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Buderus G115: what does the word "necessary" mean in your opinion

timo888timo888 Member Posts: 137
The Buderus G115 installation manual in section 3.2.1 General Operating Requirements (p. 9) talks about getting the boiler to 150F within 10 minutes when the boiler is controlled by aquastat, as ours is.


"During burner operation one should achieve the minimum boiler temperature within 10 minutes after burner start-up by means of flow reduction and one should maintain this temperature."


We have two zones: radiators and DHW. When the radiators zone is calling for heat, and if the radiators are cold, our G115/4 (aka G115/28) boiler with 98,000 btu Riello takes 20 minutes to reach 150F. If the DHW is cold and calling for heat, it takes 15 minutes to reach 150F. I haven't tested the scenario where both radiator and DHW zones are calling for heat simultaneously. It stands to reason it would take longer than 20 minutes.

The manual has a matrix that appears to state that a mixing valve is "necessary" ( page 9, section 3.2.1.) A mixing valve is on my to-do list.

As a professional installer who is familiar with installation manuals from a variety of manufacturers, would you say the Buderus manual is clear about this mixing valve requirement? About the minimum boiler temperature within 10 minutes requirement?

Or is it reasonable to argue that the manual does not make either or both requirements clear?

Me, I happen to think the word "necessary" is unambiguous, and the requirement that the boiler reach 150F in 10 minutes is also very clear, but I'm just the customer, not the installer.

The upshot of this is not simply academic--I want to deduct the cost of having the mixing valve installed from what I owe the heating oil company for the last oil delivery at $4.50/gallon, about $800. I think they've not only been wasting my money by not installing the mixing valve, they've possibly shortened the life-span of the boiler, since inadequate temperatures can lead to condensation. I think the manufacturer put this into the installation manual for a good reason, and that it must be important.

BTW, I've calculated the volume in our Direct Return system, less than 15 gallons in the pipes plus ~11 gallons in the boiler. About 25 gallons total. 192 linear feet of 1-1/4" supply and return mains, plus about a dozen convector-style radiators with 1/2" supply pipe and 1/2" return pipe, in a 3-bedroom two-storey post-WW2 brick colonial, uninsulated with old windows. Climate: near Philadelphia PA. So we aren't a "larger water volume" system, yet with the old uninsulated house with old windows, whose radiators are located on exterior walls, our house presents some of the same challenges.


  • Brad WhiteBrad White Member Posts: 2,393

    is a term I write into my specifications for things about which one cannot do without. "Essential", "must", "shall", "required", also fall into the line of command language.

    It is not "optional", nor "discretionary", nor left to others to interpret, in my opinion.

    I cannot say what you should do with that, but I think it gives you some footing, if not moral or legal authority, to pursue remedies.
    "If you do not know the answer, say, "I do not know the answer", and you will be right!"

    -Ernie White, my Dad
  • timo888timo888 Member Posts: 137
    how about "should" in the 150F

    Thanks for the opinion.

    Would you say "should" in the excerpted sentence about reaching minimum temp within 10 minutes is more of a recommendation, another way of saying "Best practice would be to..."

    Does "should" leave it up to the installer's judgment, or is that, too, more like a requirement?
  • Brad WhiteBrad White Member Posts: 2,393
    edited May 2011

    in my opinion, is (obviously to most), a softer term and would have associated qualifiers. The qualifiers would be along the lines of, "Such and Such should be done to assure that (something something happens or does not happen".

    Depending on the context and specifics, if a certain event was to be prevented (say condensation in the boiler to be avoided) and that did occur, then I would say you have some justification.

    I have to step back on this because while this is my opinion, I do not know where you intend to go with this, nor what was specifically contracted, including results and performance. This would include adhering to "best practices", a very wide door sometimes.

    In engineering and specific to best practices, there are as many ways to do one specific thing as there are engineers to stand around and point at stuff. Trust me.  In the end, what is justifiable is to demonstrate a return on investment or other means of energy savings which pays for additional costs to make those savings happen. That is the standard of care, at least to spell out such things so the decision is an informed one. From that point is where you get the different ways to get to the same result.
    "If you do not know the answer, say, "I do not know the answer", and you will be right!"

    -Ernie White, my Dad
  • meplumbermeplumber Member Posts: 678
    Having installed a lot of G115's over the years....

    I am a little confused about the mixing valve you are referring.

    Is it a tempering valve on your domestic hot water or on your heating loop?

    If it is for domestic hot, then it varies by jurisdiction.  It is best practice, but not required for all codes.

    If you are referring to heating, then that is a different story.  The practice of using mixing valves is a carry over from the European practice of using mixing valves on all installs.  You see this in the literature published by Viessmann and Buderus specifically.  It is not necessary.  If all of your radiation is high temp design (fin tube baseboard or convectors) then there is no need for the mixing valve.  The Europeans do not use fin tube.  The standard over there is flat panel radiators that are designed for lower water temps, hence the mixing valve to provide some amount of boiler protection from low water temps.

    The casting in the G115 is capable of sustaining 104 deg return water temps.  Were you using the Logamatic control instead of an aquastat, the control would hold the system circulator out until the water temp reached 104.  Then it would cycle it off and on until it held above 104.

    I would not be concerned about it taking 20 minutes to come up to temp.   Remember that 1 BTU is the amount of heat required to raise one pound of water 1 deg F.  If you have a 25 gal system at roughly 9 lbs per gallon.  That is 225 lbs of water, plus the heat absorbed by the casting, plus thermal efficiency losses.  So that is 225 Btus for every degree that you are trying to raise the water temp.  If the system is dead cold and you try to go to 150.  Then you are looking at a 100 deg rise.  With a .65 nozzle that is pretty close. 

    Unless my math is off.  Brad is better at math than I am.

  • IronmanIronman Member Posts: 3,408
    Another Opinion


    I've seen and read several of your posts here in the last couple of months and two things appear to me throughout them. First, you are a person of above average intelligence who is diligently seeking to educate himself as much as possible about his hydronic system. You are to be commended for this. Other than JDB, I don't recall a homeowner seeking as much info as you have. And, you seem to be able to quickly grasp things that many have trouble with.

    Second, you believe (and it appears so) that there are some deficiencies in your boiler installation.

    I am an independent HVAC designer and contractor. I have extensive experience with Buderus and your model of boiler. If you'll permit me, I'll try to give as an objective answer to your situation as possible with the facts at hand.

    You correctly quoted the Buderus manual about getting the boiler to 150F minimum operating temp. within 10 minutes of firing.


    "During burner operation one should achieve the minimum boiler temperature within 10 minutes after burner start-up by means of flow reduction and one should maintain this temperature."


    You then  refer to the same table about needing a mixing valve: "The manual has a matrix that appears to state that a mixing valve is "necessary" ( page 9, section 3.2.1.) A mixing valve is on my to-do list." 

    Then, you state further on:"The upshot of this is not simply academic--I want to deduct the cost of having the mixing valve installed from what I owe the heating oil company for the last oil delivery at $4.50/gallon, about $800. I think they've not only been wasting my money by not installing the mixing valve, they've possibly shortened the life-span of the boiler, since inadequate temperatures can lead to condensation. I think the manufacturer put this into the installation manual for a good reason, and that it must be important."

    The problem is that you have come to an erroneous conclusion here. Let me explain: First, the requirement for 150deg boiler temp can be accomplished by "flow reduction" as the manual states, and as you quoted. There are several ways this can be accomplished. The simplest would be to throttle the flow by partially closing a valve on the supply line. Not necessarily the best way, but the most simple. Just by doing this, your installer has met the requirement.

    Second, you are confusing a mixing valve with a diverting valve. Actually, the same valve can many times do both depending upon the way it's piped. But in hydronic terminology, a mixing valve has two inputs and one output; a diverter valve has one input and two outputs. The mixing valve that the Buderus manual is referring to is for a "mixed temperature heating circuit", not boiler bypass protection. This is to maintain a certain temp. in the heating circuit (such as radiant floor), not the boiler. If you will also notice, the same table states that there is no minimum return temperature requirement if the system has a water content of less than 115gal per 100k btu's. By your own calculations, yours has far less than this, so there is no minimum requirent for return temp.

    There is a reason that Buderus cast iron boilers don't require the usual return water temp. protection: their cast iron is more highly refined and contains silicone injected into it. It is actually flexible. They have samples formed like the concentric element of and electric stove that you can substantially flex in your hand. Much more than the stove element would flex. Trust me, I've done it. There's a pic somewhere on their web site of a guy flexing a helix coil with handles made from their c.i. He has it bent 180 deg. Because of their grade of c.i., Buderus doesn't have an issue with thermal shock or condensation as long as their instuctions are followed.

    Regarding deducting repaiars from your fuel bill: You have no legal ground for doing this. And even if they have incorrectly installed your boiler,and you were to do this, you could end up with a judgment against you and a negative credit report. Your legal recourse would be to sue them for deficiencies, but not to withhold payment of a fuel bill. Also, unless you could produce expert testimony that the install was done incorrectly, you would loose the case. Producing another contractor to condemn the first one is not considered expert testimony in most cases.

    I would recommend that you try to resolve any legitimate disputes directly with your contractor and consider the legal system only as a last resort.

    I would also recommend that you have the 2107 Logamatic control installed. It could possibly save you 20 - 30% on fuel consumption and it address all the issues of boiler temp,. condensation, etc. that you have been concerned about.

     Please don't think that I'm covering for your installer. I am known for being tough on contractors. I believer they ought to do the job correctly and have little patience for those who don't. But, I think in this instance the contractor has not violated the installation instructions. At least regarding what is in this post.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • timo888timo888 Member Posts: 137
    edited May 2011
    mixing it up


    Thanks for the detailed reply.

    As for legalities, I appreciate the advice. I don't want to take a hit on my credit rating. (I'm self-employed and need to protect my ability to borrow!) But I do want the heating oil company to have followed the manufacturer's installation instructions and to have installed the system according to local code.

    I had no idea that my installation was in any way deficient until, after posting some pictures here, was informed by forum regulars that it fell shy in a number of respects, some more serious than others.

    "That gives oil a bad name"

    "I'd be embarrassed if that were my work. It's crap".

    That news about its deficiencies coincided with a late-season invoice for delivery of oil at $4.50/gallon. Here is the mixed bag of items. Some, I understand, are minor. Some more serious. Nothing critical, as far as I know.

    1. No backflow prevention valve.

    2. No ball-valve shutoff on expansion tank and no drain, and the feed valve goes directly into the boiler, not into supply line below air separator

    3. no Fire-O-Matic valve and the fuel filter was just perched on DWH tank; no bracket;

    4. the home run from the electrical panel had no ground wire; system not grounded;

    5. no emergency shutoff switch at top of stairs -- just on top of boiler

    6. although I've been paying for service since 2006, in those 5 years they've run only two combustion tests; and on the most recent test run in late March, the test date printed on the printout was not the date of the test but was off by several months--possibly reprinted from the device's memory, or tech just forgot to set the date, don't know which

    7. the oil company has never measured the draft or adjusted the damper

    8. ever since the late March service the system was having delayed ignition problems, possibly something the tech did (it took him 30 minutes to get oil flowing through the pump, and he may have burned out the pump--a subsequent tech replaced the pump and said it looked damaged). Or it could have been a problem with the oil (air). System had been working fine until the service/delivery. Eventually I bought a TigerLoop and had it installed by another service company and the delayed ignition problems went away.

    9. The OPV valve was frequently purging water and the pressure gauge was reading 40psi; they did nothing at all to see if the water in the boiler was at the correct level, but told me simply to keep the feed valve shut at all times, although there is no LWCO on the system.

    10. the oil company never discussed boiler-room air volume requirements with me when selling the system; we are borderline, possibly under requirement if other exhaust fans in the house are operating such as dryer and bathroom exhaust, but house is quite drafty so I'm not overly concerned

    11. They never discussed how condensing boilers work, and that I'd need to install a $2000 chimney liner because of the eating away of the chimney by acids from the flue gases condensing. I found that out afterwards.

    12. The farthest radiators are always cold (we have a direct return system).

    It's the last item that go me to reading about return temperatures and how too cool a return temperature could cause condensation inside the boiler. Even though Buderus is a condensing boiler, the company does seem to warn against too much condensation. It is difficult for a consumer to find out how much is too much, and opinion seems to vary.

    Although we have a low-volume system ( much less than 115/gal per 100K btu) that arbitrary threshold of 115/100K MUST be based on certain assumptions that will vary from installation to installation, so I take it to be only a rule of thumb.

    These variables would include the presence or absence of home insulation, how drafty the area being heated is, how cold the climate is, whether or not the piping is PEX or iron, in brief, how much heat gets sinked out of the supply water as it travels through the system.

    In some respect our freezing cold radiators which are located on the exterior walls of an uninsulated brick house, directly below 1940s-era windows, have something in common with a snow-melt system. That's an exaggeration, but you get my drift. The uninsulated brick is a giant heat-sink.

    In mid-winter the radiator zone return is only lukewarm. It could be approaching 104F.

    Descrbing this problem led people here to suggest a 3-way mixing valve on a bypass leg.

    INPUT 1: supply from bypass

    INPUT 2: return from radiator zone

    OUTPUT: radiant zone water made warmer by admixture of supply

    Although I did confuse the mixing valve in the matrix (which I nw see is for multi-temperature multi-zone installations) with the mixing valve for boiler protection, there is a diagram for high-water-volume systems that shows a manual bypass without a mixing valve, which is designed to bring up the return temperature for installations where it is too low. 130F seemed to be the target temperature to shoot for.

    I do not recall seeing that return temperature as low as 104F is AOK. Not that I doubt you. I just have never seen that number in any document I've been able to get my hands on. The manual seemed to suggest that if your system wasn't able to reach 150F in 10 minutes, it's because your return water was too chilly, bringing the system temp down. That's why I had a 3-way thermic mixing valve "on my list".

    Thanks for sharing your knowledge and opinions. I am taking them in and trying my best to learn.
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