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Design advice

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Telco, Feb 15, 2008.

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  1. Telco

    Telco New Member

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    Howdy folks, found this site from a solar energy site. After reading some of the posts, you folks seem to know your water boiler systems, so I'd like to ask for some design advice. I'm planning to build a new house sometime in the next 2 years or so, and intend to use hot water for all my heating needs. I'm in Oklahoma, so we don't see the super cold days the folks in the north do, and our cold snaps normally last less than a week before we see a couple days break with sunny days in the 60 degree range. The coldest I've seen here ever is about 6 degrees.

    The house itself will be a dry block construction with blocks exposed to the inside and heavily insulated on the outside, with approximately 2000 sq ft of living space. More likely it'll be smaller, but I'm using this for a planning number for now. Insulation will wrap the entire house, including under the slab. The idea will be to bring the house to a specific temp, then use the thermal mass of the walls to help maintain that temp year round. Heating and cooling will be done with an central heat and air system that uses hot water to provide the heat. Saw it on a This Old House episode where they retrofitted a 3 story house built in the 1900s with an HVAC unit using hot water. The HVAC will also incorporate a fresh air exchange and an active humidity control. I'm willing to fork over a little extra for this seeing as I'll be building the house itself with my own hands for a HUGE labor savings. The only part I'll have to hire out is the slab, plumbing, wiring, and cabinetry.

    What I'd like to do for providing hot water is build a solar collector shed that will heat a large, heavily insulated storage tank, and here's where I start to become unsure about my design. My current plan is to build a 2000 - 3000 gallon tank, and fill it with a water/antifreeze mix. This water will be pumped through the solar collectors for heating, and will be pumped directly into the house for heat use. There will just be the one loop from the tank to the house. Inside the house the loop will split into two. One loop will put hot water into a heat exchange loop to heat the potable water, the other loop will provide hot water to the HVAC system. I also want to have an emergency backup since the sun can go away for days at a time, and I intend on a third loop that only runs through a wood fired boiler for this. All the pumps in the system will be operated by a solar power system with generator backup, off the main house grid.

    What I need to clear up is:

    - Would it be better to pump the antifreeze solution (fluid) directly from the tank to the solar collectors or would it be better to use a loop to keep the collector fluid separate from the storage fluid? I'm pretty good about keeping track of antifreeze conditions and will be using a non-toxic antifreeze (want something I can just drain on the ground if necessary, not that I would unless it was an emergency) with anti-corrosion protection.

    - How large of a storage tank do I need? Once I get the water up to temp, I'd like for it to be able to go 2-3 days in the winter before needing to fire up the boiler, if necessary. I also want to be able to maintain a constant 79 degrees in the winter, and be able to run 6 people through the shower every morning before the sun comes up. This is a worst case scenario, as the house will be occupied by 2 people and we tend to keep the house cooler than this.

    I've already gleaned some pretty good ideas from this site, for example I now plan to put the storage tank against the house next to the garage, and have the tank room vent into the garage in the winter to use the radiated heat to keep the cars warmer than they might be otherwise. Be more comfy on the morning drive to work, less warmup time for better fuel efficiency, ect. I'm also planning to use a few more inches of insulation around the tank than I was, and am going to reexamine the flat plate collectors instead of the evacuated tube collectors. I was going to go all evac tubes since they actually collect light instead of heat so can heat water with no direct sunlight, but now might either go for a mix or when I install the evacs build them inside of a flat plate style case along with a straight copper pipe to heat the case and help melt snow quickly. I figure a single pane of glass with no UV blocking qualities would heat up quickly once the water from the tank started pumping through, and I'd be able to get right on collecting sunshine earlier without having to sweep snow every morning.

    OK, be brutal with this plan, I'd much rather get a little grief now before I've set the first block than find out when the place is done that it won't cut the mustard. Any other ideas for this are welcome too, other than in-floor radiant heat. I just don't like the idea of running water through the floor inside of a plastic tube, because if the line is broken for whatever reason the floor has to come out and there's an instant multi-thousand dollar bill. Thanks.

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  2. Eric Johnson

    Eric Johnson Mod Emeritus

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    Welcome to the Boiler Room, Telco. I'm glad you've found some useful information.

    I can't address all your questions, but I'm sure they will all be addressed by people who actually know what they're talking about, shortly.

    Offhand, my only comment is that I would scrap the idea of running 2,000+ gallons of glycol mix. At a 40% (minimum) mix, that's some big bucks down the drain. Think more in terms of heat exchangers with that much water, if you can't engineer your way around the freeze potential. Infloor radiant is usually your best bet for hydronic heat.
  3. Telco

    Telco New Member

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    Wow Eric, you ain't kidding. I didn't even consider that. Just did a quick check around and found that 1000 gallons of ethylene glycol could be had in 55 gallon drums. Each drum is just over 1000 dollars, and the 18 I'd need to get roughly a 50/50 mix on a 2000 gallon tank would run about 19 grand, plus delivery. That's way more than I want to pay for this. Hmmm, was hoping to be able to just leave the coolant in the collectors every night, maybe have a small pump run the coolant in a closed loop at night just to keep it moving, but this puts a new face on things.

    OK, new thought. Looks like the big loop mess inside the holding tank might be the better way to go, since I'd just have to have antifreeze in the collectors and distribution piping instead of the whole thing. The potable water loop would also be a lot simpler since I'd not have to do a second small loop next to the heater to keep a definite separation between the drinkin' water and the heatin' water. I'm a bit paranoid about it, but I don't want any contact at all between what I drink and what I pour in my radiator. A 2000 gallon tank the way I want to do it would probably wind up being about 2500-2800 gallons of total capacity. Since the house and water storage rooms will be very close, and once it's in operation it isn't going to cool down again, it's doubtful that I'd need a second antifreeze loop between the storage and the house.

    Now on the flat panel collectors, it looks like the home made ones have just plain copper pipes in a grid going across a black background underneath a piece of glass. My questions on that is, has anyone tried going with way oversized copper tubing hammered flat to give a larger cross section to the sun, or some other flat collection surface, instead of just round pipes? And will a plastic glass like plexiglass work instead of glass glass for hail survival?

    Another question, has anyone used oil instead of water for this sort of thing? Oil will hold a lot more heat and is far less likely to freeze, and likely would never need replacing. Since oil can hold a lot more heat, it would need a smaller storage area for the same duration. Of course the boiler would have to run for days to bring the initial temp up, but once it was there it would be there forever. Comments?
  4. Eric Johnson

    Eric Johnson Mod Emeritus

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    You seem determined to sink a fortune into your heat transfer medium. Oil is not only expensive, it's a potential environmental nightmare. As a practical matter, there is no better heat storage/transfer medium than plain water. Check out flat plate heat exchangers. They allow you to run glycol in your solar panels and transfer the heat into the rest of your system with great efficiency and little cost. With a few cheap pumps, some piping and a couple of heat exchangers, you can do everything you envision in both an relatively cheap and environmentally responsible manner.
  5. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    Water is pretty good for heat storage. Oil will out-gas as it heats up, and you won't like the chemicals you will be breathing.

    As far as the heat transfer medium to the solar collectors, I'll recommend spending a few bucks on Tyfocor instead of plain antifreeze. It was specifically designed for solar use, and doesn't break down at the temperatures that solar collectors can generate. For the 5-10 gallons in the solar system, it's worth the premium price. Just don't dilute it with water - it's shipped pre-mixed.

    Joe
  6. sled_mack

    sled_mack New Member

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    Ask your solar provider about drain back systems. When the pump stops, everything gravity drains back to you tank. No need for anti-freeze.

    If your tank is right next to the house, and properly insulated, you should not need to put antifreeze in it, I don't think. Mine has just plain water. Besides, I stop using it at 120 because that is not enough to provide the heat I need for the house. From there, it takes weeks to cool down to the point of freeze up danger. You may need to just run the pump from the tank to the house occasionally to prevent the lines from freezing.

    Don't rule out radiant just yet. There are many ways of putting in radiant that don't involve having the tubes in concrete. Some would argue that they are better, since they have less thermal mass. There is a product called Roth Panel, for example, that goes on the concrete, the tubes get put in the panels, and then the flooring goes over top. The panels are made of insulation, too. I didn't use these, as they didn't fit my situation, but I would have liked to use them. I have radiant in most of my house now, and I wouldn't be without it!

    My 2700 gal tank will carry the house for 3 or 4 days when we are away. The house is 2400 sq ft, and parts of it are not insulated very well. With solar helping to maintian the tank, something about that size should carry you for 3 days. You need to do a heat loss calc to be sure.
  7. Telco

    Telco New Member

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    Heh heh, actually I'm determined to NOT sink a fortune into it. It's far cheaper to ask about many possible options than to try them.

    So, I can rule out oil then, and it sounds like a heavily insulated 2500 gallon tank will do what I want. The house itself will be very well insulated with not a lot of windows so I'm not too concerned there. If the tank will carry me 5-6 days instead of 2-3 days, so much the better.

    I'll look into the drainback system, if the main thing is it requires that the panels be higher than the tank it won't be an issue, I can just build a stubby wall to put the low point of the panel higher than the tank. Otherwise, it'll probably be the copper loops with the solar antifreeze premix. Kinda like the drainback though, sounds as if it would eliminate a lot of problems by just draining the lines every night when the sun isn't out.

    EDIT -And Sledmack is the winner! I like what I see with the drainback system. No antifreeze at all, no issues with overtemp, more efficient heat transfer, no freezing issues, no power usage at night, with the only negative being the panels will have to be built higher and the lines will need to be designed to not allow any low points. Should not be an issue at all since the panels will be on a shed, I can just lay the panels at an angle, and have the inlet and outlet pipes go vertically straight through the roof. This actually looks like it'll be simpler and cheaper than I originally envisioned.

    Still not gonna do radiant though, I like central heat and air too much and already have that part of it worked out. If a hydronic HVAC can heat a circa 1900 3 story house built when insulation was not even a word yet, then it'll work fine with the place I'm putting up.
  8. heaterman

    heaterman Minister of Fire

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    Ahem!!!

    Do NOT use ethylene glycol in a heating system. It's the wrong stuff. For regular hydronic systems the correct product is Propylene glycol. It's more stable over time and it's non toxic. Check out the websites for No Burst or Cyro-Tek.

    That being said, there is a third animal out there that is designed for use in solar based systems. It will tolerate temperatures commonly found in solar devices (>300*) without break down. It is expensive but it's the only product you should use as propylene and ethylene will both break down and turn into gorp at those temps. I would say that it's safe to say you will see temps in that range with a solar collector in Okie land. The stuff you need is about $150+ for a five gallon bucket.

    By all means use the glycol in just the solar side of things not only for cost reasons but also for the fact that you lose energy transfer capability as you add it to your system. Glycol also requires much more energy to pump than regular old H20 which is after all the heat transfer medium that God Himself designed for our use. :)
  9. Telco

    Telco New Member

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    BUMP! Been thinking on this and have a couple more questions. Since water boils into steam at 220F, how hot can these solar water heaters get the water? Would it be better to use the loops with a propylene glycol mix to take advantage of more sunshine? I'd hate to build a system that has to shut down halfway through the day, then require the backup boiler to be run.

    Next question, saw mention somewhere about using paraffin wax as a heat storage medium. I understand it would cost more than water, but aside from cost what would be the benefits and drawbacks to using paraffin wax to store heat instead of water? With water the highest I could store would be about 210 degrees unless antifreeze were used to increase the boiling point, seems like I might be able to store more heat in a given volume of wax than of water. Not finding a lot of info on this.

    Next question, how hot can a water pump normally handle and still have long term (think years) reliability? Anything I need to worry about like increased power usage with higher temps?

    Thanks for any info. This board has already provided much valuable info, even the negative stuff :cheese:
  10. slowzuki

    slowzuki Feeling the Heat

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    I'm interested too, I know you can design the max temps via the flow and how much heat you take out at your tank. Say I was directly heating the floor of my shop via solar, with a 70 F return temp to the collector, I would have to have a low flow rate to get to 300F.

    I know you have to design relief valves so if the power fails on an active solar system it will relief when it boils.
  11. wdc1160

    wdc1160 New Member

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    real quick, boiling happens at 210 at sea level pressure. normally 14.7 psi. I keep my boiler and system at 30 psi. Boiling goes up to 260. There are options. And, your pump can take the beating. Mine is rated for 250. Most have spec sheets. Some are rated for 350 for short durations. GL Bill

    PS I don't run it at 260F I run it at 210. Just so I don't get hate messages about how I am endagering my dog.
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