"Typically" varies a lot with climate.  While peak heating loads may exceed DHW loads in your neck of the woods, that would not be the case in much of the PNW or mid-Atlantic states  (or even southern New England.)

Indirects can do well, but not always. Even a 94% combustion-efficiency mod-con + indirect only may only get 60% efficiency in water-heating-only mode- see unit #11,  Table-2, p7 of this document:

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

Without a photograph of the test configuration it's hard to tell why the standby losses are so high, but assuming the folks at the Nat'l Labs configured them all similarly they're only so-so hot water heaters compared to a condensing tankless (like the one tested in the PG & E study.)  They have more detail on the system tested in Appendix 11, but is't similarly obscure. Configured as a buffer for the heating system an indirect can improve the AFUE of the system when the immediate heat load is lower than the minimum modulation of the boiler (which is typically more than 1/3 of the heating season even when the burner is perfectly sized for the peak load), more than making up for it's modest summertime HW efficiency in improved winter heating-mode efficiency.  Configured as a priority zone it improves the average efficiency somewhat by simply adding to on-time, but not as much as it might by blending zone & DHW calls if plumbed as a buffer. 

Every design will be different but the combined standby loss of the boiler & indirect can be much higher than a combi or standalone tank.  Low-mass mod-cons & tankless-combis should have lower standby losses than high-mass boiler+ indirect systems.  The latter typically underperform standalone tanks in water-heating-only mode in the above test report, but they beat boiler-embedded tankless coils by huge margin.

I was saying the domestic load is typically higher than the heating load... I should have said, in terms of BTU horsepower required to meet the demand. the majority of homes, if asked to heat their DHW load on demand, would require heaters much larger than their home heating load would require.

also, don't forget the 60% shown there for unit 11 is still exceeded only by two other units in the test, and also the 'summer average load' is a little suspect there. DHW is not an average load, it's a burst of usage a few times a day. but maybe I misunderstand something in the methodology there.

These appear to be the similar to the non-modulating Chinese point-of-use units sold by Marey (in Puerto Rico)  As point-of-use heaters they're not terrible (and you don't suffer the ~15% loss to the distribution plumbing.)  They're probably less than ideal for showering in the colder parts of the US though.  They squeak an extra coupla percent out of them by power-drafting the heat exchanger rather than relying on atmospheric draft like the low end Bosch units.  The Bosch units use a mini-magneto ignition powered by the flow itself (no batteries required.)

The non-modulating aspect would take some getting used to for most US users, but this was the typical of old-skool European point of use tankless heaters (to raise the output temp, you restrict the water flow, and conversely.)  The low end Bosch, Paloma et al atmospheric tankless units have fully modulating burner control to keep the output temps within a range across flow rates, but it's not as tight a range as the better electronically controlled tankless units (Noritz seems to be the current industry leader on tight temp control.)

Sorry to have misconstrued!

I agree, the test report begs a whole lot of questions about means & methods, and the manufacturer-bundled control algorithm of the mod-con for DHW production may have been way under-optimized.  But is WAS a standard product, and I presume the units were tested identically, if not realistically.  I wonder how the different systems would fare under the PG & E/Davis multi-profile tests.  I expect the low-mass systems would look pretty good, and the high-mass versions would fall off a cliff in the low-use/short-draw profiles.  But I was a bit surprised the mod-con + indirect didn't beat 70%, regardless of the (not fully known) test configuration.

well if they did an average load, a cycling mod/con would certainly not be as ideal as a flat out burn, and i can say the combustion efficiency of my mod/con at home, during full fire conditions, runs about 90% combustion efficiency. Jacket and fan purge losses should be similar to a tankless: the mod/con purges heat to the tank when it's done but the tankless can't: and the tank itself is better insulated than any standalone water heater with a flue. So I do find the measurements to be lacking something... then again, it is still one of the best choices in the whole test (as expected), so perhaps it's just that the whole field tests a bit low. But I think there is definitely more to it than comparing all of their test results to the tankless heaters "label rating".

perhaps if the tankless were in there as well for apples to apples comparison, but they aren't.

The amount of heat abandoned in a non heat-purging modcon or tankless is quite small compared to even a mid-mass steel or better cast-iron boiler. It's something like half the delta between the indirect's finish temp and the mod-con/tankless' final output temp multiplied by the mass of the water in the boiler loop & HX, and the thermal mass of the HX & plumbing itself- something on the order of 10lbs or less would be typical.  If the finish delta-T is say, 30F, that's ~150BTU given up by not purging, measurable, but still barely 6% of the energy in even a 5 minute burn at say, 30kbtu/h, 3% of a 10 minute burn, etc.  With reasonable tank hystersis and longer burns if it's only a temperature-maintenance burn it's still a hit (the whole burn is a hit!), but not a huge hit.  In actual usage, with the very low standby losses of an indirect, DHW burns only occur during & after large draws (showers & baths), resulting in even longer burn times, making the non-purging hit even smaller on a percentage basis.  The non-purging loss of a low mass burner pales in proportion to the heat abandoned in distribution plumbing in most houses (including mine!)

But in high-mass boilers the heat abandoned by not purging would be 3-5x that or more! (But with purging, not bad, as with the best-performance mid-mass steel boiler with purge control they tested.)

EF testing on tankess units are a joke- the numbers overstate the average efficiency by at least 8%- more if it's predominantly short-draws.  The condensing tankless only scored ~0.75EF in the PG & E low-volume use profile (but the standby losses of the standard standalone tanks were even worse) :

http://www.aceee.org/conf/08whforum/presentations/1a_davis.pdf

I would expect the Bosch 1600H to run about 78-80% in a space heating mode/combi application (violating the Bosch warranty in the process :-) ), maybe ~0.65EF in a low use DHW profile but ~0.75EF+ in high use profiles. 

Paying more for a condensing tankless for household use would be silly, since its flue-purges on short draws pull it's average WAY below it's EF test or steady state numbers. (I always laugh at the much bandied "98% efficient" claims in Navien's advertising, yet the claim seems to stick in the minds of the buyers.)  At a car wash or commercial laundry it'd make sense though (but still not as much sense as drainwater heat recovery, for even less money.)

Posted By Dana1 on 11/23/2009 2:13 PM


Enlightening input, I've been wondering about the difference between the various units since the price of these things varies widely.  The ExcelAmerica units are made in China or similar so could easily be the same unit.  They're small, low input units, so you're talking one point of use or maybe two outlets (kitchen/bath?), with probably only one in use at a time, in which case the issue of modulation is, as you say, relatively unimportant.

Now I also understand why the units get disproportionately more expensive with higher flow rates since bigger units/multiple outlet points really wouldn't work without the better control system.   This just wouldn't cut it (although I think that the point-of-demand heater for the bath in an apartment I rented as a student was little better than this).

I'm considering one of the Excel America, or similar low-end systems, as a booster for the solar radiant floor installation.  I plan to use it to heat the water before entering the floor (I assume this is standard).  $300 or so is almost the price of a heat exchanger and you get a free heater with it.  The input water temperature should be fairly constant (and high ~90°F).  Variations in the output temperature should not be that important - heating is on/off control anyway.  Any reason why I might want something with better temperature control?

It's an open solar system with a 700 gallon storage tank.  I have to check the Excel specs, but I don't really want it to turn on at all until the temperature on the incoming water is too cold to heat the house, say around 90°F  (this might take some tweaking to find out exactly where it needs to be).  Once it's on, I figure the temperature of the tank isn't going to change much since the water dumping back into it (heated by the heater) will be about the same temperature as the tank.

You do raise some good issues.  The the flow rate could vary considerably.  With every zone on, the anticipated flow rate is about 4gpm, with a worst case of about 30kBTU load.  However, it's unlikely that all zones will be running, so the flow-rate will change as zones go on or off - changing the output temperature.  Since radiant flooring is pretty tolerant of temperature variations I don't think this would be problem, but I think it would take some playing with to get it right.  Maybe better to go with a unit with slightly better control.

Is there anything else we should be looking for in these units? Is there any reason to believe that a more expensive unit will have greater longevity?  It would seem to me that the simpler units, less to fail, should last at least as long as more complex units.  A control board may be easy to replace, but 10-15 years from now it may not be available.  On the other hand, if the lower cost units are using poorly built heat exchangers they may not last 10 years (and the manufacturer may be long gone regardless of the warranty).  I don't necessarily have confidence that a brand name, at a premium price, provides me better quality.  I'm willing to be convinced of it, or perhaps I'm willing to be convinced that the off-brands are lower quality.  The brand names do seem to provide more features, better control, more displays, remote controls, etc. etc., but I'm not really sure I care about these.

The tiny li'l heaters represent a fairly high head-load to your circulation pump- it could take quite a pump! Jamming 4gpm through 'em on a high duty-cycle basis could wear stuff out (like the flow sensing for the ignition, etc.) It's better to have the radiation circulation on a different pump, and keep a separate loop to the tank for the boiler/tankless, and run it at a high-delta-T/low flow.

4gpm is ~2000lbs/hr, so assuming 30KBTU out of the heater you'd be looking at a 15F rise, but if only a single 1gpm zone is running you'd get a 60F rise. The tankless could take it, but it's way sub-optimal. Better to run the tankless at 1gpm (or whatever it's minimum ignition flow is) @ 60F rise in a boiler-loop to maintain the tank temp, that way you won't see 45F swings in temp heading out to the radiant as zones cut in/out. It'll take less pump, the tankless will last longer, and everything stays (relatively) happy. An aquastat on the tank to run the boiler-loop at your min-temp setpoint, with a ~4-10F hysteresis should keep the tankless from short-cycling, and it'll keep the output water to the radiation within a much narrower range. If the solar keeps the temp above the setpoint, the boiler loop never kicks on, and you won't be wearing it out with flow to the radiation, or having to suppress the ignition in some other way. Dumping a dribble of 1500F water into an 80F tank has a high dilution factor- the radiation will never see that 180F unless you invent some stupid plumbing tricks. ;-) Tankless heaters can handle very high delta-Ts, but try to keep it under 100F, eh?

If by "open solar system" you mean it's at atmospheric pressure, use only bronze pumps and monitor the pH of the water, buffer as-necessary. If by "open solar system" you mean you're running potable water through the thing, you're crazy- don't do it!

The problem with a separate loop is that you're using the heater/boiler to heat the 700 gallon tank.  So at 6am, the tank is at 90°F, the heater kicks in and uses a lot of BTUs (58450) to raise 700 gallons to 100+°F for the radiant heating.  A 8:30 the sun hits the solar panels so the energy put in by the heater was possibly surplus.

If I put the heat directly into the floor then as soon as the solar panels start to heat the tank then the boiler should turn off.  The boiler will heat the tank slightly, the loop loss is about 10°F and the boiler temperature rise is probably 15+°F, but most of the heat should have been used in the floor.

This, of course, presupposes that the boiler won't always be needed, if you're going to have to add BTUs everyday anyway then there's no penalty for heating the tank.  Measured output of the solar panels (courtesy of my son's science fair project) in early Jan. is just over 200K BTUs/day.

I think I should put in a manual bypass for the heater.  At each end of the heating season you have low heat load and high solar energy so the boiler should be unneeded, no point in running water through it.  In the middle of winter it should see some cycling, but since it's only about 3 months, it probably shouldn't be worse than the annual cycling on a DHW heater.  I think I need a slightly larger heater - at least one rated for 4GPM+.

Posted By Dana1 on 11/24/2009 2:22 PM
Choice 1. Excel actually have a tiny, $99, 2.2GPM heater that I was going to use, but, unfortunately it has some ferrous materials, or similar that make it unsuitable for the application (maybe pressure related).

If you route the output of your solar storage tank through a tiny electric tank HW heater and only maintain the temp of the tiny tank with the boiler, you're good. It'll only fire when the temp drops below the small tank's setpoint (with some small hysteresis), and it'll either keep up or not depending on how cold the solar storage is. Overnight it won't lose heat enough in standby to cause it to fire unless there's a flow on the heating loop and the solar store is colder than the mini-tank's setpoint. The head represented by the mini tank to the heating pumps is negligible, and you can run the tankless in a low gpm/high delta-T mode where it's most efficient and has the least amount of wear.

IIRC you need at least 8-10psi to keep oxygen out of heating water. At atmospheric it has to be an all non-ferrous show. Some tankless heat exchangers will have issues pressures under 20psi too (too many micro-bubbles on the water side forming and hanging around, insulating the water from the metal surface, lowering the heat transfer efficiency), but I'd assume mini-burners marketed to places where low pressure is the norm have factored that into the heat exchanger design. The flow restrictor on the tankless + the loop pump can probably get you the localized pressure required if that becomes a problem. Plumb it with the pump driving toward the tankless, not away from it. And listen to it while it's burning- if there's a bit of sizzle/hiss at the heat exchanger, restrict the flow until the sizzle stops. If the flow restrictor in that design happens to be on the input, not the outputyou may need to plumb in a ball valve on the output to create your own pressurizing flow restrictor.

Well, here're some losses that probably aren't accounted for in testing.  Since we installed a tankless water heater for the mother-in-law apartment in the late fall the pipes have started to freeze. We now have heat tape on the pipes and an electric well heater in the utility room to keep the water heater from freezing.

Tenant is, by-and-large happy with the water heater - when it's working - even though it's a cheapy.  Two points of use (kitchen and bathroom) no clothes washer or dish washer.  The water heater is installed in an attached, but unheated, utility room.  Since we took out the old heater (60's-70's tank) the pipes under and in the utility room have started to freeze.  So we have to heat the pipes/space to prevent freezing using a pump room heater and tape.  I'm not a big fan of heat tape, mainly a lack of life expectancy.  A well or pump room heater (I hadn't heard the term until a few weeks ago) is just a small (in this case 500W) electric heater with a thermostat range from 40°F-70°F rather than the more normal human-habitable ranges.

It's clear that the old storage heater was not very efficient and the waste heat was heating the utility room/plumbing.  However, obviously, the gains from the tankless are not as good as you might hope because we're now using electrical heating to stop the pipes from freezing. 

In this case tankless is probably still a win based on the usage patterns.  Tenants (father and son) are only there on weekdays and not during the day. A modern, efficient storage heater may also not have had enough waste heat to keep the area warm.  The freezing seems to occur over weekends when the pipes are unused so is also usage-pattern related.

I'm open for suggestions on better ways to prevent freezing.  We're in the process of improving the insulation.  I'm considering a small thermosiphon solar air heater to try and get some heat into the space during the day and throwing in a bunch of construction debris thermal mass  under the floor to maintain the temperature at night.  The goal is simply 33°F.  Probably won't hurt and is easy/cheap enough.