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How UN-green and ignorant of you!Originally Posted by Nate R
My primary reason (16 years ago) was tub-filling, followed by space savings, followed by longevity & low catastrophic leak potential compared to tanks. With standing pilot ignition and a 78-80% raw combustion efficiency it was no super-saver but it probably paid for itself in fuel savings in that time frame (and if history at this house was any guide, I would have been replacing a tank before now anyway.)
I only retired the tankless when I retired the 5x oversized cast-iron boiler, to be replaced by a Takagi KD20 tankless, used as a wall hung boiler, serving a reverse-indirect to buffer newly implemented micro-zoning and provide DHW.
Were I totally kermit-complected I might have run a mod-con instead, but the calculated fuel savings of the higher efficiency didn't go NPV+ within the anticipated lifetime of the mod-con. (Lessee, a $400 surplus tankless, or a ~$2800 mod-con, that's a $2400 cost delta... at $80-150/year annual fuel savings, assuming 3% discount rate... nope, still negative at 20 years)
And the tankless gives me WAY more first-hour gallons than an appropriately sized mod-con would. Without running it even as high as 1/2-modulation, with a drainwater heat exchanger feeding the indirect I get "endless hot water" at a ~4gpm gusher in showering mode, and can still comfortably fill the tub using the thermal mass of the indirect, maintained at a modest 130F (heating temp required by the radiation on design day, so WTF, use that temp for the DWH and forget about it). Even when pulling heat out of it at ~1.5-2x the design day with an air handler coil (the 2nd stage of a 2-stage zone) showers run forever with the tankless, but might eventually become tepid with a tiny mod-con.
So, with a tankless running as a boiler I can waste all the hot water I want- SUCCESS!!
And if the miserable Takagi beast craps out in only 5-10 years I can replace it with another $500 tankless (maybe even a cheap condensing version will be available by then) and still be $ ahead. If the price of fuel quadruples in the short term I can revisit the present-value of a mod-con against future savings, but the indirect/buffer and will stay integrated with the heating system (return water from radiation at the bottom of the tank is well in to condensing temps under a real heat load, as well as when under heavy DHW load. It'd average over 90% combustion efficiency, if not over 95% but would suffer very low cycling losses.)
But the last thing I'll be buying is another standalone tank, eh? ;-) (Not that there's anything WRONG with that...)
Take my advice, unless you can centrally locate the TWH close to each of the facets then don't do it. It will only frustrate you waiting for the hot water and/or waiting on the sandwich.
We made our acceptable with a recirc loop that turns on automatically when the HW is used but it weren't for that we'd be very unhappy.
I've never had any "cold sandwich" problems with my Takagi TK-2.
Is it perfect, no. Does it fill my large tub and allow me to have 3 showers going at once? Yes. It may get replaced in the future but I have to get something large enough for that occasional tub fill. I'm currently looking at different solutions as we added onto our house and it's too long for one heater. It takes a long time for the water to reach the addition from the old location.
The coldwater sandwich effect varies with make & model, the temperature of the incoming water, the temperature of the incoming combustion air, and the sensitivity of the user. Some have inordinately long startup times which makes it dead-obvious, but none of the big 3 (Noritz, Rinnai, & Takagi) that I've used have truly egregious models.
Old-skool 78-80% standing pilot versions have NO discernable coldwater sandwich (no flue purge, no ignition delay= no problem!)
TK2s use room air for combustion, so if it's in a 65-70F house the chilling effect of the flue purge & resulting coldwater sandwich is dramatically LESS than it's twin-sister the (sealed combustion direct-vented) KD20 in mid winter when it -5F out, but about the same in the warmer months. (Me, I don't care- my KD20 is only sending those 6-8 ounces of cooler water to a 48 gallon buffer tank of ~130F water, never to a handwashing faucet. :-) )
The coldwater sandwich issue is real, but not nearly as obnoxious in most cases as the tankless critics make it out to be. (It's a princess & pea thing, IMHO.) The flameout & loss of temperature regulation at very low flow in warmwater areas is a bigger deal, but even that isn't very significant (rare, if your lowest modulated input is 20K or less.)
My TK-2 is in my unfinished basement at a fairly constant temperature. It's not anywhere near the temps of 65-70f but it's not near outside freezing temps either.
I'd love to hear about your setup if you don't mind sending me a PM. I'm needing some hot water changes soon.
I'd appreciate advice in my situation on whether to go with a tank or tankless.
My water tank is outside in a metal shed, with outside temperatures at the lowest in winter just around freezing (32 degrees) to a high of a little over 100 degrees in summer. My water heater is located directly next to the gas meter, which the gas company doesn't like because it lies within three feet of their meter (flame source in case of leaks at the meter, I guess). Tankless prices are about $2000 to $2500 for a reputable installer installing a takagi, noritz or rinnai, utilizing one of their outdoor units. We have two bathrooms, just my wife and I live there with the occasional visitor. We don't have a high use water item except for a regular tub or shower. I think I can at most squeeze in a 50 gallon tank with 1 inch of insulation around it or a 40 gallon with two inches of insulation. Right now we run the old hot water tank to about 135 degrees in the 10 year old tank we use right now in order to supply us with enough hot water for our needs. A new one hopefully would not need to be run that hot. Until recently, I didn't even know you were supposed to drain the tank once a year. It has never been drained. What the he** do I do?
Given your relatively low water use it's unlikely that you'd ever recoup the upfront cost differential between tank/tankless in fuel savings unless natural gas sees some serious fuel price inflation over the next decade.
Still, efficiency bang for upfront $$ spent, in Glendale CA you're likely to do much better with low cost batch-solar pre-heat like the Harpiris SunCache or their competitors, and a well insulated electric tank. After CA and federal incentives the installed cost would be similar to or less than a fossil-fired tankless, and with the extremely low standby losses of an electric tank (as compared to a NG or propane tank), you'd see a greater than 50% reduction in annual water heating costs. Most of the year the electric tank would be drawing zero watts except during periods of heavy water use, as the water entering the tank woud often be hotter than what's stored there (particulaly if you timed your water use well.) The downside is, that under VERY heavy use the recovery period would be fairly long. A more expensive alternative without recovery time issues would be to use an electric tankless in conjunction with batch pre-heat (but that may require an electric panel or service upgrade.)
With either of these scenarios it would need a tempering valve/thermostatic-mixing valve at the output of the electric heater since there will be times where the batch heater is well into the "scald me now" temperature range.
Thanks for the quick reply Dana. I didn't think of a Solar pre-heat. More research. Lucky I am not time pressed at the moment. I actually thought that because I was outdoors, my standby losses were so great that tankless would pay for itself. If not, there seems no reason to do so.
Your standby losses due to being outdoors are higher in winter, lower in summer, but the average annual temp in Glendale/LA is about 65F. Even though peak days the standby can be substantially greater than non-peak, on average it's pretty much a wash- a 5F increase in delta-T over having it inside, for about a 8-10% increase in average standby. If you're currently burning 250therms/year in a 0.60EF tank it might bump to 260 or so (not more) by installing the tank outdoors. Going tankless would drop you to 175-225 therms per year, but with batch solar pre-heat you'd drop to about 50-100 therms, maybe less.
Standby losses for gas fired tanks are several times higher than electric tanks due to convection from the central flue/heat exhchanger and necessary clearance gaps in the insulation. If you opt for solar pre-heat you may get more benefit out of it with an electric tank, but a forced-draft higher efficiency tank might still be cheaper to run, depending on your utiltiy rates. Even if you burned as much as 75 therms/year (50 in standby loss) it might still be cheaper than heating the solar shortfall with electricity. If water heating is your primary gas use during spring/summer/fall (and it likely is), you can probably get a good handle on how much fuel you're using, and what your likely savings/costs would be with different options.
Thanks, Dana.
That's the kind of information that I need.
Thanks to all for the your advices! You share very useful thing for me
PEX or not PEX
Great post. Thanks for all the info...a lot of food for thought here. I haven't installed one myself, but the owner of my company is trying to push these, due to government special tax breaks...I hadn't considered a lot of the things you mentioned at all, it seems you have a valid argument against these things...at least for the time being...
Did you notice anything missing from the list? Condensing tankless is nowhere to be found. With PVC 636 venting of condensing tankless, I wonder what the installation costs comparison will look like.
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