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Condensing Boiler with Tube & Fin radiant system???

I have to replace my boiler in my 1890 victorian home. A few contractors have come out and 3 have suggested the Buderus GB142. However, one contractor said that condensing boiler was not a proper option for my situation and refuses to install one.

His reason are:

1) Tube and fin radiation, which is the type you have, will only work properly with 150* water minimum.  It is also rated for a 20* temperature drop returning to the boiler.  That means the return water is 130*, too cold for the radiation to create convection, which is how it works.

2) Once return water temperatures reach 140*, condensing begin to stop condensing.  Once they stop condensing, extra stress is put on the heat exchanger and premature equipment failure is very possible.  These units also will short cycle or cut on and off frequently trying to maintain these higher supply water temperatures they were not designed for.  Short cycling is very costly as it wastes a lot of fuel.  You probably would see your fuel usage and cost go up.

Another contractor also recommended a Rheem Commercial booster water heater with cupro nickel heat exchanger . I have no idea how this works but he said it would move the efficiency up to 95%.

I'm confused. Which type of boiler is appropriate for installation with radiant water baseboard heat.

By the way I live in Virginia.

Comments

  • Gordan
    Gordan Member Posts: 891
    I yi yi.

    1) Baseboard "radiant" - the contractor is correct in that fin and tube baseboard is designed to produce most of its heat output through convection (creating air currents over the heater element.) It does produce some of the output through radiation. Both types of output are reduced as you reduce the average water temperature through the heating element. Since, however, your baseboard is (supposed to be) sized to keep your house warm on the coldest day with a given water temperature (say, 180), it can keep your house warm on warmer days (read: most days) with less warm water. That's as much as one can say without calculating exactly what your heat loss would be, and what the required supply temperatures would be given the type and length of baseboard you've got. (One baseboard output chart I've seen shows that at 130 F average water temperature, the output is 40% of what it would be at 185 F average water temperature.)



    2) Condensing boilers are designed to be able to safely operate at lower temperatures, which cause flue gas condensation. That condensation is acidic and would, if sustained, corrode the heat exchanger of a non-condensing boiler. There is nothing in the design of condensing boilers that makes them less able to operate at non-condensing (higher) water temperatures - that's just silly. As for cycling, since just about every condensing boiler is also able to modulate (adjust its output to what's needed to keep the water temperature at a given setpoint) it is far less likely to short cycle than a boiler that's either fully off or fully on.



    What can safely be said is that yours is not the ideal application for a condensing boiler - you'll likely not be able to operate in the 90+ efficiency range all season long - but you'd still get some of the benefits, and may be able to condense some of the time. Whether the benefits would be enough to offset the added cost of a mod-con boiler (which may not be that much, given the tax rebates) is up to your contractors to calculate and you to decide.
  • CC.Rob
    CC.Rob Member Posts: 128
    go condensing, but...

    I have a house full of fin-tube. It never goes above 135F supply, even at 5F outside. It was originally sized for 180F at 5F outside. Most of the year it runs between 80-115. Works great, using an outdoor reset control and indoor feedback. House is incredibly comfortable, second only to the radiant house I grew up in.



    Now for the buts....



    1) An 1890 house has HUGE room for improvement to reduce heat loss and thus energy required to heat it. Like 50% or more. Reducing heat loss through insulation and air sealing is the simplest and most cost-effective way to save energy and improve comfort. Start with a blower door test and thermal IR imagery done by a professional contractor and develop an insulation and air sealing plan from that. There are good federal tax credits for weatherization. Often state and utility programs as well. Check www.dsireusa.org for list of things in VA.



    As you reduce the heat loss, you reduce the supply water temperature needed to heat the house. So now you can be condensing nearly all year long and reaping the extra efficiency of the modcon.



    2) Others may disagree, but IMHO the aluminum-block heat exchangers are bested by the stainless (there are many manufacturers and models from which to choose -- find a good installer). The aluminum based systems are more sensitive to system water chemistry, which needs to be checked, maintained, etc. among other things.



    Good luck.
  • CMadatMe
    CMadatMe Member Posts: 3,066
    Fin Tube

    Your contractor is clueless. Did he do a heat loss. Did he tell you that you only need the full output of the board when the temp outside is design conditions. Have plenty of mod/cons out there as Rob's house is.



    Properly installing a mod/con is not just being able to pipe the boiler. Starts with the heat loss and proper design. But estimates are free right? So you get free and the anwser you got from the contractor.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
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