Help Sizing new Mod/Con boiler to exiting HWB, part of larger basement remodel

Discussion in 'Boiler Forum' started by jbrinker, Dec 6, 2013.

  1. jbrinker

    jbrinker New Member

    Oct 13, 2009
    Upstate NY
    Here's my situation, Ive been lurking on this board for literally years. Im not a plumbing or HVAC expert by any stretch, but in getting quotes for my new remodel project, I realized that I have a better handle on some of this boiler tech and so on from reading this (and other) boards than some of the installers that I have talked to. So Id love to get a firm handle on the "right way" to do what I want to do before going any further.

    Location: Near Syracuse, NY. ODT -4F (Lets call it 0F)

    The house: 1994 built 2-story colonial. 2x6" r-19 exterior walls, plywood sheathing, housewrap, vinyl siding, 1994 vintage Peachtree double pane windows.
    1st floor: 1400 sq ft.
    Two zones - dining room/living room/powder room - 28' fin/tube; and Livingroom (cathedral ceilings)/Kitchen/Mudroom - 34' fin/tube.
    2nd floor: 1100 sq ft.
    Two zones - Front of house is master bedroom and bath - 24' fin/tube, other zone is back of house - 3 kids bedrooms and bath - 30' fin/tube.
    Basement - 1400 sq ft.
    NO heat currently. Insulated with 2" Polyisocyanurate a couple years back, and even with no heat it is pretty comfortable. I intend to add a single zone to the basement with this remodel. Flooring will be probably laminate, figured on more baseboard.

    Current boiler is a powervented 1994 Buderus G124 on Natural gas. Has been bullet-proof. I added outdoor reset control three seasons ago and it short-cycles a lot less now and the house if far more comfortable (less temp swing). Hot water provided by a powervented Bradford White 50 gal tank (1 year old). Plumbing for heat is all 3/4" copper, controls are a basic 5-zone controller, 4 wire zone valves, and the outdoor reset.

    The situation: I inherited some money, and have always wanted to actually finish off this basement. It has a LOT of potential. So now I can afford to do it. However, the boiler and HW tank take up a large area (6' x 13') at one end of the basement. Stupid placement. I want to replace the cast iron Buderus with a wall-mounted unit located more centrally, and move the HW tank to the same spot. Space is a premium. I have an area about 3.5' x 6' to work with for both, its on an exterior wall - so through wall venting will not be a problem. Basement ceiling is open, and I have access to the entire 1st floor. Plumbing for heat is all 3/4" copper. I am looking to hire out most of this job, but I do have the skills to help or do part of it myself.

    After reading another recent thread on here ( I got to thinking about the boiler sizing. Everyone so far is recommending a Mod/Con wall mounted, about the same output as the Buderus. After reading the other thread, I wonder that is probably a BAD idea. I might be better off looking at adding radiation (or more tube/fin), and going smaller. Thinking about my current boiler - it doesn't short cycle TOO much but it does sometimes. And it has all that mass of iron and water. A new mod/con will have little mass, and I would think it would short cycle more.

    In most of the rooms/loops I could add about 50% more tube/fin with little work (just go wall to wall instead of under windows for example). Basement I can just add what is needed as part of the remodel.

    Was also thinking that I could combine the whole downstairs into one big loop, or feed both loops off the same manifold and balance flow with a ball valve? Could also do the same upstairs? We really dont "differ" the zones much on each floor anyway. (We do have different setpoints and setbacks for upstairs/downstairs)

    1) Is this whole idea nuts? I really, really want to reclaim this space if I spend the $$ on the whole basement remodel. I do not have room for the Buderus where it would need to go. Has to be smaller by about 1/2 with tight clearance allowed.

    2) Assuming its doable, can you guys help me with some back-of-the envelope calculations like you did the guy in the other thread? Last thing I want to do is wind up with an oversized system preforming WORSE than the one it is replacing (which is actually quite comfortable now that I put the outdoor reset in).

    I can get more specifics if it will help. Im trying to enter all the info into Slant/Fin's online Manual J calculator app, to see how those numbers wind up looking. Thats only the first part of the process as I now realize...

    Thanks in advance!

    (also looking for installer reccomendations in this area - Syracuse and areas west)
    Last edited: Dec 6, 2013
  2. Dana

    Dana In the trades

    Jan 14, 2009
    The ACCA specs +2F as the 99% design temp for Syracuse, but OK, let's call that 0F. :)

    Forget the manual J for now- got a mid-winter gas bill, complete with the meter reading dates, and the average outdoor temp for the period? Short of an average temp, the exact billing dates and your zip code would let us look up the weather data for that period. That's a sub-2-minute calculation if I don't have to type it out, and it's a measurement, not an estimate. It'll put a hard stake in the ground for where the house is at right now. You didn't mention which size G124 you have- it comes in a number of different sizes, all run about 84-85% efficiency when tuned up, but if you look at the nameplate BTU-in and DOE BTU-out numbers it'll be good enough to use as a measuring instrument. With that we can reasonably size the boiler, but unless you have some ridiculous window/floor ratio you're looking at the smallest of any major vendors' mod-con lines, with a min-modulation output in the 15,000BTU/hr range, which is actually pretty easy to design around.

    The Slant-Fin tool is going to deliver a number at least 25% higher than reality, but it'll be useful if you want get the room-by-room temperature balancing right.

    Doing it with crusty old-schooler style rule of thumb WAG (but with more realistic rules of thumb) with 2500' of heated conditioned space with '90s vintage 2x6 & double panes, plus an insulated basement you're looking at about 35-37K of heat load at 0F, maybe 39K if you boost the basement temp to 72F up from where it's coasting at 65F or whatever. The smallest G124 has about 2x that amount of output for your, and would have short-cycling issues with stubby low-mass fin tube zones, and the bigger ones would have an even bigger issue. A zone with only 28' of fin tube puts out about 17K into the room at 180F, and the smallest G124 puts out about 4x that amount of heat.

    Rather than adding more fin-tube to the existing zones, replacing sections of it with panel radiators gives a very real boost in comfort, especially near windows, since that brings the average radiant temperature up when you have say, a 38F window surface next to a 100F radiator surface. Fin-tube just heats the air, which is fine, but us humans are more comfortable at a lower air temp if the radiant temp is higher.

    Any boiler will short cycle if you put just the minimum amount of fin tube for actually heating a basement zone with 140F water, but if you used flat panel radiators and 2-3x the minimum necessary you can probably have enough mass in that very-low load zone to keep it from short-cycling with a smallest-of-the line mod con. If you measure the amount of above-grade foundation wall area, and multiply by 5 BTU/hr that's about what your basement load is at 0F in BTU. Add another 3.5 BTU for every square foot of window. Assuming you have about 2' of exposure and a perimeter of 150' that's 300' x 5= 1500BTU/hr plus window losses, call it 2K. At at 180F it's only about feet of fin-tube, at 120F average water temp that's only 10 feet of fin tube, which is just too little mass or radiation for any boiler to deal with reasonably. But if you dropped in a Biasi B-24.71 it can deliver half the min-mod output of a tiny mod-con at 140F output, and enough thermal mass to keep it from going nuts short cycling. See:

    There is no such thing as balancing a basement heat load controlled by a first-floor thermostat- the change in heat load with temperature is just too different.

    If you want to micro-tune the room by room balance on the upper floor zones, and combine the first floor zones into one (recommended) you can use the Buderus flat panels with the tweakable-tunable thermostatic mixers right on the radiator.

    If you're going to do the work and spend the money, it's worth spending the money for an actual comfort upgrade, not just an efficiency upgrade. You may THINK you're comfortable with the fin-tube and ODR control, but it's nowhere near as nice as real radiation. (Radiant floors are even better, but an order of magnitude more expensive in a retrofit.)

    If you have the headroom it's well worth putting down 1.5-2" of EPS under any new subfloor in the basement when you refinish it, which keeps the bottom side of the subflooring above the dew point of the room air in summer. With just a vapor retarder and no insulation you're asking for a mold problem at the ~45F Syracuse style deep subsoil temps. If you can't do the foam insulation for headroom reasonse, use only hard plastic or tiles, no laminates, no wood, and don't even THINK about using throw-rugs.
    Last edited: Dec 6, 2013
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  4. jbrinker

    jbrinker New Member

    Oct 13, 2009
    Upstate NY
    Thank you very much for this very useful reply.

    Heres some more info I just looked up to help zero in the WAG method assumptions above. I just looked at the existing boiler. Its a Buderus GK224E, Max INput listed as 87900 (Max output is blank).

    Bill for last winter was 180 therm/month for Dec and 185 for Jan. Looking back thats pretty consistent last 2 years in the peak winter months. We use gas for HW too, and for the dryer. In July and August we used 15.5 and 17, so that would be the hot water and laundry amounts. So, Id say 185-17=168 Therms for heat load in the winter with the current system. Call it 170? I guess (here I go trying the math) one therm = ~100,000 BTU. So thats 17,000,000 BTU/month. 17MBTU / 744 hrs/month = 22849 BTU/Hr? Did I even come close to doing that right?

    Now that whole magic about output for each foot of tube/fin/radiator, circulation, and so on to figure out the boiler size... This is where I get lost a bit. I guess we have to have enough radiator to radiate a minimum of like 25K BTU at max temp, to keep things toasty on the coldest windiest day. Right? Plus some fudge factor, and the whole basement thing.

    Oh, The boiler is zoned as I described, no primary/secondary, just one Gundfos UP 15-42 circ pump.

    I did mention that its comfy now, at least now that I have the reset control working properly. Boiler runs less (and for longer when it does) and the house is more comfortable. Circ runs more, temps are more even. (Tekmar 256 with low limit at 145F. It would "Want" to go lower much of the time but I dont dare with a cast iron boiler)

    The basement sits right about 68 now - but thats with a fair amount of computer equipment running in my shop 24/4, and the waste heat from all the copper piping and so on as well. The house sits on a side hill, so with the slope fully 1/3 of the basement walls are above grade. The rest are 2' exposure as you said. The insulation has made the basement much more livable. The floor is cool to the touch, but I dont get chilly feet standing on it in socks - unless its below about 15F outside for a few days in a row. Only two small windows, double pane, on the leeward side of the house.

    For flooring in the basement we were considering vinyl tile that looks like wood. Like the stuff they put in medium-end department stores (not laminate, vinyl). Either that or what you said, insulate and subfloor. That has still to be decided for sure.

    As for the boiler... I guess Im looking a a far smaller unit than I figured. Could I do this in a 2-stage retrofit? Using the old tube/fin this season and retrofit baseboard radiators over the summer? (I realize this would require re-tuning things).

    And a question about these new radiators. I think I could sell my wife on the 2-row low-profile units. They look about the same as tube/fin but a lot nicer. She would not go for anything "bigger" or taller than that. Would these new radiators be "crippled" by having furniture blocking them? In almost every room, where the radiators would be there is a couch, or desk, or bed, or something blocking the "radiation" from reaching the rest of the room. With tube/fin its not a big deal, as the heated air rises. What about these?

    Finally - what do you think of this slimline stuff from AIM: ?

    That wrapped around an entire room (or one or two exterior walls) would look real nice. Not sure how it performs, and no data on the website...

    Thanks and I look forward to getting my ideas/ducks in a row before going too far with the contractors. Kinda want to know what I want before I get a lot of quotes.
    Last edited: Dec 6, 2013
  5. Dana

    Dana In the trades

    Jan 14, 2009
    Your stab at fuel-use heat load calculation has many errors, and only calculates an average, when the heat load you're interested in is at the 99th percentile bin of the binned hourly weather data, not some average load.

    To run the heat load calculation on fuel use you need the EXACT meter reading dates and your ZIP code to be able to look up the base-65F heating degree-days for the exact meter reading interval at a weather station near you. Using fuel-use uncorrelated to the exact days the error can be well into double-digit percentages.

    With that information you can convert the therms per degree-day number into BTUs per degree-hour, (100,000 BTU/therm, 24 hours/day) then derate by the efficiency of the boiler (multiply the BTU/F-hr by 0.84 for the G124, since it's steady-state efficiency is about 84%).

    Then take the difference between your 99% outside design temp (about 0F, we decided) and multiply the degree difference between your design temp and the 65F HDD base temp (65F - 0F= 65F), and you'll have a reasonable stake in the ground as to what your heat load is at the 99% design temp, which is the number you need for sizing the boiler.

    For a mid-winter month you can pretty much ignore the hot water use, especially if you have been using overnight setbacks (the 7-10% savings from the setback cancel the 7-10% energy use that was probably your hot water use.)

    The slimline radiant baseboards only put out 220BTU/ft per hour at 180F (about 1/3 what you get out of fin-tube) at condensing temps you're looking at about 55-60BTU/ft at condensing temperatures- you might be able to heat the basement efficiently with it, but is unlikely you have sufficient running wall length on the first floor to deliver enough heat at temps low enough to get the efficiency out of a mod-con, (if it can meet the 99% design condition at all.) If you're looking for a prettier solution than the big panel radiators, something like Runtal UF-2 (UF-3, UF-4, etc,depending on how tall you can tolerate) usually fill the bill (for a price.)







    These radiators are also convectors, and putting furniture in front of them doesn't dramatically reduce output, but it's better to have a couple of inches of clearance to the furniture than putting it smack up against them.

    With a gas-fired cast iron boiler with fin-tube radiation, feel free to set the low limit all the way down to 130F. It takes sustained burns with return water under 125F to get any significant condensing, and at 130F output the delta-T on that fin-tube is going to be pretty small for the brief period than the burn is starting up. Go ahead and experiment with it- and measure the return water temps. It takes at least a 20F delta-T at 140F output to get to condensing, a delta that isn't likely to be achieved (or if it does, only briefly, during the startup seconds of a burn) with your current radiation.
  6. Tom Sawyer

    Tom Sawyer In the Trades

    Nov 29, 2010
    Unless you want to spend money just for the sake of spending money, I would keep the G124 and add whatever radiation and zoning you need to it. Replacing it with a mod-con will save you a few bucks but not enough bucks to make for any sort of decent return on investment.
  7. Dana

    Dana In the trades

    Jan 14, 2009
    It's a GK224E, not a G124. The thing is probably older than the house- maybe remaindered "contractor special" from the distributor(?). The earliest online manual I could find on it was from 1992.

    It's true that it's not a priority to swap out the boiler- if it's only 20-25 years old and has be reasonably maintained it probably has another decade in it, an if it's rated at 88K-in it's output is in the low to mid 70s, and only ~2x oversized for the likely heat loads, not ~5x. As long as it's not short-cycling itself into an early grave, the efficiency hit at 2x oversizing isn't really very bad. AFUE is tested at a presumed 1.7x oversizing, but also at 140F output temp and a known- destructive 120F return temp. But if you can get the return water temps down to the ~130F range on a consistent basis the boiler is fine, and the efficiency will come pretty close to the AFUE numbers using heat-purge controllers.

    If you're currently only burning between 1000-1500 therms/year at a buck to buck-fifty/therm it's not as if bumping from 84% to 94% efficiency is going to pay of the very substantial cost of mod-con installation done right. There are probably cheaper ways to save $200-250/year in energy costs with that money than buying a mod-con. If gas were to hit $2.50/therm it's worth revisiting, but given the proximity to the Utica & Marcellus shale fields I'd expect gas prices in upstate NY to be fairly well moderated for the next decade or two. (But predicting future energy prices has proven to be a fools errand, and I've played that fool on both the up side and the down side more than once in the past 40 years. :) )

    Updating the radiation with something nicer than fin-tube, with enough of it to where it can deliver design-day heat with 140F water is an investment in both comfort & long term efficiency. When the thing is finally toast, having a good handle on the boiler output size/temp that will cover the load is worth knowing, and pretty easy to figure out from the fuel use analysis.
  8. jbrinker

    jbrinker New Member

    Oct 13, 2009
    Upstate NY
    This is all great, great information. I am trying to get the actual meter reading days (unfortunately my wife pays the bills, and insists on paper billing, which means that the online utility co website has only summary data). If/when I do I will run the calculation as you described it a couple posts above.

    I did want to point out one thing - the goal here is not to replace the boiler for efficiency. Its to move it to a different area in the basement as part of a remodel. And the new area is simply not big enough for the G124 (which is a 1991 model, good call, I just looked). If they made a "mini" version of it, that also allowed tight clearances, I could fit it. I have an area about 6' wide by 3' (max) deep to install the hot water tank and boiler/controls. It certainly could be done easily with a wall hung unit, which is the main reason I was considering them.

    If they make wall hung mid-effiiciency boilers, Id consider that too, but in the overall cost of this project (the whole remodel) the difference between a $2000 boiler and a $3500 boiler is not really significant. Plus, going forward, Id like to consider using an indirect HW tank when the (brand new unfortunately) power vent 50-gallon HW tank bites the dust. And a mod-con will work better with that in the non-heating months.

    Otherwise, yes, the current boiler seems to actually do pretty well. I think if I combined the 2 downstairs zones, and two upstairs zones, into "one zone per floor" (2 zones total) and then added a third basement zone, Id be in even better shape. Maybe. Im still entering numbers into the slant fin calc, to see how each room comes out.

    One thing I have noted - if I use night time setback (down from 70F to 65F) the family room in particular with its cathederal ceilings still "feels" cold for a long time (well into afternoon), even when the air is at 70F. If I leave the temp "stuck" to 70F 24 hours a day, the room feels much more comfortable. It has 12' (center) ceiling, and is also a "step down" from the rest of the house. It has 24' of tube fin in just that room, so it does heat up quickly. I think the baseboard radiators (UF-2) would look, and work well in that room especially.

    I really kind of like the look of those, and I think I could sell the wife on them.

    I could also do staple-up radiant in our whole first floor pretty easily. Fully 1/3 of the first floor is tile already, dining room is hardwood, and only the formal living room and this family room are carpeted. She'd love to have hardwoods everywhere.... I keep hearing both good and bad about staple up jobs. Heat spreaders, no heat spreaders, insulate under, no insulation, no carpet allowed, carpet will work.... lots of (conflicting) info.

    In my case, we have 3/4 ply, with cement board and tile on about 1/3 of the floor, 3/4 ply with hardwood (3/4) on the oteher, and carpet on the rest.

    I know, Im diverging, but this is a one time opportunity to do this type of stuff - if I want to. Probably a lot easier to just do those runtal radiators everyplace.
  9. Tom Sawyer

    Tom Sawyer In the Trades

    Nov 29, 2010
    In that case there a a whole lot of really good mod-con boilers out there but I personally prefer the Buderus GB142. Lochinavar, triangle tube, are also good choices but, whatever boiler you decide on, make sure that parts are available in your area. In other words, make sure the local supply house has parts. As for staple up, its a pretty crappy way to do radiant but sometimes its all you can do so beware that you may not be able to get enough heat with the radiant alone and will probably have to go with a couple of additional radiators.
  10. Dana

    Dana In the trades

    Jan 14, 2009
    The min-fire output of the smallest Buderus GB142 is 25K BTU/hr, which I suspect might be on the large side for this house, but the fuel use against heating degree-day data calc should tell us if that's the case. The min-mod output of many others in 15K range. Ideally you'd want the min-fire output to be under half the design-day load, so that it actually modulates most of the time rather than cycling. When it's cut up into a bunch of zones it's even more important.

    Whether or not you can get there with staple-up radiant requires a bit of careful room-by-room calculation Manual-J or I=B=R style. The SlantFin tool will be hitting a bit on the high side, but it's close enough to be useful. If a room needs more than about 25 BTU/hr per square foot of available radiant floor area it starts to get more expensive, but sometimes it's cheaper/easier to reduce the load than it is to go with a higher-output radiant solution (but probably not cheaper than adding supplemental Runtal rads.)
  11. Tom Sawyer

    Tom Sawyer In the Trades

    Nov 29, 2010
    He needs an accurate heat loss done but if you add up his current radiation, @ 180 degree water out he's running about 65,000 btu. Then again, that is split up into zones and smaller areas which unless its really cold outside probably dont run at the same time all the time. Then he's going to add more for the basement zone. I don't think 25K is going to be too small and when you add the indirect load (which I know we usually don't but it will play a factor if the boiler output is marginal. Anyway, we're spinning our wheels until someone comes up with a manual J loss for the envelope. It can be difficult to get the boiler to condense when there are too many small zones on the system and there are a couple schools of thought here. One is to combine the zones and get rid of the thermostats, holding the entire envelope at a set temperature. Very good for boiler efficiency but lousy if you have rooms that don't get used very often in which case its more economical to retain the zoning and forgo the mod-con boiler, sticking with something in the high 80 percent efficiency range. Although you lose a few percent in efficiency you gain it back in not having to heat areas of the home.
  12. Dana

    Dana In the trades

    Jan 14, 2009
    A typical 2500' two-story house of that vintage with an insulated basement comes in under 15 BTU/hr per foot of conditioned space @ 0F, which would put it at under 38K. The mean all-winter temp in Syracuse is about 25F, which would put the mean mid-winter heat load at something less than 23K. To get significant modulation out of a mod-con you'd want the min-fire output to be under that number. The therms/HDD ratio will tell us where it really lives, independent of other load calculation methods.

    Unlike Manual-J, the fuel use numbers tell us fairly accurately what the actual duty cycle of the boiler is, which is a better predictor of how the new boiler will behave than construction based heat load calculations. It won't tell us the heat load balance between rooms, or how much of the heat load was covered by electricity use, passive solar gain, & hot-bodied mammals, etc. only what was demanded of the boiler. But that's a good stake to have in the ground, and easy to place.

    There's no way the smallest GB142 would not be large enough, but if the true heat load were more like 50K (unlikely) some of the other small mod-cons out there might be cutting it close (say, the Peerless PF-50). I'd be surprised if the heat load were that high though. Even a 50K condensing boiler zoned "priority" delivers 50% more hot water to an indirect tank than a typical 40K/80% burner standalone tank, and upsizing for hot water purposes would only be needed in the most demanding of situations.

    It's pretty common to find enough pre-existing fin-tube in a place to meet the design day load with 140F water, and rarely is the true heat load of the house to be anywhere near the 180F AWT output of the fin-tube. If the 88K-in/75K out boiler was the smallest in the series (probably was), it needs that much fin-tube to avoid short-cycling the boiler into an early grave. If you assume the AFUE test standard of 1.7x oversizing, that 65K number based on the fin-tube length divided by 1.7 (=38K) is pretty close to the dirty 15BTU/foot rule of thumb max I'd use as my starting point WAG for that kind of house. It's all hinting we're kind of in the range, but true heat loads of 65K just don't happen in 2500' houses anymore without leaving some windows open.

    Micro-zoning to the n-th degree rarely works without adding a bunch of thermal mass to the system (such as massive hydraulic separators) but I'm not sure that makes any sense at all here. One zone per floor usually works pretty well on houses that size. He might be able to piggy back the basement zone onto the first floor zone with a non-electric thermostatic radiator valve & wall thermostat, only capable of heating when the first floor is calling for heat, but controlled on the high side with the valve. Depends on what the real goals are there, but with R13 continuous foam walls the basement is not exactly a heavy heat load under any outdoor condition, and the calls for heat on the first floor would be much more frequent than an independently controlled basement zone.
  13. jbrinker

    jbrinker New Member

    Oct 13, 2009
    Upstate NY
    Wow, this is great stuff. I will reply more later (Have a meeting to go to) but I was able to find the meter read dates for last winter.

    Meter read 12/17
    Estimated 1/18 @ 180.8 them
    Meter read 2/13 @ 171.5 therm

    So, 12/17 --> 2/13 180.8+171.5 = 352.3 Therm

    Looking up degree days now...

    Station ID: KSYR

    Date HDD % Estimated
    12/19/2012 26.7 0
    12/20/2012 31.9 0
    12/21/2012 28.9 0
    12/22/2012 34.8 0
    12/23/2012 32.4 0
    12/24/2012 34.1 0
    12/25/2012 40.1 0
    12/26/2012 44.9 0
    12/27/2012 40.8 0
    12/28/2012 37.5 0
    12/29/2012 37.9 0
    12/30/2012 41.6 0
    12/31/2012 33.1 0
    1/1/2013 40.1 0
    1/2/2013 46.6 0
    1/3/2013 52.2 0
    1/4/2013 33.2 0
    1/5/2013 32.9 0
    1/6/2013 29.9 0
    1/7/2013 39.2 0
    1/8/2013 40.8 0
    1/9/2013 31.7 0
    1/10/2013 29.7 0
    1/11/2013 28.9 0
    1/12/2013 23.9 0
    1/13/2013 14.9 0
    1/14/2013 25.3 0
    1/15/2013 33.8 0
    1/16/2013 33.5 0
    1/17/2013 36.9 0
    1/18/2013 44.8 0
    1/19/2013 23.2 0
    1/20/2013 30.7 0
    1/21/2013 44.7 0
    1/22/2013 52.3 0
    1/23/2013 61.5 0
    1/24/2013 60.7 0
    1/25/2013 54.6 0
    1/26/2013 55.6 0
    1/27/2013 47 0
    1/28/2013 39.2 0
    1/29/2013 29.1 0
    1/30/2013 8.4 0
    1/31/2013 29.8 0
    2/1/2013 44.5 0
    2/2/2013 47.2 0
    2/3/2013 49.2 0
    2/4/2013 46.9 0
    2/5/2013 47.4 0
    2/6/2013 42.5 0
    2/7/2013 46.6 0
    2/8/2013 38.8 0
    2/9/2013 49 0
    2/10/2013 44.4 0
    2/11/2013 27.1 0
    2/12/2013 30.9 0
    2/13/2013 32.7 0

    Total: 2167 degree days

    So, if I understand you (please correct) I take 352.3 / 2167 and get .1625 therm/degree day? * 100,000 BTU/therm / 24h/day = 677.3957 BTU-F/hr? * 65F = 44030 BTU?

    Please check my math and reasoning - as I simply read up on this, and tried to follow what you said in your post a couple up. The number at least sounds reasonable (Thats the 99% peak design number, correct?)

    If thats the case, and what was said in the above couple posts it true, Im looking to try and size a boiler in the smallest category (50-70K) that has a good turn down ratio. Some of these have 4x or 5x turn down from that range.

    As for the zoning - I see no real reason to leave the 1st floor cut into 2 zones like it is now. The original owner heated the family room with a pellet stove (so he had a reason to zone it out) - that end of the 1st floor didnt really use the boiler much. We don't use a pellet stove, its gone.

    1st floor is a fairly open floor plan, and other than possibly balancing it a little it should be fine as one big zone. We heat it that way anyway - 2 programmable thermostats set exactly the same. We do set back the 1st floor to 65 at night, but thats it. Same time, both zones.

    Upstairs, the master bed/bath/closet takes the whole front of the house (half of the upstairs, thermostat in master bedroom) and the three kids bedrooms, and their bath, take up the back half of the upstairs (a thermostat in center kids room). If I re-zoned the upstairs to one zone, id just keep the thermostat in master bedroom and try to balance it by adjusting the baseboards.

    Or could I plumb them as a sort of mini-manifold? I.e. hang both zone plumbing off one zone valve or circulator, split to a balance valve of some sort feeding the front and half on separate plumbing runs? Or even use ball valves? This might be easier to achieve balance between the kids rooms and our bedroom.

    If upstairs would be a PITA, I can leave it be as 2 zones.

    Basement wont have much load, and I was already thinking of running it secondary to the 1st floor. Either on the return side of the run, or with a bypass and a ball valve to control it (or thermostatic control as mentioned above).

    Only time the basement needs much heat is when its cold for 3-4 days in a row (like under 15F cold). Then it does. But so does the rest of the house.... Of copurse we arent using the basement much now, once finished we will, and it might "feel" colder sitting on a couch down there than it does now when I just go down to get something or do laundry.

    I emailed 4 more contractors today. Going to set up some meetings starting next week. Guy today seemed to think I was crazy, "wont achieve anything - keep the cast iron unit" not getting my point that Im NOT doing this for efficiency but for space reasons.

    Are there "small" non-condensing boilers? He mentioned a 2-stage boiler of some kind...

    A second guy called last night, he sounds really methodical. They do a lot of radiant installs. Prefers Utica SSC boilers, or Lochinvar Knight. Said they would have look and do a full load calculation before he would give me an idea.

    Thanks so much. Im still working out the slant-fin manual J app. WIll have that in a day or two.
    Last edited: Dec 11, 2013
  14. Dana

    Dana In the trades

    Jan 14, 2009
    The analysis is almost there, but forgot to derate by the ~84% steady-state efficiency of the boiler for it's OUTPUT BTU/hr, which is the heat that was actually delivered to the house (the other 16% went up the flue.)

    Multilpying that 44,030 number by 0.84 you get about 37K for a heat load at your 99% outside design temp. (And the crummy ~15 BTU/hr per square foot rule of thumb survives yet another test... just don't use it as gospel.) If it's been short cycling your heat load would be even a bit lower, in the 32-35K range, depending on the severity, but your ODR controller is probably doing heat purge on the boiler, keeping it near it's steady-state efficiency.

    At the wintertime binned hourly mean temp of about 25F the heat load is about 22,760 BTU/hr, so any mod-con with a min-fire output above that won't be modulating much, and a mod-con with a min-fire output in the 15K range will run long nearly continuous burns, and have less of an issue with short-cycling on zone calls.

    There are some 2-stage mid-efficiency low mass finned water tube boilers (RayPak, Laars Mini-Therm) that can tolerate low-temp return water, essentially "glorified pool heaters", but the 2-stage versions are in the 75-85K output at high fire, and still at or over your 37K heat load at low fire. They aren't any simpler to design around than a mod-con, but are cheaper up front. If you want a mid-efficiency low mass boiler that will really modulate in the right range you may have do drop back to using non-ASME rated non-condensing tankless hot water heater like low-end Takagi (which may not be fully legal to use strictly as a space heating boiler in your area.)

    Some likely candidates for mod-cons would be the Triangle-Tube Solo 60, Burnham ALP080, Peerless PF-50, Lochinvar KBN081, (y muchas mas). All of these have mid-fire output in the mid-teens, and high fire output more than 10% above your fuel-use/HDD calculated load, so you'd be covered well into the negative single-digits F even with the PF-050, and well into negative double digits for the slightly bigger ones.

    Many contractors will try to talk you into bumping up one size in any given line. Resist that adamantly, since that will bump the min-fire output into something that will short-cycle more readily on zone calls, and won't be modulating much- you'd see an order of magnitude bump in firing cycles per year and you won't be any warmer or more comfortable.

    Don't be surprised if the SlantFin tool spits out 50K or higher for a heat load number. More sophisticated heat load tools such as WrightSoft will come in closer to your fuel-use measured reality, maybe 10% higher rather than 25% or more. But it's not a useless exercise: If you specify your room-by-room radiation to deliver the SlantFin derived room load numbers with 140F water you'll probably be able to run in condensing mode more than 95% of the time once you've dialed in the outdoor reset.
  15. jbrinker

    jbrinker New Member

    Oct 13, 2009
    Upstate NY
    OK, finished with the slant fin tool:

    Total Home: 60044
    Main Fl 27628
    family room 9897 24
    kitchen 2210 6
    dining 5281 12
    living 4983 12
    entry 3011 0
    powder 732 4
    mudroom 1584 6

    second fl 14697
    br1 2494 12
    br2 1531 10
    br3 3410 13
    master 5447 14
    master bath 787 4
    main bath 891 4
    hallway 137 0

    basement 17719
    main area 13261 0
    "Ell" area 4458 0

    (For basement I used r13 walls - not the concrete block as listed in the slant fin calc)

    Pretty interesting. Ive listed a third column - thats the feet of tube/fin in each area now.
  16. Dana

    Dana In the trades

    Jan 14, 2009
    Since only a third of the basement is above grade, that part of the Slant-Fin calc is going to blow WAY over the top. Also, the "whole-wall" R of a 2x4 wall with R13 batts is about R9.5-10. The thermal performance of 2" of continuous polyiso unbroken by thermally bridging studs is comparable to the whole-wall R of a 2x6/R19 wall (about R13). If you reduce the wall height by half to correct for the below grade aspects, and bump the wall construction to 2x6/R19 or whatever, it'll probably spit out a basement number of about 5-6K in stead of nearly 18K, but that too would be on the high side unless it's a walk-out with a lot of windows and a big glass slider door or something. So with those corrections they'll be coming in at around 48-50K, which is about what I would expect from that tool.

    Their feet-of-fin-tube recommendations on the bigger rooms are about 400 BTU/foot, which takes an average water temp of 150F (160F out of the boiler, 140F back). To be at condensing mode most of the time you want to get the AWT down to 130F or lower (135-140F out, 120-125F back). That takes about one foot of fin tube per 250 BTU of room load. So if you multiply the Slantfin tool's length numbers by 400/250 (1.6) you'll get the full condensing performance out of a mod-con. But since their load number are a bit ahead of reality, a 1.5x multiplier would still be plenty.
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