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Does gas burner modulation cure-all??

Just had an interesting call,,, a competitor SOLD a condensing boiler based on the old unit sizing, then I was asked for a price(over the phone) for the "wet-end" piping work.

I said "who sized-it",,,, he said "it shouldn`t matter as the boilers burner modulates anyway",,,, is that correct???



I was always taught to design a boiler replacement system to design conditions, provided the rooms radiators can satisfy the load,,, IE- heat-loss. 



My question is,,,,, should we just oversize every replacement boiler now as burner modulation seems to take over?



How can firing a 200K unit(max),, in a building that only may need 85K be beneficial?



Just curious! 

Comments

  • Does not matter???

    Then I guess any contractor, at any time, can install equipment larger than necessary to get the job to cover his "butt".



    Us guys who actually do heat-loss calcs must be nuts!!  
  • Big Ed_4
    Big Ed_4 Member Posts: 1,890
    Could make Sense

    The other guy is paying more for a bigger unit too...



    I am sure in its design the efficiency is maximized in a properly sized unit .…



    You will always run into those guys set in their ways that tell you " I never had problems doing it my way " Heck !! How could you when you size them where they can heat four homes :)



    I was no better years back myself ... When a 150K boiler heats all :)
    I have enough experience to know , that I dont know it all
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    You know I am not a professional.

    "I was always taught to design a boiler replacement system to design

    conditions, provided the rooms radiators can satisfy the load,,, IE-

    heat-loss.  "



    It seems to me you were taught right, not only based on what I have read, but on my actual experience with my house and mod|con boiler.



    "My question is,,,,, should we just oversize every replacement boiler now as burner modulation seems to take over?"



    My contractor's salesman told me the same thing. Trouble is that I already calculated the heat loss of my house, so I knew the smallest boiler in the product line was somewhat too big, and so it proved. My heat loss calculated out to between 30K and 40K BTU/hr at 0F outside, and design day is only 14F around here. So the boiler I got modulates between 80K and 16K. Now if only the upstairs zone wants heat, 6K BTU/hr is enough, and the boiler goes on and off more often than I would prefer when only that zone calls for heat. Though nowhere near as fast as my old, non-modulating boiler did.



    So of course you should do a heat loss calculation. And hope that manufacturers of mod|con boilers can either modulate a much wider range, or supply boilers with smaller inputs. For me, I think a mod|con that goes 50K down to 10K would have been more suitable.
  • TonyS
    TonyS Member Posts: 849
    Dont forget your domestic load

    if your using an indirect. My home is 36000 on a design day. That would never be enough to fill the Jacuzzi. Most Triangle tubes I sell are 110s because I sell them with an 40 gal indirect. In some cases the 60000 unit would be a better choice for heating but I prefer the water output of the 110 for the domestic.
  • Steamhead
    Steamhead Member Posts: 15,348
    It's still oversized

    and I'll bet it only modulates down to a certain point, so it would cycle on and off in mild weather.



    I'd walk away from this one.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Consulting
  • TonyS
    TonyS Member Posts: 849
    The New Navien Combi unit

    is 200000-17000 btu with a domestic tankless separated with a plate exchanger. There really isnt a reason for a heat loss with a unit like that.
  • Steamhead
    Steamhead Member Posts: 15,348
    edited April 2010
    Unless

    the house is rather small. 17 MBH would be way too big for the shoulder season in some cases.



    Most of the time, a properly sized boiler has plenty of capacity left over since design conditions generally don't occur that often and don't last that long. There's plenty available for an indirect, unless you have a big Jacuzzi like Tony's.



    So there's no reason NOT to do a heat loss.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Consulting
  • "110s because I sell them with an 40 gal indirect"

    Am I missing something??,,,, a TT Solo 60 can easily supply a 40G. indirect as it has priority,, so why would you knowingly oversize?
  • Wayco Wayne_2
    Wayco Wayne_2 Member Posts: 2,472
    It's a matter of philosophy

    I prefer to do a heat load calculation and size the boiler as close as possible to that. If you want more domestic hot water capacity go up on the Indirect tank size. You maximize the efficiency of the mod con when you keep the throttling range within the size of the heat loss calc. If you need quick recovery for domestic go to a tankless water heater. Why spend more for a mod con and then compromise the efficiency you paid extra for?  
  • TonyS
    TonyS Member Posts: 849
    It depends

    your right, 60 000 is more than a standard 40 gal water heater but when you have to fill a 75 to 125 gal tub with a roman tub spout ...10gpm they come up short. You could put in an 80 gallon indirect..ouch... heat loss and upfront cost! 80 gallon a killer for a once a month bath. OK so it doesn't modulate as far down as  the sixty but we have been installing standard boilers for all our lives and now all of a sudden its a crime if it doesn't modulate down on the warmest days..that's debatable! Like Frank said, maybe 17000 is to much on a close day but look how far apart the boilers are to begin with, a 60000 then to a 110000? Thats 50000 btus, I bet I could guess that much and be right 100% of the time. I doubt you would ever get a call from a customer saying...you know that boiler cycles a bit on a warm day! But you can be sure if they run out of domestic on their new super system your getting a call
  • TonyS
    TonyS Member Posts: 849
    What if

    you do a heat loss Dave and it comes out to 67000 btus on a design day? What boiler will you use?(TT for arguments sake) Where do you draw the line and what factors make your decision?
  • Unknown
    edited April 2010
    Here Here WW,,,

    I totally agree,, needless oversizing (to-me) makes no sense!

    Seems to me the PRIME directive of any mod/con is to heat the house, DHW is a secondary.
  • plumber4
    plumber4 Member Posts: 13
    Net IBR

    If I may jump in...when you size your boiler your are actually using the Net IBR rating BTU/hr to match up to your heat loss Btu/hr. So in the case of your 36K btu heat loss the TT 60 has a net IBR of 47K so I would agree that the only benefit to using the 110 to the 60 is the first hour output of hot water would be greater with the 110. However you would get greater eff. with the lower modulation of the 60 
  • TonyS
    TonyS Member Posts: 849
    The best efficiency is not always the most comfortable

    My home is heated with radiant floors using a w/w heat pump with outside reset. Very inexpensive to operate, about 75 bucks a month during the winter. But my floors always feel cold because my house is well insulated. Many times I wished I had a big steam radiator in the room just to warm up too or throw my wet coat on. We have a cast iron gas fireplace we turn on when we come in from the cold but leaving that on just shuts down the thermostats and makes the floor colder. Sometimes comfort and efficiency are at odds with each other. I believe comfort is number one priority. Besides it doesnt hurt to have a little more boiler than you need...maybe for a future bedroom or sunroom. I enjoy doing heat loads, I use HVAC CALC and its always interesting to watch the heat load go up and down as I add insulation or change out windows. I understand that in a perfect world, perfect modulation paired with perfect water temps is suppose to make me feel perfect, but it doesnt. When im really cold I like to stand in front of an oversized warm air duct  lol
  • TonyS
    TonyS Member Posts: 849
    Plumber4

    What TT boiler would you use if the heat loss on a customers home was 58k design day?
  • TonyS

    "Net" values aside for a moment......

    The DOE capacity rates the TT Solo series @90%,, but as you know that %age will go higher based on the return water temp,, perhaps 95-98%.



    Now, the "net" IBR P&PU factor they use is (about) 1.15% less that 90% number,,,,

    if I was faced with a 58K H/L on a design-day, I would still be inclined to stay with a Solo 60.



    The domestic has "priority" anyway, and if there was a problem during those FEW days the priority-call can be timed shorter.



    Just my $.02
  • TonyS
    TonyS Member Posts: 849
    At what heat loss would you go

    to the 110?
  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    you can have comfort and efficiency

    but you have to go beyond just heat load.



    If your load is very low, then things like constant circ or floor sensing can help improve comfort. If you need to turn on "spot heat" then you are keeping your room temp below comfort levels. You could just turn up the heat a little and leave it there, or do setback/set forward schedules. Stop looking at the thermostat numbers and just adjust by comfort.



    "intense heat soak" is why man invented saunas and jacuzzis.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • Would be a "judgement-call",,

    That`s what separates the "men from the boys" LOL!  ;-)



    Depending on the water-temp the emitters can handle, the time-duration they`ll need-it and the climatic outdoor-temp of the region,,,

    A lower modulation rate is more favorable in "shoulder" seasons as opposed to constantly bumping-off & on.;-)
  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    the navien

    is a particular product for a particular situation. You still need to know your heat load for the rest of the system, whether or not you need it to size the boiler. Pipe sizing, pump sizing, zoning, water temperature curves... all require a load calc to zero in on. without one, you're just guessing.



    I believe the navien is a fairly expensive unit, isn't it? as in, couldn't the cost differential between it and others allow for a load calc to be done, and a better efficiency boiler/indirect combo used?



    it does seem like a pretty nice unit though, with a good mix of features.



    there is one element in your discussion that I think you're missing though. You were basically saying that for years we used fixed burn heat sources, and wondered why it's such a big deal to use properly sized ones now that they modulate. the answer is that fixed burn sources were high mass. modulating boilers are low mass and are much more susceptible to cycling problems even with lower modulation rates if they are mismatched to a load.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • plumber4
    plumber4 Member Posts: 13
    Tony

    Tony, I would go to the TT 110 if the heat loss came out to 58K. I recently went thru a similair problem with a heat loss coming in at 46K and when I sent it over to TT their reccomendation was to go to the 110 since it was so close to the IBR rating, especially where I was installing a Smart 40 at the same time. In most of the charts that I have read indirect manufactures suggest a 90-100K (on a 30-40 gal unit) output unit to handle the domestic loads. Of course this is probably a little overkill to reach thier sales literature ratings. Personally if thier is proof that the TT 60 would satisfy my 46K heatloss and feed a smart 40 without significant performance loss i would prefer that over the 110 so I could get a lower modulation of 16-60 rather than the 30-110 on the TT 110.
  • plumber4

    As I stated, it`s NOT possible on a TT Solo unit to supply heat & domestic at the same time,, you can only limit the length-of-call.



    Am I missing something?
  • TonyS
    TonyS Member Posts: 849
    If I turned the heat on in the floor

    and let the floor get to the point of feeling warm, my house would be 85 degrees. One night my son let a window open downstairs during winter. The floor got nice and warm but it sure wasnt cost effective. Systems I have installed in homes that were not so well insulated get warm, on newer well insulated homes, even though the stats are satisfied, never really feel warm. The btus needed per square foot of space on some of these super insulated homes is really minimal. How do you get the floor warm without turning the house into an oven?
  • plumber4
    plumber4 Member Posts: 13
    Dave

    Dave I am not stating that they will both be feed at the same time. I understand that they are set as a priorty zone. I am simply stating that the Solo 60 is has an IBR rating of 47000 btus which in my opinion is to low for a 58000 load. In addition I added that the manufacture suggests the 110 over the 60 for similar issues that  i faced. As an example the solo 60 with a smart 30 will handle 95 gallons first hr. rating while the solo 110 with the same 30 will handle 140 gal first hr. Thats it 
  • TonyS

    There most certainly seems to be a problem with the design there! :-)
  • plumber4

    OK,,, I see your point,,,,,,



    Great communication,,, I LOVE this site!!!
  • plumber4
    plumber4 Member Posts: 13
    Dave

    Dave in your opinion if you had a customer with roughly 2000 sq ft with a heatloss of 46000, and a 40 gal indirect which would feed 2-1/2 baths, 3 women and 1 man, would you chance the 60 over the 110? I am involved in the large commercial plbg and heating industry and not as much in the residential side of the business so am I wrong in looking mainly at the IBR ratings inlieu of the DOE or input ratings. Also on a second note what are your feelings on a Grundfos alphs on delta p vs the Taco on a delta t 
  • TonyS
    TonyS Member Posts: 849
    Rob

    I have my outdoor reset dialed in that I have constant circulation. My first floor is concrete slab with tile throughout, the second floor is carpet. I enjoy being barefoot but the reality is the tile is still to cold even when the stat says 72. The house itself is comfortable but the floors are far from warm( yes I know anything above -460 is warm). I dont believe the temp here dropped below 18 degrees this year. In previous years when the temp went to zero The floors did actually feel warm, but those days are not the norm. Have you had this situation before? The way I understand it is that each square foot of floor emits so many btus depending on its surface temp and the cumulative value of the btus times the square footage is what is needed to satisfy the load at any given time. Some of these new homes are so well insulated that I would be lying if I told the home owner they could lay with their baby on the floor. Of course the second floor being carpet doesnt feel cold but the tile slab does.
  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    that's unusual

    with constant circ, that doesn't mean that the floor feels warm, but cool is usually about as "cold" as it is described. "comfortably cool" we call it. tile feels colder than some other surfaces that don't pull heat from your feet as well, but even so I wouldn't normally call it "cold'.



    but, people vary. If you're already doing constant circ and it's working properly, I apologize for giving you some BS! I guess the only hedge I'd have for that is, is the floor actually uncomfortable, or are you just annoyed that it feels cool?



    You're not going to get any "consistent" comfort improvement through any other method, but I understand what you're saying about enjoying a blast of hot once in awhile. I have a wood stove and that's nice.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • TonyS
    TonyS Member Posts: 849
    Its more annoying

    I installed the system almost 20 years ago when I built the house, It works great My energy bills have always been less than my cable bills but I think its a stretch telling people they are going to have "warm floors". No doubt that if there were no heat in a slab floor with tile it would be unbearable.



    My shop is 2400 square feet with radiant and a prestige with outdoor reset.  When I first built it I didnt have any insulation for a couple of years, the floors were really warm and of course oil was cheap( had a burnham v7 at that time) Then I insulated the attic with r38.. floors were still warm, several years ago I installed the Prestige and put 4 inches of pink board on the outside and r17 garage doors, heat loss is now 27k. Now even the dog looks for something to lay on! The fuel bills are great but the floors are no longer "warm" I dont care because I dont go barefoot in the shop.
  • TMorg
    TMorg Member Posts: 23
    Domestic shortage

    I have found just the opposite. If the domestic won't keep up it IS the priority. I have seen the domestic water usage go up a lot in the recent years. If you are using a boiler under 100mbh with an indirect and a heating load over 30mbh. you better make darn sure you understand their bathing habits. Or should I say their expectations. I have had customers very interested in saving heating costs but you better give them lots of hot water. I see a lot of oversized mod-cons that cycle way too often but also keep getting calls form customers that have not enough hot water. Often its from undersized indirect pumps or piping. But Please don't spread the falsehood of "you don't need to even consider the domestic load" unless your customer is accepting low hot water usage.
  • Good point TMorg,,,

    But these are not "on demand" situations,,, with a properly sized indirect storage-tank, and boiler control flexibility how can you go wrong?
  • TonyS
    TonyS Member Posts: 849
    I disagree Dave

    Properly sizing a indirect doesnt just mean the standing capacity of the tank alone can supply the demand, the demand is also met with how much hot water is being produced while the tank is being drawn down. Most whirlpool tubs are not used that often, so it is senseless to keep a 80 gallon indirect that is losing 1 degree an hour, 664 btus! Your much better off with a smaller 40 gallon with higher recovery. Even in a home with 3 or 4 baths, very rarely will all showers be used at one time but when they are, they have to work! The standby loss on an 80 gallon is over 56 therms of gas a year or 60 gallons of propane. You are never going to save that much fuel because your boiler could modulate down another 15000 btus. Of course a tankless water heater is the most efficient, there is no standby loss.
  • Well,, I see your meaning TonyS,,,

    but at the same time I have trouble understanding how "on demand" can condense to gain efficiency(whatever it may be),, and provide any longevity of service.
  • scott markle_2
    scott markle_2 Member Posts: 611
    Low firing rates have higher efficiencies

    I hate unnecessary cycling and have viewed over-sizing as the main culprit.



    I now think that cycling is as much a failure of control algorithms than modulation limitations. Why cant the control permit supply to exceed target for a period that permits a reasonable burn and then compensate for this excess by permitting supply to fall below target for an equivalent period? Automatic differentials (Tekmar) seem to work pretty well, although they are fairly conservative in how wide of a spread they will permit. While highly variable modulation would seem to be the solution, I believe that better control strategies are as important as sophisticated burners, it's probably unrealistic to try and design a burner that can burn clean and efficiently with the modulation range you guys are asking for.



    I saw a chart recently that shows efficiency for the new Viessmann boilers at various water temperatures and firing rates. Low firing rates have higher efficiencies, especially at higher water temperatures. Looking a this chart I could see how a somewhat oversized boiler could actually have an advantage over a model with less heat exchanger surface area. This is especially true for DHW production. Most mod-cons are set to ramp up to high fire to feed the indirect, this combination of high firing rate and elevated return temps basically kills the prospects for high efficiency operation during DHW production. I did job with a solo 60 with a 80 galon smart, it condenses a tiny bit at the beginning of a DHW cycle and then goes basically dry.



    I did a much larger new Vito this fall with an 80 gal Vissmann tank and it condenses like a charm, it's also smart in that it targets supply based on tank temperature instead of just going to high fire, (one can even cap the firing rate if desired) it's in a fairly low modulation rate for most of the time and this is why it can condense healthily even with elevated returns typical of an indirect, the large surface area of the indirect HX certainly helps to.
  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    tankless is not always more efficient

    short cycles on micro demands are not efficient at all. pre and post purge cycle for 10 seconds of hot water? how many times a day does your clients' family wash their hands? Bet that offsets the standby loss of an indirect pretty fast.



    in terms of yearly energy usage, any reduction in efficiency on the heating side... unless you have practically no heating season... will vastly outweigh any increase in efficiency on the DHW side, unless the DHW load is truly huge. You're also doubling the standard standby loss of a well insulated indirect which is typically 1/2 a degree an hour, not 1 degree. You can always increase their insulation levels as well, if you like.



    Upsizing a heating appliance for more efficient DHW production is absolutely backwards of proper design methodology. Upsizing really refers to min mod though, and you're right when you say 1500 BTUs/hr in min mod is not a big deal. and there is more than min mod involved too... the more water mass you have the less min mod matters. with a big enough buffer tank or mass emitter you can upsize all you want.



    but none of this eliminates the need for heat load calculations. and perhaps unless you are using one particular water heater (which I'm still a bit skeptical of, I have to admit) or two (including the Phoenix Evolution, maybe), upsizing boilers or water heaters for DHW loads without accommodating that upsizing on the heating side is definitely a bad way to go.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • TonyS
    TonyS Member Posts: 849
    I have installed

    3 systems in my own house and shop for domestic, the first being a phase three 40 gal indirect on a low mass thermodynamics oil boiler. It worked well but I really dont know how they figure that 1 degree an hour because it usually is about 2 degrees. The next system I installed was a Navien about 2 years ago. Fuel consumption wise, its not even close. The Navien wins hands down and it condensates like a rainstorm...I know because I still didnt connect the drain, I have a five gallon bucket underneath that I dump. One day I will pipe it in  lol. So far so good, no trouble. The shop has the little tank with a coil in it tied to a prestige, The name escapes me but its sold by triangle tube, its a joke and I would never sell one. I understand the theory about modulation, I use a buffer tank on my geo w/w system because the compressor and pumps dont modulate, this doesnt mean that when they start to make modulating units my system will not work. Besides a buffer tank is alot cheaper than all those modulating controls and pumps. 
  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    you're comparing 3 systems in 3 areas?

    similar temps around the units? similar usage in all areas? are you correcting for those factors? it seems unlikely, but you obviously know enough to know that comparing 3 systems in 3 different areas with 3 different usages isn't a very useful comparison... so I'd like to hear how you are making this comparison in a little more detail?
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • TonyS
    TonyS Member Posts: 849
    The Phase Three

    was in the house and the Navien has replaced it. The Prestige with the 'mini smart" is in the shop. The mini smart  goes cold long before the boiler catches up and the standby loss is over 5 gallons of propane a month. It is unsuitable for a home. I try to test this stuff in my home for a while before I sell it. So far the Navien has impressed me, even though they have had a few problems, they have made good on them. 
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