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Boiler Modulation

Hello Everyone,

I have reading the Wall for about three or four months now. I am a solar installer, but a year ago or so I partnered up with a radiant contractor friend of mine. I love installing hot water systems. And I am always trying to look into the "true" nature of how things work. I have never really accepted what the reps. say, though I will definately listen to their pitch, and like to look at things from a different angle. Just like what I read in one of Dan's archived articles from Plumbing and Mechanical that one of the best ways to troubleshoot a system is to look at if for yourself without hearing what someone else thinks it is. At least initially. I printed out every article that is archived and have almost read them all. That it alot of reading but worth it! Sorry for the rambling, here is the question. A while back there was a thread that talked about boiler turn-down. A gentleman stated that a certain manufacturer, maybe Viessman, only goes around 3.4:1 due to innadequate burner efficiency. Kind of like a bunch of pilot lights. So is it possible that the 10:1 ratios are correct, but the burning efficiencie are really poor? Is there a way to find this information out. I love reading everyone's thoughts on the subject, and would also love some sort of documentation. I am trying to learn everything possible. I feel as though the efficiencies are calculated in a lab of course, but do not always correspond to the real world. I will stop typing now so you can help me out on this. I met a few of you I think last year in Denver at Wetstock. That was a blast and hope to meet some of you in the future. I dream hydronics now and cannot wait to learn something new. Thanks in advance for all of your input and help.
Mr. Ugly


  • Doug_7
    Doug_7 Member Posts: 233
    Modulation range and Efficiency

    There are lots of condensing boilers out there with a 5:1 modulation range. AERCO makes condensing boilers with a 14:1 and 20:1 modulation range.

    Boiler Efficiency is maintained through the modulating range with a small increase in efficiency at lower firing rates. This is the so-called inverse efficiency curve. See manufacturers literature.

    The efficiency of conventional natural draft non-condensing boilers decreases sharply as the firing rate goes down and they have to cycle ON - OFF to match heat load. The big mistake here is to assume the efficiency of these boilers is the same value at all firing rates - it isn't.

  • Justin Topel
    Justin Topel Member Posts: 65

    Thanks for the reply. I am still conducting research into the matter. I have looked at various manufacturers literature, but I am not too trusting of everything they print. I am not dis-trusting, yet I want to see some actual data. Its seems that there are some ways to "fudge" the numbers by changing some parameters. For example do they all test their numbers at the same boiler room temps which will affect radiant and convective losses, do they measure just flue gas temps, do they measure total output of BTUs to the liquid, are they just talking about the heat exchanger only? And I would like to see all of the information together at the various points of modulation. Again I do not think that they are all lying, yet I would like some more transparency on how they arrived at their stated numbers. I know that if you don't ask the right questions, you might not get the right answers. I am not interested in in lab numbers, instead I like real world parameters since that is where I will be installing them. I know that insulation values for fiberglass insulation are tested in a lab, but the presence of a small gap anywhere in the insulation significantly reduces the overall R-Value. And tests performed by the National Resource Defence Counsel showed that due to air circulation and natural convection, the R-value of blown-in fiberglass insulation decreases by as much as 50 percent as the temperature drops from 45 degrees F to 18 degrees F. But the fiberglass insulation manufacturers will never tell you that. Sorry for the long post. I just want to give my customers the best product I can for the money they are spending. Just place yourself in their posistion. If you took your vehicle into a shop, you would want the best parts for the money you spent. Its simply the right thing to do. Thanks again for the response.

  • My understanding is that the greater the turndown ratio claimed, the more unstable the flame at low fire. this is for single valves of course not staged burners.

    Based on what I have seen on units with very high turndowns (I'm looking at YOU, Ultra) vs ones that claim more modest turndowns, I believe it too.

    However as long as the flame is stable, I don't believe efficiency is reduced on low fire. I have been wrong before though, once or twice ;)
  • Doug_7
    Doug_7 Member Posts: 233
    Boiler Efficiency Ratings

    For more information on boiler efficiency rating systems, suggest you go to the GAMA web-site at:


    Look under Residential Boilers near the bottom of the page.

    A lot of effort goes into getting the boiler efficiency ratings certified as correct.

    Also see:


  • Justin Topel
    Justin Topel Member Posts: 65

    Thanks Doug,

    I quickly checked out these web pages. I am planning on contacting GAMA concerning their test parameters. I am just curious to the areas they are covering and if they run their tests throughout the modulation cycle. I will keep you posted.
  • Doug_7
    Doug_7 Member Posts: 233
    Lookup efficiency ratings - Print Certificates - GAMA

    Look up efficiency ratings – and print out certificates on specific models for your customers – with a few simple clicks of the mouse.

    Also see Gama contact info at end.

This discussion has been closed.