Can I do this to my modcon?

Discussion in 'Boiler Forum' started by ToddinMaine, Dec 16, 2012.

  1. ToddinMaine

    ToddinMaine New Member

    Messages:
    15
    Location:
    Central Maine
    Won't the circ. pump just draw water from the buffer tank until its' water temp drops enough to fire the boiler? Then the boiler's got to bring 30-40 gal. back up to the hi limit? Or am I missing the whole point.

    As an interim measure, I've tied the two downstairs zones together by running them off one t-stat ... At no real sacrifice in comfort, I might add. Given the minor demand on the second floor fin tubes, I'm essentially presenting the whole house load at each burn. Additionally, I've set the max boiler output at 60%, so it doesn't rage up to setpoint temp (180 deg) and overshoot. Now, it will typically take 5-10 min. to make setpoint then fall back to 10-20% power until the T-stat is happy, which can be a half hour or so, given the slow response of the radient floor system. That's okay for now, but when the outdoor reset starts bringing the supply water temp down from 180 deg. the long burns will probably disappear without the added mass of a buffer.
    I assume a number like 20% of max power represents 20% of the boilers' modulating range, in this case 80KBTU. So 20% power is 16KBTU plus the 20KBTU minimum rate, which = 36KBTU. Correct?
    Is setting the output limit at something less than 100% a valid way to bring an oversized boiler into line? Why else might that adjustment be provided?

    Todd
  2. Dana

    Dana In the trades

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    That is simply not so!

    Mod con boilers operate in a hysteresis high/low limit band around the outdoor reset (or fixed-temp, if so operated) setpoint. On some that hysteresis is programmable, others not. On the versions that aren't programmable the hysteresis band is typically between 5-10F wide. If you have 3 gallons in the system/loop it's about 25lbs of water and a 5F change in temp takes (25 x 5=) 125 BTU.

    At the SSC-100s' min-fire of ~20,000BTU/hr-in or ~18,000BTU/hr-out, it's dumping heat into the system at (18,000/3600=) 5 BTU per second, so it'll shut down in about (125 BTU/5 BTU per second=) 25 seconds, then fire up again when the heat emitters pull enough heat out of the loop that the system temp has fallen that 5F.

    With a 27 gallon buffer tank plus the original 3 gallons you now have 250 lbs of water, and the minimum firing time is now 250 seconds (over 4 minutes) rather than 25 seconds, which is a HUGE difference in both efficiency and wear & tear on the boiler.

    Swapping in the SSC-50 it would have half the min-fire output, and twice the minimum burn time. The 25 second short cycles now become 50 seconds, which is better, but it's still quite a hit in performance. But even 15 gallons of buffer would be enough.

    Putting enough fin-tube in the micro-zone to deliver 9000BTU/hr (the min-fire output of the SSC-50) into the small zones at 120F AWT probably isn't going to be possible (that's about 40 feet!). Combining the fin-tube zones into a single zone would work fine with the smaller boiler, but it would still short-cycle with the existing boiler.

    In the mean time, setting up the boiler to run at a fixed temp output of 160F+ would kick the burn times up to over a minute. PEX is only rated for 180F, so don't push it too far. At 170F out of the boiler you'll likely see an AWT of about 160F, and 20' of fin tube will deliver ~9000BTU/hr (about half the min-fire output of the boiler) at an AWT of 160F, which should roughly double the lengths of your short-cycles.
  3. Tom Sawyer

    Tom Sawyer In the Trades

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    Sounds good in theory but having fought this battle countless times I'm here to tell you that a buffer tank will not solve the problem. The boiler is 3x oversized for the radiation. You need to get away from the calculator and into the field.
  4. Dana

    Dana In the trades

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    I'm SO glad you're looking out for my needs, Tom! :rolleyes:

    Math works pretty good in the field too, just like gravity- it's more than just a theory. Buffers may be a band-aid, but properly applied they can work. Simply throwing a buffer at it without doing the math on the whole system out doesn't cut it though. I too prefer increasing the radiation size of the smallest zones and reducing the boiler size for a better match, but limitations of first-order simple-math models aside, add mass can (and does) work, if you do it right.
  5. SteveW

    SteveW DIY Senior Member

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    I know nothing about boilers but noticed what looks like an unsupported expansion tank on the right side of the photo - is this true?

    I would personally want to see this supported so that when the diaphragm eventually fails and the entire tank fills with water, it doesn't go to ground and turn your basement into a swimming pool.
  6. Tom Sawyer

    Tom Sawyer In the Trades

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    Agreed but from what I'm reading, I just don't think a buffer tank by itself will increase cycle times sufficiently. And I know nobody likes the thought of buying a new boiler but it's really the only way out.
  7. ToddinMaine

    ToddinMaine New Member

    Messages:
    15
    Location:
    Central Maine
    Aside from a min. fire rate of 20KBTU rather than 10, does setting the max boiler output at 50% equal a 50BTU unit? I seem to be doing okay right now with the two 1st floor radiant zones tied together and max output set at 60%. It will throttle back to 10-20% and go continuously until the t-stat is satisfied.
  8. ToddinMaine

    ToddinMaine New Member

    Messages:
    15
    Location:
    Central Maine
    Hmmmm. Good call. My only defense is that for some reason it looks a lot bigger in the photo than it really is. On the other hand, it wouldn't take much to brace it off the HW tank. Thanks for pointing that out.

    Todd
  9. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

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    Location:
    Minneapolis
    Dana is right as usual but the real rub here is the common mistake made by professional and novice alike, which is over-sizing condensing boiler. The most important thing is not making sure the boiler is big enough...it is making sure the minimum fire is low enough to serve the smallest zone load without out short-cycling (about a minute run time on modern condensing boilers). The exception to this is your typical sun room sporting the highest heat load per square foot at with the lowest load (amount of radiation). This combination inefficient in any hydronic design but deadly in an over-sized modcon.

    The thing to take away from this, is the root cause. DIY design.

    We often use condensing boilers to replace cast iron serving fin-tube baseboard but never specify or install baseboard on new construction. If you can't afford anything but fin-tube you should be looking at forced air.
  10. Tom Sawyer

    Tom Sawyer In the Trades

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    After the mistake is made is when everyone scrambles to find a cure and they invariably mess with the boiler when the problem can't be solved there. Even adding radiation won't cure the issue because the heated envelope can't shed heat fast enough.
  11. Dana

    Dana In the trades

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    That's what thermostats are for, eh? :)

    With low-mass emitters like fin-tube you're not going to get a monster temperature overshoot with just 2x the min radiation required to heat the zone, and if you did a T-stat with tweakable anticipation would usually fix it. With excess high-mass radiation you may have to resort to smarter PID algorithm thermostats if the radiation is grossly oversized & very massive, but that would be a huge boost in comfort too.
  12. Tom Sawyer

    Tom Sawyer In the Trades

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    Many years ago I had a rep that promoted ringing the exterior walls with baseboard which allows lower boiler temperature. Of course homeowners weren't keen on the plan but it does work. Unfortunately many see radiant heat and mod con's as a panacea but thermodynamics is thermodynamics regardless of the wiz bang technology. Funny that the Europeans are re thinking high mass cast iron boilers again. Maybe the old timers had it right. I install a lot of mod con's and I'll say right off the bat that sizing the boiler to the load and sizing the load to the envelope is way more critical than running copper fin tube and slapping a C.I. Boiler in ever was. I'd say that 90% of mod con/ radiant problems stem from poor or no calculations being done. Bigger is not better. I have seen far too many oversized boilers feeding insufficient radiant tubing. If the system is close it is possible to tweak thermostats and modulation settings. If it's way far off though, you are screwed.
  13. Dana

    Dana In the trades

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    A simple-designed ~2400' rancher built to current MA code min (IRC 2009) comes in under 30KBTU/hr at MA outside design temps, for which there are remarkably few boilers "right sized" to the whole house load at the 99% outside design temp, let alone what happens to individual zones at the average when they insist on micro-zoning it like this one. (It doens't take a very big napkin to sketch out the problem with THIS installation, even when writing in lipstick! :) ) The "design by hackery, no math needed" approach is guaranteed to cause issues, independent of the thermal mass of the boiler. Short cycling a high-mass boiler is at least as big a hit to efficiency as low-mass mod-cons.

    Europeans have far from abandoned modulating condensing boilers, and have quite a few tiny output mod-con options as yet unavailable in the US. I'm not sure where you're getting the impression that they're moving toward high-mass boilers on the right side of the pond. They're quite fond of low-temp radiation- panel radiators are the standard (and a good one), but high-mass low temp radiant floors are also quite popular amongst those who can afford it. But on new housing stock most European homes have a fraction of the design heat load of most new northern-US homes (milder climate, smaller houses). There is a burgeoning European interest in wood boilers and pellet boilers with high-mass thermal buffer tank storage, but for gas-fired goods mod-cons (and tankless-combis) are still dominating the market.

    With oil-fired burners as with wood-burners, the limitations of minimum burner size drive them to higher mass solutions than with gas burners. It's possible to make VERY tiny gas burners (modulating or otherwise), but with oil burners the practical low-limit for burner sizing is about 14-15kilowatts or ~50kbtu/hr, which is well over the design-day heat load of a brand new western-European house (and 2x the heat load of many newer homes even in MA.)
  14. ToddinMaine

    ToddinMaine New Member

    Messages:
    15
    Location:
    Central Maine
    Okay, that gives me some assurance that the quick 'n dirty heat loss calc. I did for my house, a new 1900 ft cape and -2 deg design temp, coming in at about 27K BTU is in the ballpark.

    I've put both radiant zones on one t-stat and have set my boiler to 65% max output. My burn times are now averaging 12 min, aided by the fact that the two small fin-tube zones upstairs rarely call for heat.
    If I back max power down to 50% and it does the job during the sub-zero weather coming up, can I be sure that swapping this 100k unit out for a 50k will be the right move? My reset is set at 180 w.t. at 25 deg o.d. temp.
  15. Tom Sawyer

    Tom Sawyer In the Trades

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    I believe a 50k would perform much better in your case so if you can swap it, by all means do.
  16. Tom Sawyer

    Tom Sawyer In the Trades

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    I didn't say abandon but they are toying with high mass, low temperature models
  17. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

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    Location:
    Minneapolis
    When teaching professional HVAC contractors to "believe" that radiant floor would heat houses (20 years ago now) I often started my seminars by asking water a particular house may present for a heat load. The results written down and passed to the front of the room ranged from 10 to 75 Btuh/sf. I have observed no improvement being in the field again these past 12 years.

    First, the block heat loads and then the room-by-room to confirm radiation and design water temperature. We do not use fin-tube on new construction, period. If you can afford to be comfortable, you can afford radiant panels.
  18. Dana

    Dana In the trades

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    I've never seen a 1900' house in New England (even an older house) with glass in the windows that actually NEEDED a 100K boiler, yet most older stock came outfitted with 150-200K or larger behemoths, and subsequent installers rarely corrected the situation. (The boiler in my ~2400' house was less than 15 years old when I moved in- a 6-plate cast iron beast with ~120K of output for a heat load under 50K, now well under 40K after tightening up the house.) Moving to a 50K boiler would DEFINITELY be a move in the right direction.
  19. ToddinMaine

    ToddinMaine New Member

    Messages:
    15
    Location:
    Central Maine
    Okay, here's the confirmation I needed. I set the boiler's max CH power at 50%. With all zones calling, it took 22 min to bring the supply temp from 140 deg. to 180, then it maintained 180 at 37% power. So. Assuming that 50% power actually means 50K BTU for this boiler (and I intend to call Utica to confirm this) I should be okay with the SSC-50.
    I could spend money on a buffer tank and the replumbing for it, and still have an oversized boiler, or I can spend maybe the same (or less) money to trade for an appropriately sized boiler, practically a drop-in job, and live happily ever after. No brainer.
  20. ToddinMaine

    ToddinMaine New Member

    Messages:
    15
    Location:
    Central Maine
    Our radiant is clip-finned pex suspended under the floor - it heats the airspace beneath the floor so the design temp is 180 deg, a good match for fin-tube. It's a rather common set-up in this area.
    Plus, we have alot of large double-hungs upstairs, so the space from the window sill-to-floor is limited making BB's a good fit.
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