is tankless feasible / advisable in upstate NY?

Hello, all. I've been reading through the archives for the tank and tankless water heater forums trying to decide how to spec a new water heater. I have a similar post on the Tank forum, but I thought I’d explore the question on this side as well.

We’re in Rochester, NY where the winter groundwater temp hovers between 40 and 45 degrees. It’s currently just me and my wife, but kids are a definite possibility in the near-term. We’re in the process of a master bath remodel and are installing a 100-gallon soaking tub, which replaces an existing 85-gallon jetted whirlpool tub. Our current heater is a 13 yr old 50-gallon natural gas tank (State, I think). It still works but is probably nearing the end of its useful life. The current water heater just barely filled the old tub, so I don’t expect it to do the job on the new tub. The new tub would be likely used a few times per month on average. So the question is tank or tankless. We heat with GFA, so indirect / boiler options are out. I’ve long been drawn to the tankless concept, but much of what I’ve read that’s relevant to our location and usage raises a lot of red flags. I’m contemplating a bid for the Rinnai R75LSi tankless with an installed cost (after NYSERTA rebate) of roughly $2,200. We’re also replacing our HVAC system which is using up the entire $1,500 federal credit so none of those dollars can offset the water heater.

My question for this forum is whether the Rinnai R75LSi tankless would be advisable for us? From a water flow standpoint I’m assuming my worst-case scenario supply (dead-of-winter, 80 degree temp rise from 40 to 120) would be 3.8 gpm. That’s sufficient for a single shower and perhaps some simultaneous hand-washing, given a 2.5 gpm shower head. I calculate 105-degree water at the shower to require 81% hot water (120 degrees) and 19% cold (40 degrees), so the hot water flow with a 2.5 gpm showerhead is 2.03 gpm (did I do that right?) If we put a low-flow shower head in the 2nd bath we could eek out 2 simultaneous showers on the coldest day of winter. In the summer when ground water temps are 50 degrees or so we’re looking at 4.3 gpm maximum flow. That’s enough for 2 showers or a shower and perhaps a load of laundry (not sure about the water flow for the washing machine – what’s a good assumption for a 10-yr old Maytag top-loader?). The tub filler flows at about 4gpm on hot only, so it looks like nothing else is happening when we fill the tub.

A few other factors – our water (Monroe County Water Authority) is “moderately hard” at between 5.7 and 7.7 particles per gallon. I understand that annual flushing / descaling is required – would we need to do so more frequently given our water hardness? Would some type of pre-heater filter help? We don’t have and won’t add a water softener.
One final question – what happens when you exceed the flow rate? Say we’re filling the tub and someone jumps in the shower – does the temperature drop off or does the pressure drop? A less powerful 105-degree shower may be tolerable. A colder one is not.

I’ll stop with my rambling and let those who know better than I weigh in. Does anyone else in a similar climate and household demand situation use this or a similary sized unit? To what effect?

I appreciate any suggestions or feedback you can offer.

Thanks!

Rob

180K input tankless works fine in coldwater states, as long as you don't need to run two showers simultaneously with other loads in the middle of winter. I lived for years with a 125K unit in Worcester MA, with midwinter incoming water temps in the high 40s. I'm now heatin my house & hot water with a single 180K tankless, but with a 48 gallon buffer tank with internal heat exchanger. I can run a single shower all day long with a full heat load and it keeps up, but with 2 it would fall short when the heating is running.

A water softener is advisable, but annual descaling would still keep up at 5-10 grains hardness.

Dana,

I appreciate your responses to both of my posts. I was still processing your response to the tank post and thought I'd thow the topic open to the tankless forum also. Based on your tankless response I take it you're a proponent of tankless. What's your experience / opinions about some of the other "shortcomings" that are often attributed to tankless - such as the "cold water slug" and the loss of efficiency for small-volume pulls (handwashing, etc)? Also, I don't know what the forum's view is on pricing discussions, but if it's not verboten does $2,300 for install sound reasonable? It's lower than a lot of the figures I've seen bandied about on the forum. Since my tank options are higher volume (75 gal) or higher output units rather than the standard 50 gallon tank I think the cost / breakeven equation may be different than the usual tank vs. tankless discussion.

Thanks.

Rob

I forgot to mention, the shortfall is only on 2 very long or continous shower flows while all heating zones are calling for heat, but part of the instantaneous load is being supported by a drainwater heat exchanger. Without the heat exchanger 2 shower flows alone would be about it. Bumping up to a 199K tankless would give you some margin, but installing drainwater heat recover would give you more margin. My return from the DWHR is almost but not quite enough to cover a peak heating system draw with ~5-6gpm flow into the drain from showers. DWHR works great for showers, but not at all in batch draws like washers & tub fills.

Quote:

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Originally Posted by RobRing

Dana,

I appreciate your responses to both of my posts. I was still processing your response to the tank post and thought I'd thow the topic open to the tankless forum also. Based on your tankless response I take it you're a proponent of tankless. What's your experience / opinions about some of the other "shortcomings" that are often attributed to tankless - such as the "cold water slug" and the loss of efficiency for small-volume pulls (handwashing, etc)? Also, I don't know what the forum's view is on pricing discussions, but if it's not verboten does $2,300 for install sound reasonable? It's lower than a lot of the figures I've seen bandied about on the forum. Since my tank options are higher volume (75 gal) or higher output units rather than the standard 50 gallon tank I think the cost / breakeven equation may be different than the usual tank vs. tankless discussion.

Thanks.

Rob

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I'm not sure I could be categorized as a "proponent" of tankless. From a sheer economics point of view the rationale isn't a no-brainer. A couple-three grand installed price isn't unusual. At $2.3K it must mean that it's easily side-vented, and you're not paying for a lot of stainless vent. (In some instances it's cheaper to go with a condensing tankless & plastic vent vs. an 82-85% unit w/ stainless vent.) I'm using my current tankless as a boiler, running non-potable heating system water in a loop to the tank- the potable water is inside the heat exchanger coils.

The coldwater sandwich issue is overstated for any of the first-tier tankless vendors (Rinnai is up there.) Poor temperature modulation or flame-out at low flow is an issue primarily for warmwater areas, ortankless units that don't modulate under 30K for coldwater areas. The Rinnai 75 goes down to ~15K.

An 0.84EF unit will actually only deliver ~75% as-used efficiency due to the short-cycling of short draws, but it'll deliver the full 84% for big draws like tub fills or showers (anything over 3 gallons, typically.) Are you going to worry about that, compared to what you get out of an 0.58EF tank?

Since you're replacing your HVAC system at the same time (presumably a forced hot-air furnace with an AC coil in the air handler) have you considered running a hydro-air system using a hydronic coil in the air handler instead of a furnace? That way you could run the heating & hot water off the same burner- you'd get a kickback for going with an indirect on the hot water, and another for going with a high-efficiency boiler. It's pretty easy to get north of 90% true efficiency out of a condensing boiler & air handler, but it takes small bit of design. (My "hail Mary" backup 2nd stage on my otherwise radiant floor heating system is a hydro-air coil in an air handler.) A condensing boiler + hydroair air handler + indirect may be cost competive or somewhat more than your condensing furnace + tankless, but probably not by huge amounts. Then you won't be peak-flow limited on your hot water (the indirect's capacity can handle the intermittency of high-flow on the washing machine, etc.), and the standby loss of an indirect is a fraction of a 50 or 75 gallon tank. A 75-80K condensing boiler with a 40-50 gallon indirect would yield similar hot water output to the Vertex 100. An even bigger condensing boiler would blow it away, but size the burner for your peak heating load, size the indirect for your tub filling needs with a "right-sized for heating" boiler behind it, zoned for HW priority. (If it means you need 80 gallons of indirect, you can bump the boiler sizing a bit, but make it as small as you can for highest efficiency.)

Hydro-air system? Huh?

Wow I'm suffering from information overload. I assumed (wrongly, I guess) that since I started with GFA I needed to stay with GFA. If Hydroair works with the existing ducts, roughly how much would it cost to provide the equivalent heat output of a 80K btu 95% gas furnace? Would I still need a separate AC unit?

Rob

80K condensing boilers run about $2.5-3K, with identical venting issues as condensing furnaces. To get the 95% efficiency out of 'em you need to use an air handler with a coil than can deliver ~75KBU or more at 140F (a commonly specified temperature, but you can run 'em lower temp for higher boiler efficiency I run mine at about 125F, at which the exit air temp at the furthest supply register is ~105F.). If you want/need AC in the same air handler, you need to spec the AC coil appropriately for the compressor & refrigeratnt used. This level of design is best left to an HVAC pro, but it's not rocket science either. A really simple-minded AH with an old-skool single speed blower might run you about a grand, a super-smart one with continuous variable air flow ECM drive motor & smart controls could be 2.5-3x that, or 2-speeds somewhere in between. Add a few hundred for pumps expansion tank, etc, then another grand for a the indirect.

Then it's all what the HVAC pro wants to charge you to design it and hook it all up.

If you haven't already, insist on a computer generated heat gain/ loss calculation based on all of the measured particulars of your house (window area & type, wall area & Rvalues etc.). It could well be that the 80K gas furnace is oversized, but with a 2-stage condensing furnace the inefficiency of oversizing was small and could be ignored. As a rule you'll be more comfortable and get better efficiency out of a smaller unit that runs with longer duty cycle, but it has to still keep up on the 1-2% coldes hours of the heating season if you always want to be warm enough. Most heating systems in the US are oversized by 2x or more. At 2x the hit in efficiency & comfort is slight, but at 3x and up it's quite noticable.

I'll give you a bit more: Unless you pulled out the flow restrictor, you ain't gettin' 2.5GPM. In fact with system pressure losses adn the fact that heads are rated in a lab at 80PSI (and you most likely have around 60PSI) You'll be lucky to be getting about 1.5GPM at the head. Also, you can't shower at 120*, you'd get cooked. You shower at about 106* so your mixing some cold in as well.

That said, the R75LSi will easily give you 2 showers +! You'll be able to fill the big tub but of course assuming it piped for max flow, you'll be maxing out the tankless while your doing it. no big deal because once it's full it's business as usual. Plus since you can set the Rinnai for 104" fill the tub up with just hot water and get right in, you'll save water and time trying to get the temp right!

You'll be just fine and love it.

Scott: A tub fill at 104F is gonna be a bit tepid, but it might be OK (not screamin' hot) for showering. Fills into a cast-iron tub need to be 110-115F to feel "hot".

If you assume two 2.5gpm showers going at 104F and mid-late winter incoming water at 44F that's 60F delta-T at 5gpm, which it 150KBTU/h, which is over the full rated output of the R75lsi by about 2%. If you assume the reported "...between 40 and 45 degrees" groundwater temp and 106F the peak load with 40F incoming water will have a delta-T of 66F, which at 5gpm is 165KBTU/h, a shortfall of ~12% , (but only ~1.5% below the full output of a typical 199KBTU/hr tankless.)

Yes, you can dial back the flow to create some margin, and most of the year incoming water will likely be above 44F, but it's a stretch to say "...the R75LSi will easily give you 2 showers +! ". It's more like " ...the R75LSi will give you 2 showers + most of the year, and you'll be happy if you understand and manage it well, but a 199KBTU/hr tankless will be more likely to fill the bill." To assume people are going to run their showers at 1.5-2gpm is a big assumption, with too many counterexamples to be able to count on it.

You missed a couple points. 1. You don't get 2.5 from a 2.5gpm head. "If you assume 2.5gpm showers...you will assume incorrectly. 2. High limit on a jacuzzi is set at 104* mandated by our heros at the US Gov. Get in a tub and take the temp with a good digital thermometer. Not one of those cheap dial things. You'll see. Same with Showering. In fact, you can take a "hotter" shower because you are not imersed in the water. It is an aereated flow which makes it "feel" cooler. You don't have to "dail back" the flow. Your standard system has already done that for you with pressure and flow losses.

I get the CI tub thing due to the fact that a CI tub will need to be "heated" by the hot water and I can not argue the temps because I have not tried that, however most folks no longer have a CI tub either. About 5 years ago I left the paper and math world for the real world. I quit "guessing" based on paper charts and began personally measuring what was going on in the field at field pressures and feild temperatures. Including the geeky actions of taking a bath and shower with a digital thermometer. I started learning stuff that isn't in the charts. Engineers oversize these things all the time because they are not allowed to do that. If the book said "step on the crack, break your momas back." They would beleive it and never step on that crack.

Don't cha love a good debate? I have to say, you have more figures than most. Your assumption, that I'm assuming is not correct though. I've been actually measuring these flows and temps in real homes in 4 states for 5 years myself. Actually doing the homework in the field is nessasary to understand these machines and how they interact in a home with it's systems. Now in old construction, you have a whole new set of rules. Pre-1980's plumbing has higher flow rates unless things have been replaced. THat's another debate altogether but valid none the less.

I don't ask anyone to fully trust me on the internet. You are encouraged to "Measure" these field conditions for yourself any time you are in doubt.

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Originally Posted by Scott D. Plumber

You missed a couple points. 1. You don't get 2.5 from a 2.5gpm head. "If you assume 2.5gpm showers...you will assume incorrectly. 2. High limit on a jacuzzi is set at 104* mandated by our heros at the US Gov. Get in a tub and take the temp with a good digital thermometer. Not one of those cheap dial things. You'll see. Same with Showering. In fact, you can take a "hotter" shower because you are not imersed in the water. It is an aereated flow which makes it "feel" cooler. You don't have to "dail back" the flow. Your standard system has already done that for you with pressure and flow losses.

I get the CI tub thing due to the fact that a CI tub will need to be "heated" by the hot water and I can not argue the temps because I have not tried that, however most folks no longer have a CI tub either. About 5 years ago I left the paper and math world for the real world. I quit "guessing" based on paper charts and began personally measuring what was going on in the field at field pressures and feild temperatures. Including the geeky actions of taking a bath and shower with a digital thermometer. I started learning stuff that isn't in the charts. Engineers oversize these things all the time because they are not allowed to do that. If the book said "step on the crack, break your momas back." They would beleive it and never step on that crack.

Don't cha love a good debate? I have to say, you have more figures than most. Your assumption, that I'm assuming is not correct though. I've been actually measuring these flows and temps in real homes in 4 states for 5 years myself. Actually doing the homework in the field is nessasary to understand these machines and how they interact in a home with it's systems. Now in old construction, you have a whole new set of rules. Pre-1980's plumbing has higher flow rates unless things have been replaced. THat's another debate altogether but valid none the less.

I don't ask anyone to fully trust me on the internet. You are encouraged to "Measure" these field conditions for yourself any time you are in doubt.

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I didn't miss the points at all. I too have been measuring stuff for awhile (decades, in fact). Bucket-tests of flow rates in real-world showers are all over the place, and to assume that they're even using un-modified 2.5gpm showerheads is presuming facts not in evidence. Many people DO pull the flow restrictors out, others have antique shower heads that they're assuming are the same as new stuff, still others have 100psi water pressure. (I my place I had all three before I went to work on it! Now it's down to 1- the 3gpm bucket-measured out of the classy antique ceramic shower head. :-) )

But even with your assumptions in place, when two people are showering and a third turns on that hot water chuggin' Maytag the BTU-rate bottom falls out (although less so with a 199K burner). whereas with an indirect the flows & temps aren't much affected by intermittent bursts of significant draw at other taps.

Having lived with an undersized tankless, I appreciate having at least a bit of margin on flow, but it's doesn't take much effort to adapt as long as it's rock-solid for at least a single shower with significant margin (which the R75LSi clearly has.) I'm pretty comfortable with the capacity I get out of a similarly sized Takagi KD20 running an indirect AND the heating system simultaneously. With the boost I get out of a 4"x4' PowerPipe on the shower drains it has plenty of margin for showering, but without the drainwater heat recovery I'd have to run the indirect as a priority zone. Tub fills into the cast iron tub are still an issue when all zones are calling for heat, but I refuse to boost the setpoint on the tank. (No regrets- just an observation. Bumping a coupla thermostats back before drawing a bath is an easy adaptation to make.)