No confusion here- the difference between the TurboMax et al and "standard" indirects is that the stored water is boiler water (as in a buffer tank), and the potable water is in the heat exchanger. Conventional indirects have it the other way around.  If your radiation temps normally need to be at or above DHW temps, using a"reverse indirect" as a heating system buffer adds mass to the heating system minimizing cycling losses from micro-zone calls, and provides DHW via the heat exchanger without making it a separate zone call. One COULD zone it and operate it in the same way as a standard indirect, but it loses the buffering function. Having a buffer-centric system allows one to micro-zone with abandon without suffering an efficiency loss- the standby losses of an indirect won't be any different by virtue of it's being used as a system buffer as well as a hot water heater.

Under heating system load the bottom of the tank returning to the boiler is roughly the radiation return temp, since the radiation return is delivered via dip-tube in some models, or via an external fitting at the bottom of the tank in others.  During the summer DHW-only use the return water temp will vary by quite a bit:  The bulk of the heat transfer happens in the bottom 1/3 of the tank since the delta-T between tank & street water starts out highest there, since the street water enters at the bottom of the heat exchanger, exiting at the top.  By the time sufficient heat is drawn from the tank in a long DHW flow that the aquastat is tripped, the boiler water in the tank is highly stratified and the return water temp initially quite low. But the turbulence of the boiler loop breaks up the stratification fairly quickly, making it no more (and possibly less) effective at delivering low temp water back to the boiler as a conventional indirect.  As long as the potable water is flowing the return water should stay well below tank setpoint, but during recovery, not-so-much.

Like mod-cons, tankless HW heaters are also not static- if output temps and boiler loop flows are set up reasonably for the load & tank setpoint they'll modulate quite a bit on the variable temperature & volume of the radiation return, and on DHW flows.  But because it's a bit bass-ackwards tweaking it for optimum efficiency is also a bit different (which is why Peerless has a separate set of controls for maintaining tanks with their mini-mod-cons).  Setting storage temps of the tank to something above the design-day radiation temp requirements seems like the wrong thing to do, but setting the tankless output temp & loop flow to something LESS than design-day BTU/H requirements stretches the modulating zone considerably.  As the temp in the tank falls under higher heating load, the tankless fire modulates up, keeping the temp at top of the tank somewhat stable.  As long as the temp at the top of the tank stays at or above design-day radiation temp (or DHW temp) at full load, it works.  My design day heat load was ~30k last year, will probably be ~25-28K this year, so if the Takagi hasn't fallen off a combustion-efficiency cliff at ~20K output I'll set up the delta-Ts & flows there.  Under full flow on the radiation the volume of radiation return will ensure a much lower return temp to the tankless than under light-load.

Unlike mod-cons, tankless HW heaters have somewhat static thermal-efficiency (=combustion efficiency minus jacket & standby losses), since the output temps are what drive jacket losses.  Standby losses on tankless systems are still quite low compared to cast iron beasts however, and made lower by running it in a modulating mode.  A tankless on it's best day will never beat a mod-con on it's worst though.  (And a standard HW tank on it's best day won't beat a tankless on it's worst either, although the gap is smaller than with condensing equipment.)

Rob:  Yeah, my bad.  At the moment my required radiation temp estimates are a combination of calculation & WAG, since it's still unclear how much of the main zones I'll be able to plate vs. suspend, and what the true performance of the building envelope tweaks will be.  I intend to start out the heating season at 115-120F and bump up only as-necessary.  Being a retofit staple-up with a lot of structural & mechanical systems interference, there are limitations to how much of the place I can rip up & rebuild at a time and remain married, eh?   Whatever the necessary temps are this season, I expect they'll be considerably lower in 10 years.   Maybe by the time the Takagi craps out there'll be suitably cheap sub-40K mod cons or something else better matched to my anticipated lower heat load.

Baron,
We distribute the Thermolec brand. link http://www.blueridgecompany.com/radiant/hydronic/325/thermolec-boilers . There are others out there but I am not to familiar with them. >.12 kw is still close to cost effective with natural Gas, oil or propane depending on your local fuel cost. In Nor Cal you may consider a solar component, Good state rebates, the thermolec can be integrated in as back up.
Dan

With separate tank HW heaters you also end up with 2x the installation cost and MORE than 2x the standby losses (since the duty cycle on both burners would be lower.)  Unless they're going flat out you'll never beat the EF numbers by much.  A combined system with one big tank (to better buffer the DHW load) and a heat exchanger for the heating water would do better upfront cost & efficiency-wise, maximizing the duty cycle on the burner (which minimizes standby time.) Summertime efficiency would be somewhere around it's EF numbers (depending on your actual hot water use), during the peak of the heating season it'll be close to the raw combustion efficiency.  Tank heater suffer considerable efficiencies at low volume use, but become quite reasonable running flat-out.  See heaters 1,2,& 3 in this set of better-than-simple EF style testing:

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

You'll be lucky to get 12 years out of a tank HW heater, but it's a lot cheaper than a boiler or tankless up front too. The heat exchangers pumps etc are generally good for 20+ years if properly specified.  If you have hard water you may need to de-lime the DHW side of the heat exchangers annually (or more often) to get that kind of life out of them.

At your utility rates it may take awhile to get payback on a drainwater heat recovery system downstream of the main shower, but not forever.  Find the city with the average ground/water temps nearest you, and plug in your utility rates here:

http://www.ceati.com/calculator/

The model and test data behind the calculations is explained in the appendices here:

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

The calculator has a number of heat-exchangers to select from, as well as a couple of plumbing configurations (preheating the HW heater intake only vs. preheating both the HW heater & cold feed to the shower.)

If you're going for a boiler, the drainwater heat recovery can pay for itself upfront in some instances by being able to downsize the boiler.  But if your heat load is truly 25K even the smallest mod-cons are 2x oversized, and even the smallest cast iron beasts are ~1.2x oversized, and cost the same as their slightly bigger twins. The drainwater heat recovery won't save you up front on them since downsizing from ~50k to 37.5K in 80-83% AFUE cast iron saves you nothing.  (A forced-draft tank HW heater with a 40-50K burner will probably meet or beat 'em in actual use efficiency too.)

Speaking to the electric solution; I still do not get why anyone would use 2 40 gallon water heaters in place of a water heater & electric boiler. You are building a home. a place to hold your assets and value for the future.
Granted the appliance may be 1/5 the cost. Life time of a tank is 1/2 or less of a boiler. The efficiency of a modulating electric boiler with outdoor reset is a huge plus, the boiler is off when not in demand, and reinstall ten years down the road will eat any savings in appliance cost of the water heater and more. Best to find the extra cash and place a boiler nest to the electric water heater. The solar can always come later if at all.
Dan

water heaters last a lot longer in heating applications than in domestic only apps, there is very little reason why one would die any faster than an electric boiler. and adding the extra volume to DHW would allow for more robust domestic loads as well if that's an issue. there are reasons to go in both directions, depending on the project.

There is practically speaking very little difference in efficiency between a tank and tankless electric heater in most heating applications; and it's not like outdoor reset matters with the electric heat source at all, it's just a question of parasitic loss and comfort. If you need reset, then the boiler with it built in is a fine choice, assuming sane comparison in costs between electricity and other energy sources.

And with a high-mass radiation like slabs outdoor reset matters even less, eh?

Outdoor reset might make a difference in keeping a low-mass radiation scheme comfortable with a high-powered electric boiler, and it makes a difference in maximizing the combustion & thermal efficiency in a fossil burner by keeping the return water temps as low as possible.  In a high mass/low temp radiation scheme like a slab I'd be more willing to spend money on PID thermostatic control than outdoor reset, which, face it, is an extremely crude model of instantaneous heat load at best.  The heat load of any structure at 0F on a sunny calm afternoon will be dramatically different than 0F during a blustery midnight storm, etc.  PID control will track and respond far better to the actual changing heat load than outdoor reset regardless of the radiation scheme, but becomes more important with the slower response times higher mass radiation.

And the higher mass of the tank does indeed make for better DHW temperature control with better peak flow and slower temperature fluctuations.  For a low heating-load combi a big ol' tank not only cheaper than an electric boiler, it's better. Only in a high cooling dominated climate might it tip the other way.  (But in those areas heat pump heating/cooling w/desuperheater DHW preheat & electric tankless or point-of-use minitank finish heat makes more sense than radiant slabs & big tanks.)

actually no, with high mass slabs, if your water temp is out of sync with your needs, you can more easily get room temperature overshoots and undershoots. reset helps a lot with that, though typically full slab water temps are low enough that if you weren't going to get any additional efficiency out of it, you'd be better off just with a good thermostat than with reset on the slab... as you say, with some PID logic.

but with higher water temperature requirements on design day, I would revise my estimation. and that can happen a lot in "medium mass" situations like overpours or slab on grade homes with a variety of floor coverings and glass coverage.

I've been working through a cost comparison, but am missing some important data:

What is the average efficiency of a standard gas tank water heater (say 0.58ef) when operated with 75-80° return water temps? All info I find is with a incoming groundwater source, which I would assume bumps up the numbers efficiency significantly.

Also, is there any mod/cons that operate in the 10K-50K Btu range?


I should add that our annual energy usage is 70MBtu for our plan in this climate.

I ballpark tank heaters at 75% efficient in heating applications. It appears to be pretty close according to what little measurement I've seen. energy factors do not apply to combustion efficiency calculations, they are a whole different type of measurement on a very small amount of usage where standby losses are a much larger fraction of the total load.

the triangle tube prestige solo 60 and Munchkin T50 are the smallest mod/cons I am aware of at this time. both in the type of range you talk about , though the T50 won't do 50k. Properly controlled, I would ballpark them at 95% efficient in low temp applications.

Ok, so here's the results with our data:


Annual Cost to Operate:

1.) Electric @ 100% = $1,260 (~$600 installed)

2.) Gas Std Tank @ 75% = $960 (~$1,200 installed)

3.) Gas Tankless @ 95% = $760 (~$3,500 installed)


One nice part about the tankless is our application will be direct vent, and many of these are are built that way.

whoa now: i hope by "tankless" you mean "mod/con boiler" but I imagine that would be more than $3500 installed if so.

A "tankless" will only run a bit over 80% if controlled well in a heating app according to the numbers dana has.

these are all ballpark numbers of course, but you can see that there is little to justify a typical tankless water heater install. A mod/con boiler would be more expensive, and in your particular case it *might* not make sense. but I don't know what's included in those numbers. heat exchangers? Indirects? or just the straight up heat source install itself?

Yes, tankless being a small mod/con boiler like the Takagi or Triangle Solo-60. That's just looking at the cost of the heater and venting, and ignoring the rest of the parts that woudl be common to all systems.

Going to the mod/con would probably mean using an indirect for the DHW. I just don't see the payback in this case vs using seperate tank style heaters for the radiant and DHW.

We've been trying to spend the money on the building shell, reduce the energy usage rather than upgrading the energy sources. I think this is one of those cases. Any recomendations for direct vent tank water heaters? We see a lot of Rheem and John Wood heaters around this area.


Back to the orginal topic: I think we'll just use some standard air sensing thermostats and run them directly to each zones pump. Again, keeping it simple and save costs.

The flat-out combustion efficiency an atmospheric-drafted 0.58EF gas fired tank is going to run about 80%, no more (and not much less), independent of return water temp, as long as we're talkin' return water temps under 130F.  At 50% duty cycle, with jacket-losses the thermal efficiency will be ~75-78%. At  25% duty-cycle you're lookin' at 70-ish.

All mod-cons will all run ~94-95% efficient at return temps under 100F at low-moderate fire, but there isn't a huge premium for going a lot lower. Dropping it to 80F returns that bumps it to the 95-96 range.

See:

http://www.csemag.com/articles/blog/1250000325/20090201/Boiler-Efficiency-vs-Temp-2.jpg

The Peerless Pinnacle T50 is a 3/1 turndown with a max input of 50K/18K. 

The Triangle Tube Prestige Solo 60 is a 4/1 turndown, 60K/16K.

The Munchkin Contender MC50 is a 3/1 turndown 50K/18K

They're out there, but I can't think any that go as low as 10K at low fire (only in my dreams! :-) )

Beyond combustion efficiency the fact that mod cons are all sealed-combustion is worth another ~5% reduction in fuel use due to the lack of necessary combustion-air coming from infiltration into the the conditioned space, adding to the load. The operational efficiency difference between a mini mod-con and an 0.58EF tank isn't as simple as the thermal efficiency test numbers might indicate. In a low-temp system like yours the mod-con will be BETTER than the 95%/75% numbers might imply, since it's duty cycle will always be higher, as well as it's thermal efficiency.  In the off season you'll be under 60%, and the mod-con will have only degraded to ~80-85% (due to cycling losses) in water-heating mode.  Mind you the difference between 82% and 60% efficiency strictly for hot water heating isn't a huge annual cost factor, but overall the mod-con with an indirect will do a lot better- 30-40% less annual fuel use. 

My Rube-Goldberg contraption using a tankless heat source should beat the tank by more than the simple numbers too, due to lower cycling & standby losses- not that it makes sense for you.  A mod-con will be able to take far greater advantage of the low return water temps. A mod-con with a standard indirect would blow my system away on efficiency with your radiation.  But it won't be cheap.  Whether you go with a hot water heater vs a mod-con is a matter of how much are you willing to spend up front to save 15-20Mbtu/year, and could the same money be spent elsewhere to achieve a similar or greater reduction.  I spent the difference between a $500 tankless and a $2000 mod-con on retrofit insulation & air sealing.  I'll find out soon enough if that was the right move, eh?

You guys are leaving out an important aspect of the Electric boiler operation, when there is no demand for heat the appliance is off. no stand by loss. no 40 gallons waiting for next time, off..........
That's among other reasons why I like the isolated electric boiler on the heat side.
Dan

standby loss is a small factor for a heating system. and operating costs are operating costs. the presence or lack of standby loss would never choose a fuel type IMHO.

Kicker: the takagi is not a mod/con boiler. it is a tankless heater. the kind of heat source not to be used. I also suggest you do not zone by pump, and instead use a single system pump with zone valves. at a minimum of 60 watts/pump for anything you're likely to use here, cutting down the number is nice.

so if I am correct an electric 40 gallon water heater cost what $45.00 a month to keep on? any one have the math?
I agree that zoning with a mz valve is a splendid choice you may save 5.00 month per additional zone as Rob has pointed out, but I still cant get away from the concept that a 40 gallon tank in the late spring - early fall running, waiting is the right choice, I guess the guy can flip the breaker or rigg a relay. Just not big on water heaters, Takagies included.
Dan

a 5' high x 1.5' diameter tank has something shy of 30 square feet of surface. with 1" of foam, If the tank is at 140 and the room is at 70, that's about 420 BTUs/hr. about 3 kwh/day, or .18 cents a day, $65/year for our friend here. and that assumes the heat is lost, not lost to heated envelope which would reduce that number significantly or even eliminate its importance completely.

still like wall mounted boiler.
Dan

it also assumes you dont add any additional insulation.

You can like whatever you like. but math tells you what you should do ;)

Math is my friend too, but I still evaluate space, visual presentation, added value to the home, design intent (in Washington builders can not use a electric water heater for heating unless it is approved and marked so on the appliance for that function buy the manufacture). A tight well designed electric boiler placed on the wall occupies almost 0 space w 24" h 60" D 12". If you take the argument of what square footage value a water heater may occupy as value you have this math water heater 24 inch diamiter (4 square feet) plus mechanical 2 square feet on wall vertical space total 6 square feet. Electric boiler vertical square feet on wall same 2 square feet.
now we have a net difference of 4 sq ft building cost 100.00 sq ft? 200.00sq ft? dead space/taxes/financing for life of building?
plus that some where less than $65.00 year stand by loss Double if you pay .12 kw ( I appreciate the math on loss by the way).
The argument for a tidy electric boiler is strong, in this application 27,000 BTU / 50 amps 8 kw the boiler cost is about 1,025. or less delivered.
Dan