Dear everyone, I'm on the fence about this choice for installing a Takagi tankless heater for both closed loop radiant and DHW: Option A. Put the Takagi on the household water side of the HX and heating the radiant closed loop via the HX. Seems like most people do this one. or Option B. Put the Takagi on the closed-loop, hydronic heating side of the HX and heating water via the HX. This is what's done by a local guy who installs radiant around here. He says he does this to prevent scale buildup in the Takagi and extend its life (running a glycol solution, though I'd think distilled water would be just as good). It's also the solution a guy who wrote an article for the Journal of Light Construction used (see attached image, from Feb '07 JLC) It seems to me that option A has the advantage that the Takagi is being used to directly heat something that's needed year round (hot water), while using the less efficient HX for heating, which is only part of the year and in an efficient house like ours, not a big drain. Also, in option B, every time you need hot water you not only have to wait for the tankless to get going but the HX needs to heat up as well. But option B could potentially buy me some years before having to replace the coils for the Takagi, or the entire unit. Any thoughts would be much appreciated. Thanks, Paul

You also have to run a relatively large pump every time you want DHW if it's on the other side of an HE, and the HE probably has a fairly significant pressure drop.

Fact is, a tank water heater is probably at least as good of a choice here, with significantly less complexity, cost, power usage, and will run very similarly in terms of efficiency. If your load is low, it's a good choice. If the load is high, then a mod/con boiler and indirect is a better choice than the takagi.

Thanks, NRT.Rob...

Regarding using a tank water heater: My design load is 30K BTU/h. Can a 40K BTU/h tank water heater really be expected to keep up on the coldest days where 30K BTU/h is required as well as providing plenty of hot water? It seems like that's cutting it pretty close. Or would we need a stronger heater?

The HE we have is a tube in tube style, not a flat plate. I don't know what the pressure drop is, but I don't think it's as bad as a plate HE.

Also, I realize that there would need to be a strong pump and more pump activity, but at 1.7 amps, it's not exactly expensive compared to having to replace the tankless because of scale, right?

Paul

That is cutting it close, but how close depends on how much hot water you're talking about. You still have the full storage capacity of the heater, you just slow down the recharge rate.

Obviously running the pump is cheaper than replacing the on demand, but the tank heater is yet cheaper still; so would a larger water heater, with no real compromise. it would perform better for most types of DHW demands, just as well for heating, using less energy and costing less all around.

I don't generally see the wisdom in trying to shoehorn a more expensive appliance into doing things It isn't really meant to do.

Also, with a 30k load, if you are in a cold climate you probably want to consider a real mod/con boiler. the additional efficiency should result in payback in a reasonable timeframe. Of course that depends partly on your fuel cost assumption.

What you're saying makes sense, Rob, but at the same time it's like you're recommending going either cheaper (tank water heater) or considerably more expensive (mod/con). Also, I know tankless heaters aren't the superefficient things they're cracked up to be by some, but what about the standby losses using a gas-fired tank heater. Even though in its dual use role it would be in use more often (reducing standby loss, i'm told, though i haven't seen numbers), for about half the year or more it would mostly be just a water heater, losing heat up the flue.

Standby loss sounds big as a percentage when you are in water heater mode, but as a real volume of heat it's pretty small. When tested the water heaters are rated on a very small amount of hot water usage, so small amounts of loss equal very significant percentages. In heating mode, the efficiencies are very similar between tank and tankless (verified in a recent discussion on the matter on these boards) as standby loss remains practically the same, but shrinks as a percentage of the overall load that is being serviced.

I am recommending either much cheaper or much more efficient as the two options. This is a "middle ground" that does neither particularly effectively. In a cold climate, I would go for efficiency. In a mild climate, I would go much cheaper if the extra cost of more efficiency is not deemed to have acceptable payback for you.

Another approach is to use a (more expensive but) highly efficient low-head heat exchanger like an Ergomax E23 it takes less pump, and adds buffering to the heating side to keep it from short cycling on smaller zone-calls, as advocated by this guy:

http://www.heatpro.us/designtree/documents/tanklesssys.htm

...which is similar to this system:

http://www.pmmag.com/Articles/Column/3a1d8c312dfc7010VgnVCM100000f932a8c0____

...or this one:

http://www.pmmag.com/Articles/Column/49425e1f0bfc7010VgnVCM100000f932a8c0____

(If you're concerned about standby losses, it's VERY easy to cover these in iso-board to reduce the standby to an arbitrarily low level. It's about the same as any indirect tank, but boiler water rather than DHW is what is buffered.)

If you configure the heat-exchanger/buffer tank essentially as a fat hydraulic separator for the primary/secondary (as in the heatpro schematic) it'll take maximal advantage of the modulating capability of the on-demand, and buy you a percent or three of overall seasonal efficiency.

Where the cost/benefit breakdown on different approaches lies is a moving target, but the buffered on-demands generally outperform any bang-bang on/off boiler, and approach mod-con performance in higher temp heating systems where condensation temps aren't possible for much of the season. (Mod-cons WILL outperform on-demands in most radiant applications, but typically only by ~10-12%, never 20%.) The best average performance you'll ever get out of a tank HW heater is about 70-75% (at most 10% above the EF rating) since their raw combustion efficiency is limited to about 78% (due to the lack of turbulence on the water side of the heat exchanger), compared to about 85-87% combustion efficiency or a typical on-demand (where the EF rating is 80-82%), or 95-98% for a typical mod-con (AFUE tested to 92-95%) A well designed low-temp heating/DHW system with a tankless for a boiler should deliver an average efficiency in the mid-80s- several percent better than it's EF rating. Combi-systems using on-demands are under intensive studies by utilities in CA, and I expect to see quite a bit more published about them over the next coupla years. For the past half-decade it's been something of an experimenter's arena, but the results have been better than anticipated by many, including this former-skeptic (scroll down or search the page for "boiler"):

http://www.profitableplumbing.com/_wsn/page5.html

But in any configuration, keeping the tankless on the heating side of the exchanger will reduce maintenance problems. A constant supply of fresh water increases the liklihood of scale formation & corrosion, requires more frequent filter changes, etc. If only heating-loop water runs through it the heater stays cleaner, lasts longer.

dana, the study you posted last time indicated 80% for a tank heater in a less than ideal heating system. The on demand, not that much higher, 2 or 3 percent. where are you getting the 70 to 75%?

and the 10 to 15% jump to mod/con territory will achieve payback in a reasonable timeframe on that load in any heating dominant climate, in most cases, over on demands or tanks... depending, of course, on your fuel cost assumptions, the biggest X factor. Even in high temp systems, with outdoor reset the boiler is in condensing mode the majority of the time (check your bin calculations :D)

As I noted the last time this came up, this is a solution looking for a problem. It's much more expensive than a tank, for similar efficiency. It's less efficient than a mod/con, and doesn't hit a sweet spot of any kind for cost effectiveness. I see no application where I would see that any differently. You can continue advocating for it I suppose, but I don't see the practical application benefits. If you want to save money, save money. If an efficiency increase will get efficiency. But half-assing both doesn't seem to make much sense to me.

Thanks for all the info. It's a shame I didn't know about the efficiency numbers for a dual-use tank water heater in a radiant system a year ago, when I bought the tankless.
At this point, with the stainless steel venting already bought and in place, it doesn't seem worth it to try and downgrade from tankless to tank, even if they would take it back (still unused)

I ran some quick calculations to see what my savings might be in efficiency by upgrading to a mod/con and this is what I got. I assumed 92% efficiency Munchkin contender, and only 80% for tankless :

Total estimated heat/DHW load: 68MBTU/year * 10$/MBTU= $680/year
92% efficiency mod/con= 680/.92 = $739/yr
80% efficiency WH = 680/.80= $850/yr

difference of $111/yr

Munchkin contender mod con is $1000 more than Takagi
9 years difference paid off.

by the way, I found and read/skimmed Rob and Dana's lengthy discussion on this subject---whew! a lot of that was beyond me, but relevant. All of the thoughts in that, though, get weighed against my situation (in the above post) with already-bought equipment and already-installed venting. Still would like to know what you think given that particular situation.

And regarding my original question. If I do keep the tankless, do I understand everybody would vote for what I called "option B"? That is, having the tankless run on the closed loop side away from the hard water, and doing DHW through the heat exchanger.

Paul

Thermal efficiency and operating efficiency are not the same thing, nor can ROI be calculated in a realistic manner from such simple information. But this is an old argument and one that won't convince those fixed on the lowest possible (Walmart) cost shopper.

I find that pretty insulting, BadgerBoiler. My point of view is not fixed. Instead, what I am is pre-commited, because most of the equipment is already bought. I'm willing to entertain the idea of attempting to return some of it, but given that I've had it nearly a year, I don't think that's likely. How about you suggest something given the reality of the specific situation that I've asked about, including limited resources and given equipment, or if my situation is distasteful to you, then move on to helping someone else.

Many here and elsewhere try to gain useful information in order to justify - or correct - their mistakes. Others ask before making the leap into a complicated and potentially expensive project, thus avoiding costly and embarrassing mistakes. It is for the latter I contribute here.

This is all true. 

With all types of burners, the raw combustion efficiency is a limiting factor.  The reason a  tank heater with ~78% combustion efficiency can't acheive steady-state numbers for average performance in a heating environment is that it can't modulate, experiencing more cycles & accrues far more cycle losses despite being better insulated than their low-mass tankless cousins.  That said, if the burner is ideally matched to its heating load, it can hit within 3% of the steady-state combustion efficiency, but probably never within 2%.   But if oversized for the load the cycling losses will drop it another several percent, which is where I get the 70-75% guesstimate.  (I've modeled it mathematically, but not verifed the model against a real system.)

With any reasonable amount of buffering and a decent system design the tankless can perform within a percent or two of its steady-state combustion efficiency, provided the average heating load is well above it's lowest modulation level.  They typically test out in the ~85-87% range for steady-state efficiency (without that level of combustion efficiency it would be impossible to score a 82% on an EF test, but they do), so in a heating environment 84-85% percent performance is possible with sufficiently cool return water.  In the comparative test NRT.Rob refers to the combustion efficiency of the tankless measured about 85-86%, with return-water probably never colder than 100F (the return from an air-handler coil), which is easy enough to hit in most radiant systems.  This has the attention of regulators & utilites in CA- as I stated above, I expect to see a lot more detail about these systems published over the next couple of years.

The delta between steady-state and in-system performance is true for their mod-con cousins too, but it takes a bit more design customization (and possibly more radiation) than with a tankless to be able to get the condensation aspect. It's far easier to screw up the system design and see mod-cons running 10% below their best-case steady state efficiency, at which case they're little better than a well-tuned tankless system.  Those 98-99% steady-state numbers are usually measured with 80F or colder return-water, which is hard to design for.  The AFUE test has 120F return water (VERY easy to design for), where they typically test in the 90-94% range, cycling losses included. Properly sized for the load, one SHOULD be able to beat it's AFUE performance, but not if it's oversized, or if higher-temp/lower delta-T radiation is required (as is often the case in retrofits.)

But it's impossible to make a simple arithmetic calc to determine the actual ROI for any of 'em based on the combustion-efficiency or AFUE-test numbers- it's VERY system-design & actual-load specific.  Tankless burners are somewhat more flexible and easier to design for than a tank, but the system will be more expensive.  Mod-cons can be easier to design around if already reasonably load-matched, but in ever-shrinking heat loads of well-insulated smaller houses their efficiency advantage starts to evaporate.

Your point is well made. Given the current state of new construction, and the vast majority of boilers - now and in the future - being fit to older homes with existing radiation, your point on proper sizing is exactly correct (This leaves the hard-core "I can do everything myself" DIYer out in the cold, so to speak). The vast majority of systems I design or retrofit to older homes will utilize return water temperatures well below 130°F and thus condense in all but design conditions, justifying condensing boilers for their purpose build role.

I could argue that a conventional power-vented water heater is the easiest to design with and very close to the operating efficiency of the overused "tankless" (non-condensing) water heater, but I won't.

While you all are involved in the more encompassing theory of using tanks, tankless, mod/cons etc., I would really appreciate feedback on my original question, which was, given the existing equipment I have, and given whatever mistakes you believe I have or haven't made, would you agree with putting the tankless on the closed loop/hydronic side while heating DHW through the heat exchanger, or on the domestic water side, heating the hydronic loop through the heat exchanger?

Paul,

You are running into difficulty with this question because putting the takagi on the closed side of the HE is practically unheard of. It might be a great idea, but I don't think you're finding many people with any direct experience with that particular setup. The vast majority of those systems are set up with the on demand on the domestic side of the system.

Thanks, Rob. I appreciate that clarification. So far, if I understand right,I have one "vote" on this forum from Dana that, at least theoretically, putting it on the closed side might be preferable. I can understand that without specific experiences with that type of setup, no one would want to weigh in on the dilemma.

I do appreciate all the thoughts and information from everyone.

I would like to point out to all that if the experts on this or any other forum wish to be helpful, they should accept the real world scenarios, decisions and mistakes that people like me bring to the forum. They might consider what the person's actual questions, needs, and time and financial limitations are and deal with them at face value, rather than belittle and complain when presented with a problem they think should have been handled differently. Consider it a test of expertise. Not everyone who's incompletely informed is looking to stay that way. This is directed not only to Badger, but to the many posters on this and other forums who take similar tones.

I am going to reply to the original question.
I have a Takgi running both radiant and DHW(open system).

I think you need to take into account how fast you will get hot water for domestic through a heat exchanger
and how hot your tankless will have to be set at to get a decent temp at the tap.

I think that you will not be happy with the combined time it tales hot water to get to the tap.

Jmagill, thanks for your reply. I plan on getting around that problem by having a buffer tank of hot water on the DHW side of things. Along with helping to prevent the Takagi from short cycling, it will ensure that there is always hot water available on tap while the Takagi and heat exchanger are getting up to speed. If you have the same problem, you might consider a similar solution. (see this article: http://www.taunton.com/finehomebuilding/how-to/articles/add-a-tank-and-pump-to-a-tankless-water-heater-to-save-energy.aspx). I'm not certain whether the heat exchanger will lower the upper limit of how many gpm the tankless can deliver to the hot water supply. But it is a tube heat exchanger and not a flat plate. I'll need to check the head loss and other stats on it to be more sure, I guess.