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Mod/condensing efficency?

KBP&H_3 Member Posts: 67
One job- 2 different ways-- 125' 5 zone baseboard system with a indirect fired w/h,, Other guy says mod / cond boiler will run at 93-95% as it is rated no matter what in this situation, I say No... There is a better chance of 86% direct vent boiler getting better than 86% w/ Honeywell AQ system than the mod cond boiler getting it's rated eff %. I've always understood that the mod / cond boilers need a 60 to 80* return temp to get their high efficency's... Input please,, going to AREE in A.C. tomorrow,, see what Boiler manuf's also say...


  • Unknown
    not true

    if the modcon gets 95% afue, that is not based on 80 degree return temps. The modcon will also have outdoor reset so for a large portion of the heating season it will be getting ultra low return temps which will make its efficiency go even higher than the afue. If you add a reset to your 86% boiler what will be your minimum temp..140? Aslo the customer is going to get back 30% of the cost of the install up to 1500 back on the modcon and he can dice up how much the modcon was and how much the baseboard was any way he wants! What are you going to offer as a rebate? energy savings? nothing but a little bit cheaper install and with the tax credit I bet he still comes in lower.
  • KBP&H_3
    KBP&H_3 Member Posts: 67
    Wasn't the question.....

    The question was eff vs eff..... Not anything else.. After 6 hrs today at the AREE in AC.. FOUR different boiler manufacturers all said that a 95% boiler will only get 87-88% with a standard baseboard installation based on a 20 T..... The only way to increase the eff is to go at least a 40* drop. The only way to do that is to increase the baseboard amount to increase the temp drop or go to a variable speed Delta T pump.. The point is that unless the boiler is installed correctly with these parameters, it will not achieve more that 90% eff nor will it condense as designed.... Four different manuf's today... Yes , there are rebates, other things, etc.....
  • hot_rod
    hot_rod Member Posts: 22,467
    to get above 87%

    the boiler has to condense. To do that the return temperature has to see return temperatures below 140F-ish. So not just the delta T per say, but the return temperature to the boiler.

    There are many times during the heating season that the building is not at design load, and the baseboard could typically meet the load at lower supply, and hense return, temperatures.

    If the return temperatures can be held below that condensing temperature you will see those 90% plus efficiency numbers. So it really depends on the distribution and control logic, not just the listed boiler efficiency numbers.

    The building heatload, amount of baseboard and the output at various supply temperatures are all part of the answer.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • CC.Rob_8
    CC.Rob_8 Member Posts: 3
    missing the boat

    If that's what the manufacturer's say, then they are missing the boat. Case in point: my house built 1979 full of fin-tube sized for 180F water at 0F outside. Granted, we have done quite a bit to the envelope to reduce the heat loss, but...

    Added ODR with indoor feedback a couple years ago. In a 4500 hour heating season, we have non-condensing return temperatures about 20 hours per year. The rest of the season, it's in the condensing range. I'll take 4480 hours of condensing any day of the week and twice on Sundays.
  • Unknown
    What return temp Hot Rod

    is AFUE based on? I assumed afue testing was based on all things being equal, including return temp.A Prestige is 95% afue. Are they testing it at a different return temp than a 86% boiler?
  • Doug_7
    Doug_7 Member Posts: 247
    Part Load and Seasonal Efficiency is the Key

    If you wish to argue about boiler efficiency, you need to consider a lot more than a single data point (like 87% at full load). The efficiency of an ordinary boiler drops off sharply at reduced heat-load, whereas the efficiency of a ModCon actually increases. Your boiler will almost never run at full load - except on a design day once every few years.

    You need to read and understand the Brookhaven Lab report on ModCon boilers with baseboards and outdoor reset to take full advantage of lower return temperatures in milder weather. Tony, hot rod and CC.Rob have discussed this point quite well.

    You also need to understand that it is well documented that fuel use often drops 30%, 40% or 50% when conventional boilers are replaced with ModCons. The ASHRAE Journal reported an average 49% efficiency gain in ten schools where conventional boilers were replaced by ModCons. How is this possible if there is only a small difference in boiler efficiency ??


    This will download the July 2006 ASHRAE article titled Boiler System Efficiency. See Table 2 on page 4.

    So - is this a simple argument about boiler test-efficiencies in the literature - or - do you actually want to reduce fuel bills and save money by improving heating system efficiency ?

    I learned a long time ago to skip the opinions and learn to respect the facts as shown by the DATA. The arguing usually stops when people look at the actual data. There is a lot of data and facts out there on this subject. And there are some deniers - usually with no data.

    My ModCon reduced my fuel use and heating bills by a well documented 32% saving me over $7,000 per year. That's good enough proof for me.

  • scott markle_2
    scott markle_2 Member Posts: 611
    good stuff!

    I think the concern that mod cons can be applied in ways that do not achieve tax credit status is worth mentioning. $1500 for a missapplied mod con is not the kind of stimulus we need.

    I paired a solo 60 and smart 80- basically no DHW condensation. I was planing a two temp system with small radiant load pulling off the DHW (buffer but revised this at the thought of not getting condensation from the low temp load.

    I'v seen short cycle make plenty of condensate. Small and efficient building envelopes are a challenge in this regard. lots of variables in defining and measuring efficiency.
  • KBP&H_3
    KBP&H_3 Member Posts: 67
    Prestige Solo

    While looking through the Prestige installation guide today at a dist., it was interesting that Triangle Tube noted a baseboard installation in only one or two diagrams. Most of the diagrams noted low temp radiant installations and snowmelt.. Regular baseboard was not even diagramed by itself.. / only with other radiant or low temp loads..
  • Mark Hunt_6
    Mark Hunt_6 Member Posts: 147

    The mod/cons are tested with a 30 degree delta I believe. Standard boilers are run at a 20 degree delta. 140 out, 120 back.
  • Unknown

    I did not know that. Thanks
  • Ted_5
    Ted_5 Member Posts: 272
    AFUE for all boilers

    AFUE is a bench test for all boilers, nothing different for mod/cons. It is a constent load, 120 EWT and 140 LWT for 9 minutes.

  • nosirra1Arrison
    nosirra1Arrison Member Posts: 57
    Actual numbers

    Actual annual propane useage. The numbers speak for themselves.
    2002 to 2005 copper fin boiler was heating staple up and in slab only(fairly inefficient radiant).
    2005 to 2008 mod/con boiler with outdoor reset heating BOTH domestic hot water and the same staple up/ in slab radiant.

    2002 = 1042 gallons...2003 = 1067 gallons...2004 = 1137 gallons...2005 = 1078 gallons....mod/con installed...2006 = 988 gallons...2007 = 816 gallons...2008 = 754 gallons

    Year one with mod/con was not so good due to the learning curve to adjust the settings from factory spec to ones that fit my system.
    Factor in heating degree days any way you would like and the numbers don't lie...I have saved a considerable amount of money and used a considerable amount less fuel.
  • Maine Ken
    Maine Ken Member Posts: 531

    Nosirra, If you calculate the roi based on your consumption, will you see a payback within the life expectancy of the mod/con? Just curious...

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  • nosirra1Arrison
    nosirra1Arrison Member Posts: 57

    ROI on mod/con is based on how long a life the boiler has. I have an Ultra so the aluminum heat exchanger is the true wildcard in the equation. If the boiler makes it past 8 years I will be ahead. That includes the extra maintenance for a mod/con. Its a gamble and a crapshoot. If I get burned I will go Prestige or a Lochinvar Solutions (w/ ODR).
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