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Oversizing a Modulating Boiler with 10 to 1 turndown

mwong00 Member Posts: 1
I bought an existing home with an oversized cast iron, non-condensate, non-modulating boiler. It was poor design as the boiler kicks on for less than 2 minutes before going off. This start stop is inefficient use of energy. The boiler managed to live 40+ years because it was cast iron, which is virtually indestructible. Oversizing a boiler was a typical mistake contractors made when installing heating systems. Now that I am staring at a $600+/month heating bill, I need to look at my options to make the system more efficient.

I found 3 options.

1. Safely reduce the output of my boiler by plugging orifices of the burners and readjust air flow accordingly. The net result is like having half the burners. I surmise that cast iron manifold will tolerate the temperature gradients. This solution poses the cheapest, quickest fix, but highest risk since nobody else seems to have tried it. I scoured the internet, and did not find a single engineering oriented discussion on the subject.
2. If I kept the same oversized boiler, I can increase the heat dissipation by Increasing the lengths of my existing finned baseboard emitters. After checking on the price of finned baseboards, and the cost of plumbing new baseboards to existing ones, I decided this is not something economically viable (baseboards and A/C contractors are not cheap)
3. Convert to a Weil McLain AquaBalance with a 10 to 1 turndown ratio.

Regardless of how big a boiler is, the max amount of heating is determined by the amount of radiant baseboard you have in your home It is easy to find out what heat dissipation you system has, as engineers has decided that a finned baseboard radiator can dissipate 600 BTU @ 180 F and 200 @ 120F). Just walk around your house, measure all the baseboard registers, add them up, and multiple the total length with those dissipation numbers. Assuming you are not going to add more baseboard or replace them all with low temperature baseboards, it is obvious that the boiler output should not exceed the maximum heat dissipation ability of your emitters. Anything bigger contributes only to constant start stopping. Something slightly smaller will actually be more efficient in a multi-zone house as not all zones will be “on" at all times. If you leave some rooms colder than others, those zones will be closed more often than zones where you want maximum warmth. Therefore a slightly UNDERSIZED boiler will be most energy efficient if you are replacing your existing boiler with a non modulating new boiler (cast iron or otherwise).

With a boiler that has some level of modulation (up to 5 to 1) , it is still wise NOT to oversize a boiler in order to get the maximum benefit of the modulation. I’ll give an example: If you doubled the size of a modulated boiler with a 5 to 1 turndown ratio, you will only be running at 2 or 3 at the coldest days. At best, you are left with only 2 levels to modulate. It will be ruining at Modulation Level 1 on an average cold day when it should be running at ¼ or less.

But, with the advent of Weil McLain’s AquaBalance that has a game changing turndown ratio of 10 to 1, you can OVERSIZE the modulating boiler by 25-30%. Why 25%? With the AquaBalance you can set the water set point to 135 F instead of a normal 145 degrees. That way the boiler can run at 70% capacity even on the coldest days. The lower temperature will lengthen the life of the boiler since the boiler is subjected to less heat. The other factor which increases the life span of the boiler is that it can run with less start-stop to fulfill the maximum heating demand. Since you are modulating at Level 7 (out of 10), you have 6 levels left on milder days. On each of the 6 levels, the system is working the heat exchanger less hard than on a perfectly sized system. With 6 levels left, this system is MORE EFFICIENT than a perfectly sized modulating boiler that only has 5 levels of turndown. Assume you have 6 Zones, and only 1 Zone is open. The AquaBalance can run at 10% (Level 1). Ideally, the pumps will run more and burners will cycle less. Depending on how well insulated or air tight your house is, you can adjust the OTR (Outdoor Temperature Reset), to further tune the system.

With a lower water temperature set point, the return water can be closer to the ideal 110 degrees or lower for maximum energy efficiency.

But what if you messed up on the heat requirements of your home and the boiler is too small? All a DIY homeowner has to do is to crank up water temperature set point in small increments until a comfortable level is reached.


  • mattmia2
    mattmia2 Member Posts: 8,889
    Add a buffer tank.

    Also make sure the 2 minute cycling is really because it doesn't have a system to dissipate the heat to, 2 minutes is really short if water is actually moving out of the boiler.
  • GGross
    GGross Member Posts: 805
    I think you have a fundamental misunderstanding of what modulation is for. All things the same and you take a burner that is on/off and one that modulates, you will use the same amount of fuel to heat a given space to a given temperature. Modulation is there primarily to eliminate short cycling, which generally lowers the life of the equipment. Longer cycles can also increase comfort level as there are less swings in the room temperature

    If you have undersized a boiler, making a change to the water temperature will not increase the boiler capacity, it will however change how many BTU/h your emitters can output. Water temp does not equal heating capacity of a heating appliance.

    In your situation you have a few options to lower fuel usage. you can lower the thermostat temperature, increase the insulation values of your home (windows walls etc), or you can increase the fuel efficiency of your heating equipment. This may be where you get the idea that modulation is strictly more efficient, you are comparing 80% equipment to 95% equipment. That being said you will only get that efficiency if you are able to keep your system temps in condensing ranges.

    I would also add that whatever you think is happening when you oversize the boiler and turn down the setpoint, that is not how it works. If you have baseboard heating, you will get the same BTU/h from the emitters at a given setpoint temperature and flow rate, whether the btu input of the equipment is properly matched, or oversized by double. Oversizing a boiler absolutely does not allow you to turn down the setpoint temperature, other factors may allow this, but boiler size is not one of them
  • pecmsg
    pecmsg Member Posts: 4,390
    How old is the home?
    Have any improvements been made, insulation, air sealing, windows, doors upgraded?

    Have a new Room by room heat load loss performed to determine exactly what size boiler you need.
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,638
    Interestingly enough, I think the short cycling = less efficiency is pretty overstated once you deploy some sort of "thermal purge" (obviously a new boiler should be sized as well as it can be, it's free/potentially cheaper to do it the right way). This old study shows some of the benefits of pumping all heat out of the boiler, regardless of size, once the heating call is over. Seems like most manufacturers now include this simple tech - the circulator simply runs even after the boiler stops firing.


    But, with the advent of Weil McLain’s AquaBalance that has a game changing turndown ratio of 10 to 1, you can OVERSIZE the modulating boiler by 25-30%.

    This is just marketing fluff- there's nothing game changing about a 10:1 turndown as that seems to be the industry's table stakes for mod-cons. A true game changing boiler might not have a larger turndown than 10:1, but would have a lower minimum firing rate than 8,000 btu. That seems to be about the floor.
  • hot_rod
    hot_rod Member Posts: 20,826
    10-1 is common, a few 12 and at least one 15-1 turndown.
    Any boiler over 10-1 they fudge a bit. I don't think they ramp down the blower any lower, they add an air shutter

    My steps would be:
    Room by room load calc. One or all the rooms may have lower loads then when built? Really no downside to knowing and identifying the high load areas. Reduce load, whenever and wherever possible!

    Measure the fin tube in each room. How does it match the load and at what temperature.

    If in fact there is more then enough fin tube, then look at output/ ft at lower temperatures.

    Numerous studies have shown reducing SWT lowers boiler operating cost, especially in condensing temperatures.

    I believe both modulation and condensing add to boiler efficiency.

    If the boiler is a bit oversized and it is a mod con, the lower the firing rate in condensing mode the more efficient it operates. The concept is you expose the boiler HX surface to low firing and you get addition condensation, more surface area.

    The holy grail is the 120F SWT or lower :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • tocker
    tocker Member Posts: 30
    edited December 2022
    Not sure if you are going to change emitters, but will add my recent experience any way..sorry for the rambling..hope it helps. For sure, oversize the emitters and go with a smaller boiler, as it provides flexibility if you can afford the space. I have a 3K SF 1955 brick and block rambler with little/no insulation on the main floor. The old boiler was rated about 110K BTU with fin-tubes. I replaced it with a Lochinvar WHB110 mod/con (same size) and 10:1 turndown, with 15 panel rads throughout on 3 zone manifolds. From the manifolds, I ran HePEX to Runtal radiators throughout. The Lochinvar is a great boiler so far, but it is far more than I need. Replacement occurred during COVID when boilers were in very short supply so a smaller one wasn't an option.

    Certainly, the heat loss calc should NOT be THE only consideration to size the emitters. I finally had a heat-loss completed by an experienced contractor who began to size the system strictly on that output, but I didn't understand the result any better than I understand Modern Monetary Theory. For instance the recommended radiators had a lower BTU output than the baseboard I was replacing and I already knew those weren't adequate from a comfort standpoint running 180 degree water. I channeled my inner-geek and built an EXCEL spreadsheet comparing my old fin tube output at 180 degree water to the various panel size options at various water temps in increments of 20 degrees (120,140, 160 etc) and based on a per SF heat load. In the end, I came to the conclusion that I needed the largest standard panels that would fit under the windows in each room in order to get to lower water temps.

    My goal for lower SWT was shaped by reading/studying from Siegnthalers book, Modern Hydronic Heating and his YouTube videos. I also watched and listened to Coffee with Caleffi and Taco pod casts and came here to ask several specific questions. Very helpful. I learned that Hot Rod, besides knowing more about this subject than I could ever know, likes fuzzy slippers and lattes and that... as a universal truth, size matters. So as he suggests, planning for lower water temps (larger emitters) will, I believe serve you well. You can adjust water temps and use/adjust variable speed pumps on a mod/con using outdoor reset. The down side to low temps: I didn't realize that the thermostats on my kickplate heaters were set at 130 degrees...which shows you how low I am running.

    The final result is that my house is very comfortable, even in the current 0-10 degree weather and my water temps are very low even with panel radiators (relatively speaking (110-140 degrees, no kidding) and it has extra capacity should the zombie apocalypse take out the sun. The boiler generally operates between 20-50% in 20 degree weather and about 40-60% in the current cold snap and I have seen it operating at 10% in the shoulder seasons. I'm very confident that given the larger emitters, a 50-70K BTU boiler would have been a little more efficient and just as effective, if it were available. My gas bill runs about 30% lower even with an oversized boiler, so I won't be losing sleep over it now.

    Recommend designing a system with a specific goal in mind from the start and plan in max flexibility. Minimize any adjustments or changes to those in the boiler room. Perform a heat-loss, but more importantly consider your current comfort. In my mind there is not one right answer, but the biggest risk is trying to plan a system around the exact right number or to have 'just enough' heat from the emitters at max water temps.