Hi, I have an older home, 2300 sf with lp boiler feeding cast rads and lp DHW heater. Both need replacing. I'm considering the benefits of going tankless or HE boiler/indirect heat tank system, maybe adding some solar at some point. I have small demands for hot water. One or two showers daily, DW twice a week, almost never wash clothes in hot. Only one zone for heating. I live in Central VA. Any thoughts?

Take a look at the Triangle Tube Solo series of heater, they are a 96% efficient, stainless steel, wall mounted gas boiler that can be used, in conjunction with a indirect tank, to provide both space heating and domestic hot water. Nice units, very user friendly.

Thanks Rob, but I'm not really asking for a brand recommendation. I'll go through that with my installer.

The question is; in my situation of using fairly little hot water, does it make sense to keep hot water standing in any tank at all? I know indirect tanks are very efficient, but it might be a day between calls for hot water and wouldn't I be better off just to make it when I need it?

Of course in the winter I need hot water for the rads, so a tankless unit would get a lot of work. Is this a reasonable scenario? It sure would be buying a lot less equipment if it would work well.

Oh, and I also have a heat pump as backup/supplementary heat.

well, remember that standby loss is standby loss; wouldn't matter if you had 10x the usage, the loss is the loss. what confuses people is that it makes the percentage number sound worse for small usage levels.

there is no doubt at all that a tankless is not the right way to go for space heating unless you're in a place where electricity makes sense. Depending on your heat load, you would want either a tank water heater or a mod/con boiler (assuming gas). In most cases, the mod/con boiler will be the best choice, even in VA. And if you have one, you might as well get an indirect, and add more insulation if you are worried about standby loss. You could do a separate on-demand instead of course, though you'd probably be in a similar efficiency in the end either way.

a mod/con and indirect is not a lot more equipment than a tankless with a heat exchanger. it's not like you need to add a mixing system or anything.

hope that helps!

Rob, I understand that standby loss is standby loss. That's why I'm considering no tank. No tank, no standby loss.

From what I understand, a con boiler holds very little water, unlike my old Vaillant. In comparison, it's almost a tankless.

Seems like if you had a two manifold tankless, you could use one side to serve your DHW, and the other to run the heating system. That would eliminate the fairly expensive indirect tank.

Why wouldn't that work?

Thanks

propane, btw

the tank loss isn't that large, especially in an indirect. That's why I was calling out the difference between actual loss and loss as a percentage of usage. Your percentage may sound high, but your loss is still the same not very high number.

mod/con boilers beat on demands by about 15% in efficiency in heating applications. they are not tankless on demands. so a tankless on demand does not replace the boiler.. not in most cases.

you can replace the indirect with an on demand, but in that comparison you aren't saving any money. for light usage, a small indirect is probably cheaper than an on demand plus venting equipment and gas connection. plus the mod/con will make DHW at 90%+ efficiency, which is better than the on demand, though that will be offset by tank loss you can again reduce tank loss by adding insulation if you wish.

so you could do mod/con and tank or mod/con and tankless, and between those two, I would typically do the tank unless it were a vacation home.

Now you got me there. I am looking at boiler efficiency in the high 90's, like 97%. How efficient can on demand units be?

in a heating app? not much over 80% in most cases. that assumes you are using one with a very low minimum modulation rate, and it does not include excess wasted pumping energy through the high head heat exchangers most of them have.

That said you're not likely to hit 97% in a mod/con boiler with your rads unless they are extremely oversized (low temp) and you are using a buffer tank. 93 to 95 maybe, depending on model, water content of the system, and water temps.

What about the Navien products? They have a Combi con tankless that's supposed to be 98% efficient and is designed for exactly what I'm proposing.

98% efficient when you are taking in 45 degree ground water, maybe (DHW production only). If that's the case it may make sense on the domestic end, though I bet you'll find the cost is not the same as other tankless models.

I have no experience with them in a heating app though and their site docs are lacking, so I can't really say beyond that.

Ground water in Central VA is 55 degrees. I'm not saying this is a better idea, I'm just prodding around for info.

And I haven't priced them yet, but I do hear good things about Navien. Even if they are more, I would be avoiding the purchase of a tank, which can be expensive.

a 40 gallon tank for light usage is not particularly expensive.

I was throwing out an arbitrary number with the 45 degrees: I dunno what they rated their unit with, but with an efficiency that high, it's definitely with cold ground water, not heating water. I hear good things about them too, but I know very little about them other than 98% efficiency is with very chilly water ;)

There are condensing versions of on-demands that have steady-state efficiencies in the high-90s, but in practice all on-demands suffer HUGE efficiency losses due to short-cycling on small volume draws, and effect not measured by the standard EF tests.   It typically takes a minimum draw of 3 gallons or more for an on-demand to approach it's steady-state numbers.  A draws of less than a gallon they're all under 50% efficient.  The EF test draws are 10.2 gallons at a time. The average efficiency of a typical condensing (0.90+ EF) on-demand/tankless HW heater for small volume users is on the order of 75-80%, not 90%+.  See:

http://www.aceee.org/conf/08whforum/presentations/1a_davis.pdf  (See p.10, the bar graphs on the right- heater #6.)

Assume there'll be a similar hit with standard efficiency (0.82-0.85 EF) on-demand/tankless, more like ~65-70% average efficiency than 80%.  That's better than standalone tank, but probably comparable to a decent indirect with well insulated plumbing.

Standby losses on indirect tanks are quite low compared to their standalone cousins for a couple of reasons- no gaps in the insulation (the burner can't be isolated & insulated well on a standalone), and no thermosiphon convection loop flue losses (no flue= no flue losses.)  Standby losses on an indirect can be be reduced to arbitrarily small levels by more insulation, both the tank, but as-importantly, the near-tank plumbing (both the boiler loop and the potable water connections, both hot and cold.)  Served by a low-mass boiler like a mod-con, the boiler standby losses are also quite small, and with the mass of the stored water to work with, domestic hot-water burns are never efficiency-robbing short cycles.

Then there's the other issues with tankless on-demands- the delayed ignition water wasting & "cold water sandwich", flameout at micro-flow, etc. that just don't happen with indirects.  The fuel use may in fact end up higher with an on-demand than an indirect due to the higher rate & longer flows.  You can buy a LOT of insulation for a tiny fraction of the installed price difference between a tankless and an indirect, making the difference in raw efficiency between the two quite small.  On-demands are great for filling the spa or other high-volume batch draws, but for most other apps you'll get more satisfactory results with an indirect, often with lower fuel use as well.

For more info on how indirect hot water heaters affect efficiency compared to separate hot water heaters &  hydronic boilers, see:

http://www.nora-oilheat.org/site20/uploads/FullReportBrookhavenEfficiencyTest.pdf

Note "tankless" in this document refers to a hot water heating coil in a mid or high-mass boiler, not a separate on-demand/tankless hot water heater.  The only mod-con tested was system #11, which only had 88% steady-state thermal efficiency, but even without going crazy on insulating the tank and near-tank plumbing it had about 58% efficiency as a hot water heater.  Assuming your boiler gets 95%+ steady-state at some reasonable output/return temp, you'll get about the same efficiency as a non-condensing on-demand (assuming it's performance ~10% or more lower than the condensing version in that first link).  If you take pains to insulate the indirect & plumbing you'll likely meet or beat a condensing on-demand's performance using a better mod-con for a boiler.

For the difference in installed cost between an on-demand and an indirect, you may also be able to install a drainwater heat recovery system and boost the apparent efficiency by a sizable fraction. It would be difficult to rationalize on fuel-cost savings alone for low volume 1-2 shower/day type use, but the payback would still be quicker than solar thermal.  I wouldn't dream of doing solar hot-water without going for drainwater heat recovery first, if at all possible (but it can be tough to do for first-floor showers in slab-on-grade construction.) See:

http://www.regie-energie.qc.ca/audiences/3637-07_2/DDR3637_2/RepDDR/B-12-GI-23Doc1-2_RepDDRSE-AQLPA_3637-2_28sept07.pdf

For more a recent comparative testing result listing of different models/manufacturers by the Canadian government see:

http://oee.nrcan.gc.ca/residential/personal/retrofit-homes/questions-answers.cfm#q45

(Note, the Eco-GFX models listed are products made by of EcoInnovation in Quebec not GFXtechnology in NY despite model name similarities.  GFXtechnology is apparently now selling re-branded Watercycle product.)

A decent sized drainwater heat recovery system is the equivalent of adding 25-30KBTU/hr to your boiler during showers or other simultaneous drain/potable-water flows (but not batch draws like tub filling- you'll need to size the tank correctly for those.)  If you're a shower-er not a soaker, this may allow you to down-size the boiler and still get reasonable hot water heating out of it.  Long hot water draws usually reprent a load several times that of a typical (or even design-day) heating load, and sometimes sizing the boiler for an indirect oversizes it for the heating load, knocking a few percent of as-used AFUE off the performance compared to a smaller unit. (The effect is larger with old-skool boilers compared to mod-cons, but it's true of mod-cons as well.)

Go with a mod-con & indirect (& drainwater heat recovery system if you can), you won't be sorry.

I laugh every time I see that 98% thermal efficiency number for the Navien tossed around. Real-world use will have a lot of short-cycles which robs it's efficiency severely. The only time you'll beat 90% in real-world use with a Navien is if you're using it as a pool heater (or hydronic boiler.) For DHW figure 85%, best-case. See comments & links above^^^^.

Edited to add: Make that "as a hydronic boiler in a low-temp high mass heating system"...

Seriously- the Navien's are a bit oversized as a boiler for most heating system for 2300' houses in the first place, and doesn't cost much less (if any) than a mod-con.  Their efficiency sweet-spot is likely to be somewhere between 1/4-1/3 of full fire, which is going to be several times your design-day heat load. At lower modulation than that you get laminar flow on the combustion-gas side of the heat exchangers and the stack temp rises (!).  A 50-60KBTU/hr mod-con has similar issues, but it's 1/4 full-fire rate is likely to be a better match to your heat load than a 180-199kbtu/hr.  If the volume of water in the system is high enough (say, 50+ gallons) it is buffered well enough to keep it from sliding too far toward a partial-load efficiency cliff even if it's not optimal, but only if it delivers the space heating at a low enough temperature:

If your cast iron radiation needs water temps to be 150F or more to get/stay warm much of the season you're unlikely to get out of the 80s in efficiency terms, even with a condensing appliance. Return water from the radiation entering the boiler need to be under 125F (the condensing temp of propane exhaust gases with 10% excess combustion air. For natural gas exhaust it needs to be under 122F.)  The theoretical limit on raw combustion-efficiency with return water temps over that is ~88-89%, and you need to subtract a percent or two for jacket losses from that to get the steady-state thermal efficiency Navien-style.  From there the rule of thumb is to subtract about 3% for every 10F the return water is over 125F (eg, if the return water is 135F, the best you're going to do is about 86%, if it's 145F, expect 83% or so etc.) 

If you've added a bunch of insulation and air-sealed the place since the original system went in, odds are you can crank down the temps a lot- a GOOD heating system installer/designer will be able to calculate how much. There are plenty of software tools out there for doing these calculations, but they're only as good as the folks who use 'em (and a lot of heating system installers don't even go THAT far.)  If you have good records of fuel purchases going back a coupla years and access to heating degree-day numbers you can do better estimates of the heat load than even the best of those tools though.  A pretty good estimator based on fuel use (and boiler modeling at the Brookhaven Nat'l Labs) is the FSA calculator downloadable here:

http://www.nora-oilheat.org/site20/index.mv?screen=home

Play with it a bit to see how oversizing the boiler affects efficiency (you'll likely find that your old boiler is/was 3-4x oversized for the load and running with less than 65% actual efficiency even if it's steady-state is 80%+, which is all too common.)  If that's the case, your radiation is also oversized, and you may indeed be able to deliver the heat at temps low enough to get some condensing benefit most of the season.

If not, the better value might be in a "right sized" mid-efficiency  ~85% boiler + indirect, applying the difference in equipment costs to your solar hot water or insulation upgrades or something. Condensing equipment really works best with low-temp radiation like radiant floors or large panel radiators.  Without the right radiation the condensing benefit is largely absent (no matter what the AFUE or steady state thermal efficiency number say.)

"Navien combi systems offer 98% thermal efficiency!"   (Only in a test lab- count on it!)

Thanks Dana, that's a lot of information. Of course there are lots of numbers being tossed around, but the layperson has to refer to something. I understand that overall efficiency of a system has to do with a lot more variables than we have talked about here, but I have so start somewhere...

I am aware of DWHR, but honestly I thought given my low volume of use I didn't think it would be that beneficial. Maybe I have to rethink that. Maybe in my situation, point of use recovery to preheat the cold input for my shower would be logical, and preheating the hot with the dishwasher makes sense, but most of my drains just don't get much hot water use.

More food for thought.

DWHR is more reliable and cost-effective than un-subsidized (or even low-subsidized) solar.  The highest efficiencies occur when it pre-heats the cold water to the entire HOUSE, including the cold feed to the shower and to the hot water heater ( "balanced flow" configuration.) If there's a single main-drain to the house, that's the ideal spot, but from a practical installation point of view it's useful to be close to the hot water heater, the primary shower, and to where the cold water enters the house (which is a rare combination.)  In a balanced flow configuration they return roughly 50% of the energy used in a shower, which is typically 40% of the total hot water use, so you're saving on the order of 20% of your total hot water heating energy with a single passive device, and the size of the solar water heater to support the load can be safely downsized by a similar fraction, the boiler can be smaller, it can pay back in several ways other than in direct fuel use. It lowers the peak load by about 50% even if it only lowers the average load 20%- both count. 

You may be able to get away with a smaller indirect as well, but if in doubt, go larger.  Standby loss doesn't increase linearly with volume, since it's an surface-area/R-value problem, 2-dimensional, whereas volume is 3.  The plumbing standby loss is a constant, independent of volume, but it's a very real fraction of the total.

If your hot water use is extremely intermittent (as in days or weeks between uses, as in a weekend cabin or something), a tankless on-demand will be the performance leader.  But in your daily dwelling, not so much- it'll usually beat a standalone tank with good margin for low-volume users, but probably not an indirect, if you take pains to further reduce losses with plumbing insulation.   Even then, if you have a 5-month or longer heating season and the boiler/indirect is located within conditioned space half the already small annual standby loss just displaces heating fuel use, whereas much of the standby loss of a standalone tank goes up the flue, not the room.  Replacing atmospheric drafted units with sealed combustion (outdoor air is ducted directly to the boiler/heater) the idle flue losses and induced air-infiltration into the building while the burner is active all disappear too.  5% here, 10% there, pretty soon you've whacked your fuel use way back.  Going from 4x oversized & atmospheric-drafted to right-sized (or modulated) and sealed-combustion is usually good for 25-30% reduction in fuel use.  If you can lower the radiation temp in to the condensing range you can often do even better.

I'm with you on all of that. I have a lot to discuss with my installer.

I'm afraid I am not a good candidate for DWHR though. As my shower and dishwasher, the largest hot water users, are both on the first floor with less than three feet of crawlspace below them. Are there any systems that don't require a vertical drain?

BTW, currently in the States, there is a 30% tax incentive on solar hot water installs. No limit.

Even a 24-30"x 4" DWHR heat exchanger does a lot of good- don't count it out just yet.

From the NRCan published test data results  (http://oee.nrcan.gc.ca/residential/personal/retrofit-homes/questions-answers.cfm#q45 ):

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Models with an efficiency ranging from 30 to 41.9% exclusively:

• ECO-GFX
  S3-40 (75 mm/ 3 inch drain at 39.0% steady state)
• Power-Pipe^®
  R3-30 (75 mm / 3 inch drain at 32.9% steady state)
  R3-36 (75 mm / 3 inch drain at 37.9% steady state)
  R4-24 (102 mm / 4 inch drain at 31.5% steady state) << R4-30 (102 mm / 4 inch drain at 40.4% steady state) <<<<<<<<<

----------------------------------------------------------------------------------------

30-40% isn't 50%, but it's still going to be a net 12-15% reduction in average HW heating fuel use (and solar panel size requirements) and a 30-40% reduction in peak load (allowing a ~25kbtu/h reduction in boiler output.)

If you lived in Canada you could just order 'em up at a big orange box store:

PowerPipe 4x24" or 4x30"

EFI is a US distributor for the PowerPipe line- you may have to call 'em to find out pricing & availabilty on the shorter models:

http://www.efi.org/wholesale/pdfs/power_pipe.pdf

Review the installation instructions to figure out the required clearances to see if it's possible in your crawlspace:

http://www.renewability.com/uploads/documents/en/home_retrofit.pdf

There was a Norwegian company with a different sort of heat exchanger that can fit under a shower, but IIRC it is both less effective and more expensive (and not widely available, if they're even still in business- couldn't find 'em in a quick google search, which may say something.)

Dishwashers tend to use hot water primarily in batch draws (take some how water pump & spray it around for several minutes, then dump, repeat) not simultaneous drain & HW flows.  Heat exchangers don't store much heat, so the heat recovery from dishwashers will be in the low single digit percentages. (Indeed, you may get more BTUs returned from room-temperature toilet flushes.) Showers are the real deal- 40% of the typical total hot water use, and 90% of the simultaneous hot-water + drain flow.

Posting a reply since my situation seems very similar, but with a little nuance. I live in a housing complex where our HOA just won a huge settlement against the master builder, Seattle Housing Authority. I'm in the somewhat awkward position of doing informal RFPs with two primary vendors who will replace our hydronic heating systems. Owners will be able to go with one vendor or the other. One is proposing a tankless Rennai and the other a Triangle Tube boiler. The boiler people say that the tankless heaters really aren't up for the job. . .they understand why they are getting used in this way, but are concerned about short cycling. The Rennai people have no such concern. Could you give your advice in the briefest and most laymen of terms? And soon, please. Things have happend almost overnight; settlement checks will be distributed on Monday. We're not going to be trying to presuade homeowners with one solution or another, but I personally would like direction and I plan to influence as many others once I've come to a conclusion informed by the clearly very bright and seasoned professionals who have posted on this thread. Details similar for all 150 affected homeowners: Many of us live in attached townhouses; sq. footage 1450-1800; well insulated and 7-8 years old. My personal situation: Townhouse is 7 year old; I share walls with a townhouse on either side of me; average therm usage is .8 May-Oct, 2.0 therm Nov.-April. 1 dishwasher load/day; 5 loads of warm laundry/week; 4 showers; 2 bathes. 4 very young children so # of baths and showers will increase over next 5-10 years. Would you go with the Rennai or with a triangle tube? Why? Thanks so much.

you say hydronic heating systems: what are the systems? radiant, air handlers, baseboard? if radiant, what kind? how many zones?