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Thread: is tankless feasible / advisable in upstate NY?

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    DIY Junior Member RobRing's Avatar
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    Default 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

  2. #2
    In the trades Dana's Avatar
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    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.

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    DIY Junior Member RobRing's Avatar
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    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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    In the trades Dana's Avatar
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    Quote Originally Posted by RobRing View Post
    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'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.)

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    DIY Junior Member RobRing's Avatar
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    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

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    In the trades Dana's Avatar
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    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.

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    In the trades Dana's Avatar
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    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.

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