconjwt,

    OK… Let’s run a bit with this info.

    A 1200 W heater powered for 10 minutes will “consume” 200 Wh of energy, or ~682 BTU’s. Assuming 95% heater efficiency, that’s enough energy to raise the temperature of 2.5 gallons (~20.8 lbs) of water by ~682 BTU’s x 95% / ~20.8 lbs = ~31 F. In order to hit the 120 F temperature target, the baseline water temperature has to be ~89 F.

    I typically use 60 F as a baseline temp for municipal water, although the annual average in my neck of the woods appears to be ~63 F. However, Tuscon Water’s website indicates that “The water at your tap can be 85 degrees or higher and can be affected by your private plumbing.” If the water supplied to your heater is indeed “85 degrees or higher”, that may well confirm your recent observations.

    But, it’ll take longer and therefore more energy to heat cooler water. For example, if your incoming water is ~60 F in the winter, the 1,200 W heater will need ~20 minutes, and ~1364 BTU’s, to heat 2.5 gallons of water to ~120 F. So, with an averge of 6 to 7 hours/day of "full" Sun in the summer, ~85 F water, and a 450 W PV aray, you may well be able to heat plenty of water for your summer needs. But, with less Sun in the winter (~5 hours/day) and colder water, the system will deliver only ~35% as much hot water.

    HTH,
    Jim / crewzer
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #74
    Re: Solar Hot Water / Heating...

    quoted by crewzer:
    "niel,

    Nahhh... I believe it's correct. My original estimate was based on a 60 degree F temp increase (i.e., from 60 F to 120 F). A 2,300 W heater running for 10 minutes would "consume" 2300 W x 1/6 hour = 383 Wh, or 1,308 BTU's, of energy which, assuming 95% efficiency, is about what's required to increase the temperature of 20.825 lbs (8 gallons) of water by 60 degrees F."

    it may have been except the 2300w you made up as it was only said to be 1200w. now as far as a gallon goes 128oz is one gallon and at 16oz per pound that is exactly 8lbs per gallon x 2.5 = 20lbs. so where does this extra .825lbs come from? as to the baseline ambient water temp i had guessed at that as i don't know what it would be in his state and if it is not buried very deeply or at all, it very well could be as high as 80+ degrees as the water will absorb or lose its heat from its surroundings before it gets to the water heater and will lose very little due to the ambient air temp inside of 79 degrees if i remember correctly. adding 35 degrees to 79 is 114 degrees which is still lower than the 120 degree mark mentioned, but did anybody measure this to be the actual temp output of the heater or was it a rough guess?
    in any case he admits it to running 10 minutes at a rate of 1200watt-hours making the total draw 200w and correct at his rate of cost for it to cost him 2.2 cents for each cycle of the heater. it is not a crime for you to admit you goofed jim as we aren't perfect.

    quoted by crewzer:
    "Energy is Watt-hour (Wh), not watt/hr."
    "Not exactly. A BTU is the amount of energy required to raise one lb. of water one degree F."

    i was wrong in the way i abriviated, but yes, watt-hours is what i meant .
    oh, and as long as you are nit-picking for correctness you are still wrong as it is the amount of energy required to raise one lb. of water one degree f in one hour. :-P
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #75
    Re: Solar Hot Water / Heating...
    ..as it is the amount of energy required to raise one lb. of water one degree f in one hour.

    BTU is the amount of energy to raise 1 pound of water's temperature 1 degree F.

    There is no time involved for BTU. Burners, for example, are rated in BTU/Hour--i.e., 50,000 BTU/HR.

    [BB covers head and runs for the hills] :-P

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #76
    Re: Solar Hot Water / Heating...

    hmmm. you guys are right about the per hour. this crow isn't cooked all of the way. :-o :cry:
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #77
    Re: Solar Hot Water / Heating...

    British vs MKS units are aways a laugh--we even had, a few, in engineering classes and at work.

    Watt*Hours=k*BTU's is kind of like talking about MPH*Hours = Distance

    One I laughed at was a program I used to convert PCB layout files to AutoCAD... Guy was careless with setting the Z axis (should have been zero inches) to a dimension that was orders of magnitude of the size of the known universe measured in inches. I could not figure out why I could not select some of the objects.

    Another was when a contractor designed a cad sheetmetal model for our computer chassis in inches--but had the cad program set to millimeters. We laughed a lot about that--a 96 port switch the size of a postage stamp (turned out to be one of many errors in the design)... The (unpaid) vendor was not near as happy.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • System2
    System2 Posts: 6,290 admin
    #78
    Re: Solar Hot Water / Heating...

    My 10 minute recovery time was at install in early March - probably average high 40 to 50s degree F ambient temperature average +/-. I will see what we get today. My guess is not much difference because I don't think the thermostat is that precise. Also, the water flow into the heater would not normally be at night, when temperatures are the coldest, and I would assume that the heater could be wrapped with some sort of extra insulation (reflectix?) to minimize the heat loss during the non-use period most of the day.
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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #79
    Re: Solar Hot Water / Heating...

    I suspect the recovery time would be based more on incoming water temp and load rather than ambient temperature. One way to really test the system would be to turn off the heater, empty all of the hot water from the tank and replace it with fresh cold water, measure the temp of the incoming water, and then turn the heater back on. I estimate it'll take about 1 minute for every 3 degree difference between incoming water temp and 120 F.

    HTH,
    Jim / crewzer
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  • System2
    System2 Posts: 6,290 admin
    #80
    Re: Solar Hot Water / Heating...

    I see that the Killawatt is showing some power consumption even though I have not seen the power indicator light on the water heater go on, and I have not yet taken a shower.
    To avoid the incoming water temperature variable, and these small all day reheating power draws, install a timer to go on just before you plan to take a shower. If you shower at 6:00 am, then set the timer for 5:45 a.m. You are then heating ambient room temperature water, because that water has been in the heater and normalized for about 24 hours -probably 73 degrees F to my desired water heater setting of 120 degree F setting. Is this logical?
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  • System2
    System2 Posts: 6,290 admin
    #81
    Re: Solar Hot Water / Heating...

    A thought about insulation
    Years ago, when I was working away in "the city", I had put an additional foot of insulation around, and over the top of my 25 US gal / 20 CDN gal electric water heater. I was thereafter rather surprised at how long the water would stay hot. The heater would be turned off Sunday morning, I would have my shower that evening, head out to work for the week, arrive home Thursday evening and still have enough warm water for another shower without turning the heater back on. This brought me to the conclusion that with the extra insulation, it just wasn't worth shutting off the heater for periods as short as a day. Wonder what an extra 2 feet of insulation would have done?
    You don't have to use energy to replace heat that has not been lost.
    Wayne
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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #82
    Re: Solar Hot Water / Heating...
    Is this logical?

    In general, I would think so.

    But, I’m biased, because that’s in effect how my whole-house water heater is set up. The water heater’s thermostatic elements are controlled by a timer that connects power from 4 AM to 8 AM, which meets morning shower requirements, and then again from 3 PM to 7 PM, which meets laundry and evening kitchen requirements. There’s manual override for high-demand conditions. This set-up has worked very well for my family of four for over 10 years, and there are usually 16 hours out of every day when we're not keeping a 50 gallon tank of hot water on stand-by.

    I believe that this sort of demand heating strategy is still popular in Europe and Asia. For example, it’s not unusual to find a small water heater installed over a bathtub, and water is heated just before it’s used for a bath.

    A possible drawback of this general approach is that the water heater’s useful life might be shortened due to mechanical stresses caused by the thermal cycling. However, my water heater’s temperature limit is 120 F, and, at 11+ years old (it was installed by the previous owner), it seems to be working just fine.

    You may have to tinker with the timer settings. For example, you might want it to shut off the heater before you start your shower so that it doesn’t start to heat the incoming water. Also, you may find that you’ll need a longer heat cycle in the winter when the incoming water is colder.

    HTH,
    Jim / crewzer
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  • System2
    System2 Posts: 6,290 admin
    #83
    Re: Solar Hot Water / Heating...

    Unless I go with a larger central storage tank, I won't have room for a foot of insulation because the point of use water heater will be installed under the sink, but it sounds like a good idea.
    I would have the timer set to power on the water heater and shut off just before shower time. Then the ambient room temperature during the day would normalize the temperature of the water in the tank, thereby making incoming water temperature not much of a factor.
    I thought about heater element failure due to excessive cycling, but if I eliminate all cycling during the day, which the water heater normally does to maintain the set temperature of 120 degrees, then I think I might actually have less cycling overall.
    Extremely hard water in Tucson will kill the water heater elements faster than cycling, so I think I might have to invest in a water softener.
    Can you describe the brand/model of timer you are using?
    Thanks.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #84
    Re: Solar Hot Water / Heating...

    The best method to save energy with an electric water heater is probably adding the thermal blanket--as thick as you can (or use those metalized "bubble wrap" blankets if there is not enough room.

    Also, remember that in terms of energy usage--the volume of water store goes up with the "cube" of the size of the water heater while the surface area only goes up with the "square"... So, as an example (ignoring losses through water pipes), a 20 gallon water heater has less surface area for heat loss vs seven 2.5 gallon point of use water heaters.

    But--on the other hand, if you have lots of water pipes--then insulating those will save heat too. Also, losing ~1-2 gallons of already heated then cooled water, just to get hot water at the tap again is an energy cost too (say a kitchen sink were lots of short hot water usages during the whole day).

    Doing things like using a small point of use hot water heater connected to the cold water pipe instead of the hot water main tank would seem to save quite a bit of energy too--assuming that a small PoU hot water heater supplies enough hot water for local use (without connecting to the main hot water heater). In my old house, I toss more heated/cooled "hot" water warming up the pipes than actual "hot water" I actually use during washing.

    I understand the issue of hard water with an electric hot water heater... We have pretty soft water here--but I take care of a small apartment building for relatives with electric hot water (built 50 years ago and uses electric hot water so the tanks are installed inside the units without venting)--and when I am replacing elements/heaters, it is surprising the amount of heating element shaped mineral build up inside the hot water heaters. I can never flush them out through the normal drain--I have to pop out one of the elements and drain through the large opening.

    I can see that using Point of Use electric hot water heaters for, say a bathroom sink (or for somebody that wastes very little water during the shower) may actually save more money than installing and using a main gas water heater (natural gas being ~1/2 the cost to heat water in our area). In my case, I have two kids that love to take long showers everyday--so volume usage favors a gas water heater in my instance.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • System2
    System2 Posts: 6,290 admin
    #85
    Re: Solar Hot Water / Heating...

    touscon

    Your idea about timing the heat for the shower is a good one. I have a heavy duty clock type timer in the house which you initiate (crank on for 30 minutes, like a bathroom fan switch) to preheat the generator in the winter...without leaving the house. This block heater is 900 watts and there's no problems. The switch is rated for 1500watts, what's the water heater's rating?

    You just mount the switch in a regular electrical switch box and find a cover plate to suite your decor (mines utility galvanized). Crank the switch, the heater is on and you don't accidentally forget to shut it off. If the teen's showers are too long, they'll learn the time limit fast!

    Ralph
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  • System2
    System2 Posts: 6,290 admin
    #86
    Re: Solar Hot Water / Heating...

    This whole POU concept does not work if people must take long hot showers. If I had hot water hog teenagers I would definitely use this method - allocation of 2.5 gallon hot water would teach conservation and benefit of Navy showers very quickly. (They can always wait 10 minutes and take another hot shower, if they get off on it that much.)
    The units are 1200 watts.
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  • System2
    System2 Posts: 6,290 admin
    #87
    Re: Solar Hot Water / Heating...

    Results of science experiment, from Killawatt readings:

    .14 kw power consumption to take one shower with 8 minute recovery time. (Note kw reading, take shower until cold water runs, let water heater power up until it shuts off, take kw reading again.)

    .51 kw power consumption for 24 hour period with water heater on for the full 24 hour period.

    Normally, I really don't use all 2.5 gallons of hot water. Wet down, press water saver button on showerhead, lather up shampoo in hair, lather up washcloth and body, release water shaver button, rinse - done - about 2 minutes and maybe 1.5 gallons total water consumption, using hot water set at 120 degrees only.

    While this plan will probably work for bathrooms and kitchen sink, hot water for the washing machine and dishwasher remain a concern.


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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #88
    Re: Solar Hot Water / Heating...

    Tusconjwt,
    Can you describe the brand/model of timer you are using?

    Click on the embedded “timer” link in my June post above… it’ll take you to Intermatic’s page on water heater timers. Looks like I’m using the model WH-21.

    For a load such as your 1200 W heater, you might consider smaller plug-in HD timer like one of these:

    http://www.intermatic.com/Default.asp?action=subcat&sid=115&cid=43&did=3
    http://www.intermatic.com/Default.asp?action=subcat&sid=114&cid=43&did=3
    Results of science experiment, from Killawatt readings:

    .14 kw power consumption to take one shower with 8 minute recovery time. (Note kw reading, take shower until cold water runs, let water heater power up until it shuts off, take kw reading again.)

    OK. It sort-of sounds like a ~1,200 W heater: 0.14 kWh energy / (8 mins / 60 mins/hr) = 1,050 W.  0.14 kWh is the energy required to increase the temperature of 2.5 gallons of water by ~23 degrees F. I’m wondering about the accuracy and/or location (hot spot?) of the tank’s thermostat.

    So, another experiment might be to heat the water, shut off the heater, take your shower, and leave the heater off until the next day. Then, turn the heater on and see how much time and energy is required to heat the water just before your next shower.

    HTH,
    Jim / crewzer
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #89
    Re: Solar Hot Water / Heating...

    quoted from crewzer:
    "So, another experiment might be to heat the water, shut off the heater, take your shower, and leave the heater off until the next day. Then, turn the heater on and see how much time and energy is required to heat the water just before your next shower.

    HTH,
    Jim / crewzer"

    that sounds like a good idea, but you will need to know the temperatures of the water to be of real use. do you have a way to measure the water temp?
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  • System2
    System2 Posts: 6,290 admin
    #90
    Re: Solar Hot Water / Heating...

    Looks like those Intermatic timers would do the trick at each heater intallation, and maybe $11 per unit for the 120 volt models covering my 1200 watt maximum load.

    Since the heater recovered in 10 minutes in March (fairly cold for Tucson), and 8 minutes in a 73 degree June environment, I think 8-10 minute is a realistic power load time range for these units, especially if I let the water acclimate ~ 24 hours by using a timer. That would put the water in the tank at a constant room temperature (73 degrees +/-). Killawatt showed about 1162-1180 actual watt draw when it was drawing power (forgot to check actual voltage, but it's always less than 120.

    I've pretty much decide on these units in every bathroom and kitchen sink, but now I have to come up with something for the washing machine and dishwasher. Looks like I might be headed to a smaller solar hot water heater (preheater) to cover that demand. Or possibly an electric demand heater just for those uses. I guess I need to find out how much hot water a washing machine and dishwasher use. Any thoughts?

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #91
    Re: Solar Hot Water / Heating...

    There are electric "demand" or "instant" hot water heaters that can handle a shower or washing machine... The complaint about those (other threads here) is that they take a lot of power (kWatts worth) and the big problem is that they cycle on/off/high/low all the time while water is flowing to keep the output temperature constant.

    And the problem with them cycling is that this causes the home's lights and such to go bright/dim with the cycling of of the heater. And even folks with very good utility power and large breaker panels--still have the problem.

    I would suggest that you install the appropriate sized tanked electric water heater for the washer (if you don't go solar). If you get a smaller water heater, you may have room for lots of insulation. A well insulated water heater is pretty close to 100% efficient.

    The other way of getting hot water would be a heat pump A/C and ground sourced heat pumps -- some manufacturers offer the option of using the heat pump to also heat hot water (almost free for A/C season, and better than "100% efficient" with ground sourced heat pumps.

    Another option--my Fisher Paykel (www.fisherpaykel.com) clothes washer only uses a few gallons of hot water and sprays soap/hot water over the cloths for a few minutes before filling the rest of the tube with cold water.

    In the end, I try not to go overboard here... I am paying $5-6 for electricity per month (gird tied solar) and $15-$50 per month for gas vs $600-$900 per month for private health insurance... Got to look at those other bills and not fixate on the trees and missing the forest.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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