First off, thanks to those of you who contribute so much knowledge here.  I've been lurking on these forums for a while (mostly because my search results tend to end up here), and learning a great deal as a result.

My 1950s 1300sf bungalow in Ottawa, Canada (design temp of -8F) is heated with mostly fin-tube baseboard heaters, as well as a couple of "extruded aluminum panels under subfloor" radiant heating zones, all tied to a fairly modern 85% AFUE cast iron natural gas boiler.  Each zone is controlled by a TRV.  The design-day heating load for the whole house is approximately 30k Btu/hr - calculated based on room-by-room heat loss calculations, and corroborated with calculations based on degree days & 2 years' worth of heating bills.  The boiler is rated for 60k Btu/hr output, although I doubt I'm getting anywhere near 85% efficiency due to the boiler setup: no primary/secondary loops, just one variable speed circulator on the boiler output feeding all of the heating zones.  The boiler is always "on" and fires whenever the water temperature drops to 135 degrees.  Needless to say, during milder weather, the boiler tends to short-cycle (burn times of 3-5min to heat to ~160 degrees).

Based largely on suggestions I had seen on these forums, I was considering replacing my aging standalone water heater with a reverse indirect (Ergomax/Turbomax) piped in as a hydraulic separator/buffer tank for the boiler.  However, I believe that for the size of my boiler, the reverse indirect approach won't yield sufficient hot water to satisfy projected peak demand (2 simultaneous "teenager" showers; 2gpm showerheads, one with heat recovery) on those cold Canadian winter mornings.

I'm now considering the possibility of using the same arrangement, but with a large (50gal+) standard indirect tank heater - probably along the lines of the Triangle Tube Smart series.  The advantage being that the stored energy for the hot water demand is not as dependent on the boiler output.  Pretty much exactly this sort of arrangement, only with the buffer tank being an indirect water heater connected with tees:

http://www.pmengineer.com/SHT/Home/Images/PME_0907_Feat2Fig10Lg.jpg

The boiler demand would be based on the indirect's aquastat, and the heating zones would use energy stored in the potable water when the boiler isn't firing.  As far as I can tell, this should work to provide sufficient hot water, as well as act as an adequate hydraulic separator/buffer tank to reduce short-cycling.

Any issues with this plan?  Any other suggestions?

well, firstly that tank is not a domestic water heater, if you read your question correctly. You can use an indirect tank as another zone off the boiler for DHW and the buffer solves some of your cycling issues. The buffer tank can have a large temp drop before calling for heat. I would also consider putting an outdoor reset control on the buffer tank to limit its max heating temp in shoulder months.

The Ergomax E23 / E24 etc and Turbomax equivalents are purpose designed EXACTLY for DHW applications:

http://www.ergomax.com/E-Info.htm

...as well as heating system buffering:

http://www.ergomax.com/New-Tanks.htm

But you may run into capacity issues running simultaneous teenage showers with only 60K of output. If combined with a decent sized drainwater heat exchanger and low-flow shower heads it might squeak by though. It would for-sure be enough to run one low flow shower, but not a 6 side-spray type. Going with the ~48 gallon versions would give you more buffer and more hot water, but they're kinda pricy, and work better with even higher-BTU output boilers. With smaller boilers the boiler flow is low enough that the turbulence inside the oversized tank isn't very high, so you don't quite get the full benefit of the larger heat exchanger.

To be sure, buffering the low-mass multi-zone system is a good idea to put a lower-bound on the length of the burns. If you kept to your 25F hysteresis, the ~ 200lbs of water of the reverse-indirect (E23 or E24) adds about 5 minutes of burn time. The hysteresis of the aquastat on an Ergomax is only about 7F though, so it takes a small amount of control design/integration to use it's aquastat to initiate the call for heat, and a strap-on aquastat (or the boiler's internal control) as the high-limit to end the burn.

Outdoor reset is the OPPOSITE of what you want to do with the buffer, since you're using thermal mass X hysteresis to maximize the burn times, and ODR would lower the hysteresis.  Putting ODR mixers on the heating loop(s) might buy you something in comfort & efficiency, but maximizing the hysteresis of the tank will be better for boiler efficiency.  The return-temp to the cast-iron boiler has to be above 130F to avoid damaging the boiler or flue, and most of the shoulder season (or even much of the heating season) you won't need heating water temps even that high- the hysteresis would be zero. Putting the ODR control on the buffer tank temp has no rationale, even if it weren't being used for DHW. (Were this a condensing boiler we might want to go there though.)

Thanks for the responses! It seems that in my effort to provide as much information as possible, I may not have been completely clear as to what exactly I'm trying to do. My plan is to use an ergomax-type setup (as per Dana1's link: http://www.ergomax.com/New-Tanks.htm ), but rather than using an ergomax, to use a tube-in-tube indirect water heater such as the triangle tube smart series ( http://www.triangletube.com/Flash_TANK.html ) in its place - I would use tees on the indirect's input and output to the boiler loop supply and return, respectively (as per the ergomax diagram).

As I understand it, this would mean that the ~50gal mass of DHW would serve as a buffer/hydraulic separator for the water on the boiler loop, and the heating loop could also use that heat when the boiler isn't firing (ie: using the indirect tank in reverse). As an "added bonus", I could use that same volume of potable water to supply my peak DHW needs. I realize that during a high heating and DHW demand, the system temperature will not be sustainable with my boiler, but the theory is that once the showers get cold, the showers will end :-) ...and the DHW tank should be able to recover at a reasonable rate as I'll still have at least ~30k btu/hr excess coming from the boiler. I expect I will need to add a mixing valve on the boiler return, or some other sort of boiler protection.

I'm planning to add waste heat recovery on one of the showers (the other is in the basement, so not really an option), but I figure that even without that, a 50 gal tank should be enough for two reasonably long showers at the same time. In any case, from my calculations, there's no reasonable way to get two endless showers with a 60k boiler in the middle of our winter, hence my reluctance to use an ergomax-type heater.

As Dana said, outdoor reset isn't really a useful option at the moment. The reasons my temps aren't lower are (a) the boiler needs to stay above 130 degrees to avoid condensation of the flue gas, and (b) I presently need a minimum of 25 degree hysteresis on the boiler control to keep the short cycling from being even shorter.

With 60K of input you can run a 1.5 gpm shower pretty much indefinitely if there are no other big loads, but a 2gpm shower would eventually draw it down unless you had a drainwater heat recovery feeding back some of the drain water heat.

With 35F incoming water and 105F shower water (70F delta-T) two 1.5 gpm showers is an instantaneous draw of about 105KBTU/hr. With a ~50% efficient drainwater heat exchanger you'd be come close to supporting two all-day 1.5 gpm showers even with 30K of space heating draw, since you'd have 60K of boiler + 50K of DWHR delivering 110K total to support a load of 100K shower + 30K space heating. But as the water temps drop, so does the fin-tube output. The kids in the shower are still fine with 105F water, but the fin-tube isn't putting out very much at all (maybe 140BTU/foot, if that), so it might support the whole thing- they won't get cold.

Plumbed as-described you won't be able to run the tank as a "priority zone" shutting off the space heating loads- think seriously about adding drainwater recovery to improve family-harmony and save a bit on fuel. IIRC there are NRCAN subsidies (and possibly others) available for retrofitting drainwater heat recovery in Ontario to take some of the sting out of it.

See: http://oee.nrcan.gc.ca/residential/personal/retrofit/13302

(oops- missed it- the ecoEnergy grant deal ended in March this year: http://oee.nrcan.gc.ca/residential/6551 )

Interesting. I hadn't realized that it was possible to get as much as 50% recovery through DWHR systems. These should really be mandatory on new construction!

I figure that if the lack of a priority heating zone for the DHW was an issue, I'm sure I could rig up something like a pressure switch on the DHW line that would shut off the heating zone circulator when there's a hot water draw, but as you said, the system will naturally regulate to some extent as the fin-tube output will drop with lower temperatures.

Also, in my case, one of the showers is in the basement, and I don't see a straightforward way to add a DWHR below the slab. I am, however, planning on installing a power-pipe on the drain from the upstairs shower, which gets used much more often than the downstairs one. Nonetheless, I don't really see a way that I can run two continuous showers with my current boiler. It's unfortunate, because I really like the design of the ergomax-type systems.

Oh, and thanks for the attempted tip. I took advantage of the eco-energy (and equivalent provincial) grants a few years ago when I did some insulation & air sealing upgrades.

With 50 gallons of buffer and a PowerPipe on the main shower you'll still have a decent amount of showering time with both showers running, just not the "endless shower" experience with both running. The amount of heat being returned by the PowerPipe would about cover the space heating load, so if you took the Triangle Tube approach to buffering it would be comparable to running two showers with a 50 gallon indirect tank without DWHR. Families live with that that all the time- they just don't expect the hot water to last forever with both showers running. But an ErgoMax E23 has only about half the volume and wouldn't cut it.