Calculating Propane Usage and BTUs for Tankless Hot Water System

Discussion in 'Tankless Water Heater Forum' started by Diavolicchio, Dec 11, 2009.

  1. Terry

    Terry Administrator Staff Member

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    I don't know how efficient some of the exchangers are for reclaiming energy,
    I did install a few in the 70's, some would reclaim from commercial washers to preheat the incoming.
    A lot of it was done from sketches on paper by hand and was left to me to figure out how to do it.

    That was a long time ago though.

    I've always liked the idea of air heat exchangers.
  2. Runs with bison

    Runs with bison New Member

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    The key thing that the GFX hit on was the falling film design. It makes perfect sense for resolving the problems of a gravity drain with venting concerns and the like. This maximizes the heat transfer coefficient on the drainwater side while minimizing the potential for plugging.

    I haven't had the opportunity to see one of these units in action. I'm skeptical of manufacturer's claims and would like to see their real world performance, especially since short ultra low flow showers and piping lag and lost volumes would hammer the overall performance.
  3. Diavolicchio

    Diavolicchio New Member

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    20
    Location:
    Union, Maine (Midcoast) USA
    Bison:

    I've found a great woodburning boiler stove for the living room (here's a better photo) that will easily heat the whole house as well as provide a 50,000 BTU/hr boiler (as opposed to the 16,000 Btu/hr boiler with the ESSE) to pre-heat the water in the storage tank. I'm just not sure what kind of a bump-up in temperature this will give me in the hot water tank if I'm starting with a full tank that's already hot from the boiler loop and bleeding the tank at the rate of 15 gallons/minute for 30 minutes at a time.

    I'll talk to the folks at TriangleTube on Monday and have them do some calculations for me. If I'm able to get the cold water that's replenishing that tank at 15 gallons/minute to warm up by at least 10 degrees before leaving the tank, I'm happy with that. A 20 degree increase would be optimal though. I just don't know what 50,000 Btu/hr would be able to do using the technology behind this particular tank. From what the TriangleTube website says, these tank-in-tank systems heat up MUCH faster than the ones with a coil-based heat exchanger. Check out this quick Flash demonstration for the TriangleTube tank and let me know what you think, would ya? It's an intriguing set-up.

    If I'm able in the cold months to get the additional 20 degree increase that I'm after from the boiler, two 30 minute/450 gallon showers in a given day will cost an average of $2.40/each in propane (which is the current cost per gallon for propane in my part of the country.) Personally, I think they'll be worth every damn nickel.

    Incidentally, without the GFX and wood boiler pre-heat they'd cost over $7.00/each--nearly 3X as much.


    John
    Last edited: Dec 13, 2009
  4. Doherty Plumbing

    Doherty Plumbing Journeyman & Gas Fitter

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    Yep. I'm a ticketed gas fitter not just some guy who searches google to come up with answer about a field he knows NOTHING about. Your math was wrong plain and simple. Not by a lot but still wrong none the less.
  5. Diavolicchio

    Diavolicchio New Member

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    Location:
    Union, Maine (Midcoast) USA
    I'll give it 10 minutes and this guy is going to start pulling someone's hair and calling him a doodyface.


    John
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    IF you're going to have multiple heating sources for an indirect (and even just one), it's good to put a tempering valve on the output to limit the temperature of the output so you don't get scalded. The only thing you don't want to do is ensure you don't get the water so hot you could start to get steam..that could be catastrophic.
  7. Diavolicchio

    Diavolicchio New Member

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    Thanks. I really appreciate the advice.


    John
  8. Runs with bison

    Runs with bison New Member

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    Your lack of even the most basic knowledge on piping losses indicates the exact opposite: you comment on things you know NOTHING about.

    You did it again with your assinine comment about that parallel GFX set up arrangement.

    Excuse me if I'm unimpressed at your attempt at a pissing contest. I'm just a chemical engineer accustomed to operating on a much larger scale, chemical plants and refineries. I've designed, modeled, troubleshot and operated various equipment, reactors, exchangers, distillation vessels. etc. I'm accustomed to using natural gas, propane, ethylene, propylene, ethane, hydrogen, carbon monoxide, O2, etc. as reactor feedstock...as well as for simple combustion.

    Now, if you want to point out where my "math was wrong", please go ahead. I don't claim to be perfect. (You might also be taking something out of context...that's my guess...I'll wait and see if there is any substance to what you say.) If I have gotten something substantially incorrect then it should be corrected.
    Last edited: Dec 13, 2009
  9. Runs with bison

    Runs with bison New Member

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    I agree.

    Diavolicchio

    I looked at the flash presentation of the tank and it still isn't clear how this is going to work into the preheating. You will start with a very hot tank so you would have to do a mix control valve to achieve some specific preheat target I suppose. Otherwise it's hot water would be used first, then the big tankless burners would kick in several minutes later. Residential boiler's are an area I don't have experience with. The tankless folks might have some experience working these in.

    Even at 50,000 Btu/hr the steady state rise (once the tank was depleted) would be ~7 deg. F.

    The project is far from the beaten path and that makes it interesting.
  10. Diavolicchio

    Diavolicchio New Member

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    Location:
    Union, Maine (Midcoast) USA
    I was just intrigued with the particular slide (the one entitled "tank-in-tank: the advantages") that showed how the tank-in-tank system preheated the same amount of water as the coil-based cylinder tank but in 1/3 the time because of the exceptionally large heating surface area inside the TriangleTube tank.

    If each shower requires 450 gallons of 110F water, and the first 100 gallons/shower are already at the proper temp inside the tank, I'm trying to identify the quickest solution for preheating the remaining 350 gallons still to enter the tank using energy solely from the wood boiler. The GFX should then give it an additional 24 degree boost prior to it hitting the pair of tankless water heaters. I guess I'm hoping that the TriangleTube technology will have some kind of edge over the tanks with the traditional coil heat-exchangers to allow me at least an extra 10 degree of preheating before the water hits the GFX.

    I'll run this all by the folks at TriangleTube tomorrow and will let you know what they have to say.


    John
    Last edited: Dec 13, 2009
  11. Thatguy

    Thatguy Homeowner

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    1 therm = 14# of wood at 100% efficiency.
  12. Diavolicchio

    Diavolicchio New Member

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    Location:
    Union, Maine (Midcoast) USA
    I've had to re-design this system today, mostly because of one factor I hadn't taken into account: the potential for Legionnaire's disease.

    My original plan was to have a 119 gallon indirect-heated water tank heated to around 110F by a boiler loop from the wood stove. This however is also within the ideal temperature range that promotes the growth of the bacteria that causes Legionnaire's disease. The water tank would only be heated to that temperature if I were home with a fire going in the boiler stove, nevertheless there could be circumstances when that water could be sitting in the holding tank for a couple of days at 110F not being used. The tank I'd planned on using really should only be used when it's connected to an energy source capable of getting the water up to 140F.

    I also learned today that the GFX system I'd planned on using was about 50% more expensive than I was anticipating, so I've chosen a smaller (and less robust) model. I'm no longer going to be using a water tank that is preheated with a boiler loop, but am simplifying to a 119 gallon uninsulated pressurized well water tank. I will be keeping the two original Noritz tankless water heaters, but scaling back from taking two 30 minutes showers daily, to two 20 minutes ones.

    One aspect of the original design I have decided to keep is the ESSE Ironheart stove with the 16,000 BTU/hr boiler loop. It's a pretty weak boiler for preheating a tank of water, but it's sufficient to accomplish three things for me six months out of every year:

    It'll provide the BTUs necessary for three hydronic radiators: 1) in the home office (7,440 BTUs); 2) in the basement mechanicals room (2,860 BTUs); and 3) on the enclosed back porch (5,430 BTUs.)

    If I'm able to keep the ambient temperature in the mechanicals room at 75F, I'm betting both the uninsulated well water tank and the copper GFX coil (when not in use) should stay around 75F, increasing their effectiveness. I can't help but think that all of that copper in a 55F room is going to take a bit longer to preheat the water going through it than the same copper would in a 75F room. The new GFX system will be comprised of two identical 80" columns exactly like this, connected at the top by a copper manifold. It will supposedly have the capacity to increase the water temperature 17 degrees (rather than the 24 degrees of the previous GFX) before it enters the pair of tankless water heaters, requiring an average annual rise in water temperature of 42 degrees (from 68F to 110F.)

    How do these changes affect the final numbers? The components of the shower/hot water system are less expensive by about $3,500, but the cost per shower goes up accordingly. My two 20 minute showers/day (300 gallons each) will now require a total of 2.4 gallons of propane/day at a cost of around $2.80/each (as opposed to the original $2.40/each for 30 minute showers.)


    * * * * *

    Does anyone see a danger in having a mechanicals room heated to 75F by a hydronic radiator?


    John
    Last edited: Dec 14, 2009
  13. Runs with bison

    Runs with bison New Member

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    What about the GFX pricing changed so much? Install or the units themselves? The payout for me just isn't there because my flows and durations are a tiny fraction of yours. I've used your links to examine the heat transfer coefficients vs. my flows and the cheapest unit still exceeds 10 years payoff for me.

    Legionaire's is a reasonable consideration. I would not consider an under 120 preheater as an all day control point. At 120+ a fired unit will easily prevent legionaire's proliferation. From what I've gathered from technical reports, electric tanks get into trouble at ~120 F setpoint (versus gas at the same setpoint) because they often scale and have sections well below 120 F. A natural gas heater is better configured to retard/prevent legionaire's growth at 120 F.

    A colder GFX coil will transer heat more easily from the drain. Don't waste heat on the utility space. I've been trying to minimize my losses there by sealing duct gap air losses and insulating ducts. I've cut them in half, putting heat/cooling where it is needed, not in the utility room. It was a waste having utility space running cold in summer, hot in winter.

    Honestly, I always considered the 1 hour shower at 15 gpm and 110 F to be very extreme and well past what I figured you would actually find comfortable. 15-20 years ago I considered a multihead shower to be ideal...and arranged showerheads to do this in college which is where I got the idea...so I know what you are getting at...I've experienced it. It was awesome. But I think the same effect could be acheived at less than 40% of the flowrate today. Efficient distrubtion can give you more bang for the buck.

    What sort of flow and design shower temp are you targeting?
  14. Diavolicchio

    Diavolicchio New Member

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    Location:
    Union, Maine (Midcoast) USA
    The folks behind the GFX units I'm considering (www.gfxtechnology.com) don't publish a price list for the more customized units they make. I knew the pricing of their basic off-the-rack models from their website and made a rough guess as to what the more elaborate models might run. I was off. Plus, with the volatile price of copper, the cost for these units probably changes on a quarterly basis.

    I'd never planned to take a 1 hour shower. It's always been two 30 minute showers, one in the morning and one in the evening.

    I've designed this system on the basis of using it to the extreme, just to cover my bases. The 6-head shower I'll be installing will have a digital thermostatic valve and digital interface so each shower can be 'designed' and pre-programmed, on the basis of which, when and how many shower heads it uses, as well the flow rate through each and how long a shower lasts. Anyone using the shower can customize and pre-program their own.

    It defeats the purpose of having a programmable system like this and ignoring it by only showering at full bore. Most likely I'll still end up taking 30 minute showers, but programmed in such a way that I'll end up using an amount of hot water comparable to a 20 minute shower running at capacity through all six heads.

    For the record, this isn't about getting clean; that's just a side benefit. It's about having a killer water massage twice a day. Pure hedonism.


    John
    Last edited: Dec 15, 2009
  15. Dana

    Dana In the trades

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    Natural Resources Canada has spent quite a bit of money chasing the answer to that question. They developed a standardized test based on 2.5gpm flows and maintain an apples-to-apples comparison of some vendors' models here:

    http://oee.nrcan.gc.ca/residential/personal/retrofit-homes/questions-answers.cfm#q44


    Percentage recovery will be higher with lower drain flow, but total recovery rate in BTUs/hr increases with increases in either drain & potable-side flows.

    Heat transfer efficiency increases with surface area, so taller fatter (and to some extent, squarer cross section on the potable wrap) is more effective than smaller bore drain & shorter length.

    More detail than you ever wanted to know about their early test methods can be found here:


    http://gfxtechnology.com/NRCAN-6_29_07.pdf


    There's more elsewhere if you want. Basically, at 2.5gpm a 4" x 48" or a 3" x 60" yields about a 50% energy return at shower drain temps. If you have the space, 70% is possible with the tallest fattest PowerPipe.

    Most of the manufacturers will give you the raw data at other flow rates & temperatures, if you beg hard enough.
  16. Diavolicchio

    Diavolicchio New Member

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    Location:
    Union, Maine (Midcoast) USA
    I've been speaking with the folks at Power Pipe too, in the hopes of getting a system from them just as effective for a little less money (or more effective for the same money.)

    I'll post how things price out once I've gotten my quote back from them.


    John
  17. Dana

    Dana In the trades

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    Beyond heat recovery issues, keep flow in mind too. High flow apps, look at how much head the potable side represents too. Some models have quite a pressure drop at 5+ gpm. The PowerPipe design seems inherently higher flow than some of it's predecessors, but development has not stopped. Ecoinnovation, vendor of the Eco-GFX line (not to be confused with GFX Technology) claims to have a lower cost high efficiency solution (as yet untested by 3rd parties. The preliminary data look pretty good for the money. (Don't be surprised if they start bad-mouthing the competition if you get 'em on the phone though- this industry has a whole lot o' back-biting & patent fighting issues.) See: http://www.ecoinnovation.ca/residential-solutions/
  18. Runs with bison

    Runs with bison New Member

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    That EcoInnovation site is amusing in that some of the pdf's have not been completely translated from Quebecois French ("fonction") and have the euro style comma/decimal inversion.

    On a more substantive note, 3/4" tubing coils vs. 1/2" is a key criteria for whole house flows. One of the things I liked about the S series GFX design was that they took a small recovery performance hit on the longer units by using two coils on the single drain tube to provide parallel flow path for the supply. Powerpipe takes this a step further apparently by doing a 4 parallel tube path...but it looks like those might be 1/2" flat faced tubes.

    The EcoInnovation savings comparison was suspect because of the water heater efficiency used. They should have been using an AFUE, not the efficiency factor which includes storage losses.
  19. Dana

    Dana In the trades

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    Yeah, I've had "issues" with the president of EcoInnovation's sometimes overzealous marketing (and his screaming about his competitors), which is why I'm awaiting 3rd party test results to show up on the NRCan list.

    Full disclosure: I went with a 4" x 4' PowerPipe in my system. I haven't instrumented it to verify effieciency, but it presents far less head to the DHW flow than a tankless HW heater(!). I'd have gone with a taller one if I had the space.
  20. Diavolicchio

    Diavolicchio New Member

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    Location:
    Union, Maine (Midcoast) USA
    I'm talking with Joel and Francois specifically at Power-Pipe. They seem to be good guys and eager to help people make this technology work for them.

    The major expense seems to be going with a copper manifold. Yet I can't wrap my head around going with PVC. These beautiful copper columns wedged into a PVC manifold look like gold detailing on a Yugo.

    I'm having them work up a quote for a three-column system using 80" columns and sticking with the copper manifold. I'll post details once I've got them.


    John
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