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ISOBoiler

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Hello, I am looking to replace a cast iron boiler with a mod-con. One contract who came by said that he had good support in the area (DC region) with NTI and Triangle Tube. I've heard mixed things about TT plus none of their boilers seem to modulate much. In terms of NTI, what are your thoughts about their lowest output boilers (I'd probably be looking at the Tx81 or even 57 given my approximate heatloss)? It looks like they have a SS heat exchanger, but nothing mentions that they're fire tube, unlike the TFT (although the 154 model only goes down to 15k) - does that imply their durability/reliability may be somewhat less?

In terms of my overall needs: I have two zones, one with 70 feet of fin tube and two sections of 4-foot cast iron baseboard and the other with 50 feet of fin tube, so not sure how low I'd need to go on the minimum modulation number. I haven't had a formal manual J as yet, but my 82% 140k-input 112k-output AFUE boiler (probably a couple points less as the damper is locked in the 'open' position) used about 800 therms over the past year (minus 80 therms for HWH and stove use) to keep my 2000 sq ft moderately well-insulated house at 60-65 degrees.

Thanks!
 

Leon82

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Well, if you take the 50 foot zone and use 120 degree water temp you get about 210 btu per foot which is 10500. So you would want a min fire no more than that.
 

Dana

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A heat load big enough to call for a TX81 for a MD location indicates either a fairly large house, or a fairly uninsulated house. The TX51 would be a better fit for most. A well insulated 2000' house is likely to come in at 30,000 BTU/hr or less, which would also be consistent with your sub-1000 therms/year fuel use. With a ZIP code for good weather data and some wintertime gas bills (to limit the effects of boiler oversizing, solar gain, and hot water/other gas use on the calculations) one could put a firm accurate upper bound on the load using these methods. If you've been keeping it at 60-65F, use base 60F for the heating degree day base.

Using annual numbers overstate reality due to those other effects, but if we use base 65F, and gross averages, Baltimore averages about 4800HDD65 in a typical year, so for 800 therms you're looking at 800/4800= 0.167 therms, or 16,700 BTU per degree day. Divided by 24 hours in a day is 696 BTU per degree hour. At an efficiency of 82% that's 571 BTU/degree-hour. At a temperature difference of 70F (say 0F outdoors, 70F in) that's 70 heating degrees, x 571 BTU/degree-hour is an absolute worst case load of 39,970 BTU/hr, which is WELL within the capacity of the TX51. Real outdoor design temps are warmer than that in MD, and the efficiency of the oversized beast probably isn't 82% given that it's ~3x oversized for the heating load, and you in fact used some amount of gas for hot water.

Even though last year was warmer than a typical year for most of the eastern seaboard, it's unlikely that your heat load is over 30,000 BTU/hr, and the TX51 puts out quite a bit more than that if you need more. There would be no point to going with the TX81. But just for grins, run the real numbers on real fuel-use against base 60F heating degree days, see where it lands.

The TX series uses stainless water-tube heat exchangers (unlike the TFT series), and come with a pre-installed internally plumbed primary loop, complete with an internal pump. It could probably be set up to pump direct if the system designer can spec a different pump than the one it comes with. NTI is a first-tier vendor with decent products- if they have good local support, go for it!
 

ISOBoiler

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Thanks, Dana. FWIW, Degreedays.net estimates that the HDD averaged over the past 2 years in the area was more like 3800 based on a 65F baseline, but the point stands. I guess the one argument for going to the Tx81 would be that having a nighttime setback is more comfortable for us, but I like to crank up the thermostat first thing in the morning - acknowledging that 180 degree water temps is decidedly inefficient for the 45 min it would run for. But 180 degrees x 150 ft of fin tube x 600 btu/hr during that time would require about 90k of output.

The manual does state that only P/S plumbing is to be used, so I'd worry that direct pumping would void the warranty.

What are your thoughts about the best pump to use for that kind of system? We have mostly 3/4" copper. Would one Grundfos alpha have enough oomph for the whole system, do you think? The plumber seemed resistant to ECM pumps for some reason.
 

ISOBoiler

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One other question - fire tube vs water tube - my impression from reading is that the fire tube reliability and lack of need for as stringent maintenance seems to be better than water tube. Would you agree?
 

ISOBoiler

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One last note - reading the manual, it looks like they only "strongly recommend" P/S piping, so I guess direct could be done. I don't know enough about what the requirements for that would be though. Assuming I'm reading the manual correctly, it looks like the primary pump only comes with the higher output combi models (Tx151 and Tx200), otherwise a Grundfos 15-58 or 26-99 is recommended.
 

Leon82

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You probably don't need 90000btu to recover from a setback.

The point of the mod con boiler is too match the heat loss as close as possible resulting in the lowest return water temperatures
 

Dana

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Thanks, Dana. FWIW, Degreedays.net estimates that the HDD averaged over the past 2 years in the area was more like 3800 based on a 65F baseline, but the point stands. I guess the one argument for going to the Tx81 would be that having a nighttime setback is more comfortable for us, but I like to crank up the thermostat first thing in the morning - acknowledging that 180 degree water temps is decidedly inefficient for the 45 min it would run for. But 180 degrees x 150 ft of fin tube x 600 btu/hr during that time would require about 90k of output.

The manual does state that only P/S plumbing is to be used, so I'd worry that direct pumping would void the warranty.

What are your thoughts about the best pump to use for that kind of system? We have mostly 3/4" copper. Would one Grundfos alpha have enough oomph for the whole system, do you think? The plumber seemed resistant to ECM pumps for some reason.

With a design heat load less than 50,000 BTU/hr (WAY less, really!) it would be insane to install the TX81, though many hacks would push you toward that or an even larger boiler. The TX51 has more than enough "extra" to recover from overnight setback, especially if you're only setting back one zone. (My system is micro-zoned- I use overnight setback on the bedrooms, and it's just fine with 125F AWT, even when it's sub-zero outside, which is below the design condition.)

But bear in mind recovery from setback requires you to use higher water temperatures in a "boost" mode during the recovery ramp, so you would not reap much (if any) fuel savings with a setback strategy, since you would have less condensing efficiency (sometimes dramatically less) during the recovery ramps.

Even with deep setbacks for sleeping comfort you can start setbacks earlier (with a smart "learning" thermostat, or a simple programmable) and never need anywhere near 180F water, even if you opted to do a setback mode. Full-out the TX51 delivers ~50,000 BTU/hr at non-condensing temperatures, divided by 150' of fin tube is 333 BTU/hr per running foot, which it would deliver with 145-150F water output, with 130-135F return water, which is above the condensing zone, but not by much- you'd be looking at ~88%, maybe even bumping on 90% efficiency.

The best kind of pump for a mod-con system is a smart ECM drive type. With a multi-zone system, use zone valves and a smart pump that operates at constant pressure, and play around with it dialing down the pressure to where it gives you ~10-15F delta-Ts across the radiation on either zone at 130F water output (115-120F return). Something like a Viridian 1816 would do it for most systems like yours. What do you have for zone pumps now?

Design by web-forum like this is fraught with potential error- I strongly urge you to hire a real hydronic designer to spec the thing if you're pumping direct. You could probably hack a P/S version that worked reasonably well, though it would use more pumping power. With a decent designer who backs their work you can pr0bably pump direct for less power use (it adds up, when you've dialed in the outdoor reset so that it runs nearly continuous burns), and save money on pumps and unnecessary plumbing labor too.
 

ISOBoiler

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Thanks, Dana, very helpful. I'll note that using 3800 HDD and a delta 70F on design day does put me at 49K BTU, but that's probably still enough of a safety margin to make the Tx51 a fine choice. My one concern is still the question of water tube vs fire tube design boilers. Other than the requirement for boiler flow, which makes fire tubes easier to pump direct (that's my understanding at least), what is your take on advantages/disadvantages of the two types, specifically the reliability and need for maintenance on water tubes vs fire tubes? Great performance is one thing, but if the boiler requires the maintenance of a 1970s Jaguar to keep working well, that's a pretty big argument against. I understand that the HTP UFT and the newest high-turndown Lochinvar are pretty new and there may not be a lot of reliability data as yet to compare to.
 

Leon82

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The firetube is easier to clean as the tubes are vertical and straight.

The water tube wraps around the combustion chamber so a flexible tool is needed.

Internet data seems to think the firetube needs less heat exchanger cleaning appointment s
 

Leon82

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The not so high turndown Lochinvar and also the triangle tube have been around for a while
 

Dana

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The industry trend seems to be toward more stainless steel fire-tube designs, and that is probably not an accident. They are easier to design systems around, and harder to screw up in a truly damaging way (though there are creative idiots out there capable of failing even the most idiot-proof systems ever invented.) My expectation is that average reliability goes up with the more forgiving heat exchangers. Fire tube heat exchangers typically have more water volume than water tube versions of equal output, but nowhere near the water volume & thermal mass of cast iron boilers. I know a couple of pretty good hydronic designers who used to swear by the Triangle Tube Prestige Solo 60, 110, etc who are now completely sold on the fire tube versions from other (including UFT series boilers) with bigger modulation. That series has now been updated, but doesn't have the same turn-down ratios of NTI, Lochinvar or HTP.

They could be wrong, of course, but it's hard to find reasons NOT to go with higher turn down ratios, less sensitive stainless HX design that's easy to specify for a single pump direct-pumped system.

The UFT boiler and it's heat exchangers have been around for longer than HTP's involvement with them, manufactured by Kiturami, a tier-1 Korean boiler manufacturer (that uses nearly all Korean first-world components and sub-systems.) I'm not sure how long NTI & Lochinvar has been making fire-tube boilers, but they are both top-tier manufacturers with a good track records.
 
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Dana

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Some of the marketing fluff supporting fire-tube heat exchangers (from Triangle Tube) reads:


A One of a Kind Stainless Steel Heat Exchanger of a Triangle Tube Prestige TriMax Solo!
text-size-heat-exchanger.jpg


  • No O-ring to wear out/fail
  • 10 Year history of reliable performance
Low pressure drop means less head pressure, smaller pump requirements and/or more flow
  • Self cleaning design
  • Higher water content:
  • Reduced hotspots/over heating risk
  • Lowers sensitivity to water flow variation
  • Easier temperature control
  • Vertical alignment and baffle system is self cleaning:
  • Reduces sludge build-up, lowers maintenance
  • Maintains efficiency over time
 
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ISOBoiler

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Thanks, Dana. Makes sense. While good local NTI support and know-how is definitely a draw, I may search around for someone who has expertise in one of the preferred fire tube boilers. What are your thoughts about the vitocrossal cu3a that modulates down to 19k? Their claim is that the overall system efficiency is excellent despite 5:1 turndown with high mass to prevent short-cycling and is easily pumped direct, but not clear to me how well that would work on a system with a 10k BTU (at condensing temps) zone.

http://www.viessmann-us.com/content...t.file/Viessmann_CU3A_Whitepaper_08072015.pdf

Looks like boiler itself runs around 4800 per the Veissmann price list, so considerably more than the Lochinvar, HTP, or NTI boilers, but maybe the ease of installation pays for itself? A different contractor I spoke to said: "Veissmann is the Mercedes of boilers" but that seemed like nonsense to me (plus I'd rather have a reliable Toyota).
 

Leon82

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If the minimum fire is double your zone load it will reach the setpoint and cycle. You may be able to delay it with programming, but your better off with a lower minimum fire rate
 

NY_Rob

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Minimum boiler BTU output of 19K BTU's for a 10K BTU load isn't a good fit.
You may be able to hold off cycling for a bit if the boiler offers stepped modulation... but, once it reaches the top step (5 steps @ 1 min intervals on the HTP for example) you'll hit the setpoint and cycle.

If the 10K BTU load is on DD with higher SWT, what is the load at 45F outdoor temp when you only need 115F SWT? Cycling could be problematic at that point.
 

Dana

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I seriously doubt your true heat load is anywhere near 49,000 BTU/hr, and a boiler that only modulates down to 19K would be a mistake, given that there are several decent boilers out there than throttle back to 10K or less. Using annual HDD and annual fuel use tends to overstate reality by at least 20%, often more than 25% (with the occasional exception to prove the rule.

Also, 19K at minimum fire is significantly oversized for your zone radiation: 19,000 BTU/hr into 50' of fin tube is 380BTU/hr per foot, which balances at an average water temperature of about 150F, WAY above the condensing zone.

Boiler2.gif
 

ISOBoiler

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So you don't buy Viessmann's argument that their specific boiler may operate better on low-load zones than a low-mass 10:1 boiler? In their white paper, they make a lot of noise about titrating the oxygen levels for the best possible combustion mix and that their high-mass system reduces the issue of short-cycling without a buffer tank, but I can't tell if that's nonsense or not.

As a side note, it's generally unlikely that the small zone would be on by itself without the bigger one as well (the house is plumbed oddly, such that the front half (two floors) is one zone and the rear half (two floors) is the other zone), so short-cycling is probably less of an issue than in a truly micro-zoned house.
 

Dana

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So you don't buy Viessmann's argument that their specific boiler may operate better on low-load zones than a low-mass 10:1 boiler? In their white paper, they make a lot of noise about titrating the oxygen levels for the best possible combustion mix and that their high-mass system reduces the issue of short-cycling without a buffer tank, but I can't tell if that's nonsense or not.

As a side note, it's generally unlikely that the small zone would be on by itself without the bigger one as well (the house is plumbed oddly, such that the front half (two floors) is one zone and the rear half (two floors) is the other zone), so short-cycling is probably less of an issue than in a truly micro-zoned house.


That's right, I'm not buying it, or at least not completely. If you have sufficient radiation to run it at condensing temperatures there is little to no gain to going with a higher mass higher-firing rate. I suppose if you tweaked it super-finely you might find some systems where a 1% increase in average combustion efficiency outweighs the higher cycling losses on some particular system. Essentially what Viessmann has done with the CU3 is combine the buffer tank with the boiler, re-inventing the high mass boiler.

Fire tube mod-cons are not nearly as low-mass as water tube mod-cons, but at the full minimum 18,000 BTU/hr out the ~110lbs of water in the CU3A 26 swings 10F in less than 4 minutes- not exactly a short cycle, but nothing to write excited letters home about. At outdoor temps when your whole house heat load is 7500 BTU/hr the TX51 could be running continuously, whereas the CU3A would be running more than 2 cycles per hour. Again, not exactly short cycling, but it's not really buying you anything.

There are limitations to modulation that can be solved with thermal mass, but in general it's better to solve them with more radiation. You already have enough radiation to deliver 10,000 BTU/hr at 95% efficiency on your smallest zone, so there's really nothing to be gained by going to a higher mass approach, given that there are many boilers that would still be modulating very nicely at 10,000 BTU/hr. A typical fire-tube boiler that modulates down to less than 10,000 BTU/hr has about 25-30lbs of water, which when combined with the thermal mass in the distribution plumbing and radiation is enough to prevent short-cycling even at every low loads, due to the much lower firing rate.

Arguments such as "The reason for this would require another entire white paper explanation but, suffice it to say, higher levels of oxygen actually decrease boiler efficiency by reducing the boiler’s ability to condense." read a lot like "There is this lethal move that I learned in karate camp last summer, but if I showed you you'd be dead." If they have that white paper, let's see it, and how closely the underlying assumptions would correlate to YOUR system and radiation.

That said, in my own house I have some low load micro-zones, and a modulating burner that only turns down to ~19K, which was fully solved by ~400lbs of water in a buffer tank at the center of it all (that also serves as the hot water heater, with an internal heat exchanger), with all zones drawing from the buffer. Had modulating boilers with sub 10K output existed back when I put that system together odds are pretty good that it not be configured this way.
 
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