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Optimal flow rate for condensing boiler

subcooler Member Posts: 140
“The Vitodens engineers were WAY to smart to let this happen!!!” Give me a break. You do realize that there is no supernatural or mystic force of nature going on here when the Vitoden maintains its target temperature. It has two options to do (1) increase or decrease water flow to stay on target or (2) increase or decrease its btu input to stay on target plain and simple. The Germans thought that up all by their selves. I swear that the engineering department must be connected to the paint shop booth, because that orange paint must have affected their thinking. So back to the original poster if you require full output from time to time and you do cut your water flow your Vito will just detune itself to match your new water flow.


  • Dan Miller_2
    Dan Miller_2 Member Posts: 11
    Optimal flow rate for condensing boiler

    I have a Vitodens 200 8-32 for a two storey brick 1914 house(of 2700 sqft including basement) heating a combination of about 350 sqft of cast iron radiation on a fat pipe gravity system converted to forced circulation as well as 400 sqft of cast iron radiation piped home-run. The heat loss for the house was calculated at 70MBH. I have balanced the flow to all loops to so that the return temperatures are around the same and every room is heated around the same temperature. I have no thermostats or TRVs other than the outdoor reset. My heating curve is 0.9 with a +3 offset.

    A low loss header and Grundfos UP 15-42F circ pump was installed to and I have a feeling that it may be overpupmping. The system is most efficient with the lowest return temperatures but it seems like the system cycles more often than it should. On the lowest modulation the supply water is about 8C higher than the return and the boiler is unable to maintain the target temperature for very long (except on cold days) as the rising return temperature causes the boiler to shut down once the aquastat in the header overshoots the setpoint by a couple of degrees.

    I have no metrics to indicate what kind of flow the system is currently operating at and what the optimal flow should be. I have estimated the system head to be around 8 to 10 which would provide between 7 and 9GMP. I don't really want to replace the pump but would consider closing the main valve to reduce flow. Can more experienced wetheads help me out?
  • Dan Miller_2
    Dan Miller_2 Member Posts: 11

  • S Ebels
    S Ebels Member Posts: 2,322
    Sounds normal

    The 8-32 is plenty of HP for your connected load and it can't ramp down far enough. You're going to see some cycling until OD temps get down far enough to make it work.

    The 6-24 goes from 22 to 73 on LP and up to around 80 on Natural. The 8-32 starts out at 33,000 and hits 101 on LP and 112 on natural.
  • Dan Miller_2
    Dan Miller_2 Member Posts: 11

    Would reducing the flow not create a greater temperature drop across the radiators thus providing a lower return temperature?

    I am aware the 6-24 would have been better suited for the space heating requirement but since the boiler is also providing heat for the indirect DHW the Viessman rep was concerned about the DHW recovery rate and recommended the 8-32 instead. I can't tell if I'm really using the extra capacity of the 8-32 for DHW but I wish I had the lower modulation rate of the 6-24.
  • Mike E_2
    Mike E_2 Member Posts: 81

    It seems funny to me that everyone is so concerned with having alot of BTU's for the indirect when the vast majority of houses in America use standard gas fired hot water tanks with a 30,000 BTU burner. After the running efficiency of about 75%, that's only 22,500 BTU going into the water.

    The 6-24 would have handled your space heating and DHW needs with ease.

    What you can do with your system to reduce the cycling is to add a buffer tank. 30-40 gallons should do alot to help your problem.

    There was a post a few months ago with a link to a program that let you enter in data about your system, and then play around with buffer tank sizes and it will show you the changes to the runtime. I don't have the link, but hopefully someone will be able to give that to you.

  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Would reducing the flow not create a greater temperature drop across the radiators thus providing a lower return temperature?

    Yes, you would get a lower return temperature, but there's MUCH more. The average radiator temperature will also be reduced, thus radiator output will also be reduced.

    For a Vitodens that means that boiler output must also be reduced--since you're already concerned about it cycling at minimum modulation this would only add to the problem!

    Don't oversize your mod-cons! To a certain degree a high peak DHW demand can be compensated with a larger storage capacity indirect without having to oversize the boiler.
  • subcooler
    subcooler Member Posts: 140
    On a Forced Boiler

    Which most of all these mod-con jobs are, the correct boiler water flow would be as much flow as practical. The whole idea is to get all the heat possible extracted from the heat exchanger. This is not a promo for lots of pumps or circulators it is just simple math. The lower the TD across the boiler heat exchanger the more btu's you are going to recover. Unless you are one of those who believe that the water can move so fast that it can't pick up the heat. In your case you don't want to lower the waterflow unless you want to give up your 90% and settle for something in the 80% plus range.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    That might be true for some mod-cons but the Vitodens takes a different approach. The primary (boiler) flow is variable speed and strictly under the control of the boiler.

    When the low-loss header is used it will vary the primary circulator speed to maintain the boiler target temperature in the mixed supply portion of the low-loss header. This target will be maintained regardless of the amount of secondary flow. HOWEVER as secondary flow exceeds primary flow, the primary temp (that in the boiler itself) must increase above that in the mixed portion of the low-loss header. This can be verified by simple math. It also prevents just the sort of problem you suggested by, "don't want to lower the waterflow unless you want to give up your 90% and settle for something in the 80% plus range." The Vitodens engineers were WAY to smart to let that happen!!!

    When the Vitodens is directly driving a system (no low-loss header) it maintain target temperature in the boiler, not at a remote location as when the low-loss header is used.

    What I believe the original poster was suggesting was to reduce secondary (emitter) flow rate. Nothing at all wrong with that when using the Vitodens--in fact, it's best if secondary flow NEVER exceeds primary flow by more than 30% or so.

    BUT he was hoping that increasing the secondary delta-t by reducing secondary flow would reduce burner cycling. Won't happen--at least not with a Vitodens. His problem is that his boiler is oversized--a 6-24 would have been more than sufficient for his space heating yet for some reason someone convinced him he needed a larger model strictly for DHW production. VERY bad advice in my book...
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Nothing supernatural or mystic about varying BOTH the flow and the burner to achieve a target, but nobody else does it... Why???

    The original poster doesn't need ANYWHERE near full ouput from the boiler for space heating.

  • subcooler
    subcooler Member Posts: 140
    Chalk up another one for the Americans

    You ask why??? Because the Americans already knew that modulation adds absolutely NOTHING to the process except for move moving parts and controls to break down…….
    You don’t gain efficiency you actually lose efficiency. But you bought into the sales pitch because it just sounded so good.

    Can I prove this? Yes and so can you. The easy way is to just look at the ratings of a 90% two stage vs. a mod. No savings there.

    Next just use simple math and a little common (not so common) sense.
    Let’s model a simple system. The system has 2 boilers one hi-lo or 2 stage and 1 mod.
    The system holds a fair amount of water lets pick 2500 gallons. For the average day the system requires 30 mbtu’s of input power. The mod. we can run at 1.25mbtu’s 24 hours a day to meet the load requirement or I can run the 2 stage for a total of 9 hours a day at a firing rate of 3.3 mbtu’s per hour. Both systems have negligible losses because of the venting arrangements. The boiler jackets losses are negligible, so now we are down to the burners and heat exchangers. Sweet.

    I hope I don’t need to explain which is running cheaper. A hint: the one that is not ripping air accost it 24 hours a day. Not to mention the added electrical costs and excess air needed to sustain that process. Plain and simple. The Americans win once again with good old simple, not so high tech. technology. Once again I won’t make it on Dan’s brownie point list or published in his columns either so there.
  • ALH_4
    ALH_4 Member Posts: 1,790
    What about this

    Just so I'm sure I understand your scenario:

    30 MBH load continuous over 24 hours
    100% output boiler vs 4:1 turndown modulating boiler firing at 30 MBH.

    with 167F supply, and 140F return:

    @ 85%, 847 MBH of fuel are consumed in one day.
    @ 97%, 743 MBH of fuel are consumed in one day.

    That's a difference of 104 MBH per day or 30kW based entirely on modulation. Two 100W/h pumps running continuously use 4.8kW. I cannot remember the power consumption of the Vitodens off the top of my head, but it is certainly nowhere near 25kW/day. Really this comparison needs to be broken into dollars, and depending on the fuel and location the economics can vary greatly. 4.8kW of electricity is not the samecost as 4.8kW of natural gas. However, on a pure energy to energy comparison, modulation wins.

    Have I been sniffing paint too?

    Slowing flow will not help short cycling. An oversized boiler is an oversized boiler no matter the flow rate. The boiler dT can never exceed the system dT. There is not much else to do besides adding mass, most likely in the form of a buffer tank.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    The numbers you gave for a "simple system" are perposterous!

    2,500 gallons of water weigh about 20,800 pounds.

    Then an average daily load of 30 mbh or 30,000 btus. Such would only be enough heat to raise the WATER TEMPERATURE IN THE SYSTEM by a mere 1 1/2F.

    If this were a radiant panel system with 1/2" tube, 2,400 gallons (giving 100 gallons to the boiler, associated piping, etc.) would fill about 185,000' of tube! At 6" spacing that would be enough for about 92,000 square feet of panel. If this were a floor that would mean an "average daily" heat requirement of 1/3 btu per square foot. In the more conventional hourly heat load, that would be only 0.014 btu/hour/square foot loss!!!!

    Where would you find a boiler that could only give off 1,250 or 3,300 btu/hour?

    Do you actually mean MILLIONS of btus per unit of time? 30,000,000 btus for an "average" day = 1,333,333 btus per hour. With a very low "typical" loss of 15 btu/hr/sqft that's 88,888 square feet! Is your "simple" system a warehouse heated by a such a large GANG of "typical" mod-cons?
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Modulation looses efficiency? I certainly agree that such is (not so common) sense...

    How about (all too common) misunderstanding?

    You completely forgot about system temperature! If you're going to supply the full daily load in 9 hours on-off firing as opposed to 24 hours of continuous firing, you'd need MUCH higher average temperature in the emitters! Combined with your "keep flow high" recommendation that means MUCH higher return temperature. MUCH higher return temperature means MUCH less efficiency from a condensing boiler!

    Modulating load to loss allows for the LOWEST POSSIBLE system temperatures which allows for HIGHEST POSSIBLE boiler efficiency from a condensing boiler!
  • Danimal
    Danimal Member Posts: 8

    It Sounds Like your unit is oversized. With controls setup up to give domestic hot water priorety there is no reason to over size a boiler because of an indirect tank
  • subcooler
    subcooler Member Posts: 140
    Did I not mention temp. shame on me.

    If you keep your flow high and have high return temps. it means you are not doing any work at your emitters. By your thinking I am thinking that if you slow the water down to get a better return temp the system is running better? By this theory everyone should oversize forget that megasize with a mod. With your last statement I do agree in theory, but I have witnessed this several times and the 24 run modulation has no advantages over a hi-lo or on-off system. And I am not compairing a 85% to a 95%. 90% vs. 90% or whatever. The eff. is lost over the burner. 24 hours vs. 9 hours. Yes this is a large structure.
  • jp_2
    jp_2 Member Posts: 1,935
    U R correct subcooler

    if the simple case is just to heat 2500gal of water you;re right, no need to modulate.

    unfortunately that is totally incorrect for modelling a heating system. you need to start at the basics.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Are you even talking about condensing boiler(s) in this or similar applications(s)?
This discussion has been closed.