All
I recently installed an on-demand hot water heater and use it for both domestic and space heating. Details are in the document at
I would appreciate your comments on this article. Please send your comments via email as I do not subscribe to this newsgroup.
Thanks! Roy Jensen
For what purpose?
Don't thank us too soon. It's hard enough to get people to read a pdf file, and then all you want are comments? Don't you have any questions? What are you trying to do and what problems do you need to solve in order to do it?
And then you want people to email you. No one will do that but me, but I don't like pdf's and I'm not going to read it.
Subscribing to a newsgroup is not as difficult as hanging from a rope and rescuing a woman from an active volcano. If Indiana Jones can do that, you can subscribe to a newsgroup.
P&M
I wonder why a guy installs a heating system, writes a whole goddamm analysis about it, and then crows about not being remunerated. Sheeit, iffin I'm gonna write a whole big shpiel about sumpn, I'd wanna be remunerated....
Inyway, after all his analysis/crowing, his own table shows efficiencies no greater than many other systems. His claim of 100% efficiency in the text is 1. impossible and 2. inconsistent with his own table, of 84-96%. Personally, I don't think any gas-fired system can come anywhere near 90%, but I could be wrong.
In addition, the btu/hr of 200,000 is fairly whopping. My near-5,000 sq ft hovel is heated by a 70,000 btu gas forced-air system, and cooled by a 5 ton (60,000 btu) system. Sep. Sears HWT, gas fired. I wonder how efficient how such a 200K system can be so efficient, esp. when the very premise of "on-demand" requires fairly high "instantaneous" energy input.
I gather that what is nice about this On-demand stuff is that you never have to worry about running out of HW, which, if you got a showering SIL like I got, can come in handy. I'm thinking of making her shower coin-operated. :)
Anyway, not saying on-demand is not efficient or a good idea, just wonder...
If I had my druthers, I'd get rid of forced air, and do a nice hot water/chilled water heating/A/C system, like what you see in some high-rise condos. Quiet, easily zoned, no big-assed noisy fan motors/duct work, etc. You then have various choices of how to heat/chill the water for the system.
Anyway, semi-inneresting read, if a little dense and hard to follow.
Mr. Jensen is obviously perty pleased with hisself. If he deigns to come back here, he should post to alt.hvac. Hope he's got thick skin. :)
I would wonder about the life expectation of a heater spec'ed for occasional use being used where it will operated a significant percentage of the time. I'm sure it won't last as long as spec'ed.
Are these unit really up to 96% efficient? When heating already warm water?
Newsgroups are for questions and answers. Not wanting to subscribe is insufficient reason to expect e-mail response.
Bob
Yeah, I have thick skin. To answer your questions:
Because I want to emphasize that it is not a biased evaluation.
The fan-coil is not listed in Table 1. The fan-coil IS 100 % efficient because there is no heat loss. Any residual heat is simply heats the room with the fan-coil.
Yep, you're wrong.
WOW, an EXECELLENT observation. Which, btw, is what got me thinking about using the on-demand unit to heat the house.
Up to 96 % efficient, according to the manufacturers.
Thanks. Where is it hard to follow?
Not particularly. Thought others might like the idea. And btw, it is DR. JENSEN. (PhD in physical chemistry, that I am pleased about!)
Roy Jensen
But as the water temp of the returning water increases, the efficiency of the water heater decreases. More heat goes out the stack because the differential is less.
Bob
Well.....
As a phys chemist, you know about Carnot efficiency, reversible processes, and alladat, and from virtually all thermodynamic considerations, the less quasi-static a process is, *necessarily* the less efficient it must be.
Now, you can do thermodynamic damage control, of course, but all other things being equal, you have *hurt your efficiency* merely by dint of the higher instantaneous energy input req'd for on-demand! Probably equal to the insulation losses of stored hot water. :)
Now, Carnot does not really apply here, as this is simple heat transfer and not a heat engine, but the same principles of quasi-staticness apply, in the form of transfer time. Thus, it is possible, in a simple heat transfer process, to get 100% heat transfer. But that requires a flue gas whose temperature is *ambient*.
How does one do that? Well, with *small* flames, low combustion gas velocity, long-assed transfer times, etc. *All inconsistent with On-demand.*
And, with one *long-assed chiminey*, jack (and yes, chiminey *is* a
3-syllable word), and one *long-assed* coil of copper tubing around sed long-assed chiminey, for heat reclamation. Which then creates draft problems, S02 problems (depending on the fuel), and just overall complexity.However, for on-demand situations, ELECTRIC energy would in fact be near-100%, as there are no flue losses. You just have to factor in the generally higher cost of electricity. The only thermodynamic consideration for electric on-demand might be the additional IR drop from high current surges. And likely shorter filament life, etc.
Note that your system here is sort of the *inverse* of the more traditional system where a hot-water tank (or coils), are driven by a hot-water or steam boiler. Here, your "furnace" is driven by the hot water heater. Which means you might have a considerably larger water mass in your HW heater than necessary for HW itself--didn't check the details.
Imo, if you are going to pump HW to a heat-transfer coil, those coils might as well be baseboard units in a room! Forced air is a pita, AND the losses from those big-assed blower fans are
*considerable*. Noisy, too. Mine is 10 effing amps, running all the goddamm time. Convert DAT to btus, bro, and calc THOSE losses!!Yes, you can argue that that energy becomes a part of the heat sent to the house, but it is "low-quality" heat, in the literal entropic sense, but moreso in the "sensible" sense, where you need heat beyond a certain temperature to create a comfort range--esp. in a low-mass/low specific medium such as effing air. So the current draw of a blow is not a total loss, just not the best use of the intended energy. imo.
Virtually the same itty bitty circulating pump feeding the fan coil in your system could feed the whole house! Wonderfully quiet, efficient, elegant. Indeed, you got more initial plumbing, but the overall efficiency, from zoning as well, proly more than makes up for it.
And, you seem willing to take the manufacturers' word on their numbers. Manufacturers are lying sacks of shit. This is like thermodynamic law. Just look at mfr's EPA claims for mpg's, and Consumer Reports' numbers.
20-30% difference, across the board.You in particular could probably well envision the experimental setups required to test all the explicit/implicit assertions in this system. Do you really think these effing mfr's go thru the trouble? Of course they don't. They fukn wing it, at your/our expense. It's called marketing. Marketing don't give a flying f*ck about facts or experiments.
Not saying this system is not worthwhile, but I think it overlooks more important HVAC/thermodynamic fundamentals than simple insulation losses of a HW tanks. It's selling point, is, I think, that it is a little different. With the nice feature that theoretically you can never run out of HW.
But, sheeeit, Jethro, take shorter showers!
Baseboard hot water heating can also double as chilled water-A/C, with a drain system built in to collect all the condensate and perhaps recycle that. A 5-ton A/C can easily produce 50 gallons/day of near-pure condensate, on a humid day. Large housing complexes indeed use this system. Super quiet, efficient, infinitely zonable.
Previous snippy comments aside, actually a very good and interesting post, altho a little hard to follow, as, first of all, you mixed too entirely different concepts, that of On Demand, and that of using the HW supply as the domestic heat source. Second, imo, I think with anything related to thermo/HVAC, you need to more clearly state the various premises, how they differ from tradition, and move along in more bite-sized conceptual increments.
DR. Violated.
As long as the combustion mixture is ideal, it doesn't matter what the flame size is. As you comment below, this is nothing more than a fancy heat exchanger. Good engineering (can't believe I just said that) using a counterflow heat-exchanger does a good job of extracting energy into the water.
Whoa! think about this Heat rises, copper coils surrounding a vertical chimney do diddly! The chimney is warm with continuous use. Horizontal mounting allows some of that heat to warm the chimney and escape into the room.
The only device I claim to be 100 % efficient is the fan coil.
The article was to emphasize a system that could easily integrate into what most homes have: forced air. I whole-heartedly agree that the same pump could be used for infloor heating, which is mentioned in the article.
All manufacturers lie. But that lie has to be based on something -- ideal conditions, most likely. Consider that some lie and quote 95 % efficiency but others can only quote 80 % efficiency for their systems. If they all exagerate by 20 %, the ODHW is still on top. Price doesn't change; the ODHW is still half the price.
As I write this sentence, I have developed a procedure to test the efficiency with a few thermometers and flow meters. It even includes several redundancy factors and checks. Now, who has money to fund it?
You don't have teen-agers, do you?
It picks up what is in the air, in a mechanical room being blown over the coils. Wouldn't call that near-pure. And near-pure water is GREAT for bacterial growth. Just ask any chemistry lab with a deionizer.
THANKS! This is exactly what I was looking for. Will post an updated version shortly.
Roy Jensen
From a chemical reaction/combustion pov, correct. From a *heat transfer* pov, absolutely incorrect. Flame size is directly proportional to gas velocity, which is inversely proportional to transfer time, and therefore inversely proportional to heat transfer per unit fuel. Thus, a system capable of pumping out 200,000 btu, to keep the same heat transfer efficiency, would have to be substantially larger in dimension than my 70,000 btu system, to accommodate 3x the combustion rate.
Thermodynamically, on-demand is *necessarily* less efficient than quasi-static (small flame) systems. Unless the system is made really large. Then, you are back to square one, cuz you now have so much volume of boiler HW, that you wind up with the same insulation losses you were trying to cure in yer HW heater!!
Yeah, but this is a trivial 100%, one step removed from where the relevant heat transfer problems exist, that is, between the combustion gas/water interface. You still have that inefficiency (shared with all heating plants), and then put the coil in hot water--really no different than steam boilers with an immersed HW coil--which in fact is an "on demand" system, around for ages, in virtually all apartment buildings.
But with questionable benefits. See above. The coil is just one step removed from the same heat transfer vicissitudes of all gas/oil fired systems.
I don't see why it would be that much cheaper. I got my sears 50 gal HW gas heater for $180. :) On sale, of course. :) :)
Not that hard. I did it, way back when. Not with air, tho, but the same principle. E = m c delta T :)
Well, not post-pubescent boys, who spend 20-30 minutes masturbating, on my HW dime. Thank gawd.... Must cost a fortune.... :)
Well, purer than lake/well water, fergodsake.
Looking forward to it. Suggest crossposting to alt.hvac. Some smart experienced guys over there, but none too flush with manners. But I'm sure this would generate an, uh, energetic thread.
Did that last week. Absolutely ZERO responses.
Roy Jensen
I am absolutely shocked and amazed--it usually takes very little for those guys to lite into someone's ass, whether or not they understand the proposition(s). Just for the helluvit.
I'm impressed!
Was it just what you posted here, or a revision?
Could also be that it's A/C season, the ones that wouuld respond are really busy. Here in the metro NYC area, it can be very very difficult to get *anyone* to come over (the whole Yellow Pages list!) during peak season.
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