I am not familar with doing my own J type heat load calc so I took my plans to a system designer and they recommended a couple of tankless units that would do my job but I didn't ask for the calculations used to determine the units they specified. Is this easy to calculate by myself?
There are various heat-loss calculator freewares out there, but this $49 package seems to get good reviews:
http://www.hvaccomputer.com/
(I haven't used it myself- can't comment on how good it really is.)
From a system design point of view, having some buffering as part of the potable water heat exchanger saves the tankless from wear & tear by minimizing the number of on/off/flue-purge cycles, and will improve the overall operating efficiency. This architecture using a "reverse-indirect" hot water heat-exchanger basically works and has some forgiveness to the design, and reduces corrosion & scale issues in the tankless:
http://www.heatpro.us/designtree/documents/tanklesssys.htm
A limitation of that architecture is that the return water isn't less than ~10-15F below the storage tank temperature, which has to be ~125 or above to get reasonably hot water out of the potable water side. This, A: limits the amount of condensing a condensing tankless can achieve (it's still pretty good though, if you can design the radiation to be able to deliver the heat at 130F.) and B: With only 115-120F return water the delta-T you can get out of the tankless, so you may need to run higher flows. (I'm not sure what the recommended max continuous flow is on the bigger Naviens & Noritz, but it's probably more than the smaller/older Takagi I've designed with.)
Another advantage to this approach is that it eliminates DHW flow and tankless "cold water sandwich" issues, since the flow is limited by the heat exchanger's plumbing, not the high-head of the tankless, and since the DHW is drawing heat from the buffer water, it never "sees" the slug of cooled water in the heat-exchanger that is created by flue purges at the end of a burn cycle.
The same guy who sketched out the tankless combi above has a hyronics primer worth absorbing, if you're committed to designing the heating system yourself:
http://www.heatpro.us/designtree/
He also has a heat-loss calcualtor freebie (and a pro version) using IBR methods (good enough for your purposes.)
http://www.heatpro.us/downprog/
(Again, haven't used it, can't really comment on it.)
There are cheaper combis out there using air handler coils, no buffering. I like the one they did with a Rinnai here:
http://dsp-psd.pwgsc.gc.ca/collection_2007/cmhc-schl/nh18-22/NH18-22-106-108E.pdf
which is similar to (but perhaps better-designed than) the one they did with a Takagi here:
http://www.toolbase.org/pdf/fieldevaluations/Tankless_Hot_Water_Heater_EvaluationSWD.pdf
Using potable water in the heating system (as in the Florida combi) is in general a bad idea (legionalla growth, more corrosion, wear & scaling issues on the tankless heat exchanger, etc.) Even if unbuffered, use a heat exchanger, and keep the tankless/heating loop at at least 12lbs pressure to A: keep efficiency-robbing sizzle out of the tankless heat exchanger, and B: keep oxygen out of the heat-loop water to limit corrosion on pumps, radiation, & burners.
Think about what you want/can-afford for radiation (radiant floors/walls/ceilings are the most comfortable and provide the lowest return-water temps, but are also more expensive. Fin-tube baseboard or an air-handler coil might be the cheapest. But low temp panel-radiators are also quite cushy, and somewhere in-between cost-wise. )
Navien sells a handy space-efficient pre-packed heat exchanger kit for doing combi-systems called the "Heating box" (
http://www.aqenergy.com/ ) but I've recently read about a few longevity issues with it. They will probably fix those issues and stand behind the product, but for the money you might be able to do better designing your own (if more bulky) system.
Whatever you do for a tankless combi system plumbing it as a primary/secondary allows you to set the tankless & radiation flows independently of one another. Trying to run it all on one pump limits the amount of modulation (and efficiency) you get out of the tankless while in heating mode, and takes a much larger pump.
In the buffered reverse-indirect model promoted by the HeatPro guy, the tank behaves as the hydraulic separator in the primary/secondary, and allows you to run the tankless at low flow/big-delta-T with tankless output temps much higher than the radiation temps. This buys you a lot more BTUs/hour out of the tankless without resorting to a 300watt pump, and without overstressing the tankless. The radiation temp is set by the aquastat on the indirect- the output temp of the tankless is whatever you program it to be, based on flow and anticipated max BTUs/hr you need. You move 1000lbs/hour for every 2gallons-per-minute you pump through the tankless. Multiply the lbs/hr times the delta-T on the tankless to get the BTUs/hour output. No matter how big the tankless, if you're pushing more than 4gpm through it, it's gonna take over 100 watts of pump. (over 200W on some tankless units.)
Even though specs for various tankless will give you "100 F rise at 6gpm" or something, don't assume that the unit will last very long if run continuously at those flow rates. It's one thing to run high flow rates for 20minutes/day for showering, quite another to run 10 HOURS per day running a heating system. Figure out from the documentation what the MINIMUM flow rate is to guarantee ignition, and if you can get the BTUs out of it that you need with flow rates merely 2-3 times that you'll be much better off in the long run.
If you're hell-bent on designing this yourself rather than hiring a pro, read up on it- a LOT. There's a huge amount of info on the web. PM Engineer mag (
http://www.pmmag.com ) has a lot of info and articles, many by an academic John Siegenthaler are VERY good, eg:
http://www.pmmag.com/Articles/Column/3a1d8c312dfc7010VgnVCM100000f932a8c0____
The "inside out tank" example is the same topology as the HeatPro combi system (substitute a tankless for the boiler.) But I'd recommend adjusting & simplyfying that schematic slightly, with the boiler loop pumping toward the tankless or boiler, not the tank, since pumping away from the boiler lowers the pressure in the heat exchanger making it more likely to form micro-bubbles that sizzle and reduce the heat-exchange efficiency. The pumps for the radiation should also pump toward the radiation, not the tank, as shown. The ErgoMax versions of the indirect have separate heating system sources/returns, distinct from the boiler loop, which simplifies the external plumbing considerably. If you can run your radiation under 140F, you won't need the complexity of mixing valves, etc either- just put a tempering valve on the DHW out to keep the scald-risk under control.
Also, you may be able to further simplify by slaving the tankless loop to the buffer-tank's aquastat only rather than firing whenever there's a thermostat call, letting the thermostats run the secondary pumps only. The primary will start up once the tank's drops below the setpoint, by then a slug of cooler return water will be in the bottom of the tank, and the modulation of the tankless will be able to adjust the flame according to the return-water temp. During simultaneous heating & DHW load will super-chill the bottom of the buffer (32-50F water entering the heat exchanger makes for the largest delta-T and greatest heat transver at the bottom of the tank), resulting in even colder boiler-loop return temps, without affecting the temp of the water entering the radiation, since the top of the tank where radiation water is source is being filled with water at the tankless output temp (which is constant). But the tankless will be firing higher to achieve that output temp with the cooler return water it is getting from the tank due to the DHW load. With the buffer setpoint and tankless setpoint temperatures optimized this can be a quite efficient system that never short-cycles, and runs near the tankless' max thermal efficiency. (Short cycles KILL the efficiency of tankless heaters & boilers- design it keep all burns longer than 10 minutes if you can, or at the very least greater than 10 gallons pumped through the tankless. The EF rating number on the tankless is based on multiple 10.2 gallon draws, not 1-2 gallon draws, so it's often overstated for "real world" situations where startup and purge cycle losses totally trash the operational efficiency of even condensing units to something less thatn 50% efficiency).
OK, too much information, I know...
(Sorry, I just designed one of these for my own place- I can't help myself. <LOL>)
