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Brad White
Brad White Member Posts: 2,398
you had already done the digging, Kevin- just had to ask!

Your design philosophy is hard to dispute- I think all would agree.

At least what you have going for you is mass. That water content can allow some rest between firings although it probably has a wider swing in the differential outside of setpoint. (Mass but not that much mass!). The 4-way at least does a good job of maintaining conditions I assume?

Short of replacing the boiler to one of the proper size (and which still has to drive the indirect), I would work with the controls to optimize differential and minimize firing cycles/maximize burn times.

It seems what you have as the largest efficiency loss is the oversizing factor (obviously) and that is hard to quantify. The "distribution side" seems to be ideal. Let's try to quantify that both in therms used and a comparable benchmark.

As for the "oil boiler on gas", is this a sanctioned application by Viessmann? Is there a catalog/factory-apporved match-up with a Riello or is it just a field conversion? May be "fine" although not "official" but I would not hold out for much warranty support should a problem occur. (Viessmann is an excellent company but no company can responsibly warrant "out of spec" applications.)

Is the original designer/installer still in business? I would contact them if possible. Diplomatic skills may be a bit pressed when approaching such a subject as you can imagine!

That all said, what is the home's SF and therms used? The cost per therm may be high but in line on the consumption side. Any ideas? I have folks reacting to high bills but are actually using less gas. Hard to explain how much more they would be using if not for their better systems.....

Before fully reacting, I would start with "fuel consumption per Degree-Day per SF" or some other empirical baseline and compare it to other such structures. I am sure these can be shared and reported by others in this forum for those that collect such things :)

My $0.01

"If you do not know the answer, say, "I do not know the answer", and you will be correct!"

-Ernie White, my Dad


  • Kevin A Gerrity_2
    Kevin A Gerrity_2 Member Posts: 27
    What to do

    I have a customer that has a problem. They called me about high heating costs. I went to their home and looked over the system, here is what i found.

    Viesmann boiler VBC-63 300,000 btu input, with Riello natural gas gun. 100 gallon Viesmann indirect hot water heater. The whole house is radiant floors (basement included). There is a 4-way mixing valve for the radiant with outdoor reset.

    The customer had the building plans and I did a heat loss with Wirsbo software. The heat loss was 84,957 btu's. I am meeting with the customer to discuse their options.

    How do you answer the question, when the customer asks is my system oversized and should the other company have known. The customer is about to spend alot of money on a solar system to off set their heating costs. I am giving them an option of replacing their six year old heating boiler. Have any of you come across this problem?

  • Brad White
    Brad White Member Posts: 2,398
    I can see the conundrum

    On the one hand everyone is complaining about high heating costs. The actual numbers are relative but ignored.

    It is a shame that such a nice boiler is apparently over-sized by nearly a factor of three but it may not have been sized for heat loss as much as for domestic production.

    Not that oversizing to drive the indirect is recommended, but if you back out the indirect load does it mitigate the "over-sizing" argument at all? The 100 -do you mean the 79 or 120 gallon heater?- takes some significant flow, (22 to 26.8 GPM and associated capacity) and that may be the basis for the sizing, whereby domestic trumps heating.

    At least they have radiant and good isolation from the domestic temperature spikes.

    In the end it may well have been over-sized but you should find out "by how much". How much less of a boiler could have been installed and still drive that indirect and whatever is connected to it? Might this be a case of a "domestic hot water heating system that also heats the building"?

    It may not be as bad as it first seems, but there is some digging to do.

    My $0.02

    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Kevin A Gerrity_2
    Kevin A Gerrity_2 Member Posts: 27

    I understand what you are saying about the hot water, but from what I have seen in the home there is no real large demand for this amount of hot water. Only four people, no large tubs or multi shower units. I try to size my systems for the heating and use priority for hot water. The customer has already spent a lot of money to try and save money, now they are going to spend more money to try and save the money they thought they where going to save? (Yogi)!

    The other thing that is real puzzling to me is the oil-fired boiler with a natural gas gun? This home was built six years ago and the system was design for this home. I am trying to understand why this type of system. I have talked to Viesmann and they're not sure either.
  • Kevin A Gerrity_2
    Kevin A Gerrity_2 Member Posts: 27

    Well, I meet with the homeowner this mourning and we discussed their options and now they want me to bid on the solar work they are looking to add. I just explained to them what the heat loss said and what I would have installed, leaving the other company out of the conversation. Nothing gained from bad mouthing

    The company that installed the system is still in business and I talked to them they said they install what the supply house spected. I know the owner and we talked about the job.

    The Riello natural gas burner was not retro it is a gas gun. I know that they are approved, I just do not understand why to use them.

  • Efficiency

    a wet-base boiler with a gas gun is maybe 6% more efficient than the typical atmospheric. Plus it allows the owner to switch fuels without buying a new boiler. But I agree, if you're going to do this it's best to use a factory- rather than a field-engineered setup.

    Our company installs these factory-engineered wet-base boiler/gas gun combinations frequently. Here's one example, on a steam system:


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  • zeke
    zeke Member Posts: 223

    Since you have all that overcapacity, why don't you use the indirect tank to supply the hot water heating and add another more modest indirect for hot water. The 100 gallons using a differential of 30 degrees will absorb 24,000 btu and take this unit about 24000/300000*.8 hrs or about 6 minutes of burn time. At full load. 80,000 BTUH, it would take 24000/80000 hrs or 18 minutes to dissipate the stored energy. At 1/2 load say, 35 deg outside temp, you would have 9 minutes of off time. I would think this beats changing boilers.
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